Flexible steam cracking process and corresponding steam cracking

Details Flexible steam cracking process and corresponding steam cracking Flexible steam cracking process and corresponding steam cracking

1997-06-25

1999-10-26

Griffin, Walter D.

Mineral oils: processes and products

Chemical conversion of hydrocarbons

Cracking

208126, 208 48R, 208 48AA, 585648, 585652, 585950, 422200, 422201, 422202, 422207, 422213, 422217, 134 2211, C10G 936, C10G 928, F28D 700, F28D 2100

Patent

active

059722061

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The invention concerns a flexible process for the steam cracking of hydrocarbons, i.e., a process which can handle a wide variety of feeds to be cracked.


BACKGROUND OF THE INVENTION

The prior art is illustrated in International patent application WO-A-90 12851 and European patent application EP-A-0 036 151.
Steam cracking is a basic process in the petrochemicals industry and consists of high temperature cracking then rapidly cooling a feed of hydrocarbons and steam. The principal operating problem arises from the deposition of carbon-containing substances on the internal walls of the facility. These deposits, constituted by coke or condensed, heavy pyrolysis tar, which is coagulated to a greater or lesser extent, limits heat transfer in the cracking zone (coils of pyrolysis tubes) and the indirect cooling zone (effluent transfer line exchanger), requiring frequent stoppages in order to decoke the facility.
Conventional cycle times (between two complete chemical decoking steps in the cracking zone, in air and/or steam) are either fixed (controlled stoppages) or variable depending on the coking in the facility, and are generally between 3 weeks and 12 weeks for feeds such as naphtha and liquefied petroleum gas.
The skilled person is aware that coking problems encountered when cracking heavy feeds (atmospheric gas oils, heavy gas oils, vacuum distillates) are far more severe than those encountered with conventional feeds such as naphtha.
As a consequence, these feeds cannot be cracked in conventional steam crackers designed for cracking naphtha, and they can only be cracked in existing processes in special furnaces typically comprising direct cooling (with pyrolysis oil) of the steam cracking effluents, which is considerably deleterious to the energy balance in the facility (no production of high pressure steam).
Known processes which are flexible as regards heavy feeds are thus incompatible with existing steam cracking facilities for conventional feeds and have a much worse energy balance.
We have proposed (EP-A-0 419 643, EP-A-0 425 633 and EP-A-0 447 527) a decoking process for use during the operation of steam cracking facilities by injection of solid erosive particles, to overcome coking problems and obtain continuous or substantially continuous steam cracking (for example with cycle periods of the order of one year).
For a particular feed, this process consists in allowing a layer of coke to form and age on the internal walls of the cracking coil, then injecting erosive particles (for example hard mineral particles with a diameter of less than 150 micrometers, which may be spherical or angular) in a sufficient quantity to substantially stabilise coking of the tubes without totally eliminating the precoat of coke which protects the tubes.
This process requires a good knowledge of the coking rates in the feed under consideration and a coil design which provides a certain amount of correspondence between the local coking rates connected to the progress of cracking along the coil and the erosion intensity connected to the rate profile along the coil and to the nature of the erosive particles. By means of simulations of coking rates and the circulation rate profile in the coil, and by means of pilot experiments, it is possible to produce substantially continuous steam cracking conditions in the feed under consideration.
Tube erosion can be maintained at a very low or zero level, and controlled by analysis of the trace metals (iron, chromium, nickel) in the recovered powder.
We have thus sought to perfect this process, which can be applied to cracking a particular feed, to a flexible furnace for the successive treatment of a large number of different feeds, under differing operating conditions (flow rate, dilution, cracking severity).
Pilot tests have been carried out and have produced several unexpected results:
Initial coking in the coil (at the beginning of the cycle) can vary very widely depending on the feed, even for feeds which only have slightly different compositions but ar

REFERENCES:
patent: 4420343 (1983-12-01), Sliwka
patent: 5177292 (1993-01-01), Lenglet
patent: 5183642 (1993-02-01), Lenglet
patent: 5186815 (1993-02-01), Lenglet

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