Mineral oils: processes and products – Chemical conversion of hydrocarbons – With prevention or removal of deleterious carbon...
Patent
1990-12-12
1993-02-16
Myers, Helane
Mineral oils: processes and products
Chemical conversion of hydrocarbons
With prevention or removal of deleterious carbon...
585652, 585950, 134 8, 208 48Q, 208 48AA, C10G 912, C10G 916
Patent
active
051868156
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method of decoking an installation for steam cracking hydrocarbons, and to steamcracking installations including means for implementing the method.
In order to remove the coke deposited on the inside walls of an installation for steam cracking hydrocarbons and comprising a steam-craking furnace generally followed by indirect-quench boiler for cooling the cracked gas, it is common practice to use a chemical decoking method based on oxidization by an air-steam mixture. To do this, it is necessary to interrupt operation of the steam-cracking installation and to isolate it from equipment situated downstream.
As an oxidizing agent, it is also possible to use steam superheated to high temperature, together with an optional addition of hydrogen. There is then no need to isolate the steam-cracking installation, but it is still necessary to interrupt its operation. In addition, decoking takes place more slowly than in the preceding method.
These two prior methods are not suitable for completely decoking the indirect-quench boiler situated at the outlet from the steam-cracking furnace. For this purpose, it is necessary from time to time to close down the installation completely, and to decoke the quench boiler by hydraulic means (water jets under very high pressure) capable of breaking up the layer of coke. A hydraulic sand blasting method is also used, with relatively large particles of sand being injected together with water under pressure in order to assist in breaking up the layer of coke, or else mechanical means may be used.
A method has also been proposed for decoking a steam-cracking installation having a single pass type furnace comprising small-diameter rectilinear tubes each extended by an individual quench heat exchanger. The method consists in chemically decoking the inside walls of the furnace tubes by means of steam, thereby causing a portion of the coke to detach from the inside walls in the form of flakes or scale which then breaks up the coke deposited downstream therefrom on the walls of the heat exchangers. This method thus simultaneously decokes the furnace and the indirect-quench means. However, it is still necessary to interrupt the operation of the steam cracking installation.
Finally, various methods have been proposed which consist essentially in injecting solid particles into the installation. A first method consists in setting up a flow of inert gas conveying metal particles of relatively large size (250 .mu.m to 2500 .mu.m) through a furnace connected to the atmosphere. Another method proposes using continuous sand blasting in the steam-cracking installation by injecting sand into the liquid hydrocarbon feedstock. The sand particles (standard sand particles having a mean diameter of 200 .mu.m-1000 .mu.m) pass through the furnace and the indirect-quench boiler and they are finally trapped by the direct-quench heavy oil. The drawbacks of this last-described method are such that it has not been possible to use it: unless a very complex and expensive system is installed for fractioning and washing particles, it is more or less impossible to separate the particles of sand from the direct-quench heavy oil without entraining the difficult-to-vaporize heavy tar contained therein, and as a result, in practice, the particles of sand are not suitable for recycling and the quench oil becomes unusable, even as a fuel; continuously sand blasting the installation also gives rise to severe, or even catastrophic erosion of the tubes through which the feedstock and the products of steam-cracking flow; and finally, injecting particles of sand into the liquid feedstock runs a major risk of solid deposits building up in the zone at the end of hydrocarbon feedstock vaporization.
The object of the invention is to provide a method of decoking a hydrocarbon steam-cracking installation which avoids the drawbacks of prior methods.
Another object of the invention is to provide a method of this type making it possible to decoke the furnace and possibly also the indirect-quench boiler of th
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Myers Helane
Procedes Petroliers et Petrochimiques
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