GB2086762A

GB2086762A - Device for mass transfer between fluids

Info

Publication number: GB2086762A
Application number: GB8133383A
Authority: GB
Original assignee: Sartorius AG
Current assignee: Sartorius AG
Priority date: 1980-11-12
Filing date: 1981-11-05
Publication date: 1982-05-19
Also published as: GB2086762B; FR2495006A1; JPS57110303A

Abstract

A device for mass transfer between fluids by means of a pressure gradient and/or concentration gradient comprises a first support fleece (21), a first membrane (5), a flow-dividing element (6) in the form of a fluid-tight foil embossed with protrusions (6'), a second membrane (7) and a second support fleece (22). These five layers are pleated together into a folded packet and are enclosed in a housing (4) in such a manner that, with the aid of sealing beads (8) in the region of the end faces of the packet, three separate flow chambers (I, II, III/III') are formed. The fleeces (21, 22) are slightly compressible and have generally planar surfaces supporting the membranes (5, 7). Under the pressure in the third flow chamber (III/III'), the fleeces (21, 22) are slightly compressed so that the flow dividing element (6) is freed from contact with the membranes (5 and 7) thereby to allow laminar fluid flow over the protrusions (6'). <IMAGE>

Description

SPECIFICATION Device for mass transfer between fluids The present invention relates to a device for mass transfer between fluids with utilisation of a pressure gradient and/or concentration gradient.

In DE-OS 26 08 758 and corresponding US-PS 3 965 015, there is disclosed a device in which a selectively permeable membrane is supported by a membrane support acting against a possible pressure gradient, the membrane support having a structure forming flow channels and the membrane and membrane support forming a common pleated unit. This is housed in such a manner in an enclosing box-shaped housing with inlets and outlets for the fluids that several flow chambers are formed, the chambers extending in one housing direction and being sealed off separately. The membrane is pleated, together with a membrane support in the form of a grid mesh, into a folded packet which is sealed off at the end side, so that two flow chambers, sealed from each other, are formed with the housing.The chamber with the mesh serves for the guidance of dialysate and the membrane surface without membrane support serves for the guidance of blood when the device is employed in the field of medicine as a dialyser or as an artificial kidney. In this case, the concentration gradient of blood to dialysate acts primarily as a transfer mechanism in order to transfer the substances to be separated from the blood through the membrane into the dialysate.

For the performance of ultrafiltration, a filter in folded mode of construction is also known (DE-OS 26 08 758, corresponding to US-PS 4 028 252), in which two compound membranes are pleated together. Each compound membrane consists of the actual membrane and a porous membrane support integrated therewith. The two membrane supports of the membranes lie congruently one on the other and are open towards one end face of the folded packet, the other end face being sealed in the plane of the membrane support. The filtrate can in that case be withdrawn towards the open end face, while the fluid to be treated flows over both the membranes in the folds open towards the side.

For the formation of three flow chambers separated one from the other it is also known (DE-AS 28 03 344, corresponding to US-PS 4 219 422) to house a folded packet, consisting of two membranes with the inclusion of a membrane support lying therebetween, in a box-shaped housing.

For performance of a combined dialysis and filtration of blood, it is also known (DE-OS 2 806 237, corresponding to GB-OS 2 014 060) to pleat a membrane especially optimised for dialysis and a membrane especially optimised for blood filtration lying congruently together into a folded packet and to push web sections as membrane supports into the laterally open pockets.

The actual membrane support is as a rule an incompressible fabric or mesh or a support plate or foil with protrusions at both sides.

It is also known in the case of dialysers and filers for use as an artificial kidney or lung to employ spacer members to separate the two membrane surfaces which face each other and which conduct the blood flow. For the avoidance of a hemolysis, these spacer members usually consist not of a fabric or mesh, but of a thin foil which, according to US-PS 3 077 268, is provided at both sides with a plurality of protrusions in the form of cones or conical frustums which keep the mutually facing membrane surfaces at a spacing from each other and which have a plurality of flow channels for the guidance of the blood to be treated.In place of such spacer members acting punctiformly, foils embossed with a plurality of parallel flow channels are disclosed in US-PS 3 490 523 and DE-OS 27 22 025, from which it is known to provide, in a folded module, a plurality of parallel flow paths on the blood-conducting side by means of an embossed foil.

