Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By c content reduction – e.g. – cracking – etc.
Patent
1994-11-18
1998-03-17
Myers, Helane
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
By c content reduction, e.g., cracking, etc.
585649, 585650, 585921, 585926, C07C 404
Patent
active
057289166
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This application is a 371 of PCT/6893100920.
This invention relates to thermal cracking, and in particular to the thermal cracking of hydrocarbons. Hydrocarbons containing 2 or more carbon atoms, e.g. ethane, propane, butane, LPG, and naphtha are generally cracked to produce olefins by passing a mixture of the hydrocarbon and steam through tubes, free of internal packing, heated to a high temperature in a furnace in the absence of a catalyst. The cubes typically have an inside diameter of 25 to 100 mm or more and the feedstock/steam mixture passes through the tubes at a high flow rate so that the flow through the tubes is extremely turbulent so as to obtain good heat transfer. Typically the flow rate corresponds to a Reynolds No. of the order of 500,000 or more.
The presence of stem aids transfer of heat from the furnace walls to the hydrocarbon: the temperature of the tube walls in contact with the process stream is typically 100.degree. C. or more above that of the gas. The steam also decreases the formation of coke and acts as a diluent to decrease the partial pressure (since the cracking reaction to olefins is favoured by low hydrocarbon partial pressures). Typically 0.3 to 0.5 tonnes of steam are employed per tonne of hydrocarbon feedstock and the outlet pressure is typically below 2.5 bar abs, for example in the range 1.7 to 2.1 bar abs.
However the use of steam is thermally inefficient and poses environmental problems. Thus steam is not completely inert under the conditions employed: normally the cracker effluent contains a small proportion of organic oxygenated compounds such as acetaldehyde, acetone, carboxylic acids, and phenols resulting from the reaction of steam with the hydrocarbon. After the cracking reaction, the effluent gas is cooled to condense the steam and as a result some of such compounds pass into the liquid water phase. While most of the water is recycled to form more steam, the presence of such compounds necessitates the addition of basic materials such as ammonia to the water to minimise corrosion. Also some of the water is bled off as a purge to avoid a build up of undesired components. This purge, typically amounting to about 10% of the condensed water, must be treated before disposal in order to avoid environmental problems.
The tubes of the cracker are normally made from steel containing a proportion of nickel in order to obtain the required mechanical properties at the temperatures encountered. Nickel, and to some extent some of the other components of the steel, catalyses the reaction of hydrocarbons with steam and also catalyses their decomposition to coke which adheres to the tube surfaces reducing heat transfer. To alleviate these problems, it is normal to introduce some sulphur compounds (which decrease the catalytic activity of nickel by acting as a catalyst poison) into the feedstock. However the sulphur compounds subsequently have to be removed from the effluent process gas: this is often effected by means of a scrubber wherein the process gas is contacted with an aqueous caustic solution. The disposal of the resultant caustic effluent also presents environmental problems.
SUMMARY OF THE INVENTION
We have found that these problems may be overcome by operating the cracking reaction in the substantial absence of stem. In order that the cracking reaction can be satisfactorily effected, various changes have to be made to the cracking process.
Accordingly we provide a process for the cracking of hydrocarbons comprising passing a hydrocarbon feedstock substantially free from steam through an externally heated catalyst-free reaction zone having a heated surface to volume ratio above 3 cm.sup.-1 at a rate such that the flow through the reaction zone is essentially laminar.
DETAILED DESCRIPTION OF THE INVENTION
The process is operated in the substantial absence of steam, although we do not preclude the presence of small amounts of steam, e.g. up to 0.1 parts by weight of steam per part by weight of hydrocarbon feedstock. Preferably the react
REFERENCES:
patent: 4636297 (1987-01-01), Uchiyama et al.
patent: 5160501 (1992-11-01), Alagy et al.
patent: 5162599 (1992-11-01), Mattuno et al.
patent: 5270016 (1993-12-01), Alagy et al.
Database WPI, Week 8304, Dec. 18, 1982, Derwent Publications Ltd., AN8308934K & JP,B,57 060 398.
Gough Arthur
Ramshaw Colin
Imperial Chemical Industries plc
Myers Helane
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