Multipass trays

Minimum diameter for multipass trays is given in Table 2. 

Multipass Tray Number of Passes Minimum Diameter (ft) Preferred Diameter (ft)... 

Adjustable weirs (Fig. 14-22 ) are used to provide additional flexibility. They are uncommon with conventional trays, but are used with some proprietary trays. Swept-back weirs (Fig. 14-22b) are used to extend the effective length of side weirs, either to help balance liquid flows to nonsymmetric tray passes or/and to reduce the tray liquid loads. Picket fence weirs (Fig. 14-22c) are used to shorten the effective length of a weir, either to help balance multipass trays liquid flows (they are used in center and off-center weirs) or to raise tray liquid load and prevent drying in low-liquid-load services. To be effective, the pickets need to be tall, typically around 300 to 400 mm (12 to 16 in) above the top of the weir. An excellent discussion of weir picketing practices was provided by Summers and Sloley (Hydroc. Proc., p. 67, January 2007). 

Antijump baffles (Fig. 14-24) are sometimes installed just above center and off-center downcomers of multipass trays to prevent liquid from one pass skipping across the downcomer onto the next pass. Such liquid jump adds to the liquid load on each pass, leading to premature flooding. These baffles are essential with proprietary trays that induce forward push (see below). 

Maldistribution Maldistribution can cause major efficiency reduction in multipass trays (>two passes). Further discussion is given under Number of Passes. ... 

High liquid rates. Multipass trays effectively lower the liquid load seen by each part of the tray. A similar trick cannot be applied with packings. The capacity of structured packings tends to rapidly fall off at high liquid rates. 

Improving liquid flow patterns. A number of special tray designs have been developed to improve liquid velocity distribution on large-diameter trays. Their main applications are vacuum distillation. In pressure distillation, liquid flows are usually high and multipass trays are used, so that stagnant zones are seldom a problem. Some means of improving the liquid flow patterns are... 

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... 

Bolles (191) correlated the reduction in efficiency in terms of the distribution ratio, i.e., the maximum-pass LfV ratio divided by the minimum-pass LfV ratio. The L and V for each pass are determined from the normal pressure balance and hydraulic relationships, applied to each pass. At high distribution ratios, a substantial drop in tray efficiency occurs. Bolles shows that if this distribution ratio is kept lower than 1.2, the loss in efficiency due to maldistribution is negligible. Bolles recommends designing multipass trays for such low distribution ratios. Detailed guidelines for achieving low distribution ratios (<1.2), thus minimizing the effects of pass maldistribution on efficiency, are contained in a companion book (1) and in Bolles s paper (191). 

Downcomer top area, ft3. (Note In multipass trays, AD is the sum of the top areas of all downcomers transporting liquid from the trey.)... 

LWtLw Outlet weir length, in (Note In multipass trays, Lw is the total outlet weir length, i.e, the sum of lengths of all outlet weirs). 

Multipass trays are designed with equal-bubbling areas. Weir lengths or heights can be adjusted to provide uniform tray hydraulics. 

Figure 13.6. Multipass trays, (a) Two-pass, (b) Three-pass, (c) Four-pass.

Feed inlet distributors are recommended for large-diameter singlepass trays (207, 354). In multipass trays, feed and reflux distributors are essential to ensure uniform distribution. The only exceptions are... 

When feed to multipass trays is to be split unevenly, such as in a three-pass tray. In such cases, a feed trough (Fig. 2.4) is often preferred to a distributor (179, 307). 

Guidelines for setting the number of passes and minimizing operating problems with multipass trays are listed below ... 

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