Process and installation for the combined production of synthetic ammonia and pure hydrogen

• US 5,409,684 A
• Filed: 06/07/1993
• Issued: 04/25/1995
• Est. Priority Date: 06/10/1992
• Status: Expired due to Fees

• Current Assignee: L'Air Liquide Societe Anonyme Pour L'Etude et L'Exploitation Des Procedes Georges Claude (L'Air Liquide SA)
• Original Assignee: L'Air Liquide Societe Anonyme Pour L'Etude et L'Exploitation Des Procedes Georges Claude (L'Air Liquide SA)
• Inventors: Fuentes, Francois
• Primary Examiner(s): LANGEL, WAYNE A
• Application Number:
  US08/072,989
• Time in Patent Office: 687 Days
• Field of Search: 423/359, 423/650, 422/129, 422/141, 422/168, 422/148, 422/170, 422/169
• US Class Current: 423/359
• CPC Class Codes: B01D 2256/16 Hydrogen
• CPC Class Codes: B01D 2259/40003 Methods relating to valve switching
• CPC Class Codes: B01D 2259/40081 Counter-current
• CPC Class Codes: B01D 2259/403 using three beds
• CPC Class Codes: B01D 53/0446 Means for feeding or distributing gases
• CPC Class Codes: B01D 53/047 Pressure swing adsorption
• CPC Class Codes: B01D 53/229 Integrated processes (Diffusion and at least one other process, e.g. adsorption, absorption)
• CPC Class Codes: C01B 2203/0405 Purification by membrane separation
• CPC Class Codes: C01B 2203/043 Regenerative adsorption process in two or more beds, one for adsorption, the other for regeneration
• CPC Class Codes: C01B 2203/0465 Composition of the impurity
• CPC Class Codes: C01B 2203/047 the impurity being carbon monoxide
• CPC Class Codes: C01B 2203/0475 the impurity being carbon dioxide
• CPC Class Codes: C01B 2203/048 the impurity being an organic compound
• CPC Class Codes: C01B 2203/0495 the impurity being water
• CPC Class Codes: C01B 3/025 Preparation or purification of gas mixtures for ammonia synthesis
• CPC Class Codes: C01B 3/501 by diffusion
• CPC Class Codes: C01B 3/56 by contacting with solids; Regeneration of used solids C01B3/508 takes precedence
• CPC Class Codes: C01C 1/0488 Processes integrated with preparations of other compounds, e.g. methanol, urea or with processes for power generation
• CPC Class Codes: Y02C 20/40 of CO2
• CPC Class Codes: Y02P 20/10 Process efficiency
• CPC Class Codes: Y02P 20/52 : using catalysts, e.g. selective catalysts

Abstract

Process and apparatus for the combined production of ammonia synthesis gas and pure hydrogen. There is withdrawn (at 21) a flow of synthesis gas which is first treated in a permeation unit (24) with the return (at 28) of the non-permeated gas, while the permeate (32)is treated by adsorption (34) (35) (36), which permits producing (in 57) very pure hydrogen. The process has a high energy efficiency.

Claims

1. Process for the combined production of synthetic ammonia (NH₃) and pure hydrogen (H₂), from a hydrocarbon base gas which comprises the following steps:

a) reforming said hydrocarbon base gas in at least two stages, of which a last stage is with introduction of air, producing from said hydrocarbon base gas and reforming air, a gaseous composite mixture consisting essentially of nitrogen (N₂) and hydrogen (H₂) with small proportions of carbon dioxide (CO₂), water vapor (H₂ O), carbon monoxide (CO), argon (Ar) and methane (CH₄);

b) washing said gaseous composite mixture so as to eliminate substantially all carbon monoxide by transformation into carbon dioxide, thereby leaving only traces of carbon monoxide, carbon dioxide and methane in the washed gaseous mixture;

c) eliminating carbon dioxide from the washed gaseous mixture, thereby leaving only traces of carbon dioxide in the resultant mixture;

d) subjecting the resultant mixture to methanization of the carbon monoxide, thereby reducing still further the content of the traces of carbon monoxide, and producing traces of methane, to obtain an ammonia synthesis gas comprising about 74-75 volume percent of hydrogen, about 24-25 volume percent of nitrogen, and about 1 volume percent of combined argon and methane;

thereafter;

e) compressing in several compression stages the synthesis gas and sending it to an ammonia synthesis reactor for producing a flow of synthetic ammonia;

