Page 242 - Introduction to chemical reaction engineering and kinetics

Chapter 9

Multiphase Reacting

Systems

In this chapter, we consider multiphase (noncatalytic) systems in which substances in
different phases react. This is a vast field, since the systems may involve two or three
(or more) phases: gas, liquid, and solid. We restrict our attention here to the case of
two-phase systems to illustrate how the various types of possible rate processes (reac-
tion, diffusion, and mass and heat transfer) are taken into account in a reaction model,
although for the most part we treat isothermal situations.
The types of systems we deal with are primarily gas-solid (Section 9.1) and gas-liquid
(Section 9.2). In these cases, we assume first- or second-order kinetics for the intrinsic
reaction rate. This enables analytical expressions to be developed in some situations for
the overall rate with transport processes taken into account. Such reaction models are
incorporated in reactor models in Chapters 22 and 24.
In Section 9.3, we focus more on the intrinsic rates for reactions involving solids,
since there are some modern processes in which mass transport rates play a relatively
small role. Examples in materials engineering are chemical vapor deposition (CVD)
and etching operations. We describe some mechanisms associated with such heteroge-
neous reactions and the intrinsic rate laws that arise.

9.1 GAS-SOLID (REACTANT) SYSTEMS

9.1.1 Examples of Systems
Two types of gas-solid reacting systems may be considered. In one type, the solid is
reacted to another solid or other solids, and in the other, the solid disappears in forming
gaseous product(s).
Examples of the first type are:

2ZnS(s) + 3O,(g) + 2ZnO(s) + 2SO,(g) 6%
Fe@dd + 4HAd + 3Fe(s) + 4H,O(g) (B)
-+ CaCN,(s) + C(s) ((3
CaC26) + N2(‘d
2CaO(s) + 2SO,(g) + 02(g) + 2CaSO,(s) (W

Although these examples do not all fit the category of the following model reaction, in
the reaction models to be developed, we write a standard form as

A(g) + bB(s) + products[(s), (s)] (9.1-1)
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