BACKGROUND OF THE INVENTION : Field of the invention The invention relates to a method for making a membrane module made up of a bundle of ceramic hollow fibre membranes with a flange at each end. Membrane modules with ceramic hollow fibre membranes are being employed more and more in separation and concentration processes. Such processes are of great economic importance for the concentration and separation of gasses and liquids. The use of ceramic hollow fibre membranes has a number of advantages. Because the surface/volume ratio is relatively great the efficiency of the concentration or separation process will be high. Moreover, the process is easy to carry out and to adapt to one's needs.

Prior art Such a method for making a membrane module is generally known. When making a membrane module care must be taken that the ends of the fibre membranes remain open. If a number of the fibre membranes become obstructed, it will have a negative effect on the membrane module's processing capacity. Because of this, the manufacturing process needs to be carried out with precision as a result of which the known method costs a great deal of time and, consequently, is expensive.

Summary of the invention An objective of the invention is to provide a method for making a membrane module of the type described in the preamble in which high-quality membrane modules can be made in a relatively simple and quick way which, for that reason, is therefore less expensive. To this end, the method according to the invention is characterized in that first of all a thin flange is made at each end which seals the open ends of the fibre membranes,

after which the rest of the flange is made and finally a layer from the end of the flange is removed having a minimum thickness equal to that of the thin flange made previously, in order to make sure that the ends of the fibre membranes are open. With the method according to the invention a membrane module of a good quality can be made in a simple way in which it can be guaranteed that all the fibres are open. Preferably, the layer is removed from the flange by sawing it off from the rest of the flange.

An advantageous, practical embodiment of the method according to the invention in which the flange can be made in a simple way is characterized in that the bundle of fibre membranes is placed in a vertical position with one end on the foot's bottom in which foot a thin layer of liquid material for the flanges is put beforehand or afterwards, after which the liquid material for the flanges hardens and then more liquid material for flanges is put into the foot which forms the flange after hardening. Preferably, the liquid material for the flanges is put in the foot beforehand and the foot is heated up to a temperature at which the material for the flanges becomes fluid.

Stiff requirements are placed on the dimensional accuracy and form accuracy of membrane modules. A further embodiment of the method according to the invention with which membrane modules with a high degree of precision can be made is characterized in that before the bundle of fibre membranes is placed in the foot, the bundle of fibre membranes is first placed in a close-fitting tube, after which this tube is placed in the foot, which is also close-fitting, the wall of which is at right angles to the bottom. In this way, it is assured that the fibre membranes are at right angles to the flange and the membrane module therefore has a high degree of form accuracy.

In order to be able to execute this last embodiment in a practical, advantageous way, still a further embodiment of the method according to the invention is characterized in that before the fibre membranes are put in the tube, a ring is placed in each end of the tube having an inside diameter which is smaller than that of the tube, after which the fibre membranes are placed in the tube and rings and protrude from the tube at one end. Then the tube, at a distance from the bottom, is placed in the foot and a first flange is made, after which the tube with the rings is slid off at the free end of the bundle, the rings are removed and an additional ring, which fits on the flange just made is placed in an end of the tube and then the tube with the additional ring is placed over the bundle and the flange and is placed in the foot, after which the other flange is made on the free end. Finally, the tube with the

additional ring is slid off the formed membrane module. With this method, a membrane module can be made in which the flanges are situated precisely straight above one another.

This makes it possible to place the membrane module in a close-fitting module housing.

The invention also relates to a membrane module made in accordance with a method according to the invention.

Furthermore, the invention relates to a membrane module comprising a bundle of hollow ceramic fibre membranes, as well as two flanges, one at each end of the bundle of fibre membranes. With respect to this membrane module, the invention is characterized in that the flanges are essentially made of one or more polymers. Preferably, the flanges are entirely or at least nearly entirely composed of polymers. There are no known membrane modules having ceramic fibre membranes and polymer flanges. These membrane modules according to the invention have the advantage that they are simple to make, in accordance with the method according to the invention.

Brief description of the drawings The invention will be elucidated more fully below by means of an example of an embodiment of the method according to the invention shown in the drawings. Shown are: Figure 1 : a side view of a membrane module made in accordance with the method according to the invention; Figure 2: a view from above of the membrane module; Figure 3: a mould for making a membrane module showing the individual parts; Figure 4: a view from above of the mould's foot; Figure 5: a cross-section of the foot; Figure 6: a cross-section of the mould's tube; Figure 7: a cross-section of the mould's ring; Figure 8: a cross-section of the mould's additional ring; Figure 9: the tube with fibre membranes in it during a first step of the method; Figure 10: the mould with the fibre membranes in it during a second step of the method; and Figure 11: the mould with the fibre membranes during a third step of the

method.

Detailed description of the drawings In figures 1 and 2 a membrane module 1 made up of a bundle 3 of ceramic hollow fibre membranes 5 with a flange 7,9 at each end is shown in two different views.

