The measurement of temperature is a vital process in the monitoring of petrochemical industries, in particular of the Fluid Catalytic Cracking Units (FCCU). Fluid catalytic cracking is a process where the high-temperature, high-molecular weight hydrocarbon components of petroleum crude oils are converted into more valuable products, such as gasoline and olefinic gases. A zeolite-based high-temperature catalyst continuously circulates through a riser, where it is mixed with the oil; the heavy hydrocarbons are then cracked into shorter-chain molecules that are used in the production of gasoline and LPG. The temperature of the process reaches around 750ºC (1400ºF), and the hot flowing and extremely erosive catalyst is kept from being in contact with the outer carbon steel by proper refractory liners. These liners are a vital element of the equipment since the mechanical strength of steel decreases dramatically above approximately 540ºC (1004ºF); above this temperature, it loses its ability to withstand internal pressure. If the refractory lining would fail the hot catalyst would quickly erode its way through the piping or vessels carbon shell or weaken it to the point where the internal pressure would cause the shell to fail. It becomes obvious that any loss of containment would pose a significant safety threat and an increased risk of fires.
If a portion of the refractory material is worn, the temperature of the outer shell increases immediately; one way to monitor the integrity of the refractory lines is by keeping record of the temperature of the outer shell. This can be done by infrared thermography (IT); this non-contact testing technique has been used for over two decades and provides a way to identify potential trouble spots and monitor the performance of critical equipment. However, this technique has also some limitations: the images can be distorted by dust, seasonal effects (winter / summer) may require reference images and some spots may not be possible to visualize with the camera.
Surface-acoustic wave (SAW) temperature sensors can be used to complement IT monitoring. They take advantage of the controlled changes of the properties of a piezoelectric material as a function of the parameter that is to be measured and, as such, are immune to effects such as dust accumulation or humidity. In addition, they can be interrogated remotely by means of an RF beam and are passive devices, requiring no external power or energy harvesting. They can be mounted on remote and/or inaccessible places or in moving parts, and give accurate temperature readings of potentially critical spots on a regular basis, without the need of human intervention. The readings can be made by a read-out electronic unit that interrogates the sensor with an RF pulse and receives and interprets the echoed signal. These data can, for instance, be fed into a finite element simulator and give the stress and tension distribution along the structure, or be used as a trigger for automatic responses once critical conditions are reached. The SAW sensors can be further coated with a nanocrystalline diamond film that will protect the devices from the hostile FCCU environment. Diamond gathers some properties that make it the best encapsulation material: it is hard and chemically inert, it is electrically insulating but thermally conductor and it is intrinsically resistant to wear. In addition, the versatility of Chemical Vapour Deposition (CVD) allows the deposition of diamond in the form of thin conformal films.
Other pieces of equipment can also be monitored using the same technique, for instance electrical (switchgears) and mechanical components, as well as high-temperature piping, heaters, and heat exchangers.