Multicomponent distillation systems

Distillation Columns. Distillation is by far the most common separation technique in the chemical process industries. Tray and packed columns are employed as strippers, absorbers, and their combinations in a wide range of diverse appHcations. Although the components to be separated and distillation equipment may be different, the mathematical model of the material and energy balances and of the vapor—Hquid equiUbria are similar and equally appHcable to all distillation operations. Computation of multicomponent systems are extremely complex. Computers, right from their eadiest avadabihties, have been used for making plate-to-plate calculations. 

Most distillation systems ia commercial columns have Murphree plate efficiencies of 70% or higher. Lower efficiencies are found under system conditions of a high slope of the equiHbrium curve (Fig. lb), of high Hquid viscosity, and of large molecules having characteristically low diffusion coefficients. FiaaHy, most experimental efficiencies have been for biaary systems where by definition the efficiency of one component is equal to that of the other component. For multicomponent systems it is possible for each component to have a different efficiency. Practice has been to use a pseudo-biaary approach involving the two key components. However, a theory for multicomponent efficiency prediction has been developed (66,67) and is amenable to computational analysis. 

In order to determine the packed height it is necessary to obtain a value of the overall number of transfer units methods for doing this are available for binary systems in any standard text covering distillation (73) and, in a more complex way, for multicomponent systems (81). However, it is simpler to calculate the number of required theoretical stages and make the conversion ... 

Total reflux exists in a distillation column, whether a binary or multicomponent system, when all the overhead vapor from the top tray or stage is condensed and returned to the top tray. Usually a column is brought to equilibrium at total reflux for test or for a temporary plant condition which requires discontinuing feed. Rather than shut down, drain and then re-establish operating conditions later, it is usually more convenient and requires less... 

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. 

The distillation of binary mixtures is covered thoroughly in Volume 2, Chapter 11, and the discussion in this section is limited to a brief review of the most useful design methods. Though binary systems are usually considered separately, the design methods developed for multicomponent systems (Section 11.6) can obviously also be used for binary systems. With binary mixtures fixing the composition of one component fixes the composition of the other, and iterative procedures are not usually needed to determine the stage and reflux requirements simple graphical methods are normally used. 

MCSTILL - Continuous Multicomponent Distillation Column System... 

In this chapter consideration is given to the theory of the process, methods of distillation and calculation of the number of stages required for both binary and multicomponent systems, and discussion on design methods is included for plate and packed columns incorporating a variety of column internals. 

If we listed all the variables in this system and subtracted all the equations describing it and all the parameters that are fixed (all feeds), we would find that the degrees of freedom would be equal to the number of sidestreams plus two. Thus if we have no sidestreams, there are only two degrees of freedom in this multicomponent system. This is the same number that we found in the simple binary colunrn. T q)ically we would want to control the amount of heavy key impurity in the distillate and the amount of light key impurity in the... 

The maximum boiling point is that temperature corresponding to a definite composition of a Iwo-coinponenl or multicomponent system al which the boiling point of the system is a maximum. At this temperature the liquid and vapor have the same composition and the solution distills completely without change in temperature. Binary liquid systems that show negative deviations from Raoult s law have maximum boiling points. See Raoult s I xiw and Van t Hoff I,aw. 

Originally, extractive distillation was limited to two-component problems. However, recent developments in solvent technology enabled applications of this hybrid separation in multicomponent systems as well. An example of such application is the BTX process of the GTC Technology Corp., shown in Figure 6, in which extractive distillation replaced the conventional liquid-liquid extraction to separate aromatics from catalytic reformate or pyrolysis gasoline. This led to a ca. 25% lower capital cost and a ca. 15% decrease in energy consumption (170). Some other examples of existing and potential applications of the extractive distillations are listed in Table 6. 

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. 

Many industrial columns use temperatures for composition control because direct composition analyzers can be expensive and unreliable. Although temperature is uniquely related to composition only in a binary system (at known pressure), it is still often possible to use the temperatures on various trays up and down the column to maintain approximate composition control, even in multicomponent systems. Probably 75 percent of all distillation columns use temperature control of some tray to hold the composition profile in the column. This prevents the light-key (LK) impurities from dropping out the bottom and the heavy-key (HK) impurities from going overhead. 

The fact that component efficiencies in multicomponent systems are unbounded means that the arithmetic average of the component Murphree efficiencies is useless as a measure of the performance of a multicomponent distillation process. Taylor, Baur, and Krishna [AIChE J., 50, 3134 (2004)] proposed the following efficiency for multicomponent systems ... 

Let us now try to extend the model of binary distillation developed in Section 12.1.1 to multicomponent systems. The extension is based on the work of Toor (1964b) and the starting point is the material balance Eqs. 12.1.4, which must now be combined in n — 1 dimensional matrix form as... 

In fact, through use of matrix models of mass transfer in multicomponent systems (as opposed to effective diffusivity methods) it is possible to develop methods for estimating point and tray efficiencies in multicomponent systems that, when combined with an equilibrium stage model, overcome some of the limitations of conventional design methods. The purpose of this chapter is to develop these methods. We look briefly at ways of solving the set of equations that model an entire distillation column and close with a review of experimental and simulation studies that have been carried out with a view to testing multicomponent efficiency models. 

Discuss how the fundamental models of mass transfer in Sections 12.1.7 (binary systems) and 12.2.4 (multicomponent systems) may be used to estimate mass transfer rates for use in a nonequilibrium simulation of an existing distillation column. Your essay should address the important question of how the model parameters are to be estimated. 

Vogelpohl, A., Murphree Efficiencies in Multicomponent Systems, The Institution of Chemical Engineers Symposium Series, No. 56, Distillation 1979, 2.1/25 -2.1/31 (1979). 

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