In DE-OS 30 05 408, it is expressely claimed that a spacer member stands in intimate contact with the active side of the membrane and is to be exposed to a supplied fluid, the spacer member being sinuous in order to form a plurality of parallel channels between the membrane and the spacer member.

It is common to all these constuctions that the spacer members actually stand in intimate contact with the membrane sides facing each other and inevitably cover a great part of the membrane surface by a plurality of punctiform to line-shaped supports and thereby make this inactive for the substance exchange. In addition, they form "dead water zones" in the region of the contact points, in which a slower fluid flow arises by reason of the lower channel height. This is particularly critical when the device is used as a hemofilter and the blood experiences a concentration and the viscosity rises. Hemofilters and hemodialysers are as a rule after completed treatment of the patient rinsed free from residual blood, for which it is endeavoured for medical reasons to rinse the filter or dialyser completely free of blood.The intimate contact of the spacer members with the blood-conducting sides of the membranes in the known constructions in fact prevents complete rinsing free and removal of the blood from the device, which is thus lost to the patient.

Since the treatment of blood in the medical field imposes strict demands on the construction of dialysers and filters and the treatment of fluids in other fields is also covered by such requirements, there is accordingly a need in the case of a dialyser or hemofilter or an oxygenator for use for the treatment of blood to eliminate the defects of the known devices, in particular by ensuring that the greatest possible surface area of the membrane is available for conduction of the flow, that the resistance on the blood side is as small as possible, and that the blood compartment after the end of the treatment can be rinsed free as free from residue as possible. In addition, with regard to industrial production, a simple manufacture as mass produced article is also desirable.

According to the present invention there is provided a device for mass transfer between fluids by means of at least one of a pressure gradient and a concentration gradient, the device comprising a housing provided with a plurality of fluid flow openings arranged at two opposite sides of the housing and with a fluid inlet and a fluid outlet arranged one at each of two opposite ends of the housing, and a laminated fluid transfer element which is arranged in the housing and which is folded into pleats, the laminae of the transfer element comprising two selectively permeable membranes arranged at a spacing from each other to define between mutually facing sides thereof a central flow chamber communicating with the fluid inlet and outlet, a flow-dividing member arranged between the membranes to maintain the spacing thereof and comprising a substantially fluid-impermeable sheet provided with deformations facing the membrane sides and defining flow channels extending in a given direction of fluid flow through the housing, and two support elements of fluid-permeable compressible material each comprising a substantially planar support surface arranged to support a respective one of the membranes at a side thereof opposite to the side facing the other membrane and each having its overlying fold portions so sealingly connected to each other and to the associated membrane in the region of the ends of the pleats as to form in conjunction with the housing a respective outer flow chamber communicating with a respective pair of the fluid flow openings thereby to provide a flow path extending in said flow direction, the support elements being so compressible by the pressure of fluid in the central chamber as to permit relative displacement of the membranes and the flowdividing member so as to free the member from contact with the membranes in the region between the sealing connections of the support elements to the membranes thereby to allow laminar flow of fluid in the central chamber over said deformations.

Through the use of a slightly compressible fleece material as the support elements and the use of a fluid-impermeable foil as the flowdividing member in the compartment of the fluid to be treated, the flow-dividing member can assume an intermediate position between the membranes so as to be free from contact with the membranes and can execute a virtually peristatic longitudinal movement in direction of the fluid flow under the fluid pressure acting thereon at both sides and the slightly fluctuating fluid pressures, whereby the danger of accumulation, congestions or the danger of the hemolysis formation in the treatment of blood is virtually eliminated.

