Ammonia synthesis

In a process for 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.sub.2 by reaction with water through catalytic shift conversion, removing said CO.sub.2 from said mixture by absorption in a liquid scrubbing medium which is regenerated by steam stripping, converting residual CO and CO.sub.2 to methane by reaction with hydrogen over a methanation catalyst, compressing said hydrogen together with nitrogen in about 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 the stream flowing to the ammonia synthesis catalyst, a method for minimizing hydrogen losses in the methanation step and hydrogen and ammonia losses in the purge step while at the same time minimizing the steam required by such CO.sub.2 removal and CO shift conversion steps which comprise the steps of:

• a. Converting from 95 to 98% of the carbon monoxide in said carbon monoxide-hydrogen mixture to hydrogen and CO.sub.2 by reaction of said carbon monoxide with water in at least two successive catalytic shift conversion zones, the last of which operates in the temperature range of from 400.degree. to 500.degree. F;
• b. Removing carbon dioxide from the mixture produced in Step (a) in a first CO.sub.2 scrubbing zone to provide a gas mixture containing not more than about 1% and not less than about 0.2% CO.sub.2, the CO.sub.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 by steam stripping thereof, whereby the bulk of said CO.sub.2 is removed with a minimum consumption of stripping steam;
• c. Subjecting the gas mixture produced in Step (b) to catalytic shift conversion at a temperature in the range of from 400.degree. to 500.degree. F to convert residual carbon monoxide to hydrogen and carbon dioxide to produce a gas stream containing not more than 0.05% residual carbon monoxide;
• d. Removing CO.sub.2 from the gas mixture produced in Step (c) in a second CO.sub.2 scrubbing zone to provide a mixture containing not more than about 100 ppm of residual CO.sub.2, the CO.sub.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 of from 90.degree. to 140.degree. F permitting the reduction of residual CO.sub.2 in said mixture at least to said level of 100 ppm and wherein the regeneration of said 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 build-up of inert gases in said loop.