Efficiency, tray flow regime

Stirred tanks are modeled assuming that both phases are well mixed. Tray columns are usually modeled as well mixed on each tray so that the overall column is modeled as a series of two-phase, stirred tanks. (Distillation trays with tray efficiencies greater than 100% have some progressive flow within a tray.) When reaction is confined to a single, well-mixed phase, the flow regime for the other phase makes little difference but when the reacting phase approximates piston flow, the flow regime in the other phase must be considered. The important cases are where both phases approximate piston flow, either countercurrent or cocurrent. 

Prior to discussing scaleup, the factors that affect tray efficiency need to be addressed. These factors are addressed in Secs. 7.3.1 to 7.3.3. Considerations relevant to the effect of flow regime were previously discussed in Secs. 6.4.4 and 6.4.5. 

When trays are operating in the spray regime, different considerations apply. The liquid does not enter the downcomer by flowing over the outlet weir. Instead, it enters when liquid droplets suspended in the vapor space descend into the downcomer (Fig. 6.16). The closer the weir is to the holes, the easier it is for these suspended droplets to reach the downcomer. Further, the liquid pool on the tray floor near the outlet weir induces liquid backmixing onto the tray, which lowers tray efficiency (333). For these reasons, the outlet weir should be placed as close to the holes as possible for spray regime operation (90,... 

From Eq. (2-109) it can be seen that the tray efficiency g , with no or very poor liquid mixing, is always larger than the point efficiency Eg. A connection between plug flow and ideal mixing in the two phase regime... 

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