Abstract:
The membrane module includes tube membranes ( 6 ), which are arranged in the membrane module in coiled, curved form ( 8 ). As a result, a greater total membrane surface area per unit of volume, simplified assembly, and processing of material mixtures with a high solid proportion are all attained. The modules are arranged to facilitate cross flow filtration over membrane surfaces disposed between a supply conduit manifold and a discharge conduit manifold. Optionally, the coils are aligned in parallel and are in coaxial relation to each other.

Description:
This application is a 371 of Application PCT/CH97/00414, filed on Oct. 31, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a membrane module of an installation for membrane separation of material mixtures comprising one or more tube membranes, a process for their manufacture by extrusion of synthetic material as well as applications of the membrane module. 
     Such membrane modules are known as structural elements in cross current filtration plants. They comprise mostly a plurality of tube membranes acting as filters. The tube membranes are designed as porous tubes either themselves designed as membrane filters or carrying filtration membranes of organic or inorganic materials on their surface. Tube membranes comprising interior or exterior membranes, are known. Their inside diameters range from some decimillimeters to about 100 millimeters. 
     In order to attain acceptable construction lengths of the membrane modules, combined with a useful filtration output, a small or larger number of straight tube membranes are installed in a straight jacket tube. This module tube has a combined entry and exit for all tube membranes for the medium to be filtered as a retentate and one or two exits for a filtrate as a permeate. 
     Membrane modules designed as so-called coil modules are also known. In this context cloth-like filter membranes are wound into an elongate roll in which the flow-through of the retentate and the drainage of the permeate is made possible by co-wound thin elastic spacers or nets. Coil modules of this design are very reasonably priced having regard to their filtration performance. However, because of their tendency to clog they are unsuitable for the separation of material mixtures having a high solids content. In contrast to this, with tube membranes having an inside diameter of some millimeters (a plurality of millimeters) even material mixtures having a high solids content such as pressed fruit juices, for example, can be processed without risk of clogging. 
     As the specific filtration output, in relation to the surface area, of known tube membranes of polysulphone or PDVF is relatively low, a number of modules, each comprising e.g. 19 tube membranes of 3 m length each, are interconnected in one installation in series and also in parallel in order to attain higher, economically acceptable filtration outputs. 
     If the number of the modules connected in series is high, up to 16 modules per series are known, the latter are interconnected by way of 180 degree pipe bends. If the group comprises as few as 5 series or passes switched simultaneously in parallel, 80 modules have to be provided in as compact a single unit as possible in order to attain a membrane filter surface area of about 180 m 2 . The individual modules are for this purpose mounted on support arms on racks and the numerous connections on the retentate side and the permeate side are brought about. This entails the following problems: 
     4 tube connections and two to three supports on support arms are required per membrane module. In an installation having 80 modules, these connections and supports result in 80 connecting bends, 85 hose connections and 320 connecting points, they further result in high installation costs thus reducing the cost efficiency of the plant. 
     Separating membranes have only a limited useful life. The membrane modules are thus parts subjected to wear and tear, having to be replaced at certain time intervals. 
     The efforts for assembly and disassembly as well as for the complex construction are thus enormous for this design. 
     SUMMARY OF THE INVENTION 
     It is therefore the object of the invention to eliminate the stated problems to a large extent. 
     According to the invention this object is attained by a membrane module of the type as set out in the opening paragraph in that the tube membranes are provided in the membrane module in a curved configuration. An advantageous modification of this membrane module is characterized in that the tube membranes are provided in the membrane module in a coiled configuration. 
     This may lead to the feature that the tube membranes in the form of at least one bundle of a plurality of tube membranes are coiled in the membrane module in rope-like twisted form (lay). 
     Further modifications of the membrane module, a process for the manufacture of tube membranes suited therefor by way of extrusion of synthetic resin as well as the use of the membrane module, are set out in the patent claims. 
