Heuristic Design of Reaction/Separation Processes

The dominant trade-off in chemical process design is between reactor size and recycle flow rate. Big reactors require larger capital investments in vessels and catalyst, but they result in smaller recycle flow rates for a given yield, which means lower capital investments and energy costs in the separation section of the process. Small reactors have reverse effects. Therefore, an economic optimum exists that balances the costs of the reaction section and the separation section of the process. This paper presents a heuristic approach to quickly determine this optimum trade-off during preliminary conceptual design. The basic idea is to start with a very large reactor and find the recycle flow rate required to meet some specified conversion/yield/selectivity criterion. This is the minimum recycle flow rate. Then a heuristic similar to that used in distillation column design is employed. The actual recycle flow rate is set equal to 1.2 times the minimum, and the reactor and separation sections are designed with this recycle. The heuristic recycle ratio has some dependence on the phase equilibrium (decreases as relative volatility decreases) and catalyst cost (increases as catalyst cost increases). The process studied has a continuous stirred-tank reactor, two distillation columns, and one recycle stream. Two consecutive reactions (A + B -> C and A + C -> D) produce a desired product C and an undesired product D. Achieving high selectivity requires low concentrations of A and C, so there is a large recycle of mostly B. Relative volatilities are assumed to be alpha A > alpha(B) > alpha(C) > alpha(D), so there is one recycle from the overhead of the first distillation column containing unreacted A and B. The second column separates C and D. A second process is also studied involving an actual chemical system.