Heuristics for Chemical Process Synthesis

A strategy is recommended that involves assembling the process operations in a specific order, as follows:

Chemical reactions (to eliminate differences in molecular type)

Mixing and recycle (to distribute the chemicals)

Separations (to eliminate differences in composition)

Temperature, pressure, and phase change

Task intregation (to combine operations into unit process) The heuristics/the rules of thumb in that strategy are: Reaction Operations

Select raw material and chemical reactions to avoid, or reduce, the handling and storage of hazardous and toxic chemcial. Distribution of chemicals

Use an excess of one chemical reactant in a reaction operation to consume completely a valuable, toxic, or hazardous chemical reactant.

When nearly pure product are required, eliminate inert species before the reaction operation when the separations are easily accomplished and when the catalist is adversely affected by the inert, but not when a large exothermic heat of reaction must be removed.

Introduce purge stream to provide exits for species that enter the process as impurities in the feed or are formed in irreversible side reactions, when these species are in trace quantities and/or are difficult to separate from the other chemical. Lighter species leave in vapor purge streams, and heavier in the liquid streams.

Do not purge valuable species or species that toxic and hazardous, even in small concentrations. Add separators to recover valuable species. Add reactors to eliminate the hazardous and toxic species.

By-product that produced in reversible reactions, in small quantities, are usually not recovered in separators or purged. Instead, they are usually recycled to extinction.

For competing reactions, both in series and parallel, adjust the temperature, pressure, and catalyst to obtain high yields of the desired products. In the initial distribution of chemical, assume that these conditions can be satisfied. Before developing a base-case design, obtain kinetics data and check this assumption.

For reversible reactions especially, consider conducting them in separation device capable of removing the products, and hence driving the reactions to the right. Such reaction separation operations lead to very different distributions of chemicals. Separations operations

Separate liquid mixtures using ditillation, stripping towers, enhanced (extractive, azeotropic, reactive) distillation, liquid-liquid extraction, crystallizers and/or absorbers.

Attempt to condense vapor mixtures with cooling water. Then use heuristic point 9 (before this point)

Separate vapor mixtures using partial condensers, cryogenic distilliation, absorbtion towers, adsorbers, and/or membrane devices. Heat removal and addition

To remove a highly exothermic heat of reaction, consider the use of excess reactant, an inert diluent, or cold shots. These affect the distribution of chemicals and should be inserted early in process synthesis.

For less exothermic heat of reaction, circulate reactor fluid to and external cooler, or use a jacketed vessel or cooling coils. Also, consider the use of intercoolers between adiabatic reaction stages.

To control temperature for highly endothermic heat of reaction, consider the use of excess reactant, an inert diluent, or hot shots. These affect the distribution of chemicals and should be inserted early in the process synthesis.

For less endothermic heat of reaction, circulate reactor fluid to an external heater, or use a jacketed vessel or heating coils. Also, consider the use of interheaters between the adiabatic reaction stages. Pressure-change operations

To increase the pressure of a stream, pump a liquid rather than compress a gas; that is, condense a vapor, as long as refrigeration (and compression) is not needed, before pumping.

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Source : Seider, W.D., Seader, J.D., and Lewin, D.R., PROCESS DESIGN PRINCIPLE, Synthesis, Analysis, and Evaluation, John Wiley and Sons, Inc., p. 133, 1999.