By repeating this type of calculation, the values of the instantaneous distillate composition given in Fig. 14-5 were obtained.
In the example on p&ge 385, about 60 per cent of the original charge could be obtained as fractions containing one of the components in at least 85 concentration. It will be noted that the least volatile material can be obtained in the highest purity because \t is taken over when the still contains the l'iast amount of other components. By operating an inverted batch ¡olumn as shown in Fig. 14-6, it is possible to remove the less volatile components and obtain the most volatile material in high purity. In this case, the batch is charged to the reservoir, and
liquid is continuously added to the top of the column from this tank. The liquid from the bottom of the column is partly vaporized, and the unvaporized portion is removed as product. The vapor is passed back up the column to strip out the more volatile components, and the overhead vapor is condensed and returned to the reservoir. The most volatile component collects in the reservoir and is obtained as the last fraction after essentially all the other components have been eliminated. An intermediate fraction can be increased in concentration by fractionating first in a normal batch operation to remove the lighter components and then in an inverted batch unit to remove the heavy constituents; or both operations can be combinec ■ simultaneously taking the lighter components overhead and the he& Wer components out the bottom with the reservoir between. This combined operation can save heat but requires additional equipment.
Finite Reflux Ratio. It was pointed out on page 384 that this case was difficult because, if the plate-to-plate calculations were carried down from the top for an assumed overhead product, the still composition would probably not correspond to any liquid composition encountered in the actual distillation. Starting with a given liquid composition in the still, it is possible to make calculations for an assumed overhead composition. If the calculated overhead composition checks the assumed value, this gives one set of corresponding distillate and still compositions. Thus a laborious trial-and-error procedure is necessary to obtain this one set of values, but the design calculation requires a number of such sets for liquid compositions that follow a definite pattern. Thus the overhead vapor corresponding to the original charge could be calculated by the above trial-and-error procedure and a small increment of this composition removed, leaving a new liquid composition. By a large number of such small steps, the distillation curve can be established, but making the trial-and-error plate-to-plate calculation for each step is almost prohibitive.
The total reflux analysis of the preceding section was relatively simple, and an approximate method for finite reflux ratios can be developed in an analogous manner. For the operating lines for any two components, the liquids on a plate are related to the vapors from below by
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