In process optimization, the chemical engineer uses statistical, factorial experiments while simultaneously varying the reaction parameters such as temperature and stoi-chiometry. Computer programs are then used to generate contour maps of yield versus temperature and stoichiom-etry, revealing the global maximum yield. Finally, the engineer optimizes for the overall process cost but not necessarily for yields. Although certain stoichiometry gives the highest yields, the costs of indicated molar excesses of reagents may be higher for it than other stoichiometries, raising the process cost overall.
The workup is a separate phase of optimization because the workup of a reaction mixture differs depending on the ratio of product to other substances. The workup is different in a reaction that gives a 60% yield than in another that gives a 95% yield.
The yields to be maximized in a workup should be chro-matographically determined yields and not isolated ones. Thus, with the yield maximized, the workup can be optimized.
Whenever feasible, distillation provides the most convenient and cheapest workup procedure. On a workup of reaction mixtures, quenching with water to precipitate an organic product or extract the aqueous layer with organic solvents should be avoided. When the organic product is precipitated by water, it is usually not in the best crystalline form. Thus, the solid is hard to filter and takes a long time to dry, which results in a long production time. Water precipitation also results in the water being contaminated with organics, which must be treated (Benforado, Riddlehover, and Gores 1991).
The organic solvent extraction of a water-quenched reaction mixture is also messy. The extraction is slow and inefficient. Things tend to be extracted nonselectively. Also, several water washings of organic phases produce a large amount of water effluent.
Instead of water quenching, chemical engineers can usually induce a solid product to crystallize by adjusting the solvent concentration and temperature (Stinson 1993). These adjustments result in a purer product, easier solvent recovery by distillation of the filtrate, and less effluent to treat.
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