Advances in synthesis of azeotropic distillation column sequences: internal secondary recyclest

An algorithm for automatic generation of sequences of distillation columns and decanters for separation of azeotropic mixtures has been developed (S.K. Wasylkiewicz, 54th Canadian Chemical Engineering Conference, paper No. 243, Calgary, October 2004; S.K. Wasylkiewicz, AIChE Spring National Meeting, paper No. 83e, Atlanta, GA, April 2005) where distillation boundaries can be crossed by moving them with pressure change, by exploring curvatures of distillation boundaries, or by liquid-liquid splits in decanters. In the first step of the algorithm, open-loop sequences are generated and primary recycles are automatically detected. Then, mass balances are calculated to finish the sequences. In this paper, we are focused on internal secondary recycles where species present in the sequence feed are introduced as separating agents. This can be a pure component produced somewhere downstream in the sequence or any other intermediate stream. On the basis of a broad knowledge about distillation regions and boundaries for the separated mixture, a preferred distillation region can be identified and a suitable recycle stream can be selected. These types of recycles can simplify tremendously, the whole sequence and reduce significantly the total cost of separation. We present in detail an example based on an industrial case where the internal secondary recycle was efficiently found and calculated during synthesis of column sequences by using the Split Generator in Distil and the mass balance calculation in MS Excel.