Vapor velocity tray efficiencies

In addition to the critical design factors for finite-stage contactors of number of theoretical trays, maximum allowable vapor velocity, column efficiency, and pressure drop as discussed earlier, a number of other factors are of importance in the development of the design. These factors are discussed in the following sections. 

This is not a good tray design, but it should operate. However, a reduced efficiency is to be expected due to low vapor velocities. 

These trays will dump liquid excessively through the perforations giving exceeding low efficiencies [47] unless a minimum vapor rate is maintained for a given liquid capacity. The smaller the holes the lower the dump point (vapor velocity). 

System limit flooding is similar to jet flooding, due to low surface tension and low density difference between liquid and vapor. Terminal velocity of some entrainment droplets is less than the upward vapor velocity, and hence they are carried up into the tray above, thus reducing tray efficiency and capacity. 

If a sieve, dual-flow, or grid-tray column is used, the only way to operate the column in a stable manner at the low initial flow rates is to blank offpart of the trays. This increases the vapor velocity through the mixing section, and assmes good contact and an efficient separation. These blanks can be removed at the time of the expansion. 

High vapor velocities, combined with high foam levels, will cause the spray height to hit the underside of the tray above. This causes mixing of the liquid from a lower tray, with the liquid on the upper tray. This backmixing of liquid reduces the separation, or tray efficiency, of a distillation tower. 

In order to improve tray efficiency, it will be necessary to increase the vapor velocity through the trays, so as to increase the pressure drop to at least 4 or 5 in of liquid per tray. If the reboiler duty were simply increased, the concentration of the heavy component—normal... 

Vapor Channeling All the correlations in this section assume an evenly distributed tray vapor. When the vapor preferentially channels through a tray region, premature entrainment flood and excessive entrainment take place due to a high vapor velocity in that region. At the same time, other regions become vapor-deficient and tend to weep, which lowers tray efficiency. 

Generally, trays work better in applications requiring high flows, because plate efficiencies increase with increased vapor velocities, and therefore increase the influence of the reflux to feed ratio on overhead composition. Column dynamics is a function of the number of trays, because the liquid on each tray must overflow its weir and work its way down the column. Therefore, a change in composition will not be seen at the bottoms of the tower until some time has passed. 

Under some conditions, vapor velocity maldistribution induced by hydraulic gradient nr tray tilt cam lead to excessive nonuniform weeping (183a Also, see Secs. 6.2.12, 6.2.13). Such excessive weeping can be detrimental to tray efficiency. 

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 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... 

If the column diameter is too large, vapor velocities will be low. The trays will operate at tray efficiencies lower than designed, and in severe cases they may not operate at all since liquid may durtp through the holes. Possible solutions include ... 

In general, the efficiency of a tray depends on the vapor velocity, which is illustrated schematically in... 

The best way to determine efficiency is to have data for the chemical system in the same type of column of the same size at the same vapor velocity. If velocity varies, then the efficiency will follow Figure 10-13. The Fractionation Research Institute (FRl) has reams of efficiency data, but until recently, most of the data were available to members only. Most large chemical and oil conpanies belong to FRI. The second best approach is to have efficiency data for the same chemical system but with a different type of tray. Much of the data available in the literature are for bubble-cap or sieve trays. Usually, the efficiency of valve trays is equal to or better than sieve tray efficiency, which is equal to or better than bubble-cap tray efficiency. Thus, if bubble-cap efficiencies are used for a valve tray column, the design will be conservative. The third best approach is to use efficiency data for a similar chemical system... 

Efficiencies can be scaled up from laboratory data taken with an Oldershaw column (a laboratory-scale sieve-tray column) tFair et al.. 1983 Kister. 1990T The overall efficiency measured in the Oldershaw column is often very close to the point efficiency measured in the large commercial column. This is illustrated in Figure 10-15. where the vapor velocity has been normalized with respect to the fraction of flooding IFair et al 19831. The point efficiency can be converted to Murphree and overall efficiencies once a model for the flow pattern on the tray has been adopted (see section 16.6T... 

The efficiency of valve tray depends upon the vapor velocity, the valve design, and the chemical system being distilled. Except at vapor flow rates near flooding, the efficiencies of valve trays are equal to or higher than sieve tray efficiencies, which are equal to or higher than bubble-cap tray efficiencies. Thus, the use of the efficiency correlations discussed earlier will result in a conservative design. 

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