Page 143 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

138

A,=H la el-H la. (73)

2.1,2.5.11. Comparison of the methods for determination of the effective surface
area
The comparison of the methods for determination of the effective
surface area of packed bed columns shows that the best of them is the method of
Danckwerts [39], and especially the variant with absorption of CO2 in solutions
of NaOH (or KOH). Good results are also obtained using the method of
Schulman [31]. It was found [40] that the results obtained using both these
methods are practically the same. The comparison is made at comparatively low
liquid superficial velocities when the effective area of the jets and drops in the
packing void fraction can be neglected. This is important mentioning because an
investigation with modern packings carried out at very high liquid superficial
2 2
velocity, up to 200 (m /m h) [5] shows, that the effective surface of the packing
is quite greater, up to two times, than its specific surface. It is clear that no
method, based on the wetted or effective surface of the packing elements, can
be useful for predicting the effective area of the packing at such conditions.
That is, the only method useful for all hydrodynamic regimes in packed bed
columns is that of Danckwerts [39]. Nevertheless, at low liquid superficial
velocities the other methods, like those of van Krevelen et al. [27], and Shulman
et al. [31], can be also used.
The methods for determination of the wetted surface area do not have
now any practical importance.
New information about the problem connected with prediction of the
effective surface by physical absorption, using the method of Danckwerts, is
given by Last and Stichlmaier [102,103]. Their investigations are carried out
with one and the same packing and liquid superficial velocity for absorption
accompanied by two different chemical processes. The first of them is
absorption of CO2 from air in 1 N KOH solution (the Danckwerts method), and
the second - absorption of CO2 in water solution of methyldiethanolamin
(MDEA). The authors [102,103] show that the effective surface area determined
by the first reaction is up to 5 times greater than in case of physical absorption.
In case of CO2 absorption in MDEA, the difference is 50%. In their
investigations Last and Stichlmaier [102-103] used own results [105, 109] for
determination of mass transfer parameters in chemical absorption. The big
difference in the effective surface for different processes is explained with
presence of stagnant zones of absorbent on the packing surface. In case of
nonequilibrium reaction (Danckwerts method), the liquid in these zones is
saturated for longer time with the absorbed gas than in case of physical
absorption. This results in determination of higher effective surface area. The
same reason explains also the difference in the second chemical reactions.