Efficiency, tray liquid entrainment

The tower diameter may be calculated for first approximation by the Souders-Brown method however, this has been found to be conservative, since it is based on no liquid entrainment between trays. Actually, some entrainment can be tolerated at negligible loss in efficiency or capacity. 

Jet Jlooding occurs due to liquid entrainment induced by vaporjets passing through the liquid flowing on the tray. The entrained droplet may carry into the tray area above and reduce tray efficiency and capacity. 

Entrainment (Fig. 14-33) is liquid transported by the gas to the tray above. As the lower tray liquid is richer with the less-volatile components, entrainment counteracts the mass-transfer process, reducing tray efficiency. At times entrainment may transport nonvolatile impurities upward to contaminate the tower overhead product, or damage rotating machinery located in the path of the overhead gas. 

Downcomer vapor underflow ("vapor entrainment" or "gas recycle1 ) is analogous to liquid entrainment. It reduces both tray capacity and efficiency (17,44,45). In low- and medium-pressure distillation systems, where gas density is significantly lower than liquid density, it takes only a small quantity of gas to generate volumes comparable to the liquid volumetric flow rate. The quantity of gas recycle is therefore small, and it has little effect on tray performance. At high pressures, the quantity of gas recycled is significant. An analysis of some FRI data (44) for iC4-nC4 distillation showed vapor entrainment increases from about 7 percent at 165 psia to about 50 to 60 percent at 400 psia on a molar basis. 

Entrainment. If entrainment is excessive, column diameter or tray spacing are usually increased. It has been recommended (2,67) that entrainment from the tray should not exceed about 0.10 lb liquid entrained per pound of liquid flow. At higher values, significant efficiency reduction occurs (34). Depending on the service, a lower or higher value can be set (4). For instance, if the column overhead stream is compressed and no knock-out drum is present, the entrainment that can be tolerated is smaller. Also, for trays operating at a high liquid-to-vapor ratio, 0.1 lb of liquid entrained per pound of liquid is an excessive quantify of entrained liquid, and a lower limit is set. 

Liquid and vapor entrainment. Both represent a recycling of lower-purity materia] which contaminates the tray liquid or vapor, both counteract the mass transfer process and lower efficiency. Liquid and vapor entrainment are discussed in Secs. 6.2.11 and 6.4.5, respectively,... 

Entrainment Corrections. Above about 80% of flood, the recirculation of liquid as entrainment between trays undermines the countercurrent action of the tray column, and efficiency therefore suffers. This is a particular problem in vacuum distillation where it may be optimum to allow a certain amount of liquid entrainment in initial design. Figure 13.41 shows an approximate method for entrainment correction to column efficiency or Murphree efficiency. The abscissa scale is the same parameter used for flooding prediction (Figure 13.32(b)). The ordinate value is used to correct from a dry to a wet efficiency (with entrainment) ... 

With increasing vapor rate, the oscillations become more violent, and liquid entrainment increases up to 70%, decreasing the tray efficiency. On sieve trays, extra weep ing occurs up to 150% compared to a stable tray. Full-wave oscillation is represented by a peak wave forming along the center of the tray with a trough at each wall. This position then reverses itself, and is called full-wave oscillation. The full-wave occurs at lower vapor rates than halfwave oscillation. Increases in entrainment and weeping also occur, and are most likely to be characteristic of medium- to smaU-sized columns, particularly those operating at reduced pressure. 

The effects of tray leakage and liquid entrainment in the vapor stream may need to be taken into account. For bubble cap trays, tray leakage is normally of no significance. Entrainment, on the other hand, can decrease separation markedly. Quantitative prediction of entrainment is sometimes possible however, the effect of entrainment is usually combined with the effect of vapor velocity on local efficiency, using the results of experimental full-scale studies, such as those performed by FRI. 

Liquid entrainment can adversely affect the tray efficiency. As liquid is entrained from one tray to the tray above, the entrained liquid, e, joins the normally flowing liquid, L, resulting in a total liquid flow of L + e. The fractional entrainment z is deflned as the ratio of entrained liquid to total liquid flow ... 

The tray efficiency is lowered by liquid entrainment because higher-boiling liquid is carried to a tray containing lower-boiling liquid, thereby countering the fractionation process. The lowering of the Murphree tray efficiency is expressed by the Colburn equation (Colburn, 1936) ... 

Murphree tray efficiency with entrainment Murphree point efficiency in liquid terms Murphree tray efficiency in liquid terms Murphree tray efficiency in vapor terms Overall tray efficiency... 

The performance curve of a crossflow tray shows a lowering of efficiency as vapor velocity approaches a flooding value (Figure 12.27). This lowering results from liquid entrainment. Figure... 

Reference has been made to liquid entrainment that occurs in increasing amounts as (he flood point is approached. The entrained liquid is recycled back to the tray abuve, negating the efleet of countercurrent contact]ag and decreasing tray efficiency. The recirculation dne to entrainment is shown in Fig. 5.7-11. 

Orifice Froude number = 1.11 > 0.5 no excessive weeping Fractional liquid entrainment = 0.024 Point efficiency = 0.760 Murphree tray efficiency = 0.795... 

Fig. 7.9 summarizes the hardware design of the RD column. Despite the large column diameter required in the reactive section (5.7 m), it is interesting to note that only 25 % of the column volume is available for holding up the liquid and carrying out the reaction. The rest of the column volume is taken up by freeboard above the tray that is necessary to minimize liquid entrainment to the tray above that, causes flooding. Distillation tray columns are not efficient devices for carrying out slow liquid-phase reactions. 

Gas flowing through the two-phase layer on a tray always entrains some liquid in form of small droplets. This liquid entrainment is unfavorable because it affects the countercurrent flow of gas and liquid within the column and, in turn, the separation efficiency of the column. Figure 5.4-6 presents an empirical correlation that summarizes nearly all data published in literature so far (Stichlmair 1978 Stichlmair and Fair 1998). 

FIGURE 5.7-12 Chart for estimating effect of liquid entrainment on crossflow tray efficiency. 

Also, a liquid entrainment check is usually made to ensure it does not exceed acceptable limits. The fractional entrainment is a function of liquid and vapor flows and densities and of the flood factor. The column should not be operated at high fractional entrainment because this reduces the tray efficiency. 

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