Filter comprising one or more ducts

Details Filter comprising one or more ducts Filter comprising one or more ducts

1997-09-09

2001-04-17

Savage, Matthew O. (Department: 1723)

Liquid purification or separation

Casing divided by membrane into sections having inlet

With membrane cleaning or sterlizing means

C210S321800, C210S321890, C210S490000, C210S496000, C210S510100, C264SDIG004

Reexamination Certificate

active

06217764

ABSTRACT:

This invention relates to filters, particularly, though not exclusively, for medium-scale applications such as water purification or food processing.
In my earlier WO 94/21362, I disclosed a way of enhancing the performance of tubular membrane filters by introducing helical flow deflectors to induce fluid mixing in the radial direction. One of the examples disclosed in the application is of a rod with a helical groove inserted concentrically within a tubular filtration membrane. Steady feed flow in the annular space between the impermeable helical insert and the concentric tubular permeable membrane provides excellent radial mixing. In a second example, the tubular filtration membrane lies concentrically inside a casing containing a helical groove. Flow patterns are created within the annular space between the impermeable casing and the cylindrical permeable membrane, which ensure high filtration performance and good mixing which prevents concentration polarisation. Both of these examples work well with a polymeric, permeable, membrane tube of diameter 12.5 mm, but it is difficult to scale up these apparatuses to provide membrane areas of the order of 1 to 10 m
2
for larger applications without resorting to bulky and expensive filter units.
Filters which have a large membrane area packed into a small volume for larger applications are available commercially. One such filter provides a large number of parallel capillaries, in a highly porous block of support material such as a ceramic, with a much tighter porous layer at the wall of each capillary. The manifolding of the capillaries for feed fluid entry and exit is provided by the porous block. Filtrate passes through the capillary walls and then through the highly porous block. The filtrate is then collected in suitable channels at the outer surface of the porous block (FIG.
1
). However, it is necessary to pump the feed flow through each capillary at velocities as high as 6 m/s in order to achieve reasonable mixing by turbulent flow and hence adequate filtration performance. Although this design is space-saving it requires very high flow rates, and hence pumping costs (both capital and running) are correspondingly high. Furthermore, damage to delicate components in the feed fluid, caused by turbulent flow is an additional disadvantage of this particular method.
One way of reducing the feed flow rate would be to place helical inserts in each tubular capillary in the porous ceramic block in a similar manner to that used in WO 94/21362. However, these capillaries generally have a diameter of 4 mm of less, and it is difficult to construct helical inserts of suitable geometry which are sufficiently robust and which are sufficiently rigid to avoid vibration and consequent damage to the capillary walls.
GB-A-2223690 discloses a filter comprising one or more substantially unobstructed ducts with porous walls, the or each duct having a helical groove in the wall; and, according to the present invention, such a filter is characterised in that the groove is a single-, double- or triple-start groove and the cross sectional area, or aggregate cross sectional area, of the groove, perpendicular to flow along the groove, is similar (as herein defined) to the cross sectional area of the lumen.
Unlike the prior art in which the porous membrane was of a cylindrical shape, the porous surface in this invention is of a helical shape. The helically grooved ducts thus provide excellent radial mixing and hence prevent concentration polarisation, but also provide increased membrane area, compared with conventional permeable membranes of circular cross-section. The present invention also avoids the high pumping costs of the prior art filter and the difficulty and expense of constructing helical inserts which cause vibration and damage to cylindrical membrane walls.
The duct(s) is (are) preferably in a porous block of support material, and a denser porous surface may be formed at the wall(s) of the duct(s).
The porous block lends itself to the convenient formation of a large member of ducts therein to create a large membrane area in a small volume for more efficient filtering. The filtrate can also be conveniently collected in a chamber or chambers at the outer surface of the porous block.
In use, the core-flow is through the central lumen of the duct with a helical flow along the duct wall which produces a vortex within the helical groove (FIG.
4
). Similar flow patterns are described in my WO 94/21362 except, of course, in the present invention the feed flow is into an open tube rather than into an annular space.
The helical groove may be single-start, or may be double- or triple-start, in order to reduce pressure drop along the ducts. Double-start helical grooves reduce the pressure drop along the duct by a factor of four and triple-start helical grooves reduce the pressure drop by a factor of nine when compared with a corresponding duct with a single-start helical groove, without compromising radial mixing. This is achieved because of the reduced length of each groove and the increased total number of sub grooves.
The lumen of the duct may be up to 20 mm in diameter, but is preferably between 3 and 5 mm in diameter with adjacent turns separated by a land, eg, substantially 1 mm wide.
Experiments have shown that the cross sectional area, perpendicular to the direction of flow along the helical groove, or the aggregate cross sectional area in the case of a multi-start groove, should be similar to the cross sectional area of the central cylindrical lumen. By similar is meant that the groove cross sectional area is within plus or minus 25%, and preferably within plus or minus 10%, of the lumen cross sectional area.
As seen in cross section perpendicular to the flow along the groove, the peripheral wall of the groove should be arcuate to promote a smooth flow pattern and an appropriate cross sectional shape is semi circular. Thus in the case of a groove cross section of semi circular shape, and applying the preferred requirement that the groove cross sectional area or aggregate cross sectional area is the same as that of the lumen, then if the diameter of the semi circular cross section of the groove is c and the diameter of the lumen is d:
n
·
(
π
⁢
 
⁢
c
2
)
8
=
π
⁢
 
⁢
d
2
4
when n is the number of groove starts.
Hence nc
2
=2d
2
.
Thus for a single-start:
c=
{square root over (2)}
d
for a double-start:
c=d
for a triple-start:
c=
{square root over ({fraction (2/3)})}
d
Thus if d=4 mm:
When n=1, c=5.6 mm and the groove depth is 2.83 mm.
When n=2, c=4 mm and the groove depth is 2 mm.
When n=3, c=3.28 mm and the groove depth is 1.64 mm.


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patent: 5006229 (1991-04-01), Espenan
patent: 5008012 (1991-04-01), Hagihara et al.
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patent: 5442968 (1995-08-01), Westlake, III et al.
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patent: 63-65908 (1988-03-01), None
patent: PCT/GB94/00561 (1994-09-01), None

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