Total tray pressure, decrease

Multichordal downcomers can increase weir length (Fig. 6). Fig. 6 shows a combined multichordal and stepped downcomer. The increased weir length decreases the liquid crest height over the weir. This decreases total tray pressure drop. The stepping is a variation of a sloped downcomer. The step increases the downcomer inlet area with a minimum decrease in tray active area. 

Because of the flow resistance of the caps and risers in bubble-cap columns, there is a decrease in liquid depth as the liquid passes across the tray. Figure 16-4 shows an extreme example in which this liquid gradient is so great that only one out of the form bubble caps is operating normally. In general, the dimensionless ratio of total liquid gradient to pressure-drop head caused by the bubble-cap assembly should be less than 0.4 in order to ensure adequate vapor distribution, and, for single-pass crossflow trays, the rate of liquid flow across the tray should be less than 0.22 ft3/(sXft) diameter. 

The effect of furfural rate on the separation of isobutane and 1-butene was determined by simulation and is shown in Figure 10.7. The column has 23 theoretical stages, a total condenser, and a reboiler. The main feed, containing 40 mole percent isobutane and 60 mole percent 1-butene, is sent to the 11th tray from the top, and the solvent is introduced at the third tray. The column operates at 3580 kPa pressure and a temperature ranging from 40.5°C at the condenser to 42°C in the reboiler. The overhead rate is maintained at 40 mole% of the isobutane-l-butene feed rate, and the column reflux ratio is maintained at 3.0. The solvent-to-feed ratio is increased from 0 to 3, and the resulting separation is observed. Figure 10.7 shows a plot of the concentration of 1-butene in the overhead and isobutane in the bottoms as a function of the solvent-to-feed ratio. These concentrations decrease sharply as the solvent ratio is increased from low values but tend to level off asymptotically at a ratio of about 2 to 3. 

---