Recycle and Purge Streams In the ammonia production example, we saw the use of recycle stream. Recycle streams are very common in industry. For example, suppose you have a reactor that is producing component B from A, but the reaction does not go to completion. Thus, you will have some A left over in the product stream, often a lot of A (Figure 1). You can then put the product stream through a separation unit and recover a stream that is mostly B and a stream that is mostly A. What do we do with the left over A? Obviously raw materials are expensive and we need the A to produce more B, so we recycle the A: we pump it back to be combined with the fresh feed so that we don't have to purchase so much A per pound of B produced. Putting unused raw material back into feed is economically important, but there are some problems that can occur. Consider the process in Figure 1 in which a side reaction produces an impurity I. The separation unit again separates out the product, and our recycle stream returns unwanted material back to the feed. Now look at the block balance for component I. Note that there is no place that I comes in or out of the block process, but it is being generated within the block by the second reaction. Thus, there will be an accumulation term. There is no way this process can operate in a continuous, steady-state mode. What happens is that there is a continuous build up of impurity until the separation unit doesn't work very well any more and our product is ruined, or until the impurity inhibits the reaction, or perhaps something worse.
Recycle Stream Figure 1: An example of a recycle stream where A & I are recycled. Therefore, whenever you design a process with a recycle stream, make sure that your balances provide for an acceptable way for impurities to be removed so that the accumulation term can be nullified by flow out additional streams. A common way to provide for this is with a purge stream off of the recycle stream as shown in Figure 2 below, Purge stream. Note how this allows the mass balance to be set up for I on the block such that the outlet of I equals the accumulation of I.
Purge Stream Figure 2: An example of a purge stream where excess A & I are purged.