key: cord-0894243-ei4rb8fr
authors: Liu, Shijie
title: Chapter 16 Combustion, reactive hazard, and bioprocess safety
date: 2020-12-31
journal: Bioprocess Engineering
DOI: 10.1016/b978-0-12-821012-3.00016-6
sha: 8d3dedbbafb9a2039dce4da46aa4b08c6ae86c33
doc_id: 894243
cord_uid: ei4rb8fr

Abstract Is bioprocess dangerous? How to maintain safety? What is reactive hazard? What is biohazard? Chemical or biotransformation means change. Any changes have irreversible consequences. Managing the changes can help us, while changes can also be harmful. The first-line of defense on any potential unwanted transformations is isolation or containment. WHO defines four-risk groups of biosafety levels. There are six key knowledge bases for biosafety. With chemical safety, substance identification is the first step. The global harmonized system (GHS) of classification and labeling is generally agreed by the world bodies. The hazards are divided into three categories—physical, health, and environmental hazards. Chemical reactivity hazard is a situation where an uncontrolled reaction could result directly or indirectly in serious harm to people, property, or equipment. Fire and explosion are common reactivity hazards. To safe guard processes and facility, one needs to first identify the threat, or screening for reactivity hazards. Combustion processes proceed with chain reactions and the reaction is highly exothermic. Combined with the high-activation energy, combustion leads to large-temperature dependence. The overall reaction products can also vary (partial oxidation versus total oxidation). Partial oxidation reactions are important in chemical industry. Pyrolysis and gasification of biomass, for example, are useful technologies in biomass conversion. An explosion is a fast, transient, exothermic reaction. Unintended and accidental explosions can occur due to (1) The accumulation of reactive chemicals (gas, liquid, or solid) in confined space; (2) Leakage of “significant amount” of reactive chemicals (gas, liquid, or solid) to atmosphere or environment; or (3) Reactor runaway. As a bioprocess engineer, you will be responsible for creating safe processes and protecting the lives and health of your colleagues.

are important as discussed in Chapter 15. We need to consider operating modes outside of the regime of optimal design and determine that all possible situations in which the system may be operated will not lead to uncontrollable consequences. Chain reactions and/or uncontrolled combustion/explosion are often associated with accidents and thus we shall have a discussion on the combustion and their applications in industry as well.

Biological agents or living organisms by nature-pose hazards to other living organisms including human. For bioprocess engineers, microorganisms are commonly of concern, although macro-organisms such as animals, insects, and plants could be more common for agricultural biological engineers and biomedical engineers. In this chapter/section, we will limit our discussions on microorganisms. The relative hazards of infective microorganisms are classified by risk group as shown in Table 16 .1, which is often used for laboratory work only.

In aligning with the four bio-risk groups, laboratory facilities are designated in meeting four biosafety levels as well. The four biosafety levels (BSL) as shown in Chapter 12 are-BSL 1 -basic; BSL 2 -basic; BSL 3 -containment; and BSL 4 -maximum containment. Biosafety level designations are based on a composite of the design features, construction, containment facilities, equipment, practices, and operational procedures required for working with agents from the various risk groups. Table 16 .2 relates but does not "equate" risk groups to the biosafety level of laboratories designed to work with organisms in each risk group. Different countries (regions) draw up their own national (regional) classification of microorganisms, by risk group, taking into account:

1. Pathogenicity of the organism. Agents not associated with disease in healthy adult humans.

(No or low individual and community risk) A microorganism unlikely to cause human or animal disease.

Risk group 2 Agents associated with human disease that is rarely serious and for which preventive or therapeutic interventions are often available.

(Moderate individual risk; low community risk) A pathogen that can cause human or animal disease but is unlikely to be a serious hazard to laboratory workers, the community, livestock or the environment. Laboratory exposures may cause serious infection, but effective treatment and preventive measures are available and the risk of spread of infection is limited.

Risk group 3 Agents associated with serious or lethal human disease for which preventive or therapeutic interventions may be available (high individual risk but low community risk).

(High individual risk; low community risk) A pathogen that usually causes serious human or animal disease but does not ordinarily spread from one infected individual to another. Effective treatment and preventive measures are available.

Risk group 4 Agents likely to cause serious or lethal human disease for which preventive or therapeutic interventions are not usually available (high individual risk and high community risk).

(High individual and community risk) A pathogen that usually causes serious human or animal disease and can be readily transmitted from one individual to another, directly or indirectly. Effective treatment and preventive measures are not usually available. 

Mode of transmission and host range of the organism. These may be influenced by existing levels of immunity in the local population, density, and movement of the host population, presence of appropriate vectors, and standards of environmental hygiene. 3. Local availability of effective preventive measures. These may include-prophylaxis by immunization or administration of antisera (passive immunization); sanitary measures, for example, food and water hygiene; control of animal reservoirs, or arthropod vectors. 4. Local availability of effective treatment. This includes passive immunization, postexposure vaccination, and use of antimicrobials, antivirals, and chemotherapeutic agents, and should take into consideration the possibility of the emergence of drug-resistant strains.

Biosafety is paramount to the public and potential of biohazard needs to be conveyed clearly to the personnel involved. Fig. 16 .1 shows the international warning sign of biohazard. Local languages are to be adopted to ensure the message can be understood. The assignment of an agent to a biosafety level for laboratory work must be based on a risk assessment. Such an assessment will take the risk group as well as other factors into consideration in establishing the appropriate biosafety level. For example, an agent that is assigned to Risk Group 2 may generally require Biosafety Level 2 facilities, equipment, practices, and procedures for safe conduct of work. However, if particular experiments require the generation of high-concentration aerosols, then Biosafety Level 3 may be more appropriate to provide the necessary degree of safety, since it ensures superior containment of aerosols in the laboratory workplace. The biosafety level assigned for the specific work to be done is therefore driven by professional judgment based on a risk assessment, rather than by automatic assignment of a laboratory biosafety level according to the particular risk group designation of the pathogenic agent to be used. Table 16 .3 summarizes the facility requirements at the four biosafety levels.

Thus, the assignment of a biosafety level takes into consideration the organism (pathogenic agent) used, the facilities available, and the equipment practices and procedures required conducting work safely in the laboratory.

The concept of Biological Safety (or biosafety) has paralleled the development of the science of microbiology and its extension into new and related areas including tissue culture, recombinant DNA, animal studies, molecular biology, synthetic biology, and biotechnology. The knowledge and skill gained by microbiologists necessary to isolate, manipulate, and propagate pathogenic microorganisms required parallel development of containment principles, facility design, and practices and procedures to prevent occupational infections in the workplace or release of the organisms to the environment.

Biosafety entails safe microbiological practices, procedures, and proper use of containment equipment and facilities. To be prudent in biosafety, one must develop knowledge of the principles of epidemiology, disease transmission patterns, risk-assessment and risk management, disinfection and sterilization, disease prevention, aerobiology and environmental control. The particular proficiencies a biosafety professional should possess as described by American Biological Safety Association (ABSA) include:

1. Disinfection, decontamination, sterilization. What is the difference between sterilization, decontamination, and disinfection? When and/or where to apply these eradication technique? How do you monitor their efficiency? How and where to use, and what are the potential hazards (explosive, flammable, corrosive, carcinogenic, and irritating) associ- ated with various disinfectants and sterilants? How to use chemicals, steam, dry heat, irradiation, filtration, ultraviolet (UV) sources, gases, or other agents to kill or inactivate microorganisms? 2. Work practices and procedures. What are the good microbiological techniques, including sterile (aseptic) techniques?

Understand the importance of developing, evaluating, and documenting exposure control procedures for biohazardous agents and materials. Develop procedures and practices to prevent release of infectious aerosols from equipment. Perform biosafety audit of work practices and procedures associated with large-scale operations. Understand and apply monitoring techniques and equipment to determine effectiveness of exposure control measures and to investigate environmental problems. Understand the use and disposal of sharps. Select and understand use of personal protective equipment. Select and understand use of respiratory equipment. Develop and implement procedures for managing biohazardous spills and releases. Assure documentation of worker exposure to biohazardous materials and preparation of an incident report. Develop comprehensive emergency response plan for biohazard areas.

associated with microbial exposure. Assess the risk of occupational exposure and infection associated with handling infectious agents. Must be familiar with routes of exposure, modes of transmission, and other criteria that determine the hazard category of a microorganism. Able to assess the risk to the community from various work environments where infectious agents or sensitizing materials may be present. Understand microbial toxins and their potential to cause work-related illness. Able to recognize the characteristics of bacteria, viruses, fungi, and parasites. Understand the hazard of exposure of service personnel to biological materials. Understand factors that may affect susceptibility, resistance, or consequences of infection. Understand the difference between risk of infection and consequences of infection. Understand the risk associated with biological aerosols in the workplace, such as ventilation, indoor air quality, recirculation, and cooling towers. Understand the risk associated with point source release of biological aerosols in the workplace, such as from homogenizers, cell sorters, centrifuges, fermenters, and lasers. Understand the risks associated with recombinant DNA technology. Know the unique biosafety conditions associated with naturally or experimentally infected animals, including nonhuman primates. 4. Regulatory aspects, standards, and guidelines. Interpret and apply the NIH guidelines for research involving recombinant DNA molecules. Interpret and apply OSHA Blood-borne pathogens standard. Interpret and apply guidelines that classify biohazardous agents according to risk. Interpret and apply guidelines for preventing transmission of Mycobacterium tuberculosis in the workplace. Interpret and apply regulations for packing, labeling, and shipping of infectious materials, diagnostic specimens, and medical waste. Interpret and apply import and export requirements associated with biological materials. Interpret and apply regulations associated with animal pathogens. Interpret and apply guidelines associated with the large-scale use of microorganisms. Interpret and apply the NSF International Standard/ American National Standard on Class II laminar flow biohazard cabinetry (NSF/ANSI 49). Interpret and apply OSHA law, standards, and directives as they relate to biohazards. Interpret and apply guidelines and regulations relating to infectious and medical waste. Familiar with agencies, such as WHO, CDC, NIH, OSHA, AAALAC, DOT, IATA, ICAO, DOD, EPA, USDA, and FDA, and their role and relationship with biosafety. Interpret and apply the CDC-NIH Biosafety in Microbiological and Biomedical Laboratories document and other pertinent CDC publications. Interpret and apply select agent regulations. 5. Program management and development. Understand the role and function of an institutional biosafety committee. Prepare and maintain a biosafety manual. Review project proposals and advise on biosafety issues. Advise on occupational health programs for persons working with biological materials. Provide and interpret biosafety resource and reference information. Organize and implement institutional biosafety compliance programs and audit their effectiveness. Institute, evaluate, and document biosafety training. Identify biological agents and materials in your institution. Develop and implement an infectious and medical waste-management program. Provide technical information and advice on products impacting biological safety. Develop and recommend biosafety policies. 6. Equipment operation and certification. Understand the use and validation of a steam autoclave. Understand the use and certification of biological safety cabinets (BSCs). Demonstrate knowledge of Class I, II, and III BSC design features, applications, and functions. Understand the calibration and use of air-measuring instruments to verify the safe operation of biological-safety equipment. Understand the design, function, and efficiency of HEPA filters. Understand the limitations in the use of equipment, such as fume hoods and clean benches, for work with biohazardous materials. Understand the use and validation of sterilizers using ethylene oxide (ETO) and vaporized hydrogen peroxide. Understand the equipment and chemicals used for space decontamination. Understand the use and applicability of animal containment equipment. 7. Facility design. Understand the functions and indications for use of primary and secondary barriers. Understand the difference and appropriateness of facility design to balance the need for hazard containment, personal product, and environmental protection. Review architectural and engineering plans and advise on biosafety issues. Verify that facilities as built meet minimum biosafety design criteria.

Before leaving this topic of biohazard, we should emphasize that biohazards or more specifically hazards of biological organisms is relative slow and have varied standards in different countries. Containment is the first-line of defense in combating biohazards. While dealing with a specific threat, the methods are similar in dealing with reactive hazards as we will learn in the subsequent sections.

While biohazards or more specifically hazards of biological organisms can be relative slow and have varied standards in different countries, chemicals hazardous to the living can be dramatic and sudden. While containment is the first-line of defense in combating biohazards, containment alone is not effective in the prevention of reactive hazards. No containment is perfect and breaching of barriers can occur and is what we term accidents. After fires and explosions, the major accident that can occur in chemical processing is the release of a chemical that is hazardous to humans. Recall that one of the green chemistry tenants is to eliminate or reduce the use of toxic chemicals in a process. Benign solvents, such as air, water, carbon dioxide, and low-volatility solvents, such as ionic liquids and molten salt, are preferred. Still, the products we seek or produce are also commonly harmful substances when improperly released. Air and water can become contaminated with chemicals that cause harm. These hazards can be acute (causing immediate and critical harm to people as in the Bhopal accident in India) or chronic (ground-water contamination which can require years and millions of dollars for cleanup). 

Major injury likely unless prompt action is taken and medical treatment is given.

Short exposure could cause serious, temporary or moderate residual injury (e.g., chlorine, liquid hydrogen, carbon monoxide)

Life-threatening, major or permanent damage may result from single or repeated overexposures (e.g., hydrogen cyanide)

Very short exposure could cause death or major residual injury (e.g., hydrogen cyanide, phosgene, methyl isocyanate, hydrofluoric acid)

Red / flammability 0 Materials that will not burn (e.g., Water)

Materials that will not burn under typical fire conditions (e.g., carbon tetrachloride), including intrinsically noncombustible materials such as concrete, stone and sand (Materials that will not burn in air when exposed to a temperature of [820°C (1500°F)] for a period of 5 minutes)

1 Materials that must be preheated before ignition will occur. Includes liquids, solids and semi solids having a flash point above 200°F (93°C) (e.g., Canola oil)

Materials that require considerable preheating, under all ambient temperature conditions, before ignition and combustion can occur (e.g., mineral oil). Includes some finely divided suspended solids that do not require heating before ignition can occur. Flash point at or above 93°C (200°F).

