Page 258 - Introduction to chemical reaction engineering and kinetics

9.2 Gas-Liquid Systems 239

(9.1-47)

Corresponding equations for the two special cases of gas-film mass-transfer control
and surface-reaction-rate control may be obtained from these results (they may also be
derived individually). The results for the latter case are of the same form as those for
reaction-rate control in the SCM (see Table 9.1, for a sphere) with R, replacing (con-
stant) R (and (variable) R replacing rc in the development). The footnote in Example
9-2 does not apply here (explain why).

9.2 GAS-LIQUID SYSTEMS

9.2.1 Examples of Systems

Gas-liquid reacting systems may be considered from one of two points of view, de-
pending on the purpose of the reaction: (1) as a separation process or (2) as a reaction
process.
In case (l), the reaction is used for the removal of an undesirable substance from a
gas stream. In this sense, the process is commonly referred to as “gas absorption with
reaction.” Examples are removal of H,S or CO, from a gas stream by contact with an
ethanolamine (e.g., monoethanolamine (MEA) or diethanolamine (DEA)) in aqueous
solution, represented by:

H,S(g) + HOCH,CH,NH,(MEA,e, --f HS- + HOCH,CH,NH; (4
CO,(g) + 2(HOCH,CH,),NH(DEA, 9 + (HOCH,CH,),NCOO- + (HOCH,CH,),NH;
(W

In case (2), the reaction is used to yield a desirable product. Examples are found in
the manufacture of nitric acid, phenol, and nylon 66, represented, respectively, by:

3NWd + &O(e) 4 2HNO,(9 + NO(g) (0

V%(e) + WYMd -+ C,H,COOH + H,O (D)
C,H,COOH + (1/2)0, + C,H,OH + CO2 I

C6H12(9 + O,(g) + adipic acid (El

The types of reactors and reactor models used for such reactions are considered in
Chapter 24. In this chapter, we are concerned with the kinetics of these reactions, and
hence with reaction models, which may have to include gas-liquid mass transfer as well
as chemical reaction.
Similar to the case of gas-solid reactions, we represent the stoichiometry of a gas-
liquid reaction in a model or generic sense by

A(g) + bB(9 + products (9.2-1)
B(9 may refer to pure liquid B or, more commonly, to B dissolved in a liquid solvent.
Furthermore, we assume throughout that B(9 is nonvolatile; that is, B occurs only in
the liquid-phase, whereas A may be present in both phases. This assumption implies
that chemical reaction occurs only in the liquid phase.
In the treatment to follow, we first review the two-film model for gas-liquid mass
transfer, without reaction, in Section 9.2.2, before considering the implications for ki-
neticsin Section 9.2.3.