In a preferred embodiment of the device, the laminae of the transfer element comprises a first support fleece, a first membrane, a flow divider in the form of a fluid-tight embossed foil, a second membrane and a second support fleece, the laminae being pleated together into a folded packet and being enclosed in the housing in such a manner that, with the aid of sealing beads in the region of the end faces of the packet, three mutually separate flow chambers are formed. Such a device is suitable as dialyser and/or as filter, especially for the treatment of blood in the field of medicine. The support fleeces are slightly compressible and have virtually planar support surfaces by means of which the membranes receive a corresponding planar support at their inactive sides.Under the pressure of fluid in the third, i.e. central, flow chamber, the support fleeces are slightly compressed so that the flow divider assumes an intermediate position free of contact between the membranes in such a manner that the protrusions facing the membrane surfaces can be subjected to a laminar flow of fluid in the third flow chamber. A housing end plate facing the fluid inlet for the third flow chamber is provided with special laminate ribs for distributing the fluid current in a common inlet passage amongst a plurality of flow ducts, which in downstream direction become smaller in cross-section and wider in correspondence with the intended fluid distribution. The ribs have a sawtooth shape in cross-section and their points bear against the facing end of the folded packet so as to separate the flow ducts from each other.

Embodiments of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a perspective elevation of a filter-dialyser device for use as an artificial kidney, the device being shown in partial section on the line 1-1 of Fig. 2, and a detail, to an enlarged scale, of a fluid transfer element of the device, Figure 2 is a horizontal cross-section on the line 2-2 of Fig. 1, Figure 3 is an exploded perspective view of a flow divider and support fleeces of the fluid transfer element, Figure 4 is a detail sectional view, to an enlarged scale, of a modified flow divider in the region of an adhesive zone at an end thereof where the support fleeces are ren dered virtually incompressible by the adhesive.

Figure 5 is a view similar to Fig. 4 but in a region behind the adhesive zone where the support fleeces are compressed by fluid pressure at both sides of the flow divider, Figure 6 is a perspective view of an embossed foil in a flow divider, Figure 7 is a detail sectional view of several fold layers of the transfer element in the region of a fluid distribution space (blood outlet), each fold layer being shown in section on the line 7-7 of Fig. 6, Figure 8 is a plan view of an embossed foil as flow divider according to a further modification, Figure 8a is a cross-section on the line Vlila-Vlila of Fig. 8, to an enlarged scale, Figure 8b is a cross-section on the line Vlilb-Vlilb of Fig. 8, to an enlarged scale, Figure 9 is a combined plan and sectional view of a flow divider according to yet another modification, Figure 10 is a horizontal sectional view, approximately on the line 10-10 of Fig. 12, of the end region of a housing of a filterdialyser device according to another embodiment of the invention, Figure 11 is a front elevation, from inside the housing, of an end plate of the housing of Fig. 10, Figure 12 is a vertical section, on the line 12-12 of Fig. 11, of the housing and plate, and Figures 13, 14, 15 and 16 are crosssections of the housing end plate on the lines 13-13, 14-14, 15-15 and 16-16, respectively, of Fig. 11.

Referring now to the drawings, there is shown in Fig. 1 a filter or dialyser which comprises a box-shaped housing 4 formed from two half shells 4a and 4b and which is provided on its upper side with a first inlet la for dialysate D or outlet for filtrate F and a first outlet 1 b for dialysate D or for filtrate F, and on the oppositely disposed side with a second inlet 2a for dialysate D or outlet for filtrate F and a second outlet 2b for dialyste D or filtrate F. The housing is also provided at its end faces with an inlet 3a for blood B and an outlet 3b for the blood B. As is evident from Figs. 1 and 2, the inlets and outlets la, 2a, 1 b and 2b for the dialysate or the outlets for the filtrate form downwardly directed distributor channels 11 in the housing 4.Both sides can thus have the function of a filter or dialyser or simultaneously, or one after the other the function of a filter and dialyser.

