In vacuum distillation, excessive pressure drop causes excessive bottom temperatures which, in turn, increase degradation, polymerization, coking, and fouling, and also loads up the column, vacuum system, and reboiler. In the suction of a compressor, excessive pressure drop increases the compressor size and energy usage. Such services attempt to minimize tray pressure drop. Methods for estimating pressure drops are similar for most conventional trays. The total pressure drop across a tray is given by ht = hd + hL
where ht = total pressure drop, mm liquid hd = pressure drop across the dispersion unit (dry hole for sieve trays; dry valve for valve trays), mm liquid h'L = pressure drop through aerated mass over and around the disperser, mm liquid
It is convenient and consistent to relate all of these pressure-drop terms to height of equivalent clear liquid (deaerated basis) on the tray, in either millimeters or inches of liquid.
Pressure drop across the disperser is calculated by variations of the standard orifice equation:
where Uh = linear gas velocity through slots (valve trays) or perforations (sieve tray), m/s.
For sieve trays, K = 50.8/C;;. Values of Cv are taken from Fig. 14-35. Values from Fig. 14-35 may be calculated from
For Sulze's fixed valve trays, Summers and van Sinderen (Distillation 2001: Topical Conference Proceedings, AIChE Spring National Meeting, p. 444, Houston, April 22-26, 2001) provided the following equation for K:
for SVG and LVG fixed valves (14-103b)
Figure 14-36 illustrates the pressure drop of a typical moving valve tray as a function of gas velocity. At low velocities, all valves are closed. Gas rises through the crevices between the valves and the tray deck, with increasing pressure drop as the gas velocity rises. Once point A, the closed balance point (CBP), is reached, some valves begin to open. Upon further increase in gas velocity, more valves open until point B, the open balance point (OBP), is reached. Between points A and B, gas flow area increases with gas velocity, keeping pressure drop constant. Further increases in gas velocity increase pressure drop similar to that in a sieve tray.
The term K in Eq. (14-101) depends on valve slot area, orifice geometry, deck thickness, and the type, shape, and weight of the valves. These are best obtained from the manufacturer's literature,
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