Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
1998-05-28
2001-08-21
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S493400, C210S321740, C210S321830
Reexamination Certificate
active
06277282
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved reverse osmosis membrane element capable of separating and concentrating various liquids, a reverse osmosis membrane apparatus using said reverse osmosis element, and a method for producing a separated liquid using said apparatus. The improved reverse osmosis membrane element of the present invention allows the treatment of a raw liquid throughout a wide pressure range from a low pressure to a high pressure without lowering the performance of the reverse osmosis membranes. The reverse osmosis element has good pressure resistance and durability.
DESCRIPTION OF THE PRIOR ARTS
Reverse osmosis membrane apparatuses used for separating or concentrating a desired component of a liquid include a spiral type apparatus in which reverse osmosis membranes are spirally wound and this overlaid on themselves with predetermined clearances kept between them, a plate and frame type in which a plurality of reverse osmosis membranes are overlaid over each other with predetermined clearances kept between them, and a hollow fiber bundle type apparatus in which hollow fibers formed of reverse osmosis membranes are bundled with predetermined clearances kept between them.
Among these, especially spiral type reverse osmosis membrane arrangements are most popularly used, since reverse osmosis membranes with a large membrane area can be packed in a limited volume.
Referring to the general structure of a spiral reverse osmosis membrane apparatus, one module is used in which a liquid passage conduit for permeated liquid is installed between two reverse osmosis membranes while a supplied liquid passage conduit is installed outside either of the reverse osmosis membranes and is wound and overlaid a plurality of times around a liquid collecting pipe, which is a hollow pipe provided with liquid collecting holes. This structure will be explained later again in reference to
FIGS. 1 and 2
.
Usually such a reverse osmosis membrane apparatus is used with a differential pressure of about 0.5 to 7 MPa kept working between the supply liquid side and the permeated liquid side for efficient liquid separation or concentration. At the working pressure, the permeated liquid passage conduit is required to support the reverse osmosis membranes without impairing the performance of the reverse osmosis membranes and to enhance the passage of the permeated liquid.
For this reason, as a permeated liquid passage conduit of a reverse osmosis membrane apparatus used at a relatively low pressure, for example as proposed in Japanese Patent Laid-Open (Kokai) No. 62-35802, a knitted single tricot fabric, made rigid by resin impregnation or heat fusion, is used. The knitted single tricot fabric has a plurality of grooves formed at certain intervals on the surface of one side of the fabric. The grooves form the main channels the passage that delivers of the permeated liquid.
In most of the presently practically used spiral configuration reverse osmosis membrane apparatuses, a knitted single tricot fabric is used to retain the shape of the permeated liquid passage. In this case, as a means to reduce the resistance to the flow of the permeated liquid, the grooves are as wide as about 300 to 600 &mgr;m, serving to to widen the permeated liquid passage area.
However, since this groove width is large for the usually used reverse osmosis membrane with a thickness of 200 to 300 &mgr;m, it happens that the reverse osmosis membrane is often deformed and caused to be depressed at the grooves blocking the permeated liquid passage when pressure is applied. The conventional spiral reverse osmosis membrane apparatuses cannot avoid the deformation of reverse osmosis membranes and are likely to decline in performance including the flow rate of the permeated liquid, the rejection of the intended substance and the concentration rate.
When the rigidity of a reverse osmosis membrane is not so high, it may be deformed at a relatively low pressure, and in the long run, performance declines even at a pressure of about 1 MPa to 2 MPa. Especially in such applications as sea water desalination and food concentration and separation where a high pressure must be applied, it is found difficult to use such an apparatus. The deformation of a reverse osmosis membrane is described later again in reference to FIG.
3
.
In a reverse osmosis membrane apparatus, to improve performance such as the flow rate of the permeated liquid, the rejection of any intended substance and the concentration rate, it is desired that the reverse osmosis membrane shell be deformed less at the pressure exerted by the raw liquid, and that the permeated liquid flow resistance by the permeated liquid passage conduit is kept small.
As a method for satisfying the requirement, Japanese Patent Laid-Open (Kokai) No. 54-31087 proposes a reverse osmosis membrane element used at a high pressure in which a 70 to 400 &mgr;m thick perforated polyester sheet having small holes of 50 to 1000 &mgr;m in diameter at about 0.1 to 200 mm intervals is provided between the reverse osmosis membrane and the grooved surface of the knitted single tricot fabric, to prevent the deformation of the reverse osmosis membrane at a high pressure for lessening the permeated liquid flow resistance. The structure will be described later again in reference to FIG.
4
.
If a perforated sheet is provided on the grooved surface of the knitted single tricot fabric, the rigidity of the perforated sheet prevents that depression of the reverse osmosis membrane at the grooves, for preventing the increase of the permeated liquid flow resistance in the permeated liquid passage formed by the grooves.
However, since the permeation area is restricted by the small holes of the perforated sheet when the permeated liquid passes through the perforated sheet, the permeation resistance increases to several to ten-odd times compared to a case without using any perforated sheet, and causes pressure loss, and at a low working pressure, the performance of the reverse osmosis membrane cannot be sufficiently manifested.
To decrease the liquid permeation resistance of the perforated sheet, it is necessary to increase the diameter of the small holes formed in the perforated sheet or to decrease the hole intervals for increasing the number of small holes. However, if the diameter of small holes is larger than the thickness of the reverse osmosis membrane, the reverse osmosis membrane is depressed into the small holes at a high pressure, and is damaged, declining in performance disadvantageously.
On the contrary, if the diameter of the small holes is kept smaller than the thickness of the reverse osmosis membrane, to prevent the depression of the reverse osmosis membrane into the small holes, the liquid permeation resistance acting when the liquid passes through the small holes becomes extremely large, in approximate inverse proportion to the 4th power of the diameter of the small holes. To inhibit the increase of permeation resistance, a very large number of small holes is necessary, making the production of the perforated sheet very difficult., and at the same time, the rigidity of the perforated sheet itself is lowered by the increase of small holes, making it difficult to obtain stable performance.
The reverse osmosis membrane element proposed in said Japanese Patent Laid-Open (Kokai) No. 54-31087 has these problems, and any a reverse osmosis membrane element which can be used stably at a high pressure is not developed yet.
A reverse osmosis membrane usually consists of three layers; a substrate formed of a fabric such as taffeta or nonwoven fabric, a porous supporting layer formed on it by a polymer such as a polysulfone or polyacrylonitrile, and a separation layer capable of separating a liquid formed on it. If the fabric used as the substrate is made thicker to prevent the deformation of the reverse osmosis membrane, the deformation can be prevented, and the reverse osmosis membrane is less likely to be damaged. However, since the fabric is thick, it lowers the porous supporting layer
Kihara Masahiro
Nishimura Kazuhiko
Yamamura Hiroyuki
Fortuna Ana
Schnader Harrison Segal & Lewis LLP
Toray Industries Inc.
Ward Richard W.
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