I haven't gotten any feedback on process feedback, which is really fortunate because it allows me to keep my new years resolution, so I will slog the blog along to the question that has been keeping people awake at night; "When is a batch a batch?"

Some say all processes like life are batches because there is beginning and an end.

Traditionally labeled "continuous processes" can significantly benefit in terms of process efficiency, time, and safety from the application of batch sequence technology for the automation of startups, grade transitions, and shutdowns. Conversely, batch processes can benefit from what has been classified as continuous control techniques.

There are some important advantages in terms of the application of PID and model predictive control by being more definitive in the distinction of batch and continuous operation. A process is best classified as "continuous" when there is both a feed flow and a discharge flow. Thus, the startup and shutdown of continuous processes, where the discharge and feed flows are zero, respectively, is better controlled if recognized as effectively a batch process. Also, fed-batch processes, which have a feed flow but no discharge flow, while termed by some as "semi-continuous" is better treated as batch.

Batch processes have an integrating (ramping) response as described in last week's blog. Such responses have different tuning rules and settings. If there is a zero load (e.g. no heat loss for temperature or conversion for concentration), the response is one sided, which means the process variable can only go in one direction. These processes will overshoot if reset action is used. Also, model predictive control (MPC) doesn't work because it assumes it can drive the controlled variable in both directions (up and down).

For more details implications for PID control of batch processes see Advanced Application Note 4, the April 2005 Control Talk column in Control magazine, and the article "Life is a Batch" in the June 2005 issue of Control magazine. For information on the need and method of translation of variables for model predictive control of batch processes, see chapter 4 in the book New Directions in Bioprocess Modeling and Control published by ISA in 2006.