Membrane module for the separation of hydrogen and method for the production thereof

A membrane module for the separation of hydrogen is configured for parallel flows and contains a plurality of planar membrane cells which respectively comprise two hydrogen-selective planar membranes respectively surrounded by a flat membrane frame. An air-permeable distancing layer is arranged between the membranes for removal of the permeate gas and a supply frame surrounding a supply area for the reformate gas. All membrane frames and supply frames have the same outer dimensions and form a stack with planar side surfaces. Two membrane frames of each membrane cell have protruding edges directed towards each other, enabling them to enter into contact with each other, except for at least one first opening towards a side surface of the stack. The supply frame is disposed, except for second and third openings towards the side surfaces of the stack, in a closely adjacent manner to the edges of the membrane frame of two neighboring membrane cells. The outsides of all membrane frames and supply frames, except for first, second and third openings, are welded or soldered to each other in a gas-tight manner.

The invention relates to a membrane module for separating off hydrogen and to a method for producing it.

BACKGROUND

Fuel cell systems, in particular those used for mobile applications, can be supplied with hydrogen by reforming hydrocarbons such as, for example, methanol, gasoline or diesel. In addition to hydrogen, the product gas formed in a reforming process also contains carbon monoxide, carbon dioxide and steam. In particular, the carbon monoxide has to be removed for use in the fuel cell, since this gas acts as a catalyst poison and leads to losses of power in the fuel cell.

Membranes, which may consist of various materials, such as, for example, ceramic, glass, polymer or metal, have long been used to separate off hydrogen. Metal membranes are distinguished by a high selectivity for hydrogen and a high thermal stability but have relatively low permeation rates.

To achieve a desired permeation rate, a large number of membrane cells each having a hydrogen-selective membrane are used, with the hydrogen-containing reformate gas flowing onto the individual membranes either in series or in parallel. The membrane cells are stacked on top of one another in order to form a compact membrane module.

Membrane modules onto which gas flows in series are described, for example, in U.S. Pat. No. 5,498,278 and U.S. Pat. No. 5,645,626.

A membrane module onto which gas flows in parallel, in accordance with the preambles of patent claims1and14, is known from WO 01/70376. Each membrane cell of the membrane module includes a plurality of oval or approximately rectangular frames stacked on top of one another as supports for hydrogen-selective, planar membranes and for an air-permeable spacer layer for discharging permeate gas, and also two feed frames, which surround feed spaces for reformate gas. All the frames have identical external dimensions and form a compact stack with smooth external surfaces. The frames include holes which are aligned with one another and form passages for the common supply and/or discharge of the process gases, namely on the one hand to supply hydrogen-containing reformate gas from an upstream reforming process, and secondly for discharging the raffinate gas, i.e. the hydrogen-depleted reformate gas, and thirdly for discharging the permeate gas, i.e. the hydrogen which is diffused through the membranes.

Such a membrane module onto which the gas flows in parallel is a very much simpler construction than a membrane module onto which the gas flows in series, since there is no need for structures which divert the permeate gas from cell to cell, as is required when the gas is guided in series.

Nevertheless, the outlay involved in producing the membrane module which is known from WO 01/70376 is considerable, since the gases are diverted within the various frames. The holes in the frames have to be produced with a high degree of accuracy, since any projecting or recessed frame parts or burrs impede the flow of gas and may make it more difficult to produce a gastight seal. An even more serious problem is that manufacturing-related inaccuracies may lead to different magnitudes of partial flows through the individual membrane cells, which has an adverse effect on the permeation rate, as will be explained in more detail below. Finally, the frames have to be connected to one another in a gastight manner over the entire surface in order for the passages and the separation spacers to be reliably separated from one another in a leaktight manner.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a membrane module which can be produced with minimum possible outlay and without the use of seals and without any risk of the gas streams leaking.

The present invention provides a membrane module in which two membrane frames of each membrane cell have raised edges which are directed toward one another and by means of which they are in contact with one another with the exception of at least one opening toward one side face of the stack. The feed frame is designed in such a way that, with the exception of openings toward side faces of the stack, it bears closely against the edges of the membrane frames of two adjacent membrane cells, and that the outer sides of all the membrane frames and feed frames are welded or soldered to one another in a gastight manner, but leaving clear the openings.

The present invention also provides a method a method for producing a membrane module for separating off hydrogen from a reformate gas, the membrane module including a plurality of planar membrane cells stacked on top of one another and connected to one another, each membrane cell including two hydrogen-selective, planar membranes that are each surrounded by a flat membrane frame, an air-permeable spacer layer disposed between the membranes and configured to discharge a permeate gas, and a feed frame that surrounds a feed space for the reformate gas adjacent one of the two membranes, the membrane frames and feed frame having identical external dimensions and being assembled to form a stack with planar side faces. The method includes providing each the two membrane frames of each membrane cell with raised edges directed toward one another and in contact with one another, providing each the two membrane frames with at least one first opening toward a side face of the stack, fitting the feed frame tightly onto the edges of the membrane frames two adjacent membrane cells except for second and third openings in the feed frame toward side faces of the stack, and soldering or welding the outer sides of all the membrane frames and feed frames in a gastight manner except for the first, second, and third openings.

