Analysis of the start-up process for reactive distillation

The start-up procedure of a distillation column is a time- and energy-consuming process. Further, the products during the start-up time are off specification and cannot easily be recycled as for conventional distillation but must costly be disposed of. In this paper, a process model to simulate the barely analyzed start-up procedure for a reactive distillation from the cold and empty state to steady state is presented. The start-up of a reactive distillation column has been modeled with gPROMS. The advantage of a cold and empty start-up is the consistent and reproducible initialization. Commercial simulators do not give the opportunity to start form a cold and empty state, e.g., a column modeled with Hysys must be shut down from a steady state to be able to model the complete start-up process, which is not possible, for example, for a batch process. Also, a change in the describing equations and discontinuities in process variables is difficult to handle within the simulation. In this paper, the start-up strategies normally used for distillation without reaction are examined and applied to reactive distillation. It will be shown that the widely used strategy of total reflux is not suitable for reactive distillation. A simplified model to derive a time constant which describes the influence of parameter setting changes, like heating power, reflux ratio and feed composition on the start-up time, is introduced and validated.