GB2078539A - Coated article for use in gas separation by selective diffusion and apparatus using such a coated article - Google Patents

Abstract

A coated article for use in separation of hydrogen from hydrogen containing gas or gas mixture comprises a refractory base material (2) with at least two flat surfaces (one of which is shown at 8), a hole (4) being formed passing through the base material (2) at one side, a grooved maze (6) being provided which extends from the hole (4) to the other side of the base material (2) and the maze (6) being coated with a hydrogen permeable membrane (14). A plurality of such coated articles can be stacked back to front, so that the outlet of one article is adjacent to the inlet of the neighbouring article. Such an arrangement, when located inside a closed furnace chamber, provides an apparatus for the separation of hydrogen from oxygen derived from the thermal dissociation of water vapour. <IMAGE>

Description

SPECIFICATION Coated article for use in gas separation by selective diffusion and apparatus using such a coated article The invention relates to a coated article for use in gas separation by a barrier for selective diffusion of gases and to apparatus using such a coated article.

Dissociation of water vapour by thermal energy into hydrogen and oxygen followed by the separation of the hydrogen with a hydrogen permeable mem brane is disclosed in U.S. Patent 4,003,725; 4,019,868; and 4,053,576, and Technical Support Package TSP 75-10314 for NASA TECH BRIEF 75 10314 (MSC-12600) entitled "Using Permeable Membranes to Produce Hydrogen and Oxygen from Water".

Solar heat is one of the ways used to thermally dissociate water vapour and this is disclosed in previously mentioned US Patent 4,019,868 and 4,053,576 and US Patent 4,030,890. Other heat sources such as nuclear energy and electrical resistance furnaces are suitable for bringing water vapour to a dissociation temperature. TSP 75-10314 and U.S. Patent 4,003,725 show resistance furnaces suitable for the present invention.

That platinum, palladium and palladium alloy metal films are permeable to hydrogen and have been used as barriers for selective diffusion of hyd rogen has been known, as disclosed by Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd edi tion, vol. 15(1968) under the section Platinum Group Metals, pages 832-860, particularly page 832 where the melting point of palladium is given as 1552 C, pages 853 and 854 where the use of palladium and a 75% Pd - 25% AG Alloy is disclosed for the separation of pure hydrogen from mixed gases by diffusion at a temperature of 350 C. in tubes of the alloy with a wall thickness of about 0.003 inches and page 855 where electrodeposition of palladium is disclosed.

TSP 75-10314 discloses, among other things, the separation of hydrogen from oxygen and watervapour at temperatures of about 6000C to 950cm and pressures from about Smm Hg (0.1 psia) to 760 mm Hg (14.7 psia) using tubes of about 0.1 mm (0.004 inches) wall thickness as the diffusion membrane.

This diffusion membrane was made from pure platinum, pure palladium, 75% palladium - 25% silver and 90% palladium - 10% silver with 90% pal ladium - 10% silver giving preferred results.

One of the many problems experienced by the prior art was to increase the surface area of contact at elevated temperatures between the platinum and palladium diffusion membranes and the dissociating water vapour or other hydrogen containing mix tures.

All prior art methods of producing hydrogen by dissociation of water vapour or from gaseous mix tures containing hydrogen have been limited by inability to separate large quantities of hydrogen from a mixture of hydrogen containing gas mixtures in a small space at elevated temperatures and at ele- vated pressures. The present invention seeks to provide a gaseous diffusion separator for hydrogen which provides a large diffusion area in a given space for separating pure hydrogen from a mixture of gases containing hydrogen.

Accordingly to the invention, there is provided a coated article for use in separation of hydrogen from a hydrogen containing gas or gas mixture, comprising a porous refractory base material having at least two flat surfaces, a hole extending through the base material at one side thereof, a grooved maze in at least one of the flat surfaces extending from the hole to the other side of the base material and a hydrogen permeable membrane coating the maze.

