Page 115 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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5. Method using x- or y-rays.
According to the first method [11-12] the whole column is weighted
with dry and with irrigated packing. The difference between the two
measurements gives the weight of the liquid phase in the packing, i.e. the
holdup. The method allows determining precisely both static and dynamic
holdups. Its disadvantages are difficulties connected with operating with a big
column, and lower measurement accuracy in presence of gas flow. In this case
the product of the pressure drop and the column cross-section is force acting
against the gravity.
According to the second method [1,14-16], called also draining method
[87], the liquid phase inlet and outlet are stopped abruptly. When the liquid
level in the bottom of the column stops changing, it is measured. The level is
measured also before the stopping. From the two measurements the volume of
the liquid in the packing is obtained. To be precise, not only the liquid volume
in the packing is obtained, but also that in the form of jets and drops between
the liquid distributor and the packing, and between the packing and the liquid
surface in the bottom of the column (end effect). To take into account this liquid
end effect, measurements are made at two different heights of the packing for a
given liquid superficial velocity. The dynamic holdup is calculated easily from
the two measurements as follows:

V -V
= j 2 , (2)
v -V

where F/ and Vj are the volumes of the liquid measured in the two experiments
3
inm ;
3
V pi and V P2 — the corresponding packing volumes in m .
From Eq. (2) it is easy to see that to increase the exactness of the
experiment we should provide greater value of V pi and smaller value of V p2,
From the same experimental results it is possible to obtain also the end
effect H e using the equation:

H^Vt-V^H,, (3)
or

H e=V 3-V p2H d. (4)