Staged columns downcomers

Problems of establishing a liquid seal in the column downcomers are common in low-liquid-rate services during startup. In such services, the downcomer liquid seal may also occasionally be lost during operation. Symptoms of these problems are excessive entrainment and poor separation. As distinct from flooding, column pressure drop may remain low. These problems, and cures that can be implemented at the design stage, are described in Sec. 6.18. 

N,n = Minimum theoretical stages at total reflux Q = Heat transferred, Btu/hr U - Overall heat transfer coefficient, Btu/hrfP"F u = Vapor velocity, ft/sec U d = Velocity under downcomer, ft/sec VD(js = Downcomer design velocity, GPM/fL Vioad = Column vapor load factor W = Condensate rate, Ibs/hr Xhk = Mol fraction of heavy key component Xlk = Mol fraction of the light key component a, = Relative volatility of component i versus the heavy key component... 

From the usual plate-type distillation and absorption columns, it was but natural to attempt to devise stagewise equipment for G/S processing, for better heat economy and better utilization of the contacting media. But instability of the downcomer for solids transfer between stages remained for many years an unsolved hydrodynamic problem, and even to this data, the stable operation of many solids downcomers still depends on mechanical devices. 

The vapor velocity in a finite-stage contactor column can be limited by the liquid handling capacity of the downcomers or by entrainment of liquid droplets in the rising gases. In most cases, however, downcomer limitations do not set the allowable vapor velocity instead, the common design basis for choosing allowable vapor velocities is a function of the amount of gas entrainment which can result in improper operation or flooding of the column. 

Plate Columns. A plate column consists of a series of plates or trays on which the liquid is retained for some period during its movement down the column. The rising vapor is bubbled through this liquid, providing intimate contact between the phases. Liquid in reflux moves downward between plates and is usually carried by a downcomer. Contact between the vapor and liquid takes place in stages. 

The multi-chamber column of Klein, Stage and Schultze [38] also has interna] downcomers. The vapour is distributed on each plate by a number of holes arranged in a circle, the intention being to produce small bubbles giving a thorough mixing of the liquid. Stage has also developed a bubble-cap column provided with. so-called... 

A schematic diagram of a distillation column is shown in Figure 7.33. The column contains a series of trays (also called plates or stages) which are usually perforated metal plates. Vapour passes up the column countercurrent to the liquid which flows across each tray and down the downcomers. As vapour bubbles through the liquid on a tray there is mass transfer between phases and... 

The design procedure sizes the column to prevent flooding caused by excessive entrainment. Flooding can also occur in the downcomers, and this case is discussed later. Excessive entrainment can also cause a large drop in stage efficiency because liquid that has not been separated is mixed with vapor. The effect of entrainment on the Murphree vapor efficiency can be estimated from... 

D3. The calculations in Example 10-3 were done for conditions at the top of the column. Physical properties will vary throughout the column, but columns are normally constructed with identical trays, downcomers, weirs, etc., on every stage (this is sinpler and cheaper). For a 12-foot diameter column, calculate entrainment, pressure drops, downcomer residence time, and weeping at the bottom of the column. The results of Problem 10.D2 are required. If the column will not operate, will it work if the gap between the tray and downcomer apron is increased to 1.5 inches ... 

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