Improving batch still operation

To the chemical engineer a continuous process is very attractive since it can be operated for most of the time at a steady state with controls that can hold the plant to given conditions. To avoid the hard work of start-up and shut-down such a plant will normally be run for several days on a consistent feedstock. A small operation which does not have round-the-clock working seldom is geared to continuous operation. 

To set against the lack of stripping capability must be set the flexibility of a batch still in dealing with multi-component mixtures. Faced with, say, a four-component solvent mixture arising from a pharmaceutical production one has either to make three (if the fourth is a residue containing no recoverable solvent) passes through a continuous column or have 

The other drawback to a batch still is that, as the most volatile component of the mixture to be separated is removed from the system, the point is reached 

To some extent these drawbacks can be avoided by using a hybrid still that has features of both a batch still and a continuous one. 

instead of charging the still kettle with feed, the feed can be introduced some distance up the column with a heel of material (probably a portion of the residue from the previous batch) already being boiled up the column, the most volatile fraction can be stripped in the fractionating stages below the feed and taken off while the batch is being charged. This not only provides stripping plates but also increases the size of the batch because the volatile fraction never needs to be accommodated in the kettle. The latter can be a substantial increase in batch size in many solvent recovery operations. The feed rate should correspond to the rate of tops product. 

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In the continuous shell still process, several shell stills are linked in series to form a battery. Fresh feed continuously enters the first still, which is kept at the lowest temperature for the lightest overload produa. The bottoms from the first still are fed to the second still, which is kept at the temperature for the next highest boiling overhead product, and so on. The number of stills required depends on the number of produas needed. If the feed and the temperature of each still remain constant, the finished product is of satisfaaory quality. Exhibit 10-3 depicts the continuous shell still process, which is an improvement over the batch shell still operation. 

The modern batch deodorizer has a capacity from 5 to 30 tonnes. It is still frequently constructed of mild steel, though stainless steel is preferred. It is heated by high-pressure steam or, in some cases, by Dowtherm or thermal oil, and its vaporization efficiency has been improved by the inclusion of mammoth pumps and effective head space insulation to prevent refluxing. A vapour scrubber can be installed to recover oil and fatty acids from the distillate vapour. The complete batch cycle time is between 6 and 10 h depending on the feedstock. The unit is not, however, suitable for physical refining and it is expensive to operate compared with continuous and semi-continuous plant. 

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