Page 14 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

Because when the pressure is low the value of p G is very small in
comparison to the liquid density, and because the values of < sjp l for practically
all liquids, except mercury, are similar, equation (13) can be simplified to the
expression:

(14)

The value F G is also called vapour (gas) capacity factor. The maximal
permissible value of the capacity factor depends on a dimensionless flow
parameter iff * equal to the ratio of values proportional to the dynamic pressures
of the liquid and the gas phase, or to be precise, to the square root of this ratio.

f

G g and L g are the mass flow rates of gas and liquid phase in kg/s;
Ggs, and L m - the mol flow rates of gas and liquid phase in kmol/s;
and M& - the average mol masses of gas and liquid phase in kg/kmol.
In practice the following relation

Fg_
f(W f) (16)

is often used.
In literature the pressure drop is one of the most important
characteristics for comparison of different packings. It is because the cost of the
energy necessary for the ventilator for the gas phase to cover the pressure drop
is usually the main part of the energy operating costs for the absorption process.
In case of distillation when thermo pumps are used for reduction of energy
consumption, the increasing of the packing pressure drop leads also to
proportional increasing of the energy consumption. Usually the pressure drop is
presented as a function of the gas superficial velocity, or better, as a function of
the gas capacity factor.