Dumping, tray deck

As the weir height of the trays is 3 in, it is a safe assumption that the low tray efficiency is due to tray deck dumping, rather than flooding. As shown in Fig. 3.3, this column has no reflux. This is a typical design for strippers when feed is introduced on the top tray, there is no need for reflux. 

The first two factors help make fractionation better, the last factor makes fractionation worse. How can an operator select the optimum tower pressure, to maximize the benefits of enhanced relative volatility, and reduced tray deck dumping, without unduly promoting jet flooding due to entrainment ... 

The problem we have just discussed—poor fractionation efficiency due to inadequate vapor and liquid initial distribution—is rather similar to tray deck dumping in trayed fractionators. And, just like trays, packed towers are also subject to flooding. 

As discussed in Chap. 2, tray deck dumping also greatly reduces tray efficiency. Unfortunately, steam strippers can have widely varying vapor rates, between the top and bottom trays of a column. 

The vapor flowing between trays was at its dew point. A sudden increase in tower pressure caused a rapid condensation of this vapor and a loss in vapor velocity through the tray deck holes. The resulting loss in vapor flow caused the tray decks to dump. 

The net effect of reducing the stripper pressure was to greatly reduce the amount of isobutane in the heavier normal butane bottoms product. Undoubtedly, most of the improvement in fractionation was due to enhanced tray efficiency, which resulted from suppressing tray deck leaking, or dumping. But there was a secondary benefit of reducing tower pressure increased relative volatility. 

Trays suffer from lost tray efficiency as a result of both flooding and dumping. Trays have some entrained droplets of liquid lifted by the flowing vapors to the trays above. This tends to blow butane up into the lighter propane product. Perforated trays always have some leakage of liquid through the tray deck to the trays below. This tends to drip propane down into the heavier butane product. 

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