Liquid purification or separation – Casing divided by membrane into sections having inlet – Each section having inlet
Reexamination Certificate
2001-03-19
2003-11-18
Walker, W. L. (Department: 1723)
Liquid purification or separation
Casing divided by membrane into sections having inlet
Each section having inlet
C210S321790, C210S321800, C210S500230, C210S500270, C210S500280, C216S056000, C427S244000
Reexamination Certificate
active
06649058
ABSTRACT:
DESCRIPTION
1. Technological Field
The invention relates to hollow membranes intended for the treatment of fluids (liquids and/or gases) with a view to separating from them one or more constituents by absorption, adsorption and/or transfer phenomena through a membrane produced in a material having properties specific to one or more of the treated fluids. It is also applicable to the transfer of material and/or heat between two fluids separated by said membrane.
The invention also relates to treatment modules for fluids that include such membranes. These modules can be used in various fields, for example, to wash acidic gases for the preparation of synthesis gases and to combat environmental pollution by purifying the gases from a furnace or by treating aqueous effluent.
The invention is also applicable to biological processes such as fermentation, the manufacture of proteins, biological oxidation processes as well to medical equipment such as blood oxygenators and artificial kidneys.
2. State of the Prior Technology
The membranes used for the treatment of fluids up to now are, either flat membranes or membranes in the form of hollow fibers.
For the latter, the possibility has been studied of producing them in the form of hollow fibers of small length and diameter as described in document 1:WO-A-95/00238. Limiting the length of the hollow fibers allows one, in particular to limit the pressure drop of the fluid circulating in them as is the case in natural capillary membranes such as those in the human lung. In effect, in these natural systems, the capillaries that supply the blood have an internal diameter as low as 7 &mgr;m, but they have low flow resistance because of their extremely short length of about 100 &mgr;m. This is the reason why natural systems are so efficient for mass transfer.
Document 1 illustrates a membrane panel of self-supporting hollow fibers comprising two base layers of a textile material encapsulated in a non-permeable material, and a multiplicity of hollow fibers of permeable material that extend between the two layers already mentioned. Hence, in this membrane with hollow fibers, the support layers have no particular property whatsoever in relation to the fluid to be treated since they are made of a non-permeable material.
Document 2: U.S. Pat. No. 4,959,152 describes an assembly of hollow fibers comprising a plurality of stacked discs in which the hollow fibers are arranged horizontally so that a fluid circulates in parallel in all of the discs. These fibers are shorter than in traditional devices but they still have a large length compared with the size found in natural systems such as the human lung.
Document 3:U.S. Pat. No. 5,104,535 describes an assembly of hollow fibers mounted between two end supports and assembled one above the other to form modules which are arranged side by side within a treatment enclosure. As in the preceding document, the hollow fibers still have lengths which are large compared with that found in natural systems used for material transfer.
With the techniques described above, the following problems have to be faced. Because of the large thickness of the membranes in the form of hollow fibers, there is always a requirement to make the surface of the pores of said membranes hydrophobic by using rather complex methods which in addition are not sufficiently reliable. So as to prevent the passage of a liquid containing the component to be transferred, through the pores of said membranes it is always necessary to make a precise adjustment of the differential pressure on both sides of said membranes. Using membranes in the form of hollow fibers having a length of the order of one meter and a length to internal diameter ratio of the order of 2000, a pressure difference between the inlet and the outlet of said fibers is obtained which is too great. If one tries to reduce the thickness of the membrane in the form of hollow fibers, the reliability of the device is reduced since the probability of rupturing said membranes is increased. Furthermore, the absence of rigidity in the hollow fibers means that these fibers have a tendency to stick to one another under the action of the fluid flows thereby causing the hydrodynamics flow conditions of the fluids to deteriorate.
In natural capillary systems like a lung, an intestine or a kidney, there is a vast number of more or less short capillaries on the surface being used for material transfer. These are the alveoli in the lungs or the epithelium, the villosities and micro-villosities in the intestines and finally the glomerulic capillaries in the kidneys which comprise fine capillaries with a length to diameter ratio of between about 10 and about 30. Numbering about 5×10
8
, the pulmonary alveoli represent a surface area of about 200 m
2
. Grouped in little clusters, the alveoli are formed from cells with a very thin wall. The transfer of gases (oxygen and carbon dioxide) is carried out through the walls of these alveoli cells. The mass of blood which passes in 24 hours in the lungs is estimated to be 10 m
3
.
Thanks to the micro-villosities found on the external surface of the wall of biological cells making up the intestinal epithelium, the geometric absorption surface area of each of these cells increases several hundred times. These intestinal villosities carry out continuous to and from movements in the liquid pulp resulting from digestion. The passage of digested foodstuff into the blood and the lymph is encouraged thanks to the turbulence in the liquid medium. In the case of the lungs, a system of capillaries is observed the diameter of which progressively decreases along the path of the aspirated air from the trachea towards the alveoli, the number of capillaries increasing in the same direction. This is why natural capillary systems are so efficient for the transfer of material from a surrounding medium into the blood, through the walls of capillaries formed by a biological route.
Material transfer in capillaries of small diameter takes place under a laminar fluid flow regime. So as to make the transfer more intensive under specific conditions it is necessary to have short capillaries of small diameter and a small intercapillary distance, on the one hand and capillaries with a thin wall on the other hand. By using hollow fibers for this purpose, the material transfer step in a gas-liquid system is limited by the rate of diffusion in the liquid phase, and the overall rate of the transfer process is proportional to the total surface area of the membrane despite the porosity of the membrane (or the wall of the hollow fibers).
When using hollow fibers, having the properties described above, one makes a distinction between two operating regimes for the membrane: a wetted membrane and a non-wetted membrane. Obtaining one or the other regime depends on the pressure used and the interaction between membrane and liquid. The strength of a membrane, the pores of which are full of a liquid phase (wetted regime) is much greater than that of a membrane the pores of which are full of a gaseous phase.
This invention proposes a resolution of the problems described above by means of a hollow membrane, with capillary tubes, the structure of which is much closer to that of natural biological systems.
DESCRIPTION OF THE INVENTION
To this end, the invention proposes a hollow membrane comprising two support layers arranged one above the other forming a space between them and a plurality of capillary tubes arranged between the support layers forming capillary channels for the flow of a first fluid, the space between the capillary tubes forming an internal cavity for the circulation of a second fluid around the capillary tubes, and the two support layers and the capillary tubes being made of an organic polymer.
This particular structure for the hollow membrane of the invention offers a number of advantages. In effect, the capillaries formed between the two support layers can have the following characteristics:
a very small length, for example from 1 to 1000 micrometers (&mgr;m) preferably from 3
Jitariouk Nicolaï
Le Moel Alain
Commissariat a l'Energie Atomique
Krebs Robert E.
Sorkin David
Thelen Reid & Priest LLP
Walker W. L.
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