AMMONIA DEHYDROGENATION

Abstract

A process for the production of compressed hydrogen in a membrane reactor, said membrane reactor comprising a first zone separated by a proton conducting membrane from a second zone, said first zone having a gas inlet and a product outlet and said second zone having a product outlet; said process comprising; a. feeding a gas comprising ammonia to said first zone via said gas inlet, and allowing a reaction to take place in said first zone so that hydrogen and nitrogen are formed; b. applying an electric field over said proton conducting membrane; c. allowing hydrogen to dissociate into electrons and protons to selectivity pass through the proton conducting membrane to said second zone where protons and electrons recombine to form hydrogen in the second zone; wherein the membrane reactor comprises a pressure regulator at said product outlet from said second zone so that, in operation, the partial pressure of hydrogen in the second zone is higher than the partial pressure of hydrogen in the first zone.

Claims

1. A process for the production of compressed hydrogen in a membrane reactor, said membrane reactor comprising a first zone separated by a proton conducting membrane from a second zone, said first zone having a gas inlet and a product outlet and said second zone having a product outlet; said process comprising; a. feeding a gas comprising ammonia to said first zone via said gas inlet, and allowing a reaction to take place in said first zone so that hydrogen and nitrogen are formed; b. applying an electric field over said proton conducting membrane; c. allowing hydrogen to dissociate into electrons and protons to selectivity pass through the proton conducting membrane to said second zone where protons and electrons recombine to form hydrogen in the second zone; wherein the membrane reactor comprises a pressure regulator at said product outlet from said second zone so that, in operation, the partial pressure of hydrogen in the second zone is higher than the partial pressure of hydrogen in the first zone.

2. A process for the production of hydrogen in a membrane reactor, said membrane reactor comprising a first zone separated by a proton conducting membrane from a second zone, said first zone having a gas inlet and a product outlet and said second zone having a product outlet; said process comprising; a. feeding a gas comprising ammonia to said first zone, and allowing a reaction to take place in said first zone so that hydrogen and nitrogen are formed; b. applying an electric field over said proton conducting membrane; c. allowing hydrogen to dissociate into electrons and protons to selectivity pass through the proton conducting membrane to said second zone where protons and electrons recombine to form hydrogen in the second zone; wherein joule heating which occurs during the application of the electric field over said proton conducting membrane is used to heat the first zone.

3. A process for the production of compressed hydrogen in a membrane reactor, said membrane reactor comprising a first zone separated by a proton conducting membrane from a second zone, said first zone having a gas inlet and a product outlet and said second zone having a product outlet; said process comprising; a. feeding a gas comprising ammonia to said first zone, and allowing a reaction to take place in said first zone so that hydrogen and nitrogen are formed; b. applying an electric field over said proton conducting membrane; c. allowing hydrogen to dissociate into electrons and protons to selectivity pass through the proton conducting membrane to said second zone where protons and electrons recombine to form hydrogen in the second zone; wherein the membrane reactor comprises a pressure regulator at said product outlet from said second zone so that, in operation, the partial pressure of hydrogen in the second zone is higher than the partial pressure of hydrogen in the first zone; and wherein joule heating which occurs during the application of the electric field over said proton conducting membrane is used to heat the first zone.

4. A process as claimed in any preceding claim wherein temperature in the first zone is 400? C. or more such as 400 to 1000? C.

5. The process as claimed in any preceding claim wherein proton conducting membrane is self-supporting.

6. The process as claimed in any preceding claim wherein the first zone comprises a dehydrogenation catalyst.

7. The process as claimed in any preceding claim wherein the hydrogen in the second zone is compressed and is at a pressure of 2 bar or more.

8. The process as claimed in any preceding claim wherein the hydrogen in the second zone is compressed and the heat generated thereby is used to heat the first zone.

9. The process as claimed in any preceding claim wherein said proton conducting membrane comprises at least one mixed metal oxide of formula (I)
AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; wherein, for each layer independently, A is Ba, Sr or Ca or a mixture thereof; the sum of a+b+c equals 1; b is 0-0.75; c is 0.05-0.5; Acc is Y, Yb, Gd, Pr, Sc, Fe, Eu,, In or Sm or a mixture thereof; and y is a number such that formula (I) is uncharged, e.g. 3-y is 2.75 to 2.95.

