In this chapter we introduce several advanced topics. The first deals with different approaches that can be taken to develop overall mass balances within a vessel. This can be used to simplify the analysis of a complex process, as we will see. We then turn to the problem of multi-phase resistances. Up to now we have been able to make assumptions about which of two phases in contact dominates the mass transfer kinetics across the interface between them. However, there are situations in which both phases may contribute to the overall mass transfer, at least over a limited range of conditions. We will develop an approach for dealing with this situation. Finally, we will consider a specific case involving topochemical reactions in porous solids.

Overall mass balance

We have used mass balances extensively throughout this book. Most often we have used them to develop an expression for the motion of an interface separating two reacting phases. In fluids, however, this process can become quite complex. Consider, for example, what happens when a gas stream is passed through or over a bed containing some reactive species. The reaction occurs slowly as the gas stream passes.