Fractionation Concepts

The concepts described in this chapter are the foundations of distillation engineering. A thorough understanding of these concepts is essential for distillation practitioner. This is one chapter that the novice can ill-afford to skip.

The author stresses and applies the visual approach (i.e., graphical methods) when introducing these concepts. This approach was deem-phasized when computers began to make rapid inroads into distillation design. For some time, graphical techniques were considered a tool of the past, never to be used again. An undesirable by-product was that the distillation column became a "black box," and the engineers' understanding of distillation suffered.

The last decade saw the pendulum swing the other way. It was appreciated that there is no conflict between computer and graphical techniques, and that the two can coexist. It was realized that the graphical techniques themselves can be programmed and used side by side with computer simulation. This hybrid approach combined the speed and accuracy of the computer with the analytical and visual value of the graphical techniques.

This chapter first discusses the stage concept, and how stages are put together in a column. It then presents the principles of the x-y diagram, which is the main graphical tool for distillation analysis. It applies this graphical technique to define and illustrate several key distillation concepts: pinching, minimum and total reflux, minimum stripping, effects of the thermal state of the feed, and column complexities (e.g., multifeed columns). The chapter then reviews the basic concepts of multicomponent distillation, and the application of graphical techniques to such systems. Finally, the chapter describes the use of graphical techniques in analyzing computer simulation results.

2.1 Theoretical Stages 2.1.1 Ideal and nonldeal stages

The ideal distillation stage is a device that meets these criteria:

1. It operates in steady state and has a liquid product and a vapor product.

2. All vapor and liquid entering the stage are intimately contacted and perfectly mixed.

3. Total vapor leaving the stage is in equilibrium with total liquid leaving the stage.

Examples. Figure 2.1 illustrates the first criterion. The system in Fig. 2. la has a vapor product and a liquid product and therefore obeys this criterion. The systems in Fig. 2.16 and c have no vapor products and therefore are not equilibrium stages. Generating a vapor phase in these systems (Fig. 2.1 d and e) renders them equilibrium stages. Figure 2. lc and e depict a total and a partial condenser, respectively. The total condenser is not a distillation stage, whereas the partial condenser is.

Figure 2.2 illustrates common distillation stage arrangements. All satisfy criterion 1. Criteria 2 and 3 determine which arrangements are ideal stages. Nonideal stages can still be modeled using the ideal stage model, but the nonideality must be accounted for.

Figure 2.2a and b shows thermosiphon reboiler arrangements. The system in Fig. 2.2a is not an ideal stage. The liquid product is made up from liquid leaving the reboiler and liquid descending from the bottom tray. Although the former is perfectly mixed with the leaving vapor, the latter does not contact the vapor and is not in equilibrium with it. The system in Fig. 2.26 is an ideal stage. Here the liquid product is made up from the liquid leaving the reboiler only. This liquid is in equilibrium with vapor leaving the reboiler, which is the vapor product from the stage.

Figure 2.2c and d shows distillation tray arrangements. The system in Fig. 2.2c is typical. It does not satisfy criterion 2, and therefore, is not an ideal stage. Further, only the vapor leaving the stage at point B can be in equilibrium with the liquid leaving the tray. Vapor leaving the tray at point A can only be in equilibrium with the tray inlet liquid, but not with liquid leaving the tray. The system in Fig. 2.2c? is rarely encountered, but it satisfies criterion 2. Here liquid composition across the tray is uniform and equals the composition of liquid leaving the tray. Vapor at both points A and £ is in contact with the liquid product stream. Providing there is sufficient time and area for

Feed

All liquid leed

All liquid leed

Liquid product

Liquid product Liquid product

Vapor product

Liquid product

Vapor product

Liquid product Liquid product

Vapor product

Vapor product

Liquid product M

Liquid product

Liquid product M

Liquid product

Figura 2.1 The distillation stage concept, to) General presentation of an ideal distillation stage; (6) and (c) single-phase product (these are not ideal distillation stages); id) and <e) two-phase product (these are ideal distillation stages).

Not an ideal

Not an ideal

_Hot "stream

Liquid product

Liquid product

_Hot "stream

Liquio from trays

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