10. The process as claimed in any preceding claim wherein the membrane reactor comprises a membrane electrode assembly which comprises, in the following layer, order: (I) a supporting electrode layer comprising a Ni composite of formula Ni-AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; (II) a proton conducting membrane layer comprising AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; (III) a second electrode layer comprising a Ni composite of formula Ni-AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; wherein, for each layer independently, A is Ba, Sr or Ca or a mixture thereof; the sum of a+b+c equals 1; b is 0-0.75; c is 0.05-0.5; Acc is Y, Yb, Gd, Pr, Sc, Fe, Eu,, In or Sm, or a mixture thereof; and y is a number such that formula (I) is uncharged, e.g. 3-y is 2.75 to 2.95.

11. A process as claimed in any preceding claim wherein water is fed to said first zone together with ammonia.

12. A process as claimed in claim 1-11 wherein feed is aqueous ammonia.

13. A process as claimed in any preceding claim wherein the proton conducting membrane is part of a membrane electrode assembly which comprises, in the following layer, order: (I) a supporting electrode layer comprising a Ni composite of formula NiBaZr.sub.aCe.sub.bY.sub.cO.sub.3-y; (II) a proton conducting membrane layer comprising BaZr.sub.aCe.sub.bY.sub.cO.sub.3-y; (III) a second electrode layer comprising a Ni composite of formula NiBaZr.sub.aCe.sub.bY.sub.cO.sub.3-y; wherein, for each layer independently, the sum of a+b+c equals 1; b is 0-0.75; c is 0.05-0.5; and y is a number such that formula (I) is uncharged, e.g. 3-y is 2.75 to 2.95.

14. A process as claimed in any preceding claim wherein the proton conducting membrane is part of a membrane electrode assembly which comprises, in the following layer, order: (I) a supporting electrode layer comprising a Ni composite of formula NiBaZr.sub.aCe.sub.bY.sub.cYb.sub.dO.sub.3-y; (II) a proton conducting membrane layer comprising BaZr.sub.aCe.sub.bY.sub.cYb.sub.dO.sub.3-y; (III) a second electrode layer comprising a Ni composite of formula NiBaZr.sub.aCe.sub.bY.sub.cYb.sub.dO.sub.3-y; wherein the sum of a+b+c+d equals 1: b is 0.05-0.75 c is 0.05-0.25 d is 0.05-0.25; and y is a number such that formula (I) is uncharged, e.g. 3-y is 2.75 to 2.95.

15. A process as claimed in any preceding claim wherein the proton conducting membrane is part of a membrane electrode assembly which comprises, in the following layer, order: (I) a supporting electrode layer comprising a Ni composite of formula NiBaCe.sub.0.7Zr.sub.0.1Y.sub.0.1Yb.sub.0.1O.sub.3-y; (II) a proton conducting membrane layer comprising BaCe.sub.0.7Zr.sub.0.1Y.sub.0.1Yb.sub.0.1O.sub.3-y; (III) a second electrode layer comprising a Ni composite of formula NiBaCe.sub.0.7Zr.sub.0.1Y.sub.0.1Yb.sub.0.1O.sub.3-y; wherein, y is a number such that formula (I) is uncharged, e.g. 3-y is 2.75 to 2.95.

16. A process for the production of hydrogen in a membrane reactor, said membrane reactor comprising a first zone separated by a proton conducting membrane from a second zone, said first zone having a gas inlet and a product outlet and said second zone having a product outlet; said process comprising; a. feeding a gas comprising ammonia to said first zone via said gas inlet, and allowing a reaction to take place in said first zone so that hydrogen and nitrogen are formed; b. applying an electric field over said proton conducting membrane; c. allowing hydrogen to dissociate into electrons and protons to selectivity pass through the proton conducting membrane to said second zone where protons and electrons recombine to form hydrogen in the second zone; wherein the membrane reactor comprises a membrane electrode assembly which comprises, in the following layer, order: (I) a supporting electrode layer comprising a Ni composite of formula Ni-AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; (II) a proton conducting membrane layer comprising AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; (III) a second electrode layer comprising a Ni composite of formula Ni-AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; wherein, for each layer independently, A is Ba, Sr or Ca or a mixture thereof; the sum of a+b+c equals 1; b is 0-0.75; c is 0.05-0.5; Acc is Y, Yb, Gd, Pr, Sc, Fe, Eu,, In or Sm, or a mixture thereof; and y is a number such that formula (I) is uncharged, e.g. 3-y is 2.75 to 2.95.

