C2. Optimum pressure and concentration gradients in tubular reactors

Abstract

In tubular reactors (or in stirred reactors in series) it is possible to have the conditions vary along the tube. In this way better results can often be obtained. The effect of a pressure gradient is of interest in equilibrium gas reactions where the number of moles increases (e.g. dehydrogenation reactions). High pressures are favourable in the first part of the tube, where the reaction mixture is far from equilibrium, whereas low pressures are favourable in the last part, where equilibrium concentrations are approached. It appears that the optimum pressure is roughly proportional to the local reactant concentration. The reactor volume required for a certain conversion is two to three times smaller than it is without a pressure gradient. In a reaction system with two feed components, the concentration gradient can be influenced by introducing one of the components A along the tube and the other component B at the beginning (cross-stream reactor). In this way favourable effects are obtained in systems where the desired reaction is first-order in A and there is an undesired side reaction of a higher order in A. The lower the concentration of A, the higher the selectivity of the process; optimum selectivity is obtained in a cross-stream reactor where the concentration of A is kept constant over the entire reactor. If the conversion is high the yield of desired product is optimum at optimum selectivity; at lower conversion levels the yield can be increased at the expense of selectivity. Results are obtained by analogue computation for various ratios of the reaction rate constants and are compared with those of the one-stage and the co-current tubular reaction systems.