Patent GB-733636-A (GB)
Title: Cracking crude petroleum oil
IPC: C10G11/18
Filed: 1952-10-03
Abstract: <PICT:0733636/III/1> Crude petroleum oil is contacted with hot cracking catalyst whereby the oil is completely vaporized and a substantial proportion of the heavy constituents are cracked to gas oil with only moderate production of coke and light gases, the effluent from the craking zone is passed to a fractionation zone wherein gasoline, gas oil and a heavy bottoms fraction are separated, the gas oil is subjected to catalytic cracking in a second cracking zone, the effluent from the second cracking zone is passed back to the fractionation zone, and a major portion of the heat required for the process is derived by burning carbon formed during the cracking. Crude oil, supplied through line 1, divides into two portions, 55-90 per cent being passed to preheater 5 and the remainder being introduced through line 9 into the lower part of fractionator 11. The oil is preheated to 500-550 DEG  F. in preheater 5, and is then mixed in line 13 with regenerated catalyst particles (50-100 microns) at 1050-1150 DEG  F.  A low activity catalyst such as activated clay is employed, the proportion being such (5 to 30 lb. per lb. of oil) that a dry catalyst/oil vapour suspension at 850-1100 DEG  F. enters reactor 15. This reactor comprises a vertical transfer line with a ratio of length/diameter of 8 : 1 to 12 : 1, and the suspension passes up the line at a linear velocity of 10 to 100 feet/sec., whereby the oil vapours are retained therein for a relatively short time (1-25 seconds).  Under these conditions the heavy fractions are cracked to form gas oil, only small quantities of gasoline, coke and gas being formed.  The suspension is passed from line 15 into gas/solids separator 19, and the separated catalyst is withdrawn through standpipe 21 wherein it is stripped and aerated by inert gas introduced through taps t.  The carbonized catalyst travels round U-bend 22 and is then suspended in air, supplied via lines 23, and passed up riser line 25 into regenerator 30, wherein a dense fluidized bed of the particles is maintained.  While the carbon is burned off the catalyst, the latter is raised to 1050-1150 DEG  F. The hot regenerated catalyst is stripped and returned to line 13 via standpipe 33 and U-bend 34.  U-bends 22 and 34 contain catalyst particles at maximum fluid density and act as gas seals in the catalyst circulation system.  Part of the catalyst in circuit must be replaced by fresh catalyst (0.2 to 1 lb. per barrel of crude) in order to compensate for permanent de-activation by the deposition of inorganic ash constituents. The spent catalyst is withdrawn via line 37, and the fresh catalyst is supplied via line 35.  The vapours leaving separator 19 are passed via line 39 to fractionator 11, the temperature of the bottom section of which is maintained at about 700-800 DEG  F. by feeding in cold crude oil via line 9.  Gas and gasoline are withdrawn from the head of the fractionator, heavy naphtha is withdrawn via line 49, heating oil (B.R. 400-550 DEG  F.) is withdrawn via line 47, and gas oil (B.R. 550-1200 DEG  F.) is withdrawn via line 57.  The heavy fraction withdrawn from the base of the fractionator is recycled via line 43 to reactor 15, or all or a part of it may be withdrawn via line 45 for use as a heavy fuel oil. The gas oil withdrawn through line 57 is mixed in line 61 with catalyst particles at 1050-1150 DEG  F., the amount of catalyst (5-25 lb. per lb. of oil) being such that the mixture enters reactor 60 at 900-1000 DEG  F.  The catalyst is preferably a high-activity material, such as a synthetic silica-alumina gel containing 10 to 20 per cent of alumina and having a particle size of 50 to 100 microns. The catalyst is maintained as a dense fluidized bed in rector 60, and the cracking therein is more severe than in the first stage. Contaminated catalyst is regenerated in regenerator 70, wherein a dense fluidized bed of catalyst is also maintained; and the regenerated catalyst is returned via standpipe 72 to line 61.  The cracked oil vapours withdrawn from reactor 60 are passed via line 76 to the base of fractionator 11.  In a modification, catalyst withdrawn from the second cracking stage may be used as the catalyst for the first stage.  In this case, catalyst is passed from regenerator 70 to line 13 via line 77; and fresh catalyst is supplied to the second stage via line 79.  If the volatility of the product gasoline is too low, a small fraction of heavy naphtha may be branched off line 49 via line 80, and introduced into reactor 60 for cracking into lower boiling products; in this case, both cracking stages may be operated at lower temperatures, e.g. 825-925 DEG  F.