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Constant reflux

Figure 12. The second method is to increase the reflux ratio during the run in order to maintain a steady top product composition.

stage, xn + 1 is the mole fraction of the MVC in the liquid arriving at the nth stage, xd is the mole fraction of the MVC in the top product, D is the molal flow rate of top product, V is the molal vapor flow rate in the column, and L is the molal liquid flow rate in the column. The reflux ratio R is given by

(LAO

With the VLE data and the top and bottom compositions, it then is possible to calculate graphically by the

Table 1. Computer Program Modes

Mode

Specified

Program calculation

Top-product and still composition

Top-product still composition, reflux ratio Top-product and still composition Number of theoretical stages Top-product composition required Initial and final still composition Still charge Batch time

Number of theoretical stages Average top-product composition Initial and final still composition Still charge Batch time

Number of theoretical stages

Minimum number of theoretical stages Minimum reflux ratio Number of theoretical stages Reflux ratio

Reflux ratio at stat and end of batch

Boiling rate Product quality

Reflux ratio required constant throughout batch

Boil-up rate Product quality

McCabe and Thiele procedure the minimum reflux ratio, minimum number of theoretical stages, and other such parameters. In batch distillation in particular, these procedures are tedious and time-consuming because, of course, the composition of the liquid in the still changes with time and it is necessary to repeat the calculations many times. Obviously, the procedures become even more time-consuming with multicomponent systems.

For this type of design work, computer programs can be used to enable the engineer to produce an efficient design very quickly. One program is extremely flexible and operates in a different series of modes, as is charted in Table 1. Naturally, the VLE data have to be specified for all modes. The program further assists in the determination of the number of theoretical stages. Other modes also are available to provide different permutations of operation.

To help determine the sizing of the column diameter, various programs can be used to incorporate different proprietary methods, including those of Fractionation Research Inc. (FRI) of California.

Column Internals

The choice of internals for the column depends mainly on the product being processed and the size of the column to be used. To meet virtually every parameter, packed columns as well as sieve, bubble, and ballast trays are available. As a general rule, sieve trays are not used frequently in batch columns because the turndown ratio of most trays of this type is only about 1.5 to 1. This reduces one of the main advantages of batch columns, mainly flexibility. Usually, small batch columns are packed because the efficiency of trays of less than 2-ft diameter often decreases rapidly. Ballast trays, although expensive, are often used for larger columns because they are efficient and have turndown ratios of up to 9 to 1.

Control

The control of batch columns is very simple, and, therefore, required instrumentation usually is quite inexpensive. Constant reflux operation can be handled by a ratio controller or by using a timed deflection of condensate. For operation with variable reflux, the control of the reflux ratio must be tied to some property of the top product that undergoes a sufficiently large change in value for change in composition. Manual control of this type of reflux operation is not feasible.

Numerous batch distillation systems have been supplied to customers who require the separation of many different components. To simplify the operation of these systems, there are proprietary computers specifically designed for process control. For example, use of certain microprocessors enables the operator to switch from one type of separation to another without having to make many manual adjustments on the control panel. In addition, the computer can be programmed to automatically take care of all necessary tails cuts as well as to clean the system during interims between the various separations.

Batch Systems/Continuous Distillation

In a number of duties, it is essential that the base product contain only very small amounts of one component. For example, in the recovery of solvents from water, it is mandated that the water may retain only trace quantities of solvents before being pumped to waste. Because one disadvantage of a batch system is that there is no stripping section, it is necessary to boil the batch pot for relatively long periods of time in order to reduce the residual solvent to trace quantities.

To resolve this problem, a number of batch distillation systems with the capability of being operated in the continuous mode have been supplied. By incorporating one, two, or three feed points on the column, the feed can be pumped directly to the column instead of to the batch tank. Furthermore, a separate small holdup/reboiler is furnished in certain cases so that the batch tank and its associated reboiler need not be used for the continuous operation.

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