Key components in multicomponent distillation

Stupin, W. J., and F. J. Lockhart, The Distribution of Non-Key Components in Multicomponent Distillation, paper presented at the 61st Annual Meeting of AIChE, Los Angeles, California, December 1-5, 1968. 

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. 

In multicomponent distillation, A and B are the light and heavy key components respectively. In this problem, the only data given for both top and bottom products are for m-and p-xylene and these will be used with the mean relative volatility calculated in the previous problem. Thus ... 

In multicomponent distillation, there are three or more components in the products, and specifying the concentrations of one component in each does not fully characterize these products. However, if the concentrations of two out of three or three out of four components are specified for the distillate and bottoms products, it is generally impossible to meet these specifications exactly. An increase in reflux ratio or number of plates would increase the sharpness of the separation, and the desired concentration of one component in each product could be achieved, but it would be a coincidence if the other concentrations exactly matched those specified beforehand. The designer generally chooses two components whose concentrations or fractional recoveries in the distillate and bottoms products are a good index of the separation achieved. After these components are identified, they are called key components. Since the keys must differ in volatility, the more volatile, identified by subscript L, is called the light key, and the less volatile, identified by subscript H, is called the heavy key. 

In multicomponent distillation neither the distillate nor the bottoms composition is completely specified because there are not enough degrees of freedom to allow complete specification. This inability to completely specify the distillate and bottoms compositions has major effects on the calculation procedure. The components that do have their distillate and bottoms fractional recoveries specified (such as component 1 in the distillate and component 2 in the bottoms in Table 6.3) are called key components. The most volatile of the keys is called the light key (LK), and the least volatile, the heavy key (HK). The other components are nonkeys (NK). If a nonkey is more volatile than the light key, it is a light nonkey (LNK) if it is less volatile than the heavy key, it is a heavy nonkey (HNK). 

Later, these columns were independently rediscovered (Petlyuk, Platonov, Slavinskii, 1965 Platonov, Petlyuk, Zhvanetskiy, 1970) on the basis of theoretical analysis of thermodynamically reversible distillation because this distillation complex by its configuration coincides with the sequence of thermodynamically reversible distillation of three-component mixture (see Chapter 4), but in contrast to this sequence it contains regular adiabatic columns. The peculiarities of Petlyuk columns for multicomponent mixtures are (1) total number of sections is n(n - 1) instead of 2(n - 1) in regular separation sequences (2) it is sufficient to have one reboiler and one condenser (3) the lightest and the heaviest components are the key components in each two-section constituent of the complex and (4) n components of a set purity are products. 

But that s only for a binary feed composition. In multicomponent distillation, the ratio of the key components in the feed will typically not coincide with the ratio of the key components in the liquid on the tray, even though the tray temperature is the same as the feed at its bubble point temperature. 

Simple analytical methods are available for determining minimum stages and minimum reflux ratio. Although developed for binary mixtures, they can often be applied to multicomponent mixtures if the two key components are used. These are the components between which the specification separation must be made frequendy the heavy key is the component with a maximum allowable composition in the distillate and the light key is the component with a maximum allowable specification in the bottoms. On this basis, minimum stages may be calculated by means of the Fenske relationship (34) ... 

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%. 

The first objective of a shortcut method is selection of two key components and then setting all conditions around these components. Call the light key component LK and the heavy key HK. The fractionation objective is to separate these two key components per a given specification. The distribution (distillate product analysis and bottoms product analysis) of a multicomponent system is also accomplished in this proposed shortcut fractionator program, Hdist. 

If one or more unit operations have been given infeasible specifications, then the flowsheet will never converge. This problem also occurs with multicomponent distillation columns, particularly when purity specifications or flow rate specifications are used, or when nonadjacent key components are chosen. A quick manual mass balance around the column can usually determine whether the specifications are feasible. Remember that all the components in the feed must exit the column somewhere. The use of recovery specifications is usually more robust, but care is still needed to make sure that the reflux ratio and number of trays are greater than the minimum required. A similar problem is encountered in recycle loops if a component accumulates because of the separation specifications that have been set. Adding a purge stream usually solves this problem. 

The graphical-based shortcut methods for binary batch distillation may be applied to multicomponent distillation only when the separation is between two key components to produce one distillate product and the residue. In this case the calculations may be approximated by lumping the other components with either of the key components and treating the system as a pseudo-binary. 

For multicomponent feeds, specification of two key components and their distribution between distillate and bottoms is accomplished in a variety of ways. Preliminary estimation of the distribution of nonkey components can be sufficiently difficult as to require the iterative procedure indicated in Fig. 12.1. However, generally only two and seldom more than three iterations are necessary. 

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