Materials which must be moderately heated or exposed to high ambient temperatures before ignition will occur. Includes liquids having a flash point at or above 100°F (38°C) but below 200°F (93°C) (e.g., Diesel fuel).

Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur (e.g. diesel fuel) and multiple finely divided suspended solids that do not require heating before ignition can occur. Flash point between 38 and 93°C (100 and 200°F).

Materials capable of ignition under almost all normal temperature conditions. Includes flammable liquids with flash points below 73°F (23°C) and boiling points above 100°F (38°C), as well as liquids with flash points between 73°F and 100°F.

Liquids and solids (including finely divided suspended solids) that can be ignited under almost all ambient temperature conditions (e.g., gasoline, acetone). Liquids having a flash point below 23°C (73°F) and having a boiling point at or above 38°C (100°F) or having a flash point between 23 and 38°C (73 and 100°F).

Flammable gases, or very volatile flammable liquids with flash points below 73°F (23°C), and boiling points below 100°F (38°C). Materials may ignite spontaneously with air (e.g., Propane).

Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily (e.g., acetylene, propane, liquid hydrogen). Includes pyrophoric substances. Flash point below 23°C (73°F). While there are differences among different agencies and countries, the general messages are similar. To avoid confusion and promote trade, United Nations has created a universal identification system, global harmonized system of classification and labeling of chemicals (GHS), beginning in 1992. Fig. 16 .2 shows a sample GHS chemical label. The GHS label includes the substance name, id, and eye-catching pictograms to show the potential dangers, warnings, and descriptions on how to handle it. Table 16 .5 shows the pictograms of the different hazard classes. As shown in Table 16 .5 the GHS hazard classification also includes three categories-Physical, health, and environment. As compared with HMIS and NPFA labeling, GHS combined the flammability hazards into physical hazard category while one additional category is added-environmental hazard.

The adoption of the GHS is expected to facilitate international trade by increasing consistency between the laws in different countries (and jurisdictions) that currently have different hazard communication requirements. There is no set international implementation schedule for the GHS. United States of America published the final rule for implementation of GHS on March 26, 2012. It required product manufacturers to adopt the standard by June 1, 2015; product distributors to adopt the standard by December 1, 2015, while workers had to be trained by December 1, 2013.

The physical hazards are assigned to 9 different hazard classes largely based on the United Nations Dangerous Goods System. These include: by brief contact with an ignition source, such as a burning match, and if the flame spreads rapidly. it is further divided into a. flammable solids, b. polymerizing substances and c. self-reactive substances, that is, thermally unstable solids liable to undergo a strongly exothermic thermal decomposition even without participation of oxygen (air) other than materials classified as explosive, organic peroxides, or as oxidizing agents. Spontaneously combusting solids or pyrophoric solids are solids that, even in small quantities, are liable to ignite within 5 minutes after coming into contact with air. Substances and mixtures of this hazard class are assigned to a single-hazard category on the basis of the outcome of the UN Test N.2. Self-heating substances are solids or liquids, other than a pyrophoric substance, which, by reaction with air and without heat supply, is liable to self-heat. Substances and mixtures of this hazard class are assigned to one of two hazard categories on the basis of the outcome of the UN Test N.4. Substances which on contact with water emit flammable gases are liable to become spontaneously flammable or to give-off flammable gases in dangerous quantities. Substances and mixtures of this hazard class are assigned to one of three hazard categories on the basis of the outcome of UN Test N.5, which measures gas evolution and speed of evolution. Flammable aerosols can be classified as Class 1 or Class 2 if they contain any component, which is classified as flammable. 5. Oxidizing substances and organic peroxides contain category 1 oxidizing substances and category 2 organic peroxides, organic liquids or solids that contain the bivalent -O-O-structure and may be considered a derivative of hydrogen peroxide, where one or both of the hydrogen atoms have been replaced by organic radicals. The term also includes organic peroxide formulations (mixtures). Substances and mixtures of this hazard class are assigned to one of seven "Types," A to G, on the basis of the outcome of the UN Test Series A to H. 6. Toxic and infectious substances 7. Radioactive substances 8. Substances corrosive to metal are substances or mixtures that by chemical action will materially damage, or even destroy metals. These substances or mixtures are classified in a single hazard category on the basis of tests (Steel: ISO 9328 (II): 1991 -Steel type P235; Aluminum: ASTM G31-72 (1990) -non-clad types 7075-T6 or AZ5GU-T66). The GHS criteria are a corrosion rate on steel or aluminum surfaces exceeding 6.25 mm per year at a test temperature of 55°C. 9. Miscellaneous dangerous substances which includes environmentally dangerous substances.

The health hazards are classified based on 12 areas. These include: l Acute toxicity includes five GHS categories from which the appropriate elements relevant to transport, consumer, worker and environment protection can be selected. Substances are assigned to one of the five toxicity categories on the basis of LD 50 (oral, dermal) or LC 50 (inhalation). l Skin corrosion means the production of irreversible damage to the skin following the application of a test substance for up to 4 hours. Substances and mixtures in this hazard class are assigned to a single harmonized corrosion category. l Skin irritation means the production of reversible damage to the skin following the application of a test substance for up to 4 hours. Substances and mixtures in this hazard class are assigned to a single irritant category. For those authorities, such as pesticide regulators, wanting more than one designation for skin irritation, an additional mild-irritant category is provided. l Serious eye damage means the production of tissue damage in the eye, or serious physical decay of vision, following application of a test substance to the front surface of the eye, which is not fully reversible within 21 days of application. Substances and mixtures in this hazard class are assigned to a single-harmonized category.

l Eye irritation means changes in the eye following the application of a test substance to the front surface of the eye, which are fully reversible within 21 days of application. Substances and mixtures in this hazard class are assigned to a single harmonized hazard category. For authorities, such as pesticide regulators, wanting more than one designation for eye irritation, one of two subcategories can be selected, depending on whether the effects are reversible in 21 or 7 days. l Respiratory sensitizer means a substance that induces hypersensitivity of the airways following inhalation of the substance. Substances and mixtures in this hazard class are assigned to one hazard category. l Skin sensitizer means a substance that will induce an allergic response following skin contact. The definition for "skin sensitizer" is equivalent to "contact sensitizer." Substances and mixtures in this hazard class are assigned to one hazard category. l Germ cell mutagenicity means an agent giving rise to an increased occurrence of mutations in populations of cells and/ or organisms. Substances and mixtures in this hazard class are assigned to one of two hazard categories. Category 1 has two subcategories. l Carcinogenicity means a chemical substance or a mixture of chemical substances that induce cancer or increase its incidence. Substances and mixtures in this hazard class are assigned to one of two hazard categories. Category 1 has two subcategories. l Reproductive toxicity includes adverse effects on sexual function and fertility in adult males and females, as well as developmental toxicity in offspring. Substances and mixtures with reproductive and/or developmental effects are assigned to one of two hazard categories, "known or presumed" and "suspected." Category 1 has two subcategories for reproductive and developmental effects. Materials, which cause concern for the health of breastfed children, have a separate category, Effects on or Via Lactation. l Specific target organ toxicity (STOT) category distinguishes between single and repeated exposure for target-organ effects. All significant health effects, not otherwise specifically included in the GHS, that can impair function, both reversible and irreversible, immediate and/or delayed are included in the nonlethal target organ/systemic toxicity class (TOST). Narcotic effects and respiratory tract irritation are considered to be target organ-systemic effects following a single exposure. Substances and mixtures of the single exposure target organ toxicity hazard class are assigned to one of three hazard categories. Substances and mixtures of the repeated exposure target organ toxicity hazard class are assigned to one of two-hazard categories.

l Aspiration hazard includes severe acute effects such as chemical pneumonia, varying degrees of pulmonary injury or death following aspiration. Aspiration is the entry of a liquid or solid directly through the oral or nasal cavity, or indirectly from vomiting, into the trachea and lower-respiratory system. Substances and mixtures of this hazard class are assigned to one of two hazard categories this hazard class on the basis of viscosity.

And the environmental hazards are classified in two areas: l Acute aquatic toxicity means the intrinsic property of a material to cause injury to an aquatic organism in a short-term exposure. Substances and mixtures of this hazard class are assigned to one of three toxicity categories on the basis of acute toxicity data: LC 50 (fish) or EC 50 (crustacean) or ErC 50 (for algae or other aquatic plants). In some regulatory systems these acute toxicity categories may be subdivided or extended for certain sectors.

l Chronic aquatic toxicity means the potential or actual properties of a material to cause adverse effects to aquatic organisms during exposures that are determined in relation to the lifecycle of the organism. Substances and mixtures in this hazard class are assigned to one of four toxicity categories on the basis of acute data and environmental fate data-LC 50 (fish) or EC 50 (crustacea) or ErC 50 (for algae or other aquatic plants) and degradation or bioaccumulation.

Besides the chemical labeling, detailed data must be provided for safe-handling of the particular chemical. The safety data sheet (SDS, formerly known as material safety data sheet or MSDS) is specifically aimed at use in the workplace. It provides comprehensive information about the chemical product that allows employers and workers to obtain concise, relevant, and accurate information that can be put in perspective with regard to the hazards, uses, and risk management of the chemical product in the workplace. The SDS contains 16 sections. While there are some differences in existing industry recommendations, and requirements of countries, there is widespread agreement on a 16 section SDS that includes the following headings in the order specified:

1. Identification 2. Hazard(s) identification 3. Composition/information on ingredients 4. First-aid measures 5. Fire-fighting measures 6. Accidental release measures 7. Handling and storage 8. Exposure control/ personal protection 9. Physical and chemical properties 10. Stability and reactivity 11. Toxicological information 12. Ecological information 13. Disposal considerations 14. Transport information 15. Regulatory information 16. Other information.

The GHS SDS headings, sequence, and content are similar to the ISO, EU and ANSI MSDS/SDS requirements. The SDS provides a clear description of the data used to identify the hazards.

The current industry consensus defines chemical reactivity hazard as a situation where an uncontrolled chemical reaction could result directly or indirectly in serious harm to people, property, or the environment.

The Environmental Protection Agency (EPA) issued an Alert in 2004 [EPA 550-F-04-004 (5104A) www.epa.gov/ceppo] as part of its ongoing effort to protect human health and the environment by preventing chemical accidents. EPA is striving to learn the causes and contributing factors associated with chemical accidents and to prevent their recurrence. Major chemical accidents cannot be prevented solely through regulatory requirements. Rather, understanding the fundamental root causes, widely disseminating the lessons learned, and integrating these lessons learned into safe operations are also required. EPA publishes Alerts to increase awareness of possible hazards. It is important to review this information and consider whether additional action is needed to address the hazards. The purpose of this Alert is to introduce small-and medium-sized facilities to a simple method developed by the Center for Chemical Process Safety (CCPS), essential practices for managing chemical reactivity hazards, to screen facilities for chemical reactivity hazards. The CCPS preliminary screening method as illustrated in Fig. 16 .3 is a tool to help one identify where chemical reactivity hazards are likely to occur in a facility and may be applicable to a wide range of activities including warehousing, repackaging, blending, mixing, and processing. Many materials used in industrial facilities can pose chemical reactivity hazards. Reactivity hazards may not be as easy to identify as other hazards such as toxicity or corrosivity. A facility is at a higher risk of having an uncontrolled release if not all the existing chemical reactivity hazards are identified. Table 16 .6 shows the typical causes of reactivity incidents. One can infer that chemical reactivity hazards can be categorized as follows: 1. Self-reacting or unstable chemicals. A single-unstable compound can undergo uncontrolled decomposition, rearrangement, or polymerization. For example, organic peroxides can pose fire and explosion hazards. 2. Runaway reactions. Intended reactions of chemicals to produce desired products can become uncontrollable. For example, the reaction of phenol with formaldehyde to produce phenolic resins is subjected runaway. 3. Incompatibilities. Inadvertent mixing of two or more process chemicals may lead to an unintended chemical reaction.

For example, mixing acids with cyanide salts generates highly toxic hydrogen cyanide gas, and mixing silver salts with ammonia in the presence of a strong base may generate an explosively unstable solid. Also, process chemicals may react with materials present in the process such as water, air, materials of construction, lubricating oils, utility fluids, etc. For example, acetylene reacts with copper to form explosive compounds, and phosgene reacts corrosively with stainless steel, which may result in loss of containment.

Most chemical reactions release heat, and many reactivity incidents involve the sudden release of heat in quantities or at rates too high to be absorbed by the immediate soundings. Potential consequences or reactivity incidents include 1. Overpressurization and rupture of closed containers and vessels, which can result in an explosion and projectiles that can cause injuries and property damage and/or the dispersal of hazardous materials and toxic exposures, fires, and violent explosions; 2. Generation of gases (toxic or combustive) or other hazardous materials (e.g., the generation of hydrogen sulfide by the inadvertent mixing of an acid with a sulfide solution); 3. Generation of heat that causes thermal burns and ignition of combustible materials; 4. Fire in the absence of an additional ignition source; 5. Initiation of other chemical reactions.

In order to avoid or manage the chemical reactivity hazard, the first step in managing chemical reactivity hazards is identifying those facility operations and chemicals that represent a potential chemical reactivity hazard. Fig. 16 .3 shows a schematic flow diagram of steps to follow in identifying chemical reactivity hazards in a given facility. The screening method is based on a series of twelve "yes-or-no" questions to help one determine, if there are chemical reactivity hazards in the facility.Once the chemical reactivity hazards are identified in the facility, a hazard management system can be set-up to properly address them. Regulatory process safety and risk management systems such as the OSHA Process Safety Management Standard, and EPA's Risk Management Program (RMP) incorporate elements that are applicable to the management of reactive chemicals and can be used as a basis. EPA believes that facilities have a general duty to address significant reactive chemical hazards under the general duty clause of section 112(r)(1) of the Clean Air Act.