17. A membrane reactor comprising a first zone separated by a membrane electrode assembly from a second zone, said first zone having a gas inlet and a product outlet and said second zone having a product outlet wherein said second zone product outlet is provided with a pressure regulator; a power source adapted to pass an electric field over the membrane electrode assembly; and wherein said membrane electrode assembly comprises, in the following layer, order: (I) a supporting electrode layer comprising a Ni composite of formula Ni-AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; (II) a proton conducting membrane layer comprising AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; (III) a second electrode layer comprising a Ni composite of formula Ni-AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; wherein, for each layer independently, A is Ba, Sr or Ca or a mixture thereof; the sum of a+b+c equals 1; b is 0-0.75; c is 0.05-0.5; Acc is Y, Yb, Pr, Eu, Pr, Sc or In, or a mixture thereof; and y is a number such that formula (I) is uncharged, e.g. 3-y is 2.75 to 2.95.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0294] FIG. 1 is a micrograph of a fractured surface showing the structure resulting from the use of a single co-sintering step to produce the MEA architecture. Also included is a porous current collector layer deposited on the cathode surface.

[0295] FIG. 2 shows conversion of anhydrous ammonia as a function of hydrogen recovery. Here hydrogen recovery is defined as the percentage of hydrogen measured in the exit of second zone divided by the total hydrogen available from fed NH.sub.3 to the first zone. Zero hydrogen recovery yields the open circuit conversion. It is observed that ammonia conversion increases with increased hydrogen recovery and reaches 100% at a hydrogen recovery of ?60%

[0296] FIG. 3 shows conversion of aqueous ammonia as a function of hydrogen recovery. Here hydrogen recovery is defined as the percentage of hydrogen measured in the exit of second zone divided by the total hydrogen available from fed NH.sub.3 to the first zone. Zero hydrogen recovery yields the open circuit conversion. It is observed that ammonia conversion increases with increase hydrogen recovery and reaches >98% at hydrogen recovery of >95%.

[0297] FIG. 4 shows increasing hydrogen partial pressure within the membrane reactor during ammonia dehydrogenation.

[0298] FIG. 5 shows a process flow diagram for a 1 tonne/day hydrogen production facility operating on anhydrous ammonia.

[0299] FIG. 6 shows a process flow diagram for a 1 tonne/day hydrogen production facility operating on aqueous ammonia (35% NH.sub.3 solution).

EXAMPLES

Membrane Preparation:

[0300] A tubular asymmetric membrane support of 60 wt % NiBaZr.sub.0.7Ce.sub.0.2Y.sub.0.1O.sub.3-? (BCZY27) with a 30 ?m dense membrane was synthesized using a reactive sintering approach.

[0301] Precursors of BaSO.sub.4, ZrO.sub.2, Y.sub.2O.sub.3 and CeO.sub.2 were mixed in stoichiometric amounts (metal basis) together in a Nalgene bottle on ajar roller for 24 h. The material was dried in air and sieved through a 40 mesh screen. This forms a first precursor mixture.

[0302] Two portions of the precursor mixture were mixed additionally with 64 wt. % NiO. One of those portions (the first portion) was then blended with water soluble acrylic and cellulosic ether plasticizer to prepare the extrusion batch.

[0303] Green tubes were extruded using the extrusion batch on a Loomis extruder. The extruded tubes were then dried and spray coated with the first precursor mixture.

[0304] After a second drying step the tubes were dip coated in a solution of the previous second portion (containing NiO). The tubes were co-fired by hang-firing in air at 1600? C. for 4 h. This process creates an internal NiO-BCZY27 layer. The sintered tubes were then treated in a hydrogen mixture (safe gas) at 1000? C., to reduce the NiO to Ni and give the necessary porosity in anode support structure and outer cathode. A Ni current collector was deposited on the outer cathode. A scanning electrode micrograph of the cell cross section is given in FIG. 1.