When, for example in operation as dialyser, the inlet 2a is throttled in terms of flow and an underpressure is applied to the outlet 2b, then, apart from the concentration gradient from blood to dialysate, a pressure gradient appears, so that filtration also occurs.

Arranged in the housing 4 is a fluid transfer element in the form of a folded packet consisting of two membranes 5 and 7, associated support elements or fleeces 21 and 22, and a flow divider 6 enclosed by the membranes and support elements. These five layers lying one on the other, namely support element 21, membrane 5, flow divider 6, membrane 7 and support-element 22, are, after location of the endless webs at the edges, pleated in common according to Fig. 3 and compressed into a folded packet matched to the height of the housing. During the folding, the end regions at the end faces of the folding grooves are glued together by a bead of synthetic resin 8 so that a hoselike flow path, which is formed by the flow divider 6 and the membranes 5 and 7 lying congruently thereon, is left open only at the end face.

For the formation of the three flow chambers I, II, and Ill/Ill', as indicated above the end region of the packet is provided with a bead 8 which, as shown in Fig. 2, sealingly contacts the sides of the housing 4 and thereby bounds and seals off blood distributor chambers 9 and 10 from the rest of the housing 4. In addition, both longitudinal edges of the end folds 12 are provided over their entire length with a glueing or sealing 8' so that the packet is open at both end faces only in the region of its middle layer, i.e. the flow divider 6. The flow chamber Ill/Ill' is formed as a result, the chamber having two sections III and Ill' separated by the flow divider 6.

To enhance flow guidance in this chamber, the flow divider 6 has a zig-zag-shaped or sinuous cross-sectional structure extending transversely to the parallel flow paths, the structure defining wave crests 6' and wave grooves 6" which, in the direction of the parallel flow paths, additionally have a zig-zagshaped or sinuous transfer displacement.

Thus, the wave crests 6' lie in one common reference plane and the wave grooves 6" lie in another common reference plane. The grooves therefore run parallel to each other but in a zig-zag to sinuous shape.

This structure is provided by appropriate formation of the flow divider 6 from a fluidtight foil, as schematically shown in detail in Figs. 1 and 3.

For formation of the other two flow chambers I and II, the longitudinal edges of the packet sealingly contact the longitudinal walls of the housing 4 in the region between the inlets and outlets la, 2a, 1b and 2b and the upper side and lower side of the packet sealingly contact the housing bottom and housing ceiling. Dialysate D ingressing via the inlets 1 a and 2a passes through the distributor channels 11 into the region of the individual fold grooves, which are open towards the respectively associated longitudinal housing wall, and is forced according to Fig. 2 to penetrate into the depth of the folds and to flow in direction of the respective outlet 1 b and 2b.

In the present embodiment, the dialysate D introduced into the inlet 1 a thus flows through the flow chamber I and leaves this through the outlet 1 b, while the dialysate D introduced into the second inlet 2a in like manner leaves the flow chamber II through the outlet 2b. Blood B preferably flows in counterflow manner through the flow chamber lil/lil', entering the housing 4 through the inlet 3a and the distributor chamber 10, passing to the distributor chamber 9, and leaving this chamber through the outlet 3b.

By reason of the concentration gradient between blood B and dialysate D, urinary substances to be withdrawn from the blood diffuse through the membrane 5 and 7 into the hollow space within the membrane support elements 21 and 22 and are guided away from this. If an underpressure pump is connected to the outlet 1 b and/or the outlet 2b for the dialysate D and the flow through the inlet is and/or the inlet 2a of the dialysate D is throttled or entirely blocked by a hose clamp, then ultrafiltration from the blood towards the dialysate takes place by virtue of the pressure gradient. In this manner, greater quantities of blood water can be withdrawn from the blood over a short time.

The device can be used as pure filter unit when the dialysate D is dispensed with. The inlets necessary for the dialysate guidance are in this case closed off or serve as filtration outlets.

Membranes of high permeability are particulary suitable for both operations.

