Measuring temperature across a process unit — for example, a furnace, desalter, hydrofiner, fixed-bed reactor or hydrocracker in refineries or similar process units in chemical plants — leads to improved process control, increased catalyst life, more efficient production, fewer process upsets and decreased emissions. Known as temperature profiling, this technique allows acquisition of temperature data from multiple sensors and transmission of the data to an automation system. There, control and monitoring software analyzes the data and takes action. This article covers the sensing devices used to acquire temperature profile data.
Temperature Profiling a Hydrofiner
A hydrofiner is a typical application for temperature profiling. New environmental standards for low sulfur fuels are driving the need for temperature profiling in the refining industry. This set of new regulations requires plants to reduce air pollution emissions, particularly NOX, and is leading to equipment retrofits and upgrades.
For example, the sulfur content in mineral oil products must be limited, and typically, this is achieved by catalytic desulfurization in a hydrofiner. Heated to 572 to 752°F (300 to 400°C) and raised to a pressure of 362 to 870 psi (2.5 to 6 MPa), the oil is mixed with hydrogen and reacts with the catalyst. The sulfur molecular connection then is converted to H2S and hydrogen carbides.
One of the issues with this process is performing temperature profile monitoring at the different catalyst layers of a hydrofiner (figure 1). Only a limited number of access points is available in a hydrofiner, so multiple sensors must be inserted at each access point across the layers.
One way is to insert a special sensor array with multiple sensors, all connecting to a single transmitter. The instrument shown in the top left corner of figure 1 has multiple sensors that can be applied as needed based upon the application. Individual thermocouples are mounted in a single nozzle. The unit’s “tentacles” can be positioned within the reactor as needed to monitor the layer. Using data from multiple sensors, the control and monitoring software can construct a 3-D image of the catalyst layer.
Temperature-sensing elements can be replaced individually to minimize maintenance. The sensing elements are encased in a guiding tube that remains in the reactor. This allows a faulty insert to be easily exchanged for a new one (figure 2).
Temperature profiling with a sensor array also is useful in fixed-bed reactors. Due to the solid state of the catalyst, it may not be possible to achieve a homogeneous reaction mixture. Formation of hot spots and cold spots may occur, and coke formation can quickly lead to deactivation of the catalyst. With multiple measuring points per process connection, sensors can be freely positioned in the reactor to detect problems. As a result, operations can quickly become aware of a degrading process situation and take appropriate action.
Sensors are routed to a process transmitter or terminal block that can accept multiple sensors and then output 4 to 20mA signals or thermocouple signals to the control system. Either the terminal block or the transmitter typically is housed in an enclosure (figure 3).
Using Multipoint Thermometers for Temperature Profiling
Another method for creating a temperature profile is with a multipoint thermometer: a temperature sensor with a long thermowell containing multiple sensors. The instrument shown in figure 4, for example, contains multiple sensors arranged along the length of the thermowell. The thermowell can be longer than 100 feet, allowing it to extend vertically through a reactor or process vessel.
While figure 4 shows the sensor mounted across a reactor vessel, the sensor also can be mounted vertically or at an angle as needed to obtain different temperature profiles. Theoretically, the maximum immersion length of each individual multipoint thermometer can be equal to the reactor’s internal diameter. Supports may be necessary to reinforce the end of the multipoint thermowell at the opposite side of the process connection.
Using Temperature Profiling to Measure Processes Safely
In the oil and gas, refining and chemical industries, safety always is a priority. Therefore, great care has to be taken when making multiple sensor connections because in almost all cases, the measurements are being made in a hazardous area or the contents of the vessel are hazardous. In the case of the sensor array, all of the process connections are potential leak points as they leave the vessel. With multiple connections, this can pose a problem.
For both the sensor array and multipoint thermometer, one solution is to install a safety chamber between the flange and junction box. This prevents process gases and liquids from escaping the vessel if any of the process connections leak.
After long-term usage under demanding process conditions such as high corrosion rates, pressures and temperatures, combined with a series of known and unpredictable process phenomenon such as turbulence, highly exothermic local chemical reactions and others, a crack in the thermowell wall might occur, allowing the process media to fill the internal volume of the thermowell. In such a case, the process media can be contained by second and third barriers.
By using a pressure vent on the multipoint flange, the leak can be detected and monitored via a pressure transmitter. The second barrier protects the safety chamber from any attack by highly corrosive process media as well as from high process pressures or temperatures. Even after a breakthrough of the second safety barrier, there is no need for a plant shutdown because the safety chamber is in accordance with PED/ABSA-pressure directives and their harmonized rules and calculation codes. This allows it to continue safe operations until the next planned downtime.
Applications for temperature profiling in the oil and gas and petrochemical industries are characterized by demanding process conditions in terms of hazardous media with high pressures and temperatures combined with elevated corrosion rates, turbulence and vibration. Specialized instrumentation such as array sensors and multipoint thermometers are available to make required measurements, but care must be taken during installation and use.