Note: Descriptions are shown in the official language in which they were submitted.
CA 02247187 2004-04-30
Separation Method and Apparatus
Field of the Invention
The present invention relates to a method and device based on cross flow
membrane
filtration for separation of separable constituents from a liquid medium.
Related Art
In a cross flow filter a liquid is supplied to a chamber comprising at least
one dynamic
membrane. The liquid being supplied is divided into a permeate flow
penetrating the
dynamic membrane and a reject flow flowing along the filter without
penetrating said
filter.
Cross flow filter apparatuses are described in the patent literature, for
example, in the
Swedish Publication Prints SE 451429, 457607, 459475, 463241 and 465040.
Methods based on the use of such apparatuses are successfully applied for
separation
of solid constituents from a liquid phase and for separation of liquid from
liquid, for
example, for separation of oil from water.
Figure 1 shows a section of a separation device of the type disclosed in SE
451429.
The device comprises a chamber 10 in which plates 1 1 provided with center
holes 12
are arranged. A dynamic filter membrane 13 (also referred to just as "a
filter") is
fastened on the plates. A rotor shaft 14 provided with rotor wings 15 runs
through
the center holes of the plates. The liquid to be prepared is supplied into the
rotor
chamber through the channel or inlet 16 whereby part of it (the permeate)
passes
through the filter and is collected via channels 17 in the plate to be
eventually
discharged through a permeate outlet. The portion of the liquid flow (the
reject) which
does not permeate the filter is discharged through the channel of outlet 18.
By means
of the rotor wings 15 the liquid is pushed across the filter surface 13 and a
possible
porous layer applied onto it.
SE 459475 describes a separation device comprising a number of adjacently
arranged
units or cells according to SE 451429. The liquid to be treated is supplied
through a
joint channel formed by through-holes in the plates. The collected permeate is
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discharged through a common outlet and the reject is discharged through a
common
conduit for the outgoing reject.
The filter membrane itself can also be provided' with one or more layers of a
porous
material, for example, of the kind described in the above mentioned patent
publications.
Clogging of the pores of the filter membrane or the porous material applied
onto it due
to deposit of separated constituents is a big problem. Due to the clogging the
separation capacity rapidly diminishes and because of this it is necessary to
regenerate the filter with regular intervals. According to the publication SE
451429
the regeneration is carried out in such a way that elements (brushes, valves
or the
like) connected to the rotor wings are brought into contact with the porous
layer on
the filter membrane. Hereby the constituents separated from the liquid flow
are
removed and a portion of the porous material as well. New porous material is
added
when necessary.
Mechanical regeneration of this type, which involves touching of the filter
membrane
or the porous layer on it, is always connected with the risk of disturbing the
wanted
pore size. Therefore it is highly desirable to avoid regeneration methods of
this kind.
Summary of the Invention
According to the present invention a separation method and a separation device
is
obtained in which the filter is kept free of deposition of separated
substance.
The advantages of the invention are based on the supplied liquid medium being
subjected to a strong turbulence created by a suitably formed rotor, passibly
in
connection with sound waves, preferably ultrasound, and/or an electric field,
whereby
the depositition of separable particles on the filter is prevented or
essentially reduced.
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According to one embodiment of the rotor it is designed in such a way that the
essentially plane surfaces of the wings directed towards the filter are
provided with
irregularities deviating from the plane of the wing surface. The
irregularities may, for
example, be through- holes, recesses or slots, or bosses.
According to another advantageous embodiment the surface of the rotor wings
directed towards the filter is curved so that the distance between the surface
of the
wing and the filter is small at the inner point of the rotor and that said
distance
gradually grows towards the periphery of the rotor.
According to a third embodiment the rotor wings are designed in such a way
that they
in a plane parallel to the filter form an S-shaped piece, whereby the S-shape
is
continuous.
An especially advantageous embodiment of the rotor is achieved by combining
the S-
shape with any of the above mentioned embodiments of the surface of the rotor
directed towards the filter.
According to another embodiment, a method for separation of constituents,
separable
in a filter, from a liquid medium containing such constituents, whereby a
separation
device is used comprising a cell or a stack of a plurality of adjacent cells,
whereby
each cell is provided. The method comprises: a plate unit containing at least
one
filter; at least one inlet for supplying the liquid medium into a chamber; at
least one
outlet far discharging part of the liquid medium as reject, whereby said inlet
and outlet
are positioned on a first side of the filter; at least one outlet on a second
side of the
filter for discharging a liquid medium as permeate which has passed through
the filter,
whereby the liquid medium supplied to the chamber is subjected to a strong
turbulence created by a rotor; whereby deposition of separable constituents on
the
filter is reduced, wherein the rotor comprises a plurality of
circumferentially distanced
rotor wings having one of following shapes an essentially plane surface of the
rotor
wings directed towards the filter comprises at least one area deviating from
the plane
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of said surface the surface of the rotor wings directed towards the filter is
curved and
the distance between the surface of the wing and the filter is small at an
inner
portion of the rotor and that said distance gradually increases towards a
periphery of
the rotor, and a section of the rotor wings in a plane parallel with the
filter forms an
essentially S-shaped surface, whereby the S-shape is continuous.
