Application of COSMO-SAC in Complex Phase Behavior: Vapor-Liquid-Liquid Equilibria

Introduction

An integrated part of chemical engineering is the calculation of phase equilibria. The importance of phase equilibria calculations is realized in designing the process equipment and optimizing separation and purification processes. Phase equilibria involves either homogeneous phases, that is, two liquid phases or heterogeneous phases, for example, liquid with vapor or solid phases. The main objective of these calculations is to recognize the presence of one vapor or liquid, two vapor-liquid or liquid-liquid, or multiple phases. Thus, the knowledge of thermodynamic properties of pure and mixture fluids is absolutely essential to identify these regions. For decades, researchers and process engineers established phase equilibria calculations both experimentally and computationally. Though experimental database are well established for binary and ternary vapor-liquid equilibria (VLE) and liquid-liquid equilibria (LLE), they are time consuming and expensive. Moreover, database is not so strong for complex-phase equilibria processes. The lack of experimental data and the search for a priori prediction motivate researchers to develop thermodynamic models for various chemical and pharmaceutical processes. The modern-day process simulators such as ChemCAD and ASPEN run these models to get a priori phase equilibria. In Chapter 2, we have discussed computation techniques of LLE with three state-of-the-art industrial problems. This chapter aims to provide a detailed analysis of complex-phase behavior involving two liquid phases and one vapor phase simultaneously present, better known as the vapor-liquid- liquid equilibria (VLLE).

Information regarding temperature, pressure and composition in VLE and LLE is crucial for designing distillation and extraction processes. However, thermodynamic properties are necessary in complex-phase behaviors like VLLE. If the presence of heterogeneous liquid mixtures is not correctly accounted for the systems that exhibit a miscibility gap, there may be multiple solutions to vapor-liquid phases, which in turn are a possible reason for multiple steady states in heterogeneous distillation. VLLE is frequently encountered in heterogeneous azeotropic distillation which is widely found in hydrocarbon industry. The heterogenic azeotropic distillation separates azeotropic mixtures into their components. The thermodynamic models, used to compute heterogeneous azeotropic distillation, should describe VLE and LLE as accurately as possible (Wyczesany, 2014). In this chapter, we will highlight the experimental and computational techniques used for VLLE. A priori based COSMO-SAC model (discussed in Chapter 3) will be used to predict complex-phase behaviors of VLLE and the calculation procedure will be elaborated with eight state-of-the-art industrial problems.

 
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