Mineral oils: processes and products – Chemical conversion of hydrocarbons – Cracking
Patent
1991-02-04
1992-08-18
Morris, Theodore
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Cracking
208 48Q, 585650, 422197, 422198, 422207, C10G 914, C07C 402, F28D 2100
Patent
active
051396505
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method and to an installation for steam cracking hydrocarbons to produce ethylene and other unsaturated low hydrocarbons.
It is common practice to steam crack hydrocarbons in a furnace having tubes for circulating a steam-hydrocarbon mixture which is raised to high temperature, generally about 850.degree. C. to 880.degree. C., for a very short period of time, about 0.07 seconds (s) to 0.3 s.
Use is made, in particular, of furnaces in which the tubes form sinuous paths comprising a plurality of straight lengths in series. The tubes are connected via an outlet manifold to quenching means, either by direct contact with oil or else by indirect contact in a heat exchanger or in a boiler, with quenching being intended to stop chemical reactions in the steam cracking effluents, and in particular secondary pyrolysis reactions of the olefins formed by the steam cracking.
In these furnaces, the tubes are of great length and large diameter (e.g. an inside diameter of 75 mm to 150 mm), which prevents the effluents leaving the furnace having a high temperature (the rate of temperature rise in large-diameter tubes is insufficient, and obtaining high outlet temperatures for the effluents would lead to over cracking thereof). The time required for collecting and transferring the effluents to the quenching zone also makes it impossible to have a high effluent temperature at the outlet from the furnace since that would give rise to over cracking of the effluents and thus to a drop in yield.
However, there exists a hydrocarbon steam cracking method in which the reaction temperatures in the furnace and the temperatures of the effluents at the outlet from the furnace are very high (e.g. 880.degree. C. to 900.degree. C. for a feedstock of naphtha), and this is achieved by using a plurality of small-diameter tubes (e.g. 30 mm) with cracking being performed during a single pass through the furnace. In order to avoid effluent over cracking downstream from the tubes, the ends of the tubes are directly connected to a plurality of small quenching heat exchangers which are disposed on extensions of the furnace tubes, with each quenching heat exchanger corresponding to one of the furnace tubes or possibly to two of the furnace tubes. The hydrocarbons are thus cracked at very high temperature and they are quenched without significant transit time between the furnace and the quenching heat exchanger. Good yields of ethylene, propylene, and butadene are thus obtained. However, this technique requires special quenching heat exchangers to be implemented which are much less compact than the conventional quenching heat exchangers of the first technique mentioned above and which require a special decoking procedure which is rather difficult.
These two prior techniques cannot be combined since they are incompatible: merely replacing a quenching heat exchanger associated with a conventional furnace by a plurality of elementary heat exchangers situated on extensions of its tubes would not suffice, since this would require the tubes to pass through the roof of the furnace, and this cannot be done on existing furnaces. It is also impossible to replace the large-diameter tubes by a plurality of small tubes while retaining the quenching heat exchangers: in order to avoid over cracking in the transfer line going to the quenching heat exchangers, it would be necessary to limit the cracking temperatures which would loose the advantages related to using small tubes with a very high cracking temperature (ethylene yields increased by about 15%).
A particular object of the invention is to provide a method and apparatus for steam cracking hydrocarbons which enables the advantages of the two above-mentioned prior techniques to be combined without suffering from their drawbacks.
Another object of the invention is to provide a method and an installation for steam cracking hydrocarbons enabling a higher yield to be obtained in olefin production.
Another object of the invention is to provide a method and an installation of this type ena
REFERENCES:
patent: 3607153 (1971-09-01), Cijer
patent: 4714109 (1987-12-01), Tsao
Chemical Abstracts, vol. 87, No. 12, Dec. 19, 1977 "Processing of Hydrocarbon Raw Materials," Lobanov, V. A., p. 171.
Hailey P. L.
Morris Theodore
Procedes Petroliers et Petrochimiques
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