Process for producing thermally cracked products from hydrocarbo

Details Process for producing thermally cracked products from hydrocarbo Process for producing thermally cracked products from hydrocarbo

1996-08-21

1998-12-15

Yildirim, Bekir L.

Mineral oils: processes and products

Chemical conversion of hydrocarbons

With prevention or removal of deleterious carbon...

208 48AA, 208130, 208131, 585648, 585650, 585950, C10G 916

Patent

active

058491767

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The invention relates to a process for producing thermally cracked products from hydrocarbons and simultaneously reducing the coking on heat exchange surfaces in reactors for the thermal cracking of organic compounds.


BACKGROUND OF THE INVENTION

To produce ethylene and other alkenes, hydrocarbons or hydrocarbon mixtures are thermally cracked in externally-heated reactors of metal materials. The cracking furnaces preferably consist of high-temperature-resistant tubes containing chrome and nickel.
The effectiveness of tubular reactors depends heavily on whether coke deposits that interfere with heat transfer form on the inner surfaces of the reactor tubes during operation. This generally happens in the reactors in use today. After operational periods of between one week and three months, the deposits formed on the reactor interiors that come into contact with the hydrocarbons become so extensive that the resulting decline in performance necessitates shut-downs and expensive cleaning procedures. The coke deposits are usually removed by means of gassing with a mixture of hot steam and air, which uncovers the metal surfaces and permits the desired heat flow to again be achieved. However, in spite of the thorough removal of deposited coke, new deposits can form and necessitate renewed shut-down and decoking after relatively short periods of operation. Because the oxidative decoking procedures that are used lead to high-temperature corrosion of the tubes, which in turn creates reaction products that catalyze coke formation, the deposit of coke accelerates after each decoking. For this reason, the operational periods between decokings grow shorter as the number of decoking cycles increases. This is undesirable from the technical as well as the economic point of view, because it interferes with steady long-term operation, reduces plant efficiency and requires more frequent cleaning expenditures. Attempts have therefore been made for years to find solutions that counteract the rapid formation of coke on the inner surfaces of tubular reactors. In some plants, a treatment with alkyl-ammonia sulfonates is used to achieve this object; however, this treatment does not result in reduced coke formation U.S. Pat. No. 4,105,540 describes the addition of small amounts of phosphoric and phosphorous acid esters, while U.S. Pat. No. 4,551,227 describes the addition of a combination of tin and phosphorous compounds; phosphorous and antimony or tin compounds; or antimony and phosphorous compounds as anti-foulants. U.S. Pat. No. 3,647,677 and U.S. Pat. No. 3,531,394 propose the use of elementary phosphorous and phosphorous compounds as anti-foulants in refinery plants and steam cracking plants, respectively. This also applies analogously to U.S. Pat. No. 4,835,332 (tri-phenyl-phosphine), U.S. Pat. No. 4,900,426 (tri-phenyl-phosphine-oxide) and U.S. Pat. No. 4,842,716, which describes the addition of a combination of a phosphorous anti-foulant and an inhibitor (imidazoline) that favors film formation. Finally, gaseous or vaporous sulphur-containing compounds, as described in the publication by Boene, K.: Oil Gas J. 81 (1983) 93, and nitrogen-containing compounds are described as additives with an anti-fouling effect. All of these additives evidently lead to a limited reduction in the coking tendency, but without inhibiting the carburization of the surface layers of the material, which must be understood as the start of an undesirable reaction cascade. Because catalytically active centers are created at the end of this process, the use of such anti-foulants addresses the symptoms, but not the causes, of coke formation.
It is known that the carburization of metal surfaces is inhibited by relatively high silicon contents in these surface layers, as described in the publication by Norton, J. F. and Barnes J.: Corrosion in Fuel Systems, The Electrochemical Soc., Pennington N.J. (1983) 277, whereby this inhibition evidently gradually declines under the conditions of thermal cracking by depletion of the silicon s

REFERENCES:
patent: 4024049 (1977-05-01), Shell et al.
patent: 4692234 (1987-09-01), Porter et al.
patent: 4756819 (1988-07-01), Bousquet et al.

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