Design of separation processes

Several equations have been developed to represent the dependence of activity coefficients on liquid composition. Only those of most use in the design of separation processes will be given. For a detailed discussion of the equations for activity coefficients and their relative merits the reader is referred to the book by Reid et al. (1987), Walas (1984) and Null (1970). 

The UNIQUAC equation developed by Abrams and Prausnitz is usually preferred to the NRTL equation in the computer aided design of separation processes. It is suitable for miscible and immiscible systems, and so can be used for vapour-liquid and liquid-liquid systems. As with the Wilson and NRTL equations, the equilibrium compositions for a multicomponent mixture can be predicted from experimental data for the binary pairs that comprise the mixture. Also, in the absence of experimental data for the binary pairs, the coefficients for use in the UNIQUAC equation can be predicted by a group contribution method UNIFAC, described below. 

The use of digital computers to carry out complete calculations in the design of separation processes has been the goal of many. To do this effectively, suitable methods for phase equilibria and tray-to-tray distillation calculations are required. Results calculated by the application of such methods to dehydrate aqueous ethanol mixtures using ethylene glycol as the extractive distillation solvent is discussed below. A brief review of the methods used for phase equilibria and enthalpies is followed by a discussion of the results from distillation calculations. These are compared for extractive distillation with corresponding results obtained by azeotropic distillation with n-pentane. 

Fien, G. A. F., and Liu, Y. A. Heuristic Synthesis and Shortcut Design of Separation Processes Using Residue Curve Maps A Review, Ind. Eng. Chem. Res. 33, 2505-2522 (1994). 

For the right choice of the selective solvent and for the development and design of separation processes, a reliable knowledge of the phase equilibrium behavior (extractive distillation vapor-liquid equilibrium (VLE) extraction liquid-liquid equilibrium (LLE), absorption gas-liquid equilibrium (GLE)) is required. This information is available from phase equilibrium thermodynamics. 

The design of separation processes would be facilitated greatly by accurate mathematical models of the thermodynamics. Unfortunately, most standard equations of state do not represent adequately the phase behavior In the near supercritical region and In general were not developed with highly asymmetric solu-... 

The point to note here is the important role of thermodynamics, in that the schematic design of the process to separate the feed mixture into two strearns of specified purity was based completely on vapor-liquid equilibrium. The fact that an azeotrope occurred dictated that the desired separation could not be obtained with a single distillation column. Then how the azeotropic composition changed with temperature was the basis for whether a second column of higher or lower pressure should be used. This example is just one illustration of the very central role of thermodynamics in general, and vapor-liquid equilibrium in particular, in the design of separations processes in chemi-. cal engineering. 

Today the design of separation processes is performed almost exclusively with the aid of process simulators by solving the balance equations. In the case of separation processes, apart from the data of the pure substances, reliable information on the phase equilibria of the multicomponent system that is to be separated is required. 

Fien, G. J.A.F. and Y.A. Liu, Heuristic synthesis and shortcut design of separation processes using residue curve maps A review. Industrial Engineering Chemistry Research, 1994, 33(11) 2505 2522. 

The knowledge of the phase equilibrium behavior is not only important for the design of separation processes, but also for other applications, like the design of biphasic reactors, for example, gas-liquid reactors, the estimation of the fate of persistent chemicals in the environment, and so on. 

This leads to the fact that all K-factors and all relative volatilities show a value of 1 at the azeotropic point and that the system cannot be separated by ordinary distillation. A reliable knowledge of all azeotropic points for the system to be separated is of essential importance for the synthesis and design of separation processes. 

Most important for the application of group contribution methods for the synthesis and design of separation processes is a comprehensive and reliable parameter matrix with reliable parameters. The present status of modified UNIFAC is shown in Figure 5.84. Today parameters are available for 91 main groups. In the recent years new main groups were introduced for the different types of amides, isocyanates, epoxides, anhydrides, peroxides, carbonates, various sulfur compounds, and so on. In the last year the range of applicability was even extended to systems with ionic liquids [56]. 

Particularly when polar groups are present in liquid mixtures, azeotropes are often formed. For the design of separation processes like distillation, the knowledge of the azeotropic composition at different thermodynamic conditions is of critical importance. In this context, molecular simulation offers a powerful route to predict azeotropic behavior in mixtures. The prediction of the vapor-liquid equilibrium of the mixture CO2 + C2H6 is presented here as an example. 

Gmehling, J. VLE data for the design of separation processes AIChE Symp. Ser. 1985,81(244), 121-129... 

Applications include the use of ionic liquids as electrolytes in electrochemical devices, as solvents in chemical synthesis and catalysis, separation technology, as lubricants and heat-transfer fluids. For the design of separation processes... 

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