Ethylene is produced in the petrochemical industry by steam cracking. In steam cracking, a gaseous or liquid hydrocarbon feed like naphtha, LPG or ethane is diluted with steam and briefly heated to 750-950°C, inducing radical reactions followed by immediate quench to stop these reactions. The cracking process converts saturated hydrocarbons into smaller, often unsaturated, hydrocarbons. Ethylene is separated from the resulting complex mixture by repeated compression and distillation. The products produced in the reaction depend on the composition of the feed, the hydrocarbon to steam ratio and on the cracking temperature and furnace residence time.
The process also results in the slow deposition of coke on the reactor walls that degrades the thermal efficiency and wear of the reactor. An ethylene cracking furnace usually requires de-coking every a few months. In the de-coking process, the hard solid coke layer on the wall of coils is heated by steam or air and converted to carbon monoxide and carbon dioxide.
Critical Control of Process Gas
To monitor the reactions in cracking furnace and improve combustion efficiency of the fuel, the ethylene production process often requires to detect oxygen(O2) on radiant section of the cracking furnace.
The reaction mixture exiting the transfer line exchangers (TLE) reflects the status of cracking reactions in heat coils, while during the decoking process the TLE off-gas reflects how completely the coke deposit is being burned. Monitoring carbon monoxide(CO) and carbon dioxide(CO2) in the TLE off-gas is used as input to optimize the processes of cracking and de-coking. By measuring hydrogen sulfide(H2S) outlet of caustic scrubber to control the process of acid gas removal.
Challenge of Instrumentation
Traditionally zirconia probes (O2) and extractive analyzers (CO, CO2) are used for monitoring process gases of ethylene production. Zirconia probe measures gas concentration in a limited extension neighboring to the probe. The accuracy of measurement value is inevitably affected by gas turbulence. Zirconia probe and extractive analyzers both are limited by their low adaptability to particulates, corrosives, high temperature and changing of fuel sources, resulting in more consumables, high cost and not a real continuous operation.
FPI’s cross-duct LGA measures average gas concentration along the laser beam path, resulting in much less effect from gas turbulence and gas sources change. The real-time precise measurement of combustion process helps improve thermal efficiency of the cracking furnace and reduce coke deposition in the reactor.
The fine tuned laser spectrum intentionally eliminates methane(CH4) absorption that is capable of detecting CO content with no interference in all combustible gases either fuel oil or natural gas. The non-contact laser and sensor of the in-situ LGA combined with continuous purge results in virtually maintenance free, and performs well in high temperature up to 1500°C. High-speed response (down to 1s) of the LGA highlights its reliability for the process control of cracking and de-coking.
Radiantsection of cracking furnace
Outlet of caustic scrubber
NOTES: Listed are typical data FPI experienced and backed by field references. Alternate gases and ranges may be available or customizable on request.