Reactor Cooling and Heating Systems – Part 3

Heat exchangers in a process recirculation steam are also used for reactor temperature control. The high velocities on the process side in the exchanger increase the heat transfer coefficient and reduce fouling. For cascade control the reactor temperature PID output is the setpoint of the exchanger outlet temperature. For the exchanger to do its job the recirculation flow must be high and a wide range of returning process temperature must be tolerated. The response of the exchanger outlet temperature has a 2 stage response. The 1st stage is a relatively fast self-regulating response of the exchanger. The 2nd stage response is a protracted integrating response from the recycle effect of the process fluid in the vessel changing temperature. For large vessel volumes, the 2nd stage response ramp rate is slow enough for an exchanger temperature loop fast tuning based on the 1st stage response. Open loop tests that wait on the exchanger outlet temperature to settle out at a new operating point may be confused by the ramping from the 2nd stage response. Plants may inadvertently tune for the 2nd stage response rather than concentrating on having the PID quickly deal with the 1st stage response (see slide 6 in Reactor-Cooling-and-Heating-Systems excerpt from my ISA AW tutorial).

The speed of response of control of heat exchanger outlet temperature can be significantly increased by throttling a bypass around the exchanger and keeping the utility flow constant (see slide 7). This mode of operation bypasses the thermal lag of the heat exchanger. The response is as fast as the blending of the streams bypassing and going through the exchanger. Due to the change in heat transfer coefficient with velocity, there is an additional response from the change in heat transfer. Consider the response of the heat exchanger with cold water. An increase in cooling demand will cause a decrease in bypass flow and an immediate decrease in exchanger outlet temperature. The higher velocity through the exchange will increase the heat transfer to the cooling water making a further slower reduction in temperature. If the measurement and valve are fast enough, the PID can be tuned for faster rate of response from blending providing further separation between the first stage self-regulating response and the second stage integrating response. Heat exchanger bypass control provides a much faster secondary loop for reactor temperature control.

Faster tuning of the secondary loop can compensate for utility temperature and pressure disturbances before they affect the primary reactor temperature loop. A faster secondary loop also makes the primary loop ultimate period faster enabling more aggressive reactor temperature control and better compensation of process disturbances (e.g. reactant temperature and concentration).

At low cooling or heating requirements, the hot or cold utility valve can be throttled to extend the rangeability of the exchanger. A valve position controller (VPC) can be used to prevent the bypass valve from going wide open by cutting back on the utility flow (see slide 8). A VPC can increase the turndown of the heat exchanger bypass control and prevent the process velocity through the exchanger from going below a minimum reducing the degree of fouling and frosting. This minimum flow also keeps the process side heat transfer coefficient from dropping too low. A low output limit on the VPC can be used to prevent fouling and deterioration heat transfer coefficient on the utility side. The VPC should be the enhanced PID to minimize interactions. Steam heat exchanger bypass control is not used because of steam blowing into the condensate system at low heating requirements.