Process for producing compressed hydrogen in a membrane reactor and reactor therefor

Abstract

A process for direct compression of hydrogen separated from a hydrocarbon source is described herein. The process comprises a first zone wherein a hydrocarbon reaction that produce hydrogen occurs, a ceramic proton conductor which under an applied electric field transport hydrogen from said first zone to said second zone, and a second zone where compressed hydrogen is produced. The heat energy generated by ohmic resistance in the membrane is partially recuperated as chemical energy in the hydrocarbon reforming process to generate hydrogen.

Claims

1. A process for compressing hydrogen in a membrane reactor; said membrane reactor comprising a membrane electrode assembly, said membrane reactor further comprising a first zone separated by a hydrogen transport 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: feeding a gas comprising a hydrocarbon to said first zone, and allowing a reaction to take place in said first zone so that hydrogen is formed; applying an electric field over said hydrogen transport membrane; and allowing the hydrogen to disassociate into electrons and protons in said first zone and allowing protons to selectively pass through the hydrogen transport membrane to said second zone, where protons recombine to form of 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 said membrane electrode assembly comprises, in the following layer order: a supporting electrode material comprising a Ni composite of formula Ni-AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; the hydrogen transport membrane layer material comprising AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; and a second electrode material 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, Ca or a mixture thereof; the sum of a, b, and c equals 1; b is from 0 to 0.45; c is from 0.1 to 0.5; Acc is Y, Yb, Pr, Eu, Pr, Sc, In, or a mixture thereof; and y is a number such that formula (I) is uncharged.

2. The process as claimed in claim 1, wherein the gas added to the first zone comprises the hydrocarbon and water, the process produces reaction products at the product outlet of the first zone, and said reaction products comprise CO and/or CO.sub.2.

3. The process as claimed in claim 1, wherein the membrane reactor is operated at a temperature above 400° C.

4. The process as claimed in claim 1, wherein the hydrogen extracted from the first zone shifts the reaction equilibrium towards the product side.

5. The process as claimed in claim 1, wherein heat emitted from ohmic losses in the hydrogen transport membrane is used to heat the first zone.

6. The process as claimed in claim 1, wherein the hydrogen transport membrane catalyzes a dehydrogenation reaction.

7. The process as claimed in claim 1, 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 claim 1, wherein the gas fed to the first zone is a mixture of methane and steam in a molar ratio of from 1:2 to 1:3.

9. The process as claimed in claim 1, wherein said hydrogen transport membrane comprises at least one mixed metal oxide of formula (I)
BaZr.sub.aCe.sub.bY.sub.cO.sub.3-y  (I) wherein the sum of a, b, and c equals 1; b is from 0 to 0.45; c is from 0.1 to 0.5; and y is a number such that formula (I) is uncharged.

10. The process as claimed in claim 1, wherein 2.75≤y≤2.95.

11. The process as claimed in claim 9, wherein 2.75≤y≤2.95.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the increasing pressure within the membrane reactor during steam reforming.

(2) FIG. 2 is a general schematic of a membrane electrode assembly (MEA) showing an anode at which reactions occur that generate electrons, a cathode at which reactions occur that consume electrons, and an electrolyte which conducts ions but is insulating towards electrons. In this case, the overall reaction is the oxidation of hydrogen to generate water.

(3) FIG. 3 is a micrograph of a fracture surface showing the structure resulting from the use of a single co-sintering step to produce the MEA architecture. Note that the electronically active component of the electrodes (dark grey phase) has undergone a further reduction step.

(4) An electrode support structure is illustrated in FIG. 4. The first layer comprises the electrode support. The second layer comprises the membrane. The third layer comprises the second electrode. In a further embodiment, a catalyst layer can be adhered to the surface of the MEA or lie freely on top (not shown).

(5) A tubular membrane-support structure/design is illustrated in FIG. 5. Two general designs are possible. One with a catalyst layer on the surface of electrode layer 3 and one with the catalyst layer on the inside of the tube. An arrangement with the catalyst layer on the surface of the tube is advantageous if the dehydrogenation reaction is slow. If the reduction reaction of O.sub.2 and/or the water formation reaction and/or the diffusion of water/O.sub.2 to the membrane are the slowest the arrangement with the catalyst layer inside the tube will be advantageous.

(6) FIG. 5 shows the support layer (1) with membrane layer (2) and outer electrode layer 3.

(7) An embodiment for a planar reactor design is illustrated in FIG. 6. Modules of catalyst-membrane-support assemblies are stacked horizontally arranged so that the support faces a support of a second assembly, and the catalyst faces a catalyst of a third assembly and so on. This stacking form channels for the reactant gas and the purge gas respectively. Each assembly is sealed at the end with suitable sealing material, such as a glass which is non-catalytically active towards coke formation. The embodiment shown has a counter-current gas flow. This configuration has a similar hydrogen pressure gradient AP in the two end segments. The first segment is located at the inlet of the reactant gas. The hydrogen concentration will be highest at this point, while the oxygen content in the purge gas will be the lowest. In the other end, of the air inlet, the hydrogen pressure will be at the lowest point, while the oxygen pressure will be at the highest. The pressure gradient in the two ends will be approximately equal, which is also true for the part between the two ends. This will ensure a homogeneous dehydrogenation along the membrane, which furthermore will stabilize the conversion towards carbon formation. In this way a constant thickness of the membrane can be used throughout the reactor.