f) removing between two of said compression stages a flow of synthesis gas under medium pressure, whereby substantially all the hydrogen is separated from the other constituents;

nitrogen, argon, methane, to produce a gaseous flow having a relatively high hydrogen content,

wherein step f) for production of hydrogen comprises at least two purification substeps, namely;

f₁) an initial substep of permeation delivering under said medium pressure an effluent consisting essentially of hydrogen and nitrogen with substantially all the argon and all the methane, which effluent is recycled to the ammonia synthesis gas; and

a permeation gas under lower pressure consisting essentially of hydrogen with a residual content of nitrogen and traces of argon and methane;

f₂) an initial adsorption substep of the permeation gas on molecular sieves of a pressure swing adsorption system delivering under said lower pressure a production gas constituted by hydrogen with only traces of argon in an amount equal to or less than 100 ppm and traces of nitrogen, methane and water vapor, each in an amount equal to or less than 1 ppm, and under an even lower pressure, a residual gas containing a considerable amount of hydrogen, a significant amount of nitrogen, and small amounts of argon, methane and water vapor.

2. Process for the combined production of synthetic ammonia and pure hydrogen, according to claim 1, further comprising returning a portion of the residual gas after recompression to the permeation gas.

3. Process for the combined production of synthetic ammonia and pure hydrogen, according to claim 2, further including a regeneration step of said molecular sieves with depressurization comprising a terminal phase in countercurrent relative to a co-current adsorption direction followed by a final regeneration phase with countercurrent elution with a gas having a substantial hydrogen content, wherein the portion of the residual gas returned to the permeation gas is exclusively the gas issuing from the elution, and wherein a purge of the final depressurization gas is conducted countercurrently.

4. Process for the combined production of synthetic ammonia and pure hydrogen, according to claim 1, wherein the reforming step is controlled so as to increase the H₂ :

N₂ ratio at the inlet of the compression stages so as to achieve an optimum H₂ ;

N₂ ratio in the synthesis reactor.

5. Process for the combined production of synthetic ammonia and pure hydrogen, according to claim 4, wherein the H₂ :

N₂ ratio at the inlet of the compression stages is greater than 3;

1.

6. Installation for the combined production of synthetic ammonia and pure hydrogen, comprising:

a) a reforming unit for performing at least two reforming stages, of which a last stage includes means for introducing air, said reforming unit further including means for producing from hydrocarbon base gases and reforming air, a composite gaseous mixture consisting essentially of nitrogen and hydrogen, with small proportions of carbon dioxide, water vapor and carbon monoxide, argon and methane;

b) a unit for substantial elimination of carbon monoxide by transformation into carbon dioxide leaving only traces of carbon monoxide, carbon dioxide and methane;

c) a unit for the elimination of carbon dioxide, leaving only traces of carbon dioxide;

d) a methanization unit of the carbon monoxide for reducing the content of the traces of carbon monoxide, and for producing methane;

e) compression means in several compression stages including a conduit for sending synthesis gas to a reactor for ammonia synthesis to produce the flow of synthetic ammonia;

f) a gaseous separation unit operating on a stream withdrawn between two stages of the compression means for separating hydrogen from other gaseous constituents comprising nitrogen, argon and methane;

wherein the gaseous separation unit comprises

f₁) a gaseous permeation unit including a compartment under pressure having an inlet connected to the synthesis gas conduit downstream of a stage of said compression means, an outlet connected to said synthesis gas conduit, downstream of the connection point of said inlet, said outlet including means for circulating gas, and a permeation compartment under lower pressure, with an outlet for permeate;

f₂) an adsorption unit with molecular sieves, each connected successively;

by an inlet in co-current with a supply conduit for transmitting a gaseous mixture comprising the permeate of the permeation unit;

by a countercurrent outlet fluidly connected to a residual gas conduit.

7. Installation for the combined production of synthetic ammonia and pure hydrogen, according to claim 6, further comprising a recycling conduit between the residual gas conduit and the supply conduit of the adsorption unit, said recycling conduit including a recycling compressor.

8. Installation for the combined production of synthetic ammonia and pure hydrogen, according to claim 6, further comprising means for adjusting the reforming operation by increasing the H₂ :

N_a ratio at the inlet of the compression stages so as to achieve an optimum ratio in the synthesis reactor.

9. Installation for the combined production of synthetic ammonia and pure hydrogen, according to claim 8, wherein the adjustment means permit increasing the H₂ :

N₂ ratio above 3;

1.