A first step in accident prevention is to examine how the reactor performs under all possible feed conditions. What are the possible ranges of pressure, temperature, feed rate, feed compositions, and other variables that the system may possibly experience, and what will be their responses in the reactor and effluent? The bioprocess engineer needs to continuously be aware of these upsets or drifting away from the desired set point(s).

Efficient handling of upsets requires the knowledge of steady-state and transient behaviors of the reactor system. We have only discussed steady-state behavior except for (1) brief considerations in Chapters 4, 11, 12, 15, and (2) feed and thermal transients in CSTRs in Chapter 15. Numerical tools, such as ODEXLIMS, are useful in aiding the analysis.

It is important to build a model of the bioprocess early in process design and use this model to simulate all possible upsets. The bulk of this text deals with the kinetics or kinetic model of chemical and biotransformations, and thus there is no excuse not be able to establish a reasonable mathematical model for the process under concern. After each possible upset has been examined individually, combinations of upsets need to be considered. What would happen when pressure, temperature, and feed substrate concentration suddenly rise at the same time?

Fires and explosions are involved in most industrial accidents. Later in this Chapter we will examine some of the issues involved in flames, fires, and explosions, and these are some of the major issues that need consideration in accident prevention. Where are the most likely trouble spots and what are the consequences of an incident?

Heat, fire, explosion, and pressure generation are all features of undesired reactions. Reactive chemicals and materials, monomers, resins, peroxides, application interface's batteries and explosives-all have the potential to produce heat by exothermic reactions, these typically are unwanted. If the heat is not removed there is the potential for a runaway reaction. There is the need to understand the likelihood and to assess the impact of "undesired reactions." This can be done in the laboratory on a smaller scale with the aim to simulate the scenario of what can happen in real life on any larger industrial scale. Such an understanding of heat release from chemical reactions and the potential for runaway reactions is vitally important in many branches of the chemical & allied industries. When the heat generated by a chemical process is greater than the possible heat removal, the temperature will rise with perhaps catastrophic effect! An example will be shown at the end of the Chapter to illustrate importance of heat removal. Now, you are equipped with the ability to conduct detailed reactor analysis that can simulate such unknown exothermic runaway reactions, mimicking what can happen on large scale and giving worst-case data; data obtained under zero-heat loss conditions. What happens if a chemical or system self heats? Initially (near onset of self-heating) this reaction will be slow. In chemicals this gives a loss of yield, a shelf life issue. At this stage heat loss is likely to pose little potential of a runaway and an explosion. However should the reaction accumulate heat it will accelerate and as temperature raises the rate of reaction will also increase. If the reaction is not kept under control, a safety problem will occur, which may result in explosion, loss of property, even human injury or death. It is necessary to remove this heat of reaction to control the reaction. To maintain control of the reaction the heat loss must be greater than the heat production. Fig. 16 .4 shows a schematic of a simple-reactor system. Heat-exchanging capability is illustrated with jacket having a "cooling" (or "heating") fluid passing through. If the reaction mixture volume is constant, we obtain from the first-law of thermodynamics [see Eq. (3.134)] similar to that in Chapter 15,

Combustion, reactive hazard, and bioprocess safety Chapter | 16 733

The energy of stirring is dissipated into the reaction mixture, ≤ W 0 s . Heat transfer into the reactor is accomplished by the jacket:

where (T a -T) m is the mean heat transfer temperature difference. Normally, log mean temperature difference is used. 

The temperature will rise if the heat removal is not sufficient (as compared with the heat generation). At steady state,

As the rate of reaction r increases with temperature exponentially (Arrhenius law), Eq. (16.6) predicts that the heat produced will increase exponentially with temperature. However, the heat loss from vessel will only increase linearly with temperature as predicted by Eq. (16.5).

Scales of reaction and/or amounts of individual chemicals are different in R&D process development, manufacturing plants and transportation storage sites. The difference in the amount of chemicals poses different level of dangers. A small amount of any chemical may not be of acute danger to humans and environment, whereas large amount of any chemical can cause significant harm. The 2015 explosion in the port city of Tianjin, China, for example, shows that improper storage of chemicals (in this case nitrocellulose, and ammonium nitrate) can cause extreme harm. Over one hundred lives lost in the explosion. For all potential runaway reactions, heat loss possibility and environmental conditions are important. Therefore, scale is important. A small vessel will lose more heat than a large vessel due to the available heat transfer area per volume of the reaction mixture; the large vessel is more adiabatic. A reactive material in a smaller vessel will be stable to a higher temperature than the same material in a larger vessel. The same sample may be safe in a beaker or drum -but may runaway in a tanker causing explosion. A battery may be stable as a single unit -but as a battery pack in a black cased Notebook alongside heat-producing electronic components heat output may raise the battery temperature causing disintegration and fire. To simulate the real-life or large-scale scenario, the sample under investigation must be in an environment without heat loss (or gain). The temperature where the heat production exceeds the heat loss is the temperature of no return (TNR). There is not just one TNR, this value will change as heat-loss potential changes. Vary the environment and the TNR can be reduced significantly.

There is a continuing extensive literature on accidents and loss prevention in the chemical industry. This involves extensive use of statistics and probabilities in assessing damage and probable outcomes which are beyond what can be covered in this text. Chemical engineering progress runs a section of process safety each issue; the readers are encouraged to keep refreshing up. Safety consideration is a life-long learning issues that needing update as we understand more.

It is paramount to design a chemical process plant with sufficient redundancy able to respond to fluctuations that may occur. As a primary issue in reactor design is minimization of costs, any redundancy will clearly add cost and complexity to the process. Design with transients and accidents in mind are clearly an extremely complex subject that requires careful analysis and consultation before deciding on a final design of a process.

It is also important to determine the possible outcomes that could arise if the worst possible sequence of events should occur. Hurricanes can be accounted for and responded to as weather forecast is generally able to predict their occurrences. However, earthquakes and tornadoes are more difficult because there are no reliable forecasts available early enough to implement a procedure to shut-down the plant. Sabotage and intentional damage caused by insiders are very difficult to factor in because they require a response to conscious damage by people who are potentially well qualified to understand the maximum damage that could occur. How should one respond to a terrorist attack or a deliberate bombing act? 

The concepts of chain reactions have been discussed in Chapter 6. Free-radical reactions, polymerization, and depolymerization/hydrolysis are examples of chain reactions as depicted in Chapters 6 and 8. If one recalls, the essence of these reactions are their three characteristic steps-(1) initiation, (2) propagation, and (3) termination. Combustion processes are fast and exothermic reactions that proceed by free-radical chain reactions. Combustion processes release large amounts of heat, and they have many applications in the production of power and heat, in incineration, automobiles, and even in rockets.

These processes combine many of the complexities of the earlier chapters-complex kinetics, mass transfer control, and large-temperature variations. They also frequently involve multiple phases because the oxidant is usually air while fuels are frequently liquids or solids such as coal, wood, and oil drops.

In addition to the importance of combustion reactors in chemical processes, uncontrolled combustion reactions create the greatest potential safety hazard in nature and the chemical industry. Therefore, all bioprocess engineers need to understand the basic principles of combustion reactors to recognize the need for their proper management and to see how improper management of combustion can cause unacceptable disasters.

We have introduced chain reaction in Chapter 6, Section 6.4, with a classic example of the decomposition of acetaldehyde to methane and carbon monoxide, Eq. (6.59). In this chapter, we choose another example, the decomposition of diethyl ether. (16.10)

One may mistake the reaction as "elementary" as the rate law "agrees" with the stoichiometry. However, the reaction is in fact a complex reaction with free radicals. This is indeed a simple chain reaction, and it is a prototype of many chain reactions. The mechanism by which this reaction proceeds is thought to involve four major steps (16.14)

Reaction (16.11) is the initiation step, two free radicals are generated. Naturally, this is the slow step, requiring the breakage of a methyl bond from the diethyl ether. For a chain / radical reaction, initiation step is important as it is the starting point. Radicals are generated in the initiation step to allow the reaction to occur. In many applications, the initial radicals can be generated elaborately by using an initiator or substance that requires low energy to break into radicals. UV irradiation, reactor wall-surface construction and high-temperature (or heat) are common methods of promoting the initiation step. Table 16 .7 shows some of the bond energies that can be exploited for free-radical initiation. The carbon-iodate bonds are generally weak. The tertiary C-H bond in isobutane is fairly weak (404 kJ/mol) compared to other bonds in the molecule. Allyl and the benzyl bonds are of the lowest energy compared with others due to their π-electron sharing capabilities.

Steps (16.12) and (16.13) are propagation steps, with reactions occurring but a different free radical is produced while consuming one. The free radicals are not reduced by these reactions. The propagation steps are generally fast. Therefore, it is known that chain reactions are unlikely to stop until the reactants are exhausted. The last step, (16.14) is the termination step, when two free radicals are combined to form a stable molecule. In this case, the termination step is the reverse step of the initiation step.

While the free radicals in the chain reactions are unstable and very active, there are substances that can adsorb free radicals and stop it from propagation. These substances are known as scavengers. Scavengers can be employed to stabilize chemicals that tend to decompose through chain reactions.

Chain reactions can also be very sensitive to the materials used for constructing the walls of the reactor because the adsorption and reaction properties of the walls in promoting or retarding termination and initiation steps depend on the nature of these surfaces. These effects have caused much difficulty in establishing the "true" rates of chain reactions and, more important, have led to accidents when chain reactions propagated unexpectedly because of unpredicted kinetics, which can be quite different from those obtained in a small lab reactor. One common difference is that lab data are frequently obtained in glass-walled vessels, which are quite unreactive, while large-scale reactors are frequently made of steel, whose walls have different reactivities.

Bioprocess engineers need to be strongly aware of the size effects in scaling-up any process that might involve chain reaction processes, for example, pyrolysis, gasification, and the more simple ones discussed earlier. The lab data, even if carefully obtained, may not explain large-reactor behavior. For a given reactor geometry, the wall surface area per volume is higher for smaller reactors. The rate of a chain reaction may be much higher in a large reactor. This might be good news unless the rate is too high and results in reactor runaway, where the temperature cannot be controlled. Similarly, one may be planning to run a process that does not occur by chain reactions, but in a large reactor a "side reaction" that occurs by a chain reaction could take over and dominate the process. Combining complicated kinetics with the possibility of thermal runaway can make engineers old before their time.

Everyone who works in a bioprocess or chemical laboratory is warned that organic chemicals should never be stored for long times because they can explode without warning or apparent provocation. Bottles of organic chemicals should always be disposed of within a few years, and, if a chemical is found that might be very old, a "bomb squad" of safety experts should be called to remove it and properly dispose of it.

Why do all organic chemicals just sitting around the laboratory at room temperature have the potential to explode spontaneously? The answer is that they can react with oxygen, in the air at room temperature in chain reactions to form organic peroxides, which can spontaneously react explosively upon shaking or opening the cap. Organic peroxides are examples of compounds that have fuel (C and H atoms) in the same compound with the oxidant, and these are solid and liquid explosives similar to dynamite and TNT.

In this section we will consider the mechanism by which a "harmless organic molecule" can react with oxygen at room temperature to form a potentially lethal system. Then we will discuss positive applications of this process, which are the primary processes by which such common chemicals such as acetone, phenol, isobutylene, styrene, and propylene glycol are made commercially. 

Consider any organic molecule, which we will call R-H, where R• could be an alkyl or any fragment containing C, H, and O atoms. In the presence of O 2 this molecule can undergo the autooxidation reaction

This reaction never happens significantly as a single step at any temperature because the R-H bond strength is > 330 kJ/ mol for any C-H bond as shown in Table 16 .7. However, consider the chain reaction sequence

If we add the second and third steps, we obtain the chain reaction

by which a hydrocarbon is converted into a peroxide. The first step is a very slow dissociation step, with an activation energy > 330 kJ/mol. However, once the radical R• is made, it will rapidly react with O 2 to form the alkylperoxy radical, which is also very reactive and can abstract an H atom from any organic molecule in the solution to form the relatively stable (until someone shakes the bottle) alkyl peroxide.

We call this type of reaction autooxidation because it is an autocatalytic process (the reaction generates radical intermediates that propagate chain reactions) and it is an oxidation that converts alkanes into alkyl peroxides.

This chain reaction is just like those discussed previously, except now the reactants are an organic molecule and O 2 , and the product is a potentially explosive hydroperoxide. It can be sketched as a chain in Fig. 16 .6. The features of this chain reaction that make it so explosive are 1. Organic peroxides have high-exothermic heats of decomposition because they carry both fuel and oxidant in one molecule. We will discuss this part of the process (the explosion) later in this chapter.

The reactions can only occur if O 2 is present. A bottle of an organic chemical is perfectly safe for centuries if the cap is impermeable to oxygen, and is kept tightly fastened. (A cork is not a suitable cap.) However, one never knows whether someone may have left the cap off or loose for some time to allow the peroxide to form significantly. 3. This reaction depends crucially on the initiation step, and different molecules have vastly different capabilities of dissociation. The major problem here is that the initiation step can be initiated by a trace amount of an impurity that no one knows is in the bottle. As examples, all tertiary C-H bonds are fairly weak, as are the C-H bonds in aldehydes and alkenes. The reaction can be initiated by traces of these compounds, and the chain propagated throughout all molecules in the otherwise quite unreactive liquid. 4. The initiation step can be photoinduced. If a bottle is sitting in sunlight, UV photons (fluorescent lights are also more dangerous than incandescent lights) can cause photodissociation to initiate the chain reaction much faster than in the dark (this is the reason why many chemicals are sold in brown bottles).