The raised edges in the membrane frames have a certain width, so that they bear flat against one another, and the feed frames are narrower at the edge than the raised edges in the membrane frames and are provided with matching recesses such that they fit accurately into the raised edges from behind.

The membrane frames can easily be produced by punching or stamping flat material, such as for example metal sheet, and the feed frames can be shaped by chip-forming or chipless processors.

After all the frames of a stack have been placed on top of one another, which does not require any excessive degree of accuracy, the stack is simply joined and made gastight by welding or soldering of the outer sides, with the openings for supplying and discharging the process gases being kept open. Associated openings then lie above one another and can be connected in a simple way to a matching feed or discharge passage. Production is particularly simple if all the frames consist of metal, since they can then be connected to one another and to the feed and discharge lines by welding.

In the same operation as that used to produce the raised edges, it is also possible to stamp structures into the membrane frames which cause the reformate gas to be distributed uniformly through the feed space, these structures preferably being webs which are directed toward the reformate gas opening in the feed frame and extend, in particular, in a radial distribution from the reformate gas opening in the direction of a membrane edge.

One significant advantage of the membrane module according to the invention is that only a small number of different components are needed, and these components can be punched or stamped in a simple way. Cross sections which determine the flow resistance and therefore the gas flow rate then result through stamping, which can easily be carried out with a high level of accuracy, so that the individual part-streams through the membrane cells are practically identical and therefore a high overall efficiency is achieved. The overall membrane module can be welded, so that there can be no leakage flows of carbon monoxide into the permeate gas, which cannot always be ensured by seals. In each case only one additional component is required to supply and discharge the reformate, raffinate and permeate gases. The membranes can be used in rectangular form, so that the membrane scrap can be kept at a low level, and they do not have to be perforated in order for permeate gas to pass through them. The parallel routing of the reformate and raffinate gases means that there is no need for any structures for diverting gas from membrane cell to membrane cell, as is required in the case of series flow routing, and consequently the dimensions of the membrane module can be kept small.

DETAILED DESCRIPTION

As shown inFIG. 1, a membrane module comprises a multiplicity of planar membrane cells2which are arranged above one another and each include two hydrogen-selective, planar membranes4, between which there is an air-permeable spacer layer6.

The membranes4are preferably metal foils made from palladium, palladium alloys or refractory metals, such as vanadium, niobium and tantalum and alloys thereof. They ensure a virtually infinite selectivity for hydrogen and therefore a purity of the permeate gas which is sufficient to supply fuel cells. Alternatively, it is possible to use composite membranes, for example, hydrogen-selective membranes on a porous support structure which may, for example, consist of ceramic or porous stainless steel.

In each case two membrane cells2are separated from one another by feed spaces8, into which pressurized hydrogen-containing reformate gas10, which is obtained in an upstream reforming process, for example, from methanol, gasoline or diesel, is fed from one side of the stack of membrane cells2.

Some of the hydrogen contained in the reformate gas10diffuses through the membranes4into the air-permeable spacer layer6when the hydrogen-containing reformate gas flows along the membranes4. This means that the hydrogen content or the hydrogen partial pressure is reduced while the gas is flowing along the membranes4and emerges from the stack of membrane cells2as hydrogen-depleted raffinate gas12at the opposite side from the reformate gas inlet, as indicated by arrows inFIG. 1.

The permeate gas which has diffused through the membranes4is high-purity hydrogen which is collected in the spacer layer6and discharged laterally (not shown inFIG. 1).

At the upper and lower ends of the stack of membrane cells2, adjacent to the last feed space8, there is in each case a single hydrogen-selective membrane4, an air-permeable spacer layer6and an end plate14for sealing and supporting the last layers with respect to the internal gas pressure of the membrane module.FIG. 1shows just one upper end plate14.

It has been found that the maximum permeation rate which is theoretically possible is achieved not only in the case of membrane modules onto which gas flows in series, as described, for example in U.S. Pat. No. 5,498,278 and U.S. Pat. No. 5,645,626 but also when the gas flows onto the modules in parallel, as shown inFIG. 1, provided that the individual part-streams through the feed spaces8, indicated by arrows, are of sufficient magnitude. Furthermore, it was established by simulation for ten part-streams that even if the part-streams differ with deviations of 10% from one another, the permeation rate, under simulation conditions close to those encountered in practice, is only lower by at most 3% than the maximum permeation capacity which is theoretically possible. On the other hand, if the cross sections of the reformate gas feed line and the raffinate gas discharge line are designed suitably, it is possible to ensure even with gas which flows in parallel that the part-streams are virtually equal (since the flow resistance is equal in all the flow passages).

A membrane module with which virtually identical part-streams can be ensured yet which nevertheless can be produced with little outlay will now be described in detail with reference toFIGS. 2 to 5, in which components which are functionally identical to those shown inFIG. 1are denoted by the same reference numerals.

