Systematic Framework for Environmentally Conscious Chemical Process Design: Early and Detailed Design Stages

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This paper presents a systematic and hierarchical approach for incorporating environmental considerations into all stages of chemical process design. The complexity of the environmental and economic assessments increases as the design proceeds. The design method integrates computer-aided process simulation with a suite of environmental impact, economic, and decision analysis software tools. At the earliest design stage, the environmental assessment includes emission estimates from major process equipment, considers pollution control efficiency, and generates nine risk-based environmental impact indices. The economic assessment is based on the cost of raw materials and reaction stoichiometry. This assessment method was applied to the selection of alternative feedstocks for a reaction step: benzene versus n-butane for maleic anhydride (MA) production. Using known yields and selectivities, the n-butane route was shown to be superior to the benzene route in both the raw material cost and environmental impacts. After synthesis of a process flowsheet and simulation, a series of "process diagnostic summary" tables was generated to identify early process improvement strategies. An "improved" base-case flowsheet was developed, simulated, and assessed for profitability and environmental impacts. The improved n-butane process flowsheet after implementation of heat integration yielded an 80% reduction in utility costs and a 13% reduction in the aggregate environmental index compared to the base-case flowsheet. Then, several design tasks, such as equipment sizing, scaled gradient analysis, and multiobjective optimization using a genetic algorithm, were applied. After optimization, the economic performance of both the n-butane and benzene flowsheets was substantially bettered over the previously improved base-case flowsheets. The aggregate environmental index for the n-butane process was decreased to the greatest extent as a result of early design activities and negligibly as a result of detailed design and optimization. The opposite was observed for the benzene process. The reasons for this difference in system behavior are explained. The early screening environmental and economic assessment methods were shown to be accurate when compared to the more rigorous assessment methods. Optimum operating conditions of the processes (reaction temperature and pressure, mass separating agent flow rate, etc.) change significantly when the objective function for optimization is either an aggregate environmental index or a profitability index.

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Industrial and Engineering Chemistry Research