Azeotropic distillation design method

This book studies a broad spectmm of real azeotropic distillation separation methods for a variety of industrially important chemical systems. Economically optimum rigorous steady-state designs are developed for many of these chemical systems. Then practical control structures are developed that provide effective load rejection in the face of typically large disturbances in throughput and feed composition. Trade-offs between steady-state energy savings and dynamic controllability (product quality variability) are demonstrated. 

Whereas there is extensive Hterature on design methods for azeotropic and extractive distillation, much less has been pubUshed on operabiUty and control. It is, however, widely recognized that azeotropic distillation columns are difficult to operate and control because these columns exhibit complex dynamic behavior and parametric sensitivity (2—11). In contrast, extractive distillations do not exhibit such complex behavior and even highly optimized columns are no more difficult to control than ordinary distillation columns producing high purity products (12). 

Baird [Comp. Chem. Engng., 9, 593 (1985)]. Since then, they have been applied successfully to problems involving interlinked distillation (Wayburn and Seader, op. cit.), azeotropic and three-phase distillation [Kovach, 111 and Seider, Comp. Chem. Engng., 11,593(1987)], and reac tive distillation [Chang and Seader, Comp. Chem. Engng., 12, 1243 (1988)], when SC and inside-out methods have failed. Today, many computer-aided distillation-design and simulation packages include continuation techniques to make the codes more robust. 

The future of azeotropic distillation may well be in the development of new and more efficient azeotrope formers for the specific separations desired. Design methods and equipment for azeotropic processes are essentially the same as for ordinary fractionation hence, substantially all developments in that field will be applicable to azeotropic distillation. 

In conclusion, recent developments in solvent selection, phase nonideality description, and tray-to-tray calculation schemes have greatly facilitated the design of extractive and azeotropic distillation schemes, and use of salts give new methods for extractive distillation separations. Finally, the work of Gerster (30), Black and Ditsler (29), and Black et al. (25) compare these two schemes. 

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. 

Azeotropic distillation has often been discussed in recent literature (I, 2, 3). Methods for providing phase equilibria for azeotropic and extractive distillation have been studied extensively (I, 4, 5, 6, 7, 8, 9). Some have discussed the design (10) or calculation of azeotropic distillations (2, 3) others only discussed choosing the entrainer for azeotropic distillation processes (11). 

Axial flow pumps, 134, 136, 140 applicafion range, 150 Azeotrope separation, 387,388,420-426 Azeotropic distillation, 420-426 acetonitrile/water separation, 422 commercial examples, 421-424 design method, 424 ethanol/water/benzene process, 424 n-heptane/toluene/MEK process, 424 vapor-liquid equilibrium data, 421, 423, 425,426... 

Design. When the vapor-liquid equilibria are known, in the form of UNIQUAC parameters for instance, the calculation of azeotropic distillation may be accomplished with any of the standard multicomponent distillation procedures. The Naphthali-Sand-holm algorithm (Fig. 13.19) and the 0-method of Holland (1981) are satisfactory. Another tray-by-tray algorithm is illustrated for azeotropic distillation by Black, Golding, and Ditsler [Adv. Chem. Ser. 

Methods for determining equilibrium stages for azeotropic distillation are the same as those discussed in Section 5.3. As indicated in Fig. 5.5-3, the distillation path may cross over into the two liquid phase legion, and one mast be cantious about the design of contacting equipaienl when this condition is present. Research has shown that with good aeration, conventional trays may be used when two liquid phases are present. ... 

We start the chapter by explaining the graphical thermodynamic representations for ternary mixtures known as Residue Curve Maps. The next section deals with the separation of homogeneous azeotropes, where the existence of a distillation boundary is a serious obstacle to separation. Therefore, the choice of the entrainer is essential. We discuss some design issues, as entrainer ratio, optimum energy requirements and finite reflux effects. The following subchapter treats the heterogeneous azeotropic distillation, where liquid-liquid split is a powerful method to overcome the constraint of a distillation boundary. Finally, we will present the combination of distillation with other separation techniques, as extraction or membranes. 

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