Pressure drop, limiting velocity and calculation of column dimensions

11 Pressure drop, limiting velocity and calculation of column dimensions 

In the laboratory it is usual to employ columns having diameters of 10—30 mm for distillations at normal pressures and of 20—50 mm for distillations at reduced pressures. For very large throughputs, even at normal pressure, a diameter greater than 50 rnm is necessary. The recent view that a column should be loaded to just below the point of flooding makes it necessary to settle on its diameter beforehand, so that the desired throughput may be realized. 

Plate columns have the advantage that they can be used at very low loads and that the separating effect may then be a maximum, whilst packed columns require a minimum load to ensure a proper countercurrent exchange. In large-scale operation columns are often controlled by the pressure differential, so that flooding ma be avoided and an optimum separating effect maintained. The same method can be used with advantage for laboratory columns (c/. section 8.4.2). 

The term lower limiting velocity is used to denote the vapour velocit - (referred to unit cross section of the empty column) below which the effectiveness of the column begins to fall off, the term upper limiting velocity for the vapour velocitj at which flooding is so intense that, in plate columns, the layer of liquid on a j)late is entrained upwards, and in packed columns a quantity of spraying liquid rises from the foot of the column to the top. Since the fundamental publications by Mach [219], 

Type of packing Material Diameter Height Wall Surface Column 

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