V

_ Hot "stream

Liquid to Iray below

Liquid to Iray below

Liquid to tray below

Liquid to tray below

Figure 2.2 Ideal and nonideal stages in distillation systems, (a), (6) Thermosiphon reboiler arrangements; (c), Id) fractionation tray arrangements; (e), if) inter-condenser arrangements. Arrangements (¿>, (d), and (f) are ideal stages. Arrangements (a), (c), and (e) are n on ideal stages.

vapor-liquid contact, equilibrium will be established. Criterion 3 will be satisfied, making arrangement 2.2d an ideal stage.

Figure 2.2e and f shows intercondenser arrangements. The system in Fig. 2.2e is not an ideal stage. Liquid leaving the stage is made up from liquid condensed in the intercondenser and liquid from the top column. While the condensate is in equilibrium with vapor leaving the stage, the liquid from the column does not mix with this vapor and is not in equilibrium with it. Mixing the column liquid with the vapor-

Figure 22 (Continued)

Figure 22 (Continued)

liquid mixture leaving the condenser (Fig. 2.2f) converts the nonideal into an ideal stage, provided the line downstream of the mixing point is sufficiently long.

Stage efficiency. This accounts for the nonideality of a stage. The number of ideal stages is equal to the number of nonideal stages multiplied by the stage efficiency. The nonideality may lower or enhance the separation; if it enhances the separation, the stage efficiency can exceed 100 percent. Stage efficiencies are discussed in Chaps. 7 and 9.

2.1.2 Stripping, rectification, and fractionation

Vapor leaving a distillation stage is richer than the feed in the more-volatile components. Liquid leaving the stage is richer than the feed in the less-volatile components. In order to improve the separation, multiple stages are used.

Stripping stages (Fig. 2.3a) concentrate the less-volatile components in a liquid stream. A vapor recycle vaporizes ("strips") the more-volatile components from the liquid. To generate the vapor recycle, heat is supplied to vaporize a portion of the bottom-stage liquid. This vapor recycle is termed boilup.

Vapor product

Feed F

Vapor product

Feed F

Distillation With Intercondenser

Liquid product

Liquid product

Liquid product

Liquid product

Feed

Feed

Liquid product

Figure 2.3 Multiple-stage arrangements, (a) Stripping; (6) rectifying; (c) fractionator.

Vapor

Liquid product

Vapor

Liquid product

Figure 2.3 Multiple-stage arrangements, (a) Stripping; (6) rectifying; (c) fractionator.

Rectifying stages {Fig. 2.36) concentrate the more-volatile components in a vapor stream. A liquid recycle condenses the less-volatile components from the rising vapor. To generate the liquid recycle, cooling is applied to condense a portion of the overhead vapor. The liquid recycle is termed reflux.

The stripping and rectifying stages shown in Fig. 2.3a and b can be combined into a single separation process with internal recycle (Fig. 2.3c). This process is termed fractionation.

In a single-feed fractionator, stages above the feed are rectifying

Pooling '

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