Page 133 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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K Ga= w o(ln^--ln^) .-(11,-11,) (40)

and

r
(41)

Here A Koa is the end effect for the gas-side controlled mass transfer and the
additional indices, 1 and 2, are for the first and for the second packing height.
The mass transfer coefficient Kaa in Eq. (40) is free from the influence
of the end effect.
Before the method of Danckwerts for determination of the effective
surface area of the packings, the gas-side controlled mass transfer coefficient
was measured using sublimation of naphthalene from packings covered with
this material. The quantity of the sublimated naphthalene can be determined
measuring the weight of the elements before and after the experiments [80] and
measuring the concentration of the naphthalene [26, 81] by a
spectrophotometer.
Another type of a model system, used for determination of the gas-side
controlled mass transfer coefficient, is evaporation of liquids (usually water) in
a gas stream [82]. The disadvantage of this method is the strong dependence of
the equilibrium partial pressure of the evaporated liquid on the temperature, and
the fact that because of the high value of the evaporation heat, it is practically
impossible to carry out the process isothermally and therefore to determine
precisely the equilibrium partial pressure of the liquid. This leads to a
significant error of this method, especially in comparison with that of
nonequilibrium absorption accompanied by instantaneous chemical reaction.

2,1.2.4,2. Determination of the mass transfer coefficients for the equations of
the diffusion model
To determinate the mass transfer coefficients in a given model the
experimental results have to be calculated using the same model. The method
[74, 77] proper for packed bed columns for dispersion model is based on the
analytical solution of Sleicher [78], and Miyauchi and Vermeulen [79]. This
solution gives the possibility to calculate the concentration profile in the
packing, knowing the Bodenstein numbers and the mass transfer coefficient.
Using an iteration procedure, it is possible, at given initial and end
concentrations, to obtain also the respective mass transfer coefficient. The