An embodiment of the method according to the invention for making such a membrane module will be described by means of the following description which refers to the figures.

In figure 3 a mould 11 is shown for making the membrane module, in which the individual parts are also displayed. In figures 4 through 8 inclusive the parts are shown separately for the sake of clarity. The mould 11 consists of a foot 13 and a tube 15 which can be slid into the close-fitting foot. At the tube's ends 17,19 the inside diameter has been increased along a distance in order to be able to place rings 21 or an additional ring 23 in it.

The foot 13 has a bottom 25 and a wall 27 and is made, for example, of polypropylene, Teflon, aluminium coated with Teflon or any other material with which the casting material used to make the flanges does not interact or adhere to. There are openings 31 in the upper part 29 of the wall 27 for pouring or injecting into the foot the casting material from which the flange is made. The lower part 33 of the wall 27 is slightly conical on the inside 35 in order to facilitate removal of the flange to be made in it.

The inside 37 of the wall's upper part 29 is stepped, in which the inside diameter of the upper step is the same as or slightly greater than the outside diameter of the tube 15. In this way it is assured that the tube 15 never can completely close off the openings 31 and that there will always be openings through which the casting material can be put in the foot 13. There is a threaded hole 39 in the bottom 25, in which a bolt can be screwed in order to be able to remove from the foot the flange which is to be made in it later.

The inner wall 41 of the rings 21 is conical at one end and the inner wall 43 of the additional ring 23 is conical and has the same shape as the inner wall 35 of the lower part 33 of the wall 27 of the foot 13. The tube 15, the rings 21 and the additional ring 23 are preferably made of the same material as that of the foot 13.

In the first step of the manufacturing process, see figure 9, a ring 21 is slid into

each end of the tube 15 and the tube 15 is slid into the foot 13, after which the entire unit is laid in a horizontal position. Beforehand a socket head bolt 45 is screwed into the hole 39 in the bottom 25 of the foot 13 and the hole above the socket head bolt is filled with a plastic material, for example, putty 47, in order to seal the hole. Then the space inside the tube and the rings is filled in layers as fully as possible with fibre membranes 5 which have been sawed off to length. The conical shape at the end of the inner wall 41 of the rings facilitates placing the fibre membranes 5.

After that, the foot 13 is removed and heated up in an oven. A casting material is made by mixing the proper amount of binder and hardener and placing it in the oven for a period of time at a temperature at which the casting material becomes castable. Then the casting material is poured into the foot 13 in which the bottom 25 is covered by a thin layer of casting material 49, for example, several millimetres thick. After that the tube 15 with the bundle 3 of fibre membranes in it is slid into the foot 13 and the entire unit is placed in a vertical position, as a result of which the ends of the fibre membranes 5 enter the layer of casting material 49 until reaching the bottom 25 of the foot, see figure 10, and the liquid casting material 49 enters the ends of the fibre membranes.

The entire unit is now placed in a vertical position in the oven and heated until the casting material 49 has hardened, so that the ends of the fibre membranes 5 are sealed.

Then more casting material is poured into the foot 13 through the openings 31, see arrow 51, reaching the lower edge 53 of the openings. The entire unit is then placed in the oven again and heated up until the casting material has hardened. The hardened casting material is now attached to the bundle 3 of fibre membranes and forms a flange.

After that, the entire unit is taken out of the oven and placed in a horizontal position. The socket head bolt 45 is now screwed in, as a result of which the flange just made is pushed out of the foot 13. The tube 15 with the rings 21 is then slid off the bundle 3 of fibre membranes, and the rings 21 are taken out of the tube 15. Next, the socket head bolt 45 is screwed out, the hole 39 is filled again with putty 47 and the foot 13 is placed in the oven and filled with a new layer of casting material 55. The additional ring 23 is slid into one end of the tube 15 and the bundle 3 of fibre membranes is slid into the tube again, as a result of which the flange just made 7 is clamped in the additional ring 23.

The tube 15 with the bundle 3 of fibre membranes is slid in a vertical position into the foot 13, as a result of which the free ends of the fibre membranes 5 hit the bottom

25 of the foot 13 and the casting material 55 enters the ends of the fibre membranes 5, see figure 11. The entire unit is heated up again in the oven until the layer of casting material 55 has hardened and the fibre membranes are sealed. Additional casting material is now poured into the foot 13 through the openings 31, see arrow 57, reaching the lower edge 53 of the openings 31. After this has also hardened again in the oven, the additional flange 9 can be removed from the foot 13 by using the socket head bolt 45. Next, the tube 15 with the additional ring 23 is slid off from the bundle 3 of fibre membranes, after which a layer is sawed off from each end of the flanges 7,9, having a minimum thickness equal to that of the first thin layer in order to assure that the fibre membranes 5 are open.

Although in the above the invention is explained on the basis of the drawings, it should be noted that the invention is in no way limited to the embodiment shown in the drawings. The invention also extends to all embodiments deviating from the embodiment shown in the drawings within the context defined by the claims.