According to another embodiment, a device for separation of constituents,
separable
in a filter, from a liquid medium containing such constituents, whereby said
device
comprises a cell or a stack of a plurality of adjacent cells, whereby each
cell is
provided. The device comprises: a plate unit provided with a through going
center
hole containing at least one filter; at least one inlet for supplying the
liquid medium
into a chamber; at least one outlet for discharging part of the liquid medium
as reject,
whereby said inlet and outlet are positioned on a first side of the filter; at
least one
outlet on a second side of the filter for discharging a liquid medium as
permeate which
has passed through the filter, and a rotor positioned on the first side of the
filter, said
rotor comprising a plurality of circumferentially distanced rotor wings which
are
movable along a surface of the filter, whereby the rotor includes a rotor
shaft running
through the center hole of the plate unit, whereby rotational movement of the
rotor
wings causes strong enough turbulence in surrounding liquid medium to reduce
precipitation of separable constituents on the filter, wherein said rotor
wings having
one of following shapes an essentially plane surface of the rotor wings
directed
towards the filter comprises at least one area deviating from the plane of
said surface,
the surface of the rotor wings directed towards the filter is curved and a
distance between the surface of the wing and the filter gradually increases
from an
inner portion of the rotor towards a periphery of the rotor, and a section of
the rotor
wings in a plane parallel with the filter forms an essentially S-shaped
surface,
whereby the S-shape is continuous.
Brief Description of the Drawings
The invention will be described in greater detail with reference to the
accompanying
drawings, wherein
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Figure 1 shows a section of a separation device according to prior art,
Figure 2 shows a previously known rotor intended for the device in Figure 1,
Figure 3 shows a rotor according to the present invention in one embodiment,
Figure 4 shows a rotor in a second embodiment,
Figure 5 shows a rotor in a third embodiment,
Figures 6A to 6C show a cross section of the rotor wings in Figure 4,
Figures 7A to 7D show a cross section of the rotor wings according to a
further
embodiment, and
Figure 8 shows an embodiment of the rotor wing according to Figure 3.
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Detailed Description of the Preferred Embodiments
Figure 2 shows a previously known rotor viewed against the plane of the filter
13.
The rotor consists of two rotor wings 15 fastened to the rotor shaft 14. The
rotor
wings 15 are massive, almost rectangular prisms.
Figure 3 shows an example of a rotor according to the invention. The rotor
wings 15
are curved in such a way that their section in a plane parallel with the
filter 13 forms
an S-shaped surface. When operating the rotor rotates in the direction of the
arrow.
Rotors of the known type according to figure 2 mainly cause a liquid flow in
the
rotation direction. In test runs with a rotor according to figure 3 it has
been shown
that the concave sections on the rotor wings cause a strong liquid flow also
in the
radial direction. When a conventional rotor is being used the circular speed
diminishes
when the rotor shaft is approaching. As shown in the figure the rotor forms a
continuous S-form projecting diametrally through the rotor shaft. Due to the
radial
liquid flow caused by said continuous S-form a higher turbulence is achieved
also in
the vicinity of the rotor shaft. As a result of this a sufficiently strong
turbulence is
achieved over the entire filter surface in the surrounding liquid medium in
order to
prevent separable constituents from depositing on the filter. Another
advantage of the
S-form is that the convex portions of the rotor wings encountering the
surrounding
liquid medium are effectively kept free from deposits. An essential advantage
of said
S-form is that the pumping effect of the rotor, which in this connection is
undesirable,
will be compensated for by the increased liquid turbulence.
Figure 8 shows an especially advantageous embodiment of the wing profile
according
to figure 3. The convex profile 15a of the rotor wing is formed so that the
angle a
between the line L drawn from the shaft 14 to an arbitrary point P on the
profile 15a
and the tangent T to the profile 15a in said point, is always almost constant
and thus
independent of the position of said point P. The angle a is preferably between
45 ° to
80°, advantageously about 60°. Due to this embodiment the convex
surface is kept
free from deposits at the same time as the rotor needs very little power.