     It has been found that a lay-out according to the invention may be attained with conventional tube membranes which can be bent at bending radii less than 20 times the inner diameter of the tube without damaging the membrane layer. For this purpose tube membranes of organic materials in the required lengths are commercially available. Known tube membranes of an inorganic material, such as sintered metal, can be produced in the required lengths from commercially available tube pieces of up to 1 m length by bending, welding together and application of the membrane by coating. 
     Compared with conventional membrane modules, membrane modules according to the invention offer the advantage of a high packing density and a simpler design. Compared with the abovementioned group of 80 conventional membrane modules, such a group may be replaced by a membrane module according to the invention having an outer diameter of about 1.40 m and a structural height of 1.40 m having the same membrane filter surface area of about 180 m 2 . Instead of the aforesaid 320 connections only about 3 connections will still be necessary, depending on the design. For this purpose, the membrane filter surface in the membrane module is formed by 150 parallel membrane tubes having an inner diameter of about 7 mm and a length of 55 m each. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are elucidated in more detail in the specification which follows and in the figures of the drawing. There are shown in: 
     FIG. 1 a  a membrane module according to the invention for the membrane separation in a vertical section B—B according to FIG. 1 b,    
     FIG. 1 b  a horizontal section A—A of the membrane module according to FIG. 1 a,    
     FIG. 2 a  a partial section C—C according to FIG. 2 b  of a collecting pipe for tube membranes at the beginning or at the end of a winding, 
     FIG. 2 b  an end facing view of the collecting pipe for tube membranes according to FIG. 2 a,    
     FIG. 3 a modification of a membrane module according to the invention for the membrane separation in a vertical section, 
     FIG. 4 a  a further membrane module according to the invention for the membrane separation in a vertical section, in combination with a permeate tank, 
     FIG. 4 b  a horizontal partial section D—D according to FIG. 4 a  across a collecting pipe for tube membranes at the beginning or at the end of a winding, 
     FIG. 5 a cross-section through a bundle of a plurality of tube membranes of a winding in a membrane module according to FIG. 1 a  comprising spacers between the tube membranes, 
     FIG. 6 a section across spacer elements molded onto a tube membrane, 
     FIG. 7 a view of a modification of spacer elements molded onto a tube membrane, 
     FIG. 8 a view of a wire-like spacer element, wound around a tube membrane, 
     FIG. 9 a view of three tube membranes for coiling into a form produced as a longitudinally contiguous unit, 
     FIG. 10 a  a radial section across two disk-like flat tube membrane modules each containing a single continuous tube membrane, 
     FIG. 10 b  a winding pattern for flat winding compromising two layers to form a membrane tubing unit into two coils according to FIG. 10 a,    
     FIG. 11 an axial partial section across a membrane module comprising a horizontal winding axis for the tube membranes, 
     FIG. 12 a winding pattern for two windings of tube membranes disposed side by side in a membrane module, and 
     FIG. 13 a view of a tube membrane produced as a flat tube for conversion into a coiled body, 
     FIG. 14 a membrane module according to the invention for membrane separation, having a vessel as a tank for the material that has been separated, 
     FIG. 15 a diagram of a system for membrane separation of material mixtures, having a membrane module of FIG. 14, 
     FIG. 16 a membrane module according to the invention, in which a plurality of tube modules are disposed with their winding axes vertically one above the other, 
     FIGS. 17 a ,  17   b  two views of a membrane module in which a plurality of tube membranes are disposed interchangeably between support plates in drawerlike compartments, 
     FIG. 18 a membrane module having a plurality of tube membranes, which can be disposed vertically and horizontally with a tipping device, 
     FIG. 19 a membrane module according to the invention, in which a plurality of tube modules are disposed with their winding axes horizontally side by side, in a side view, 
     FIG. 20 the membrane module of FIG. 19 in an axial view, 
     FIGS. 21 a ,  21   b ,  21   c  a membrane module according to the invention, having a plurality of tube membranes, with a retractable vessel, in various views, 