Since one cannot be sure about the history of any old bottle of a chemical in a "common" laboratory, one should always dispose of old chemicals very carefully. FIGURE 16.6 Chain reaction for alkane oxidation by autoxidation in which the alkyl and alkyl peroxy radicals propagate the chain, which is fed by the alkane RH.

Although not as dramatic as explosion, spoilage of food is a very important bioprocess issue in the food industry. This has two main forms-biological organism (bacterial, fungi or yeast) growth and oxidation.

In biological organism growth, improper sterilization during processing leaves organisms that reproduce and cause problems by creating large concentrations of organisms that taste bad or, even worse, make the consumer sick. In fact, as we will learn in Chapter 19 that no sterilization is absolute or 100%. Living organisms grow exponentially, so the concentration of microorganism biomass in foods are inherently unpredictable.

These organisms have two varieties, aerobic and anaerobic, signifying one grows with or another grows without oxygen. Aerobic microorganism growth is basically an oxidation process, but now the organism can be regarded as a catalyst for oxidation. Anaerobic organisms are in fact more dangerous than aerobic organisms because they tend to produce more dangerous toxins, such as botulism and salmonella.

Another significant spoilage mechanism of food is autooxidation. Here O 2 from air at room temperature causes oxidation of ingredients in food to produce molecules such as aldehydes and organic acids, both of which generally taste bad, although they are not usually toxic. The mechanism of food turning rancid is the same as we have discussed in this section: peroxide formation, which is autocatalytic, leading to rapid propagation. As in all these processes, the rate is controlled by traces of particular compounds that initiate peroxide formation, which then propagates throughout the food sample.

No amount of "sterilization" will prevent or even slow autooxidation, and there are only two defenses: removal of O 2 and addition of inhibitors. Oxygen barriers in food packaging are a major topic in the engineering of polymer films. The barrier properties of various polymers are very important in food applications, and many of these are multilayer polymers that have a thin layer of an impermeable polymer (such as polyacrylonitrile and ionic polymers) on a cheaper but O 2permeable polymer such as a polyolefin, which gives mechanical strength to the film.

The other method of oxidation prevention in foods is the addition of antioxidants. Nitrite solutions on meat or lettuce preserve the red or green color and make the package look fresher. The long lists of "additional ingredients" on packages of foods are either antioxidants or chemicals that inhibit bacterial growth. The phrase "all natural ingredients" on a package means that you need to be especially careful regarding spoilage.

One example of autooxidation you have experienced is the room-temperature oxidation of alcohol in wine. Within days and sometimes within minutes of opening a bottle of wine, the taste begins to deteriorate because of autooxidation. This converts the ethanol into acetaldehyde and to acetic acid. Wine lovers talk about letting the wine "breathe" after opening; so apparently some oxidation actually improves the taste. Distilled vinegar is made by the intentional oxidation of the alcohol in fermented apple juice into acetic acid, which can then be distilled from the juice and pulp.

Both biological reactions and autooxidation reactions in food are complex problems because both are highly autocatalytic. Their occurrence is therefore very variable between nominally identical samples of food. When a reaction is both unpredictable and intolerable, the bioprocess engineer must be especially alert.

There is considerable hype in the popular culture about the role of antioxidants in promoting human health, from preventing cancer to slowing aging. These molecules presumably act to prevent autooxidation within our bodies. The processes are very complex and not well understood because they are different in different regions of our bodies and because the foods and additives we consume may be rapidly transformed into other molecules that may or may not be antioxidants. These molecules act as freeradical scavengers within our bodies, which prevent the chain reactions of autooxidation. Because these processes are so complex, no one knows how a particular chemical will act when ingested, and therefore additives can be sold that have no beneficial effect.

Room-temperature oxidation processes supply the energy by which we exist, but it is important that only the proper oxidation reactions occur in our bodies and not the ones that cause our skin to wrinkle and our brains to perform badly on exams. If you could come up with a chemical to assist in either of these, you could become very wealthy if you can avoid getting in trouble with the FDA.

We will next consider the complexities of combustion, which are caused by the fact that combustion reactions are chain reactions. Combustion reactions are extremely fast and exothermic, and therefore, once the reaction is ignited, the process proceeds very quickly and becomes very nonisothermal. We will consider two exothermic chain reactions involving gases, the oxidation of H 2 and alkanes. Then we will consider combustion of liquid and solid fuels, a gas-liquid process. These processes all occur very rapidly and at very high temperatures.

A "simple" prototype of combustion reactions is hydrogen oxidation, All reactions are bimolecular. The 19 reverse reaction rate coefficients kare found from the equilibrium constants k -= k / K eq . A simplified reaction scheme captures many features of this reaction. First, the reaction is initiated by generation of free radicals, with the lowest activation energy process being H 2 dissociation, and add all four of the propagation reactions, we see that all the radical species cancel, leaving only the "simple" reaction

which is the stoichiometric reaction producing water from hydrogen and oxygen at high temperatures, although hydrogen peroxide H 2 O 2 is also produced in measurable amounts in low-temperature processes. The reactions in this chain reaction are shown in Fig. 16 .7. This reaction sequence is similar to that described for diethyl ether decomposition to ethane and acetaldehyde: Reactions that produce stable products actually occur only through radicals attacking the parent and eventually producing the stable products (C 2 H 6 , CH 3 CHO, and H 2 O in these examples).

However, the complete reaction mechanism of the hydrogen oxidation reaction is much more complex, both in its number of reaction steps, number of intermediates (OOH and H 2 O 2 ), and observed behavior. A mixture of H 2 and O 2 can sit in a dry bulb for many years with absolutely no H 2 O detected. However, if water is initially present, the reaction will begin, and if a spark is ignited or a grain of platinum is added to the mixture at room temperature, the reaction will occur instantly and explosively.

The reaction cannot begin without initiation of free radicals, and H 2 dissociation is very slow. Therefore, the few H atoms produced by the initiation steps may diffuse to the walls of the vessel and recombine before they can begin chain reactions. However, the presence of H 2 O accelerates the rate of the reaction because it causes formation of traces of H 2 O 2 , which easily dissociates and forms more H• and •OH, which initiate the reaction by attack of H 2 and O 2 , and these reactions rapidly produce more radicals, which strongly accelerate the process.

A spark generates many radical species in the spark gap, and these can propagate radicals throughout the vessel. A platinum surface catalyzes the reaction (H 2 and O 2 readily dissociate on a Pt catalyst surface to begin the reaction), and the surface reaction then heats the Pt to a high temperature, where homogeneous reaction begins rapidly. Thus we see that premixed H 2 and O 2 mixtures behave very unpredictably. They are usually quite unreactive, but moisture or a trace chemical such as H 2 O 2 , a spark, a trace of a catalyst (perhaps undetectable), or a hot surface can cause rapid and uncontrolled reaction.

The explosion limits of the H 2 + O 2 reaction have been studied thoroughly, and these limits exhibit an interesting variation with pressure. At low pressures the reaction is stable because radicals diffuse to the walls of the chamber and prevent chain branching, which leads to an explosion. At very high pressure radical quenching by diffusion to the walls is slowed down, and the heat released by the reaction leads to a thermal explosion (Fig. 16.8) .

The boundaries between stable flames and explosions are very "fuzzy" because they depend on traces of chemicals in the vessel, which may act as initiators or scavengers, and they depend strongly on the size and composition of the walls of the vessel. These diagrams are shown to illustrate the complexities in predicting the behavior of any chain reaction, even a "simple" one such as H 2 + O 2 . 

In the reaction steps for H 2 oxidation the first two reactions are chain branching reactions, with one radical species producing two other radicals every time a reaction event occurs. The first of these probably proceeds by the steps 16.27) and the hydroperoxy radical •OOH reacts immediately at high temperatures but can be observed in finite concentrations at low temperatures. Note that this reaction produces two reactive free radicals from one. We call these chain branching reactions because the propagation steps produce more radical species than they start with. Thus the propagation steps can also increase the concentration of free radical species, and further destabilize the kinetics. We call a propagation step in which one radical species is produced for each radical reacting a linear chain, while a process that produces more radicals is called a branched chain. These are sketched in Fig. 16.9 .

Most combustion reactions involve chain branching reaction steps. Under conditions where these steps are less significant than linear chain reactions, the reaction appears to be stable, but when the chain branching steps dominate, the overall reaction rate can accelerate uncontrollably.

Recall that in autooxidation discussion, hydroperoxide species were sufficiently stable at low temperatures that only linear chains formed. When the bottle of peroxides explodes, chain branching steps such as (16.28) begin to become significant. This is a chain branching reaction because one radical-prone molecule produces three radicals. The rapid rise in the concentration of radical species can accelerate the decomposition reaction of a bottle containing peroxide rapidly and catastrophically. FIGURE 16.9 A schematic of chain branching reactions where the propagation steps produce more than one radical so that the process tends to grow exponentially, leading to a rapid acceleration of the reaction and possibly a chain-branching explosion.

One of the most important combustion reactions in industry and in home heating is natural gas combustion,

The transportability of natural gas in pipelines and the clean combustion in its oxidation make it the fuel of choice in many applications.

As one might guess from our previous example of H 2 oxidation, this reaction is extremely complex. The standard model involves over 300 reaction steps among approximately 30 chemical species. Here even the stable product species can be complex, with CO, HCHO (formaldehyde), and soot (carbon) being among the highly undesired pollutants in CH 4 combustion. Since H 2 , O 2 , and H 2 O are involved as intermediates and products, the 38 reaction steps in the H 2 + O 2 reaction listed previously are in the set of the reactions that involved in CH 4 oxidation.

We will not attempt to describe the mechanism of this process except to note that an important initiation step is CH 4 dissociation, Mixtures of natural gas and air are similarly unpredictable, being quite stable and unreactive until a spark or a hot surface ignites reaction, and then thermal and chain branching autocatalysis takes off and so does the building.

Similar mechanisms (but even more complex) operate in the combustion of higher hydrocarbons. Initiation is easier because the C-H bonds can be weaker. The CH 4 + O 2 reactions are a subset of all hydrocarbons since CH 4 is one of the possible decomposition products of higher hydrocarbons.

One much employed example is combustion of gasoline in the engine cylinders of an automobile. Here the fuel is injected into the combustion chamber as a vapor with air at nearly the stoichiometric composition for complete combustion, and a spark ignites the mixture to create pressure, which forces the piston down. Details of ignition reactions determine issues such as engine knock or flooding, with branched aliphatic and small fuel molecules being advantageous for both of these problems. High-octane alkanes produce fewer chain branching radicals during the compression of the air-fuel mixture, and this avoids engine knock, which will occur if ignition occurs spontaneously before the spark plug fires to ignite the alkane-air mixture at the start of the power stroke.

In the diesel engine, the jet engine, and kerosene combustion, liquid fuel is sprayed into the combustion chamber with air, and the liquid is not totally vaporized before ignition. Therefore, in drop combustion there are flames around the liquid drops that shrink as the reaction proceeds.

We note finally that for most of these processes no one knows their kinetics. Rates have been measured with great precision in the laboratory under specific conditions, but in engineering applications the processes are so complex that these kinetics are not useful. In almost all situations the rates of reaction are controlled by mass and heat transfer rates, and the fluid flow is turbulent. As the actual reaction steps depend on mass transfer (ignition by initiators and termination reactions at surfaces), it is a very complicated problem.

The complexity and importance of combustion reactions have resulted in active research in computational chemistry. It is now possible to determine reaction rate coefficients from quantum mechanics and statistical mechanics using the ideas of reaction mechanisms as discussed in Chapter 6. These rate coefficient data are then used in large computer programs that calculate reactor performance in complex chain reaction systems. These computations can sometimes be done more economically than to carry out the relevant experiments. This is especially important for reactions that may be dangerous to carry out experimentally, because no one is hurt if a computer program blows up. On the other hand, errors in calculations can lead to inaccurate predictions, which can also be dangerous.

Most chemical accidents have occurred because some of these effects were not expected or accounted for, and most of the research on combustion processes originated in an effort to explain the unexpected and potentially hazardous. Thermal ignition contributes another safety hazard in combustion processes.

Our first example of a chain reaction, the decomposition of diethyl ether to ethane and acetaldehyde, is endothermic; so the reactor tends to cool as reaction proceeds. However, the oxidation of H 2 is exothermic by 238 kJ/mol of H 2 , and the oxidation of CH 4 to CO 2 and H 2 O is exothermic by 803 kJ/mol of CH 4 . Thus, as these reactions proceed, heat is released and the temperature tends to increase (strongly!). Thus, thermal ignition is very important in most combustion processes.

In addition to ignition of gases, liquids and solids can also exhibit ignition phenomena. A classic example is the "spontaneous combustion" of oily rags containing paint solvents or automobile oil. One does not want to store large quantities of these rags together, because they can suddenly heat up and catch fire. Another example is the "spontaneous combustion" of coal piles and of underground coal fields. A large pile of coal will almost inevitably catch fire and burn, and barges of coal must be transported and unloaded quickly or the barge will begin to burn, a quite unpleasant experience for the sailors, especially if far from port.

For a pile of wood, coal, or hay, the burning stability is dependent on the temperature, moisture, and spark. Assuming that the pile is reasonably large, the limitation would be oxygen availability. Fig. 16 .10 shows some of the examples of uncontrolled or open biomass burning. In this case, one could use a simple CSTR model to describe the burning of a pile of biomass or coal. The feed (reactant) to the reactor is air or O 2 , and the products are CO 2 , CO, and H 2 O.

We have already dealt with nonisothermal reactors and their effects in Chapters 16 and a brief review in Section 16.3. The major and potentially most dangerous application of these effects is in combustion reactors. Consider our standard CSTR with parameters as indicated in Fig. 16.11 .