To carry out separation, a mixture of gases containing hydrogen may be prepared by the dissociation of water vapour at temperatures between about 3500C and 12500C, with a range of 5500C to 950 C preferred. This gaseous mixture may then be passed through the maze which has a hydrogen permeable platinum group metal membrane, preferably an alloy of 90% palladium - 10% silver, and hydrogen will then be separated to leave an oxygen enriched gas.

One embodiment of the gaseous diffusion maze may comprise a plurality of wafers comprising the above coated article, each of which has an inlet on one side connected with the beginning of the maze traced on the surface of the wafer and an outlet at the end of the maze on the opposite side of the wafer from the inlet. The porous refractory materials are coated with a film of the hydrogen permeable membrane (eg. 90% Pd-10% Ag) so that the maze for the transport of the dissociating water vapour is surrounded by the hydrogen permeable membrane.

A plurality of these wafers may be stacked back-to front with the respective outlet to inlet indexed so that a continuous path is created from wafer to wafer.

Hydrogen separated through the membrane may be passed through the porosity of the refractory and may be directed to a particular section of the wafers for transport and collection. In one embodiment, the whole wafer is coated with 90% Pd-10% Ag except for a circle at the centre of the wafer to which hydrogen is directed. In another embodiment the edges are left uncoated and hydrogen passes through these edges as water vapour is introduced at the beginning of the maze and oxygen is separated and collected at the end of the maze.

The invention will now be described in greater detail, by way of example with reference to the drawings in which:~ Figure 1 shows a front elevation of one embodiment of an individual integral wafer of the present invention having a gas diffusion maze therein; Figure 2 is a sectional side view taken along the line 11-11 of Figure 1; Figure 3 is a rear elevation of the wafer of Figure 1 showing the inlet hole and hydrogen passage therein; Figure 4 is an exploded, partial sectional side view showing one embodiment of a wafer combination of the present invention in assembly with an outlet wafer on the left, an inlet wafer on the right, and one example of the plurality of intermediate wafers of Figure 1 to 3 there between; Figure 5 shows another embodiment of the assembly of Figure 4 which takes advantage of countercurrent circulation of the hydrogen produced;; Figure 6 is a perspective view of an assembly of wafers in the form shown in Figure 4; Figure 7 shows the assembly of wafers of Figure 6 mounted in an electric resistance furnace; Figure 8 shows another embodiment of an assembly of wafers mounted in an electric resistance furnace where the porous edges of the wafers are uncoated with the hydrogen permeable membrane and hydrogen being produced from the porous edges; and Figure 9 is a detailed cross-sectional view of the end wafer of Figure 4 with a stainless steel or palladium outlet conduit mounted therein for the hydrogen and oxygen.

With particular reference to Figures 1 to 3, an integral porous refractory wafer 2 is shown having an inlet hole 4 through the wafer from back-to-front.

A maze 6 is acid etched, molded, pressed, sintered or gouged across at least one flat surface 8 of the wafer. The maze has a plurality of turns or corners 10 ending at outlet 12 which faces out in Figure 1.

The front, back and edge of the wafer are coated with a hydrogen permeable membrane 14 such as 10% Ag-90% Pd with the exception of concentric circles 16 and 18 where the porous refractory material, such as Awl203, shows through.

The wafer 2 of Figures 1 to 3 is shown in the partial exploded view of Figure 4 as the second wafer from the right. Wafers 20 and 22, which are the first and last wafers in an assembly, differ from the plurality of wafers 2 making up the composite assembly. In the embodiment of Figure 4, the first wafer 20 has no hole 18 on the outside wall and the last wafer 22 need not have a maze on either flat surface. Wafer 20 has water vapour inlet 3 and wafer 22 has oxygen outlet 5.

Figure 5 shows an embodiment different from Figure 4 wherein first wafer 24 has circle 18 with the porous refractory material showing through the outside wall. Last wafer 28 has no circle in the outside wall as does wafer 22 of Figure 4.

