As reactor size increases, the thermal lag from reactor contents to reactor wall increases and the heat-transfer area per reactor volume decreases. Temperature control is characterized by sustained errors between setpoint and measurement during heatup and cooldown and by varying thermal responses.
A typical control system has the reactor contents’ temperature primary controller output being cascaded to the jacket/coil temperature secondary controller setpoint.
The primary control modes normally are proportional (P) plus integral (I) plus derivative (D), with P typically set in the range 35% to 50%, the I mode set slower than the overall time constant and the D mode set at I/4. The I mode should only be activated when the measurement is within the proportional band; it should be set conservatively, ensuring that energy is not driven into the process at a rate faster than the process can accept it, to prevent oscillation.
The secondary control mode normally is a P-only controller, with P set <less than equal to>< 25%, as the I mode slows down the response.
Distillation boilup is determined by the temperature difference between jacket/coil and reactor contents. Boilup is controlled by the jacket/coil inlet temperature; the secondary controller will require I mode to be activated to eliminate offset.
For high-accuracy temperature measurements, use a resistance sensor with a Smart transmitter to provide flexibility when setting ranges. The thermal lag associated with the sensor is minimal. However, there can be a significant thermal lag associated with the thermowell if it is incorrectly designed or installed, and this can lead to an uncontrollable system. Fast response designs are available and should be used.
Satisfactory performance depends upon the selecting a control valve with the appropriate operating characteristics. A valve has an inherent characteristic (relationship between flow and stroke at constant ΔP) and an operational characteristic where the inherent characteristic is modified by the process pressure conditions. An equal-percentage operating characteristic tends toward a linear characteristic as ΔPmax/ΔPmin increases. A linear operating characteristic tends toward a quick opening characteristic as ΔPmax/ΔPmin increases.
An equal-percentage valve characteristic normally is used for temperature control, although situations might arise where a linear characteristic provides better control. The operational characteristic of a valve can be modified by controller output-signal characterization.
Use pneumatic control-valve actuators with positioners. The calibration for split-range operation of the valves should be achieved at the positioners, not with scaled multiple controller outputs, to ensure loop integrity is maintained under all failure modes.
Three options are available:
Direct heat/direct cool. The appropriate supply and return services are connected directly to the reactor jacket/coils. Temperature ranges from -20°C to +180°C (-4°F to 356°F) with water, steam or ethylene glycol/water are possible with pressurized systems. Arrangements vary from totally manual to fully automatic and include forced circulation with steam/water mixing facilities. Combined heating/cooling facilities require automatic valve sequencing and jacket/coil blowdown routines when changing services. This configuration exhibits good thermal response. Potential operational problems include cross-contamination of services, jacket fouling, corrosion, thermal shock of glass-lined equipment and product degradation from high wall temperatures.
Indirect jacket heat/direct cool. This uses a single HTF, with the coolant being injected into the reactor circulating loop. Heating is provided by a heat exchanger with steam on the service side. Changeover between heating and cooling mode is seamless using control valves in split range. However, in a multiple-reactor facility, this system does not provide complete segregation of the reactor service system from the other reactors. This could result in an extended shutdown of the total facility in the event of water breakthrough due to a single heat exchanger failure.
Indirect jacket heat/indirect cool. This is probably the most common arrangement. As shown in Figure 4, a three-way valve at the steam heat exchanger provides fast-response bypass control by eliminating the thermal lag associated with the heat exchangers . Steam can be applied continuously to the heat exchanger shell at full pressure, eliminating problems associated with condensate lift and return, preventing freezing when operating below 0°C (32°F) and providing excellent linear control characteristics. Thermal response on cool is slower than direct injection due to the added thermal lag of the cooling heat exchanger. This exchanger allows for a less expensive fluid for the cooling service, which might provide cost benefits over a centralized refrigeration facility involving the use of significant volumes of an HTF. In such a system, take care to allow for thermal expansion throughout the loop.
This system also allows for segregation of the reactor service system from other reactors, which enables rapid identification of water breakthrough problems at a facility with several reactors.
John Edwards is a Senior Consultant for P & I Design Ltd., Thornaby, England, where he is responsible for process modelling and engineering. E-mail him at email@example.com.
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