Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By c content reduction – e.g. – cracking – etc.
Patent
1993-12-09
1995-08-29
McFarlane, Anthony
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
By c content reduction, e.g., cracking, etc.
585649, 585650, 585651, 585652, 585653, 208 52CT, 208132, C07C 404
Patent
active
054462290
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a process for converting hydrocarbon feedstocks to olefins by thermal cracking in high temperature cracking or pyrolysis furnaces. In a particular aspect, the invention relates to such a process including steps for reducing formation of coke in pyrolysis furnaces and associated equipment such as transfer line exchangers.
Olefins such as ethylene and propylene are produced by pyrolysis (thermal cracking) of petroleum gases (butane, ethane, propane, etc.) or of distillates such as naphtha, n-hexane and gas oil. The cracking reactions occur in a thermal reactor comprising tubes or coils inside a high-temperature furnace. During manufacture of olefins, steam can be added to the hydrocarbon feedstocks to reduce hydrocarbon partial pressure to promote production of olefins and to minimize rate of coke deposition. The cracking reactions occur under a reducing atmosphere in which there is substantially no oxygen present.
Pyrolysis tubes are made of metal alloys which can withstand the high temperature and stress-strain relationship of production conditions and that are stable against carburization. A special 25% chrome-20% nickel alloy has been a preferred tube material for reactor coils. Materials having a still higher nickel content, which may be alloyed with minor amounts of carbide stabilizers such as tungsten and niobium (columbium), also are used. Such materials contain catalytic sites which promote the formation of catalytic coke. Typically, other portions of the cracker are made of lower grade steels which are less corrosion-resistant.
Illustrative of olefins production, cracking of petroleum gases or distillates to produce ethylene is accomplished by passing the hydrocarbon feedstock at a positive pressure into a high-temperature cracking furnace composed of a feed section, a convection heating coil section and radiant coil cracking section; cracking the feed in the cracking furnace at a temperature in the range of from about 650.degree. C. to about 930.degree. C. and in the presence of steam, with the steam-to-hydrocarbon ratio typically in the range of from about 0.2:1 to about 1.5:1; pressurizing the cracking furnace effluent in a compressor from substantially atmospheric pressure; and fractionating the pressurized cracking furnace effluent, wherein the pressure profile within the cracking furnace is optimized to maximize the yield of ethylene. Modern ethylene plants are normally designed for near-maximum cracking severity, a term used to describe the depth of cracking or extent of conversion, because of economic considerations. Hence, the production of ethylene from hydrocarbon feedstocks is accompanied by the production of other olefins which entails co-production of many industrially important petrochemicals.
The effluent from the pyrolysis furnace is provided to a transfer-line exchanger (TLE) located adjacent to the furnace. The TLE provides rapid cooling of the heater effluent to prevent further reactions of the effluent from continuing thereby impairing the yield of primary products.
The gaseous effluent from the pyrolysis furnace leaves the TLE at temperatures in the range of 300.degree. C. to 600.degree. C. to enter the separation and recovery portion of the ethylene plant. In this portion, effluent is separated into the desired products by compression in conjunction with condensation and fractionation at gradually lower temperatures. Typically, all the C.sub.2 's are separated from C.sub.3 's and higher. Acetylene must be removed from the C.sub.2 fraction to meet ethylene specifications. Most frequently, the acetylene is eliminated by hydrogenation of the C.sub.2 fraction in the presence of an effective acetylene conversion catalyst.
During operation of olefin furnaces, there is a buildup of coke on the inside of the furnace tubes or coils into which the hydrocarbon feedstocks are fed and on the inside of the tubes or coils in the transfer line heat exchangers into which the effluent from the furnaces is fed. This coke buildup in the ethy
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Allen Lynn M.
Schroeder Myron E.
Taylor Don M.
Amoco Corporation
McDonald Scott P.
McFarlane Anthony
Oliver Wallace L.
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