Stagewise Contact in Perforated Plate Columns

Two partially miscible liquid phases may be contacted for extraction purposes in a discontinuons or stage-wise manner in cofuntus where the stages take the fomi or perforated pietes, fitted with dowacomers to facilitate flow of the continuous phase. Bubble-cap plates have been found ineffective in extraction, because of the lower density diffntunce. lower Interfacial tension, and higher vjsersity of the dispeise phase, compared to gas-liquid systems. 

When flow rates of the disperse phase are low, drop formation and detachment occur at the perforations on each plate. At higher flow rates, however, drops form at the lipa of jets emerging from the perforations. Either form of operation is accommodated by the design procndure, bat the jetting mode is the more desirable, since throughputs are higher and plate efficiency increases up to 2.5-frld,7 

In contrml to Section 7.3, where the two phases were symbolized by G and L wjihont regard to which was the disperse phase, it is now convenient to identify the contirreous and disperse phases as C and D, respectively, denoting the flow rales of ench phase through the column cross section in moles/tiare. 

This is the sum of the mass transfer rates to tha jets (subscript f), during drop formation at the tip of each jet (subscript /), during Tree rise (subscript r). and during coalescence (subscript c) henenth plate n -I- I. But 

Plates n + I and n in a performed plate liquid-liquid extraction column. 

Skelland and co-woikers have developed a procedure for the design of perforated plate extraction columns. This eliminates the need for experimentally measured stage efficiencies, which are usually costly and troublesome to obtain. Additionally, the validity of such efliciencies in scaled-up application is fir quently uncertain. Currently, the procedure involves use of rate equations for mass transfer during drop formation either at the perforations or at the end of jets issuing from the perforations, during ftee rise or fall of the drops, and during coalescence beneath Mcb i le, to locate a pseudoequilibtium curve. The latter is employed instead of the actual equilibrium curve on the - a distribution diagram in a stepwise 

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