DESIGN AND SYNTHESIS OF HETEROGENEOUS AZEOTROPIC DISTILLATIONS

A method of approach for developing a systematic design procedure for heterogeneous azeotropic distillation sequences is presented. A simple technique for calculating liquid-liquid-vapor equilibrium is developed. The technique is capable of distinguishing between metastable and absolutely stable liquid phases and therefore gives reliable predictions about the number of coexisting equilibrium liquid phases. The procedure has been used to calculate residue curve maps and to design heterogeneous azeotropic distillation columns. A residue curve map analysis for heterogeneous mixtures is developed to aid in the sequencing of heterogeneous distillation columns. In the analysis, distillation boundaries and the projected liquid boiling envelope are superimposed to divide the triangular composition diagram into distinct distillation regions. Classes of residue curve maps which give rise to feasible column sequences are identified. Several kinds of sequences are presented and it is shown that the conventional sequence frequently requires much larger reflux ratios than other alternatives. A simple technique is developed for discovering the underlying geometric conditions which govern the calculation of minimum reflux ratios in heterogeneous azeotropic distillation columns. A set of finite difference mass balance equations which describe the steady-state composition profile are derived and used to demonstrate the design technique. A differential approximation to these equations are developed for calculating minimum reflux ratios and for studying the effect of design variables of these complex columns. The results show that a small change in the operating conditions can cause a significant shift in the column composition profile while maintaining the product compositions essentially constant. The design procedure, therefore, provides rather unique information about the operability of these columns and the study suggests that the composition profile sensitivity must be taken into account in control system design. Finally, a procedure for calculating minimum reflux ratios for heterogeneous distillation columns is presented. The method can be extended to calculate minimum reflux ratios in homogeneous distillation columns with non-sharp splits