(8) FIG. 7 shows the membrane reactor in action. The membrane separates a first zone from a second zone. The first zone contains methane and steam. Upon dehydrogenation of the methane, hydrogen in the form of protons passes through the membrane to the second zone. Current is applied across the membrane to encourage transport.

EXAMPLE 1

(9) Membrane Preparation

(10) A tubular asymmetric membrane of 60 wt. % Ni—BaZr.sub.0.7Ce.sub.0.2Y.sub.0.1O.sub.3-y (BZCY72) support with a 30 μm dense membrane was synthesized using a reactive sintering approach (Coors et al. Journal of Membrane Science 376 (2011) 50-55). 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 precursor mixture.

(11) Two portions of the mix were mixed additionally with 64 wt. % NiO. One of those portions was then blended with water soluble acrylic and cellulosic ether plasticizer to prepare the extrusion batch.

(12) Green tubes were extruded using a Loomis extruder. The extruded tubes were then dried and spray coated with the precursor mixture after drying. 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—BZY support layer with dense membrane layer and outer NiO—BZY 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.

(13) Dimensions of the cell tube are ˜25 cm long with an outer diameter of ˜10 mm, an inner diameter of ˜9.8 mm and a membrane thickness of ˜30 μm.

(14) Catalyst

(15) The anode support structure consisting of the 60 wt. % Ni-BZCY72 provides sufficient catalytic activity

(16) Reactor

(17) The reactor consists of a 1″ metallic outer tube (800 HT). The ceramic tube above was mounted on a membrane tube fitting with a o-ring (Kalrez) seal and placed inside the outer tube. Inner gas tube and current collector consisted of at Ni-tube (O.D. 4.6 mm). To ensure contact with the support, Ni wool was put into the end of the tube. The temperature was monitored with 3 thermocouples, two outside the reactor at heights corresponding to the top and bottom of the electrode, and one inside ceramic tube at a height corresponding to the top of the electrode. For cathode current collector Cu wire was wrapped around the electrode. Gas analysis was performed using a micro GC (Model 490, Varian) measuring the concentrations of He, H.sub.2, CH.sub.4, CO and CO.sub.2 in the product and sweep outlet gas lines.

EXAMPLE 2

(18) Membrane Preparation

(19) 2 tubular membranes, A and B, of 6 cm each where prepared as described in Example 1. Both segments have outer electrode areas of 14.1 cm.sup.2. Segment A is connected with segment B in series. The two segments are connected in series together using a glass ceramic based interconnect. Segment A is further sealed to an alumina riser of 25 cm length using a glass ceramic seal. Segment B is capped with a glass ceramic seal.
Catalyst
The anode support structure consisting of the 60 wt. % Ni-BZCY72 provides sufficient catalytic activity
Reactor and Setup
The reactor consists of a 1″ metallic outer tube (800 HT). The segmented ceramic tube on riser was mounted with Swagelok fittings seal and placed inside the outer tube. Inner gas tube and current collector consisted of at Ni-tube (O.D. 4.6 mm). To ensure contact with the support, Ni wool was put into segment A. The temperature was monitored with 3 thermocouples, two outside the reactor at heights corresponding to the top and bottom of the electrode, and one inside ceramic tube at a height corresponding to the top of the electrode. For cathode current collector Cu wire was wrapped around the cathodes. Current wires where taken from cathode of segment B. Gas analysis was performed using a micro GC (Model 490, Varian) measuring the concentrations of He, H.sub.2, CH.sub.4, CO and CO.sub.2 in the product and sweep outlet gas lines. A Hameg 4040 power source with 4 terminals was used in galvanostatic mode for the hydrogen removal, compression and production of heat.
Process
The reactor was operated at 800° C. using the reactor and membrane electrode assembly as described above. A gas mixture consisting of 28% CH.sub.4, 17% H.sub.2 and 55% H.sub.2O was fed to the first zone where a reforming reaction occurs to convert methane and steam to hydrogen to thermodynamic equilibrium. An external electric field of 5 A was applied to the membrane. A back pressure regulator at the outlet of second zone was adjusted so that the pressure increased in a step-wise manner. The continuous transport of hydrogen through the membrane allows for a continuous increase in pressure as shown in FIG. 1. A total pressure difference of 11 bar was obtained.

(20) A membrane electrode assembly of 18.9 cm.sup.2 with a area specific resistance (ASR) of 0.8 Ω cm.sup.2 operating at a current density of 328 mA/cm.sup.2 will emit 2.7 W. A bundle of 215 tubes will generate 14 kWh during 24 h operation. This heat will balance the heat required to steam reform 6 Nm.sup.3 of CH.sub.4 at a conversion of 98%.