Method for the preparation of ammonia synthesis gas

Abstract

Method for the preparation of ammonia synthesis gas by a combination of ATR or secondary reforming process using oxygen from an air separation unit and electrolysis of water for the production of ammonia synthesis gas.

Claims

1. A method for the preparation of ammonia synthesis gas comprising the steps of: (a) providing a gaseous hydrocarbon feed stock; (b) separating atmospheric air into a separate oxygen containing stream and into a separate nitrogen containing stream; (c) preparing a separate hydrogen containing stream and a separate oxygen containing stream by electrolysis of water; (d) autothermal or secondary reforming at least a part of the gaseous hydrocarbon feed stock with the oxygen containing stream obtained by the separation of atmospheric air in step (b) and the oxygen containing stream obtained by the electrolysis of water in step (c) to a process gas comprising hydrogen, carbon monoxide and carbon dioxide; (e) treating the process gas withdrawn from the auto-thermal or secondary reforming step (d) in one or more water gas shift reactions; (f) removing the carbon dioxide from the water gas shift treated process gas; (g) purifying the process gas from step (f) to obtain a purified hydrogen stream; and (h) introducing the nitrogen containing stream obtained by the separation of atmospheric air in step (b) into the purified hydrogen stream in an amount to provide a molar ratio of the hydrogen to the nitrogen of 2.7-3.3 in the mixed hydrogen and nitrogen gas stream to obtain the ammonia synthesis gas.

2. The method according to claim 1, wherein the separating of atmospheric air in step (b) and the electrolysis of water is powered by renewable energy.

3. The method according to claim 1, wherein the purified hydrogen stream in step (g) is obtained by a liquid nitrogen wash.

4. The method according to claim 1, wherein the separating of atmospheric air in step (b) is performed by cryogenic separation.

5. The method according to claim 1, wherein at least a part of the hydrogen containing stream from step (c) is added to the purified hydrogen stream in step (h).

Description

(1) The present invention is based on a combination of the ATR process or the secondary reforming process using oxygen from an air separation unit and the electrolysis of water for the production of ammonia synthesis gas.

(2) Thus, this invention is a method for the preparation of ammonia synthesis gas comprising the steps of

(3) (a) providing a gaseous hydrocarbon feed stock;

(4) (b) separating atmospheric air into a separate oxygen containing stream and into a separate nitrogen containing stream;

(5) (c) preparing a separate hydrogen containing stream and a separate oxygen containing stream by electrolysis of water;

(6) (d) autothermal reforming or secondary reforming at least a part of the gaseous hydrocarbon feed stock with the oxygen containing stream obtained by the separation of atmospheric air in step (b) and the oxygen containing stream obtained by the electrolysis of water in step (c) to a process gas comprising hydrogen, carbon monoxide and carbon dioxide;

(7) (e) treating the process gas withdrawn from the autothermal reforming or secondary reforming step (d) in one or more water gas shift reactions;

(8) (f) removing the carbon dioxide from the water gas shift treated process gas;

(9) (g) purifying the process gas from step (f) to obtain a purified hydrogen stream; and

(10) (h) introducing the nitrogen containing stream obtained by the separation of atmospheric air in step (b) into the purified hydrogen stream in an amount to provide a molar ratio of the hydrogen to the nitrogen of 2.7-3.3 in the mixed hydrogen and nitrogen gas stream to obtain the ammonia synthesis gas.

(11) Purification of the process gas obtained in the autothermal reforming step can be performed by subjecting the gas to water gas shift reaction of CO to CO.sub.2 for more hydrogen production and CO.sub.2 removal with a liquid solvent being rich in potassium carbonate or amine and thereby selectively absorbing carbon dioxide in the liquid solvent as known in the art.

