Question

When considering the evolution of a process flowsheet, it was noted that there are three forms of recycle structure for unused reactants, given as a–c below. For each case, carefully explain under what conditions you would consider or implement each strategy.

1. Separate, purify, and recycle

2. Recycle without separation and use a purge

3. Recycle without separation and do not use a purge

Answer 1

(a) Good when product(s) and reactant(s) are easily separated and purified (most often by distillation.) Any inerts in the feed or byproducts can be removed by some unit operation and thus recycle does not require a purge.

(b) When unused reactant(s) and product(s) are not easily separated (for example when both are low boiling point gases) and single pass conversion of reactant is low.

(c) This is only possible when no significant inerts are present and any byproducts formed will react further or can reach equilibrium.