Page 12 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

constant at constant liquid superficial velocity and the lines for dry and irrigated
packing in double logarithmic coordinates are parallel. The lines for co-current
and countercurrent flow coincide.
Under point A the gas velocity is very low and the influence of the
liquid velocity on the relative gas velocity cannot be neglected. In case of
countercurrent flow the effect of the liquid superficial velocity leads to
significant increasing of the relative gas velocity and, that is why, to increasing
of the ratio of the pressure drop of irrigated packing to the pressure drop of dry
packing. The increasing of the liquid superficial velocity leads to increasing of
this ratio. The opposite effect is observed at a co-current flow.
Over point B the influence of the friction force between gas and liquid
in case of countercurrent flow leads to increasing of the liquid holdup, i.e. to
additional increasing of the real gas velocity and of the pressure drop. That is
why after point B, called loading point, the slope of the line for irrigated
packing is greater than that for dry one. Just on the contrary, in case of co-
current flow, the friction between gas and liquid acts in the gravity direction, i.e.
in direction of decreasing of the holdup. That is why the slope of the line for
irrigated packing in this case is lower than that for dry one.
The increasing of the liquid holdup with the gas velocity for
countercurrent flow, and the increasing of the gas velocity with increasing of
the liquid holdup lead to additional increasing of the slope of the lines for the
pressure drop. For countercurrent flow in point C, called flooding point, this
leads to transition of the disperse phase into disperse medium and vice versa. In
case of co-current flow it leads to additional decreasing of the line slope in
comparison to that for dry packing.
As already mentioned, the greatest advantage of the packed bed column
is the possibility for carrying out the processes in a countercurrent flow.
The flow rates in case of a co-current flow, as expected and seen from
Fig. 2, are not limited by loading and flooding points. But this regime is not
proper in case of equilibrium because of the strong reduction of the driving
force. However, in industry there are a lot of processes for which the
equilibrium is not of importance. Just for these processes the co-current packed
bed columns are very proper. The maximal gas and liquid superficial velocity
for these apparatuses are limited only by economical considerations, namely by
the strong increasing of the pressure drop when increasing the gas velocity.
Besides the countercurrent and co-current flow regimes, the packed bed
column can operate in a crossflow. A principle scheme of such a column [41] is
presented in Fig. 3.