Inline measurements of temperature profiles in spatially confined applications place special demands on the measurement technology. This especially applies for determination of the temperature changes in tube and tube-bundle reactors. A measuring system for fiber-optic temperature measurement has been developed for this purpose, allowing a greater number of temperature points while simultaneously reducing the protective tube in the reactor. This system has been successfully implemented in an application at Evonik in Marl, Germany.
Reliable determination of the temperature profile within the catalyst filling has far-reaching significance for the catalytic conversion of gases and liquids in tube and tube-bundle reactors. They influence:
- The course of the reaction.
- The quality of the material conversion.
- The aging of the catalyst.
The identification of hotspots — areas with excessive temperatures that can occur in the filling — plays an important role.
Matthias Huning of Evonik Industries, specialist in electrical measurement and control technology in the high performance polymers business sector, describes the problem in his plant as follows: “We use tube-bundle reactors in our production plant for Laurolactam, a starting material for Vestamid L. The challenge is to install a sufficient number of temperature-measurement points in a small space within a single-tube reactor in order to quickly detect high temperatures and undertake countermeasures. In this way, we can prevent destruction or the accelerated aging of the catalyst due to overheating. This avoids plant shutdown, which would be required due to the complicated procedure for replacing a catalyst.” Evonik uses the name Vestamid to summarize a group of high quality polyamides, including polyamide 12, 612, 610, 1010, polyamide 12 elastomers, polyphthalamide and bio-based polyamides.
Due to the small diameter of the reactor tubes, the necessary number of measurement points and the demands on the speed of acquisition, it was not possible to use a conventional measuring system such as RTDs or thermocouples. Instead, Evonik employed fiber-optic temperature sensing based on fiber Bragg grating technology.
Fundamentals of Optical Temperature Detection
Fiber Bragg gratings (FBGs) are optical periodic structures inscribed in optical fibers. Because a particular wavelength of incident light is reflected while all others are passed, each grating acts as a narrow-band filter. If a light beam with a broad spectrum is sent through an FBG, the reflections of each section of the changing refractive index only affect a specific wavelength of light to any substantial degree. This is called Bragg wavelength, and it is calculated by formula 1:
The Bragg wavelength depends on the distance of the reflectors within the grating (Ù). Multiple gratings may therefore be placed on a fiber. Changes in length of the fiber by force or heat deform the grating and result in a shift of the reflected wavelength. This is mainly due to change to the refractive index of the quartz glass by the thermo-optic effect.
The first part of the expression describes the effect of strain on the change in wavelength. The second part takes into account the effect of temperature on the change of the wavelength.
Effect of Strain and Temperature
As the second formula shows, both temperature and strain cause a change in the wavelength. To eliminate the influence of the strain, the fiber Bragg grating may not be subjected to mechanical stress when used as a temperature sensor.
One company uses the characteristic of the wavelength change as a function of temperature in its measurement system. In such cases, the system consists of a transmitter to which up to four fiber-optic measuring probes with up to 48 FBG can be connected. This makes it possible to synchronously measure the temperature at up to 192 locations per measuring system.
In the present application, FBGs are inscribed every ~7.87” (20 cm). One manufacturer offers measuring probes tailored to the application in regard to length, number of sensors and the sensor positions.
Detailed Profile Provides Knowledge
In this application, each measuring probe with a diameter of approximately 1 mm records temperatures in a measurement range of 32 to 752°F (0 to 400°C) with a measuring error of <0.5 K. It is also characterized by a very fast response time. The T90 time is under 4 sec.
Due to the measured value transmission (reflection of light), which takes place in the same fiber, no additional cables are necessary. This means the required diameter of the protective tubes for the measurement setup is reduced substantially.
On the one hand, a larger cross-section and, hence, volume is available for the reaction in the reactor, which has a positive effect on the throughput. On the other hand, the response times of the sensors are reduced because the damping effect of the air gap between the fiber with its inscribed gratings and the tube walls is kept low.
“Our plant personnel can detect the development of hotspots or the effectiveness of the catalyst in good time with the detailed recording and visualization of the complete temperature profile in the reactor,” Huning says. “We use this information to initiate measures to reduce the temperature, for example, in the first scenario. In the second, we can perform maintenance procedures such as replacing the catalyst when necessary due to its age.” Both applications extend the life of the catalyst in the reactor, which means cost-effective, preventive maintenance procedures are performed based on need.
Advantages for Installation and Maintenance
The applications of contactless measuring procedures with fiber-optic sensors are becoming increasingly common in the chemical industry. The sensors are not sensitive to electromagnetic interference and are chemically resistant. Another advantage is the possibility to couple the optical signals.
Joachim Kolsch, product manager at Siemens Process Industries and Drives, explains this. “For the installation at Evonik, we used a glass-fiber coupler to connect the sensing fiber in the reactor and the transmission line to the transmitter. This coupler can simply be disconnected for maintenance purposes, for example, when the reactor cover needs to be opened. The measuring probe can be easily pulled out before revision of the reactor or before replacing the catalyst and rolled into a spindle. The latter also enables it to be easily and safely transported.”
The transmitter provides the determined values for analysis in control systems via a Profibus DP interface. It also makes them available for management of the assets and optimization of the process.
Precise and rapid detection of temperature profiles for gas-phase reactions in fixed-bed reactors, for example, effectively help to detect the thermal load of the catalyst filling and maintain efficiency by introducing countermeasures. With the implementation of optical measured value acquisition using Bragg gratings along fiber-optic media, the sensor company provides its customers with a way to simultaneously record and process a range of temperatures for monitoring and optimization measures. Customers can detect faults and optimize the reaction processes efficiently, thereby achieving higher product throughput in the plant.
Evonik now has implemented the applications described in the first production plant after a thorough testing phase.