Liquid purification or separation – Casing divided by membrane into sections having inlet – Cylindrical membrane
Reexamination Certificate
2000-02-28
2002-03-26
Kim, John (Department: 1723)
Liquid purification or separation
Casing divided by membrane into sections having inlet
Cylindrical membrane
C210S321790, C210S321800, C210S321880, C210S323200
Reexamination Certificate
active
06361693
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to fluid filters with a filter unit of a porous ceramic medium, and particularly to those useful in minute or micron fluid filtration, ultra fluid filtration and reverse-osmosis fluid separation, and gas separation.
2. Description of the Background Art
Conventionally, in processes for manufacturing foods, chemicals, semiconductors and the like, filter units (also referred to simply as “units” hereinafter) formed of porous organic membranes are used to filter fluids. An organic membrane, however, is less resistant to heat, pressure and chemicals and it is thus being replaced with a membrane formed of a porous ceramic medium which is highly resistant to heat, pressure and chemicals. Furthermore, in recent years such ceramic membrane has also been applied to a bio-reactor such as a carrier for microorganism incubation, a catalyst carrier, and the like.
Recently there has been an increased demand particularly for such a porous ceramic membrane that has a significantly high permeability and also has a high resistance for stress capable of resisting a pressure of a fluid even if the membrane has a reduced thickness. In order for a membrane to have an enhanced permeability the membrane must have its porosity increased to reduce its resistance given to a fluid permeating therethrough and also have its pores reduced in diameter to allow finer components in the fluid to be separated. In other words, the membrane requires enhanced separation capability. Normal ceramic conventionally used, such as alumina (Al
2
O
3
), can, however, only provide for a porous medium having a porosity of at most 40%, normally at most 30% or therearound. This results in more of pressure drop through the membrane when it is filtered. Thus such a unit as only making use of its pores is inefficient and thus does not meet the demand as described above.
To overcome such disadvantage, a porous ceramic medium can be molded in the form of a cylinder having a multi-layered structure to allow a fluid to in practice permeate through a membrane of a reduced thickness and thus minimize its pressure drop. More specifically, a collection of units, or a filter module (also referred to as a “module” hereinafter), formed of the porous ceramic medium, has a multi-layered structure configured of a porous membrane with filtration capability, a base supporting the membrane, and an intermediate, posed between the membrane and the base if required.
To put such modules to practical use, a unit must be designed to have a structure to allow a fluid permeating through a membrane to have an increased flow per unit permeation-area of the membrane and it also must have its external dimensions minimized. To achieve this goal, a monolith structure has been developed. Such unit has a cross section, as shown in FIG.
23
. In the figure, a filter unit
1
is in the form of a lotus root, with a porous ceramic medium
2
having provided therein a large number of channels or cells
3
passing a feed fluid. Note that a “feed fluid” refers to a fluid before it is filtered and a “permeated fluid” refers to a fluid after it is filtered. Exemplary monolith structures are disclosed in Japanese Patent National Publication Nos. 1-501534 and 3-500386 and Swiss Patent CH 681281. A monolith structure also basically has a multi-layered structure having a porous membrane and a base supporting the membrane. Providing a multi-layered structure internal to a channel of a module having such a cross section as shown in
FIG. 23
, however, requires a rather complicated process and hence an additional cost. Instead, if a porous medium of the above-mentioned normal ceramic is provided having a mono-layer structure to allow a fluid permeating through a membrane with small pressure drop, the membrane must have a reduced thickness. In practice it is no more than 1 mm, although such thickness significantly reduces the membrane's resistance for stress.
WO 94/27929 (Japanese Patent National Republication No. 06-827929) discloses a porous medium of silicon nitride based ceramic (a ceramic containing silicon nitride and/or sialon as a main component(s)) normally having a high porosity of no less than 50% as well as high resistance for stress. This porous ceramic medium is formed of silicon nitride and/or sialon grains each in the form of a column having an aspect ratio of no less than three, and of a binder of oxide, and the column grains of silicon nitride and/or sialon are bound together directly or via the binder of oxide to form a three-dimensional, randomly expanding network structure having a 3-point bending strength of at least 100 MPa. Such porous ceramic medium ensures that a thin membrane of a mono-layer structure, such as that described above, can have high resistance for stress. It should be noted, however, that simply replacing a monolith unit with a unit of silicon nitride based ceramic having a mono-layer structure, does not result in the unit having a filtered flow increased as expected, since as shown in
FIG. 23
by a curved line a fluid must move a long distance particularly from a channel located around the center of the unit to the outer periphery of the unit and the fluid thus results in more of its pressure drop, so that the unit has its total permeated flow determined mainly by the permeated flow only from the channels adjacent to the surface of the unit.
In processes for manufacturing foods, chemicals and the like, such monolith module is normally used in cross-flow filtration in various manners. In this filtration system, as shown in
FIG. 24
, a feed fluid
5
in a source-fluid vessel
4
is fed by means of a feed pump
6
to a filter provided with a module
1
, and a permeated fluid is discharged and thus collected through an outlet of the casing while a feed fluid that has passed through a channel or cell internal to the module is returned via a circulatory line
7
to the source-fluid vessel and re-fed to the filter repeatedly and the fluid is thus filtered.
The present inventors have also proposed in Japanese Patent Laying-Open No. 11-123308 a module having a small resistance against a permeating fluid and thus suitable for the above filtration system, formed of a thin membrane having a mono-layer structure of the above silicon nitride based ceramic medium.
For a filter employing such monolith filter module, it is important to increase its desired permeated flow (a flow rate of a permeated fluid per unit time, i.e., filtration rate). In order to do so, a collection of units, or a module, is required to pass a fluid with less of the fluid's pressure drop. However, the module is also required to have a membrane having high separation capability depending on its pores' diameter, as has been described above. Thus the two requirements must be well-balanced. Furthermore, since normally a large number of units are arranged and housed in a casing of a fixed volume their volumes cannot be arbitrarily increased to provide an enhanced permeated flow.
In order to meet the demands as above, it is essential to, with units arranged in a casing, (1) increase the amount of a fluid permeated per unit surface area of a membrane, (2) reduce the fluid's pressure drop caused at a discharging route, and to (3) increase the membrane's surface area effective in filtration. Exemplary modules with collected and arranged units are disclosed, e.g., in Japanese Patent Publication No. 6-11370 and Japanese Patent Laying-Open No. 5-146609, although they mainly address issue (3) only and also have each module providing for a limited permeated flow. On the other hand, the module proposed in Japanese Patent Laying-Open No. 11-123308 has its individual units overcoming issues (1) and (2), although it fails to satisfactorily address issue (3).
SUMMARY OF THE INVENTION
In order to overcome the above three disadvantages simultaneously, the present inventors, while making use of the module structure proposed in Japanese Patent Laying-open No. 11-123308, have tried to
Fasse W. F.
Fasse W. G.
Kim John
Sumitomo Electric Industries Ltd.
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