Nonlinear Behavior of Reactor−Separator Systems with Azeotropic Mixtures

Abstract

In this work, the behavior of a coupled nonlinear reactor−separator system is analyzed. The reactor is modeled as a continuous stirred tank reactor (CSTR) that sustains a first-order reaction of the form A→B. The separator is modeled as a flash. The effluent from the reactor is fed to the separator. Here, we assume that the liquid stream from the separator is recycled back to the reactor. The primary interest is to investigate the system when the vapor−liquid equilibrium (VLE) has an azeotrope. Under these conditions, the recycle stream can be either reactant-rich or reactant-lean depending on the feed composition to the flash. The focus is on pure mass recycle, as the two units are assumed to be decoupled energetically via heat exchangers. Two different modes of operations are considered. In the first mode of operation, the fresh feed flow rate is fixed. In the second mode of operation, the effluent flow from the reactor is fixed. In practice, these different modes of operation can be achieved using a suitable control strategy. When fresh feed to the network is flow-controlled, there is a region of dynamic instability in the solution branch corresponding to the recycle of the reactant-lean stream. In addition, coexistence of states with the recycle of reactant-rich and reactant-lean streams is possible. Our analysis indicates the presence of large regions of static and dynamic instabilities when the reactant-lean stream is recycled to the reactor and the effluent from the reactor is flow-controlled. Our results imply that the region in which the separator operates is very important in determining the behavior of the coupled system. Consequently, startup of the system is critical when the VLE of the system has an azeotrope.