Batch vs. Continuous Control and Optimization – Part 4

Batch and fed-batch reactors are designed so that the product concentration is always increasing in the reaction phase of the cycle whereas continuous reactors are designed to hold a constant product concentration. These fundamental differences have enormous implications in terms of composition control.

Conventional control of product concentration in a batch and fed-batch reactor is not possible because the PID expects to be able to decrease besides increase what it is trying to control. The response of batch and fed-batch product concentration is one sided always increasing unless we have inadvertently triggered a reverse reaction. If we change the process variable (PV) to rate of product formation than the PID can make decreases as well as increases in the PV. The slope is the batch profile and would be computed by simply passing the at-line analyzer PV through a deadtime block with a deadtime equal to the loop deadtime to give a rate of change as was mentioned for an effective method of identifying the initial ramp rate of the PV response in Part 2. The setpoint for the profile is essentially product formation rate. Slides 3 and 4 show the control systems for chemical and biological reactor in the excerpt Batch-Profile-Control from my tutorial on reactor control at ISA Automation Week 2011.

If there is no at-line analyzer, then heat transfer rate can be used as an inferential measurement of conversion rate. The inlet jacket or coil temperature is synchronized with the outlet jacket or coil temperature by passing the inlet temperature through a deadtime block whose deadtime is the residence time in the jacket (jacket volume divided by jacket flow) which for the plug flow volume of the jacket or coil is a transportation delay as shown in slide 5 of the excerpt. For continuous reactors, the heat transfer rate or conversion rate would be kept constant. For batch and fed-batch, the conversion rate would be a profile with a setpoint as a function of batch time. For biological reactors, oxygen uptake rate is used an inferential measurement of cell growth rate.