Advanced Temperature Measurement and Control

Unit 6 - Reactors

UNIT 6

Reactors

This unit describes control strategies and tuning requirements for reactors.

Learning Objectives When you have completed this unit you should:

Recognize when and how a poor coil, jacket or thermowell design can make exothermic reactor control impossible.
Be able correct the most common mistake in tuning temperature controllers for reactors.
Determine how to adapt the controller tuning for the different operating conditions of batch reactors.
Know how to set up the best cascade loop.

6-1. Process and Equipment Design Considerations

The process time constant is normally much larger than other time constants and dead times in the temperature control loop for a properly designed reactor. This assumes the following sources of dead time total less than ten percent of the process time constant:

The mixing turnover time (approximated as the volume divided by the sum of throughput, recirculation, and agitator pumping flow rates).
The heat transfer surface thermal time constant (approximated as the product of the surface mass and heat capacity divided by the product of the heat transfer coefficient and area).
The coolant transportation delay (approximated as the coolant volume divided by the coolant flow rate).
The thermowell - sensor time constant (see Section 2-3).

For this situation, the loop dead time is small, and excellent reactor temperature control is possible for constant coolant flow. Conversely, insufficient agitator pumping rate, a large heat transfer surface mass-to-area ratio, a low coolant flow, a large or glass-coated thermowell, or a loose sensor fit in a thermowell create enough equivalent dead time from secondary lags or pure dead time to cause poor control despite the best of tuning. A variable volume or a variable coolant flow will cause a process gain nonlinearity. The effect of changes in coolant flow are particularly disastrous due to the additional changes in coolant transport delay, as described in Section 5 for heat exchanger temperature control. To keep the coolant flow fixed, a coolant pump and recirculation system are needed. The coolant heat removal rate is changed by throttling a coolant makeup flow to change the coolant temperature. Normally, as shown in Fig. 6-1, the output of the reactor temperature controller sets the remote set point of a coolant temperature controller. The cascade control of reactor to coolant temperature has excellent dynamics, in that the inner (slave) loop is much faster than the outer (master) loop. The inner loop corrects for disturbances in makeup coolant temperature or pressure before they affect the reactor temperature and provides a more linear process gain for the outer loop.

Fig. 6-1. The coolant temperature loop compensates for coolant upsets before they affect the reactor temperature and makes the process gain more linear for the reactor loop.

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Unit 6 - Reactors

Learning Objectives When you have completed this unit you should:

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