Cfs

Top section

Bottom section

Downcomer area. Use Table 6.5 to obtain a preliminary estimate of downcomer top area. For a depropanizer at 315 psia, Table 6.5 recommends a velocity of 90 to 110 gpm/ft2. Select 100 gpm/ft2. No derating is required here, since the factors in Table 6.5 already take the foaming tendency into account.

Top section

Bottom section

Tower diameter. Total tower area is given by

Top section

Bottom section

Roughly, when the difference in calculated diameter between the top and bottom section exceeds 20 percent, different diameters for the top and bottom sections are likely to be economical (32). In the present example, the diameter difference is 3 percent, so column diameter should be uniform. The preliminary column diameter is the larger of the two, i.e., 6.4 ft. This diameter is rounded to the next highest half foot, giving 6.5 ft.

6.5.4 Preliminary tray layout

As stated in the introduction to Chapter 6, detailed tray layout is performed in the secondary phase of tray design. A preliminary layout, however, is needed at the primary phase, because the layout influences column size. In the present example, the preliminary layout will be based on tray layout guidelines detailed in a companion book (1).

Number of tray passes. Set this so that the liquid flow rate does not exceed 7 to 13 gpm/in of outlet weir (1). At this preliminary point in the calculation, assume that the weir length is 0.8 times the tower diameter, i.e~,

Top section

Bottom section

The weir load is satisfactory for a one-pass design for the top section, but a two-pass tray is needed for the bottom section.

Tray spacing. For the bottom section, 24 in is appropriate (1). For the top section, there may be some spare capacity available, so that a smaller tray spacing (18 in) is preferred. Assume

Top section

Bottom section

Hole diameter. A good choice of size for clean services is 3/ie in, while in is preferred for fouling services (1). Assuming that the deprop-anizer is a mildly fouling service, V2-in holes are appropriate.

Others. Based on Kister's guidelines (1), the following are appropriate preliminary choices for other tray layout parameters.

Af = Fractional hole area = 0.1

hw = Outlet weir height = 2.0 in hd = 1.5 in t - 10 gage (for carbon steel) = 0.135 in

Pitch. For an equilateral triangular arrangement, the hole pitch can be calculated from the following geometrical relationships (1).

For dH = 0.5, Af = 0.1,/? = 0.951 (0.5/V0l) = 1.5 in

Downcomer layout. The total tower area of a 6.5-foot tower is 33.2 ft2. In the top section, downcomer area is 3.8 ft2, i.e., the downcomer is about 3.8/33.2 = 11.5 percent of tower area. This is a relatively small fraction, and there is little incentive to reduce it further. On the other hand, in the bottom section the downcomer occupies 8.5/33.2 = 25.6 percent of the tower area. There is an incentive to reduce this area by using a sloped downcomer. A 2:1 top-to-bottom area ratio is appropriate (1). Therefore, downcomer top area in the bottom section is 8.5 ft2, while downcomer bottom area is 8.5/2 = 4.25 ft2. In summary

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