Entrainment Effect on Tray Efficiency

The extent of entrainment of the liquid by the vapour rising over a plate has been studied by many workers. The entrainment has been found to vary with the vapour velocity in the slot or perforation, and the spacing used. Strang 60-1, using an air-water system, found that entrainment was small until a critical vapour velocity was reached, above which it increased rapidly. Similar results from Peavy and Baker 6 11 and Colburn 62 have shown the effect on tray efficiency, which is not seriously affected until the entrainment exceeds 0.1 kmol of liquid per kmol of vapour. The entrainment on sieve trays is discussed in Section 11.10.4. 

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. 

Lockett et al. (12,128,129) present rigorous methods for allowing for the effects of entrainment and weeping. Colwell and O Bara (58) and Banik (57) also present rigorous methods for allowing for the effects of weeping on tray efficiency. 

Eduljee s [19] correlation of literature data appears to offer a route to evaluating the effect of entrainment on tray spacing and efficiency. It is suggested as another check on other methods. Figure 8-117 may be used as recommended ... 

Hole Sizes Small holes slightly enhance tray capacity when limited by entrainment flood. Reducing sieve hole diameters from 13 to 5 mm ( to 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. 

The dry Murphree efficiency calculated thus far takes into account the vapor and liquid resistances and the vapor-liquid contact patterns, but is uncorrected for the effects of entrainment and weeping. This correction converts the dry efficiency to a "wet or actual Murphree tray efficiency. Colburn [Eq. (14-98), under "Entrainment ] incorporated the effect of entrainment on efficiency, assuming perfect mixing of liquid on the tray. 

This means both vapor and liquid loadB are raised and lowered simultaneously. Increasing vapor rate reduces efficiency, while increasing liquid rates raises efficiency. The two effects normally cancel each other, and efficiency is practically independent of load changes (assuming no excessive entrainment or weeping). Figure 7.106 shows a typical dependence of tray efficiency on vapor and liquid loads for a commercial-scale distillation column. Anderson et al. (97) show a similar dependence for several different valve trays. 

Entrainment has an adverse effect on the column operation in that it reduces the concentration change per tray and, consequently, decreases the efficiency. Nonvolatile impurities can be carried up a tower by entrainment, resulting in off-color or impure overhead products. 

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. 

The effect of entrainment on separating efficiency depends on each case. In general, entrainment is more harmful in rectifying sections than in stripping sections, because it carries less volatile components toward the draw point for the volatile product. It is, in fact, possible to estimate the reduction in tray efficiency caused by a given amount of entrainment. The amount can be estimated roughly by the method of Simkin, Strand, and Olney. ... 

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

Problem 12.6 deals with the dimensioning of a large ethylbenzene/styrene tray-type fractionator. The overall column efficiency for the sieve trays is to be estimated, based on the same conditions given for Problem 12.6. Two methods will be used (a) the O Connell (Figure 12.60) approximate method and (b) the Chan/Fair more basic method [Equation (12.84)], corrected for entrainment and liquid crossflow effects. 

Figure 5.7-12 appears to be useful for estimating valve tray entrainment, although tha available daia are scarce.1 Absolute values of entrainment obtained from the figure mey not be very accurate, but the predicted effect of the entrainment on efficiency appears quite reliable,... 

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