EFFECT OF RECYCLE ON CHEMICAL REACTOR CONTROLLABILITY

Abstract

This paper studies temperature control of continuous stirred-tank chemical reactors (CSTR's) when they occur in a series configuration with and without a downstream separation section with recycle. The dynamic controllability of the reactor section is a key factor in the dynamic performance of the process because of the large impact that temperature control has on product quality, yield and safety of the entire plant. Also in a combined reactor/separation process, temperature control in the reactor can affect reactor compositions which in turn can drastically affect the load on the separation section. The number of reactors in the reactor system is a major design parameter which affects the steadystate economics (operating and investment costs). The number of reactors also has a strong impact on the dynamic controllability of not only the reactor system itself but of the whole combined system.

The results of this study demonstrate that the use of multiple reactors results in poorer temperature control, with or without recycle, when the kinetic system is a simple first-order irreversible reaction A →B. Also with this reaction, temperature control is most difficult in the first reactor of the series. However, for a kinetic system with consecutive reactions A→B →C, the reverse is true: temperature control is improved by the use of multiple reactors, and the last reactor in the series is the most difficult to control. This is true for most disturbances, but the multiple reactors are shown to be more sensitive to feed thermal effects.

KEYWORDS:

• Recycle
• Control
• Chemical reaction engineering.

Additional information

Notes on contributors

WILLIAM L. LUYBEN

Author to whom correspondence should be addressed.