Two-pass tray

DIVIDE LIQUID FLOW RATE BY 2 -- 13) OBTAIN DIAMETER FROM TWO-PASS TRAY LINE... 

Figure 3.1 Cross section of two-pass tray (conventional).

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. 

Downcomer backup. For side downcomers of two-pass trays, the force balance of Eq. (6.19) makes the dear liquid height in the downcomer... 

Special considerations In multipass trays. In a multipass tray, vapor distribution between the passes is largely determined by the hole area, while liquid distribution is largely a function of the weir height and length. If the geometry of the passes is perfectly identical, the distribution of vapor and liquid is the same for each pass, and tray efficiency is uniform. This is readily achievable in two-pass trays, where the design of each pass is identical to the other, but not so when a... 

In vacuum columns, the calculations furthermore are based on conditions at the column top, where vapor density is lowest and vapor velocity highest. The F-factor method used here is quite satisfactory for single- and two-pass trays it has an accuracy well within 15 percent. The F factor Fc is defined as follows ... 

Fig. 9 Two-pass trays 1) capped areas 2) center liquid downcomers 3) side liquid downcomers 4) disengagement zones 5) seal pan and 6) cross-sectional view.

Eqs. (14-16) are for one-pass trays. For two-pass trays the total downcomer area is halved before calculating the weir length. The weir length is then doubled to account for both downcomers. Two-pass tray weir loads are limited by the liquid flow into the downcomers on the tower edge (outboard downcomers). 

For two-pass trays, Fqs. (14) and (16) define Hi for the outboard downcomer and half the total downcomer area Apc is used. 7/3, the center downcomer width, can be calculated by assuming the center downcomer is a rectangle. This gives good results for nearly all towers ... 

Use the entire area for one-pass trays for two-pass trays, use half the total area for Apc- For the inboard (center) area of two-pass trays use the approximation ... 

Although the general model focuses on single-pass trays, multi-pass trays are also used. In multi-pass trays the liquid splits and flows in opposite directions so that any liquid element travels only a certain fraction of the tray width. Liquid flows down from one tray to the tray below through more than one downcomer, except on alternating trays in two-pass trays where liquid flows down one central downcomer every other tray. Figure 14.3 illustrates tray arrangements and liquid paths in two-, three-, four-, and five-pass trays. 

For high liquid flow cases, it may be necessary to split the flow into two or more passes. This creates separate contacting zones, as shown in Figure 12.44. For the two-pass tray shown, liquid flows side to center and center to side on successive flays this means that the effective weir crests differ because of differing weir lengths. The center weir is about the same length as the column diameter, whereas the side weir may be only 70% or so of the diameter. 

Pure liquid feeds into two-pass trays, which can be introduced into the central downcomer by arrangements similar to those shown in Fig. 2.1 (arrangements a, b, d, g, h) and Fig. 2.2 (arrangement a). 

This guideline need not be adhered to when the quality of inlet stream distribution is not of major importance, e.g., when distributing feed to a single-pass tray or to the center area of a two-pass tray. 

Figure 4.7 Reboiler trapout-pan arrangements, (a) Single-pass trays (once-through reboiler) (6) two-peiss trays, single reboiler (once-through reboiler) (c) two-pass trays, two reboilers (once-through reboiler) (d) single-pass trays (recirculating reboiler).

Unlike the internals discussed in previous chapters, poor tray layout of one- or two-pass trays rarely causes spectacular column failures such as flooding at 50 percent of design rates or extremely poor separation. Ill effects resulting from poor tray layout seldom extend beyond suboptimum design or performance a moderate reduction in capacity, efficiency, or turndown and some increases in capital or... 

To avoid excessively short liquid paths, several designers recommend that two-pass trays should only be installed when column diameter exceeds 4 to 6 ft (38, 48, 73, 138, 172, 211, 257, 319, 371, 409) three-pass trays should only be installed when column diameter exceeds 7 to 9 feet (38, 138, 211) and four-pass trays should only be installed when column diameter exceeds 10 to 12 ft (38, 138, 211). 

Items where levelness is critical are indeed level. As a general rule of thumb, all items that contain notches and all gravity distributors and parting boxes must be thoroughly checked. It is a good practice to also check levelness in pressure distributors. Since tray levelness is normally less critical on one- and two-pass trays (Sec. 7.9), and tray-by-tray levelness checks are tedious, it is often satisfactory only to spot-check their levelness. 

Olefins Water-wash tower A welding rag left in the column reflux line found its wt to and partially blocked the reflux distributor to two-pass trays. EhGoessive entrainment resulted. Inspect for debris at points where lines leadingto the column were modified. 

Sin e-pass trays were replaced by two-pass trays in a oqxicity revanqi. Hitler cffxictly was adiieved, but reducing flow path fixnn 36 to 18 in lowered efficiency to an extent that original tn had to be remstalled. 

Use "anti-jump" baffles on the center downcomer of two-pass trays. 

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