Figure 6 is a perspective view showing an assembly of Figure 4 defining the endothermal water decomposition unit 38 of the present invention having a plurality of wafers 2 sandwiched between first and last wafers 20 and 22.

Figure 7 shows the assembly of Figure 4 mounted in an electric resistance furnace 28 having stainless steel walls 30 and a plurality of heating elements 32.

A water vapour inlet 34 of stainless steel tubing enters on the left and the water vapour is passed through 10% Ag-90% Pd tubing 36 into the assembly 38. The 10% Ag-9û% Pd collared tubing 40 connects with the 2 outlet of wafer 22 for distribution to stainless steel outlet 42 for 02. 10% Ag-90% Pd collared tubing 44 conducts H2 from circle 16 on wafer 22 through the wall of the furnace to H2 collecting tube 46.

In the embodiment of Figure 8, the wafers have edges which are porous and uncoated so that hyd rogen produced in the endothermal water decom position unit 50 can diffuse to the outside of the unit and be picked up by an inert carrier gas such as nitrogen which enters through stainless steel conduit 48. The hydrogen produced is swept out of the furnace by the inert carrier for collection through stainless steel conduit 52.

Figure 9 shows one way of mounting conduits 44 and 40 in the hydrogen and oxygen outlets otthe last wafer 22' of the assembly. The wafer 22' has holes 58 and 60 with respective recesses 62 and 64 molded therein in the green state for holding the collared conduits 44 and 40. Of course, a collared conduit 36 can be mounted in the same way in the first wafer 20.

The method of carrying out the present invention will now be described, by way of example with particular reference to Figures 4, 5,7 and 8.

Water vapour is introduced into the furnace of Figure 7 by way of stainless steel conduit 34 and conduit 36 or inlet 3 into the endothermal water decomposition unit 38. Inside the furnace the temperature is conveniently maintained between about 3500C and 1250 C and pressures of from 5 mm Hg to superatmospheric can be maintained.

Even though the platinum group metal membrane is applied in a thickness of about 0.0005 - 0.005, preferably 0.001 - 0.003, a considerable pressure, superatmospheric, can be applied inside the endothermal water decomposition unit because the porous refractory wafer backing of the membrane is strong in compressive load. Of course, it is possible to do away with the 90% Pd-10% Ag. conduits 36,40 and 44 in high pressure applications by having the inlet 2 and the outlets 5 and 16 register with conduits 34,42 and 46 under compressive load from the walls of the furnace.

As shown in Figure 4, the water vapour enteres at inlet 3, travels through the platinum or palladium metal coated maze of wafer 20 and H2 is diffused through the membrane in the grooves of the maze into the porosity of the refractory of wafer 20. H2 diffuses simultaneously through the membrane on the right of wafer 2 opposite the maze and grooves of wafer 20. H2 moves through the porosity of wafer 20 to the uncoated center hole 16 where it is chanelled from wafer to wafer through the centres thereof.

After passing to the outlet 12 of wafer 20, the oxygen enriched water vapour now passes through inlet 4 of wafer 2 and proceeds through the maze of wafer 2 where the water vapour becomes more enriched with oxygen. Hydrogen passes through the membrane into the porosity of the wafer and proceeds to the collection area in the centres of the wafers. The same mechanism of integral wafer 2 takes place through the stack o wafers until the last wafer 22 is reached and oxygen exists from outlet 5 and hydrogen exits from outlet 16.

Figure E showsthe invention operated with countercurrent flow of the hydrogen. This is accomplished by hazing a circle of uncoated refractory 18 on the first wafer 24.

The process of Figure 8 is carried out by leaving the porous edges of the wafers uncoated so that the hydrogen is passed directly into the body of the furnace. At elevated pressures, there is no need for an inert carrier gas but when the separation is carried out at less than atmospheric pressure, it is necessary to pass an inert carrier gas such as nitrogen or argon therethrough.

Specific Examples The wafers of the present invention may be made from refractory materials or from refractory metals.