     FIG. 22 a disklike flat winding or tube membrane module comprising only a single continuous membrane tubing unit in a boxlike container for retaining and replacing the winding, 
     FIG. 23 a  a disklike flat winding comprising only a single continuous tube membrane, with a support plate to improve stability, 
     FIGS. 23 b ,  23   c  two variants of support plates of FIG. 23 a,    
     FIG. 24 a diagram of a two-layered disklike flat winding comprising two continuous tube membranes to increase the diameter and the filter area of the winding, 
     FIG. 25 a circuit diagram of three groups, each comprising three tube modules, with collecting pipes between the groups that act as mixing chambers, 
     FIGS. 26 a,    26   b,    26   c  details of a fast-action closure connection of a tube membrane with a collecting pipe as in FIG. 17, 
     FIG. 27 an embodiment of a connection of a coiled bundle of tube membranes with a collecting pipe as in FIG. 2 a , in which a plurality of tube membranes of equal length are arranged such that their ends are staggered, 
     FIG. 28 an interruption, embodied as a mixing chamber, in a coiled bundle of tube membranes in a membrane module of FIG. 1 a,  in an arrangement of tube membranes of equal length in partial bundles, with the ends of the tube membranes staggered. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 a  and FIG. 1 b  each illustrate a section of a membrane module in which tube membranes are wound around a vertical winding axis. The membrane module comprises a closed vessel  1  comprising a lower portion  2  and an upper portion  3  fitted to the latter in sealing relationship but removably. As is apparent in particular from FIG. 1 a,  two collecting pipes  4 ,  5  for a plurality of tube membranes are introduced laterally into the upper portion  3 . As can be seen, a portion of each collecting pipe  4 ,  5  is inside the closed vessel  1 . The tube membranes  6  form a continuous bundle  7  connecting the collecting pipes  4  and  5 , wound around a vertical winding core  9  serving as a support in a coiled body  8  or a coil. 
     FIGS. 2 a  and  2   b  illustrate an end of a collecting pipe  4  or  5  in a partial section and in an axial view. As is apparent from FIG. 2 a , the plurality of tube membranes  6  in the bundle  7  is wound in rope-like twisted configuration with a so-called lay. This serves the purpose of permitting the bent guidance of the bundle  7  in the coil  8  without damaging the tube membranes  6 . The tube membranes  6  are fixed to the end of the collecting pipe  4  by casting with a casting compound  10 . 
     The material mixture to be separated, e.g. a pressed fruit juice, is fed under pressure to one of the collecting pipes  4 ,  5  where it flows parallel against the plurality of tube membranes  6 . At the corresponding other collecting pipe  5  or  4  the material mixture is discharged again as retentate. On the way through the tube membranes  6  a portion of the material mixture of correspondingly fine particle size is separated in a manner known per se through the membranes, reaching in this manner the free space of the vessel  1 , encompassing the configuration  8  according to FIG. 1 a,  as a permeate or filtrate. 
     From this space the permeate reaches an outlet  11  for the separated material through the winding core  9  serving simultaneously as collecting pipe. As illustrated in FIG. 1 a,  the winding core  9  is disc shaped in its lower region for supporting the coiled body  8 . In order to keep the separating membranes of the tube membranes  6  wetted at all times, the outlet  11  is provided at the top. The entire coiled body  8  of the bundle  7  is removable for controlling purposes with the collecting pipes  4 ,  5  and  9  from the lower portion  2  of the vessel  1 . Likewise for control purposes, a viewing glass  12  is provided in the lower portion  2 . For draining the permeate, the vessel comprises a sealable outlet  13  below while an air outlet  14  is provided at the top. 
     In a modification of the membrane module according to FIG. 3 reference numerals already explained with regard to FIG. 1 a refer to structural elements having a corresponding function. The collecting pipes  4 ,  5 , serving as connections for the retentate, are in this case provided in the lower portion  2  of the vessel  1  while an outlet  11  is provided for the permeate at the top of the upper portion  3 . For operations on the coiled body  8  only one permeate duct towards the outlet  11  needs be disassembled in this case with the upper portion  3 . The coiled body  8  is stabilized by a holding strap  17 . The winding core  9 ′ is provided with apertures  15  for collecting the permeate. 
