Ammonia synthesis

• US 3,947,551 A
• Filed: 05/29/1974
• Issued: 03/30/1976
• Est. Priority Date: 05/29/1974
• Status: Expired due to Term

• Current Assignee: UOP Des Plaines IL A NY General Partnership
• Original Assignee: Benfield Corporation
• Inventors: Parrish, Roger W.
• Primary Examiner(s): Vertiz, Oscar R.
• Assistant Examiner(s): Wheelock, Eugene T.
• Application Number:
  US05/474,232
• Time in Patent Office: 671 Days
• Field of Search: 423/359, 423/360, 423/361, 423/362, 423/363, 423/437, 423/228, 423/229, 423/650, 423/651, 423/652, 423/653, 423/654, 423/655, 423/656, 23/260, 260/449 M
• US Class Current: 423/359
• CPC Class Codes: C01B 3/025 Preparation or purification of gas mixtures for ammonia synthesis

Abstract

An improved ammonia synthesis train utilizing hydrocarbon starting materials which are converted to hydrogen under superatmospheric pressure in a series of steps which include conversion of the hydrocarbons to hydrogen-carbon monoxide mixtures, shift conversion of the carbon monoxide to hydrogen and CO₂, and removal of the CO₂. In such a system, reduction in process gas losses is achieved through an improved arrangement and integration of the shift conversion and CO₂ removal steps. The carbon monoxide is first shifted in a series of shifts including a final low temperature shift; the bulk of the CO₂ is removed by an essentially isothermal hot potassium carbonate CO₂ removal system; the small residual amount of carbon monoxide is shifted to very low levels in a final low temperature shift; and the small amount of residual CO₂ is then removed to very low levels in a second non-isothermal scrubbing system employing an aqueous solution of a chemical absorbent. Methanation, compression and ammonia synthesis follows. Losses of process gas in the methanation step and in the purge gas from the ammonia loop are sharply reduced with concomitant high thermal efficiency.

Claims

1. 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 CO₂ by reaction with water through catalytic shift conversion, removing said CO₂ from said mixture by absorption in a liquid scrubbing medium which is regenerated by steam stripping, converting residual CO and CO₂ 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 CO₂ 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 CO₂ 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 350°

to 550°

F;

b. Removing carbon dioxide from the mixture produced in Step (a) in a first CO₂ scrubbing zone to provide a gas mixture containing not more than about 2% and not less than about 0.1% CO₂, the CO₂ 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 CO₂ 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 350°

to 550°

F to convert residual carbon monoxide to hydrogen and carbon dioxide to produce a gas stream containing not more than 0.1% residual carbon monoxide;

d. Removing CO₂ from the gas mixture produced in Step (c) in a second CO₂ scrubbing zone to provide a mixture containing not more than about 200 ppm of residual CO₂, the CO₂ 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°

to 140°

F permitting the reduction of residual CO₂ in said mixture at least to said level of 200 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.

2. A process in accordance with claim 1 in which the steam for the steam stripping of said scrubbing solutions in Steps (b) and (d) is supplied by transferring heat present in said gas mixture to said scrubbing solutions.

3. A method in accordance with claim 1 in which the process gas stream is heated and saturated with water prior to the final low temperature shift conversion in Step (c) by direct contact with a circulating stream of water which is heated in turn by direct contact betwen the circulating water stream and the hot process gas.

4. A method in accordance with claim 1 wherein the hot process gas leaving Step (a) is cooled prior to Step (b) by direct contact with a circulating stream of water and wherein the heated water thereby generated is utilized to heat and saturate the process gas stream leaving Step (b) by directly contacting such process gas stream with said heated water prior to introducing said mixture into the final low temperature shift conversion zone in Step (c).

5. A method in accordance with claim 1 in which from 95% to 98% of the carbon monoxide in the carbon monoxide-hydrogen mixture is converted to hydrogen and CO₂ in Step (a).

6. A method in accordance with claim 1 in which the removal of carbon dioxide in Step (b) is carried out to provide a gas mixture containing not more than about 1% and not less than about 0.2% CO₂.

7. A method in accordance with claim 1 in which the CO₂ removal in Step (d) is carried out to provide a mixture containing not more than about 100 ppm of residual CO₂.

8. A method in accordance with claim 1 in which the temperature of the last shift conversion in Step (a), and the temperature of the shift conversion in Step (c) is in the range of from 400°

to 500°

F.

9. A method in accordance with claim 1 in which the absorption temperature in Step (b) is in the range of from 220°

to 250°

F.

10. A method in accordance with claim 1 in which the absorption temperature in Step (d) is in the range of from 100°

to 130°

F.

11. A method in accordance with claim 1 in which the scrubbing solution employed in Step (d) is an aqueous solution of an alkanolamine.

12. 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 CO₂ by reaction with water through catalytic shift conversion, removing said CO₂ from said mixture by absorption in a liquid scrubbing medium which is regenerated by steam stripping, converting residual CO and CO₂ 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 CO₂ 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 CO₂ 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°

to 500°

F;

b. Removing carbon dioxide from the mixture produced in Step (a) in a first CO₂ scrubbing zone to provide a gas mixture containing not more than about 1% and not less than about 0.2% CO₂, the CO₂ 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 CO₂ 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°

to 500°

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 CO₂ from the gas mixture produced in Step (c) in a second CO₂ scrubbing zone to provide a mixture containing not more than about 100 ppm of residual CO₂, the CO₂ 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°

to 140°

F permitting the reduction of residual CO₂ 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.