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01 February 2004

Fiber-optic sensors spread out

Tube reactors depend on accurate temperature profile for quality product.

By John Berthold and Richard Lopushansky

Measuring the temperature profile at various radial locations along the length of a catalyst tube in an ethylene oxide (EO) reactor provides vital information for the safe and efficient operation of the reactor.

Here are the results of a test of a 32-channel fiber-optic temperature measurement system in an EO reactor.

This organization tested the 32-channel fiber-optic system in a 42-foot-high ethylene oxide tube reactor to demonstrate its suitability for use in industrial applications.

This 32-channel fiber-optic temperature measurement system tested along the length of a 42-foot high ethylene oxide reactor.

The sensors provided discrete measurement points along the length of the reactor. Monitoring the temperature along the length of the reactor gave the reactor operator vital information to safely and efficiently operate the reactor, including detection of hot spots that could compromise the integrity of the reactor and improved prediction of the end of life for the catalyst in the reactor.

Conventional multipoint thermocouple probes are considered unsatisfactory for measuring the temperature of small-diameter catalyst tubes for the following reasons: the large cross section of thermocouple probes adversely affects the flow and heat transfer dynamics of the catalyst reaction in the small diameter tubes, fewer than 10 discrete measurement points can be included in a typical thermocouple probe, and the life of conventional thermocouple probes is often less than six months.

The multipoint fiber-optic temperature sensing system tested in this application overcomes these shortcomings. This fiber-optic temperature measurement system is ideal for monitoring the temperature profile in tube reactors, along the length of furnace tubes, or in any application that requires a robust, high-resolution measurement with a very small cross section.

Fiber-optic temperature sensing probes can range in length from several inches to more than 50 feet and can be packaged in sheaths smaller than 1/8 inch in diameter.

One can space the individual temperature sensors as close as 1 inch apart. Laboratory experiments have shown that unsheathed fiber-optic sensors respond to temperature changes in excess of 400°F in less than one second.

Sensor location Notice that for comparison purposes the fiber-optic sensors are on the left and clustered about the reference thermocouples, which are on the right.

It is now possible, with fiber-optic sensing systems, to make 32 or more temperature measurements along the length of a catalyst tube with a system that is rugged enough to survive the high-velocity gas flow in the dome of a reactor. Because of the small cross section and flexibility of optical fiber and because of the innovative packaging of the sensors, installation time of the sensor probe is minimal. It is even possible to install or replace a sensor probe while the reactor is in operation and to remove a sensor probe at the end of a run for use in the next turnaround cycle.

Rugged multipoint connectors quickly and reliably connect the sensors to the signal conditioner. The signal conditioner provides a seamless interface with existing control systems including 4–20 mA, 0-5 volt, RS-485, Modbus, and fieldbus and can be located safely hundreds of feet away from the hazardous process. High-resolution signal conditioners can multiplex with 32 or more sensors, providing a cost-effective solution for multipoint temperature measurements in hazardous environments.

Optical signals to temperature

The temperature measurement system consisted of a 46-foot long probe, a 100-foot long cable, and a 32-channel signal conditioner. The probe was made of 1/4-inch diameter 316 stainless steel. The probe actually contained 37 fiber-optic temperature sensors spaced 12 inches apart, beginning 3 feet from the end. The 37 optical fibers exited the 1/4-inch tube and were terminated in a multipoint fiber-optic connector.

This multipoint connector enabled the probe to easily connect and disconnect and assured the integrity of the system configuration. The connector mated with a 100-foot armor sheathed cable and linked the 37 temperature sensors to the signal conditioner. The signal conditioner converted the optical signals to temperature readings and transmitted these readings to the process control computer via standard 4–20 mA current loops.

The fiber-optic sensor probe centered in a catalyst tube that itself centered inside a 6-inch diameter reactor. The 2-inch annular space between the internal tube and reactor wall contained kerosene. At ambient temperatures, the kerosene level was 29 feet from the top of the reactor.

The catalyst tube contained inert material and was not actual catalyst. During the test, nitrogen gas flowed through the inert catalyst and served as a heat transfer medium.

The organization ran only 16 4–20 mA lines from the control room to the signal conditioner, and so monitored only 16 of the 37 sensors in the probe. Five reference thermocouples were located in the annular space between the jacket and the catalyst tube, and they registered readings for comparison purposes.

These thermocouples sat at elevations (measured from the top of the reactor) of 1, 5, 15, 29, and 39 feet. The 16 active sensors were from locations near the reference sensors to provide a distribution of temperature measurements along the length of the reactor near the reference thermocouples. Note that two of the reference thermocouples—the thermocouples located at 29 feet and at 39 feet—were in the kerosene liquid, and the others were in the kerosene vapor phase.

