Azeotropic distillation vapor-liquid equilibrium data

This study was undertaken to obtain the necessary vapor-liquid equilibrium data and to determine the distillation requirements for recovering solvent for reuse from the solvent-water mixture obtained from adsorber regeneration. Previous binary vapor-liquid equilibrium data (2, 3) indicated two binary azeotropes (water-THF and water-MEK) and a two phase region (water-MEK). The ternary system was thus expected to be highly nonideal. 

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... 

EPAR ATION and purification processes account for a large portion of the design, equipment, and operating costs of a chemical plant. Further, whether or not a mixture forms an azeotrope or two liquid phases may determine the process flowsheet for the separations section of a chemical plant. Most separation processes are contact operations such as distillation, gas absorption, gas stripping, and the like, the design of which requires the use of accurate vapor-liquid equilibrium data and correlating models or, in the absence of experimental data, of accurate predictive methods. Phase behavior, especially vapor-Uquid equilibria, is important in the design, development, and operation of chemical processes. 

The separation of benzene (1) and cyclohexane (2) by distillation is complicated due to the formation of an azeotrope (Example 2.5). Vapor-liquid equilibrium data for this binary are required for the design of a workable separation process. As a first step, find the azeotropic composition and temperature at 100 kPa pressure. Use the van Laar equation for activity coefficients with parameters A12 = 0.147, A21 = 0.165. The computations can be made with assumptions consistent with low pressure conditions. The vapor pressures of benzene and cyclohexane can be represented by the Antoine Equation 2.19 with the following constants Al = 13.88, Bl = 2788.5, Q = -52.36, A2 = 13.74, = 2766.6,... 

The design of azeotropic or extractive distillation columns, as with con-A ventional columns, demands a knowledge of the vapor-liquid equilibrium properties of the system to be distilled. Such knowledge is obtained experimentally or calculated from other properties of the components of the system. Since the systems in azeotropic or extractive distillation processes have at least three components, direct measurement of the equilibrium properties is laborious and, therefore, expensive, so methods of calculation of these data are desirable. 

The vapor-liquid equilibrium (VLB) and liquid-liquid extraction (LLE) correlations in Aspen Plus are not always as accurate as possible. This can cause significant errors, particularly near pinch points in distillation columns. If data is available, Aspen Plus will find values of the parameters for any of the VLB or LLE correlations by doing a regression against the data you input. This is illustrated to obtain an improved fit for the non-random two-liquid (NRTL) VLB correlation for the binary system water and isopropanol (IPA). VLB data for water and isopropanol is listed in Table B-1. This system has a minimum boiling azeotrope at 80.46°C. The Aspen Plus fit to the data with NRTL is not terrible, but can be improved. 

The problem of solvent selection is relatively complex and a thorough treatment requires considerable information. In addition to basic liquid-liquid equilibrium data, knowledge of the phase densities, viscosities, and the liquid-liquid interfacial tension is also important. Moreover, the economics of IXE systems are often dominated by the solvent regeneration costs. If, for example, solvent regeneration b to be accomplished by extractive or azeotropic distillation, then vapor-liquM equilibrinm data for the ternary system must also be available. Insofar as the most interesting LLE systems ate often ffiose whidi are least ideal, the generation of a physical property data base to complete cost analysis is usually a sigruficant problem. 

Fig. 18.4-2. Distillation versus pervaporation. The vapor-liquid equilibrium is given by the dotted line and the pervaporation into vacuum is given by the solid line. The data are for ethanol-water using a polyvinyl alcohol membrane. Note there is no azeotrope for pervaporation.

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