Looking Ahead In this chapter we discuss material balances involving recycle instances in which material from down stream of the process, returns and reenters the process again. Cases with and without reaction will be discussed. Purge and bypass will also be explained along with the industrial uses of material balances. 2.1 Introduction In Chapters and 10 we restricted the discussion and examples to a single unit with stream inputs and outputs as illustrated in figure 12.la. In Chapter 11 you encountered multiple units but the stream flows still were all in a forward direction representing serial sequences as in Figure 12.1b. In this chapter we take up processes in which material is recycled, that is fed back from a downstream unit to an upstream unit, as shown in Figure 12.lc. The stream containing the recycled material is known as a recycle stream.