Temperature control is very important to the application of process safety since it can prevent runaway reactions, fires and explosions, and equipment failure. Process temperature is often monitored by temperature control systems that are in place to achieve and maintain a set point or target temperature. These control systems are made up of several components that send and receive signals that result in adjustment of heat transfer rate through the process. When control systems cannot keep process temperature within safe limits, Safety Instrumented Systems (SIS) can provide a second layer of protection.
In 2006, a confined vapor cloud explosion at a CAI/Arnel manufacturing facility injured at least 10 people and damaged over 20 homes beyond repair. The incident happened when the temperature of a 2000-gallon tank increased to a dangerous level, which led to the formation of a large vapor cloud in the unventilated facility. This cloud exploded when it came in contact with an undetermined ignition source. The likely cause of the temperature increase in the tank was that a steam valve remained open overnight. This is an incident that may have been avoided if there were a temperature control system in place. The temperature control system used at this facility was entirely dependent on operator supervision. If an operator forgot to shut off the steam valve at the end of a shift, there was no backup control system. If a basic process control system (BPCS) had been in place, the temperature of the tank would have been monitored by a sensor/transmitter that would have sent a measurement signal to a controller that would have compared that signal with another signal representing the set-point. A change in the controller signal would have resulted in an automatic change to the output/load from the valve positioner that could have closed the steam valve when the temperature approached a dangerous level.
(Courtesy of the U.S. Chemical Safety Board)
Temperature Control Systems
Temperature control systems consist of transmitters, controllers, and control elements. Transmitters contain sensing elements that measure the process temperature and send a corresponding signal to the controller. The controller compares the actual temperature to the set point temperature and sends a control signal to the control element. the control element alters heat transfer rates to adjust the process temperature to match the set point temperature.
Below is a generic model of a temperature control system. This model illustrates the feedback control loop in many control systems. The diagram below shows a temperature control system for a one-piece reactor jacket.
C ontrol Signal
A ctual Temperature
The temperature inside the process reactor is recorded by a sensor-containing transmitter, which sends a signal to a temperature controller, which then compares that temperature with a set point value. The controller sends a new signal to the control element for either hot or cold fluid to be injected into the reactor jacket. The temperature inside the reactor will change, and a new temperature signal will be generated by the transmitter. This control loop will continue until the sensor signal matches the set point signal sent to the controller.
(Copyright Chemical Engineering, Access Intelligence, LLC)
Transmitters in temperature control systems send an electrical signal to a controller so the controller can alter the operation of a control element. The most common sensors that are contained in temperature transmitters are thermocouples, thermistors, and resistance temperature detectors (RTDs).
Thermocouples , as shown below, are the most commonly used temperature measuring device in the chemical processes industry (CPI), and function by converting a measured voltage into a temperature value. Thermocouples record the difference between two dissimilar metals and a reference junction.
(Copyright Chemical Engineering Department, University of Michigan, Ann Arbor, MI)
Thermistors measure temperature by recording electrical resistance changes that are caused by temperature differences. These devices are made up of metal oxides with epoxy coatings. Thermistors have applications that include monitoring heat loss in pipes. The picture below shows a thermistor in the cross section of a pipe.
(Copyright Lawrence Berkeley National Laboratory)
Resistance temperature detectors , pictured below, are most commonly referred to as RTDs, and they measure a change in resistance due to temperature change. RTDs are composed of pure metals, are very accurate, and have a fast response time. These devices are very common in the CPI, and have applications in equipment such as distillation columns and reactors.
(Courtesy of Emerson Process Management)
Controllers receive signals from transmitters that are compared with set point signals. Controllers use control calculations to create an output signal which is sent to control elements. These devices can be analog or digital. Digital controllers often need the use of analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) to change between each type of signal depending on the compatibility of the sensors, controllers, and control elements. Controllers come in many control modes, including proportional, integral, derivative, and proportional-integral-derivative or PID control.
A temperature controller that regulates a valve to control heat transfer fluid temperature is pictured below, to the left. This device can be connected to certain heat exchangers and heaters. To the right of the temperature controller is an all-in-one control valve/temperature controller, also called a PID controller, that can directly receive analog inputs from sensors and make adjustments to its own control valve operation.
