Influence of Tangent Pinch Points on the Energy Demand of Batch Distillations: Development of a Conceptual Model for Binary Mixtures

This contribution explores the influence of tangent pinch points on the performance of batch distillations of highly nonideal binary mixtures and its incorporation into a conceptual modeling framework under the assumption of a rectifier with an infinite number of stages. First of all, the y-x diagram is divided into three regions taking into account both the inflection point (IP) in the phase diagram and the slope of the equilibrium curve at x(D) = 1. After that, limiting values for the reflux ratio and distillate composition; namely, r* and x(D)*, are calculated for the instantaneous still composition with the aid of region-dependent algorithms, which incorporate the tangency condition in different ways. Finally, the instantaneous column performance is estimated taking into account the selected operation mode; i.e., constant reflux ratio or constant distillate composition. Applied to the mixture acetone water, results for complete simulations are presented in terms of both rectification advance and operation time. A brief comparative study on the minimum energy demand required for a given separation as estimated from models with and without tangent pinches is also carried out.