Effect of design on the operation of homogeneous azeotropic distillation

Abstract

In homogeneous azeotropic distillation, the entrainer feed flowrate represents a design degree of freedom which does not exist in simple distillation. This degree of freedom has a significant impact on the control properties. For high ratios of entrainer feed to azeotrope feed and low internal flows, the stripping and rectifying sections dominate the extractive section, and the control properties we obtain are similar to those of simple distillation. In particular, internal flows have a positive impact on separation. When the ratio of entrainer to azeotrope feed is low and the internal flows are high, the extractive section dominates and we obtain control properties which have no equivalent in simple distillation. In this case, the internal flows have a negative impact on separation. Furthermore, it is shown that economical steady-state optimization of the column design and operation may lead to relatively complex control problems. However, a simpler control situation can be obtained by increasing the entrainer feed flow above its economically optimum value. Thus, integrating design and control is essential in the case of homogeneous azeotropic distillation. These properties are completely general and will be observed for any homogeneous separation of a minimum boiling azeotrope with a high boiling entrainer which does not introduce any new azeotrope in the mixture.