     FIG. 4 a  shows another modification of the membrane module. In this case a closed vessel  1 ′ is designed as a permeate tank. In the vessel  1 ′ a plurality of tube membranes  6 , positioned in a row side by side and parallel to one another, is wound around an axis  16 . These tube membranes  6  terminate in this case transversely to the axial direction into collecting pipes  4 ′ and  5 ′ for the material mixture to be separated where, similiarly to the embodiments according to FIGS. 1 a  and  3 , they receive the flow in parallel. This requires no twisting of the tube membranes  6 , and as a result a high packing density of the filtering surface is attained in the space available. The collecting pipes  4 ′,  5 ′ for the retentate may furthermore be kept small in diameter as the tube membranes  6  terminate transversely, as shown in particular in the section D—D according to FIG. 4 b.    
     The tube membranes  6  are cast into the collecting pipes  5 ′ by means of a casting compound  10 ′, as shown in FIG. 4 b . From the tube membranes  6  the permeate reaches the vessel  1 ′ which in this case likewise has the function of an integrated permeate tank  18 . A permeate outlet  11 ′ is provided below on the vessel  1 ′. The coiled body  8 ′ is stabilized by holding straps  17 , as is apparent from FIG. 4 a.    
     FIG. 5 in an axial view corresponding to that of FIG. 2 b , illustrates a bundle of tube membranes  6 , the position of the tube membranes  6  being stabilized by holding straps  17 . As can be seen, the holding straps  17  simultaneously serve as spacers between the tube membranes  6 . An improved drainage of the permeate exiting from the tube membranes  6  is made possible in that the holding straps  17  are net-like. 
     A further possibility to bring about a suitable spacing between the tube membranes  6  in a coiled body is shown in FIG. 6 in that the tube membrane  6  comprises spacer elements  27  molded on as peripheral bulges. An embodiment of such spacer elements  27 ′ extending in an axial direction on the outside of a tube membrane, is shown in FIG.  7 . According to FIG. 8 such spacer elements  27 ″ may also be wound in wire-like form helically around a tube membrane  6 . 
     FIG. 10 a  shows a cross flow filtration apparatus that includes two spirally coiled bodies  8 ″ of tube membranes  6  having only two layers and thus having a disk-like configuration. The bodies  8 ″ are connected on both sides to collecting pipes or manifolds  4 ″,  5 ″ for the retentate, adapted to accommodate still further bodies, not shown, of the same type. The permeate may be collected in known manner from the surroundings of the coiled bodies  8 ″. FIG. 10 b  illustrates a winding pattern of a tube membrane  6  for a tube membrane module or body  8 ″. The membrane tubing unit making up a tube membrane module  8 ″ has first and second end portions  31  and  32  and a central portion  33 . The central portion  33  is wound about an axis  16 ′ in two side-by-side co-axial coils  33   a  and  33   b  joined together on the inner periphery of the coils by a connector portion  33   c.  The end portions  31  and  32  extend outwardly from the peripheries of their respective coils  33   b  and  33   a  in opposite circumferential directions for efficient connection to the manifolds  4 ″ and  5 ″ for supplying and discharging the mixture which is being separated. In contrast to the embodiments of the filtration modules according to FIGS. 1 a,    3  and  4   a , the embodiment according to FIG. 10 a  permits the replacement of individual bodies or tube membrane modules  8 ″ and therefore of individual tube membranes  6 . Sintered metal pipes are particularly suitable for this purpose. 
