ANALYSIS OF INSTABILITY IN AN INDUSTRIAL AMMONIA REACTOR

The starting point for this study was an incident in an industrial pla nt, where the ammonia synthesis reactor became unstable with rapid tem perature oscillations (limit cycles) in the range from about 300 degre es C to 500 degrees C. A simple dynamic model reproduces this behavior In industry a steady-state van Heerden analysis is often used to anal yze the stability, but a more careful analysis for this reactor system reveals that instability occurs when there still is a positive steady -state margin, namely as a pair of complex conjugate poles cross the i maginary axis (Hopf bifurcation). This is consistent with the observat ions where the instability manifests itself as oscillations rather tha n extinction of the reaction. This somewhat unusual behavior can be ex plained by the presence of an inverse response for the temperature res ponse through the reactor beds combined with the positive feedback cau sed by the preheater.