Other Factors Affecting Tray Efficiency

Weir height. Taller weirs raise liquid level on the tray in the froth and emulsion regimes. This increases interfacial area (1, 137) and vapor contact time. Larger interfacial areas and contact times enhance efficiency, especially when the mass transfer resistance is concentrated in the vapor (most distillation systems). In the spray regime, weir hei t affects neither liquid level nor efficiency (Sec. 6.4.4). 

In distillation systems, the improvement of tray efficiency due to taller weirs is sm l (6), Koch Engineering (8), Ereis and R b (28), and Kalbassl et al. (184) observed little effect of weir height on distillation tray efficiency for weirs 1.5 to 3 in, 1 to 2 in, and 0,5 to 1 in tall, respectively. Finch and Van Winkle (185) reported an efficieniy increase of the order of 5 to 10 percent as weir height is raised from 1 to 3 in a similar increeise was reported by Prado and Fair (110,144) in humidification and stripping tests. 

Length of liquid flow path. Longer liquid flow paths enhance the liquid-vapor contact time, the significance of liquid plug flow, and, therefore, raise efficiency. However, flow-path increases are coupled with col- 

Fractional hole area. Efficiency increases with a reduction in fractional hole area (23,28,110,144,186). Yan and Sakata (23), experimenting with commerdal-scale towers, show a 10 to 15 percent increase in tray efficiency when firactional hole area was lowered from 14 to 8 percent of the bubbling area (Fig. 7.10a). Kreis and Raab (28j show an identical increase for N2/O2 separation, and an even larger increase (20 to 25 percent) when fractional hole area was lowered from 8 to 5 percent of the bubbling area. Prado and Fair (110,144) showed an efficiency increase of the Older of 5 percent as fractional hole area was reduced from 11 to 6 percent of the bubbling area in humidification and stripping tests. The above data were collected both in the froth and spray regimes. 

Vapor4Iqukl loads. A higher vapor load reduces the vapor contact time but also increases the interfadal area (136,137). These two factors have counteracting effects on tray effidency. Usuedly, the x ntact time dominates, and effidency decreases with hi er vapor rates (185). A higher liquid load increases tray effidency (185) because it increases tray liquid holdup, and therefore vapor contact time. 

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Errors in relative volatility are the most underrated factor that affects both tray and packing efficiency. The effects are direct when VLE errors affect separation stage requirement at a constant reflux ratio, and indirect when VLE errors affect the reflux ratio requirement (which in turn affects the stage requirement). Since higher relative volatility lowers both stage and reflux requirements (and vice versa), the direct and indirect effects complement each other and do not counteract each other. The discussion below applies to hoth tray and packed towers. 

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