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Eog,wet = Eog1 +Eog(y/(1 2 y))(17)
where y is the fraction of the liquid entering the collector zone that is not removed (i.e., the fraction that is entrained to the tray above). It should be noted that entrained liquid is considered to be recycled as part of the downflow from the tray above and thus adds to the liquid/vapour ratio on the tray under consideration. While there is the expected detrimental effect of the recycle on efficiency, this is partly compensated by the additional liquid dispersion (and increased interfacial area).
Collector Efficiency
The collector comprises a series of collector/baffles, as shown in Figure 1. A standard arrangement of these units has evolved through both distillation and air-water simulation tests. The dimensions for this arrangement, for two adjacent levels of collector troughs, are shown in Figure 3. For this arrangement, and three rows (vertically), collection efficiencies, as measured, are shown in Figure 4. These data are for a superficial F-factor of 8.3 m/s(kg/m3)0.5; lower values of the F-factor appear to have only a minor effect on
Figure 3. Standard dimensions of collector elements.
collection efficiency, so long as the vapour flow rate is above the rate for the onset of weeping. On the other hand, liquid rate has a definite influence on collector efficiency, as may be observed in Figure 4. For the range of liquid rates shown, the collection efficiency may be represented by
gs
where g0 =overall collection efficiency (fractional)
g1 =collection efficiency for one row (fractional)
N =number of rows
It has been found that three rows of collectors are normally sufficient. For a single row, air-water tests show
Gs
where L9 =mass flow rate of liquid through the collector openings, kg s2 1m2 2 . Combining equations (18) and (19) gives
gs
Equation (20), based on air-water data, appears to apply equally well to the hydrocarbon distillation conditions described in Figure 6. At very high liquid loadings the collection efficiency declines and at some point the equivalent of ‘flooding’ occurs. The high entrainment recycle then underscores its deleterious effect on efficiency.
Figure 4. Removal efficiency of collector elements.
Pressure Drop
The pressure drop across a cocurrent tray may be taken as the sum of the ‘dry drop’, static head of liquid in the contacting zone, and the drop for two-phase flow through the collector elements. In terms of height (in mm) of clear liquid (density = rL) being processed,
Gs
The dry drop hd may be computed from the conventional orifice relationship:
Gs
where the orifice coefficient depends on the geometry of the inlet liquid dispersion devices. For perforated channels with rounded entries for the gas flow, Cv< 0.90; however, the value of the coefficient is adjustable, depending on the method of liquid dispersion into the upflowing vapour.
The pressure loss through the contacting zone is based on the equivalent clear liquid contained in the dispersion:
Gs
with ∅L obtained from equation (7).
The pressure loss for flow through the collectors is complicated by the changing ratio of liquid to vapour as liquid removal occurs. One approach is to consider hc as a residual, and make use of the measured overall pressure drop coupled with the calculated values of hd and hL (equations (22) and (23)). A more rigorous approach is to account for changing liquid/vapour ratio for flow through the collector, changes of direction, and energy dissipation resulting from impact on the collector elements; this approach has not yet been investigated. Experimental results, shown later, indicate 70 –80% of the total tray pressure drop occurs in the collector section.
Weeping Dumping
Weeping or dumping occurs when there is insufficient pressure loss through the liquid disperser to maintain a head of liquid over the dispersers, i.e. in the contacting zone. Again, technology developed for crossflow sieve trays may be adapted, since the mechanism should be the same for either device. From sieve tray weeping correlations (Fair, 1997)12 , the dry pressure drop at the incipient weep point hd is obtained from