Fig. 3 illustrates the guidance of the fluids in a plurality of parallel, zig-zag-shaped flow paths. If such a zig-zag-shaped flow path according to Fig. 1 or Fig. 3 is sectioned in longitudinal direction, then the flow divider 6 appears as cut wave, similar to that shown in horizontal section in the encircled detail of Fig. 2, in the longitudinal sectional illustration according to Fig. 4.

With use of an embossed foil the flow divider, the wave structure may have a transverse channel 16 formed at both sides, for example in the region of the distributor channels 11, i.e. the wave crests 6' are upset, in the region of the inlet channels 11, at both sides through about 1/3 at about their middle, as illustrated in Figs. 6 and 7. Alternatively, the transverse channel 16' is formed, as shown in Fig. 8, by replacing the wave structure in the region of the inlet channels 11 by a plurality of individual protrusions 17 and individual depressions 17' (Figs. 8 and 8b). This structure on the one hand enables a good and uniform distribution of the fluid from the distributor channels 11 into the depths of the individual folds of the membranes 5 and 7 and on the other hand does not obstruct the fluid flow on-the flow divider 6 itself.

The mode of construction of the transverse channels 1 6 and 16' according to Figs. 6 to 8 serves to distribute the fluid flow entering at the end face, in the present case the blood flow, uniformly-over the entire fold depth directly behind the entry openings at the end face. However, it could happen that in spite of careful manufacture individual entry openings between the flow divider 6 and the two membranes 5 and 7 are partially or wholly clogged, during compression of the folded packet, by remnants of adhesive substance displaced from the adhesive bead 8. This can, however, be prevented by provision of a stamping cut at the end face of the entire packet in the plane of the membrane support elements 21 and 22 after glueing together of the folds.

Preferably, the wave crests 6' are provided over their length with a plurality of interruptions 23 making possible fluid communication in the wave grooves 6", as is illustrated in Fig. 8. These interruptions 23 form static mixers to ensure that the fluid flowing over the flow divider 6 is continuously intermixed and subjected to a diagonal flow from the distributor chamner 10 (inlet) to the distributor chamber 9 (outlet). The fold edges indicated at 18 in Fig. 8 show that through the zig-zag-shaped course of the embossings on the flow divider 6, the wave crests 6' and the wave grooves 6" are superimposed on each other crosswise after the folding. The flow divider 6 provides a plurality of defined flow paths between the two membranes 5 and 7.

The flow divider 26 illustrated in Figs. 4 and 5 is comparable with the flow divider 6.

The flow divider 26 is formed of an embossed foil of plastics or other synthetic material, which in cross-section has a double sawtooth profile in which the outer sawtooth spines 27 and sawtooth notches 28 have steeper flanks than the central sawtooth portions 29 connecting the spines and notches. This shape results in the possible contact points of the flow divider 26 with the facing membranes 5 and 7 being restricted to a minimum during operation, when fluid pressure may possibly fluctuate at both sides of the flow divider 26.

The same applies to the embodiment of the flow divider 26 according to Fig. 9. In this case, the protrusions arranged at both sides consist of streamline bodies 27 of the form well-proven in general flow technology. At the inlet side and at the outlet side the streamline bodies 37 can be arranged one after the other and displaced one relative to the other in the general overflow region so that the effect of static mixing is also provided by this arrangement. The unitary flow divider 36 illustrated in Fig. 9 can also be achieved through a twopiece construction in which two deeply embossed foils lie congruently on each other by the concave parts of their streamline bodies and are welded together at the edges.

Since the flow divider 6 reaches up to the end face of the folded packet, and can even protrude somewhat, a plurality of entry openings for the fluid, in this example the blood B, are formed in simple manner so that the flow resistance remains very small.

In the illustrating embodiment, the transverse channels 16 and 16' of the flow divider 6 run at right angles to the fold edges, designated by 18, of the membranes 5 and 7 and the membrane support elements 21 and 22. The zig-zag embossing of the fluid-tight foil is shown at 17" in Fig. 8a.

