PRO/II-Application Briefs

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Methods and DataThe K-values for the system are computed using a special ALCO-HOL thermodynamic data package. This package uses a unique set of NRTL binary data. Enthalpies and densities are calculated using library values. Vapor phase properties are taken to be ideal, and heat of mixing effects are taken into account.Simulation ModelThe separation column is modeled as an ordinary distillation col-umn with 11 theoretical trays (including the condenser and reboiler). The feed enters on stage 5. The presence of two liquid phases in the separation column is handled by the CHEMDIST algo-rithm.Rigorous VLLE stage-by-stage calculations are CPU intensive and in this case, unnecessary (except in the condenser). For this reason, the overhead bubble point condenser model uses VLLE thermody-namics, while the rest of the column uses VLE thermodynamics.

Two operating specifications are imposed upon the column. Each specification requires an independent variable to provide the neces-sary degrees of freedom for the model to converge.The first specification sets the minimum purity of di-isopropyl ether (DIPE) in the bottoms product (stream 4). The reboiler duty is var-ied to meet the DIPE recovery in the bottoms product.Although the water-rich phase from the condenser is totally with-drawn, half of the DIPE-rich phase is returned as reflux to the col-umn. A second performance specification sets this reflux rate of the DIPE- rich phase. The condenser duty is the second variable that changes value in order to satisfy the reflux rate specification.