Catalyst:

[0305] The anode support structure consisting of 60 wt. % Ni-BCZY27 provides sufficient catalytic activity for ammonia dehydrogenation.

Cell Assembly:

[0306] The ceramic cell above was sealed to a ceramic alumina riser with an outer diameter of ? using a glass ceramic seal designed to thermally match the thermal expansion coefficient of the cell assembly. The ceramic riser enables positioning of the ceramic cell in a uniform temperature zone during experiments. The other end of the tubular ceramic cell was capped using a similar glass ceramic sealant material, yielding a leak free cell assembly.

Reactor and Setup:

[0307] The tubular reactor set-up consists of the inner cell assembly and an outer steel reactor tube (Kanthal APMT, ID=20.93 mm). The cell assembly were assembled onto a 316 SS Swagelok-based system providing electrical contact and feedthrough for thermocouples and gases. Thermocouples were placed inside the tubular cell and outside the reactor tube at the top and bottom of the ceramic cell. By utilizing these thermocouples, the heating zones of the reactor furnace were adjusted to an axial temperature difference of less than 10? C. A Ni tube (O.D.=4.6 mm) served as the gas feed and current probe for the inner first zone. To ensure contact between the tubular cell and Ni tube, Ni wool (American Elements) was inserted into the first zone to ensure contact between the Ni tube and first electrode. The outer second electrode was contacted with with Ag wires (diameter=0.25 mm) wrapped around the tubular structure. Gas analysis was performed using an Agilent 7890 gas chromatograph measuring the concentrations of He, H.sub.2, N.sub.2 and NH.sub.3 in the product and sweep outlet gas lines. A Hameg HMP4040 power source was used in galvanostatic mode for the hydrogen removal, compression and production of heat.

Process 1 Anhydrous NH.SUB.3 .Dehydrogenation

[0308] A cell assembly was mounted in the reactor setup, both described above. The active cell area was 32.4 cm.sup.2. A gas flow consisting of 105 mL/min N.sub.2 and 20 mg/min H.sub.2O was fed to the second zone, while a gas flow consisting of 26.2 mL/min of He and 20 mg/min of NH.sub.3 was fed to the first zone where the dehydrogenation reaction of NH.sub.3 occurs and hydrogen is transported through the membrane when an external bias is applied. The reaction temperature was 600? C. Helium was used as an internal standard and to identify possible leaks through the membrane. The ammonia conversion obtained at open circuit voltage (OCV) was equal to 99.5%. When applying an external electric field of 3.2 A over the membrane the ammonia conversion reached 100%. The ammonia conversion increases with the amount of hydrogen transported through the membrane, corresponding to increasing the current and obtained hydrogen recovery as shown in FIG. 2.

Process 2 Aqueous NH.SUB.3 .Dehydrogenation

[0309] A cell assembly was mounted in the reactor setup, both described above. The active cell area was 15.39 cm.sup.2. A gas flow consisting of 105 mL/min N.sub.2 and 20 mg/min H.sub.2O was fed to the second zone, while a gas flow consisting of 15.1 mL/min He, 10 mg/min NH.sub.3 and 32 mg/min H.sub.2O, corresponding to a 75% H.sub.2O 25% NH.sub.3 aqueous ammonia mixture was fed to the first zone where the dehydrogenation reaction of NH.sub.3 occurs and hydrogen is transported through the membrane when an external bias is applied. The reaction temperature was 600? C. Helium was used as an internal standard to identify possible leaks through the membrane. The ammonia conversion obtained at open circuit voltage (OCV) was equal to 76%. When applying an external electric field of 3 A over the membrane the ammonia conversion reached 98%. Similarly to the anhydrous case, the ammonia conversion increases with the amount of hydrogen transported through the membrane, corresponding to increasing the current and obtained hydrogen recovery as shown in FIG. 3.