Insofar as a "fluid-tight" foil or sheet as flow divider 6 is referred to in the description and claims, it is sufficient for the foil to be substantially fluid-tight, i.e. micro leakages are harmless. The flow divider 6 must be at least so tight that no short-circuit paths are possible therethrough A relatively cheap, slightly compressible, virtually planar support fleece, preferably of threads of plastics or other synthetic material, is preferably used as membrane support element. The threads of plastics material interlaced and felted together form a multipoint support for the two membranes 5 and 7.

Because of the irregularity of the support points, the usual press stud effect provided by webs and meshes of mutually crossing threads plays no part. The fleece of threads of synthetic material ensures a good intermixing of the dialysate and has a relatively low resistance to the dialysate flow or to the withdrawal of the filtrate.

Since the support elements 21 and 22 serve not only as membrane supports, but also for the flow guidance of the filtrate or dialysate in the flow chambers I and II, they must on the one hand be as loose as possible in structure in order to reduce the flow resistance, but on the other hand be planar and incompressible above a certain compression level in order to provide a good support for the membranes 5 and 7. The two requirements are mutually contradictory so that the most favourable form of support fleece must be chosen for each particular application. In the present embodiment, a support element is used, which in one layer has a mesh thickness of 370 microns. By virtue of the approximately 40 to 50 overlying folds and the assembly pressure, each layer of the support element is slightly compressed to about 330 microns.Because of the hardened adhesive bead 8, a further compression is not possible in the region of the end faces of the packet and the entry openings of the flow chamber Ill/Ill', illustrated in Fig. 4, arise in conjunction with the flow divider 26.

When the third flow chamber Ill/Ill', which is formed by the two membranes 5 and 7 and the flow divider 26, is filled and a difference pressure of 0.2 to 0.4 bars is established at both sides of the flow divider 26 with respect to the pressure in the flow chambers I and II, then each layer of the support element 21 and 22 is compressed to about 260 microns compared with the unloaded original thickness. With a difference pressure about 0.4 bar, the support elements 21 and 22 are virtually incompressible.The embossed foil of the flow divider 26 or of the dividers 6 and 36 has a mean thickness of about 40 microns and the foil structure also yields somewhat under the fluid pressure according to the form of cross-sectional embossing, so that an intermediate position of the flow divider 26 results between the two facing membrane surfaces of the membranes 5 and 7 through the compression of the support elements 21 and 22. This allows the pressure-loaded fluid in the flow chamber Ill/Ill' to also flow over the outer points of the flow divider 6, 26 or 36 and thus flow over the entire available membrane surface, so that this is exploited in an optimum manner for the substance exchange. As a result, "dead water zones", in which blockages, curds or other deposits can collect, cannot form in the region of the convex structure of the flow divider.This is particularly important in the use of the device as a hemofilter, in which a concentration of the blood takes place in the flow chamber Ill/Ill'.

The actual channel heights between the membrane surface, the membrane 5 and the flow divider 26 on the one hand and the membrane 7 and the flow divider 26 on the other hand, or the flow divider 6 or 36, keep within the limits which have proved themselves expedient by reason of the long experiece in the artificial kidney and lung field and which are part of the general knowledge in this field.

In order to further reduce the flow resistance on the blood inlet side, the housing illustrated in Figs. 1 and 2 can have a modified housing end plate 4c as shown in Figs.

10 to 16. The inlet 3a for the blood B is arranged at the side and, as shown in Fig. 11, passes over into an inlet channel 30 reducing in cross-section and rising in downstream direction. At the same time, the inlet channel 30 is divided up into a plurality of laminate channels 31, which in downstream direction reduce in cross-section and become wider, as is apparent from the sectional illustrations of Figs. 12 to 16, the individual channels 31 being bounded by laminate ribs 32. The ribs 32, which in corss-section have a sawtooth shape, have sharp spines which press against a planar surface, produced by a stamping out, of the folded packet. The individual channels 31 or the ribs 32 have deflecting curves in direction of the inlet 3a or the inlet channel 30.It is secured by this particular shape that blood entering into the housing is uniformly distributed over the entire end face of the packet and can enter uniformly into the individual inlet channels formed by the flow divider 6, 26 or 36 without short-circuit paths or without formation of preferred flow channels at the side. The principle of this flow guidance can, of course, also be applied to layered hose membranes which are flowed towards from the end face.

