Cw

Lime

FIG. 3.8.11 Optimum recovery determined by trading-off the effluent treatment costs and raw materials costs against the costs of separation.

gives a curve which shows the minimum cost at a particular acetone recovery.

Again, the unrecovered material from the separation becomes an effluent that requires treatment before it can be discharged to the environment. Thus, adjusting the raw material cost to the net value involves adding the cost of waste treatment of the unrecovered material. In fact with some separations, such as that between acetone and water, separation is possible to a level that is low enough for discharge without biological treatment.

Figure 3.8.12 shows an example of improving a process by recycling the excess reactant and solvent used in the reaction. Initially, the total waste generated from this process amounts to 0.8 lb/lb of product produced. After the changes, this figure drops to 0.1 lb/lb of product produced, and manufacturing costs drop by more than 20%.

Product

Solvent

Solvent

Product

FIG. 3.8.12 Changing a process to recycle excess reactants. The change from the process on the left to the process on the right not only slashed the outlet of waste, but also lowered the manufacturing costs significantly.

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