Recall from Chapter 15 that the steady-state energy-balance equations in a CSTR can be reduced to a single equation, which we wrote by considering the rates of heat generation and removal. We write this, from Eq. (16.1), as

where U is the effective heat transfer coefficient, A H is the heat transfer area, A B is the surface area of the biomass (or coal) pile, r″ is the rate of reaction based on surface area, and reactant (or species) A refers to oxygen (O 2 ). We then rearranged this equation to become Eq. (16.5) or and we constructed a graphical solution by plotting the left-hand side H R (T) and the right-hand side H G (T) versus T, as repeated in Fig. 16 .11. Consider an oxidation reaction in this CSTR. Suppose we begin at the lower steady state, where the reaction rate and the rate of heat generation are very low. If the feed temperature T 0 and the wall temperature T a are low, there is little reaction, and we would detect negligible temperature change. However, if we increase T 0 or T a , slightly (and the straight line moves right ward in parallel as shown in dashed line), the system can jump to the upper steady state. Alternatively, if the feed rate is decreased and the straight line has decreased slope (also shown as dashed line passing through T 0 ), the system jumps to the upper steady state. These situations have been discussed in Chapter 15.

If this is a combustion process in which the rate has a very large effective activation energy, H R is very large and negative, and QC j0 C Pj is small (undiluted gases), then these effects become very large. One therefore observes a situation where the rate is effectively zero for one temperature because the effective activation energy is very large, but a slight change in any parameter can cause the system to jump to the upper steady state, where the reaction goes to completion and the temperature goes to the adiabatic flame temperature.

For H 2 or CH 4 in air near stoichiometric compositions (28% H 2 , in air and 9.5% CH 4 in air, respectively), the temperature jumps from ambient to >2000° (it does not make much difference whether one uses °C or K units for this calculation), and the reactor almost certainly explodes.

For a coal pile, dry wood pile or a barrel of oily rags, there is a slow flow of air through the pile or reactor with a large residence time, and processes are slow enough (an autooxidation process) that the composition is nearly uniform. A slight increase in temperature (perhaps a warm day) can cause the rate of oxidation of the alkane in the oil or the carbon in the coal or the biomass in wood to oxidize, and at some T 0 , the "reactor" will suddenly ignite and burn. Once this happens, natural convection forces air rapidly through the barrel or coal pile and "fans" the flame. Underground coal fields are especially difficult to extinguish because there are many holes in the ground which act as inlets and outlets for the "reactor". While the CSTR model is extremely primitive, the ideas of size dependence and differences in rate for air flow before and after ignition explain many of the features of these troublesome chemical reactors. where reaction rate (constant) increases exponentially with increasing temperature. When heat is generated (exothermic reactions), temperature rises in the reactor and thus increases the reaction rate. In return, faster heat generation is expected. This is the thermal autocatalysis. FIGURE 16.11 Heat generation and removal for combustion reaction in a CSTR. When the feed temperature increases (or feed rate decreases) slightly, the rate can go from nearly zero to nearly completion when the heat removal (straight) line becomes tangent to the heat generation curve, and the steady-state temperature jumps to a high value.

In hot-water extraction example described in Chapter 8, the catalyst proton H + (or H 3 O + ) was generated during the extraction reactions. In Chapters 13 and 14, we examined the cell growth. Cell is produced and it is also the catalyst for more growth. These reactions can be modeled as

because the reaction generates the catalyst after we added the catalyst to initiate the process. Another example was the chain branching reaction such as

just described in hydrogen oxidation. The bioreactions were nearly isothermal, but combustion processes are both chain branching and autothermal, and therefore they combine chemical and thermal autocatalysis, a tricky combination to maintain under control and of which bioprocess engineers should always be wary. This suggests immediately the definition of autocatalysis. A reactive intermediate or heat can act as catalysts to promote the reaction or more precisely increase reaction rate. However, in contrast to conventional catalysis, we do not add the catalyst from outside the system, but the catalyst is generated by the reaction. We may add promoters or heat to initiate the process, which then accelerates by autocatalysis. Conversely, we may add inhibitors or cool the reactor to prevent both types of autocatalysis.

The best inhibitor of autocatalysis is to have a small system. Surfaces act as both sinks of free radicals to increase termination and as sinks of heat (larger heat transfer area) to prevent thermal runaway, and small systems have large surfaceto-volume ratios to prevent these runaway possibilities.

In this section we consider the combustion of premixed gaseous fuel and air mixtures. Consider first the laboratory Bunsen burner one can see frequently in a biosafety cabin / workstation, shown in Fig. 16 .12. Natural gas from the gas supply system enters the bottom of the burner, where it is mixed with air, with flow rates adjusted by the gas valve and holes in the bottom of the burner, where air is sucked in by natural convection. The premixed gases travel up the barrel of the burner (a tubular reactor), and, if flows are suitably adjusted and a match has been used to ignite the mixture, a stable flame forms at the top of the tube.

If the flow rate is too low, the flame will flash back and propagate back up the tube to the mixing point and extinguish. If the flow rate is too high, the flame will lift off the top of the tube and either extinguish or burn as a turbulent lifted flame.

You have probably noticed that is it nearly impossible to maintain a stable flame within the tube of a Bunsen burner; it will either burn at the top or flash back to the mixing point. There is usually no stable steady state for a flame reaction in a straight tube. This points out several major characteristics of premixed flames-(1) They cannot be controlled stably within a constant-diameter tubular reactor configuration; and (2) the chain branching reactions and the heat release tend to make them either blow out or flash back into the tube and extinguish.

Flames are characterized by a flame velocity u F , which is the velocity with which a flame front will move down a tube containing stagnant premixed gases. The upper and lower compositions where the flame velocity goes to zero are called the flammability limits, and the flame velocity is usually its maximum near the stoichiometric composition for CO 2 and H 2 O formation.

The reason why it is difficult to sustain a flame within a tube is that the flame will only be stable if

where u is the flow velocity for the PFR. It is virtually impossible to adjust the flow velocity to be exactly u F . Even if this occurs, the flame is only neutrally stable; it will drift up or down the tube. Flow in the tube will be parabolic if the flow is laminar (Re < 2100), and thus gases near the center of the tube may propagate downstream, while gases near the wall propagate upstream toward the mixing point. This further destabilizes a premixed flame within a tube.

Premixed flames can only be stabilized by providing an expansion section in the tube where the cross section increases so that u slows, creating a zone where u = u F and a flame is stable. In a Bunsen burner this is achieved quite simply by having the flame height vary as u changes within the tube so that a burning surface (roughly a cone) is maintained stable over a large range of flow rates. At the exit of the tube the flow velocity decreases over a wide range so that the flame is quite stable.

The other stabilizer of flames is cold surfaces. These act to lower u F in the region of the surface and thus create a stable edge to the flame at the wall of the burner. This occurs both by thermally quenching the reaction near the surface and by quenching free radicals by adsorption on the surfaces, which stops the combustion reactions.

One very important device that uses this principle is the flame arrestor, which is inserted in pipes to prevent flames from propagating between pieces of gas-handling equipment. These are usually metal shavings (steel wool) inserted in tubes at strategic distances to stop any flames that may travel down the tube.

When the air inlet is shut off in a Bunsen burner, the flame can still be maintained, but it switches from the blue color of a normal CH 4 flame to the yellow of a diffusion flame. This is the situation of a gas stove. Now pure CH 4 flows up the tube, and the O 2 from outside the tube is mixed with CH 4 just above the tube, with flow of air driven by natural convection below the hot product gas mixture.

This burner, a butane lighter, a candle, a burning log, and a match (after ignition) are all examples of diffusion flames where one generally provides the fuel and relies on natural convection of air to provide the oxidant.

Diffusion flames are much more stable than premixed flames because the rate of the reactions is limited by the supply of air and its diffusion into the fuel region to form the flame front. Several examples of diffusion flames are given in Fig. 16.13 . These all typically have cylindrical symmetry, with air surrounding the fuel in the center. The cylindrical laminar flame is simplest to sketch because it is two dimensional in composition (direction of flow z and radial position from the center r), with composition profiles as shown in Fig. 16 .14. Gaseous fuel and air are admitted through separate slots, and the reaction proceeds along the line of contact. The gases are convected in the vertical flow z-direction, and horizontal diffusion (x) between the fuel and air create the flammable mixture.

One can model this system in principle using the differential mass-and energy-balance equations written with the principles shown in Chapter 3 with flow in the z direction and diffusion in both the flow direction and the lateral x direction to obtain profiles of C A (z, x) and T(z, x). One can see immediately that this will be a very complex mathematical problem to solve because there are many species (at least 30 for natural gas flames), but the problem will be made even more complex because of natural convection. Since the temperature in the flame varies from 25 to >2000°C in short distances, the density in and above the flame decreases dramatically (by nearly a factor of 10), and this causes large buoyancy effects, which accelerate the gases in the vertical direction. This provides the air flow in many diffusion flames, but it can also create turbulence, which causes greatly enhanced mixing or flow-induced diffusion / dispersion above that which would occur by molecular diffusion.

There is another distinction between flames: laminar versus turbulent. Small flames are generally steady and laminar, while large flames exhibit visible turbulent eddies. Candles and fireplaces are well-known examples where in still air the flame is steady, but, when there is wind or one blows on the flame, the flame flickers.

One can describe these phenomena through the Reynolds number (forced convection) and Rayleigh number (natural convection), but the reader can see immediately that the situations are so complicated that correlations in elementary texts on fluid flow are not easily applicable to predicting flame behavior. Reactive flows are among the most complex problems in modern engineering.

Heat considerations give the first estimate of the characteristics of combustion. As discussed in Chapters 3 and 4, the adiabatic reaction temperature (frequently called the adiabatic flame temperature) gives an approximation to the final temperature,

We need to be especially careful in using this equation to try to describe combustion accurately. Since the final temperature is so large, the heat capacity C P is not at all constant. Further, there are many reactions and products in flames, and one has to use the multiple-reaction version of this equation for accurate calculations. The intermediates, which are at negligible concentrations at low temperature, have significant concentrations at high temperatures. For example, the water dissociation reaction

has an equilibrium constant that predicts considerable H• and •OH in the high temperature of most flames. 

Another consideration in flames is radiation. The light that one sees in a flame is mostly fluorescence from the radiation of particular radical species formed in electronically excited states. (The blue color from CH 4 flames is CH• emission). Gases also radiate blackbody radiation, primarily in the infrared. The glow from burning wood or coal is blackbody emission radiated from the surface. The emission observed from cracks in a burning wood surface is brighter because the emission from a cavity has the equilibrium blackbody distribution, which is independent of the emissivity of the surface. Also most of the heat from a campfire is from radiation if you are beside the fire. The air beside the campfire is close to the ambient temperature because natural convection is drawing it up into the rising flame.

Most of the emission from combustion processes is in the infrared and is not visible, but a large fraction of the energy carried off from combustion processes is in the form of radiation heat transfer.

In practice there is only a certain range of compositions where premixed gases will burn. There is obviously a great deal of interest in this composition because it sets the limits of operation of many processes. The ignition limit is only loosely defined because it depends sensitively on temperature, flow conditions, and the presence of trace chemicals in the fuel, which can act as flame initiators or inhibitors by enhancing or slowing either rate of initiation or rate of termination.

A very important consideration in flammability is the presence of surfaces. Cold surfaces act as free-radical quenchers and maintain the gas temperature below the ignition temperature, while hot surfaces can act as ignitors.

Many of the industrial accidents in the chemical industry have originated from reaction systems that were quite stable in small containers but exploded when larger reactors were used.

One of the traditional ways of generating heat and power for human use is by burning a fuel. Solid wood and coal have been very important fuels since our cave-dwelling ancestors, and both are still important fuels in heating and power generation. Their combustion has characteristics that are common to most solids combustion processes.

The reduction of solid mass or liquid drops during pyrolysis/gasification/combustion can be modeled based on surface reactions. Two scenarios could emerge during pyrolysis: shrinking core (or systematic burning of biomass) or selective depletion (slow pyrolysis or carbonization). Fig. 16 .15 shows the "Shrinking-core" model of biomass peeling away during pyrolysis -gasification process. It is not difficult to imagine that the reaction rate is simply proportional to the exposed surface area if the molecules are assumed to be identical, i.e.

where r X is the biomass generation rate; k is the specific reaction rate constant; S is the surface area (exposed), and R is the remaining radius of the biomass particle (assuming spherical shape).

The "shrinking-core" idealization of biomass gasification / liquefaction. The molecules located on the extreme outer layer of the surface is shown as dark-blue X, the layer immediately down the first is shown as red X, while the layer next to the second layer is shown as X; … The molecules can be "peeled" away one layer at a time as shown on (B), right view. Total exposed surface molecules are not changing if the surface is "flat" as illustrated on (B), left view;Peeling away of surface molecules. If all the molecules are equally "peelable" and not affected by the "peeling" of other molecules, the overall process may be modeled as a "Shrinking-core" shown on (A) for a round particle; Shrinking-core or decreasing particle size.

In most applications, we do not need the detailed particle size change. In this case, the particle size change can be related to the conversion easily

where R 0 is the initial particle size. That is where S 0 is the initial surface area of the biomass particle.

As illustrated in Fig. 16 .16, the pyrolysis residual biomass yield decreases with time, while the available vaporizable components also changes with time. As a result, the residual biomass increasingly becomes a porous matrix of carbonized materials. Similar to Eq. (16.38), one can envision that the reaction rate or loss of biomass rate is proportional to the exposed surface area. However, the exposed surface area is no longer a simple relation as shown in Eqn (16.38) . At the start of the reaction, there is a portion of the particle surface that is equally peelable, which goes true for the out boundary of the "non-touched" core shrinks. However, there are surface molecules exposed within the remaining matrix that are left-over that are still peelable. Therefore, ε π ε ( )

where x is the radius of the "shrinking-core" of biomass that has not subjected to pyrolysis peeling; ε 0 is the initial portion of exposed molecules that are "equally" peelable; a is the (internal) surface area of the porous matrix after pyrolysis; and n is the power-law index of the fraction of peelable molecules on the internal surface. The application of Eq. (16.42) is more difficult than Eq. (16.38) for the case where no "skeletal" matrix leftover on top of the shrinking-core. Because of the "skeletal" matrix, the conversion of biomass can be computed by

where f max is the maximum conversion can be achieved and α is the fraction of "peelable" components left inside the "skeletal" remains. The value of α should lie between 0 and (1f max )(1 -ε 0 ). A first order approximation leads to 

which can be employed to approximate the rate expression to yield 

The first step when coal or wood particles (we call them chunks or powder and logs or chips, respectively) burn is pyrolysis, in which heat causes the water and volatile organics to evaporate. These gases then burn in oxygen in the boundary layer in the first stage of burning.