The advantage of the described embodiments is that a number of variations are possible in the field of mass transfer between fluids with the interposition of membrane unit with the aid of a standard housing, wherein large exchange surfaces are present for the fluids, even with small housing dimensions, by virtue of the five-layer fold material.

The device described as dialyser and/or filter can be used analogously as an oxygenator for oxygen enrichment of blood when an appropriate membrane is used. Equally well, the device can also be used for plasma pheresis and for the separation of milk content substances when the device is equipped with membranes suitable for this purpose. The membranes 5 and 6 can be of the same kind and structure, or different commercially usual membranes can be combined in order to optimise a mass transfer between the fluids in the field of medicine, in the laboratory or in the industrial field. The device is of course also suitable for the treatment of water.

In all cases, the adhesive or sealing bead 8 acting as a seal is additionally loaded by a fluid pressure emanating from the fluid flow in the third flow chamber Ill/Ill' so that, apart from the adhesive seal, a press seal also still occurs in the planes of the support elements 21 and 22. Even membranes which after their moistening partially detach from the adhesion, are pressed by the fluid pressure sealingly against the adhesive or sealing bead 8, which represents a significant constructional advantage relative to the known forms of construction.

Claims

1. A device for mass transfer between fluids by means of at least one of a pressure gradient and a concentration gradient, the device comprising a housing provided with a plurality of fluid flow openings arranged at two opposite sides of the housing and with a fluid inlet and a fluid outlet arranged one at each of two opposite ends of the housing, and a laminated fluid transfer element which is arranged in the housing and which is folded into pleats, the laminae of the transfer element comprising two selectively permeable membranes arranged at a spacing from each other to define between mutually facing sides thereof a central flow chamber communicating with the fluid inlet and outlet, a flow-dividing member arranged between the membranes to maintain the spacing thereof and comprising a substantially fluid-impermeable sheet provided with deformations facing the membrane sides and defining flow channels extending in a given direction of fluid flow through the housing, and two support elements of fluid-permeable compressible material each comprising a substantially planar support surface arranged to support a respective one of the membranes at a side thereof opposite to the side facing the other membrane and each having its overlying fold portions so sealingly connected to each other and to the associated membrane in the region of the ends of the pleats as to form in conjunction with the housing a respective outer flow chamber communicating with a respective pair of the fluid flow openings thereby to provide a flow path extending in said flow direction, the support elements being so compressible by the pressure of fluid in the central chamber as to permit relative displacement of the membranes and the flowdividing member so as to free the member from contact with the membranes in the region between the sealing connections of the support elements to the membranes thereby to allow laminar flow of fluid in the central chamber over said deformations.

2. A device as claimed in claim 1, wherein said fluid openings comprise inlet and outlet flow passages extending at right angles to the fold edges of the pleats.

3. A device as claimed in either claim 1 or claim 2, comprising a respective fluid distribution chamber arranged at each of said ends of the housing between the central fluid chamber and the associated one of the fluid inlet and the fluid outlet.

4. A device as claimed in any one of the preceding claims, wherein the deformations are so arranged that the flow channels have a substantially zig-zag-shaped or sinuous course and the sheet has a substantially zig-zagshaped or sinuous cross-section transversely of the channels.

5. A device as claimed in any one of claims 1 to 3, wherein the deformations are so arranged that the cross-section of the sheet transversely of the channels comprises a respective plurality of pointed projections directed towards each membrance, the projection of the two pluralities being interconnected by connecting portions extending a a shallower angle with respect to a central plane of the sheet than the flank portions of the projections.

