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Oct
12

Reactor Cooling and Heating Systems – Part 2

Whether the secondary loop uses coil or jacket inlet or outlet temperature is often a matter of tradition for a particular company or process industry. The dynamic response of the cascade control system to reactant disturbances such as feed and reaction rate are the same for jacket inlet and outlet temperature control. However, the coil or jacket inlet temperature control will correct for changes in cooling or heating utility supply temperature and pressure sooner than the transportation delay through the jacket. The jacket loop process deadtime is also less by the amount of this delay allowing a faster reset time setting and faster correction of valve nonlinearities. At the coil or jacket inlet the mixing of the recirculation with the hot or cold makeup flow may be incomplete and the discontinuity in the transition from hot to cold more abrupt. The location of the temperature sensor on the jacket outlet offers time for mixing and volume for smoothing transitions. Less measurement temperature noise can translate to a higher controller gain and less overreaction to the discontinuity at a split range transition (see slides 3 and 4 in Reactor-Cooling-and-Heating-Systems excerpt from my ISA AW tutorial).

The difference between the reactor and the jacket outlet temperature (approach temperature) can provide an inferential measurement of the heat transfer coefficient (U) for a constant jacket circulation flow, a given production rate, and heat transfer area (A). The approach temperature increases as UA decreases. For residence time control the increase in level will increase the heat transfer area covered by reactants offsetting the increase in heat release with production rate eliminating the need for production rate and level correction. For fed-batch reactors and continuous reactors without residence time control, a correction for level is needed to compute U from the UA.

For the coil or jacket temperature control to provide rapid adjustments of cooling and heating for disturbances to the jacket and setpoint changes from the reactor temperature control, the coil or jacket PID response needs to be fast achieved by a fast sensor and fast tuning.

 A tight fitting sensor bottomed in a thermowell with a high thermal conductivity metal and tapered tip provides a fast measurement. Spring loading can ensure the sensor sheath is bottomed. The clearance between the sheath outside diameter and the thermowell inside diameter must be minimized and the fluid velocity maximized. While grounded thermocouple sensors are a few seconds faster than resistance temperature detectors (RTD) sensors, the difference is insignificant compared to the effect of thermowell design and fluid velocity.  The thermowell design details are also important for reactor temperature. The greater sensitivity and lower drift of the RTD is important for jacket besides reactor temperature measurements. The lower drift reduces maintenance and the higher sensitivity provides faster recognition. The use of RTDs also facilitates more accurate online heat transfer computation for process diagnostics and inferential measurements of reaction rate. Thermocouples are preferred for temperatures above 400 oC where RTD insulation resistance and sensor integrity become problematic.

A high controller gain and a small reset time provide fast tuning. The jacket temperature controller is self-regulating process with a maximum PID gain that is about half of the open loop time constant divided by the product of the open loop gain and deadtime as detailed in the section on controller tuning. The minimum reset setting is about 4 times the loop deadtime for the jacket temperature PID.

Most temperature loops tend to develop an oscillation across the split range point in making the transition between heating and cooling. The transition creates a discontinuity from the change in utility fluid and the increase in friction and loss of sensitivity in valve seating or sealing. The worst case is often the transition between steam and coolant due to the huge difference in temperatures and the creation of bubbles in coolant or water droplets in steam. Steam valves tend to have higher seating and sealing friction due to the higher temperatures and pressures.  Hot and cold liquids and tempered water systems offer a much smoother transition than steam and coolant and reduce nonlinearities.

 Steam does not provide uniform heating in coils or a jacket. Steam typically collects in the top of the jacket and condensate in the bottom. Hot spots can develop around inlets. Thermal shock and steam hammer can damage glass lined vessels. The time required to drive steam completely out of the jacket before cooling water is introduced introduces a considerably delay in the control system. Improper trap design or operation can cause condensate buildup.

 The addition of hot water instead of steam directly into the jacket  provides a more uniform heat distribution, a dramatically smoother transition between heating and cooling, and a more efficient and maintainable system.  For rapid heating, the use of direct steam injection heaters and pressurized water can provide hot water temperatures well above 100oC (see Slide 5 ).  If the injection heater has hundreds of small orifices, the bubbles are extremely small and are rapidly and quietly mixed into the water. Variable orifice steam injection heaters are not as quiet and the mixing is not as complete. The use of jacket outlet temperature reduces the possibility of bubbles hitting the sensor.