TABLE OF CONTENTS E.Dendy Sloan, Ph.D.
Professor
Chemical Engineering Department
Colorado School of Mines
Golden, CO
Chemical engineers make reaction-equilibria calculations to find the potential yield of a given reaction, as a function of temperature, pressure, and initial composition. In addition, the heat of reaction is often obtained as an integral part of the calculations. Because the calculations are made from thermodynamic properties that are accessible for most common compounds, the feasibility of a large number of reactions can be determined without laboratory study. Normally, thermodynamic feasibility should be determined before obtaining kinetic data in the laboratory, and if a given set of reaction conditions does not yield a favorable final mixture of products and reactants, the reaction-equilibria principles indicate in what direction one or more of the conditions should be changed.
The basic sequence of equilibria calculations consists of (1) determining the standard-state Gibbs free-energy change ? G for the reaction under study at the temperature of interest (since this step commonly includes calculation of the standard enthalpy change of reaction ? H , an example of that calculation is included below), (2) determining the equilibrium constant K from ? G , (3) relating the equilibria compositions to K, and (4) evaluating how this equilibrium composition changes as a function of temperature and pressure. A frequent complication is the need to deal with simultaneous or heterogeneous reactions or both.
[*]Example 4.10, Calculating Reaction Conversions, Selectivities, and Yields, is from Smith, Chemical Process Design, McGraw-Hill.
