Page 138 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

133

rj - rate constant of the reaction - 1/s;
2
D- diffusivity of the absorbed component, CO2, in the liquid phase, m /s.
The conditions determining that the rate of the absorption is independent of the
hydrodynamics of the gas and liquid phase are given by the equations:

(50)
and
(51)
k r«mkc. t

where ki is the liquid-side controlled mass transfer coefficient, m/s;
kg- gas-side controlled mass transfer coefficient, m/s;
m- equilibrium constant of the absorption process, not accounting for the
chemical reaction.
More precise investigations [66] show that expression (50) can be
replaced with:

(52)

Different variants of the method of Danckwerts use different types of
chemical reactions. Especially proper is the absorption of CO 2 from air in
NaOH solution, used by many investigators [40, 41-59]. The method is
practically free from disadvantages. An additional advantage is that the rate of
absorption of CO2 in NaOH is a packing characteristic used some times for
comparison of different packings [60]. That is why the method is described in
details.
In case of pseudo-first order chemical reaction in absorption of CO2 in
NaOH solution, the constant rj is given by the equation:

ri = r n. COM (53)

Here r# is the rate constant of a second order chemical reaction and CQH is the
concentration of the chemically active component in the solution, in our case -
NaOH, or more precisely, the OH ions.
It has been found out [61, 62], that r n can be calculated by the equation:

(54 )
or