The Influence of Tangent Pinch Points on the Performance of Batch Rectifications: Development of a Conceptual Model for Ternary Mixtures

KARINA ANDREA TORRES; JOSÉ ESPINOSA

Salvador, Bahía, Brasil

Congreso; 10th International Symposium on Process Systems Engineering - PSE2009; 2009

University of São Paulo, SP, Brazil

This contribution explores the influence of tangent pinch points on the performance of batch distillations of highly non ideal ternary mixtures and its incorporation into a conceptual modeling framework under the assumption of a batch rectifier with infinite number of stages. Two algorithms are developed to determine the maximum feasible distillate composition on the line of preferred separation and its corresponding limiting reflux ratio. The first one is based on a region elimination method (REM) and relates the occurrence of a saddle-node bifurcation to the appearance of a maximum in the curve reflux vs. molar liquid fraction of any of the components of the mixture. As it can be seen, this method takes into account the bifurcation analysis of reversible distillation profiles. The second method is totally analytical and consists in solving an equation system that represents two facts: the liquid compositions of the bifurcation point, together with its vapor in equilibrium and its distillate must be aligned and a turning point occurs. The resolution of the equation system is simplified because of the fact that in batch distillation of ternary mixtures operating at reflux below or equal to the limiting one, the bifurcation point coincides with the instantaneous still composition in the reboiler. Then, the dependence of feasible distillate mole fractions on reflux ratios above the limiting one is calculated by solving a non linear equation system, which incorporates the tangency condition and provides the unknown compositions of tangent pinches. A numerical method, based on the improved memory method, is implemented here to solve this non linear equation system. The conceptual model replaces the one based on the assumption of the invariance of a controlling saddle pinch, which is valid for ideal systems and non ideal mixtures without tangent pinch points, and emphasizes the variation of the composition of the tangential pinch points for reflux above the limiting one. Results obtained from the conceptual model for instantaneous column performance are in excellent agreement with those calculated from rigorous simulation in Hysys and highlight the effectiveness of the proposed algorithm. Two highly non ideal ternary mixtures are studied along the contribution: the systems methanol/2-propanol/water and acetone/chloroform/benzene.