# Patent

## Title
Head plate

## Abstract

The invention relates to a head plate, fixing the end of a tube bundle with a number of, in particular, porous tubes with a membrane in sealing manner. The head plate is made from a metal or a metal alloy with a melting point lower than the lowest failure temperature for a tube material and/or the membrane.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This is a U.S. national stage of International Patent Application No. PCT/DE2007/000020 filed Jan. 5, 2007, claiming priority to German Patent Application No. 10 2006 000984.3 filed Jan. 5, 2006.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a head plate holding a tube bundle having a plurality of tubes with a membrane, in particular porous, at one end and sealing them tightly.
2. Description of the Related Art
Tubes of said type, e.g., tubes, capillaries or the like made of a plastic, a plastic composite, optionally also with a laminar structure, made of a ceramic or a metal have a variety of applications.
For example, in the case of fuel cells, a fuel component is conveyed in such tubes of a tube bundle, which is why such tubes designed as a microreactor are regularly also provided with a membrane by which the fuel components reacting with one another are separated.
Another area of application of such tube bundles is for refining fuels of biorenewable materials, e.g., bioethanol. Bioethanol flows through a tube designed as a semipermeable membrane and the water present in the bioethanol is removed through the membrane.
In such tube bundles, there is fundamentally the problem of holding and sealing the tubes at one end. So-called potting or gripping of the ends of these tube bundles together and in particular also for a sealing connection with a housing, a pipeline or the like is usually accomplished by synthetic resin, e.g., epoxy resins, casting techniques, although these are not without problems. For example, the different thermal expansion coefficients at a working temperature of up to 150° C. in low-temperature fuel cells and/or up to 250° C. in medium-temperature fuel cells lead to substantial thermal burdens, which can lead to more cracks, leakage and the like in the area of the head plate, which usually results in failure of the entire apparatus.
SUMMARY OF THE INVENTION
Against this technical background, one object of the present invention is to make available a head plate which can be designed to withstand higher mechanical loads, to securely grip the individual tubes of the tube bundle and to hold them with a seal.
It has surprisingly been found that these technical problems are solved by a head plate in which the emphasis is on the head plate being made of a metal or a metallic alloy of a lower melting point than the failure temperature of the tube material and/or the membrane.
Low-melting metals, solders or mechanical alloys are usually extremely brittle and coarse-grained after solidification of their melts. This is regularly associated with a great mechanical instability. Therefore, such materials are usually not suited for gripping the individual tubes of a tube bundle, in particular those of a small diameter, and sealing them tightly.
Nevertheless, the present invention makes available such metals and/or metallic alloys of a low melting point. This is also true in particular with regard to the wide range of variation in diameters of the tubes of the tube bundle, said diameters often varying from less than 1 mm up to the centimeter range.
With the choice of the metal or the metallic alloy, its melting point, which of course must be lower than the failure temperature of the tube of material and/or that of the membrane but on the other hand is higher than the operating temperature of a finished device, should be taken into account. This can often be achieved if the melting point is between 100° C., in particular 120° C., and 250° C.
The preferred metal for use is bismuth, chemical symbol Bi, or contains the alloy bismuth.
Bismuth is a reddish white, shiny and moderately hard heavy metal. Like water, bismuth contracts in volume on melting and expands by 3.32% on solidification. Consequently, on cooling, bismuth and melts containing bismuth are excellent for penetrating into even the tiniest interspaces between the tubes of a tube bundle. In addition, bismuth has a very high chemical stability. For example, it is insoluble in nonoxidizing acids.
However, the melting point of pure bismuth is 271.3° C., which is too high for many applications. It is therefore preferable to use a bismuth alloy whose melting point can be lowered significantly when such a solution contains approximately 14% to 60% bismuth, 20% to 30% lead or up to 45% tin or antimony, cadmium, indium, zinc, tellurium, mercury or thallium.
With the applications mentioned in the introduction, however, it is preferable for the alloy or the metal to be lead-free in particular.
A preferred alloy is a eutectic bismuth-tin alloy which has a melting point of 138° C. and a density of 8.58 g/cm3.
The metals of a eutectic alloy are advantageously completely miscible in a molten state, and the melt solidifies in the form of a pure substance at a uniform temperature. On solidification, the components separate and crystallize side-by-side in an extremely fine crystalline form, the eutectic structure. It is also advantageous that a eutectic has the lowest solidification point or melting point that is possible in the substance system in question, referred to as the eutectic temperature or eutectic point respectively in the melting diagram.
A tube material will usually be a plastic, in particular a polymer, whereby the idea is in particular that a membrane may also be a polymer membrane. The failure temperature at which there is no longer any functionality of the tube and/or membrane is often slightly less than 200° C., so that such a polymer tube and/or such a polymer membrane will definitely not be damaged by the metal melt and/or the melt of the alloy.
Alternatively, a tube, in particular porous, may be made of a ceramic and/or a metal, in particular a sintered metal. Such tubes are most suitable for being provided with a layer of zeolite which forms a membrane. On the basis of their crystalline pore structure, such membrane layers are suitable for size-selective and shape-selective separation of liquid and gaseous substance mixtures. In addition, the hydrophilic/hydrophobic hydrophobic character can be adjusted through the choice of the Si/Al ratio in the zeolite crystal. Thus a hydrophobic zeolite membrane can be made available for a selective separation of organic solvents such as ethanol from water, with the help of pervaporation at approximately 100° C.
In the case of tubes with a small diameter in particular, they may be bundled chaotically and in contact with one another, e.g., for refining bioethanol. Nevertheless, due to the special properties of bismuth in particular, the tubes are held securely and with a seal. However, for the sake of safety, the axial length of a casting should be designed to be longer than the diameter of an enveloping curve of a tube bundle. Then the individual tubes may be encased with a seal everywhere over the axial length.
As an alternative, of course, the tubes of a tube bundle may also be aligned at a distance from a predefinable grid, as is customary with fuel cells, for example.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be explained in greater detail on the basis of the drawings, in which exemplary embodiments are depicted schematically. The drawings show:
FIG. 1: a lateral view of a casting of the tube ends of tubes of a tube bundle, and
FIG. 2: an enlarged detail along line II, II in FIG. 1.



