Distillation multicomponent

In multicomponent distillation, the equilibrium depends on all components. Complete composition of top and bottom products required trial-and-error 

The condition A = 0 is often assumed to be valid in multicomponent distillation calcula- 

The general case of mass transfer in a mixture where one component has a zero flux is known as Stefan diffusion. This situation is very common. 

Let us denote that component with zero flux as species n. Thus, N =J +x N, = 0 

the relation that allows the calculation of the nonzero from the 7, is 

Your objectives in studying this section are to be able to  

Explain why analysis of multicomponent distillation problems is always by trial and error. 

Make appropriate assumptions and solve the overall material balances. 

Many of the distillations of industry involve more than two components. While the principles established for binary solutions generally apply to such distillations, new problems of design are introduced which require special consideration. An important principle to be emphasized is that a single fractionator cannot separate more than one component in reasonably pure form from a multicomponent solution, and that a total of C - 1 fractionators will be required for complete separation of a system of C components. Consider, for example, the continuous separation of a ternary solution consisting of components A, B, and C, whose relative volatilities are in that order (A most volatile). In order to obtain the three substances in substantially pure form, the following two-column scheme can be used. The first column is used to separate C as a residue from the rest of the solution. This residue is necessarily contaminated with a small amount of B and an even smaller amount of A. The distillate, which is necessarily contaminated with a small amount of C, is then fractionated in the second column to give nearly pure A and B. 

Another significant difference between multicomponent and binary distillation problems arises from a degree-of-freedom analysis around the column (Wankat, 1988). Assuming constant pressure and negligible heat losses in the column, the num- 

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Multiple Products. If each component of a multicomponent distillation is to be essentially pure when recovered, the number of columns required for the distillation system is N — 1, where AJ is the number of components. Thus, ia a five-component system, recovery of all five components as essentially pure products requires four separate columns. However, those four columns can be arranged ia 14 different ways (43). 

C. D. HoUand, Fundamentals of Multicomponent Distillation, McGraw-HiU Book Co., Inc., New York, 1981. 

V. Julka,M Geometric Theory of Multicomponent Distillation, Ph.D. dissertation, University of Massachusetts, Amherst, 1992. 

RIGOROUS METHODS FOR MULTICOMPONENT DISTILLATION-TYPE SEPARATIONS... 

Open-loop behavior of multicomponent distillation may be studied by solving modifications of the multicomponent equations of Distefano [Am. Inst. Chem. Eng. J., 14, 190 (1968)] as presented in the subsection Batch Distillation. One frequent modification is to include an equation, such as the Francis weir formula, to relate liquid holdup on a tray to liquid flow rate leaving the tray. Applications to azeotropic-distillation towers are particularly interesting because, as discussed by and ihustrated in the Following example from Prokopalds and Seider... 

McCormick [97] presents a correlation for Gilliland s chart relating reflux, minimum reflux, number of stages, and minimum stages for multicomponent distillation. Selecting a multiplier for actual reflux over minimum reflux is important for any design. Depending on the com-... 

Multicomponent distillations are more complicated than binary systems due primarily to the actual or potential involvement or interaction of one or more components of the multicomponent system on other components of the mixture. These interactions may be in the form of vapor-liquid equilibriums such as azeotrope formation, or chemical reaction, etc., any of which may affect the activity relations, and hence deviations from ideal relationships. For example, some systems are known to have two azeotrope combinations in the distillation column. Sometimes these, one or all, can be broken or changed in the vapor pressure relationships by addition of a third chemical or hydrocarbon. 

Yaws [124] et al. provide an estimating technique for recovery of each component in the distillate and bottoms from multicomponent distillation using short-cut equations and involving the specification of the recovery of each component in the distillate, the recovery of the heavy key component in the bottoms, and the relative volatility of the light key component. The results compare very well with plate-to-plate calculations. Figure 8-46, for a wide range of recoveries of 0.05 to 99.93% in the distillate. 

Example 8-22 Multicomponent Distillation by Yaw s Method [124] (used by permission)... 

Assume a multicomponent distillation operation has a feed whose component concentration and component relative volatilities (at the average column conditions) are as shown in Table 8-3. The desired recovery of the light key component O in the distillate is to be 94.84%. The recovery of the heavy key component P in the bottoms is to be 95.39%. 

Figure 8-47. Short-cut solution of Fenske-Underwood-Gilliland theoretical trays for multicomponent distillation. Used by permission, Frank, O., Chem. Eng. Mar. 14 (1977), p. 109.

This, method for multicomponent distillation involving more than one feed and more than one side stream requires a reliable minimum reflux. 

Material Balance for Estimated Multicomponent Distillation Recoveries for Example 8-28 Using Method of Yaws, Fang, and Patel... 

Figure 8-51. Working chart for Yaws, et. al short-cut method for multicomponent distillation for estimating component recovery in distillate and bottoms. Used by permission, Yaws, C. L. et al., C/iem. Eng., Jan. 29 (1979), p. 101.

Multicomponent distillation is by far the common requirement for process plants and refineries, rather than the simpler binary systems. There are many computer programs which have been developed to aid in accurately handling the many iterative calculations required when the system involves three to possibly ten individual components. In order to properly solve a multicomponent design, there should be both heat and material balance at every theoretical tray throughout the calculation. 

There are several valuable references to developing and applying a multicomponent distillation program, including Holland [26, 27,169], Prausnitz [52, 53], Wang and Henke [76], Thurston [167], Boston and Sullivan [6], Maddox and Erbar [115], and the pseudo-K method of Maddox and Fling [116]. Convergence of the iterative trials to reach a criterion requires careful evaluation [114]. There are sever-... 

This example summarizes a typical short multicomponent distillation using the techniques previously cited (see Computer Printout). 

Holland, C. D., Unsteady State Processes with Applications in Multicomponent Distillation, Prentice-Hall. 

Hengstebeck, R. J., An Improved Shortcut for Calculating Difficult Multicomponent Distillations, Chem. Eragjan. 13 (1969), p. 115. 

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