Ask Greg McMillan

Greg's Response:

Temperature loops control the energy balance and the reaction rate through the Arrhenius equation. A cascade temperature control system offers the greatest linearity and responsiveness to coolant pressure and temperature upsets. The reactor temperature PID manipulates the setpoint of a jacket inlet temperature control that in turn manipulates makeup coolant flow. The coolant exit flow, such as cooling tower water (CTW) return flow, equals the coolant makeup flow to the jacket by piping design and in some case by pressure control in the recirculation system. The resulting constant jacket coolant flow eliminates the increase in dead time, process gain, and fouling from a decrease in jacket flow. The temperature controller PID gain must be maximized for highly exothermic reactors even to the point of causing oscillations in the jacket temperature loop. Highly exothermic reactors can have a runaway response due to positive feedback where too low of a PID gain causes excessive acceleration in temperature response to the point of no return. 

In a liquid reactor a coated sensor or a sensor tip that does not extend past the nozzle into the vessel or a sensor hidden behind a baffle, could cause a measurement time constant larger that is than the thermal time constant. A sensor in a baffle with a glass coating in a small liquid volume will also have an excessive measurement lag due to the poor baffle surface thermal conductivity.

The temperature sensor location is also important for the secondary jacket temperature loop. The sensor should be in the coolant recirculation line rather than in the jacket. The higher velocities and turbulence in the pipeline provide a faster measurement with fewer fluctuations from level and phase changes and cold or hot spots from product sticking on the reactor wall.

A dynamic simulation that includes the positive and negative feedback process time constants, mixing delays, measurement and valve 5Rs, and all thermal time constants from heat transfer surfaces and sensors is critical for detailing and tuning the best control strategy. 

For much more knowledge, see the ISA book Advances in Reactor Measurement and Control (use promo code ISAGM10 for a 10% discount on Greg’s ISA books).

Top Ten Mistakes made in Liquid Single Phase Reactor Temperature Control

1. Throttling reactant feeds for temperature control causing inverse response.
2. Throttling jacket flow for temperature control causing poor heat transfer, high secondary loop process gain, and large dead time at low production rate.
3. Not using Coriolis flow meters on reactant feeds causing stoichiometry errors.
4. Use of thermocouples instead of RTDs causing drift and poor accuracy and sensitivity.
5. Sensor not spring loaded and thermowell tip not reaching into well mixed zone of reactor.
6. No lead-lag compensation of ratioed setpoints to provide simultaneous changes preventing momentary stoichiometry errors for changes in production rate that accumulate over time.
7. Reactant feed controllers not tuned for maximum disturbance rejection (e.g., pressure upsets).
8. Direct addition of steam into jacket causing bubbles and droplets in jacket heater from transitions between heating and cooling instead using steam injection heater.
9. Not maximizing PID gain and rate action and minimizing reset action in reactor temperature controller (most reactor temperature PID reset times are one to two orders of magnitude too small).
10. Not narrowing jacket temperature controller scale range to operating range causing a loss in jacket temperature loop performance.