Electrochemical Compression

[0310] A cell assembly was mounted in the reactor setup, both described above. The active cell area was 14.45 cm.sup.2. A gas flow consisting of 15.1 mL/min He, 65 mg/min NH.sub.3 was fed to the first zone where the dehydrogenation reaction of NH.sub.3 occurs and hydrogen is transported through the membrane when an external bias is applied. The gas flow in second zone was decreased from 105 mL/min N.sub.2 and 20 mg/min water in two steps, first to 10 mL/min N.sub.2 and 20 mg/min H.sub.2O and then to 20 mg/min H.sub.2O during the experiment. The continuous transport of hydrogen through the membrane allowed for a corresponding increase in hydrogen partial pressure, showing a higher partial pressure of hydrogen in the second zone (cathodic pressure) compared to the first zone (anodic pressure) as shown in FIG. 4.

Process Flow Diagram 5 Anhydrous Ammonia

[0311] For compressed hydrogen production from anhydrous ammonia a process flow diagram is given in FIG. 5.

[0312] Anhydrous ammonia is fed via line (1) through a pump to heat exchanger-1. This heat exchanger-1 can be heated via hydrogen extracted in line (5) from the membrane reactor. A second heat exchanger-2 can be used before ammonia passes into the membrane reactor via line (4). Any unreacted starting material and retained nitrogen can be recycled to heat exchanger-2 via line (10) and nitrogen extracted via line (11).

[0313] If required water can be added to the permeate side of the reactor via heat exchanger-3 which can also be heated via hydrogen via line (6). The hydrogen water mixture from heat exchanger-3 can be removed and condensed via 7. Water can be recycled and hydrogen taken for storage via line (9). If required water can also be further heated via lines 15 and 16 and the heater there between.

[0314] ASPEN software is used to simulate a 1 tonne H.sub.2 per day production facility using process flow diagram described above. Reaction conditions are 650? C. with a reaction pressure of 27.9 bar (giving a hydrogen partial pressure of 20.9 bar assuming full conversion). Produced hydrogen is electrochemically compressed to 25.4 bar. Operating at a current density of 0.517 A/cm.sup.2 a membrane area of 214 m.sup.2 is needed. Heat generated by the operation of the membrane, Joule heating, is supplied to the endothermic ammonia dehydrogenation reaction and for heat exchange of fed anhydrous ammonia. The benefit of heat integration yields an overall energy efficiency of 92.1%.

Process Flow Diagram 6 Aqueous Ammonia

[0315] For compressed hydrogen production from aqueous ammonia a process flow diagram is given in FIG. 6.

[0316] Aqueous ammonia is fed via line (1) through a pump to line (2) and passes through a series of heat exchangers forming a so called heat recovery loop. Line (3) containing the reaction mixture is fed to heat exchanger-1, which can be heated via line (11) from the retentate of the membrane reactor. A second heat exchanger-2 can be used before aqueous ammonia enters the membrane reactor via line (5). This heat exchanger-2 can be heated via hydrogen extracted in line (6) from the membrane reactor permeate. Any unreacted starting material and retained water and nitrogen can be recycled to heat exchanger-1, heat exchanger-3 and the heat recovery loop via line (11) and the remaining water, nitrogen mixture is extracted via line (15).

[0317] If required, water can be added to the membrane via line (17) fed first to heat exchanger-3, which can be heated via the retentate from line (11), followed by heat exchanger-4, which can be heated by line (6) containing a hydrogen water mixture from the permeate. The hydrogen water mixture from heat exchanger-4 can be removed and condensed via 7. Water can be recycled and hydrogen taken for storage via line (9). If required water can also be further heated via lines (19) and (20) and the heater there between.

[0318] ASPEN software is used to simulate a 1 tonne H.sub.2 per day production facility using process flow diagram described above. Reaction conditions are 650? C. with a reaction pressure of 27.9 bar (giving a hydrogen partial pressure of 7.3 bar assuming full conversion). Produced hydrogen is electrochemically compressed to 25.4 bar. Operating at a current density of 0.664 A/cm.sup.2 a membrane area of 167 m.sup.2 is needed. Heat generated by the operation of the membrane, Joule heating, is supplied to the endothermic ammonia dehydrogenation reaction and for heat exchange of fed aqueous ammonia (35% NH.sub.3 solution). The benefit of heat integration yields an overall energy efficiency of 82.7%.