Inline measurements of temperature profiles in spatially confined applications place special demands on sensing technology. This especially applies to determining temperature changes in tube and tube-bundle reactors. Yet, getting such data can help optimize the reaction. That’s why Evonik, Marl, Germany, opted for innovative fiber-optic temperature measurement technology.
Reliable determination of the temperature profile within the catalyst filling has far-reaching significance for the catalytic conversion of gases and liquids in tube/tube-bundle reactors. This profile substantially influences the course of the reaction, the quality of the material conversion, and the aging of the catalyst. The identification of hotspots — areas with excessive temperatures that can occur in the filling — plays an important role in minimizing problems.
Evonik’s Matthias Hüning (Figure 1), a specialist in electrical measurement and control technology in the company’s 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 a plant shutdown, which would otherwise be required due to the complicated procedure for replacing a catalyst.”
The small diameter of the reactor tubes, the necessary number of measurement points and the demands on the speed of data acquisition ruled out the use of a conventional measuring system, i.e., resistance temperature detectors (RTDs) or thermocouples. So, working with Siemens, Evonik decided to install a Sitrans TO500 system. It features fiber-optic temperature sensing based on Fiber Bragg Grating (FBG) technology. This allows a greater number of measuring points while simultaneously reducing the protective tube in the reactor. Initial implementation took place in 2013; due its success, an additional system was installed in a similar application this year.
Optical Temperature Sensing
Contactless measuring procedures with fiber-optic sensors are becoming increasingly common in the chemical industry. The sensors are insensitive to electromagnetic interference and also chemically resistant. Another advantage is the possibility to couple the optical signals.
FBGs enable optical temperature detection. They 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 goes 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 the Bragg wavelength, λb; it is calculated by:
where n is the effective refractive index of the fiber core, and Λ is the distance between the gratings, also referred to as the grating period. A fiber may contain multiple gratings.
Changes in length of the fiber from force or heat deform the grating and result in a shift of the reflected wavelength. This mainly stems from alteration of the refractive index of the quartz glass by the thermo-optic effect [1,2]. Because both strain and temperature cause a change in the wavelength, the FBG when used as a temperature sensor must not experience any mechanical stress, to eliminate the influence of strain.
The Sitrans TO500 system uses this wavelength change to determine temperature. The system consists of a transmitter to which as many as four fiber-optic measuring probes — each with up to 48 FBGs — can be connected. This enables the system to synchronously measure the temperature at up to 192 locations. Measuring probes can be precisely tailored to the application in regard to length, number of sensors and the sensor positions.
In the Evonik application, FBGs are inscribed every 20 cm (Figure 2). Each measuring probe has a diameter of approximately 1 mm and records temperatures within a range of 0ºC to 400ºC, with a measuring error of <0.5 K. Response time is very fast; the T90 time is under four seconds .
Because the measured value transmission (reflection of light) takes place in the same fiber, no additional cables are necessary, which substantially reduces the required diameter of the protective tubes for the measurement setup. This provides two key benefits: 1) a larger cross-section and, hence, volume is available for the reaction in the reactor; and 2) the small air gap between the fiber with its inscribed gratings and the tube walls keeps the gap’s damping effect low and, thus, cuts the response time of the sensors.
At Evonik, a glass-fiber coupler connects the sensing fiber in the reactor and the transmission line to the transmitter. This coupler just needs disconnecting when maintenance requires, for example, opening the reactor cover. The measuring probe itself can be easily pulled out when necessary and rolled onto a spindle. The latter also enables it to be easily and safely transported.
The Sitrans TO500 transmitter provides the determined values for analysis in control systems via a Profibus DP interface and makes them available for management of the assets and optimization of the process.
“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,” notes Hüning. “We use this information to initiate measures to reduce the temperature, for example, in the first scenario. In the second scenario, we can perform maintenance procedures, such as replacing the catalyst when necessary due to its age.” Both extend the life of the catalyst in the reactor, which means cost-effective, preventative maintenance procedures occur based on need.
Optical temperature measurement using FBGs along fiber-optic media provides an elegant way to simultaneously record and process a wide range of temperatures to enhance monitoring. It allows efficiently detecting faults and optimizing reaction processes, thereby achieving higher product throughput in the plant.
JOACHIM KOELSCH is a product manager at Siemens Process Industries and Drives, Kalrsruhe, Germany. Email him at Joachim.firstname.lastname@example.org.
1. Egner, F., Bank, R., Verbeek, D., Lainer, M. and Bauer, A., “Distributed Temperature Measurement,” German Patent DE102014018825 (2016).
2. “Fundamentals of Fiber Bragg Grating (FBG) Optical Sensing,” National Instruments, Austin, Texas, (2016).
3. von Dosky, S., Ens, W., Grieb, H., Hilsendegen, M. and Schorb H., “Optical Fiber Temperature Measurement for Process Industry,” presented at AMA Conference, Siemens, Karlsruhe, Germany (2013).