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The calculated HETPs (Sec. 9.4.4) are 27 in for the top section (1.5-in Pall® rings) and 36 in for the bottom section (2-in Pall* rings). Compared to the data, the calculated HETPs appear optimistic.
Could the Table 11.1 data have been measured under mal distribution? The ratios of column to packing diameters range from 12 to 15; at these ratios, lateral mixing should have mitigated the pinching effects. The bed heights were low, and did not violate good redistribution practice (except for one case—and this case gave the lowest HETP!). There is no evidence to suggest maldistribution. The data therefore imply that the high HETPs are possibly system-related. These data, therefore, serve as a better basis for HETP in this example than Eq. (9.34).
The Table 11.1 data suggest that the top sections operate at HETPs of the order of 35 to 38 in with 1.5-in Pall* rings. The bottom sections have an HETP of about 29 in with 1.5-in Pall® rings, and 40 in with 2-in Pall® rings. The difference (11 in) is similar to the difference predicted from Eq. (9.34), and therefore, makes sense. It follows that for design purposes, an HETP value of 38 in is a good estimate for the top section of the depropanizer in this example, while an HETP of 40 in is a good estimate for the bottom section. Note that these values are not conservative; they match the available data.
Minimum wetting. As with a tray tower, this turndown check is performed at the lowest loaded stages (8 and 9; see Table 6.10) at turned down conditions. As per Example 6.1, the problem requires turndown to 60 percent of the expected design loads. Therefore, the vapor and liquid loads shown in Table 6.10 for stages 8 and 9 are multiplied by 0.6 for the minimum wetting check.
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