Multicomponent Nonideal Distillation Column

As a more realistic distillation example, let us now develop a mathematical model for a multicomponent, nonideal column with NC components, nonequimolal overflow, and inefficient trays. The assumptions that we will make are  

Vapor and liquid are in thermal equilibrium (same temperature) but not in phase equilibrium. A Murphree vapor-phase efficiency will be used to deseribe the departure from equilibrium. 

Multiple feeds, both liquid and vapor, and sidestream drawoffs, both liquid and vapor, are permitted. A general nth tray is sketehed in Fig. 3.13. Nomenela-ture is summarized in Table 3.1. The equations deseribing this tray are  

An appropriate vapor-liquid equilibrium relationship, as discussed in Sec. 2.2.6, must be used to find y j. Then Eq. (3.96) ean be used to calculate the y j for the inefficient tray. The yj- i, y would be calculated Irom the two vapors entering the tray Fj( i and V .  

Additional equations include physical property relationships to get densities and enthalpies, a vapor hydraulic equation to calculate vapor flow rates from known tray pressure drops, and a liquid hydraulic relationship to get liquid flow 

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Develop the state model for a multicomponent (C components) nonideal distillation column with N trays. Use the general nomenclature developed in Example 4.13 for the ideal binary distillation. 

Even if you were only half awake when you read the preeeding chapter, you should have recognized that the equations developed in the examples eonstituted parts of mathematical models. This chapter is devoted to more complete examples. We will start with simple systems and progress to more realistic and complex processes. The most complex example will be a nonideal, nonequimolal-overflow, multicomponent distillation column with a veiy large number of equations needed for a rigorous description of the system. 

The equipartition principle is mainly used to investigate binary distillation columns, and should be extended to multicomponent and nonideal mixtures. One should also account for the coupling between driving forces since heat and mass transfer coupling may be considerable and should not be neglected especially in diabatic columns. 

Heat and mass transfer interactions in distillation columns with nonideal multicomponent mixtures, azeotropic mixtures, and so on. 

For an extensive discussion of the mathematical modeling of an ideal, binary distillation column and of a nonideal multicomponent column, the reader can consult the books by Douglas [Ref. 1], Luyben [Ref. 2], and Friedly [Ref. 3], An... 

When a multicomponent fluid mixture is nonideal, its separation by a sequence of ordinaiy distillation columns will not be technically and/or economically feasible if relative volatiK-ties between key components drop below 1.05 and, particularly, if azeotropes are formed. For such mixtures, separation is most commonly achieved by sequences comprised of ordinary distillation columns, enhanced distillation columns, and/or liquid-liquid extraction equipment. Membrane and adsorption separations can also be incorporated into separation sequences, but their use is much less common. Enhanced distillation operations include extractive distillation, homogeneous azeotropic distillation, heterogeneous azeotropic distillation, pressure-swing distillation, and reactive distillation. These operations are considered in detail in Perry s Chemical Engineers Handbook (Perry and Green, 1997) and by Seader... 

In the example distillation system considered in Chapters 3 and 4, we studied the binary propane/isobutane separation in a single distillation column. This is a fairly ideal system from the standpoint of vapor-liquid equilibrium (VLE), and it has only two components, a single feed and two product streams. In this chapter, we will show that the steady-state simulation methods can be extended to multicomponent nonideal systems and to more complex column configurations. 

Bausa, J., von Watzdorf, R., Marquardt, W. Shortcut methods for nonideal multicomponent distillation 1. simple columns. AIChE Journal 44, 2181-2198 (1998)... 

Marquardt, W.R., Watzdorf, R.V. andBausa, J. (1999) Shorcut methods for nonideal multicomponent distillation 2. Complex columns. AIChE Journal, 45, 1615-1628. 

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