Page 261 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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of the column [56-58, 74, 75] and thus the mass transfer coefficient too. The
main effect of the surfactants [59] is connected with the fact that the surface
tension of the moving film is not constant in all points. At the moment of
building of the new film surface, the surface tension is equal to that of the pure
liquid. With time the surfactant defuses from the bulk of the liquid to its surface
which leads to difference in the surface tension along the film. This difference
leads to a surface force which reduces the surface velocity. According to all
mass transfer models this leads to reduction of the mass transfer coefficient. For
a packed bed column there is an additional effect, namely covering of the
moving liquid film with very small bubbles which hinder the diffusion process.
The investigations about the influence of surfactants on the liquid-side
controlled mass transfer [59] are carried out with Raschig rings 26.5x25x4.1
mm and 16.3x15.5x0.5 mm of ceramics, respectively of glass. The results show
that up to the flooding point, the surfactants reduce the mass transfer coefficient,
and the reduction is smaller with increasing ofRe L. To calculate the volumetric
mass transfer coefficient in presence of surfactants, the following equation is
proposed [59];

(159)
10 Be,

where K La M is the mass transfer coefficient in presence of surfactants.
The constant A depends on the type of the surfactant and its
concentration, but not on the packing and can be obtained experimentally. This
gives the possibility to use the results obtained by experiments with a given
packing and surfactants to calculate a column with a different packing and the
same liquid.
Over the flooding point the adding of surfactants leads to increasing of
the mass transfer coefficient, when the comparison is made at the same liquid
superficial velocity. It is determined that the effect of the surfactants in this case
is connected only with increasing of the pressure drop. Le. the volumetric
liquid-side controlled mass transfer coefficient is equal to that for pure liquid,
when the comparison is made at the same liquid superficial velocity and the
same pressure drop. In both cases, with and without surfactants, the gas
velocities are different.

3.2.1.2.1.6.2.2. Coefficients for the diffusion model
Using the method described in part 2.1.2.4.2 and equation (133) for
calculating the Bodenstein, respectively the Peclet, number and also data for the