6. A device as claimed in any one of the preceding claims, wherein the deformations are shaped to provide fluid flow passages interconnecting the flow channels and arranged at spacings along the length thereof.

7. A device as claimed in any one of claims 1 to 5, wherein the deformations comprise protrusions having a streamlined shape and arranged at both sides of the sheet.

8. A device as claimed in claim 7, wherein the protrusions are arranged in a staggered formation in said flow direction and immediately downstream of the fluid inlet.

9. A device as claimed in any one of the preceding claims, comprising a plurality of ribs so arranged in contact with an end of the transfer element at the housing end with the fluid inlet and extending substantially at right angles to the planes of the fold portions of the transfer element as to define a plurality of flow ducts for directing fluid into the central chamber from the fluid inlet, the flow ducts communicating with the fluid inlet by way of a common inlet passage.

10. A device as claimed in claim 9, wherein the inlet passage extends substantially at right angles to the flow ducts, the flow ducts communicating with the inlet passage by way of curved inlet sections which curve towards the fluid inlet.

11. A device as claimed in either claim 9 or claim 10, wherein the inlet passage decreases in cross-section in a direction away from the fluid inlet.

12. A device as claimed in any one of claims 9 to 11, wherein each of the flow ducts decreases in cross-sectional area but increases in width in a direction away from the inlet passage.

13. A device as claimed in any one of claims 9 to 12, wherein the ribs in crosssection define a generally sawtooth-shaped profile, the points of which bear against said end of the transfer element.

14. A device for mass transfer between fluids, the device being substantially as hereinbefore described with reference to Figs. 1 to 3, 6 and 7 of the accompanying drawings.

15. A device for mass transfer between fluids, the device being substantially as hereinbefore described with reference to Figs. 10 to 16 of the accompanying drawings.

16. A device as claimed in either claims 14 or claim 15 and modified substantially as hereinbefore described with referrence to Figs.

4 and 5, Figs. 8, 8b and 8a or Fig. 9 of the accompanying drawings.

17. A device for mass transfer between fluids with the utilisation of a pressure gradient and/or concentration gradient, comprising a selectively permeable membrane supported by a membrane support acting against a possible pressure gradient and having a structure permitting a flow of fluid, the membrane and membrane support forming a common pleated unit and being housed in such a manner in an enclosing box-shaped housing with inlet and outlets for the fluids that several flow paths extending in one housing direction are formed in flow chambers sealed off from each other, wherein the membrane support is formed by a slightly compressible support fleece with a substantially planar surface, a second unit comprising a further support fleece and a further membrane is associated with the first unit axially symmetrically in such a manner that the membrane sides without support face each other and are held at a spacing by a flow divider in the form of a fluid-tight foil which follows the folding of the membranes, the unit, pleated together into a folded packet from the layers of the first support fleece, the first membrane, the flow divider, the second membrane and the second support fleece Iy- ing one on the other, is provided in such a manner with an adhesive or sealing bead in the planes of the membrane supports in the region of the end faces that the folds of the support fleece lying one on the other are sealingly connected to each other and to the associated membrane so as to form, in conjunction with the housing, a first and a second flow chamber, with each of which is associated a respective inlet and outlet arranged at right angles to the fold edges on oppositely disposed housing sides in the region of the end faces of the folded packet, the flow divider holding the two membranes at a spacing has open flow inlets, which extend to the facing ends of the folded packet and are embossed at both sides, and flow channels, which are formed by protrusion of depressions of the foil and extend in flow direction of the fluids, for a further fluid for the formation of a third flow chamber which ends at each end face in a respective distributor chamber provided with an inlet or outlet, and the flow divider, immediately behind the adhesive or sealing bead and under the influence of fluid pressure acting thereon and on the support fleeces, assumes an intermediate position free of contact with the facing surface of the membranes so that the protrusions of the flow divider which face the membrane surfaces can be flowed over laminarly by fluid in the third flow chamber.