     A membrane module in which the winding axis of the coiled tube membranes is horizontal, is shown in FIG.  11 . This membrane module likewise comprises a closed vessel  1 ″ having an inlet and an outlet  4 ′″, and  5 ′″ for the retentate and two outlets  11 ″ for the permeate. The tube membrane is wound around a horizontal axis  16 ′ either as a single tube or parallel in a plurality of tubes as bundle  7 ′ connecting the inlets and outlets  4 ′″ and  5 ′″ for the retentate. The body  8 ″ is kept in the vessel  1 ′ via spacers  30  provided with apertures for the discharge of the permeate from the coil  8 ″ towards the outlets  11 ″. 
     With a view to repairing tube modules and to meet different requirements as to a filter surface area, an embodiment of the membrane module comprises at least two separate windings of tube membranes  6  in a vessel having separate inlets and outlets for the retentate. A winding pattern for such separate windings  8 ′″ onto a combined winding core  9 ″ with an axis  16 ″ is illustrated in FIG.  12 . 
     A variant of the membrane filtration module described in conjunction with FIG. 4 a  is shown in FIG.  14 . In this embodiment as well, a plurality of coiled tube membrane units  6 , not shown individually, are disposed in a container one above the other about a common axis  16 . The tube membrane units  6  here are each wound, for instance as described in conjunction with FIG. 10 a , such that its central portion forms two coaxial planar coils which communicate with one another at the inner edges of the coils and from the outer peripheries of their connector end portions extend to orifices into the collecting pipes or manifolds  5 ″ and  4 ″ for supplying and discharging the material mixture which is being separated. The collection tubes themselves, are diametrically opposite one another. While in the exemplary embodiment of FIG. 4 a  the catch vessel  1 ′ for the permeate communicates with the permeate tank  18  only via an overflow with a venting valve  14 ′, a vessel or container  40  for the tube membranes  6  of FIG. 14 acts simultaneously as a tank for the separated permeate. 
     The container or permeate tank  40  includes an opening or outlet  11 ″ 0  at the bottom for discharge of the permeate, which can be aspirated away by a pump  41  connected there. In operation of the separation system, as FIG. 15 shows, the permeate is advantageously pumped out of the permeate tank  40  down to a minimal level  43  only once it has reached a maximal level  42 . 
     The container  40  includes a removable upper part  44 , which is joined to a lower part  45  by a releasable flange  46 . For maintenance work that does not require removal of the upper part  44  itself, this upper part has a closable manhole  47 . Finally, a spray head  48  is provided on the top of the container  40  for cleaning the tube membranes  6 . 
     FIG. 15 is a diagram of a system for membrane separation that includes a membrane filtration module  50  according to the invention as shown in FIG.  14 . This system also, in a manner known per se, includes a tank  51 , which via a line  52  receives a material mixture  53  to be separated. Connected to the tank  51  at the bottom is an outlet valve  54  for the material mixture  53 , which is delivered to a product inlet  58  of the membrane module  50  via a pump  55  and a regulating valve  57  controlled by pressure sensor  56 . Along with the outlet valve  54 , an inlet valve  59  for a rinsing fluid, which can be supplied to the membrane filtration module  50  instead of the material mixture  53 , is provided. 
     The material mixture  53 , as already described in conjunction with FIG. 14, flows through the membrane filtration module  50  and leaves it as retentate at an outlet  60 . From the outlet  60 , the retentate flows via a line  61  and a regulating valve  62  either back into the tank  51  via a valve  63 , or leaves the system for membrane separation via a valve  64 . The material mixture  53  accordingly circulates in a retentate loop in the course of separation operation in the system. As already described in conjunction with FIG. 14, the material separated off in the tube membranes  6  leaves the permeate tank  40  as permeate via the outlet  11 ″ and is aspirated from the system via the connected pump  41 . 
     The operation of the pump  41  is controlled by level sensors  65 ,  66  for the level of permeate in the permeate tank  40 , via a control line  67 , as already indicated with regard to FIG.  14 . To measure the pressure of the material mixture  53 , a respective pressure sensor  68 ,  69  is provided at the product inlet  58  and at the outlet  60 . The pressure at the product inlet  58  is adjusted via the regulating valve  62 , which receives a pressure report from the pressure sensor  68  via a control line  70 . 