Kirk-Othmer, ibid., discloses in vol. 17 (1968) suitable refractories, with the exception of silica # refractories which poison the platinum group membranes, on pages 227-267, particlarly page 243 which discloses aluminum oxide, beryllium oxide, calcium oxide, magnesium oxide, thorium oxide, titanium oxide, uranium oxide, and spinel.

The refractory metals are disclosed on pages s 267-284, particularly page 267 which discloses that iridium, rhodium, chromium and plantinum are resistant to air at 1400 C.

The refractory materials can be processed into the wafers of the present invention using the techniques disclosed in Kirk-Othmer, ibid, Supplement Volume (1971), p. 150 where the cold pressing and isotactic pressing of aluminium oxide ceramics is disclosed.

The refractory metals can be processed into porous wafers using the techniques of powder metallurgy as disclosed in Kirk-Othmer, ibid, vol. 16 (1968), pages 401-435, particularly page 415 where sintering is disclosed and page 425 where the preparation of porous sintered materials is disclosed.

The porosity in the manufacture of the porous refractory wafers of the present invention can be controlled by using the techniques disclosed for the manufacture of aluminium oxide abrasive grinding wheels as disclosed in Kirk-Othmer, ibid, vol. 1 (1963), page 32, where a chart of the grain sizes used is given, pages 34 and 35, where the control of open structure is disclosed and pages 35 and 36 where the methods of manufacture are given.

In addition to the above procedures for fabricating the wafers of the present invention, it is also possible to usethetechniques disclosed in U.S. Patents 3,344,586; 3,428,476 and 3,499,265, but with the addition of the step of molding a maze into at least one side of the wafer when the ceramic or refractory materials is in the green state.

Example 1 Having all the above in mind, a porous refractory wafer can be produced for the present invention as follows: A dry mixture of 25% Georgia Kaolin, 15% Tennessee ball clay, 55% nepheline syenite, and 5% silica is made by tumbling these ingredients in a bottle for several hours after which 200 mesh carbon black is added and thoroughly mixed in byfurthertumbling for about 8 hours.

The porosity of the finished piece is largely determined by the amount of carbon black used and for a porosity of 40-50%, which is preferred, the proportion of carbon black to the other ingredients is about 20-40%.

When the dry mixing is complete, the mixture is dampened with a fluid which serves as a binder and lubricant. The moisture content is preferably about 25-30% which supplies the moisture necessary to be able to press the mass into the desired disk shape satisfactorily. The fluid is suitably 3-10% glycerine as the binder and the remainder water as the lubricant.

The fluid is mixed in to dampen the mixture thoroughly.

Then the mixture is molded in a round mold having a plunger design which produces the maze as shown in Figure 1 of the present invention. Although wafers of 3 inch diameter and about 118 inch thick were produced, any suitable size can be made.

These wafers are then air dried over-night and fired in a periodic furnace which is raised to a temperature of about 11 00 C prog progressively over a period of about 24 hours.

The porous ceramic wafers are first coated on the maze side with palladium by brushing on a solution of palladium resistance dissolved in oil of peppermint and chloroform and containing 4.5% Pd by weight. Twelve coats are applied with each fired at about 3500C in air to thermally decompose the resinate to metal. After 12 coats, a palladium film about 1.2 microns thick is on the substrate. The film is then fired to 1 0000C in air with a one hour soak to compact the film and bond it to the substrate.

The uncoated circle indicated at 16 in Figure 1 is maintained by placing a cardboard circle thereover and then removing it after all layers have been applied.

A silver naphthenate solution having a viscosity suitable for application by brushing was made as follows: Grams Silver naphthenate (32% Ag) ..................... 3.35 Toluene ........................................ 11.13 14.48 The silver naphthenate was dissolved to a stiff gel by stirring mechanically for 2 hours at room temper ature. 0.52 Grams of t-octyl amine were then added and, with stirring continued for a few more minutes.

