n x HETP

Comment. The recommended published criteria surveyed by Kister (40) suggest that it is best to redistribute liquid approximately every 20 ft, and to have no more than 10 theoretical stages per bed. Both of these criteria suggest that the bottom section of the tower should have two packed beds with a redistributor in between, while the top section should contain one packed bed.

Substituting 1.5-in PallĀ® rings for 2-in rings in the bottom section will lower the packed height to 25 ft [Eq. (9.34)], possibly at the price of only a slight increase in tower diameter. This may make it possible to get rid of the redistributor and get away with a single packed bed in the bottom section. Such a design will be in slight violation of the recommended distribution criteria (above), and will also have a higher column to packing diameter ratio. Both factors will render such a design more sensitive to maldistribution. Little is known about how detrimental maldistribution is to multicomponent distillation efficiency at high pressure. The author feels that a relatively small investment (in a redistributor) is justified here in order to keep the design less sensitive to maldistribution. The author would therefore prefer two beds of 2-in PallĀ® rings in the bottom section.

The flood-point calculations above show that there is some spare capacity in the top section. This can be utilized for reducing the height of the top section by using smaller packings. The number of stages in the top bed, and the lower bed height, will remain well within the above guidelines for good redistribution practice. The bed to packing diameter ratio will increase somewhat; however, going from 2 in to 1.5 in would only increase the ratio by 33 percent, and this should be tolerable.

In summary, the second trial will use the following:

Dt = 6.25 ft dp = 2 in in bottom section, 1.5 in in top section

9.4.4 Column sizing example: second trial

Top section

Bottom section


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