Steady-state transitions in the reactive distillation of MTBE

Abstract

The reactive distillation process for MTBE synthesis can produce multiple steady states which reduce column operability and controllability, particularly during start-up. A rigorous dynamic simulation of a MTBE reactive distillation column has been developed for a column which was suspected to exhibit multiplicity. Three distinct steady states were found at the nominal design point. Certain perturbations in the feed composition or the manipulated variables were found to cause transitions between parallel steady states and these were plotted using the dynamic capabilities of the column model. Steady-state transitions were only found when neither product flow rate was used as a column input (i.e. when an energy balance control configuration was used). Appropriate closed-loop control was also found to essentially prevent steady-state transitions. No practical mechanism of controlling a transition from an undesirable steady state to a desirable steady state was found: the controlled perturbations of a manipulated variable, the initiation of closed-loop control and the direct manipulation of the column material balance were all found to be either ineffective or impractical. A catastrophic shift can be contrived by manipulating an input variable but destabilisation of the column operation is a potential consequence of the rapid change in internal flows which occurs during the transition.