The solution became fluid. The amber brushing solution then contained 7.15% Ag by weight or approximately .01 mole of silver naphthenate and .004 mole of amine.

The silver naphthenate solution is then applied by brushing over the palladium in several coats with each fired at about 2000C in air. When a silver weight equal to 1/3 the palladium weight has been added, the coated ceramis is heated for 4 hours at 6000C in hydrogen to form a 75:25 Pd:Ag alloy in situ.

The other side of the wafer and the edges are then coated as above to produce a wafer coated except for bare circles 16 and 18 on the front and back as shown in Figures 1-3.

Example 2 The method of Example 1 is carried out for molding and preparing wafers prior to coating. In this example, only the maze 6 of Figure 1 is coated and the back of the wafer is coated following the techniques of Example 1. Such wafers are useful in the apparatus shown in Figure 8.

Example 3 The method of Example 1 is modified slightly to prepare the wafer 20 of Figure 4. No circle of cardboard is used in the coating of the right side of wafer 20 to prevent a coating.

Example 4 One wafer from Example 3 and a plurality of wafers from Example 1 are secured together to make an assembly by brushing powdered glaze material, such as "Pemco frit P-1701" on a small portion of adjacent flat sides taking care not to coat the maze.

The glaze is fired to fuse the wafers together.

Claims (12)

1. A coated article for use in separation of hydrogen from a hydrogen containing gas or gas mixture comprising a porous refractory base material having at least two flat surfaces, a hole extending through the base material at one side thereof, a grooved maze in at least one of the flat surfaces extending from the hole to the other side of the base material and a hydrogen permeable membrane coating the maze.

2. A coated article as claimed in Claim 1, wherein the base material comprises aluminium oxide, beryllium oxide, calcium oxide, magnesium oxide, thorium oxide, titanium oxide, uranium oxide, spinel, iridium, chromium or platinum.

3. A coated article as claimed in claim 1 or 2 wherein the hydrogen permeable membrane comprises platinum, palladium or alloys or silver and palladium.

4. A coated article as claimed in any of claim 1 to 3 wherein circular portions at the centres of the flat surfaces are uncoated and the remainder of the flat surfaces and the edges of the base material are coated with the hydrogen permeable membrane.

5. A coated article as claimed in any one of Claims 1 to 3, wherein a circular portion at the centre of the flat surface having the maze is uncoated at the remainder of the flat surfaces and the edges of the base material are coated with the hydrogen permeable membrane.

6. A coated article as claimed in any one of Claims 1 to 5, wherein the hydrogen permeable membrane has a thickness of about 0.0005 - 0.005 inches.

7. A coated article as claimed in Claim 6, wherein the hydrogen permeable membrane has a thickness of about 0.001 - 0.003 inches.

8. An apparatus for separating hydrogen from a hydrogen containing gas or gas mixture comprising a plurality of wafers as claimed in any one of claims 1 to 9, stacked back-to4ront, wherein the hole at said one side defines an inlet, an outlet is provided at the termination of the maze and the outlets of successive wafers are registered with the inlets of succes sivewafers in said plurality.

9. An apparatus as claimed in claim 8, wherein the plurality of wafers are located in a closed furnace chamber, heated by heating means and having means for introducing water vapour into the chamber; means for removing hydrogen from the chamber and means for removing oxygen from the chamber; the first of the inlets of the wafers being connected with the means for introducing water vapour and the last of the outlets being connected to the means for removing oxygen from the chamber.

10. An apparatus as claimed in claim 9, wherein a circle of uncoated base material is located on the two flat surfaces at the centre thereof and the remainder of the flat surfaces and the edges of the base mater ial are coated with the hydrogen permeable mem brane, the said circles being connected to the means for removing hydrogen from the chamber.

11. A coated article substantially as described herein with reference to the drawings.

12. An apparatus for separating hydrogen from a hydrogen containing gas or gas mixture substantially as described herein with reference to the draw- ings. ~~~~~~~~~~~~~~~