Design and control of a gas-phase adiabatic tubular reactor process with liquid recycle

Previous studies have explored the design and control of processes with gas-phase adiabatic tubular reactors that feature a gas recycle and a simple separation section consisting of a single ideal separator vessel. The gas recycle leads to high compressor capital and operating costs. This paper extends this work to the case in which a distillation column is required in the separation section and the recycle stream is liquid. The liquid recycle means that there are no compressor costs to counterbalance the reactor costs. However, there are large capital and energy costs associated with the vaporization/condensation of the recycle stream. For the numerical case studied, the liquid recycle process is more expensive than the gas recycle process, and it is more difficult to control. The basic reaction is A + B --> C. Three reaction systems are considered: case 1 (irreversible with moderate activation energy), case 2 (irreversible with high activation energy), and case 3 (reversible). The optimum steady-state designs for cases 1 and 3 can be effectively controlled by the same control structure. The optimum steady-state design for case 2 cannot be controlled, and the process has to be redesigned to prevent reactor runaways. The concentration of one of the reactants has to be reduced so that it becomes a limiting reactant, thus providing self-regulation. This self-regulation in the liquid recycle process is not as effective as that in the gas recycle system because of the slower changes in concentrations due to the larger holdups of material in the liquid phase.