A sharp profile
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A sharp profile Optical determination of temperature gradients within the smallest space enables optimized reaction 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. An innovative measuring system for fiber-optic temperature measurement has been developed for this purpose. It allows a greater number of measuring 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 / tube bundle reactors. They significantly influence the course of the reaction, the quality of the material conversion as well as the aging of the catalyst. The identification of hotspots – areas with excessive temperatures that can occur in the filling – plays an important role. Matthias Hüning of Evonik Industries in Marl, Germany, 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 a plant shutdown, which would otherwise be required due to the complicated procedure for replacing a catalyst.” The multipoint measurement system SITRANS TO500 transmits temperatures and temperature profiles reliably and fast. By indicating the spatial temperature distribution, the system improves visibility into operating conditions to optimize quality, yield and lifetime. Due to the small diameter of the reactor tubes, the necessary number of measurement points and the demands on the speed of data acquisition, it was not possible to use a conventional measuring system (RTDs or thermocouples). Together with Siemens, Evonik therefore employed fiber-optic temperature sensing based on Fiber Bragg Grating technology.

Fundamentals of optical temperature detection Fiber Bragg Gratings (FBG) are optical periodic structures inscribed in optical fibers. Since a particular wavelength of incident light is reflected while all others are passed, each grating acts as a narrow-band filter. Fiber Bragg Gratings (FBG) are optical periodic structures inscribed in optical fibers. Since 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...

Detailed profile provides knowledge In this application, each measuring probe with a diameter of approximately 1 mm records temperatures in a measurement range of 0 ºC to 400 ºC with a measuring error of < 0.5 K. It is also characterized by a very fast response time; the T90 time is under four seconds [6]. 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 substantially reduced. On the one hand, a larger cross-section and...

References: [1] Press release Evonik: VESTAMID® L – Polyamide 12 http:// corporate.evonik.de/de/presse/suche/pages/news-details. aspx?newsid=4980 [2] HBM: https://www.hbm.com/de/4596/was-ist-ein-faser-bragg-gitter/ [3] elektronik messen+ testen 2/2010 Grundlagen der optischen Sensormessung mit Faser-BraggGittern (Fundamentals of optical sensor measurement with Fiber Bragg Gratings) [4] Optical fiber temperature measurement German Patent DE102014018825 [5] Fundamentals of Fiber Bragg Grating (FBG) Optical Sensing http://www.ni.com/white-paper/11821/en/ [6] Optical Fiber Temperature...