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Figure 4 shows another shape of the rotor wings. The rotor wings 15 are bodies
essentially shaped as prisms and their surface directed towards the filter 13
is
essentially plane. Said surface is provided with one or more areas 20
deviating from
the plane of said surface. Figures 6A to 6C, which show a cross section of the
rotor
wings in figure 4, show alternative shapes of the irregularities 20. In figure
6A said
irregularities consist of bosses 21, in figure 6B of recesses 22 and in figure
6C of
through-holes 23. The irregularities 20 of the rotor wings may also be a
combination
of one or more holes 23, one or more recesses 22 and/or one or more bosses 21.
The presence of irregularities, especially recesses or through-holes, reduces
the
unwanted pumping effect of the rotor due to higher turbulence. Simultaneously
the
recesses and holes cause a smaller contact surface between the rotor blade and
the
surrounding liquid which in its turn reduces friction and thus also the need
of power.
Figures 7A - 7D, which show a cross section of the rotor blade 15, show an
especially advantageous embodiment of the bosses 21 which in this case are
composed of another material than the rotor wing itself. The bosses 21 consist
of
separate bodies mounted in holes running through the rotor blade.
Alternatively said
bodies can be mounted in recesses in the rotor blade or be fastened onto its
surface.
Said bodies can be lipuid or gas impermeable compressable pieces of, for
example,
foam plastic with closed cells, a gas-filled cover etc. The advantage of this
construction is that it is possible to act on the size of the bodies by means
of the
surrounding pressure. It is, for example, possible to choose such a material
for the
body 21 which is strongly compressed at the normal working pressure of the
separation device (Fig. 7A), whereby the boss above the plane of the rotor
surface is
very slight or non-existent and thus the power need of the rotor is slight.
When
needed the working pressure can be changed to p 1, which is lower than p0,
whereby
the body 21 expands (Fig. 7B). Hereby the distance to the surface of the
filter is
reduced and the turbulence increases. Therewith the cleansing effect of the
filter
membrane increases. It is also possible that the body 21 expands so much that
it will
come into contact with the filter membrane whereby mechanical cleansing is
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achieved. It is also possible to fet the bodies 21 act as regulating means
acting on
mechanical devices 24 in the form of flexible elements or the like attached to
the rotor
blade which elements in their turn come into contact with the filter membrane
(figures
7C and 7D).
Figure 5 shows a rotor type according to a third embodiment in a section
perpendicular to the plane of the filter 13. The surface 19 of the rotor wing
15 which
is directed towards the filter 13 is curved in such a way that the distance
between
the surface of the wing and the filter is small at the inner portion of the
rotor, i.e. in
the vicinity of the rotor shaft 14 and that said distance gradually grows
towards the
periphery of the rotor. By means of this embodiment of the rotor wings it is
possible
to achieve a good and even turbulence over the entire filter surface in the
liquid
medium positioned in the chamber 10. When rotor wings with an even thickness
are
being used the circular speed (and thereby turbulence) is low in the vicinity
of the
rotor shaft. By reducing the distance between the rotor surface 19 and the
filter 13
according to the construction in figure 5 in this area the turbulence is
increased. The
circular speed and thereby also the turbulence is high at the periphery of the
rotor
wings. In this area it is advantageous to increase the distance between the
surface 19
of the rotor wing and the filter 13 in order to further counteract increase of
the
turbulence. If the distance between the rotor wing and the filter were to be
small also
at the periphery of the wing this would lead to an undesired power increase in
order
to achieve the rotation.
The rotor wings according to figure 5 can also be provided with irregularities
of the
same kind as the rotor wings according to figure 4, for example through-holes,
bosses
and recesses.
It is known that the use of sound waves, especially ultrasound, furthers
filtration and
prevents clogging of the filter pores (see e.g. T Touri et al., "Filtration
technique
enhanced by elektro-acoustic methods", Process Tecnology Program, TEKES
report,
Jyvaskyla 31 March 1995). The rotor formed according to the present invention
may
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also be combined with a sound wave transmitter, especially ultrasound, and~or
devices for obtaining an electric field. The combination of sound waves and
electric
fields which are designated as an electro-acoustic method, has proven to be
very
efficient in preventing filters from getting clogged during a filtration
process. A highly
effective filtration is achieved by means of an electro-acoustic apparatus in
combination with the special rotor design described above.
The separation device according to the present invention can be used to
separate both
solid particles, solved high- molecular substances and colloids from a liquid
and to
separate a liquid from another liquid.
The filter material is chosen on the basis of the separation process. The
filter
membrane can, for example, be a porous textile cloth, a paper cloth or a
membrane of
the type described in one of the patent publications mentioned in the
introduction.
The filter membrane itself can also, if wished, be provied with one or more
layers of a
porous material, e.g. of the type described in the above patent publications.
The filter
may also comprise a ceramic filter known per se or some other rigid and porous
material.
It is evident to a person skilled in the art that the various embodiments of
the
invention may vary within the scope of the enclosed claims.
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