Coal and wood decompose to give many small organic molecules such as alkanes, aromatics, alcohols, aldehydes and ketones, and organic acids. As an example, methanol is still sometimes called wood alcohol because it was first obtained by heating wood in a kiln and condensing the products, one of which was methanol. In the nineteenth century, coal and wood were the major sources of many chemicals such as acetic acid and acetone, and coal was a major source of chemicals such as benzene. As noted previously, one of the types of products from coal was highly colored chemicals, and these were called coal tars, from which highly colored dyes could be extracted. The search for reliable and cheaper sources of these dyes produced the development of organic chemistry for the systematic search for new colored dyes, and this led to the giant European chemical companies such as AkzoNobel and BASF.

Pyrolysis of biomass has gained interest again in the twenty-first century with the public awareness of CO 2 and climate change. Biomass can be pyrolyzed to produce liquefied carbohydrates, which can be further converted to liquid fuel and chemicals. Slow pyrolysis carbonizes more while fast pyrolysis produces more vapor products as shown in Fig. 16.16 .

Woody biomass has an approximate stoichiometry of CH 1.4-1.6 O 0.65-0.7 . It still requires oxygen to burn. In the biomass energy applications studied today, the energy value is desired to be kept in the final products and thus less oxygen addition is desired. Addition of oxygen is only used to provide heat for raise the system temperature. Thermal or hydrothermal treatment of woody biomass can be applied in a wide temperature ranges. Torrefaction occurs between 200°C and 320°C, when the solid material is partial carbonized by devolatilizing the biomass. Commercial pyrolysis is normally carried out under oxygen-lean environment, below a temperature of 600°C. As the temperature is increased, more gas products are produced. Gasification is normally conducted between 700°C and 1000°C.

Pyrolysis of woody biomass produces char (solid residue), liquid (bio-oil or oil), and gas (or syn-gas). Fig. 16 .17 shows the typical yields of pyrolysis as a function of pyrolysis temperature. Table 16 .9 shows typical compositions and yields of pyrolysis of switchgrass.

During pyrolysis, each component pyrolyzes differently. Cellulose undergoes cracking of glycosidic bonds to produce levoglucosan and isomers (up to 80% conversion in the absence of alkali metals) as well as ring-opening and reforming to produce low-molecular weight oxygenates (acids, aldehydes, ketones), furfural and HMF, heterocyclic compounds, etc. Hemicellulose thermal cracks to furans and to sugars in the absence of alkali metals. Lignin is rendered to pyrolytic lignin (relative large molecules). Pyrolysis reactions of biomass are catalyzed by alkali metal salts. One can expect less liquid production when alkali metal salts are present. Table 16 .10 shows the elemental composition of the whole pyrolysis oil (aqueous phase + oil phase) as well as those of the original switch grass. One can observe that the oil is slightly less oxygenated than the switchgrass itself. The two phases of the oil has very different densities: 1100 kg/m 3 for aqueous phase and 920 kg/m 3 for the oil phase. The composition of the char varies with temperature and residence time as well during the pyrolysis. Table 16 .11 shows that as the pyrolysis time increases from 1 hour to 3 hours, the concentration carbon increases in the char. As the pyrolysis temperature is increased, there is a marked increase in the carbon concentration in the char and a decrease in the concentrations of hydrogen and oxygen. Fig. 16.18 shows that the stoichiometry of the (residue) solid material after pyrolysis of biomass is close to that of coal as pyrolysis temperature and residence time increased.

Once the volatile products have been pyrolyzed, one is left with a material that is mostly carbon, and the residual solid is called coke (from coal or wood) and charcoal (from slow pyrolysis of wood). Coke was of course used in blast furnaces to make iron (the volatiles would make the process less reproducible in reducing Fe 2 O 3 ) by our ancestors, while charcoal was used in soap (mainly the alkalis in the ash) and as an adsorbent.

Coke and charcoal are highly porous, coke forms from wood with the holes left behind when the volatile components evaporate, and charcoal remains from the cellular framework of wood fibers. Addition of limited air in pyrolysis produces more volatile products and makes a more porous carbon residue. Active carbon is made this way.

These carbon residues then can be burned with air in the second stage of combustion. There are two modes of carbon combustion, with and without flames. If there is no flame over the carbon, then reaction occurs by surface reaction. There are multiple steps for the surface reaction: mass transfer of oxygen to the surface, (dissociative) adsorption of oxygen on the carbon active centers (C σ ), reaction or transformation to form a CO or CO 2 molecule, and release of the gas molecule into the surroundings (exposing carbon active centers from below).

if the reaction is deficient in O 2 . When there is a flame, burning is by both surface and homogeneous reactions. The surface reaction is primarily (16.49) and the homogeneous reaction is primarily

although this is in fact a free-radical chain reaction with many steps. This creates a reacting boundary layer over the coke or charcoal particle with reactant and product composition profiles as illustrated in Fig. 16.19 .

There is therefore typically a progression of burning of solids involving first a homogeneous reaction, where volatile gases are driven off and burn, followed by a homogeneous-heterogeneous reaction, where both homogeneous and surface reactions occur in a boundary layer, followed by a surface reaction, where glowing coals react only on the surface.

Almost everyone has seen a campfire or charcoal grill. When a log is placed in a campfire, it first sputters and pops as water trapped within the solid is released as the temperature rises, and then flames are observed around it (flame jets form as the organics escape from cracks in the log) but the surface remains dark. Then after most of these organics have escaped, the wood surface begins to glow as the surface reaction begins with the flame continuing. Finally, after the fire has "died down", one observes no flame at all but just the glowing embers due only to surface reaction. Stirring the coals or blowing on the fire causes the flame to reignite for a while until it returns to the pure surface reaction mode. Finally, the heat release is too small to sustain reaction or the wood turns completely to ashes.

The burning process as one can observe from a campfire or charcoal grill is a perfect example of the pyrolysis, combustion and/or gasification. In order for the reactions to occur, high temperature is needed. Burning provides heat to raise the "solid fuel" temperature. While energy is needed to raise system temperature, pyrolysis and gasification processes are usually desired to keep more energy content in the final product (solid, liquid, or gases).

While less common today, solid wood and/or coal was the fuel for home heating prior to 1960's. Today wood and coal are burned in electric power plants, where fast and efficient combustion is desired. Therefore, coal plants grind large coal FIGURE 16.19 Concentration profiles around a burning carbon particle, rendering a homogeneous reaction in the boundary layer around the particle. chunks into fine powder and then blow it with air into burner jets, where it reacts with O 2 first by pyrolysis, and then the remaining coke burns, leaving behind an ash, which is a few percent of the weight of the original coal.

Most of us have never seen the inside of a coal-fired boiler, but the sparks rising from a campfire are very similar to the second stage of a coal-burning process. When a campfire is stirred, small particles of charcoal are broken off the log, and buoyancy carries them upward. They glow brightly by the heat released by heterogeneous carbon oxidation for several seconds until the carbon is all consumed.

In these processes the reactant O 2 flux is mass transfer limited around the reacting solid sphere, and the temperature of the particle is determined by heat transfer around the sphere, as shown in Fig. 16.16. Example 16.1 Most of us have poked a campfire or seat by a fireplace at night and watched the sparks fly up. They glow a bright red, which indicates that they are emitting blackbody radiation from a source with a surface temperature of 1000-1200°C. You have also noticed that their color does not change until they suddenly disappear. The time for this reaction varies, but they seem to glow for a few sec and then suddenly disappear. a. Estimate the temperature of a campfire spark. b. How long does it take to consume a fiber bundle of φ0.2mm × 1mm pine wood fiber (particle)?

Let us construct mass and energy balances on the particle, which we assume to be a cylinder of dry pine wood particle of radius R. The reaction is

where X stands for the pine wood biomass (normalized by carbon), and ν H/C is the mole fraction of hydrogen over carbon in the biomass. The elemental composition of dry pine wood can be found in weight percent as C: 51.8%, H: 6.3 %, O: 41.3% and N: 0.1%. The lower heating value of dry pine wood is given by -∆H R = 21.03 MJ/kg. Based on the elemental composition of pinewood, one can find the Roels' formula for pinewood as CH We do not know, nor do we need, the exact kinetics of the reaction explicitly. It is a surface reaction between the biomass (carbon and hydrogen) on the surface of the particle and O 2 from the air.

A mass balance on O 2 around the particle yields

where r″ is the rate of reaction based on the surface area. We have simply equated the rate of reaction of O 2 at its surface with the loss of moles of biomass particle, and the rate of mass transfer of O 2 to the surface. An energy balance on the particle yields Heat released due to reaction = heat lost by conduction + heat lost by radiation

where all terms are familiar except the radiation term, with ε the emissivity of the surface, σ the Stefan-Boltzmann radiation constant, T s , the carbon particle temperature, and T a the ambient air temperature (300 K or whatever it was that night 

Burning or combustion can produce maximum amount of heat if CO 2 and H 2 O are the only products for carbon and hydrogen in the solid fuel. In some applications, we seek to render solid fuel to easily transportable gas mixtures retaining the majority of the energy value. Coal gasification and liquefaction were evaluated in the twentieth century especially during the petroleum crises. Biomass gasification is gaining prominence in the twenty-first century. Gasification is a versatile technique for coal and/or biomass conversion. Gasification of biomass is usually carried out between 700°C and 1200°C in a fraction of a second. Fig. 16 .20 shows schematics of typical gasification reactors. During the gasification process, the biomass undergoes drying / torrefaction and pyrolysis before gasification (oxidation of the carbonized materials). The principles of these processes have been discussed earlier in this section. The process can be semi-batch as shown in (a) updraft and (b) downdraft reactors, or continuous as shown in (c) fluidized bed gasifiers.

The importance of gasification in a future economy lies on its potential to facilitate the use of biomass in the existing petroleum based industry. Fig. 16 .21 shows some of the commodity chemicals can be produced through gasification of woody biomass. Besides these catalytic routes, syngas has been subjected to bioconversion as well. Certain bacteria, or enzymes can be applied to convert syngas to chemicals similar to sugars. Therefore, gasification has been a well-studied method in thermochemical conversions. However, as expected, the bioconversion efficiency is much lower for gaseous substrates than solid / liquid substrates. Table 16 .12 shows the typical compositions of gases in the syngas as a function of the oxygen addition. In anaerobic conditions, the gasification process is endothermic. Large amount of heat is needed to maintain the high temperature (700°C -1000°C) as well as to provide energy for breaking the polymer binds. The benefit of anaerobic gasification is the high heating (or energy) value of syngas. Table 16 .12 shows that the low heating value (LHV) of syngas produced with steam as heating medium is 12-14 MJ/Nm 3 , while it is only 4-6 MJ/Nm 3 for syngas produced by partial burning with air. The high-energy content in the syngas is desirable.

The energy requirement of gasification can be satisfied by adding oxygen in the feed. When oxygen is available, biomass will burn or partially burn to provide energy to the system. However, the burning of biomass will reduce the energy content in the final product gas stream. Therefore, oxygen content should controlled at low level, just enough to provide heat to maintain the desired gasification temperature.

Gasification is a process in which the carbon and hydrogen raw material sources are first converted to basic building block chemicals: CO and H 2 and polymerize together to produce desired products, such as alkene:

This process was discovered and developed primarily in Germany during World Wars I and II to provide synthetic liquid fuels to compensate for the Allies blockade of crude oil shipment. (The argument can be made that both world wars were essentially fought over access to petroleum in the Caucusus region of Russia and the Middle East. The US blockade of Japanese access to Far East crude oil was a major factor in Japan declaring war on the United States with the bombing of Pearl Harbor. These arguments are summarized in the book The Prize by Daniel Yergin).

Fisher-Tropsch (FT) technology has been practiced on a large scale in South Africa because of the need for an independent source of liquid fuels during their political isolation, and they had abundant coal to make syngas. However, these processes are potentially an exceedingly important replacement for liquid fuels to use when crude oil supplies are depleted. Using coal (several hundred years supply in the United States alone) and natural gas (more proven reserves than petroleum), we can be assured of sources of liquid fuels for transportation through this technology long after supplies of crude oil and even natural gas have been used up. The more prominent use of Fisher-Tropsch technology is the renewable biomass feedstock, as petroleum, natural gas, and coal will eventually be depleted. Biomass as we know it is renewable and will not be depleted as long as the Sun is sending out rays.

Fisher and Tropsch found that when a mixture of CO and H 2 was heated to about 250°C at high pressures over an iron catalyst, polymer would form, and under suitable conditions this had the appropriate molecular weight for gasoline and diesel fuel. Different metal catalysts give different molecular weights (Ni produces CH 4 ) and different amounts of alkanes (Fe and Re), olefins (Ru), and alcohols (Rh); so catalysts and process conditions can be altered to produce a desired molecular weight and distribution.

