Reactor Cooling and Heating Systems – Part 4

Heat exchangers in a coil or jacket recirculation system are used to provide tempered water. This allows the use of colder or hotter utility streams as inputs to exchangers rather than as inputs to a coil or jacket, moderating temperature extremes  that could cause heat transfer surface coatings or product degradation from localized cold and hot spots and thermal shock cracking a glass lining. Mammalian bioreactors are particularly vulnerable to temperature extremes because of metabolic sensitivity. There is greater propensity for localized temperature variations from heat transfer surface extremes in these bioreactors due to less mixing from the reduction in agitation to avoid cell rupture. Mammalian cells unlike bacterial cells have membranes rather than cell walls making them more susceptible to damage from agitation.

Separate exchangers are used for steam and chilled water (see slide 9 in Reactor-Cooling-and-Heating-Systems). The coil or jacket temperature response to a change in controller output may exhibit a temporary initial change (lead) followed by a ramp (integrating response) before approaching a final steady state value (self-regulating response). The integrating response originates from the recycle of jacket water. For example, an increase in heat exchanger outlet temperature makes a loop through the coil or jackets and comes back to the heat exchanger as an increase in inlet temperature. The ramp rate (integrating process gain) increases as the coil and jacket volume decreases. A lead in the opposite direction of the response can be caused by a blocked-in exchanger (closed control valve).  There is a thermal lag from the UA of the heat exchanger and an increase in process gain and deadtime at low utility flow in the self-regulating. Coil and jacket temperature control by the manipulation of a utility stream to a heat exchanger has a slower and more nonlinear and irregular response than the direct steam injection and blending of hot and cold water in a constant coil and jacket circulation flow.

The coil and jacket exchanger thermal lag can be passed by the manipulation of an exchanger bypass flow creating a faster temperature loop that is easier to tune (slide 10). A VPC can be used to reduce utility flow during low loads and increase temperature loop turndown (slide 11). The potential benefits are similar to those discussed for bypass control of a process exchanger in the reactor recirculation line.

A constant jacket flow provided by recirculation is preferred to the throttling of total flow to the jacket. The process gain and deadtime for the jacket temperature controller are inversely proportional to flow. At low jacket flows the combination of high process gain and high process deadtime can cause sustained oscillations. Also, at low jacket flows the heat transfer coefficient is greatly reduced and the fouling rate greatly increased decreasing the capability of the reactor temperature control to do its job.

All of the above control schemes can benefit from the use of an enhanced PID for jacket and reactor temperature control. The enhanced PID will prevent limit cycles from valve stiction particularly problematic near the split range point and unnecessary crossing of the split range point. The enhanced PID can reduce the overshoot by a providing a smarter time for the valves coming off output limits for large setpoint changes in batch operation and continuous operation transitions and startups. The deadband setting (threshold sensitivity) setting of the PID can reduce the reaction to noise enabling a higher controller gain. Finally, the dynamic reset limiting of the PID offers the ability to prevent a runaway by the addition of directional velocity limits to provide a faster response to the demand for cooling for highly exothermic reactors.