Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Patent
1995-11-20
1997-05-13
Fortuna, Ana
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
2103232, 210420, 210456, 21032178, 21032187, 2103219, B01D 6100, B01D 6300
Patent
active
056289090
DESCRIPTION:
BRIEF SUMMARY
This invention relates to membrane filters particularly, but not exclusively, of the cross-flow type.
The performance of membrane filters is limited by the efficiency of fluid mixing to bring as much as possible of the fluid being filtered into contact with the membrane.
Various solutions have been proposed. One involves pulsing the fluid with an oscillatory flow along a channel at least partly defined by the membrane, the membrane either being furrowed or dimpled or the channel containing spaced obstructions, in order to produce eddies and other secondary flows. A difficulty of this is in the complexity of the mechanism required to produce the oscillatory flow. Other proposals have involved the use of narrow channels but have required high pressure drops from inlet to outlet, caused by frictional losses in the channels. Typical inlet and outlet pressures are 3.5 and 1 bar, respectively, and these produce uneven filtration and demand high pumping power. A third approach has been to use large bore tubes lined with thin layers of membrane which are perfused at flow rates high enough to ensure turbulent flow. This may solve the problem of poor mixing but at the expense of high pumping power and possible damage to components, such as proteins or blood cells, in the feed fluid.
What is required is a membrane filter which uses laminar secondary flows to provide gentle, efficient mixing, which can be scaled up easily and which requires modest pumping power.
An approach which goes some way to meeting these requirements is to induce a force in a direction tending to throw the particles being filtered away from the membrane to thus prevent clogging of the membrane. In a tubular filter this may be achieved by defining a helical path along which the fluid being filtered may flow. Such an arrangement is described in U.S. Pat. No. 3,768,660 which discloses a reverse osmosis cell comprising an elongate hollow porous core having an osmotic membrane on its surface. Fluid to be filtered is passed into the annulus formed between the core and an outer tubular shell. A flexible helical member is mounted within this annulus to thus define a helical path for the fluid flowing in the annular space. The circular flow thus induced creates a centrifugal force which acts to keep particles away from the membrane, to prevent clogging.
The present invention seeks to provide a filter which utilizes helical flow to induce centrifugal forces, but having improved performance by inducing a further component of motion in the fluid flowing around the helix, namely a corkscrew vortex component, which has been found to give much improved flushing of the membrane surface whilst at the same time maintaining substantially laminar flow to prevent damage to the particulate components in the fluid being filtered.
According to the present invention, there is provided a filter comprising a tubular, substantially cylindrical, porous membrane arranged coaxially with, and radially spaced from, a generally cylindrical profiled surface, which surface is formed with at least one helical groove, the arrangement being such that, in use, a fluid to be filtered is passed from one axial end along the passage defined between the membrane and the profiled surface. The invention is characterised in that means are further provided for inducing or enhancing, in the helical flow of the fluid to be filtered, a corkscrew vortex flow. It will be understood that the word corkscrew is intended to give the idea of a corkscrewing motion of the fluid in the existing direction of the helical flow--in other words a further helical component of fluid motion in addition to the principal helical flow defined by the helical groove. Preferably the vortex flow is such as to substantially fill the whole cross section of the helical flow path of the fluid being filtered. This is achieved by careful design of the geometry of the tubular membrane and profiled surface. In particular, those parts of the profiled surface where it most closely approaches the membrane between adjacent turns of
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Pitera, Edward W. et al., "Convection Promotion in Tubular Desalination Membranes", Ind. Eng. Chem. Process Des. Develop., vol. 12, No. 1, 1973.
Fortuna Ana
Isis Innovation Limited
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