Primary Tray Considerations

Number of Passes Tray liquid may be split into two or more flow passes to reduce tray liquid load QL (Fig. 14-21). Each pass carries 1/Np fraction of the total liquid load (e.g., j in four-pass trays). Liquid in each pass reverses direction on alternate trays. Two-pass trays have perfect symmetry with full remixing in the center down-comers. Four-pass trays are symmetric along the centerline, but the side and central passes are nonsymmetric. Also, the center and offcenter downcomers only partially remix the liquid, allowing any maldistribution to propagate. Maldistribution can cause major loss of efficiency and capacity in four-pass trays. Three-pass trays are even more prone to maldistribution due to their complete nonsymmetry. Most designers avoid three-pass trays altogether, jumping from two to four passes. Good practices for liquid and vapor balancing and for avoiding maldistribution in multipass trays were described by Pilling [Chemical Engineering Progr., p. 22 (June 2005)], Bolles [AIChE J., 22(1), p. 153 (1976)], and Kister (Distillation Operation, McGraw-Hill, New York, 1990).

Common design practice is to minimize the number of passes, resorting to a larger number only when the liquid load exceeds 100 to 140 m3/(h m) (11 to 15 gpm/in) of outlet weir length [Davies and Gordon, Petro/Chem Eng., p. 228 (December 1961)]. Trays smaller than 1.5-m (5-ft) diameter seldom use more than a single pass; those with 1.5- to 3-m (5- to 10-ft) diameters seldom use more than two passes. Four-pass trays are common in high liquid services with towers larger than 5-m (16-ft) diameter.

Flooding

Flooding

Liquid flow rate per weir length

FIG. 14-19 The flow regime likely to exist on a distillation tray as a function of vapor and liquid loads. (From H. Z. Kister, Distillation Design, copyright ©1992 by McGraw-Hill; reprinted by permission.)

Liquid flow rate per weir length

FIG. 14-19 The flow regime likely to exist on a distillation tray as a function of vapor and liquid loads. (From H. Z. Kister, Distillation Design, copyright ©1992 by McGraw-Hill; reprinted by permission.)

Tray Spacing Taller spacing between successive trays raises capacity, leading to a smaller tower diameter, but also raises tower height. There is an economic tradeoff between tower height and diameter. As long as the tradeoff exists, tray spacing has little effect on tower economies and is set to provide adequate access. In towers with larger than 1.5-m (5-ft) diameter, tray spacing is typically 600 mm (24 in), large enough to permit a worker to crawl between trays. In very large towers (>6-m or 20-ft diameter), tray spacings of 750 mm (30 in) are often used. In chemical towers (as distinct from petrochemical, refinery, and gas plants), 450 mm (18 in) has been a popular tray spacing. With towers smaller than 1.5 m (5 ft), tower walls are reachable from the manways, there is no need to crawl, and it becomes difficult to support thin and tall columns, so smaller tray spacing (typically 380 to 450 mm or 15 to 18 in) is favored. Towers taller than 50 m (160 ft) also favor smaller tray spacings (400 to 450 mm or 16 to 18 in). Finally, cryogenic towers enclosed in cold boxes favor very small spacings, as small as 150 to 200 mm (6 to 8 in), to minimize the size of the cold box.

More detailed considerations for setting tray spacing were discussed by Kister (Distillation Operation, McGraw-Hill, New York, 1990) and Mukherjee [Chem. Eng. p. 53 (September 2005)].

Outlet Weir The outlet weir should maintain a liquid level on the tray high enough to provide sufficient gas-liquid contact without causing excessive pressure drop, downcomer backup, or a capacity limitation. Weir heights are usually set at 40 to 80 mm (1.5 to 3 in). In this range, weir heights have little effect on distillation efficiency [Van Winkle, Distillation, McGraw-Hill, New York, 1967; Kreis and Raab, IChemE Symp. Ser. 56, p. 3.2/63 (1979)]. In operations where long residence times are necessary (e.g., chemical reaction, absorption, stripping) taller weirs do improve efficiency, and weirs 80 to 100 mm (3 to 4 in) are more common (Lockett, Distillation Tray Fundamentals, Cambridge University Press, Cambridge, England, 1986).

Adjustable weirs (Fig. 14-22a) are used to provide additional flexibility. They are uncommon with conventional trays, but are used with some proprietary trays. Swept-back weirs (Fig. 14-22&) 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).

Downcomers A downcomer is the drainpipe of the tray. It conducts liquid from one tray to the tray below. The fluid entering the downcomer is far from pure liquid; it is essentially the froth on the tray, typically 20 to 30 percent liquid by volume, with the balance being gas. Due to the density difference, most of this gas disengages in the downcomer and vents back to the tray from the downcomer entrance. Some gas bubbles usually remain in the liquid even at the bottom of the downcomer, ending on the tray below [Lockett and Gharani, IChemE Symp. Ser. 56, p. 2.3/43 (1979)].

Downcomer Backup

FIG. 14-20 Distillation flow regimes: schematics and photos. (a) Froth. (b) Emulsion. (c) Spray. [Schematics from H. Z. Kister, Distillation Design, copyright © 1992 by McGraw-Hill, Inc.; reprinted by permission. Photographs courtesy oof Fractionation Research Inc. (FRI).]

FIG. 14-20 Distillation flow regimes: schematics and photos. (a) Froth. (b) Emulsion. (c) Spray. [Schematics from H. Z. Kister, Distillation Design, copyright © 1992 by McGraw-Hill, Inc.; reprinted by permission. Photographs courtesy oof Fractionation Research Inc. (FRI).]

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