Swept-back Weir

Weir Liquid Crest Distillation Column
FIG. 14-21 Flow passes on trays. (a) Single-pass. (b) Two-pass. (c) Three-pass. (d) Four-pass.

The straight, segmental vertical downcomer (Fig. 14-23a) is the most common downcomer geometry. It is simple and inexpensive and gives good utilization of tower area for downflow. Circular downcom-ers (downpipes) (Fig. 14-23b), are cheaper, but poorly utilize tower area and are only suitable for very low liquid loads. Sloped downcom-ers (Fig. 14-23c, d) improve tower area utilization for downflow. They provide sufficient area and volume for gas-liquid disengagement at the top of the downcomer, gradually narrowing as the gas disengages, minimizing the loss of bubbling area at the foot of the downcomer. Sloped downcomers are invaluable when large downcomers are required such as at high liquid loads, high pressures, and foaming systems. Typical ratios ofdowncomer top to bottom areas are 1.5 to 2.

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).

Clearance under the Downcomer Restricting the downcomer bottom opening prevents gas from the tray from rising up the down-comer and interfering with its liquid descent (downcomer unsealing). A common design practice makes the downcomer clearance 13 mm (0.5 in) lower than the outlet weir height (Fig. 14-25) to ensure submergence at all times [Davies and Gordon, Petro/Chem Eng., p. 250 (November 1961)]. This practice is sound in the froth and emulsion regimes, where tray dispersions are liquid-continuous, but is ineffective in the spray regime where tray dispersions are gas-continuous and there is no submergence. Also, this practice can be unnecessarily restrictive at high liquid loads where high crests over the weirs sufficiently protect the downcomers from gas rise. Generally, downcomer clearances in the spray regime need to be smaller, while those in the emulsion regime can be larger, than those set by the above practice. Seal pans and inlet weirs are devices sometimes used to help with downcomer sealing while keeping downcomer clearances large. Details are in Kister's book (Distillation Operation, McGraw-Hill, New York, 1990).

Hole Sizes Small holes slightly enhance tray capacity when limited by entrainment flood. Reducing sieve hole diameters from 13 to 5 mm (2 to jg in) at a fixed hole area typically enhances capacity by 3 to 8 percent, more at low liquid loads. Small holes are effective for reducing entrainment and enhancing capacity in the spray regime (Ql < 20 m3/hm of weir). Hole diameter has only a small effect on pressure drop, tray efficiency, and turndown.

On the debit side, the plugging tendency increases exponentially as hole diameters diminish. Smaller holes are also more prone to corrosion. While 5-mm (gg-in) holes easily plug even by scale and rust, 13-mm (j-in) holes are quite robust and are therefore very common. The small holes are only used in clean, noncorrosive services. Holes smaller than 5 mm are usually avoided because they require drilling (larger holes are punched), which is much more expensive. For highly fouling services, 19- to 25-mm (|- to 1-in) holes are preferred.

Adjustable Weir Plate Sweptback Weir Picture

Downcomer plate

Swept-back weir

Picket Fence Weir Distillation

FIG. 14-22 Unique outlet weir types. (a) Adjustable. (b) Swept back. (c) Picket fence. (Parts a, c, from H. Z. Kister, Distillation Operation, copyright © 1990 by McGraw-Hill; reprinted by permission. Part b, courtesy of Koch-Glitsch LP.)

FIG. 14-22 Unique outlet weir types. (a) Adjustable. (b) Swept back. (c) Picket fence. (Parts a, c, from H. Z. Kister, Distillation Operation, copyright © 1990 by McGraw-Hill; reprinted by permission. Part b, courtesy of Koch-Glitsch LP.)

Similar considerations apply to fixed valves. Small fixed valves have a slight capacity advantage, but are far more prone to plugging than larger fixed valves.

For round moving valves, common orifice size is 39 mm (117/32 in). The float opening is usually of the order of 8 to 10 mm (0.3 to 0.4 in).

In recent years there has been a trend toward minivalves, both fixed and moving. These are smaller and therefore give a slight capacity advantage while being more prone to plugging.

Fractional Hole Area Typical sieve and fixed valve tray hole areas are 8 to 12 percent of the bubbling areas. Smaller fractional hole



Shaded area is cross-sectional shape of downcomer

Shaded area is cross-sectional shape of downcomer

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  • rowan
    What is crest in distillation column?
    6 years ago
  • jaska
    1 year ago

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