Perhaps the most extreme examples of separation and recycling are purge streams. Purges deal with both feed impurities and the by-products of the reaction. Purifying the feed can reduce the size of some purges. However, if purification is not practical or the purge must remove a byproduct of the reaction, additional separation is necessary.
Figure 3.8.10 shows the recovery of acetone from an aqueous waste stream by distillation. As the fractional recovery of acetone increases when the reflux ratio is fixed, the cost of column and auxiliary equipment increases. Alternately, fixing the number of plates in the column eliminates additional column cost, and increasing recovery by increasing the reflux ratio increases the energy consumption for separation.
For each fractional recovery, a tradeoff exists between the capital and the energy required to obtain the optimum reflux ratio. The result is that the cost of separation (capital and energy) increases with increasing recovery (see Figure 3.8.11). On the other hand, increasing recovery saves the cost of some of the lost acetone. Adding the cost of raw materials to the cost of separation and recycling
FIG. 3.8.10 Process improved by recycling the excess reactant and solvent used in the reaction.
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