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FIG. 13-99 Distillate composition profile for a batch distillation of a four-component mixture.

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FIG. 13-99 Distillate composition profile for a batch distillation of a four-component mixture.

the next batch. For a multicomponent mixture, two or more intermediate cuts may be taken between product cuts.

2. Constant overhead composition, varying reflux. If it is desired to maintain a constant overhead composition in the case of a binary, the amount of reflux returned to the column must be constantly increased throughout the run. As time proceeds, the pot is gradually depleted of the lighter component. Finally, a point is reached at which the reflux ratio has attained a very high value. The receivers are then changed, the reflux is reduced, and an intermediate cut is taken as before. This technique can also be extended to a multi-component mixture.

3. Other control methods. A cycling procedure can be used to set the pattern for column operation. The unit operates at total reflux until equilibrium is established. Distillate is then taken as total draw-

off for a short period of time, after which the column is again returned to total-reflux operation. This cycle is repeated through the course of distillation. Another possibility is to optimize the reflux ratio in order to achieve the desired separation in a minimum of time. Complex operations may involve withdrawal of sidestreams, provision for inter-condensers, addition of feeds to trays, and periodic charge addition to the pot.

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