Design and Control of Gas-Phase Reactor/Recycle Processes with Reversible Exothermic Reactions

A gas-phase reactor with reversible exothermic reactions is the classical example of the many tradeoffs between operating costs and capital costs. Because per-pass conversion is limited by chemical equilibrium, recycle is almost always needed, and this gas recycle is expensive because of the high capital cost and high energy cost of gas compression. At low temperatures, conversion is limited by small kinetic reaction rates, and large reactors are needed. At high temperatures, conversion is limited by the decrease in the chemical equilibrium constant, and large recycle flowrates are needed. Thus, the optimum design of this type of system requires finding the best set of design optimization variables: reactor temperature, reactor size, and gas recycle flowrate. In systems containing inert chemical components that must be purged from the system, an additional design optimization variable is present: purge flowrate or inert composition in the recycle gas. Higher inert compositions require larger reactors and more recycle. Lower inert compositions result in larger losses of reactant components in the purge gas. These systems present intriguing dynamic control problems because of their integrating character and control loop interaction. In this paper we consider the design and control aspects of a ternary system with the gas-phase reversible, exothermic reaction A + B S C occurring in an adiabatic tubular reactor packed with solid catalyst. Reactor effluent is cooled, and the product C is removed in a separator. Gas containing unreacted A and B (and inert I in the second part of the paper) is mixed with fresh feed and recycled back to the inlet of the reactor. The optimum design of this system is obtained, and the performances of several alternative control structures are explored.