The invention relates to a membrane filtration element comprising at least one flow duct having a membrane wall, a housing enclosing the flow duct, a fluid supply which is connected to one end of the flow duct, a retentate discharge connected to the other end of the flow duct and a permeate discharge connected to the housing, the flow duct extending helically around an imaginary axis.

A membrane filtration element of this type is disclosed in US-A-5 202 023. This known membrane filtration element employs a plurality of hollow fibre membranes as flow ducts. In use, one end of the hollow fibre membranes is supplied with a liquid which, as it flows through the hollow fibre membranes, partially escapes through the membrane walls and is thus separated into a permeate, to be collected in the housing, and a retentate, to be discharged at the other end of the hollow fibre membranes . In one embodiment of the known membrane filtration element the bundle hollow fibres is wrapped helically over a cylindri¬ cal core to obtain a short cylindrical configuration. The object of the invention is to provide an improved membrane filtration element which makes a higher packing density of the flow ducts possible and in which the membrane wall does not rapidly oul. Fouling of the membrane wall reduces the permeate yield. Fouling of the membrane wall occurs particularly if the flow in the flow ducts is laminar or not very turbulent, since the substances accumulating at the membrane walls are not removed effectively in that case. While highly turbulent flows in the flow ducts do result in more effective removal of these substances, the energy consumption in terms of permeate produced per membrane surface unit area is then very high, however.

According to the invention this object is achieved by a membrane filtration element of the abovementioned type wherein the shape of the helix formed by the flow duct is such that, if an axial main stream is established in the

flow duct, a secondary flow is produced whose flow direction is substantially transverse to the direction of the main stream, and that the ratio c/a is less than 2.4, where a is the hydraulic radius of the flow duct and c is the radius of the helix.

As a result of this secondary flow a flow along the membrane wall is achieved which ensures that substances accumulating at the membrane wall are removed, resulting in less rapid fouling of the membrane wall. Moreover, the fluid in the flow duct is thoroughly mixed by this secondary flow and as a result the concentration differences in the fluid of the substances dissolved or suspended in the fluid and retained by the membrane in their entirety or in part, which have been produced as a result of the permeate escaping through the membrane wall, are eliminated. This is particularly true if the diffusion velocity is low with respect to the velocity of the secondary flow, because the evening-out of the concen¬ tration differences by this convection then proceeds more rapidly than via diffusion. Particularly for a Reynolds number which, based on the internal diameter of the flow duct and the axial flow velocity, is less than 20,000, it is thus possible to obtain a relatively high permeate yield per membrane surface unit area in conjunction with low energy consumption. By making c/a relatively small, i.e. smaller than 2.4 it is possible to achieve a high packing density of the flow ducts.

The article by K. Tanishita "Tightly wound coils of microporous tubing: progress with secondary-flow blood oxygenator design" in Transactions American Society for Artificial Internal Organs, Vol. XXI, Washington, D.C., April 1975, pages 216 to 223, discloses the use of coiled tubes with inherent secondary flow in a blood oxygenator. In this article the pitch of the coiled tube is the tightest possible and the minimum value of the ratio c/a is 2.4.

Preferred embodiments of the membrane filtration element according to the invention are defined in dependent claims 2 to 9.

The invention will be explained in more detail with reference to the accompanying drawings in which:

Figure 1 is a schematic sectional view of a membrane filtration housing containing a helically extending flow duct;

Figure 2 is a side view of an embodiment of a helical flow duct;

Figure 3 is a cross-section of the flow duct of Figure 2, a secondary flow being depicted therein; Figure 4 is a side-view of three parallel helical flow ducts having a common axis;

Figure 5 is a side-view of three twisted flow ducts;

Figure 6 is a cross-section of the twisted flow ducts of Figure 5; and

Figure 7 is a side view of two parallel and both helically and spirally extending flow ducts having a common axis.

