Spiral reverse osmosis membrane element, reverse osmosis...

Details Spiral reverse osmosis membrane element, reverse osmosis... Spiral reverse osmosis membrane element, reverse osmosis...

2000-02-18

2003-12-02

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S641000, C210S321740, C210S321830, C210S493400, C210S195200

Reexamination Certificate

active

06656362

ABSTRACT:

This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/JP99/03274 which has an International filing date of Jun. 18, 1999, which designated the United States of America.
TECHNICAL FIELD
The present invention relates to a spiral reverse osmosis membrane element capable of producing permeated water with a small quantity of input energy at a low operating cost, a high salt rejection and a high water permeation rate or water flow rate when applied to seawater desalination, and relates to a reverse osmosis membrane module using the aforementioned element, a reverse osmosis separation apparatus incorporating therein the module, and a reverse osmosis separation method.
BACKGROUND ART
A reverse osmosis separation method is a method for obtaining permeated liquid having a low solute concentration by passing a solution through a reverse osmosis membrane at a pressure higher than its osmotic pressure to thereby separate and remove part of a solute. The reverse osmosis separation method is adopted in technical fields such as seawater desalination, brackish water desalination, production of ultrapure water, concentration of waste water, and recovery of valuables from waste water.
Especially in seawater desalination, the reverse osmosis separation method is coming into wide use since it does not entail a phase change, needs less energy and is easier to control as compared with a conventional evaporation method.
To perform the reverse osmosis separation method, a spiral reverse osmosis membrane element, for example, is used whose structure will be described below referring to drawings.
FIG. 1
is a schematic perspective cutaway view of the element, and
FIG. 2
is a cross sectional view taken along line II—II of FIG.
1
.
The element has a hollow pipe
1
arranged at the center of the element and having a surface thereof formed with a plurality of through-holes
1
a
. Reverse osmosis membranes
2
, permeated liquid passage members
3
, and feed liquid passage members
4
are wound around the outer surface of the hollow pipe
1
in a manner described below.
Each reverse osmosis membrane
2
has a bag-like shape as a whole, and a permeated liquid passage member
3
is arranged therein. The bag-shaped reverse osmosis membranes
2
are attached to the outer surface of the hollow pipe
1
with their openings
2
a
enclosing through-holes
1
a
formed in the hollow pipe
1
so that the interior of the reverse osmosis membranes
2
and the permeated liquid passage members
3
may communicate with the through-holes
1
a.
Each feed liquid passage member
4
is arranged between reverse osmosis membranes
2
associated therewith, and frame members
5
configured to allow liquid to pass therethrough are attached to both ends of the membrane and passage member assembly, whereby the spiral structure is attained.
The above-mentioned element is arranged in a pressure vessel and is adapted to be supplied at its one end (upstream side) with feed liquid
6
at a predetermined pressure.
As the feed liquid
6
flows along the feed liquid passage members
4
, it undergoes reverse osmosis separation by the reverse osmosis membranes
2
, to be separated into permeated liquid and a solute. The permeated liquid, passing through the reverse osmosis membranes
2
and having a low solute concentration, flows into the through-holes
1
a
and gathers in the hollow pipe
1
. The permeated liquid
6
a
is then taken out from the downstream side of the element.
The feed liquid which has not passed through the reverse osmosis membranes
2
continues flowing along the feed liquid passage members
4
to the downstream side. In the course of flowing, the feed liquid takes in the solute separated from the feed liquid and left on the membrane surfaces, to become concentrated liquid
6
b
having a high solute concentration.
There is a critical problem in operating the element such that the element performance lowers due to concentration polarization.
The concentration polarization is a phenomenon that fouling substances, such as impurities and contaminants contained in the feed liquid, are enriched on the membrane surfaces of reverse osmosis membranes
2
which are in contact with feed liquid passage members
4
, so that the solute and fouling substance concentration of the feed liquid becomes higher on the membrane surface. As a result, the osmotic pressure becomes higher.
When the concentration polarization occurs, the quantity of permeated liquid decreases, and impurities such as gel and scale precipitate on the membrane surface. For this reason, the reverse osmosis membrane cannot develop its capability and the performance of the element lowers.
The occurrence of the concentration polarization can be suppressed by making the flow of the feed liquid on the membrane surface turbulent. For example, the turbulent flow occurs more easily by using the feed liquid passage member
4
of a smaller thickness to increase the linear velocity of the feed liquid on the membrane surface, so that the concentration polarization layer may be thinned.
With the feed liquid passage member
4
having a smaller thickness, however, the passage defined by the feed liquid passage member
4
is easily clogged with fouling substances contained in the feed liquid such as impurities and microorganisms. As a result, the element performance lowers and the pressure loss in the feed liquid increases. To keep up the quality and quantity of permeated liquid, the operating pressure for the feed liquid needs to be raised, and hence a high-pressure pump requiring electric power to operate and pressure pipes must be provided, resulting in increased liquid production costs.
Conventionally, a net member having a net structure as shown in
FIG. 3
is widely used as a feed liquid passage member
4
.
The net member has meshes in the form of series of quadrilaterals formed by linear members
7
a
and
7
b
crossing each other. The net member is arranged between reverse osmosis membranes
2
and is wound around the hollow pipe in a manner that opposite two cross-points
7
c
,
7
d
out of four cross-points
7
c
,
7
d
,
7
e
,
7
f
of the linear members
7
a
,
7
b
are in line in the flow direction of the feed liquid
6
, i.e., in parallel with the axial direction of the hollow pipe
1
. This kind of net structure is effective in decreasing the concentration polarization, because the feed liquid
6
flowing along the net structure forms a turbulent flow effectively.
In this conventional net member, the quadrilateral mesh is formed into a square shape, that is, the four sides of the quadrilateral mesh have the same length, and the distance (X) between the cross-points
7
e
,
7
f
is equal to the distance (Y) between the cross-points
7
c
,
7
d
. The absolute value of an angle (&agr;) between the line connecting the cross-points
7
c
,
7
d
and the linear member (mesh leg)
7
a
(
7
b
) is 45°.
When the conventional net member having the size and shape described above is used as a feed liquid passage member, the flow of the feed liquid can be made turbulent. However, there is a problem that the pressure loss in the feed liquid increases, so that the operation cost and the equipment cost may increase.
Japanese Provisional Patent Publication No. 5-168869 discloses a special net member which comprises first linear members arranged in parallel with the flow direction of the feed liquid and second linear members crossing the flow direction of the feed liquid at an angle smaller than 45°. When this net member is used as a feed liquid passage member, the pressure loss in the feed liquid can be decreased as compared with the aforementioned net member, but the quality and quantity of permeated liquid lower. Further, the pressure-loss decreasing effect produced is too small to be practical under a flow velocity condition of the feed liquid in the actual operation. Furthermore, this net member is expensive because it requires an advanced net-making technique.
A tricot member is generally used as a permeated liquid passage member
3
, w

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