1) PLATE TOWERS FOR GAS ABSORPTION
Bubble-cap columns or sieve trays, of similar construction to those described in on distillation, sometimes used for gas absorption, particularly when the load is more than can be handled in a packed tower of about 1 m diameter and when there is any likelihood of deposition of solids which would quickly choke a packing. Plate towers are particularly useful when the liquid rate is sufficient to flood a packed tower. Since the ratio of liquid rate to gas rate is greater than with distillation, the slot area will be rather less and the downcomers rather larger. On the whole, plate efficiencies have been found to be less than with the distillation equipment, and to range from 20 to 80 per cent. The plate column is a common type of equipment for large installations, although when the diameter of the column is less than 2 m, packed columns are more often used. For the handling of very corrosive fluids, packed columns are frequently preferred for larger units. The essential arrangement of such a unit is shown in Figure 4, where:
L m is the molar rate of flow per unit area of solute free liquid,
G m is the molar rate of flow per unit area of inert gas,
n refers to the plate numbered from the bottom upwards (and suffix n refers to material leaving
plate n),
x is the mole fraction of the absorbed component in the liquid,
y is the mole fraction of the absorbed component in the gas, and
s is the total number of plates in the column
Figure 5 Plate tower—nomenclature for fluid streams
A material balance for the absorbed component from the bottom to a plane above plate
n gives:
(32)
Or (33)
If the equilibrium curve can be represented by the relation ye = mx, then the number of
plates required for a given degree of absorption can conveniently be found by a method
due to KREMSER and SOUDERS and BROWN. The same treatment is applicable for
concentrated solutions provided concentrations are expressed as mole ratios, and if the
equilibrium curve can be represented approximately by Ye = mX.
Figure 6 Diagrammatic representation of changes in a plate column
A material balance over plate n gives:
(34)
For an ideal plate, yn = mxn;
(35)
Applying this relation over the whole column and putting n = s gives:
(y0 ? ys) = actual change in composition of gas, and
(y0 ? ys+1) = maximum possible change in composition of gas, that is if the gas leaving
the absorber is in equilibrium with the entering liquid (or ys = mxs+1).
(36)
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