Page 154 - Entrophy Analysis in Thermal Engineering Systems

CHAPTER TEN

Entropy and chemical equilibrium

10.1 Introduction
Many practical applications involve complex processes with numerous
chemical reactions, e.g., combustion of fuel, gasification of biomass, steam
reforming of hydrocarbons. The byproducts of the reactions often consist of
several substances. The product of biomass gasification, as an example, is a
mixture of mainly H 2 , CO, CO 2 ,CH 4 , and H 2 O with trace of other heavy
hydrocarbons such as C 2 H 4 ,C 2 H 6 ,C 6 H 6 [1]. To correctly and accurately
predict the gasification process, one needs to account for various transport
mechanisms coupled with the kinetics of the reactions involved. Develop-
ment of a mathematical model that includes these effects and the numerical
solution of such models is a challenging task, but the outcome is rewarding.
A solution of a coupled transport-kinetic model applied to a reactive flow
allows prediction of species concentrations, temperature, and velocity along
the flow path. For instance, the predicted composition of the gasification
products, also called producer gas, using such models is shown to be highly
accurate [2,3]. It is important to realize that the transport-kinetic models rest
on the laws of nature—the primary reason for their high accuracy. The
transport equations are indeed analytical expressions of the conservation
laws. The kinetic models that are employed to predict the rates of formation
or consumption of chemical compounds are also based on experimental
observations.
Now the question is whether one may determine the composition of a
chemical reaction at the state of chemical equilibrium by mere laws of ther-
modynamics. This problem was investigated in the late 19th century by
Gibbs, and later by Nernst who examined the theory of Gibbs through a
limited number of experiments. The criterion proposed by Gibbs postulates
that at chemical equilibrium the Gibbs energy of a system maintained at
fixed temperature and pressure is a minimum.

dG ¼ 0 (10.1)

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Entropy Analysis in Thermal Engineering Systems 149
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