Page 268 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

10.4 Design illustration 269

difference is compensated by the difference between H TOL and H TOG , and the column height can be
determined by either approach.
Even if the individual coefficients (k G a) and (k L a) are relatively constant for any given case, the
overall coefficients vary with m for nonlinear equilibrium curve. Therefore, the overall coefficients
should be used only when m and molar density of the phase (r m ) are nearly constant.

10.3.6 Absorption accompanied by chemical reaction
In many commercial absorption processes, a chemical reaction between the solute and the solvent
provides a more complete removal of solute, e.g., removal of CO 2 from the air by using mildly alkaline
solutions or scrubbing of ammonia using a dilute acid solution. The chemical reaction is employed to
favorably alter the gaseliquid equilibrium relationship for the enhancement of mass transfer rate. The
reaction in the absorbent phase affects the liquid-phase mass transfer coefficient (k L ), which is
influenced by reaction kinetics, as well as by factors influencing physical mass transfer at the inter-
phase. The enhanced rate due to chemical reaction also occurs from a greater effective interfacial area
since absorption can take place in the nearly stagnant regions, as well as the dynamic liquid hold up
and is usually incorporated by introducing an enhancement factor (E) in the rate expressions where
k L
(10.23)
E ¼ o
k
L
o
k L is the actual mass transfer coefficient and k is the mass transfer coefficient in the absence of re-
L
action under the same circumstances.
When absorption is accompanied by a very slow chemical reaction, the apparent values of k G a may
be lower than with absorption alone, e.g., in the absorption of chlorine in water followed by hydrolysis
of dissolved chlorine, the slow hydrolysis reaction essentially controls the overall rate of absorption. In
o
the “slow reaction regime,” there is no enhancement effect, and we assume k L ¼ k and E ¼ 1. In this
L
case, the only effect of the chemical reaction is to enhance the driving force from the interface to the
bulk, which is kept higher due to the low concentration of solute in the bulk liquid.
In the case of a fast chemical reaction (instantaneous reaction regime), the mass transfer rate is
independent of chemical kinetics and depends on factors affecting the physical transfer of reactants
and reaction products. E can be very large, particularly when the concentration of the reactant in the
liquid is high. Between the “slow reaction” (E ¼ 1) and “instantaneous reaction” EzN (mass transfer
independent of reaction rate) regimes, there is a broad range of conditions termed as the fast reaction
o
regime. In this case, k L depends on the reaction rate, and while both k L and k are affected by the
L
hydrodynamic conditions, E is relatively independent of these factors. Based on the value of E, the
effective mass transfer coefficient can be estimated and used in the design calculations. In many cases,
the effective equilibrium data with chemical reaction are obtained from experimental and pilot plant
data. This data can be directly used for the process design.

10.4 Design illustration

Problem 10.1. Design a packed bed absorber to remove at least 90% of the ammonia present in an
airstream by scrubbing with water. The air stream containing 10% ammonia (by volume) has a flow
o
3
rate of 1500 Nm /hr and is available at 30 C and a pressure slightly above atmospheric. Ceramic