Phase equilibrium residue curve maps

For the synthesis of heterogeneous batch distillation the liquid-liquid envelope at the decanter temperature is considered in addition to the residue curve map. Therefore, the binary interaction parameters used in predicting liquid-liquid equilibrium are estimated from binary heterogeneous azeotrope or liquid-liquid equilibrium data [8,10], Table 3 shows the calculated purity of original components in each phase split at 25 °C for all heterogeneous azeotropes reported in Table 1. The thermodynamic models and binary coefficients used in the calculation of the liquid-liquid-vapour equilibrium, liquid-liquid equilibrium at 25 °C and the separatrices are reported in Table 2. 

A residue curve map (RCM) consists of a plot of the phase equilibrium of a mixture submitted to distillation in a batch vessel at constant pressure. RCM is advantageous for analyzing ternary mixtures. More exactly, a residue curve shows explicitly the evolution of the residual liquid of a mixture submitted to batch distillation. 

Table 8.1 describes the steps of the methodology in more detail. The procedure starts with the Problem definition production rate, chemistry, product specifications, safety, health and environmental constraints, physical properties, available technologies. Then, a first evaluation of feasibility is performed by an equilibrium design. This is based on a thermodynamic analysis that includes simultaneous chemical and physical equilibrium (CPE). The investigation can be done directly by computer simulation, or in a more systematic way by building a residue curve map (RCM), as explained in the Appendix A. This step will identify additional thermodynamic experiments necessary to consolidate the design decisions, mainly phase-equilibrium measurements. Limitations set by chemical equilibrium or by thermodynamic boundaries should be analyzed here. 

Figure 8.4 presents the residue curve map of simultaneous phase and chemical equilibrium at normal pressure. Special coordinates, Xx (acid + water) and X2 (acid + ester) enable tbe representation of all four components in a bidimensional... 

Figure A.2 (right) emphasizes a particular position where phase equilibrium and stoichiometric lines are collinear. In other words the liquid composition remains unchanged because the resulting vapor, after condensation, is converted into the original composition. This point is a potential reactive azeotrope, but when the composition satisfies chemical equilibrium too it becomes a true reactive azeotrope. Some examples of residue curve maps are presented below. Ideal mixtures are used to illustrate the basic features, which may be applied to some important industrial applications.

FIG. 13-96 Residue curve maps for the reactive system methanol—acetic aci(d-methyl acetate-water in phase and chemical equilibrium at 1-atm pressure, a) Calculated by Barbosa and Doherty [Chem. Eng. Sci., 43,1523 (1988)]. (b) Measured by Song et al. [Ind. Eng. Chem. Res., 37,1917 (1998)]. 

Residue curve maps (RCMs) have been long used as a tool for analyzing a given ternary system s phase equilibrium behavior. These maps, originally pioneered by Schreinemakers in 1902 [1], enable design engineers to quickly scan possible separation trains or sequences, and also to identify areas of difficult separation due to azeotropes. 

To calculate residue curve maps for the synthesis of TAME one has to proceed in the same manner as the MTBE example and calculate phase equilibria bet veen liquid and vapor phases, chemical equilibrium constants in the liquid phase, and kinetics of the chemical reactions. 

First simulation results on steady state multiplicity of etherification processes were obtained for the MTBE process by Jacobs and Krishna [45] and Nijhuis et al. [78]. These findings attracted considerable interest and triggered further research by others (e. g., [36, 80, 93]). In these papers, a column pressure of 11 bar has been considered, where the process is close to chemical equilibrium. Further, transport processes between vapor, liquid, and catalyst phase as well as transport processes inside the porous catalyst were neglected in a first step. Consequently, the multiplicity is caused by the special properties of the simultaneous phase and reaction equilibrium in such a system and can therefore be explained by means of reactive residue curve maps using oo/< -analysis [34, 35]. A similar type of multiplicity can occur in non-reactive azeotropic distillation [8]. 

The design equations would include, in addition to the usual heat and mass balances and vapor-liquid equilibria, equations for chemical equilibria and/or reaction kinetics. The occurrence of a chemical reaction can severely restrict the allowable ranges of temperatures and phase compositions by virtue of the additional equations for chemical equilibrium/kinetics. This effect can be quantitatively analyzed by constructing a residue curve map (RCM). It explicitly shows the shifting of distillation boundaries in the presence of reaction and defines the limits of feasible distillation column operation. We illustrate this (Venimadhavan et al., 1994) by considering the reaction... 

As illustrated throughout this section, process simulators have extensive facilities for preparing phase-equilibrium diagrams T-x-y, P-x-y, x-y,... ), and residue curve maps and binodal curves for ternary systems. In addition, related but independent packages have been developed for the synthesis and evaluation of distillation trains involving azeotropic mixtures. These include SPLIT by Aspen Technology, Inc., and DISTIL by Hyprotech (now Aspen Technology, Inc., which contains MAYFLOWER developed by M.F. Doherty and M.F. Malone at the University of Massachusetts). 

While choosing entrainers for separation of binary azeotropic mixtures, the structure of phase equilibrium diagrams (residue curve maps) of ternary mixtures formed at the addition of entrainer is of great importance. 

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