Page 11 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN001H-01 May 7, 2001 16:18

8 Absorption (Chemical Engineering)

In a manner similar to the concentrated solutions case,
the coefficients in Eqs. (8) can be expressed in terms of
the concentration-independent coefficients using relation-
ships similar to those of Eqs. (7), that is,

k G = k y fm (9a)
G

k L = k x fm (9b)
L
K OG = K y ∗ (9c)
OG fm
4. Absorption with Chemical Reaction
When the solute is absorbed into a solution containing a
reagent that chemically reacts with it, the concentration
profile shown in Fig. 3 becomes dependent on the kinet-
ics of the reaction and the concentration of the reacting
reagent in the liquid.
Figure 5 shows concentration profiles that commonly
occur when solute A undergoes an irreversible second-
order reaction with component B, dissolved in the liquid,
to give product C,

A + bB → cC (10)
The rate equation is
r A = k 2 C A C B ; r B = br A (11)

Figure 5 shows that a fast reaction takes place only in the
liquid film. In such instances, the dominant mass transfer
mechanism is physical absorption and the diffusion model
above is applicable, but the resistance to mass transfer in
the liquid phase is lower because of the reaction. On the
other hand, a slow reaction occurs in the bulk of the liq-
uid, and its rate has little dependence on the resistances to
FIGURE 5 Vapor- and liquid-phase concentration profiles near
diffusion in either the gas or liquid film. Here the domi- an interface for absorption with chemical reaction.
nant mass transfer mechanism is that of chemical reaction;
therefore, this case is considered part of chemical reaction
technology, as distinct from absorption technology. in the bulk of the liquid, and the contactor behaves as a
The Hatta number Ha is a dimensionless group used to reactor, not an absorber. Here, the main consideration is
characterize the speed of reaction in relation to the diffu- providing sufficient liquid holdup for the reaction to take
sional resistance to mass transfer, place.
The effect of chemical reaction on rate of absorption is
max. possible conversion in the liquid film described in terms of an enhancement factor φ which is
Ha =
max. diffusional transport though the liquid film used as a multiplier:

◦
D A k 2 C B0 φ = k L k ,
= (12) L
2
k ◦
L where k is the physical mass transfer coefficient.
◦
L
When H a 1, all the reaction occurs in the film, and the The enhancement factor can be evaluated from equa-
process is that of absorption with chemical reaction. As in tions originally developed by Van Krevelen and Hoftijzer
the case of absorption with no reaction, the main consid- (1948). A convenient chart based on the equations is
eration is to provide sufficient surface area for diffusion. shown in Fig. 6. The parameter for the curves is φ x − 1,
On the other hand, when H a
1, all the reaction occurs where φ x is the enhancement factor as Ha approaches