Eq. (6.8)

% Qd, ««




100 x Qo/Qo m(ut

'Center-to-side Dow. tSide-to-center flow.

'Center-to-side Dow. tSide-to-center flow.

Summary. According to the above criteria, the tower appears oversized. The next trial should be performed with a smaller tower diameter, say 6 ft.

6.5.6 Second trial

Throughout this trial, vapor and liquid loads and physical properties are taken from Table 6,10. Tray layout parameters are discussed below and shown in Table 6.11.

Layout. First, determine downcomer areas. The calculation in Sec. 6.5.5 shows that the area for the top section downcomers is about right. On the other hand, the bottom section downcomers appear somewhat oversized. Table 6.5 suggests that for the depropanizer (high foaming tendency, 18- and 24-in tray spacing) up to 110 gpm/ft2 is an appropriate velocity. Resize the downcomer for 110 gpm/ft2.

Now retain the number of passes, tray spacing, hole diameter, weir height, and clearance under the downcomer as per Sec. 6.5.5.

Recalculate weir and downcomer dimensions by the same procedure as in Sec. 6.5.4. The results are summarized in Table 6.11.

Comment Note that the length of the flow path (LFP) will restrict any further reductions in column diameter. As stated previously (Sec. 6,5.4), flow path length smaller than about 18 in is best avoided. This limit is reached in the bottom section, center-to-side flow.

Flooding check. The calculation in Sec. 6.5.5 is now repeated.

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