Control of a feed-effluent heat exchanger / reactor system: assessment by dynamic simulation

Feed-effluent heat exchangers (FEHE) are commonly employed in the chemical industry to preheat the feed to exothermic adiabatic tubular reactor systems by recovering heat from the hot reactor effluent. While this approach enhances thermal efficiency and reduces both capital and operating costs, it introduces complex, non-linear dynamics that can lead to open-loop instability, reactor quenching, or significant temperature increases. This Thesis investigates various FEHE/reactor system designs and control strategies through dynamic simulations, with a particular focus on how feed flow disturbances (which are common in industrial plants due to fluctuations in production demand or raw material availability) affect system performance. The study begins with an analysis of the base design configuration, highlighting the critical role of controlling the reactor inlet temperature for both steady-state and dynamic performance. It then examines a modified design featuring a bypass around the heat exchanger, introducing a manipulated variable to control the reactor inlet temperature. The impact of the bypass selection on process controllability is assessed, revealing that a larger bypass results in a larger heat exchanger area, which enhances control robustness and allows efficient preheating of the reactor feed even under anomalous high throughputs compared to nominal conditions. Subsequently, the Thesis explores a conventional design that incorporates a furnace to enable process start-up, addressing the absence of an external heat source in both the base and bypass-only configurations. The performance of this new design and the relevant control strategy, which uses two manipulated variables (i.e., the bypass fraction and furnace duty) is compared to an alternative design and control structure proposed by Luyben (2012, Ind. Eng. Chem. Res. 51, 25, 8566-8574.). The alternative design demonstrates improved control performance and reduced operational costs under low throughput conditions, though it requires higher furnace duty at elevated throughputs.