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

Proper inventory of impurities in a large plant is a combined flowsheet/equ ipment design and plantwide control problem. This problem can be solved eff iciently by managing positive and negative feedback effects and by taking p rofit from interactions through recycles. Chemical conversion is an effecti ve way to counteract the positive feedback through recycles. Changing the c onnectivity of units can modify the effect of interactions. This article pr oposes a methodology for evaluating the dynamic inventory of impurities tha t consists of a combination of steady-state and dynamic flowsheeting with c ontrollability analysis. This approach is used to assess the best design al ternative and/or to propose subsequent modifications. A VCM process with fo ur recycle alternatives illustrates the approach. Taking advantage of the e ffect of interactions enables three impurities to be kept under control wit h only two controllers. The approach is generic and can be used to improve the environmental performances of chemical processes.