Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
1997-12-08
2001-02-20
Walker, W. L. (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S321760, C210S321770, C210S321850, C210S321860, C210S407000, C210S411000, C210S437000, C210S439000, C210S448000, C210S457000, C210S487000, C210S493100, C210S497010, C210S772000
Reexamination Certificate
active
06190557
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spiral wound type membrane element used in a membrane separation device such as a low-pressure reverse osmosis membrane separation device, an ultrafiltration device or a microfiltration device, and a method for running the same and a method for washing the same.
2. Description of the Background Art
With the recent trend toward applications of membrane separation technology to water-purification technology, the membrane separation technology is being applied as pretreatment for reverse osmosis membrane separation systems used to turn salt water into fresh water, for example. While microfiltration membranes and ultrafiltration membranes which provide large permeate flow rates are mainly used for such membrane separation, reverse osmosis membranes providing large permeate flow rates at ultra-low pressures of 10 kgf/cm
2
or lower are being developed these days.
As for membrane elements used for this kind of membrane separation, hollow fiber membrane elements are often used in view of membrane area per unit volume (volumetric efficiency). However, the hollow fiber membrane elements have the disadvantage that the membranes are easily broken. If the membrane is broken, raw water is mixed into permeate to lower the separating performance.
As for membrane elements providing large membrane area, there are spiral wound type membrane elements. As compared with the hollow fiber membrane elements, the spiral wound type membrane elements are more advantageous in that they can maintain high separating performance and thus provides higher reliability.
FIG. 8
is a partially cutaway perspective view of a conventional spiral wound type membrane element and
FIG. 9
is an external perspective view of the conventional spiral wound type membrane element.
As shown in
FIG. 8
, the spiral wound type membrane element
21
includes an envelope-like membrane (a bag-like membrane)
23
formed by putting separation membranes
26
on both sides of a permeate spacer
25
and bonding them together on three sides. The opening of the envelope-like membrane
23
is attached to a water collection pipe
22
formed of a perforated hollow pipe, and it is spirally wound around the water collection pipe
22
together with a net-like raw water spacer
24
.
The raw water spacer
24
is provided to form a passage through which the raw water passes between the envelope-like membrane
23
. If the thickness of the raw water spacer
24
is small, the separation membranes
26
can be efficiently accommodated but they will suffer from clogging with suspended substances. Accordingly, usually, the thickness of the raw water spacer
24
is set to about 0.7 to 3.0 mm.
A spiral wound type membrane element using a corrugating type raw water spacer (a so-called corrugating spacer) is already known, which is formed in a zig-zag shape to treat raw water, e.g., river water, containing a large quantity of suspended substances.
As shown in
FIG. 9
, the peripheral surface of the spiral wound type membrane element
21
is covered by a sheath
27
formed of FRP (Fiber-Reinforced Plastics), a shrink tube, or the like, whose two ends are each equipped with a packing holder
28
called an anti-telescope.
FIG. 10
is a cross section showing an example of a method for running the conventional spiral wound type membrane element. As shown in
FIG. 10
, a pressure vessel (a pressure-resisting vessel)
30
is formed of a tubular case
31
and a pair of end plates
32
a
and
32
b
. One end plate
32
a
has an inlet
33
for raw water and the other plate
32
b
has an outlet
35
for concentrate. The other end plate
32
b
also has an outlet
34
for permeate in the center.
The spiral wound type membrane element
21
, to which a packing
37
is attached on the peripheral surface in the vicinity of one end, is accommodated in the tubular case
31
and both of the opening ends of the tubular case
31
are sealed with the end plates
32
a
and
32
b
. One opening end of the water collection pipe
22
is engaged with the permeate outlet
34
in the end plate
32
b
, and an end cap
36
is attached to the other opening end thereof.
When running the spiral wound type membrane element
21
, raw water
51
is introduced from the raw water inlet
33
of the pressure vessel
30
into a first liquid chamber
38
. As shown in
FIG. 8
, the raw water
51
is supplied from one end of the spiral wound type membrane element
21
. The raw water
51
flows in the axial direction along the raw water spacer
24
and is discharged as concentrate
53
from the other end of the spiral wound type membrane element
21
. The raw water
51
passed through the separation membranes
26
while flowing along the raw water spacer
24
flows into the water collection pipe
22
as permeate
52
along the permeate spacer
25
and is discharged from the end of the water collection pipe
22
.
The permeate
52
is taken out from the permeate outlet
34
of the pressure vessel
30
shown in FIG.
10
. The concentrate
53
is taken out through the concentrate outlet
35
from a second liquid chamber
39
in the pressure vessel
30
.
When the membrane element is operated, the membrane is clogged with suspended substances in the raw water, which reduces the flux of permeate. Then the clogging substances are removed by chemical washing to recover the flux of permeate, which raises the problem that the chemical washing requires troublesome work and cost. Accordingly, with a hollow fiber membrane element, for example, it is periodically cleaned by back wash reverse filtration with permeate or air to prevent clogging. However, applying back wash reverse filtration to the conventional spiral wound type membrane element
21
causes the following problems.
FIG. 11
is a partially cutaway perspective view showing back wash reverse filtration operation with the conventional spiral wound type membrane element. As shown in
FIG. 11
, permeate
52
is introduced from an end of the water collection pipe
22
. Since the peripheral surface of the envelope-like membrane
23
wound around the water collection pipe
22
is covered with the sheath
27
, the permeate guided out from the peripheral surface of the water collection pipe
22
permeates through the envelope-like membrane
23
to flow in the axial direction inside the membrane element
21
along the raw water spacer
24
and is discharged from the end of the membrane element
21
. Hence, contaminants such as suspended substances causing clogging are likely to be caught by the raw water spacer
24
before discharged to the end of the membrane element
21
, causing the problem that they are not sufficiently removed.
Furthermore, as shown in
FIG. 10
, the gap between the inner peripheral surface of the tubular case
31
of the pressure vessel
30
and the spiral wound type membrane element
21
forms a dead space S, which causes the fluid to stay (fluid stay). When the spiral wound type membrane element
21
is used in a long time, the fluid staying in the dead space deteriorates. Especially, if the fluid contains organic matter, various germs such as microorganisms propagate and decompose the organic matter to produce a bad smell, or may decompose the separation membranes, leading to reduction in reliability.
Moreover, since the raw water is supplied from one end of the spiral wound type membrane element
21
and is discharged from the other end, the conventional spiral wound type membrane element
21
requires the packing holders
28
to prevent the envelope-like membrane
23
wound around the water collection pipe
22
from being transformed into a shape like bamboo shoots. Further, pressure loss due to the raw water spacer
24
and pressure loss due to clogging produce a difference in pressure between the inflow of raw water and the outflow of concentrate, which deforms the spiral wound type membrane element
21
. In order to prevent deformation, the peripheral surface of the envelope-like membrane
23
wound around the water collection pipe
22
is covered
Hisada Hajimu
Nishida Yuji
Akin Gump Strauss Hauer & Feld L.L.P.
Fleming Michael A.
Nitto Denko Corporation
Walker W. L.
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