(12) Compared to prior art methods using electrolysis of water for hydrogen production and air separation for nitrogen production, the oxygen product from electrolysis of water and from air separation is advantageously used for partial oxidation in the autothermal reformer or secondary reformer resulting in a reduced size of the ASU, which is a costly and energy intensive unit and process. For minimizing energy loss of the ASU, the size of the ASU can be reduced to a level where just sufficient amounts of nitrogen are produced as required in the ammonia synthesis. When the stoichiometric ratio of hydrogen and nitrogen for ammonia synthesis is produced in the ATR or secondary reforming and water electrolysis, the ASU size will be at its minimum and thus will not vent any excess of nitrogen.

(13) However, depending on the availability of power for water electrolysis and the efficiency of the water electrolysis, the design of the ASU can be changed to provide oxygen in excess, in order to substitute a part of the hydrocarbon feedstock with hydrogen produced by the water electrolysis.

(14) Still an advantage of the invention is that energy for operating the electrolysis unit and ASU can be renewable energy generated by windmills, solar cells, hydraulic energy or other renewables.

(15) Thus, in a preferred embodiment of the invention, the electrolysis of water and the separation of air is powered by renewable energy.

(16) The method for air separation employed in the method according to the invention is preferably fractional distillation in a cryogenic air separation unit to provide nitrogen and oxygen. Alternatively, other methods such as membrane separation, pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA), can be used.

(17) The advantage of using cryogenic air separation is that a part of the separated nitrogen is in liquid form. Liquid nitrogen is preferably used in step (g) in a nitrogen wash unit for the removal of methane, argon and carbon monoxide by-products from the reforming step.

(18) After the liquid nitrogen wash the ammonia synthesis gas will then be essentially free of inerts and more efficient in the ammonia synthesis, in that purge gas can be avoided.

(19) One of the major advantages of the method according to the invention is a considerably increased efficiency of the electrolysis unit by nearly 50%, compared to the efficiency in the prior art processes employing solely electrolysis and air separation, without ATR or secondary reforming.

(20) Reported efficiencies of commercialized technologies for water electrolysis are between 40% to 60%. The efficiency of water electrolysis is defined as the Lower Heating Value (LHV) of hydrogen produced divided by the electrical power consumed. No energy value is given to oxygen produced since it has no thermodynamic heating value.

(21) The synergy in combining water electrolysis and ATR or secondary reforming technology for ammonia synthesis gas production, results in overall savings of hydrocarbon feedstock and fuel for the partial oxidation process and reduced power savings in the ASU due its reduced size.

(22) In Table 1 below, key figures are given for a 2200 MTPD ammonia plant for comparison of syngas technologies for ATR with ASU and ATR with ASU combined with water electrolysis.

(23) TABLE-US-00001 TABLE 1 Natural gas ASU power Power for CO2 Technology for consump- consump- electrol- footprint, syngas tion, Nm.sup.3/h tion, MW ysis, MW Nm.sup.3/h ATR with ASU 65,506 30.3 0 79,700 ATR with ASU & 53,807 12.9 195.3 65,470 water electrolysis

(24) By means of the process according to the invention, when utilizing 195.3 MW power for water electrolysis with an efficiency of 50%, the saving of natural gas is 129 MW (LHV=39771 KJ/Nm.sup.3) and 12.9 MW power for the ASU. The overall efficiency of the water electrolysis has then increased from 50% to 72.6%. That is nearly an increase of 50%.

(25) Since the natural gas consumption has been reduced by 22% the CO.sub.2 emission has been reduced correspondingly.

(26) When used in revamp or for increasing capacity of ATR or primary and secondary reforming based ammonia synthesis gas plants, the method according to the invention provides the further advantages of reducing specific consumption of the hydrocarbon feed stock and as a result thereof production of CO.sub.2. As known in the art, CO.sub.2 must be removed from the ammonia synthesis gas in an upstream process by sour gas wash with amines or a potassium carbonate solution. That process is costly and reducing the amount of CO.sub.2 in the raw ammonia synthesis gas reduces the overall process cost.