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Oct
06

Reactor Cooling and Heating Systems – Part 1

The cooling and heating system is critical for reactors since temperature plays such a huge role in determining reaction rate and selectivity (formation of desired product).  The limit to what a valve position controller (VPC) can do to increase production rate depends upon the capability of the cooling and heating system. The system can also be the source of discontinuities and nonlinearities. Most reactor temperature control problems beyond tuning can be traced back to deficiencies in the reactor coil or jacket temperature system design (see Reactor-Cooling-and-Heating-Systems excerpt from my ISA AW tutorial).

Heating is required for endothermic reactions (reactions that consume energy), liquids that are being vaporized, or bringing a reactor up to operating temperature. Cooling is needed for exothermic reactions (reactions that product heat), vapors that are being condensed, and bring a reactor down to operating temperature. Often both heating and cooling are required. Split ranged control is used to go back and forth between heating and cooling. In split range control for an exothermic reactor with a fail open cooling valve, as the temperature controller output increases from 0% to the split range point, the coolant valve goes from wide open to completely closed. As the temperature controller output increases from the split range point to the 100%, the heating valve goes from closed to wide open. The split range point is traditionally set at 50% but would be better set so that the change in temperature for a change in flow is about the same for each valve.  While cooling and heating may be done by coils besides jackets, many of the control considerations are the same. Jacketed reactors will be used for purposes of discussion and illustration. Cascade temperature control is the most prevalent strategy where the primary reactor temperature PID provides a setpoint to a secondary jacket temperature PID for the throttling of hot and cold fluids. For highly exothermic reactors BFW is added under level control and the reactor temperature PID output is the jacket outlet steam pressure PID setpoint. The jacket temperature control schemes are suitable for batch besides continuous.

The use of cascade control where the reactor temperature controller output is the setpoint of a coil or jacket temperature loop offers considerable performance improvements. The jacket temperature controller can correct for disturbances to the jacket before they affect the reactor temperature. The jacket temperature loop also isolates process and valve gain nonlinearity from the reactor loop. For a negligible increase in the heat of reaction compared to heat transfer capability, the process gain for the reactor temperature loop approaches unity.

Coils generally offer a faster temperature response than a jacket by a decrease in the volume and an increase in the velocity. Both of these work to decrease the process deadtime that is the coil volume divided by the utility flow rate. The increase in velocity increases the heat transfer coefficient but this is partially offset by a decrease in the surface area. An increase in the product of the heat transfer coefficient and surface area (UA) will decrease the secondary process lag in the thermal response. The transition in split range operation is faster which is useful for a valid transition between hot and cold utility streams but can be problematic for inadvertent transitions from stick-slip and an integrating response in the process or PID.