For processes with a rather limited conversion or separation performance, recycles can be introduced to recover unused raw materials. For batch processes this can be done in a similar way as for continuous processes. The main problem is to determine when and how much recycle can be profitable. The degree of difficulty describes the process in a qualitative way, whereas the degree of exhaustion is a quantitative measure. The more difficult the operation, the more limited the performance.
More difficult operations require more time or energy.
The exhaustion can be expressed in the fraction of the raw materials which is processed. This extract can then be reused by adding it to the feed of the next batch, provided that the raw material fraction of the extract is higher than that of the feed. Figure 1 shows a basic scheme for recycling. For the process operation, two phases can be distinguished. In the first phase the product is produced while in the second phase the feed is exhausted up to an acceptable level.