GB1513267A - Ammonia synthesis - Google Patents

Abstract

1513267 Ammonia synthesis BENFIELD CORP 13 Nov 1975 46946/75 Heading C1A In the synthesis of ammonia from a hydrocarbon starting material involving the steps of generating under superatmospheric pressure a mixture comprising carbon monoxide and hydrogen from said hydrocarbon, converting the carbon monoxide in said mixture to hydrogen and CO 2 by reaction with water through catalytic shift conversion, removing said CO 2 from said mixture by absorption in a liquid scrubbing medium which is regenerated by steam-stripping, converting residual CO and CO 2 to methane by reaction with hydrogen contained in said mixture over a methanation catalyst, compressing the gaseous mixture so produced together with nitrogen contained in or added to said mixture in the molar ratio required for ammonia synthesis, passing said compressed mixture over an ammonia synthesis catalyst, recycling unconverted synthesis gas to the ammonia synthesis catalyst, and purging sufficient gas from the recycle stream to maintain a desired concentration of methane and other inerts in such stream, hydrogen losses in the methanation step and hydrogen and ammonia losses in the purge step are minimized while at the same time minimizing the steam required by such CO 2 removal and CO shift conversion steps which comprise the steps of: (a) converting from 90% to 99% of the carbon monoxide in said carbon monoxide-hydrogen mixture to hydrogen and CO 2 by reaction of said carbon monoxide with water in at least two successive catalytic shift conversion zones, the first of which is a high-temperature shift conversion zone and the last of which utilizes a lowtemperature shift conversion catalyst; (b) removing carbon dioxide from the mixture produced in step (a) in a first CO 2 scrubbing zone to provide a gas mixture containing not more than 2% and not less than 01% CO 2 , the CO 2 removal in said first zone being carried out by contacting said mixture with a scrubbing solution comprising an aqueous solution of potassium carbonate, wherein at least the major portion of said scrubbing is carried out at a scrubbing solution temperature in the vicinity of the atmospheric boiling temperature of said solution, and wherein the regeneration of said scrubbing solution is carried out at a reduced pressure in the vicinity of atmospheric pressure by steam-stripping thereof, whereby the bulk of said CO 2 is removed with a minimum consumption of stripping steam; (c) subjecting the gas mixture produced in step (b) to low-temperature catalytic shift conversion to convert residual carbon monoxide to hydrogen and carbon dioxide to produce a gas stream containing not more than 0À1% residual carbon monoxide; (d) removing CO 2 from the gas mixture produced in step (c) in a second CO 2 scrubbing zone to provide a mixture containing not more than 200 ppm of residual CO 2 , on an anhydrous basis, the CO 2 removal in said second zone being carried out by contacting said gas mixture with a scrubbing solution comprising an aqueous solution of an alkaline chemical absorbent wherein said scrubbing is carried out at an absorption temperature substantially below the atmospheric boiling temperature of the solution, permitting the reduction of residual CO 2 in said mixture to a low level and wherein the regeneration of such scrubbing solution is carried out by steam-stripping thereof; (e) converting the residual carbon monoxide and carbon dioxide in the gas mixture from step (d) to methane in a catalytic methanation zone, and thereafter introducing said gas mixture into an ammonia synthesis loop including a purge for preventing the buildup of inert gases in said loop.