The products are essentially all linear molecules, which for olefins and alcohols have the double bond or the OH group on the end carbon (α-olefins and α-alcohols). The mechanism of this polymerization process is thought to be similar to (but also very different from) Ziegler Natta (ZN) polymerization of ethylene and propylene on Ti. It is thought that CO adsorbs and hydrogenates (perhaps to form the CH 2 group) on an adsorption site adjacent to an adsorbed alkyl R, as shown in Fig. 16 .22. If the CH 2 inserts between the metal and the adsorbed R, an adsorbed RCH 2 -is obtained, which can add another CH 2 to form RCH 2 CH 2 -, and the chain repeats itself indefinitely until the adsorbed alkyl dehydrogenates to olefin, hydrogenates to paraffin, or hydrates to α-alcohol. The CO and H 2 (syngas) for FT synthesis were initially made by the gasification of coal, which was plentiful in Germany. Fig. 16 .23 shows the equilibrium compositions for the gasification of carbon as a function of temperature. One can observe that the gas mixture consists of mostly CO and H 2 at 1:1 at high temperatures (greater than 900°C). At lower temperatures, equilibrium favors the formation of CO 2 and CH 4 in the gas mixture instead of H 2 O.

Besides coal and biomass, syngas can also be made using naphtha or other hydrocarbon feedstocks, such as the methane from natural gas,

Note that coal produces CO and H 2 in a 1:1 ratio, naphtha in a 1:2 ratio, and methane in a 1:3 ratio. Because of the need in thermal energy to shift the reactions forward, oxygen (or air) is usually supplied to partially oxidize the carbon (and hydrogen) to maintain energy balance. An excess of H 2 is thus usually desired; so alkanes are the preferred feedstock.

Existing syngas plants operate by direct oxidation of natural gas

using pure O 2 from a liquid air plant. This process, called autothermal reforming, uses this exothermic reaction in an adiabatic reactor and produces the 1:2 ratio of CO:H 2 that is ideal for methanol or FT processes. Today, more attention is paid to the gasification of renewable biomass. The energetics in gasification of coal and lignocellulosic biomass are similar on the same carbon mass basis. Another feedstock of interest is the black liquor in a chemical pulp mill. Currently, the black liquor is being burned to recover the pulping chemicals and thermal energy that is needed in the pulp and paper making processes. The black liquor gasification is therefore not going to make a significant contribution to our needs in liquid fuel or hydrocarbon, but it can improve the pulp mill energy efficiency. In gasification, O 2 is usually added for energetics reasons, as shown in Table 16 .12. The gasification reactions without oxygen are endothermic, requiring energy to complete. Partially burning H 2 to H 2 O or CO to CO 2 can produce the heat needed for the reactions to complete. To increase the availability of H 2 , the steam shift reaction,

can be carried out. Gasification is employed today, for example, in ammonia production. Ammonia has wide applications. It is a precursor for urea, a commonly used fertilizer in agriculture. The ammonia synthesis reactions will be discussed in Chapter 8 (Section 8.3). The hydrogen gas is provided by gasification.

This FT process together with gasification may one day be the dominant method by which we will obtain some of our liquid fuels from renewable biomass, and coal. This technology is capable of supplying at least 200 years of liquid hydrocarbons at current consumption rates from known proven reserves of coal. The biomass as renewable resources, on the other hand, can supply our needs for liquid fuel "indefinitely." Two types of reactors will be discussed, a straight-through transport reactor, which is also referred to as a riser or circulating fluidized bed, and a packed-bed reactor (PBR), which is also referred to as a fixed-bed reactor.

Riser. Because the catalyst used in the process decays rapidly at high temperatures (e.g., 350°C), a straight-through transport reactor is used. This type of reactor is also called a riser and/or a circulating bed. A schematic diagram is shown in Fig. 16 .24. Here the catalyst particles are fed to the bottom of the reactor and are shot up through the reactor together with the entering reactant gas mixture and then separated from the gas in a settling hopper. The volumetric gas feed rate is at 3 × 10 5 m 3 /h. A schematic of an industrial straight-through transport reactor used at Sasol are shown in Fig. 16 .25 together with the composition of the feed and product streams. The products that are condensed out of the product stream before the stream is recycled include Synoil (a synthetic crude), water, methyl ethyl ketone (MEK), alcohols, acids, and aldehydes. The reactor is operated at 25 atm and 350°C and at anyone time contains 150 tons of catalyst. The catalyst feed rate is 6 to 9.5 tons/s, and the gas recycle ratio is 2:1.

Packed bed. The packed-bed reactor used at the Sasol plant to carry out Fischer-Tropsch synthesis reaction is fed at a syngas rate of 30,000 m 3 /h (STP) at 240°C and 27 atm. The reactor contains 2050 tubes, each of which is 5.0 cm in diameter and 12 m in length. The iron-based catalyst that fills these tubes usually contains K 2 O and SiO 2 and has a specific area on the order of 200 m 2 /g. The reaction products are light hydrocarbons along with a wax that is used in candles and printing inks. Approximately 50% conversion of the reactant is achieved in the reactor. 

In combustion of solids such as charcoal, a reaction occurs between carbon and O 2 from the air in diffusion-limited reactions. In the year AD 142, a Chinese named Boyang Wei wrote that when charcoal was combined with sulfur and potassium nitrate (obtained by crystallizing liquid animal wastes in a process too messy to describe), the mixture of solids when ignited had quite different characteristics. This mixture would fizz with a flash of light. Thus were born explosives, which Westerners were the first to turn into materials of warfare in the fifteenth century.

As mentioned previously, the animal waste sources of fixed nitrogen were too limited for conducting a large-scale war, and chemical engineers developed the Haber process for making ammonia from N 2 and the Ostwald process to oxidize NH 3 into NO and thus to HNO 3 . The explosives business then evolved through nitroglycerine, nitrocellulose, and TNT to the modern plastic explosives and rocket fuels.

A solid or liquid explosive is simply a chemical or a mixture of solids that contains fuel and oxygen in the condensed state so that no diffusion processes slow the rate. The reaction of TNT can be written approximately as which produces a lot of heat and gases. There is not enough oxygen in the molecule to convert all C to CO 2 or H to H 2 O, but the products quickly are heated to sufficient temperatures that they react instantly with O 2 at contact. If the solid is in an open container, the reaction just fizzes, but if it is confined long enough to build up pressure, it generates a pressure wave, which causes accelerated propagation of the combustion wave, which is heard as a bang. If the container is confined on all but one side (the nozzle), reaction creates a rocket.

The art and engineering of propellant and explosive manufacture consists in finding suitable combinations and treatments of solids that will produce a "stable" compound that will not ignite until desired and will then burn with the appropriate rate to produce the desired blast wave or thrust. Obviously these are dangerous experiments, and they should be attempted after one figures out all the possibilities and what to do about them.

Since an explosive solid contains fuel and oxidant in close proximity, the exothermic reaction can occur very quickly and without diffusion control, just as for separate fuel and oxidant systems. Premixed flames and explosions with gases in- is highly exothermic. Many acetylene tanks and pipes exploded before we learned to add stabilizers (chemicals such as acetone, which scavenge free radicals, which would otherwise promote chain-branching reactions). Even then, acetylene is sufficiently dangerous that it is seldom used in large quantities in the chemical industry. Acetylene is widely sold as the fuel for welding torches, and it is stored in large cylinders at high pressures in many welding shops. In fact, this acetylene is mixed with acetone, which has been found to be an effective scavenger of acetylene decomposition, so that these tanks are relatively safe. Table 16 .13 lists the heats of formation of several of these potentially dangerous hydrocarbons.

In the previous section we described one type of reaction that can cause an explosion, namely, the situation where fuel and oxidant are mixed in solids or liquids that can release large amounts of energy very quickly. These processes can also occur in premixed gases. An example of useful explosions in gases is the repeated explosions that occur in the automobile engine. However, most examples of explosions in gases are in fact disasters in which products burst walls of containers and cause havoc.

An explosion can be defined as a fast, transient, exothermic reaction. It needs exothermicity to generate energy and must be fast to generate this energy very quickly in a transient pulse. We can also distinguish between events in which the reaction propagates at subsonic velocity as an explosion and one in which the reaction propagates with sonic or supersonic velocity as a detonation.

In practice, we call an explosion a reaction that makes noise. This can be a "whoosh," a "thump," or a "bang" in order of increasing rate of energy release and time of confinement. The bang is caused by a buildup of pressure that is released very quickly as the confinement system ruptures or the reactor ruptures. The parameter that distinguishes these characteristics is the flame velocity u F , which is the velocity with which the reaction front propagates down a tube or across a container. The velocity of sound in air at 300 K is approximately 1100 ft/s or 300 m/s. If the velocity of propagation is less than the velocity of sound, the wave is called a deflagration, and if it is at or above the velocity of sound, it is called a detonation. Detonation can do much more damage because of the shock wave. We call a subsonic reaction wave a deflagration and a supersonic wave a detonation.

If CH 4 and air at the stoichiometric ratio (9.5% CH 4 in air) at atmospheric pressure are ignited at one end of a pipe, the pressure will ultimately rise by a factor of ∼ 10, and this pressure wave will travel at 300 m/s down the pipe. The heat of reaction is -803 kJ/mol of CH 4 , and all this heat will be released and the pressure will rise very quickly.

Explosions in solids and liquids can be even more violent than in gases because the energy content per mole is comparable, but the density of solids and liquids is approximately 1000 times the density of gases at atmospheric pressure. As such, accumulation of dust in grain storage, grain processing facilities and metal processing facilities can lead to spectacular explosions that are very dangerous.

These considerations in fact underestimate the damage an explosion can cause as the flame speed becomes sonic and generates a shock wave. The pressure wave travels so fast that it carries considerable momentum, and this can burst walls even more effectively than a static calculation of wall strength would indicate. Sonic waves also reflect and focus their energy, so that quite unpredictable and extreme localized damage can occur from an explosion.

The chemical industry is one of the safest industries in the United States in terms of accidents and fatalities, with approximately 10 fatalities per year for 100,000 workers. Your chances of getting killed in driving to and from work are much higher than while at work in the most hazardous job you could have in the chemical industry.

Nevertheless, accidents in chemical industry can be spectacular, with great potential for loss of life and income for many people. Most importantly, chemical engineers (you) will be responsible for creating safe processes and protecting the lives and health of your colleagues.

A great deal can and should be said about safety, but we have only been able to outline the principles by which chemical reactors may go out of control. We will conclude our discussion of safety by listing some of the notorious accidents that have occurred in the chemical industry (see Table 16 .14).

The Texas City Disaster was perhaps the first "modern" chemical disaster, and it is still the US chemical accident with the most casualties. It occurred when a barge of ammonium nitrate (NH 4 NO 3 or AN) fertilizer was being unloaded in Galveston Bay near Texas City. A fire started in the crew quarters of the barge. The fire department was spraying water on the fire, and many people on the shore were watching the action. The latest explosion involved with AN was in West, Texas, in 2013. Suddenly the barge exploded, taking with it a sister barge, a sulfur plant on the shore, and two airplanes flying overhead. No one expected this disaster, and all procedures were being followed according to the "standard safety practices" of 1947. AN was a fertilizer, not an explosive! No one had "done the experiment" until the fire happened to get hot enough with a large enough quantity of AN to cause its detonation.

By current practice, large quantities of AN are never accumulated or exposed to heat or fires. However, fertilizer storage warehouses still explode occasionally, and AN has become a popular, inexpensive, and readily available material for terrorists, for example, in the World Trade Center bombing in New York City in 1992 and the Oklahoma City bombing of 1995.

A second explosion occurred in a chemical plant in Texas City, Texas, some years later, and this was called the Texas City Disaster II. This explosion occurred when a distillation column used to purify butadiene from other C4 hydrocarbons was operating improperly and the feed was shut off, as was the product stream from the reboiler. The column suddenly blew up, sending the column like a rocket -a mile into the town. After some investigation, it was surmised that the reboiler accumulated a high concentration of vinyl acetylene, which is a strongly endothermic compound. The initial explosion was the reaction

304.5 kJ/mol 2 2R

(16.58)

The hydrogen pressure burst the reboiler and the contents of the column then ignited, turning the distillation column into a primitive but effective rocket that took off. Fortunately, no one was around at either the launch site or landing site. However, the town's residents heard a loud bang, and they had been highly sensitized from the previous one when the AN barges blew up a few years before.

The third Texas City disaster came with the refinery explosion on March 23, 2005, when a hydrocarbon vapor cloud exploded at the isomerization process unit at BP's Texas City refinery, killing 15 workers and injuring more than 170 others. The isomerization plant at the site was designed for the conversion of low octane hydrocarbons, through various chemical processes, into higher octane rating hydrocarbons that could then be blended into unleaded petrol. The leakage of large quantity of hydrocarbon into the air caused explosion.

Yes. Sugars can explode. The 2008 Georgia sugar refinery explosion was an industrial disaster that occurred on February 7, 2008 in Port Wentworth, Georgia. Fourteen people were killed and 42 injured when a dust explosion occurred at a sugar refinery owned by Imperial Sugar.

Flixborough, the Phillips polyethylene plant explosion, Norco Shell refinery explosion and the Texas City refinery explosion were vapor cloud explosions (VCE) of hydrocarbons that escaped through leaks in chemical reactors. In Flixborough disaster near the villege of Flixborough, North Lincolnshire, England, Nylon 6 was being made by oxidation of cyclohexane to produce caprolactam.

In the Phillips plant low-density polyethylene was being produced in a process that uses isobutane solvent at very high pressures. These liquids escaped and vaporized without combustion because no ignition sources were nearby. After the hydrocarbon vapors had spread large distances and had become diluted with air (O 2 ), a cigarette or pilot light somewhere ignited the premixed alkane-air mixture creating a VCE, with disastrous consequences to the plant, to the neighborhood, and in the latter case to people.