The plan was to operate the reactor through the following temperature cycle, ranging from ambient to approximately 500°F.

• Ramp the temperature to 500°F, and maintain the reactor at 500°F through the first day.
• Shut down the reactor at the end of the first day; allow it to cool passively overnight.
• Ramp the temperature to 500°F, and maintain the reactor at 500°F through the second day.
• Shut down the reactor at the end of the second day; allow it to cool passively overnight.
• Open the reactor and remove the probe on the third day.

The slow rate of passive cooling during the night provided a great amount of information and an excellent basis for evaluating the performance of the fiber-optic sensing system.

Test results

The first test demonstrated the signal conditioner survived shipping, handling, and installation. This test also showed the seamless linkage between the fiber-optic signal conditioner and the existing control system via the conventional 4–20 mA communication protocol. It demonstrated the simultaneous communication to other computers via the RS-232 serial digital protocol. A separate system monitor and calibrated sensor standards provided a stable and immediate comparison between the readings displayed at the control system and the signal being sent from the signal conditioner.

The test demonstrated the following: ruggedness of the system to survive shipping, handling, and installation; seamless communication via the 4–20 mA; consistent readings of the control system with those at the signal conditioner; consistent communication via the RS-232 digital interface with the 4–20 mA readings; and demonstrated upscale burnout reading.

The second test demonstrated the ease of installing and connecting the sensor probe to the signal conditioner. This involved positioning the 46-foot-long temperature sensing probe into the reactor and completing the connection via the multipoint fiber-optic connector. Once the system was installed and operational the functionality test began.

This test of the fiber-optic temperature measurement system has shown great potential for overcoming the shortcomings of conventional thermocouples for applications where high-resolution multipoint measurements are required. The small cross section of the fiber-optic probe provided reliable temperature profile measurements with minimal effect on the process, which should lead to safer and more efficient reactor operations. The fiber-optic instrumentation can be used for detecting hot spots that could compromise the integrity of the reactor and predicting the end of life for the catalyst.

This beta test demonstrated:

• the capability of packaging 37 fiber-optic temperature sensors in a probe with a 1/4-inch outer diameter,
• the durability of the cabling, probe, and instrumentation to survive shipping and handling during installation,
• a convenient connector/cable design that provided quick and reliable installation and configuration of the sensors relative to the signal conditioner,
• a 32-channel, time-based multiplexing capability with an update rate of ten seconds for each channel,
• a seamless and direct interface between the signal conditioner and the existing control system via standard 4–20 mA loop outputs,
• the capability to provide the required scaling, engineering units, and upscale burnout readings, and
• the capability to communicate the measurement results to a remote host computer via standard serial communication channel (RS232, RS422, and RS485). CP

Behind the byline

John W. Berthold, technical director of Davidson Instruments (www.davidson-instruments.com), has thirty-five years of experience in optics research and sensor development and holds 35 U.S. patents. He has a Ph.D. in optical sciences from the University of Arizona and is a fellow in SPIE. Richard L. Lopushansky, president of Davidson Instruments, has twenty-five years of experience in the start-up and development of technology-based businesses serving the government and the oil and gas industry. He has a B.S. in industrial engineering and an MBA.

Ethylene oxide kills good, kills bad Photo courtesy of NASA The Environmental Protection Agency (EPA) first registered ethylene oxide as an antimicrobial pesticide in 1948. Ethylene oxide works to sterilize hospital items, to treat processed spices and seasonings, and to treat commercial food processing, handling, and storage facilities. When used directly in the gaseous form or in nonexplosive gaseous mixtures with nitrogen or carbon dioxide, ethylene oxide can act as a disinfectant, fumigant, sterilizing agent, and insecticide. Ethylene oxide (EO) is also an ingredient or an intermediate in the production of several other chemicals including ethylene glycol and polyester. Ethylene glycol is primarily in automotive antifreeze, and polyester is in fibers, film, and bottles. Ethylene oxide is also part of the process to produce nonionic surfactants in household and industrial detergents. A surfactant is a chemical that reduces the surface tension of water. During October 2001, the U.S. government identified incidents of contamination of several government and commercial buildings throughout the U.S. with spores of Bacillus anthrasis, commonly known as anthrax. Because ethylene oxide is effective in killing spore-forming bacteria, the EPA has identified EO as a potential decontamination agent. Although ethylene oxide is not officially for use against anthrax spores, the EPA decided that under certain emergency conditions EO can be sold for that purpose.