New Sensing System Optimizes Chemical Industry Processes

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A petrochemical plant in Heide, Germany. A new fiber-optic sensor measures temperatures inside chemical reactor tubes, thus enabling the detection of critical temperatures. | Download

Siemens has, for the first time, managed to adapt the fiber-Bragg grating temperature measurement system in the form of a sensor for industrial applications. Called Sitrans TO500, the new sensor measures the temperature inside the chemical reaction mixtures in tubes that are several meters long. The sensor enables measurements to be taken at up to 48 points along a fiber-optic cable, making it possible to create a precise temperature profile. Previous measuring probes enabled measurements to be taken at only a few points. Thanks to the new technology, process engineers can now obtain unprecedented insights into reaction processes and control them more efficiently. The sensor has been successfully tested by customers from the chemical industry and was presented at the SPS IPC Drives trade fair in November 2016.

Good-bye to Bulky Systems
Tube-bundle reactors consist of externally cooled reaction tubes in which a gas mixture, for example, is chemically converted with the help of a catalyst. The tubes are several meters long and between two and five centimeters in diameter. The temperature changes along the length of the reaction volume, depending on factors such as the amount of gas that flows into the reactor and where. Such reactions typically occur at temperatures ranging between 250 – 600 degrees Celsius. To measure temperatures, the sensor is inserted into a protective tube and placed into the reactor tube. Conventional sensors such as resistance thermometers and thermocouples require a separate component and cable for every measuring point. As a result, associated protective tubes have to be several centimeters wide even if only a few measuring points are used, and processes have be controlled on the basis of theoretical models based on only a few measurements. However, chemical reactors could be operated more efficiently if the temperature profiles taking place inside them were more precise. Reactions could then be controlled in such a way that catalytic converters would age as slowly as possible, resulting in a diminished need for replacements.

Ultrathin Optical Fiber Sensor
Although the new fiber-optic sensor from Siemens has up to 48 measuring points, it nevertheless fits into a protective tube less than two millimeters wide. The sensor consists of an optical fiber less than one millimeter thick. Optical periodic structures known as Bragg gratings are placed along the fiber. When light is sent into the fiber, a specific wavelength is reflected, depending on the spacing of the Bragg grating. Because a medium’s refractive index depends in part on its temperature, the wavelength of the reflected light at various temperatures deviates from the previously measured reference value. In order to take measurements at several points, the fiber has multiple Bragg gratings, each for a different wavelength. The laser light used for this sensor also has a variety of wavelengths, which extend over a bandwidth of 100 nanometers. As a result, the sensor can measure the temperature at each Bragg grating.

Flexibility and Temperature Resistance
In order to adapt fiber-Bragg grating technology to industrial applications, Siemens engineers had to make the inherently brittle fiber somewhat flexible, as this is the only way the sensor can be inserted into a reactor and extracted again when, for instance, a facility is undergoing maintenance. Flexibility also enables the meter-long sensor to be rolled up for transport. Another challenge faced by engineers was the high temperatures inside the reactor. Not only does the fiber have to withstand this heat, it has to be adjusted in advance to prevent temperature drift in measurements.

Contact:
Mr. Dr Norbert Aschenbrenner
Editorial Office
Siemens AG
norbert.aschenbrenner@siemens.com

Original Internet Article:
https://www.siemens.com/innovation/en/home/pictures-of-the-future/industry-and-automation/digtial-factory-fiber-optic-measures-temperature.html

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