Effect of Design and Kinetic Parameters on the Control of Cooled Tubular Reactor Systems

The steady-state design of cooled tubular reactor systems involves tradeoffs between reactor costs and compression costs, which are both affected by kinetic parameters and by tube geometry (diameter, length, and number). Peak temperatures occur in cooled reactors, which makes temperature control more challenging and which might necessitate the use of multiple temperature measurements along the length of the reactor. This paper considers the impact of these parameters on the dynamic controllability of the system. The exothermic, irreversible, gas-phase reaction A + B → C occurs inside the tubes of a packed tubular reactor. Steam is generated on the shell side to remove heat. The process consists of a feed−effluent heat exchanger, furnace, reactor, partial condenser, separator, and recycle compressor. Cases are explored that have different kinetic parameters (specific reaction rates and activation energies), different design parameters (tube diameters and reactant concentrations), and different control structures (number of temperature sensors). A conventional selective control scheme is used that controls the highest of several temperatures along the length of the reactor by manipulating the steam pressure in the reactor shell. Results show that dynamic controllability worsens as specific reaction rates, activation energies, and tube diameters increase and as the number of temperature sensors decreases. Designs with large reactor heat-transfer areas are easily controlled. Tube diameter has a greater impact on controllability than does reactant dilution. Reactant dilution appears to be less effective in cooled tubular reactors than in adiabatic tubular reactors.