Each membrane cell2includes two flat membrane frames16one of which is shown inFIG. 2and which each bear a membrane4. Furthermore, each membrane cell2includes an air-permeable spacer layer6between the membranes4and an annular feed frame18(FIG. 3), which has the same external dimensions as the membrane frames16, the external dimensions in this case being formed by a rectangle with rounded corners. An individual membrane cell2in the assembled state is shown inFIG. 4.

As shown inFIG. 2each membrane frame16is a substantially planar component, which has been stamped from stainless steel sheet, in the shape of a rectangle with rounded corners, which in the center has a rectangular opening20for the membrane4and into which a raised edge22has been stamped. The raised edge22has a certain width over which it is planar and extends parallel to the plane of the inner region of the membrane frame16, offset at a short distance therefrom, specifically downward inFIG. 2. In a section in the center of a narrow side of the membrane frame16, the metal sheet is not stamped to form the raised edge22, but rather extends in the same plane as the inner region of the membrane frame16, so that in this section the raised edge22includes a recess24through which the permeate gas26is discharged.

Four webs28project from the upper surface of the membrane frame16, as seen inFIG. 2, which webs extend approximately from the center of the narrow side of the membrane frame16which lies on the opposite side from the narrow side comprising the recess24in a star shape virtually as far as the rectangular opening20in the membrane frame16. The webs28together with the webs28of an adjacent membrane frame16form a distributor structure which distributes the reformate gas10, which is supplied on this side of the membrane frame16, uniformly over the entire width of the membrane4.

The webs28can be stamped into the metal sheet of the membrane frame16in the same operation as that in which the raised edge22is stamped, and the stamping can be carried out in the same operation as that in which the membrane frame16is punched out of sheet-metal material.

The feed frame18shown inFIG. 3takes the form of a continuous annular strip which closes off a feed space8(FIG. 1) for reformate gas10, toward the sides of the membrane module. The annular feed frame18is slightly higher than the total thickness of a membrane frame16, narrower than the raised edges22in the membrane frames16and is provided with recesses30which correspond to the recesses24in the membrane frames16, so that it accurately fits into the raised edges22when it is assembled with the membrane frames16of two adjacent membrane cells4. The feed frame18can be produced, for example, by stamping or by milling, bending and welding together strip material.

The state in which a feed frame18(in a position turned about its longitudinal axis with respect to the position shown inFIG. 3) bears on the membrane frame16is shown inFIG. 4. A further membrane frame16is placed onto the opposite side of the membrane frame16(in a position turned about its longitudinal axis with respect to the position shown inFIG. 2). As can be seen, the recesses24in the two membrane frames16form a permeate outlet passage for discharging the permeate gas26.

As can be seen inFIG. 3, the feed frame18also includes recesses32and34which, together with the adjoining membrane frame16, form an inlet passage for the reformate gas10and an outlet passage for the raffinate gas12, which can be seen inFIG. 4.

Before the membrane frames16are joined together, as shown inFIG. 4, the membranes4, which are slightly larger than the rectangular opening20in each membrane frame16, are welded in a gastight manner onto the edges of the openings20, preferably on the side of the spacer layer6, so that the membrane4bears directly on the spacer layer6. The membranes4can be connected to the respective membrane frames16by means of various welding processes, for example, electron beam welding, laser beam welding, ultrasound welding or resistance roll seam welding, or by means of soldering processes. A spacer layer6, the thickness of which is double the offset of the raised edge22of the membrane frame16with respect to the inner region of the membrane frame16, is placed between each pair of membrane frames16.

The spacer layer6consists, for example, of a stainless steel mesh or nonwoven or comprises a multilayer structure and has the function of supporting the membranes4against the trans-membrane pressure difference between reformate gas10and permeate gas26in operation and of discharging the permeate gas26which has diffused through the membranes4toward the permeate outlet passage parallel to the membrane surface.

To produce a complete membrane module, a multiplicity of the membrane cells2shown inFIG. 4are stacked on top of one another. In each case one single membrane frame16with installed membrane4and a spacer layer6are arranged at the upper and lower ends of the stack of membrane cells2. Last of all, stable end plates14are fitted, holding the membrane module together with respect to the internal gas pressure.

This type of cell closure structure, which has already been described in general terms with reference toFIG. 1, can be seen in more detail fromFIG. 5, which shows an exploded view of the topmost part of the membrane module, including two membrane cells2. The cell closure structure with a single membrane frame16beneath the spacer layer6and the end plate14ensures that reformate gas part-streams which are passed through the membrane module closest to the end plates14flow over the same membrane surface area as all the other part-streams.

Once the stack of membrane cells2has been assembled, the membrane frames16, the feed frames18and the end plates14are welded together, with the result that a compact stack is formed, which is gastight apart from the inlet and outlet passages for the reformate gas10, the raffinate gas12and the permeate gas26. The inlet and outlet passages for the reformate gas10, the raffinate gas12and the permeate gas26in each case rest accurately above one another and in each case form a rectangle onto which a matching feed or discharge tube or the like, is welded.