E3

^ Vapor product

Heat

Feed

Liquid product Figure 2.3 (Continued)

Liquid product

Liquid product

and those below it are stripping (Fig. 2.3c). In multifeed fractionators, the more precise functional criterion below is used to distinguish the rectifying from stripping sections.

The stripping section has a net downflow of material. The vapor serves only as a recycle stream to remove lights from the liquid. Therefore, the quantity of liquid exceeds the quantity of vapor in the stripping section. The converse applies in the rectifying section, This section has a net upflow of material, and the quantity of vapor exceeds the quantity of liquid.

Figure 2.4 shows a multifeed fractionator. The top three sections have a net upflow of material and are therefore rectifying. The bottom three sections have a net downflow of material, and are therefore stripping.

2.1.3 Material and energy balances

For a single binary distillation stage (Fig. 2.5a) the following equations apply:

Section 2

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