A batch process is best described as which of the following?

• A. Mass crosses the system boundaries continuously during operation
• B. Between feed addition and product removal, no mass crosses the system boundaries
• C. The process maintains a constant volume at all times
• D. The system exchanges energy but not mass during operation

Answer: (B)

Batch processing means you load a fixed amount of material into the system, let the processing occur with no mass entering or leaving during that step, and then remove the finished product afterward. That is, between the time you add the feed and the time you remove the product, no mass crosses the system boundaries, though energy can be exchanged (heating, cooling, stirring) during the batch. This distinguishes it from a continuous process, where material is constantly flowing in and out and mass crosses the boundaries throughout operation. Volume is not required to stay constant in a batch, since reactions or phase changes can change the system’s volume, so a constant-volume description isn’t defining.