GB709645A - A process for the conversion of carbon dioxide with hydrogen - Google Patents

Abstract

Aliphatic hydrocarbons of more than one carbon atom per mol and oxygen-containing organic compounds are made by contacting a synthesis gas containing carbon dioxide and hydrogen in the ratio by volume of 0.2-1.5 CO2 : 1H2 with a hydrogenation catalyst comprising a metal of the 8th group of the periodic system, which contains an alkali-metal compound in an amount, calculated as the alkali-metal monoxide, of 0.1-6 per cent by weight, the catalyst being provided as a fixed bed or as a suspension in finely divided condition in a liquid medium, the synthesis gas being contacted with the catalyst at a gauge pressure of 1-100 atmos. and at 150-380 DEG C., and at a rate of at least 100 normal cubic metres per cubic metre of catalyst space per hour, a greater part of the water and of the heavier hydrocarbons formed during the synthesis being removed from the exit gases which are thereafter contacted with the same or a similar catalyst. An iron catalyst at 240-350 DEG C. and 10-30 atmos. with a flow rate of the gas mixture not greater than 5000 normal cubic metres is preferred. Exit gases may be recycled or the synthesis effected in a series of stages. Water may be removed by cooling but preferably by chemical, e.g. calcium chloride, or physical, e.g. activated charcoal, alumina, silica gel, bauxite, fuller's earth and bentonite, sorption. The latter type of compounds also separate some of the normally gaseous hydrocarbons. The alkali-metal compound used as promoter preferably has an alkaline reaction in aqueous solution, particularly oxides, hydroxides, carbonates, silicates, phosphates, borates and organic acid, e.g. acetic acid, salts of sodium and potassium. It is desirable that a particle size of more than 1 mm. is used in the fixed bed. When using catalysts in a liquid medium 90 per cent or more of higher molecular hydrocarbons may be obtained in the products. With heavily alkalized catalysts solid hydrocarbons may be obtained. Exit gases are preferably mixed with fresh synthesis gas before passing to another stage. In examples: (1) a gas of composition 19.9 per cent CO2, 0.4 per cent CO, 58.3 per cent H2, 0.4 per cent CH4 and 21 per cent N2 is passed at 280-340 DEG C. and 20 atmos. at a flow rate of approximately 100-250 normal litres per litre per hour through a reduced iron oxide catalyst containing about 0.5 per cent of Cu and 0.8 per cent K2CO3 of particle size less than 0.2 mm., the fresh gas being mixed with 3 times its volume of exit gas which is cooled to 50 DEG C. or less, water and higher hydrocarbons being separated, whilst a quantity of exit gas, corresponding to the contraction of gas volume during the synthesis, is passed over active charcoal to adsorp low molecular hydrocarbons, the final products comprising small quantities of methane, ethane, ethylene, C3 and C4 hydrocarbons, C1-C4 water soluble alcohols and relatively larger amounts of hydrocarbons of B.R. 15-200 DEG C., of B.R. 200-320 DEG C. and boiling above 320 DEG C.; (2) a gas of approximate composition 20 per cent CO2 and 63 per cent H2 with some N2 and CH4\t is passed at 280-300 DEG C. (rising eventually to 320 DEG C.) and 20 atmos. at a flow rate of 600 normal cubic metres per cubic metre per hour through a reaction chamber containing an iron catalyst prepared by precipitating ferric nitrate with sodium carbonate and reducing and containing 0.5 per cent of Cu and 1 per cent of K2O, of particle size 0.05-0.5 mm., the exit gas flowing through three other reaction chambers, water and higher hydrocarbons being largely removed between each chamber, the final products being similar to those of (1) and (3) is similar to (2) except that the catalyst is suspended in synthesis oil of B.R. 300-340 DEG C. to give a suspension containing about 20 per cent of iron and the gas flow rate is about 300 normal cubic metres, a better yield of higher hydrocarbons being obtained.