Vyi Lx0 V

This operating line relates the composition streams after m stages.

Figure 7 shows the cascade in terms of a more conventional representation in a distillation column. At the top of the column we need liquid to feed the cascade. This is produced by condensing and returning some of the vapour which leaves the top stage. We also need vapour to feed the cascade at the bottom of the column. This is produced by vaporizing

Figure 5 A cascade of separation stages.
Figure 6 Mass balance on a countercurrent cascade.

and returning some of the liquid leaving the bottom stage. The feed to the process is introduced at an intermediate stage; products are removed from the condenser and the reboiler (vaporizer).

The method by which the vapour and liquid are contacted with each other in distillation columns falls into two broad categories. Figure 8 shows a plate or tray column. Liquid enters the first tray at the top of the column and flows across what is shown in Figure 8 as a perforated plate. Liquid is prevented from weeping through the holes in the plate by the upflowing vapour. In this way the vapour and liquid are contacted. The liquid from the first tray flows over a weir and down a downcomer, to the next stage and

Figure 7 Refluxing and reboiling.

so on. The design of stage used in Figure 8 involving a plate with simple holes is known as a sieve tray. Many other designs of tray are available involving, for example, valve arrangements for the holes in the trays. In practice, the column will need more trays than the number of equilibrium stages as mass transfer limitations prevent equilibrium being achieved on a tray.

The other broad class of contacting arrangement is that of packed columns. Here the column is filled with a solid material which has a high voidage. Liquid trickles across the surfaces of the packing and vapour flows upward through the voids in the packing, contacting the liquid on its way up the column. Many different designs of packing are available.

The design of a distillation column like the one shown in Figure 8 involves a number of steps:

1. Set the product specifications.

2. Set the operating pressure.

3. Determine the number of theoretical stages required and the energy requirements.

4. Determine the actual number of trays or height of packing needed and the column diameter.

5. Design the column internals, which involves determining the specific dimensions of the trays, packing, liquid and vapour distribution systems, etc.

6. Carry out the mechanical design to determine wall thicknesses, internal fittings, etc.

Let us start by considering the simplest case of binary distillation.

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