     FIG. 16 shows a structural variant of the membrane filtration module of FIG. 14, in which the same reference numerals indicate corresponding components. FIG. 16 shows the disk like tube membrane units  6 , disposed in a container  40  and arranged vertically one above the other about the common axis  16 . The end portions  75 ,  76  of each of the tube membrane units  6  extend to the manifolds or collecting pipes  5 ″ and  4 ″ for the material mixture to be separated. An outlet opening  11 ″ in the bottom of the container is provided for the discharge of permeate separated from the mixture flowing through the internal passages of the tube membranes  5  and collecting in the container. 
     FIGS. 17 a  and  17   b  show an embodiment of a membrane module which allows coiled disklike tube modules  6  to be replaced by disconnection from the common collecting pipes or manifolds  4 ″ and  5 ″ of the kind described in conjunction with FIG.  16 . FIG. 17 b  is a section crosswise to the axis  16  of the membrane module of FIG. 17 a,  taken along the line A—A. In addition to the manifolds or collecting pipes  4 ″,  5 ″ for the material mixture to be separated, a parallel support tube  80  is also provided. The tubes  4 ″,  5 ″,  80  all have a radially inward-pointing slotted strip  81 , into the slots  82  of which perforated support plates  83  are thrust. In this way the support plates  83  form drawerlike compartments, in which the disklike tube membrane units  6  are interchangeably retained. 
     The end portions  75 ,  76  of the tube membrane units  6  for connection to the collecting pipes  5 ″ and  4 ″ are advantageously provided for replacement purposes with connector fittings for fast-action closure connections, as shown in detail in FIGS. 26 a,    26   b,    26   c.  The permeate emerging through the tube membranes  6  can easily flow out through the perforated support plates  83 . 
     FIG. 18 schematically shows a further example of a membrane module with a plurality of tube membranes  6  and a permeate tank  40  of the type described in connection with FIG.  16 . In this case, the permeate tank  40  is supported with a bearing  86  so it may pivot between a vertical position  87  and a horizontal position  88 , as indicated by the arrow  89 . In the horizontal position  88 , replacement of individual membrane modules  6  can be done especially simply, as the arrow  90  indicates, while the vertical position  87  has advantages during operation of the system. 
     A membrane filtration module in which a plurality of disklike tube membrane units  6  are disposed in a container  40  side by side with a horizontal common axis  16 ′ is shown in side view in FIG.  19  and in axial view in FIG.  20 . The manifolds or collecting pipes  4 ′,  5 ′ for the material mixture to be separated are disposed diametrically and parallel to the common axis  16 ′, as shown particularly in FIG.  20 . An outlet  11 ″ for the permeate is disposed at the bottom of the permeate tank  40 . As FIG. 20 particularly shows, the permeate tank or container  40  has an upper part  44 , which can be hinged open at a hinge  96  for servicing. The end portions of the tube membranes  6  for connection with the manifold or collecting pipe  4 ′ are identified by reference numeral  75  in FIGS. 19,  20 . 
     FIGS. 21 a ,  21   b ,  21   c,  for a membrane filtration module with horizontally arranged axes of the tube membranes  6 , shows one possibility of horizontally retracting a removable part  44 ′ of the permeate tank  40  through a rolling device  101 , so that the tube membrane units  6  become accessible for servicing. 
     FIG. 22 shows an advantageous embodiment of a tube membrane  6  as a disklike flat winding, of the kind that can be used particularly in the membrane modules of FIGS. 16-21. The coiled central portion of the tube membrane  6  is disposed in a boxlike container  106 , which serves the simultaneous purposes of retaining in position the windings of the two coils of the central portion of tube membrane  6  and providing access in the replacement of the coiled tube membrane  6 . The end portion  75  and  76  of the tube membrane protrude circumferentially from the round boxlike container  106  as shown for coupling to the supply and discharge manifolds. The wall of the boxlike container  106 , as FIG. 22 shows, is provided in some regions with many openings  107 , which act to drain off the separated material as permeate. For work on the tube membrane  6  itself, a cap  108  can be removed from a lower part  109  of the container  106  by means of a snap closure  110 . 