A membrane filtration element depicted in Figure 1 comprises a helically extending flow duct 10 which is accommodated in an encasing housing 11. At one end of the flow duct 10 a fluid can be supplied, here indicated by the arrow 12. The flow duct 10 has a membrane wall, and a portion of the fluid flowing through the flow duct 10 escapes through the membrane wall and thus passes into the housing 11 from which it can be discharged via a permeate discharge, here indicated by the arrow 13. That part of the fluid which is retained by the membrane wall and is also referred to as retentate exits at the other end of the flow duct 10, as indicated here by the arrow 14.

Figure 2 depicts a helically extending flow duct 20 with the dimensions a, b and c, which are of particular interest; a here is the radius of the flow duct, 2τrb is the pitch of the windings and c is the radius of the helix. If the ratio between a, b and c stays within certain limits, an axial main flow through the flow duct 20 will produce a secondary flow in the flow duct.

One form of this secondary flow is depicted in Figure 3. The flow profile shown here is achieved if

where Re is the Reynolds number of the axial main flow, based on the internal diameter of the duct and the axial flow velocity prevailing in the duct.

The secondary flow has the effect of stabilizing the flow, the transition point from laminar to turbulent flow in a helical flow duct consequently being at a higher Re number than in a straight flow duct. Moreover, the thickness of the hydrodynamic boundary layer is reduced. As a result, better mixing of the boundary layer with the fluid takes place and mass transfer proceeds more rapidly. As can be seen, the secondary flow is formed here by two vortices 31 having opposite directions of rotation, which are substantially transverse to the direction of the main stream. Here the vortices 31 have the same circumference. This is achieved if in particular the following condition is met :

If the condition

is met, the one vortex will be larger than the other vortex. Four different zones can be distinguished across the cross-section shown of the flow duct: two zones A where the two vortices 31 flow along the membrane wall 32; a zone B which is situated between the points where the respective vortices 31 detach from the membrane wall 32;

a zone C which is situated between the points where the respective vortices 31 attach to the membrane wall 32; and a zone D which is situated in the central section of the flow duct .

As a result of permeate escaping through the membrane wall 32, concentration differences, seen across the cross-section of the flow duct, are produced in the remaining fluid, the concentration increasing from C to A to B and then decreasing from B to D to C. As a result of mixing by the vortices 31, these concentration differences are advantageously largely eliminated again. Thus it is easier for permeate to escape, the permeate yield being increased as a result . Figure 4 depicts three parallel helical flow ducts

40, 41 and 42 having a common axis. This configuration enables a high packing density of flow ducts per unit volume of the housing to be achieved.

A high packing density can be achieved if the value of the ratio c/a is smaller than 2.4. Such a small value is possible by making the pitch of the helix greater than the diameter of the flow duct (space between the windings) . Preferably, 0,1 ≤ c/a ≤ 1.5 and more preferably 1 ≤ c/a ≤ 1.5. Figures 5 and 6 depict an embodiment thereof, which can be fabricated by three flow ducts 50, 51, 52 being twined or twisted round one another. Of course, the number of flow ducts may be varied. The flow ducts support each other which improves the stability. In Figure 7, flow ducts 70 and 71 extend both helically and spirally. As can be seen, the helix diameter decreases from the top downwards. Thus it is possible for a plurality of such flow ducts to be nested inside one another in a simple manner. To obtain the above-described secondary flow together with an axial main stream it is also possible to use a flow duct having an oval cross-sectional shape. In that case too a flow duct is obtained in which a helical shape extending around an imaginary axis can be discerned.

In case of a flow duct with an oval cross-section in the formulas shown above the parameter a is the hydraulic radius which is defined as 2A/1, where A is the cross- sectional area of the flow duct and 1 is the circumference of the flow duct. For a round cross-section the hydraulic radius is of course equal to the radius. Moreover, the secondary flow can be generated by a flow duct which has an oval shape and is twisted solely around its centre line. In this case the ratio c/a is zero. The membrane filtration element according to the invention can thus be used, thanks to the helically wound flow duct, to obtain a high permeate yield in conjunction with low energy consumption and a high packing density. Advantageously, a plurality of membrane filtration elements are connected in series. Supplying the retentate from the one element in turn as a fluid to the next element makes it possible to achieve more comprehensive filtration.