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Chemical and Process Engineering Resources

Sizing of Packed Towers in Acid Plants

Dec 13 2010 10:20 AM | Guest in Separation Technology -----

Discussion of Results

Table 3 shows that there is a wide discrepancy in tower diameter prediction when different sizing methods are used. From practical experience and discussion with colleagues, the GPDC approach appears sound but the packing factors published for large packings appear questionable. This is likely due to the use of the "void fraction effect" in small pilot towers. An adjustment of packing factors for all three of the larger size saddles would appear to be in order and could result in at least a doubling of the packing factors for the 3" saddle. Possibly the published packing densities of Figure 2content_link.gif and the basis Leva equation would give sufficient guidance. The two proprietary programs gave similar results which suggested a similar logic basis but the details of the program were not available. Again, these programs are only as good as are the data used in the correlations. The question is, are these data obtained from a pilot tower or a full scale tower.

The last design approach listed in Table 3 is based on data collected by CECEBE and NORAM in a full size tower using the actual void fractions. There is some concern about designing to relatively high pressure drops as with any correlations when the pressure drop approaches 0.75" W. C. There is no benefit to the owner if a supplier is overly optimistic on packing performance only to have the owner ultimately suffer the consequences. Nevertheless, higher quality packing will give measurable and economically quantifiable performance advantages.

In Table 4, the void fractions have been adjusted to larger tower diameters and a number of packings have been evaluated on the basis of the CECEBE design techniques. As can be seen, larger standard saddles give smaller tower diameters just as one would expect. For the HPTM saddle, field data are available and a structured ceramic packing, for which a proprietary program was available, is also listed. Interestingly, the diameter associated with the HPTM saddle was essentially the same as that predicted from several of the programs for the standard saddle and well below that which one can expect for the standard 3" saddle. The structured ceramic program predicted a slightly smaller diameter but it is not backed by any published field data. However, the nature of this structured packing suggests that it is not significantly affected by tower size. As mentioned already, its major disadvantage is that it requires special attention to liquid distribution. No capacity benefit is gained if liquid distribution is implemented through a layer of standard 3" saddles.

If one is considering installation of new towers, the cost of the tower is one issue. The cost of packing is a second. Standard packing is sold normally on the basis of a definition of a piece density which falls short of what is needed to cover a "settling allowance". Often fifteen percent extra packing may be needed. Combining this with a large diameter tower can result in as much as fifty percent more packing being needed over more recent high performance packing to get an equivalent result. Combining this with the cost of the tower shell, sound economics would suggest that the best packing will probably give the best economic solution and the cheapest packing, possibly the worst solution. The difficulty has been the wide discrepancy in the ability to predict pressure drop in large packed towers. In several cases recently, HPTM saddles have been supplied to owners who were concerned about pressure drop and power consumption and could justify the expense on the power saved. With rising power costs, this incentive will become even more powerful. The discussion above on pressure drop offers data relevant to this issue.

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