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Unit Operations and Pollution Prevention |
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by David R. Shonnard In developing a flowsheet for the production of a chemical, it is desirable to consider the environmental ramifications of using each unit operation in the process rather than postponing this consideration until the flowsheet is finished. This "front end" environmental assessment is more likely to result in a chemical process that has less potential to cause environmental harm. In many instances, this environmentally benign design will also be more profitable, the improved design will require lower waste treatment and environmental compliance costs and will convert a higher percentage of raw materials into salable product. In considering pollution prevention for unit operations in the design of chemical processes, the following considerations are important. |
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In the following sections, we apply this framework for preventing pollution in unit operations by considering choices in materials, technology selection, energy consumption, and safety ramifications. In Section 9.2, material choices that are generic to most chemical processes, like process water and fuel type, are analyzed with respect to in-process waste generation and emission release. Other process materials that are more specific to various unit operations are discussed in subsequent sections of this chapter. Chemical reactors are the topic in Section 9.3.The environmental issues related to the use of reactants, diluents, solvents, and catalysts are discussed first. Then, the effects of reaction type and order on product yield and selectivity are covered. The effects of reaction conditions (temperature and mixing intensity) on selectivity and yield are illustrated. Finally, the benefits of additional reactor modifications for pollution prevention are tabulated. In Section 9.4, the most important topics include the choice of material (mass separating agent) to be used in separations, design heuristics, and examples of the use of separation technologies for recovery of valuable components from waste streams, leading eventually to their reuse in the process. Separative reactors are the topic of discussion in Section 9.5. These hybrid unit operations have special characteristics to help achieve higher conversions and yields in chemical reactors compared to conventional reactor configurations. In Section 9.6, methods for reducing emissions from storage tanks and fugitive sources are discussed. The safety aspects of pollution prevention and unit operations are the topic of Section 9.7.It will be shown that many pollution prevention efforts tend to make chemical processes more complex, necessitating a higher level of safety awareness. In making pollution prevention decisions that include choices of materials, unit operations technologies, operating conditions, and energy consumption, it is very important to consider health and environmental risk factors. It is also of high importance to incorporate cost factors and safety ramifications. In Section 9.8, review a method for evaluating health risk into unit operations decisions by considering the optimum reactor operating conditions as an example application. Although no generally accepted method exists for these risk assessments, the method outlined in Chapter 8 and applied in Section 9.8 is useful for incorporating multiple risk factors into decisions regarding item operations. Finally, it is also important to introduce the concept of "risk shifting." Pollution prevention decisions that are targeted to reduce one kind of risk may increase the level of risk in other areas. For example, a common method for conserving water resources at chemical manufacturing facilities is to employ cooling towers. Process water used for cooling purposes can be recycled and reused many times. However, there is an increased risk to workers who may be exposed to the biocides used to control microbial growth in the cooling water circuit. Also, in some cooling water processes, hazardous waste is created by the accumulation of solids-for example, from the use of hexavalent chromium (a cancer causing agent) as a corrosion inhibitor. Another example of shifting risk from the environment and the general population to workers involves fugitive sources (valves, pumps, pipe connectors, etc.).One strategy for reducing fugitive emissions is to reduce the number of these units by eliminating backup units and redundancy. This strategy will reduce routine air releases but will increase the probability of a catastrophic release or other safety incidents. Simply put, the objective of pollution prevention is to reduce the overall level of risk in all areas and not to shift risk from one type to another. |
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