GB836909A - Improvements in and relating to the production of unsaturated hydrocarbons and methanol - Google Patents

Abstract

A liquid or gaseous hydrocarbon feedstock, the components of which boil at not greater than 150 DEG C., a gas comprising one or more normally gaseous hydrocarbons and/or hydrogen, and oxygen insufficient in amount to bring about complete combustion are subjected to reaction in one or more stages so that a temperature in excess of 1200 DEG C. is attained whereby both cracking and combustion occur so that the product comprises substantial amounts of acetylene, ethylene, methane, hydrogen, carbon monoxide and carbon dioxide, the hydrogen: carbon monoxide volume ratio being greater than 1 : 1; the product is treated successively for the removal of carbon dioxide, acetylene and ethylene; the residual gas comprising methane, hydrogen and carbon monoxide, the hydrogen : carbon monoxide volume ratio being greater than 1 : 1, is passed to a methanol converter in which substantial proportions of the hydrogen and carbon monoxide are converted in the presence of a catalyst comprising zinc oxide and an oxide of chromium at elevated temperature and pressure to methanol; the methanol produced is condensed and in part used to scrub under elevated pressure unreacted gas leaving the methanol converter; and the solution of methane in methanol so obtained is removed from the system and its pressure decreased, methane being liberated together with other dissolved gases. The use of lower hydrocarbons as feedstock causes a decrease in the ratio of hydrogen to carbon monoxide. It is desirable that substantially pure oxygen should be used in the process. The gas mixture is freed from carbon dioxide, for example, by washing with aqueous ammonia. Acetylene may be removed by heating the gas mixture with liquid ammonia at -70 DEG C. and the ethylene by increasing the pressure of the gas mixture and treating it with a suitable solvent, e.g. a C5-C6 naphtha, at a temperature of -20 DEG C. The gas remaining after the ethylene removal may be fed unchanged to the methanol converter together with gas recirculated from the converter or part may be withdrawn and used, for example, as fuel gas for pre-heating the reactants or reacted with steam at 400-500 DEG C. in the presence of an iron catalyst to convert the carbon monoxide to hydrogen and carbon dioxide; the hydrogenrich gas so obtained may be recycled to the initial reaction zone or mixed with the gas stream being passed to the methanol converter. The pressure of the gas mixture fed to the methanol converter should be increased, preferably to 200-400 atm. The gas mixture is preferably passed through activated carbon containers to remove any naphtha vapours before being passed into the methanol converter. The methanol conversion is conveniently carried out at 300-450 DEG C. The product leaving the methanol converter is scrubbed with methanol, the methanol solution may then be passed into a vessel in which the pressure is decreased to atmospheric, thus liberating methanol and other dissolved gases and part of the resulting methanol is purified, the rest being used to scrub gases coming from the converter. Alternatively the pressure may be reduced to atmospheric in two stages. In an example the process is carried out on a mixture containing light naphtha boiling above 30 DEG C., oxygen and a combustion gas containing hydrogen and methane obtained by reacting part of the gas from the combustion zone after separation of carbon dioxide, acetylene and ethylene with steam.