Patent GB-722369-A (GB)
Title: Improvements in and relating to the cracking of heavy hydrocarbon oils
IPC: C10G9/00
Filed: 1952-08-15
Abstract: <PICT:0722369/III/1> Heavy hydrocarbon oils are cracked by causing the oil to flow down a plate-type fractionating column or similar contacting device wherein it is brought into direct contact with a countercurrent stream of a portion of the light gas oil vapours withdrawn from the upper part of the column, the vapours being compressed and heated (without cracking) prior to being introduced into the column so that they contain sufficient heat to effect thermal cracking of the heavy oil to vaporous products.  The vapours withdrawn from the upper part of the column contain only minor amounts of gas and gasoline (about 5 per cent), the chief product being gas oil suitable for catalytic cracking.  The residue obtained at the base of the column is suitable for blending to fuel oil.  A reduced crude, fed through line 38, is preheated to about 400 DEG C. in the convection section 11 of heater 12, and is then introduced into the upper section of fractionating tower 14 which is supplied, through line 15 and one or other of lines 16, 17 and 19, with overhead vapours, from the same tower, with a dew point up to about 400 DEG C., the vaporous streams being introduced into the tower at suitable temperatures, and at a pressure of about 2 atm. above that of the feed oil, so that the temperature in the intermediate sections of the tower is maintained at about 450 DEG C. while that in the lower section is maintained at about 420 DEG C.  The rate of flow of the heavy hydrocarbon feed is adjusted to give a residence time in the tower of 20 to 30 minutes, whereby nearly all the heavier oils are converted to lighter oils, but a residual fraction containing heavy oil components remains in order to avoid a coking-tendency in the lower section of the tower.  The residual fraction, withdrawn from the base of the tower, is passed to flash separator 28, which is maintained at reduced pressure, and the distillate which is evolved is returned to the tower by lines 33 and 38.  The residue from separator 28 may be blended with a relatively heavy catalytic cracked gas oil to make a fuel oil of suitable viscosity.  The overhead vapours from the tower are withdrawn through line 20 at a temperature of 400 DEG C., and about 70 per cent is passed through centrifugal compressor 22 to one or more of the coils (not shown) in the radiant sections of heater 12, wherein it is heated to about 440 DEG C. prior to recycling to the tower.  Or, instead of using part of the total overhead in line 20, a vaporous stream can be withdrawn from the tower by line 21, and used as the recycle stream.  In order to ensure that the compressed recycle stream in line 24 is in the vapour state, a separator is provided for removing any liquid formed during compression.  That part of the overhead vapours not recycled is introduced, via line 26 and centrifugal expander, into fractionator 29, the energy thus made available being used to run compressor 22. Light gas oil is withdrawn from the bottom of the fractionator, and is passed forward to the catalytic cracking plant.