Steady-state and dynamic effects of design alternatives in heat-exchanger/furnace/reactor processes

Feed-effluent heat exchangers (FEHE) are widely used in industry to preheat the feed to adiabatic tubular reactors. The hot reactor effluent is passed through a FEHE to recover heat. The positive feedback of energy introduces the potential for open-loop instability. Heat-exchanger bypassing is typically used to control the reactor inlet temperature. Previous papers(1-4) have explored the control of this type of process. A furnace or heater following the FEHE may or may not be required under normal operation but is always needed for startup. Therefore, a design alternative exists in which both the reactor inlet temperature and the furnace inlet temperature are controlled, using the two manipulated variables: heat-exchanger bypassing and furnace firing rate. This paper explores the impact of these alternative designs on both the steady-state economics and the dynamic controllability. The exothermic, irreversible, gas-phase reaction A + B --> C occurs in an adiabatic tubular reactor. A gas recycle returns unconverted reactants from the separation section. Steady-state economics favor the use of only a FEHE with bypassing. Dynamic controllability strongly favors the use of both FEHE bypassing and furnace firing, particularly when the reactor gain (K-R = Delta T-out/Delta T-in) is large.