Although some of the reactions considered in this book are truly homogeneous in that they take place within a single phase (normally a fluid phase), many are heterogeneous. A heterogeneous reaction involves more than one phase and normally takes place at or near the interfaces between these phases. Typical phase combinations encountered in reactors have been listed in Section 1.2. Many reactions involve solid and fluid phases. In some cases (as in the combustion of coal in air described in Chapter 8) components from both these phases are used up in the reaction. A more complex but very important example of a heterogeneous reaction involving solid and fluid phases is that which occurs when fluid reactants combine on the surfaces of a solid catalyst to form fluid products. This form of catalysis, which for obvious reasons is termed heterogeneous catalysis, is very commonly encountered in the heavy chemicals industry. The catalyst affects the rate of reaction but is not used up in the reaction and does not affect the equilibrium. If several reactions are thermodynamically possible, a good catalyst will selectively accelerate the one which gives the desired product or products. An introduction to the kinetics and mechanisms of heterogeneous catalysis is given in Chapter 3 and examples of catalytic reactor layouts have already been given in Chapter 1.