Bhopal, the site of the largest chemical accident in the past 50 years, wasn't an explosion but rather an uncontrolled reaction. The plant in Bhopal, India, owned jointly by Union Carbide and the Indian government, was producing methyl isocyanate as an intermediate for a pesticide. Water was admitted into a storage tank containing this chemical, which caused it to hydrolyze and produce CO, HCN, and unreacted MIC, which spread downwind into a crowded slum nearby and caused many deaths and casualties. The desired reaction to synthesize MIC was all of which are highly toxic. The Bhopal chemical plant was used to produce agricultural pesticides and herbicides. These chemicals are very important in agriculture to maintain insect and weed populations under control so that crop yields can be improved. One wants to produce pesticides and herbicides that are very selective in destroying a particular insect or plant but are not also toxic to other organisms (such as humans). Here the objectives are to produce a chemical that is toxic to specific plants or animals but not to others. Some desirable characteristic of these chemicals is that they (1) attack only the desired pests, (2) do not decompose during storage and shipment, and (3) decompose soon after application so that pesticide residues do not remain on food or contaminate ground water. It is obvious that considerable creative engineering goes into formulating and testing these agricultural products.

At Bhopal they were manufacturing SEVIN, also called carbaryl, a broad-spectrum insecticide that was and still is used on many crops in most countries throughout the world. The reaction steps to manufacture SEVIN are In the reaction steps leading to SEVIN, a common agricultural pesticide, all of the reactants are quite inert and harmless except phosgene, MIC, and the final product.

Xiangshui chemical plant explosion, Yancheng Industrial Park, Jiangsu, China. The March 21, 2019 explosion occurred at a local time of 14:48 (06:48 GMT). 78 people were killed and at least 94 were severely injured, 32 of whom were critically injured. Around 640 people, included children at a local kindergarten, required hospital treatment and were taken to 16 hospitals.

The force of the blast started numerous fires in Yancheng, knocked down several buildings, and reportedly destroyed windows kilometers away. The fire was controlled by 03:00 local time next morning. Considerable damage was caused to nearby factories and offices; including the roof of Henglida Chemical Factory, 3 km from the explosion, fell in. At least one of the people killed was in another building destroyed by the blast. Windows are reported to have been blown out up to 6 km away from the explosion, and houses and other buildings were damaged in the nearby village-level administrative divisions. This explosion was so strong that it even registered on the earthquake sensors and the devastation could be seen by satellites. According to the South China Morning Post article entitled "Devastation at blast site after China chemical plant explosion leaves at least 64 dead, 640 injured" discusses the point of the surrounding three rivers were polluted with exceedingly levels of dichloroethane and dichloromethane by 2.8× and 8.4× the national water quality standards. The 170 m wide crater and 2 m deep required filling along with neutralizing the soil to prevent contamination to the surrounding community. "This chemical plant was flattened along with the surrounding 16 factories also have varying degrees of damage."

On April 4, 2019, the Standing Committee of the Yancheng Committee of the Communist Party of China had a meeting at which it was decided to definitively close down the Xiangshui Chemical Industry Park.

University lab can explode too. While the ferocity of an accident is proportional to the amount and concentration of the chemicals caused it, bad accidents can occur in a university lab. On the morning of December 26, 2018, a fire/explosion occurred at the waste treatment lab, Environmental Engineering Department of Beijing Jiaotong University in China. Three students died at the accident, where magnesium was mixed with phosphoric acid to prepare for the wastewater treatment of waste filtrate liquor. The hydrogen produced from the reaction vessel exploded due to the spark generated by the (mixer) shaft and motor.

Example 16.2 Reactor runaway (This is a story retold based on Fogler, 1999 ).

An explosion occurred at Monsanto plant in Sauget, Illinois, on August 8, 1969 at 12:18 am. The blast was heard as far as 10 miles away and people were awakened from their sleep. The culprit was a batch reactor producing nitroanaline from ammonia and o-nitrochlorobenzene (ONCB):

The reaction is normally carried out at 175°C and about 500 psi. The ambient temperature of the cooling water in the heat exchanger is 25°C. The reactor temperature could be changed by adjusting the coolant rate. At the plant setting, the reactor tem-perature could be maintained at 175°C at the maximum coolant rate if the ambient temperature of 25°C could be maintained throughout the heat exchange.

On the day of the accident, two changes in normal operation occurred: 1. The reactor was charged with 9.044 kmol of ONCB (nearly three times of the normal 3.17 kmol), 33.0 kmol of NH 3 (instead of 43 kmol in normal operations) and 103.7 kmol of H 2 O (about the same as the normal of 103.6 kmol). 2. The reaction is normally carried out isothermally at 175°C over a 24-h period. Approximately 45 min after the reaction was started, cooling to the reactor was turned off for 10 min. It was not unusual not to cool in that 10 min or so on period when the normal charge of 3.17 kmol of ONCB was used and no ill effects occurred. The reactor had a safety relief valve with a disk rupture pressure of about 700 psi. Once the disk ruptures, the water would vaporize, and the reaction would be cooled (quenched) by the release of the latent heat of vaporization.

Plot the temperature-time trajectory up to a period of 125 min after the reactants were mixed and brought up to 175°C. Show that the following three conditions had to have been present for the explosion to occur: (1) Increased ONCB charge; (2) Reactor cooling stopped for 10 min; and (3) relief system failure.

Additional information: The reaction rate is r k k C C , with 0.00017 m /(kmol min) at 188 C ONCB ONCB NH 3 3 − = = ⋅°T he reactant volume for the charge of 9.044 kmol of ONCB is 3.265 m 3 ONCB/NH 3 + 1.854 m 3 H 2 O = 5.119 m 3 . The reactant volume for the charge of 3.17 kmol of ONCB is 3.26 m 3 . Thermodynamic and kinetic parameters are-∆H R = -5.9 × 10 5 kcal/kmol-ONCB; E a = 11.273 kcal/mol; C P,ONCB = 40 cal/(mol·K); C P,H 2 O = 18 cal/(mol·K); C P,NH 3 = 8.38 cal/ (mol·K).

Assume ∆C P = 0, UA H = 35.85 kcal/(min·°C) with T a = 298 K.

The stoichiometry of the reaction may be represented by which is more than sufficient to control the temperature in the reactor. One can conclude that the explosion would not have happened if the cooling was not turned off. and UA H = 35.85 kcal/(min·°C). Fig. E16 .2.2 shows the simulated temperature profile for the reactor operation. We can observe that the temperature continue to rise. More clearly, from Fig. E16.2. 3, by 110 minutes (or 10:45 pm), the reactor temperature had reached 515.49 K or 242.3°C. The temperature began increasing even more rapid after this point. One can observe that 120 min after the batch reactor was started the temperature increases sharply and the reactor explodes at a few minutes before midnight. If the mass and heat capacity of the stirrer and reaction vessel had been included, n i C Pi term would have increased by about 5% and extended the time until the explosion occur by 15 minutes or so, which would predict the actual time of the explosion occurred at 12:18 am.

When the temperature reached 300°C, a secondary reaction, the decomposition of nitroaniline to noncondensable gases such CO, N 2 and NO 2 , occurs, releasing even more heat. The total energy released was estimated to be 6.8 × 10 9 J, which is enough energy to lift the entire 2500 Mg building 277 m straight up.

The pressure relief disk should have ruptured when the temperature reached 265°C or 538.15 K (ca. 700 psi), but it did not and temperature continued to rise. If it had ruptured, the liquid water would vaporize, a maximum of 10 6 kcal would be able to be drawn from the reaction mixture, thereby lowering the temperature and quenching it.

If the disk had ruptured at 265°C (700 psi), the maximum mass flow rate out of the 2-in orifice to the atmosphere (1 atm) would have been 830 kg/min at the time of rupture. This leads to a maximum heat removal of which is much greater that the heat generation at that time H G = 26520.8 kcal/min. The reaction could have easily been quenched.

United Nations has created a universal identification system, global harmonized system of classification and labeling of chemicals (GHS), beginning in 1992. The GHS label includes the substance name, id, eye-catching pictograms to show the potential dangers, warnings and descriptions on how to handle it. GHS hazard classification also includes three categories: Physical, Health and Environment. As compared with HMIS and NPFA labeling, GHS combined the flammability hazards into physical hazard category while one additional category is added: environmental hazard. Chemical reactivity hazard is defined as a situation where an uncontrolled chemical reaction could result directly or indirectly in serious harm to people, property, or the equipment. As such, the first come into mind of the reactivity hazards is fire and explosion, which usually involves strong exothermic reactions. To safe guard processes and facility, one needs to first identify the threat, or screening for reactivity hazards. When the "threats" are identified, one can then find the remediation(s) for the particular "threats" by adding barriers or change the design if possible to avoid the "threats

Bhopal Catastrophe 1984: Causes and Consequences (in Reference Module in Earth Systems and Environmental Sciences)

Elements of Chemical Reaction Engineering

Chemical Process Safety: Fundamentals with Applications

Combustion

The Engineer Is Human: The Role of Failure in Successful Design

Design Paradigms: Case Histories of Error and Judgment in Engineering

World Health Organization, Laboratory biosafety manual

Because of these factors, chain reactions are inherently unpredictable. Chemical and thermal autocatalysis makes the overall rate not a simple function. Chain reactions can also be very fast so that the reaction may be limited by mass transfer processes.Partial oxidation reactions are important in chemical industry. Pyrolysis and gasification of biomass, for example, are useful technologies in biomass conversion. We frequently do not have reliable reaction rate expressions for chain reactions, but we can compensate for this lack by designing and operating the reactor to manage the overall course of the reaction by properly dealing with mixing, mass transfer, promoters and inhibitors, and the presence of surfaces.An explosion can be defined as a fast, transient, exothermic reaction. It needs exothermicity to generate energy and must be fast to generate this energy very quickly in a transient pulse. We can also distinguish between events in which the reaction propagates at subsonic velocity as an explosion and one in which the reaction propagates with sonic or supersonic velocity as a detonation. Unintended and accidental explosions can occur due to (1) The accumulation of reactive chemicals (gas, liquid, or solid) in confined space; (2) Leakage of "significant amount" of reactive chemicals (gas, liquid, or solid) to atmosphere or environment; or (3) Reactor runaway. As a bioprocess engineer, you will be responsible for creating safe processes and protecting the lives and health of your colleagues and community. c. Sketch C j (z) in the cigarette for O 2 , CO 2 , CO, and tar assuming a steady-state gas velocity and that the length of the cigarette L changes slowly. d. How does the heat generation and absorption vary with position? Sketch T(z). e. Illustrate u(t) and T(z, t) during a puff? Sketch L(t).Estimate transient temperatures, velocities, and concentrations versus time with as much accuracy as possible.16.6 Model a campfire spark as a spheric wood particle (ρ = 500 kg/m 3 ) surrounded by a boundary layer of thickness 2R (Sh = 2) at 1000°C. a. Summarize in several sentences and equations the model of the combustion of a campfire spark. b. Why will a small particle be lifted from the fire? c. One part of this problem is analogous to a hot air balloon. Formulate a model with which you could calculate the size and density for lift and the velocity of the rising spark particle. where the first is the initiation step, the second and third are propagation steps, and the fourth is the termination step. Derive a rate expression for this reaction r = k[CH 3 OCH 3 ] n assuming pseudo steady state on radical intermediates. c. Find an expression for the reverse reaction rate.

Consider the combustion of a coal pile of diameter D and height H. Before ignition the air flow is driven by wind, and after ignition by natural convection. The heat of combustion of carbon is -393 kcal/mol. Set up a qualitative energy balance (no numbers) to explain the dependence of combustion on weather and on D and H. What size and shape of coal piles do you recommend to minimize spontaneous combustion?

If a container filled with each of the molecules in Table 16 .13 at room temperature and atmospheric pressure suddenly decomposed into C (s) and H 2 , calculate the temperatures and pressures in the container.

A bottle that you thought contained ethanol had oxidized to 10% ethylhydroperoxide. If the contents of this bottle suddenly went to chemical equilibrium, estimate the temperature and pressure in the bottle (for a short time). The heat of formation of ethylhydroperoxide is -172 kJ/mol and the heat of carbon oxidation to CO 2 is -280 kJ/mol. Btu/lb-ammonium nitrate at 500°F; C P = 0.38 Btu/(lb-ammonium nitrate·°F); C P = 0.47 Btu/(lb-steam·°F) and r = k C A , with k = 0.53 h -1 at 510°F and k = 5.03 h -1 at 560°F. The enthalpies of water and steam are: H L (200°F) = 168 Btu/lb and H G (500°F) = 1202 Btu/lb.

Explore the ONCB explosion described in Example 16.2. Show that no explosion would have occurred if the cooling was shut off for 10 min after 9.04 kmol charge of ONCB or if the cooling was shut off for 10 min after 45 min of operation for the 3.17 kmol ONCB charge. Show that if the cooling had been shut off for 10 min after 12 h of operation, no explosion would have occurred for the 9.04 kmol charge. Develop a set of guidelines as to when the reaction should be quenched should the cooling fail. Perhaps safe operation could be discussed using a plot of the time after the reaction began at which the cooling failed, t 0 , versus the length of the cooling failure period, t f , for the different charges of ONCB. Parameter values used in this example predict that the reactor will explode at midnight. What parameter values would predict that the reactor would explode at the actual time of 18 min after midnight? Find a set of parameter values that would cause the explosion to occur at exactly 12:18 am. For example, include heat capacities of metal reactor and/ or make a new estimate of UA H . Finally, what if a ½-in rupture disk rated at 800 psi had been installed and did indeed rupture at 800 psi (270°C)? The explosion still would have occurred. (Note: The mass flow rate varies with the crosssectional area of the disk. Consequently, for the conditions of the reaction the maximum mass flow rate out of the ½-in disk can be found by comparing it with the mass flow rate of 830 lb/min of the 2-in disk.

http://knovel.com. http://www.aiche.org/ccps/. http://www.aiche.org/ccps/topics/process-safety-technical-areas/chemical-reactivity-hazards. http://www.aiche.org/academy/courses/ela904/chemical-reactivity-hazards. http://www.chemsafety.gov/. http://www.osha.gov.