Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D65/00—Accessories or auxiliary operations, in general, for separation processes or apparatus using semi-permeable membranes
  • B01D65/08—Prevention of membrane fouling or of concentration polarisation
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D63/00—Apparatus in general for separation processes using semi-permeable membranes
  • B01D63/02—Hollow fibre modules
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D63/00—Apparatus in general for separation processes using semi-permeable membranes
  • B01D63/02—Hollow fibre modules
  • B01D63/025—Bobbin units
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D63/00—Apparatus in general for separation processes using semi-permeable membranes
  • B01D63/02—Hollow fibre modules
  • B01D63/027—Twinned or braided type modules
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D63/00—Apparatus in general for separation processes using semi-permeable membranes
  • B01D63/06—Tubular membrane modules
  • B01D63/068—Tubular membrane modules with flexible membrane tubes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2321/00—Details relating to membrane cleaning, regeneration, sterilization or to the prevention of fouling
  • B01D2321/20—By influencing the flow
  • B01D2321/2008—By influencing the flow statically
  • B01D2321/2016—Static mixers; Turbulence generators

Definitions

• 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.
• 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 .
• 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.
• 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.
• 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.
• 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, 2 ⁇ rb 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.
• 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.
• 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.
• the vortices 31 have the same circumference. This is achieved if in particular the following 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 .
• 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.
• the helix diameter decreases from the top downwards.
• a plurality of such flow ducts to be nested inside one another in a simple manner.
• 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.
• 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.
• the hydraulic radius is of course equal to the radius.
• 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.
• 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.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Separation Using Semi-Permeable Membranes (AREA)

Applications Claiming Priority (3)

Publications (1)

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ID=19762351

Family Applications (1)

Country Status (5)

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• 1996
  • 1996-02-20 NL NL1002397A patent/NL1002397C2/nl not_active IP Right Cessation
• 1997
  • 1997-02-19 CA CA002246675A patent/CA2246675A1/en not_active Abandoned
  • 1997-02-19 EP EP97904651A patent/EP0881941A1/de not_active Ceased
  • 1997-02-19 WO PCT/NL1997/000074 patent/WO1997030779A1/en not_active Ceased
  • 1997-02-19 AU AU17362/97A patent/AU1736297A/en not_active Abandoned

Non-Patent Citations (1)

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