110266 Navigate Phase Equilibria

# 110266 Navigate Phase Equilibria - Reactions and...

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66 www.cepmagazine.org November 2002 CEP Reactions and Separations esidue curve maps (RCMs) have attracted inter- est from the academic and conceptual engineer- ing design communities. Despite their useful- ness, RCMs have received little attention from practicing process engineers. In addition to being useful as a separation synthesis tool, RCMs can also be used by the practicing engineer to visualize and investigate vapor/liq- uid/liquid equilibrium (VL(L)E) issues affecting modeling of distillation and liquid/liquid extraction columns. [VL(L)E denotes systems that contain two liquid phases and a vapor phase in equilibrium]. Other areas of applica- tion are column troubleshooting and control analysis. This article reviews the physical significance and termi- nology of RCMs, and illustrates their use to evaluate the underlying thermodynamics of ternary systems. Four ex- amples cover: using data outside their intended range; em- ploying VLE calculations instead of VL(L)E; using param- eter sets optimized to different regression objectives; and choosing the appropriate model for the vapor phase. Wa- sylkiewicz and Shethna (1) present additional examples of using RCMs for data evaluation. RCMs are readily generated using commercial process simulation software, such as Aspen Plus or DISTIL. A residue curve represents the composition of the residue of a simple batch distillation involving three components over time (Figure 1). Mathematically: d ξ i / dx = x i y i (1) where ξ is a nonlinear time scale that spans the duration of the experiment, x and y are the familiar mole frac- tions in the liquid and vapor, respectively, and i is the i th component. By integrating forward and backward in time from differ- ent starting positions, a family of curves — the residue curve map, is obtained. The usefulness of RCMs lies in the fact that the composition profiles of continuous distillation columns approximate the composition trajectories of the residue curves (RCs). Fien and Liu (2) provide a more-de- tailed discussion of this relationship. Figure 2 presents the RCM terminology. Nodes represent the starting and end points of RCs and can be pure components, or binary or ternary azeotropes. The nodes and saddle points are used to These maps provide a rapid, graphical means to visualize the separation possibilities and constraints of azeotropic ternary systems. They also help check the soundness of models used to predict equilibrium data. Navigate Phase Equilibria via Residue Curve Maps R Waldo E. de Villiers, Raymond N. French and George J. Koplos, Shell Global Solutions (US) Inc. Vapor at y ( ξ ) High-boiling Component Feed Composition Intermediate-boiling Component Low-boiling Component Residue at x ( ξ ) Heat Experimentally: Figure 1. RCMs, using experimental or generated data, show the composition of the residue of a simple batch distillation over time.

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CEP November 2002 www.cepmagazine.org 67 define distillation boundaries and associated regions. Residue curves point toward increas- ing temperature and decreasing volatility.
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