(Copyright Alfa Laval, Richmond, VA)
(Copyright Flowserve Corporation, Irving, TX)
Control elements receive a signal from controllers to change the heat transfer rate of the system, which in turn raises or lowers the temperature toward the set point or target value of the system. Control elements are also be referred to as actuators , which are used to open and close valves, the control valves themselves, or the control elements can be heat transfer equipment such as reactor jackets, heat exchangers, and heaters.
Actuators are used to control valves. Actuators can be pneumatic, meaning they can use pressurized air to regulate valve opening and closing. Actuators can also use electric power for regulation.
The picture below, on the left, shows a pneumatic diaphragm actuator attached to the top of a valve. When pressure below the diaphragm of this actuator becomes greater than the pressure above it, the diaphragm will move upward and the valve will open. To the right of the pneumatic diaphragm actuator is an electro-mechanical actuator, which controls valve opening by receiving an electrical signal which activates a motorized gear that changes the valve opening position.
(Copyright Valtorc International, Kennesaw, GA)
(Copyright Columbus McKinnon Corporation, Amherst, NY)
Temperature Control Valves
Temperature control valves are composed of a valve body, trim, seat, and actuator. Temperature control valves are actuated by a change in temperature recorded by a sensor connected to the valve which causes fluid relief in order to raise or lower system temperature. These valves are critical for temperature control systems since they can control the amount of fluid which will pass through them in order to reach heating or cooling units, and this amount of fluid will control the amount of heat transfer that can take place.
Heat exchangers are used to maintain a constant temperature in stirring tanks for mixing or reacting components. Heat exchangers may also be used for heating or cooling a fluid stream as it moves through a process, which is important to control stream temperatures through equipment with varying temperature thresholds. The picture below shows a shell and tube heat exchanger.
(Copyright Chemineer, Inc., North Andover, MA)
One-piece jackets surround reactors and are injected with heat transfer fluid. The heat transfer fluid will disperse some of the heat released in the reactor to prevent a runaway reaction. The heat transfer fluid is transferred to the jacket tangentially to promote mixing and heat dispersion effects. Half-coil jackets consist of a series of pipes. In constant heat flux jackets, heat transfer channels are aligned outside a reactor with a variable area of heat transfer based on the number of channels in service. One-piece jackets do not have great dispersion of the heat transfer fluid inside, which can cause temperature variations along the reactor walls. Half-coil jackets offer faster and more effective cooling than one-piece jackets since the distribution of the heat transfer fluid is greater through the piping elements that make up the half-coil jacket. Constant-flux jackets are made up of multiple jacket elements that can be controlled by a temperature control valve. These elements can be turned on and off independently to allow for very accurate control of reactor temperature. The response time is also faster for constant-flux jackets than for either one-piece or half-coil jackets. For more information on the heat transfer jackets mentioned above see the batch reactor module, where temperature control is discussed.
The picture below shows a one-piece reactor jacket, on the left, and a half-coil jacket, to the right. Both include multiple heat transfer fluid injection ports for better control of heat transfer fluid flow rates.
(Copyright Chemical Engineering, Access Intelligence, LLC)
Safety Instrumented Systems
Safety instrumented systems (SIS) are needed to provide a second layer of protection over the basic process control system (BPCS). These systems are designed to be independent of the BPCS so that there is no interference when they are activated. SISs activate automatically when temperature exceeds the upper alarm limit for the process. This SIS activation may lead to the starting of a pump which sends a cooling fluid to the jacket of a reactor, or shuts a steam valve or a heating element.
Alfa Laval , Richmond, VA
Chemical Engineering , Access Intelligence, LLC
Chemical Engineering Department, University of Michigan, Ann Arbor, MI
Chemineer, Inc. , North Andover, MA
Columbus McKinnon Corporation , Amherst, NY
Flowserve Corporation , Irving, TX
Valtorc International , Kennesaw, GA
Cross, James, personal communication, 2016.
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Fogler, Scott H. Essentials of Chemical Reaction Engineering. Westford, MA: Prentice-Hall,  2011. Print.