Pressure drop in trays

P = Pressure drop in inches of water SG = Specific gravity of the liquid on the tray at the appropriate temperature T = Number of trays T, = Tray spacing, in. 

Fv = valve tray F-factor, ft- /min/valve Fvm = valve tray F-factor at the beginning of the valve open region, fr /min/valve g = gravitational constant, ft/s he = clear liquid height, in. ho = dry tray pressure drop, in. 

This is based on the correlation of Mayfield [45] where hjt (weep) = dry tray pressure drop at tray weep point, in. liquid. 

Vh = vapor velocity through valve holes, ft/sec P = tray aeration factor, dimensionless AP = tray pressure drop, in. liquid pvm = valve metal density, tj = tray deck thickness, in. 

Total vapor pressure drop per tray, in. liquid (wet tray)... 

Actual tray pressure drop, in. liquid Prandtl number dimensionless Fractional opening in the circumference or a valve or. Pi... 

Dauphine, T. C. Pressure Drops in Bubble Trays, Sc. D. Thesis, Mass. Inst. Technology (1939). 

Fair reports that the data for mass transfer in spray, packed, and tray columns can be used for heat-transfer calculations for these columns. The pressure drop in these types of columns is usually quite low. 

In a packed column, liquid and vapor flow counter-currently and separation between the liquid and vapor phases takes place continuously. In contrast, in a column with trays, separation occurs in stages. In a packed column, vapor does not bubble through the liquid as in the columns with trays. For this reason, and due to the absence of the vapor-flow orifices, packed columns operate at a much lower pressure drop. In addition, because liquid and vapor contact in a packed column is less agitated than in a trayed column, packed columns are less likely to foam. 

This approximate approach is admittedly crude, but I have used it quite effectively for several distillation simulations. At each point in time the pressure Pp at the top of the column is calculated from Eq. (5.33), and new pressures on all the trays are calculated using a constant pressure drop per tray. 

The reason for the disparity in performance of such devices in the two services has been clearly outlined by Hachmuth (HI). Bubble-tray towers for distillation, for example, use as the source of energy for dispersion of the gas and for developing the desirable turbulent flow conditions both the expansion of the vapor as it experiences a pressure drop in flowing through the tray, and the liquid head available between trays. In liquid extraction only the liquid head is available. When it is considered that the difference in densities of the contacted phases in distillation may be of the order of 50 to 60 lb./cu. ft., whereas in extraction it is more likely to be of the order of 5 or less, it is easy to understand that in the latter case there is simply insufficient energy available from this source to provide for adequate dispersion and interphase movement. Interfacial area between phases remains small, turbulences developed are of a low order, and mass transfer rates are disappointingly small. 

It is a characteristic of process equipment, that the best operation is reached, at neither a very high nor a very low loading. The intermediate equipment load that results in the most efficient operation is called the the best efficiency point. For distillation trays, the incipient flood point corresponds to the best efficiency point. We have correlated this best efficiency point, for valve and sieve trays, as compared to the measured pressure drops in many chemical plant and refinery distillation towers. We have derived the following formula ... 

A field measurement indicated a pressure drop of 2.0 psi. Assuming a specific gravity of 0.50, then the pressure drop per tray, in inches of liquid is ... 

The narrow-trough vapor distributor shown in Fig. 7.4 is intended to disperse the vapor evenly across the bottom of the packed bed. The width of the chimney does not exceed 6 in. The older-style chimney trays, which may have had a few large round or square chimneys, reduced the separation efficiency of the packing. To work properly, the vapor distributor has to have a reasonable pressure drop, in comparison to the pressure drop of the packed bed. For example, if the expected pressure drop of a 12-ft packed bed is 10 in of liquid, the pressure drop of the vapor distributor ought to be about 3 to 4 in of liquid. 

Reduce the pressure drop in the column by using packings instead of trays, if possible. 

Several factors are required first in order to calculate the tray pressure drop in inches of clear liquid. This liquid pressure unit is that liquid density referred to as liquid density used DI, lb/ft3. First calculate the weir length Lm ... 

The columns pressure deserves special attention. The top of both columns is set at 0.066bar with a pressure drop per tray at 500 Pa, a conservative value for vacuum operation. The total pressure drop is accounted for in real stages with 75% overall efficiency. Koch flexitrays with 4 passes are adopted as internals, since these give smaller diameter. However, relatively large values are obtained, of 2.5 and 3.2 m for the bottom and top trays respectively, due to the high throughput and deep vacuum. 

Pressure drop. Wet pressure drop on the tray is determined by the resistance of the aerated mass to vapor flow (Sec. 6.3.3). As the nature of the dispersion is different, one would expect a different mechanism to cause pressure drop in the spray regime. This has been confirmed by experiments (88,114). To date, not enough is known about the nature of this mechanism. 

Bolles (191) correlated the reduction in efficiency in terms of the distribution ratio, i.e., the maximum-pass LfV ratio divided by the minimum-pass LfV ratio. The L and V for each pass are determined from the normal pressure balance and hydraulic relationships, applied to each pass. At high distribution ratios, a substantial drop in tray efficiency occurs. Bolles shows that if this distribution ratio is kept lower than 1.2, the loss in efficiency due to maldistribution is negligible. Bolles recommends designing multipass trays for such low distribution ratios. Detailed guidelines for achieving low distribution ratios (<1.2), thus minimizing the effects of pass maldistribution on efficiency, are contained in a companion book (1) and in Bolles s paper (191). 

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