GB768107A - Cracking gas-oils - Google Patents

Abstract

<PICT:0768107/III/1> Gas oil is cracked by contacting it with a catalyst at a temperature of 850 DEG to 1000 DEG F. until about 40 per cent of the gas oil is converted into gasoline and other hydricarbons boiling below 430 DEG F. together with a small amount of coke, and the vaporous hydrocarbon mixture withdrawn from the catalytic cracking zone is passed to a soaking zone where it is heated to a temperature of 850 DEG to 1150 DEG F. for sufficient time to produce additional hydrocarbon components boiling below 430 DEG F. The duration of the soaking should be about 0.5 to 10 seconds at 1150 DEG F., and about 0.5 to 5 minutes at 900 DEG F. The process is particularly suitable for cracking gas oils having a high aromatic or nitrogen content which tend to produce heavy coking on the catalyst. The product from the thermal cracking zone may be subjected to mild treatment with a catalyst, preferably with some of the partially spent catalyst from the first step of the process, in order to decrease the diolefine-content therein. Preheated gas oil is introduced through conduit 14 into transfer line reactor 12 where it is mixed with regenerated silica-alumina catalyst at 1100 DEG F. from standpipe 13. The catalyst/oil ratio on a weight basis is approximately 11 to 1; and the resultant mixture oil vapours and catalyst has a temperature of 980 DEG F. and is at a pressure of 20 p.s.i.g. Line 12 is 6 feet in diameter and 50 feet long, and the space velocity of the hydrocarbon vapours therein is 40 wt./wt./hour, whereby 40 per cent of the gas oil is cracked while flowing through it. The mixture is discharged tangentially into separating zone 17 in vessel 10. The separated catalyst passes down to stripping zone 19, where it is maintained as a dense fluidized bed by steam-introduced through lines 20. The major part of the stripped catalyst is transferred to regenerator 11 via standpipe 22 and riser 23, air being supplied via conduit 24. The remainder of the catalyst is taken off through standpipe 31, and transferred by steam up riser 34 to the top section of vessel 10. The vapours remain in separating zone 17 for about 10 seconds; they leave it through the opening of annular plate 18, and enter soaking zone 29. The vapours are maintained for 30 seconds at a temperature of 950 DEG F. and a pressure of 15 p.s.i.g. in zone 29, thereby causing an additional 10 per cent conversion of the gas oil. The vapours leaving zone 29 are now contacted in zone 30 with the partially spent catalyst which has been introduced into vessel 10 through riser 34. As it flows down through zone 30, the catalyst passes through three separate dense fluidized beds (each about 8 inches deep) supported on perforated trays 36, 37, 38, provided with downcomers 39, 40, 41 respectively. The temperature in zone 30 is 950 DEG F. and the pressure is 14 p.s.i.g. The stabilized product vapours pass out of the apparatus through cyclone 42, and the catalyst is returned to separating zone 17 via downcomer 41. In embodiments wherein the thermal cracking is effected at a higher temperature than the catalytic cracking, the soaking zone may be a heat exchanger which is heated by hot combustion gases, e.g. the flue gases from the regeneration zone. The combustion gases exhausted from this heat exchanger may be used to preheat the gas oil.