Heuristic Design of Reaction/Separation Processes

The dominant trade-off in chemical process design is between reactor size and recycle flowrate. Big reactors require larger capital investments in vessels and catalyst, but they result in smaller recycle flowrates 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 flowrate required to meet some specified conversion/yield/selectivity criterion. This is the minimum recycle flowrate. Then a heuristic similar to that used in distillation column design is employed. The actual recycle flowrate 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), catalyst cost (increases as catalyst cost increases) and the number of recycle streams and distillation columns.

Two different cases are explored using hypothetical components. In the first, the process has a CSTR, 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 in this first case are assumed to be α_A> α_B> α_C> α_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.

In the second case, relative volatilities are assumed to be α_A> α_C> α_B> α_D, so there are three distillation columns and two recycle streams. One recycle containing unreacted A goes overhead in the first distillation column. Product C goes overhead in the second column. A second recycle stream of unreacted B goes overhead in the third column with product D going out the bottom.

An actual process (production of methoxy-methyl-heptane) using real components is also studied to verify the results of the proposed heuristic procedure.