Plate columns hydraulic gradient

Figure 11 shows a t pical liquid collector plate for a column that uses one side downcomer to withdraw the liquid. The maximum diameter for such a design is about 12 ft, which is limited by the hydraulic gradient necessary for such a liquid flow-path length, For larger diameter columns, two opposite side downcomers or a center downcomer normally is used unless the total amount of liquid collected is relatively small. 

Khamdi, A. M., A. I. Skoblo, and Yu K. Molokanov, "Some Questions Concerning the Hydraulic Gradients of Plate Columns, Khim Tekhnol Topliv Mosel 8(2), 1963, p. 31. 

The distillation operation is embodied in a vertical column to take advantage of the difference in density between the liquid and vapor phases. Thus, in stagewise distillation, the more-dense liquid phase L moves down vertically via downcomers at the side of the plate and next to the column wall. The liquid phase is thereby introduced at or on each plate and flows across the plate also by gravity for example, by virtue of the hydraulic gradient that builds up in the downcomer. Meanwhile, the less-dense vapor phase V moves upward, passing up and through the liquid on the plate via holes or bubble caps (or nozzles) and overall moves countercurrently to the liquid phase, as represented schematically in Figure 4.1. 

In small-diameter columns, D/Ha is usually so small that the effect of cross flow is negligible, but in large columns it may become so great that the kinetic head equivalent to Vcf may be significant in terms of liquid head. Under these conditions, the pressure in the vapor space is not constant across the cross section, and its variation is such that it forces an increase in the hydraulic gradient. In order to make Hs as large as possible, any beams or projection on the bottom of a plate should be positioned to aid the vapor cross flow. Good vapor distribution on the plate will eliminate the effect of vapor cross flow. 

Various modifications have been made to improve the operation of perforated plates. One of these modifications involves adding distribution slots or louvers across the plate to minimize the hydraulic gradient of the liquid flow for large diameter columns. Another modification which extends the operating range of perforated plates is to install self-regulating valves or caps over larger... 

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