DETAILED DESCRIPTION OF THE EMBODIMENTS
FIG. 1 shows a tube bundle 1 with a plurality of tubes 2 shown from the side, e.g., polymer membranes with an outside diameter of approximately 0.5 mm. The tubes 2 are chaotically bundled and are touching one another. The outside contour of the tube bundle 1 is therefore merely indicated by a dash-dot enveloping curve 3. It is nevertheless possible to form a metallic head plate 4, which holds the individual tubes 2 at one end among one another and with respect to a housing, for example, and does so reliably and with a seal (see also FIG. 2).
To do so, a metal or a metallic alloy of the type defined in the introduction is liquefied in a suitable pot shape and the tube bundle 1 is immersed in the melt at one end. The tubes 2 may be sealed to one another at one end in a known manner. However, this is not usually necessary with tubes 2 having a small inside diameter, because the molten metal and/or the molten alloy penetrates axially into a tube 2 only to a slight extent.
To ensure that all the interspaces of the axial length of the head plate 4 are also sealed, the axial extent of a casting 5 is usually designed to be greater than its diameter and/or than the enveloping curve 3.
The head plate 4 is manufactured by a cut, e.g., along line II, II, by means of which the tube ends are then exposed.
FIG. 2 shows a subsequently diagramed polished section of such a cut on an enlarged scale. Larger interspaces between the individual tubes 2 are homogeneously sealed by a metal or a metallic alloy according to the invention with extremely fine pores and homogeneously in this enlarged diagram. If tubes 2 are extremely close to one another, or are in contact, there remains a slight space 6 (see arrows 6) between the tubes 2. However, these spaces are reliably sealed over the axial length of the casting 5 and/or the head plate.
After appropriate machining of the lateral radial surface 7 of the head plate 4, the tube bundle 1 is then gripped with a seal and held at one end for a further application.
Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

## Claims

1. A head plate assembly comprising:
a tube bundle comprising a plurality of tubes extending along an axial length defining an external contour; and
a head plate having an axial length in the direction of the axial length of said tube bundle and gripping ends of the plurality of tubes and encasing the tubes with a seal over the axial length of the head plate;
wherein the head plate comprises a low-melting metal or a low-melting metallic alloy material having a melting point between 100° C. and 250° C.,
the head plate being formed from a casting having an axial length longer than a diameter of an enveloping curve of the external contour of the tube bundle.

2. The head plate assembly according to claim 1, wherein the metallic alloy contains bismuth.

3. The head plate assembly according to claim 1, wherein the metallic alloy is a bismuth alloy.

4. The head plate assembly according to claim 1, wherein the metallic alloy is a eutectic bismuth-tin alloy.

5. The head plate assembly according to claim 1, wherein the metal or the metallic alloy is free of lead.

6. The head plate assembly according to claim 1, wherein at least one of the plurality of tubes is made of a polymer.

7. The head plate assembly according to claim 1, wherein at least one of the plurality of tubes is made of a membrane.

8. The head plate assembly according to claim 7, wherein the membrane is a polymer membrane.

9. The head plate assembly according to claim 7, wherein the membrane is a zeolite membrane.

10. The head plate assembly according to claim 1, wherein at least one of the plurality of tubes is made of a ceramic and/or a metal material.

11. The head plate assembly according to claim 1, wherein the plurality of tubes are bundled chaotically to one another and in contact with one another.

12. The head plate assembly according to claim 1, wherein the plurality of tubes in the tube bundle are aligned with a distance between them on a predefinable grid.

13. The head plate assembly according to claim 1, wherein the plurality of tubes are porous.

14. The head plate assembly according to claim 1, wherein the metal comprises bismuth (Bi).