     A further means for improving the stability of the coiled tube membrane is shown in FIG. 23 a.  Here, a likewise disklike support plate  116  is placed into the disklike tube membrane  6  having the axis  16 . One of the two coils in the central portion of the elongated tube membrane lies on one face of the support plate  116 , while the other coil lies on the opposite face of the plate  116 . The ring assembly comprising the tube membrane  6  and the support plate  116  is retained by radially oriented holding straps, of which only the holding strap  117  is indicated in the sectional view of FIG. 23 a.  Better permeate drainage is permitted by variants of the support plate  116  as shown in FIG. 23 b  and FIG. 23 c.  The support plate  116  of FIG. 23 b  has ribs  117 ′ on both sides, the spacings between which correspond to the spacing between adjacent windings of the tube membrane  6 , such that the tube membrane  6  rests on the ribs  117 ′, resulting in enlarged drainage conduits. 
     The support plate  116  of FIG. 23 c  has holes  118 , which likewise make it easier for the permeate to drain off. 
     If, given a restricted length of the tube membranes in view of the pressure gradient, it is desired that a flat winding of only two axial layers with a large filter surface area be realized, then the winding can be formed of a plurality of radially side by side membrane tubes  6 ′,  6 ″. FIG. 24 schematically illustrates a suitable winding pattern for the incorporation of two tube membranes  6 ′ and  6 ″ into a two layer disklike body of enhanced diameter. In this diagram, first end portions  123 ′ and  123 ″ of the respective membrane tubing units  6 ′ and  6 ″ are shown as being coupled to a manifold  4 ″ for supplying the mixture which is to be separated. Second end portions  124 ′ and  124 ″ of the respective tube membranes  6 ′ and  6 ″ are provided for coupling to a discharge manifold  5 ″ for receiving the flow exiting the module. The central portions of the two tube membranes are wound together into each of two coils in layers  121  and  122 , with connections  125 ′ and  125 ″ between the coils at their inner peripheries. Within each of the two radially extending coils, the two tube membranes  6 ′ and  6 ″ are disposed in the same radial plane. In order for there to be approximately equal-length terminal portions of the membrane tubes  6 ′,  6 ″ at the collecting pipes  4 ″,  5 ″, it is expedient that the radial order to membrane tubes  6 ′,  6 ″ in one layer  121  be reversed from the other layer  122  at the central transition in the winding diagram of FIG. 10 b,  as shown in FIG.  24 . With this crossover, the succession of the parallel tube membranes in a radial direction from the central winding axis will be  6 ″ followed by  6 ′ in one of the planar coils (e.g.,  121  in FIG. 24) and will be  6 ′ followed by  6 ″ in the other of the planar coils (e.g.,  122  in FIG.  24 ). The thereby increased diameter D of the winding is no problem in many cases. 
     The collecting pipes described thus far for the material mixture to be separated have the function not only of collection or distribution but also a mixing function. In membrane tubes with an oncoming parallel flow, such a mixing function for certain tube lengths can serve to avert excessive thickening and hence clogging of the material mixture in individual membrane tubes. FIG. 25 schematically shows how mixing tubes in the form of collecting pipes  134 ,  135  are provided, one mixing tube per two groups, between groups  126 ,  127 ,  128  of tube membranes  6 . These mixing tubes are closed toward the outside in the separation mode and have rinsing valves  136 ,  137  that are externally accessible only for cleaning purposes. 
