Selected response surfaces are visualised in Figure . The optimal settings are found using constrained optimisation using Sequential Least Squares Programming (SLSQP), and minimum LCOM occurs with a purge/vent rate of 2.26%, a pressure of 99 bar and 9680 reactor tubes. As seen from Figure , a higher purge rate shifts the optimum towards fewer tubes i.e. less catalyst volume, due to a lower recycle rate and accumulation of inerts and vice versa for a lower purge rate. The optimal purge rate is very close to the value of 2.2% found by Luyben using a one-factor-at-a-time (OFAT) approach. Luyben found very little difference between 9,000 and 10,000 tubes, with a slightly better economy at the high end. Luyben selected 110 bar based on an analysis of return of investment (ROI), whereas a lower optimal pressure is suggested in this study. Despite the different objectives chosen (levelised cost, vs income/ROI), the conclusions are very similar. The reason for a slightly lower pressure found to be optimal in the present study may also, at least partially, be due to a slightly better conversion due to the differences in kinetic parameters applied.