Enhancement of production rates in a bistable chemical reaction

Abstract

We present experiments and calculations which show that a dramatic enhancement of the production rate may be achieved by external periodic forcing of chemical reactions with threshold properties if the product yield of the reaction decreases with an increase in the flow rate through a continuous flow stirred tank reactor (CSTR) at constant temperature. Since the production rate (PR) is represented by the (mathematical) product of the flow rate and the reaction yield, the optimal PR enhancement is achieved at high flow rates with short but large periodic pulses to lower flow rates where the yield is high. Here we investigate a bistability as a threshold at constant temperature in the Minimal–Bromate reaction and we reach experimental PR enhancements of up to 31% over the maximum PR in the unperturbed system. NFT model calculations are in agreement with the high enhancements observed in the experiments. In contrast, only a small PR enhancement (1%) is encountered for a first order thermokinetic reaction in which an existing bistability or subcritical Hopf bifurcation is crossed by an external periodic flow perturbation. Here the nonlinearity resides in the exponential Arrhenius dependence of the rate constant on temperature. As a consequence, narrow PR pulses occur which lead to only small enhancements in the averaged production rate. This is demonstrated by employing the Sal'nikov model with CSTR boundary conditions.