     In membrane modules with bundles of tube membranes as in FIG. 1 a , instead of the mixing tubes  136 ,  137  of FIG. 25, interruptions of the coiled bundle  7  that are embodied as mixing chambers  141  are provided, as shown in FIG.  28 . The tube membranes  6  in the partial bundles  7 ,  7 ′ located between the mixing chambers  141  all have the same length, but because of the coiling of the partial bundles their ends are staggered, as shown in FIG.  28 . Connections  146  serve to deliver and drain off a rinsing agent for cleaning the tube membranes once a separation operation has ended. Like the bundles  7  coiled for instance as in FIG. 1 a , the mixing chambers  141  are also located inside the vessel  1  for the separated material in the form of permeate; in the separation mode they are therefore surrounded on the outside by permeate, and the connections  146  for the rinsing agent discharge into connecting means, not shown, that lead through the permeate to the outside. 
     Like FIG. 28, FIG. 27 also shows a staggered arrangement, because of the coiling, of equal-length tube membranes  6  in a coiled bundle  7  inside a collecting pipe  4  as in FIG. 2 a.    
     In conjunction with the embodiment of a membrane module in accordance with FIGS. 17 a  and  17   b,  which allows especially easy individual replacement of coiled tube membranes  6  by disconnection from the common collecting pipes  4 ″ and  5 ″, as explained in conjunction with FIG. 16, reference has already been made to advantageous fast-action closure connections of the tube membranes  6  to the collecting pipes  5 ″ and  4 ″. Such connections are shown in FIGS. 26 a,    26   b,    26   c.    
     FIG. 26 a  shows an end portion  76  of a tube membrane  6  which is connected to a manifold or collecting pipe  4 ″ shown in longitudinal section, with the interposition of a coupling comprising a connector fitting or transition piece  151  attached to the membrane tubing end and a receiving flange  152  on the manifold  4 . As shown particularly by the cross section of FIG. 26 b  along a line A—A of FIG. 26 a,  the connector fitting  151  on the tubing end portion  76  has an external annular groove  153 , which is engaged by a U-shaped clip  154 . The clip  154  is inserted through bores  155  along secants in the receiving flange  152  and retained in such a way that the transition piece  151 , after introduction into the receiving flange  152 , is retained in rotatable but captive fashion. An O-ring  156  provided on the face end of the transition piece  151  assures a liquid-tight connection between the connection  76  of a tube membrane  6  and a collecting pipe  4 ″. FIG. 26 c  shows a fragmentary section through a variant of the transition piece  151  of FIG. 26 a,  in which the same reference numerals indicate corresponding components. 
     As already mentioned above, the described membrane modules can be manufactured with commercially available tube membranes permitting being bent at bending radii less than 20 times the inner tube diameter without damaging the membrane layer. Simpler than such bending processes may be a process for the manufacture of tube membranes by extrusion of synthetic resin using an extrusion nozzle having an annular aperture. Due to varying control of the flow-through velocity of the synthetic resin alongside the annular aperture the tube membrane may be manufactured in bent form by thermal or mechanical means. Using extrusion nozzles of suitable shapes multiple tube membranes  36  may also be extruded thus as a unit as shown in FIG. 9 or tube membranes  37  may be manufactured as a flat tube as apparent from FIG.  13 . For the common collecting pipes for the parallel delivery and removal of the material mixture to be separated to and from the tube membranes, in particular collecting pipes  4 ″,  5 ″ in accordance with FIG. 14 with an oncoming flow at right angles to their axis, variants in which the collecting pipes have a plurality of separate parts, each with its own feed line, can offer advantages. 
     Membrane modules according to the invention can be used in cross current processes for the separation of fruit juices, foodstuff or waste waters. In this context, depending on the type of the component to be separated from the material mixture, one will select membranes with separation limits in the range applicable to reverse osmosis, nanofiltration, ultra filtration or micro filtration. For appropriate applications the membrane modules can also be simply adapted to function in an operation with dead-end filtration. In comparison with membrane modules with linear tube modules, according to the invention larger filter surface areas of about 180 m 2  can be attained in modules.