Extractor vessel and internals

1 Provision of extractor baskets. In some situations it is desirable (or even necessary) to enclose the solids which are to be extracted in a set of baskets which are then placed in the extraction vessel. This procedure is necessary in the case of materials which tend to form compact agglomerates when extracted in a deep bed. Such agglomerates can be virtually impossible to extract and, furthermore, the compressed bed containing them does not flow 

Agglomerates can also be formed in beds of moist green coffee, but the effect is much less marked than in the case of tea. Practical experience has shown, however, that it is possible in this case to avoid the difficulty by careful control of the pressure gradient in the extractor and by depressurising reasonably slowly. In view of this baskets are not used in large-scale coffee decaffeination plant since they add to the cost of the operation. It may, however, be convenient to use them in smaller scale units, as discussed later. 

Care should be taken when designing tea extraction units to ensure that 

The above are the principal expenses and amount in all to an increase of about 50% in the cost of the extractor unit. This increase does not change appreciably with vessel volumes in the range 2 to 25 m (This range is typical of large-scale plant.) It adds about 8% to the total investment on the plant. The advantage in the use of baskets is that it enables the extractor to be on-line for a higher proportion of the time. There are two reasons for this. 

A further discussion of the precautions necessary when installing and operating baskets is given in section 8.3.4. 

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