Recycle interaction effects on the control of impurities in a complex plant

Proper inventory of impurities in a large plant is a combined flowsheet/equipment design and plantwide control problem. This problem can be solved efficiently by managing positive and negative feedback effects and by taking profit from interactions through recycles. Chemical conversion is an effective way to counteract the positive feedback through recycles. Changing the connectivity of units can modify the effect of interactions. This article proposes a methodology for evaluating the dynamic inventory of impurities that consists of a combination of steady-state and dynamic flowsheeting with controllability analysis. This approach is used to assess the best design alternative and/or to propose subsequent modifications. A VCM process with four recycle alternatives illustrates the approach. Taking advantage of the effect of interactions enables three impurities to be kept under control with only two controllers. The approach is generic and can be used to improve the environmental performances of chemical processes.