Effect of weir height

In distillation systems, the improvement of tray efficiency due to taller weirs is small (5). Koch Engineering (8), Kreis and Raab (28), and Kalbassi 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 efficiency increase of the order of 5 to 10 percent as weir height is raised from 1 to 3 in a similar increase was reported by Prado and Fair (110,144) in humidification and stripping tests. 

Clear liquid height at the froth-to-spray transition, corrected for the effect of weir height on entrainment, given by Eq. (6.29), in of liquid. 

Effects of weir height (reactive holdup) on the TAC with 10-cm weir height as... 

Taller weirs (28,48,63). Lockett and Banik (56) observed that taller weirs increased the weeping tendency, except (1) at high liquid rates and high weirs (2) at high vapor rates, low liquid rates, and low weirs. In both of these exceptions, weir height had little effect on weeping. 

Determine the effects of the physical properties of the system on column efficiency. Tray efficiency is a function of (1) physical properties of the system, such as viscosity, surface tension, relative volatility, and diffusivity (2) tray hydraulics, such as liquid height, hole size, fraction of tray area open, length of liquid flow path, and weir configuration and (3) degree of separation of the liquid and vapor streams leaving the tray. Overall column efficiency is based on the same factors, but will ordinarily be less than individual-tray efficiency. 

Umholtz, C. L. and M. Van Winkle, Effect of Hole Free Area, Hole Diameter, Hole Spacing Weir Height, and Downcomer Area, Pet. Ref. V. 34, p. 114 (1955). 

The weir height, 7/w. usually varies from 1 to 3 in. (2.5-7.5 cm). Two inches (5 cm) is a typical value. For heights above 15 /o of the tray spacing reduce the effective tray spacing for jet flood by the excess over 15 /o ... 

Picket fence weirs are used in low-liquid-rate applications (Fig. 8). Picket fence weirs can serve two purposes at low liquid rates. First, they reduce the effective length of the weir for liquid flow increases the liquid height over the weir. This makes tray operation less sensitive to out-of-level installation. Second, pickets can prevent liquid loss (blowing) into the downcomer by spraying. This occurs at low liquid rates when the vapor is the continuous phase on the tray deck. Picket fence weirs should be considered if the liquid load is less than 1 gpm per inch of weir (0.0267 ft /sec/ft, 0.00248 m /sec/m). At liquid rates lower than 0.25 gpm per inch of weir (0.00668 ft / sec/ft, 0.000620 m /sec/m) even picket fence weirs and splash baffles have a mixed record in improving tray efficiency. Operation at liquid rates this low strongly favors the selection of structured packing. 

The effects of total pressure, height of submergence, and height of empty section on the number of plates have been studied as a function of the outgoing NO concentration (up to 20 ppm level). Figures 3-A, 3-B, and 3-C show the effect of height of empty section at weir heights of 0.025,... 

The pressure drop is assumed to remain constant after the scale-down because the weir height is not changed. In practice, there is an additional contribution to the pressure drop due to gas flow through the tray orifices. This would change if column flows changed, but the pressure drop through the orifices is small and should be a minor effect. Examples 19.4 and 19.5 illustrate how the performance of the reboiler... 

Hydraulic head In the perforated region of the tray, the liquid is in the form of a froth. The equivalent depth of clear liquid is an estimate of that which would obtain if the froth collapsed. That is usually less than the height of the outlet weir, decreasing with increased gas rale. Some methods of estimating A use a specific aeration factor to describe this [87). In Eq. (6.38), which is the recommended relationship (49), the effect of the factor is included as a function of the variables... 

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