David R Hart

RUST International Corporation Birmingham, Alabama

Although neither as widely used nor as sophisticated as their continuous counterparts, batch stills play an important role in the chemical process industries. Batch stills are typically used

1. Where the compositions of the materials to be separated vary over wide ranges

2. Where the separation only needs to be performed infrequently, such as in pilot-plant operations

3. Where the materials to be separated are produced in relatively small quantities, such as in semiworks or small-scale commercial facilities

4. Where the main product contains only relatively small amounts of light and/or heavy impurities

Selection of a batch still usually involves evaluating the performance of an existing distillation system to determine whether its performance is acceptable. However, it may be necessary to design a new system for the specific separation to be performed.

With an existing system, the following are common concerns:

1. What product compositions would be obtained c. without reflux, or b. with reflux when using,

(1) a constant reflux ratio, or

(2) a varying reflux ratio?

2. How much time would be required to obtain some specific product composition at some constant boil-up rate, or what boil-up rate would be required to obtain some specific product composition within some specified time under conditions of, a. constant reflux ratio, b. varying reflux ratio?

The design of a new system for a specific separation involves determining a minimum reflux ratio and selecting a control protocol (fixed or variable reflux ratio) and an amount of time to be allowed for distilling a batch of some given size.

This chapter addresses each of these concerns in order. Before passing on to that discussion, however, the distillation nomenclature will be briefly restated, both because batch stills are analyzed after the same fashion as continuous stills, and for completeness within this chapter. The overall material balance is

where F = feed, moles

D = distillate, moles B = bottoms, moles

(in the case of batch stills, the bottoms are generally called residue). A material balance on the "light" (lower boiling) component in a binary mixture, or the light "key" component in a more complex mixture is

where ^subscript = mole fraction in the liquid phase A material balance around a total condenser gives

where V = vapor from column

L = liquid returning to column (equals zero in "simple" distillation)

By definition

LID = R = reflux ratio (sometimes called external reflux ratio)

L L but from the definition of reflux ratio

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