Servomex  - a Spectris Company

Experts in ethylene solutions

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Courtesy of Servomex - a Spectris Company

The precise and rapid analysis of process gases within an ethylene plant is vital to ensure safe and efficient operation, being used to optimize process efficiency while maintaining the highest levels of safety. These efficiencies can be achieved in a variety of ways, including shortening the residence time in the cracking furnace; rapid adaptation to changing feed-stocks; improving combustion control; optimizing fractionation; and improving process reliability. Safety can also be improved as a result of fast, reliable process analysis technologies with high measurement availabilities.

The multiple processes involved in manufacturing ethylene require a range of gas analyzer measurements to help manage the efficient control and safety of the process, optimize combustion, maintain product quality and manage emissions to comply with environmental legislation; more than thirty different points of gas analysis are present in total. With considerable differences between the measurements required – for instance the type of measurement needed, the gas being measured or the relative concentration to be sensed - specific analyzer technologies are designed for and perform better at different points in the process. Traditionally these measurements have been predominantly made by infrared (IR) and gas chromatograph (GC) analysis techniques, supported by techniques for combustion and emissions analysis. However, recent advances in in-situ Tunable Diode Laser and in-line Tunable Filter infrared technologies have expanded the measurement technologies available for faster, safer or more accurate control of the process at certain points. IR analysis techniques provide key measurements across the whole of the ethylene manufacturing process; for example, IR analysis is crucial to cracking an ethane or mixed light hydrocarbon feedstock. The majority of IR process analyzers employ a single beam, dual wavelength measurement technique that offers high performance, repeatability and long-term stability. In particular, the measurement is virtually unaffected by the build-up of sample cell contamination since it influences both wavelengths equally. A 50% loss of signal due to obscuration of the sample cell windows produces no more than 3% error in the reading. Analyzers using gas filter correlation (GFC) technology, such as the Servomex SERVOTOUGH SpectraExact 2500 infrared analyzer, can measure trace levels of carbon dioxide or carbon monoxide with exceptionally low crossinterferences from hydrocarbons. Combustion control for the cracking furnace is provided through monitoring both oxygen and combustibles in flue gases. This is most commonly achieved by utilizing a close-coupled extractive combustion analyzer capable of measuring both oxygen (O2) and carbon monoxide (COe). In these analyzers, sensors are housed in a sensor head mounted on the process wall, to which a sample is aspirated via a sample probe giving fast in-phase response of both O2 and COe. The range of probes available enables a fast, dynamic and accurate analysis in high temperature combustion applications. However the advent of Tunable Diode Laser (TDL) technologies now allows a path-averaged combustion measurement of O2, CO and CH4 to be made in-situ of the process.

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