GB655827A - Improved hydrocarbon synthesis - Google Patents

Abstract

In a Fischer synthesis process using a dense fluidized iron catalyst in which the catalyst becomes contaminated with waxy material formed during the synthesis, a portion of the catalyst is continuously or intermittently withdrawn from the synthesis zone and mixed with a liquid hydrocarbon fraction boiling in the gas oil range to form a slurry which is <PICT:0655827/IV (b)/1> hydrogenated in a hydrogenation zone, partly reduced and dewaxed catalyst and hydrogenated slurry oil being recovered from the hydrogenation zone, and the catalyst after separation from the hydrogenated slurry oil recycled to the synthesis zone. The liquid hydrocarbon fraction boiling in the gas oil range may be recovered from the synthesis products. In the drawing, a synthesis gas feed mixture of hydrogen and carbon monoxide (molar ratio between 0.7 and 1.8 : 1), is introduced into reactor 2 through line 8 and flows upwardly through grid 4. In the reactor a mass of powdered iron catalyst such as alkali metal promoted sintered reduced pyrites ash is maintained as a dense fluidized bed, all the material being finer than 100 mesh. This catalyst may be supplied from the hopper 12. The pressure in the reactor may be from 250 to 750 p.s.i.g. and the reaction temperature which may be controlled by cooling is kept at 450 DEG to 750 DEG F. The catalyst is preferably promoted with about 0.3 to 1.5 per cent by weight of an alkali metal salt such as K2CO3, Na2CO3 or KF. When the gas enters the enlarged section of reactor 2 most of the coarser catalyst particles entrained in the gas drop back into the fluidized mass and the bulk of the entrained catalyst fines is removed in the cyclone 14 and returned to the fluidized bed by pipe 16. The reaction products and the remaining catalyst fines and other contaminants such as entrained iron salts of fatty acids are passed through cooler 18 into oil scrubber 20 where the remaining catalyst powder is removed by circulating product oil in known manner. The temperature in the scrubber is preferably above 175-250 DEG F. and the overhead gases are then passed to cooler 24 and partial condenser 26, both of which are operated at such temperatures that reaction products boiling above 400 DEG F. are condensed in condenser 26, but lighter reaction products pass overhead as vapour to cooler 30 and then to separator 32, wherein normally condensible synthesis products are withdrawn and sent to the liquid products p recovery system. Non-condensible tail gas from separator 32 is removed through line 36, a portion recycled to reactor 2 via line 38 and the balance withdrawn through line 40 for further use, e.g. as fuel or as recycle to gas generator. The oily condensate from separator 26 comprises synthesis products boiling above 400 DEG F. and is sent as required to scrubber 20, but a portion may be sent to the recovery system. A stream comprising scrubbing oil, scrubbed catalyst and other condensed reaction products is withdrawn from scrubber 20 and passed to slurry mixing tank 48 and into this tank is also introduced through tube 6 a stream of catalyst from reactor 2, the catalyst being withdrawn intermittently or continuously as the wax accumulates. The catalyst oil slurry from tank 48 is then pumped to the hydrogenation reactor 56 which may be packed with non-catalytic material such as Raschig rings, pumice or activated carbon, but no extraneous hydrogenation catalyst is necessary. Hygrogen is supplied through line 58 and reactor 56 is under a pressure of about 50 to 750 pounds per square inch and a temperature of 500-800 DEG F. The oxygenated organic compounds in the slurry are thus reduced and the olefinic constituents saturated while the waxy catalyst deposits are removed partly by solution in the slurry oil and partly as a result of the hydrogenation and any oxidized catalyst is partially reduced. Hydrogen and volatile hydrogenation products are withdrawn through line 55 and the hydrogen may be recycled. Condensible material is cooled in cooler 57 and separates in separator 61 as a liquid comprising lower molecular weight hydrocarbons and water. A slurry containing cleaned reduced catalyst and saturated oil is withdrawn from the hydrogenator through line 60 to filter 62 to remove the solids. The liquid product is withdrawn through line 69 and is suitable for use as a diesel fuel while the catalyst is conveyed to the hopper 12 for re-use in the system.