Filtering membranes on the basis of welded polymer...

Details Filtering membranes on the basis of welded polymer... Filtering membranes on the basis of welded polymer...

2002-01-29

2004-08-10

Walker, W. L. (Department: 1723)

Liquid purification or separation

Casing divided by membrane into sections having inlet

Each section having inlet

C210S500210, C210S321840, C210S488000, C156S250000, C156S269000, C055S529000, C096S011000

Reexamination Certificate

active

06773590

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to filtering membranes on the basis of welded polymer structures and to a method for manufacture such membranes. The membranes of the present invention are intended for separation of liquid and gaseous mixtures and may find application in chemical industry, bioengineering, medicine, food industry, etc.
BACKGROUND OF THE INVENTION
At the present time, separation of liquid and gaseous mixtures with the use of separation membranes find wide application in various industries. Thus, such membranes are used in the chemical industry for separation of azetropic mixtures, cleaning and concentration of solutions, cleaning and isolation of high-molecular compounds from solutions that contain low-molecular components, etc. In bioengineering the separation membranes are used for isolation of active substances, such as vaccines and ferments. In food industry such membranes find application in the production of juice concentrates, milk, and high-quality sugar. In oil industry the separation membranes are used for separation of gaseous products from wastes for synthesis of polymers. Other fields for efficient use of separation membranes are treatment of water and solutions, purification of potable water and sewages, etc.
One of greatest achievements in the development of membrane technique was a membrane method for desalination of sea water invented by S. Loeb and S. Sourirajan in 1960 (S. Loeb and S. Sourirajan, UCLA Dept. of Eng. Rep. 60-60 [1960]). This method (hereinafter referred to as L-S method) determined for many years the strategy in development of membrane production technology. The authors of this method, S. Loeb and S. Sourirajan, were awarded a Nobel Prize. Many years of experience gained in the improvement of the original L-S method are reflected in many patents and publications. The double-layered structure proposed by S. Loeb and S. Sourirajan was composed of a thin dense cellulose acetate film applied onto a thin porous substrate. The upper dense layer with a thickness not exceeding 1 &mgr;m possessed selectivity to water and therefore allowed isolation of sodium chloride therefrom. The porous substrate with a thickness exceeding 100 &mgr;m imparted mechanical strength to the structure. If both layers are made from the same polymer, the membranes are called anisotropic membranes (AM), and if they are made of different materials the membranes are called composite membranes (CM). Depending on their shapes, the membranes can be flat, spiral, tubular, or in the form of hollow fibers (First Demonstration of Reverse Osmosis by UCLA SEAS. See in Internet: http://www.engineer. Ucla. Edu/history/osmosis. Html).
Modern membranes comprise thin hollow fibers with the walls made from a porous material without any selectivity. Selectivity is acquired by coating the membrane walls with another material, which possesses selectivity and determines productivity of the membrane operation.
Processes of membrane separation of fluids, which find industrial application, are the following: microfiltration, ultrafiltation, and reverse osmosis. These processes are realized with the use of semipenetrable two-layered membranes produced in the form of AM and CM structures with different sizes of pores.
Microfiltration retains particles in the range of 0.1 to 10 microns (1,000 to 100,000 Angstroms). Particles in this range, such as paint pigments or bacteria, are retained and concentrated by the membrane. Microfiltration can be used to remove bacteria and small suspended solids or clarify beverages.
Ultrafiltration retains particles in the range of 0.001 to 0.1 microns (10 to 1000 Angstroms). Protein and sugar molecules are in this size range. Ultrafiltration can be used to reduce the biochemical oxygen demand (BOD) of waste water by removing substances such as sugar. Ultrafiltration can also be used to separate oil from waste water so that the oil may be recycled.
Reverse osmosis makes it possible to retain particles as small as 0.001 microns (10 Angstroms) or smaller. It also retains ionic substances such as dissolved salts or metal ions, Reverse osmosis can be used to concentrate rinse water from plating operations. The concentrated rinse water then can be used to replenish the plating bath.
It is known that porosity of membrane produced in accordance with the L-S method has the following values expressed in percentages: microfiltration—70%; ultrafiltration—60%; reverse osmosis—50% (Ralph E. White, Peter N. Pintarno. Industrial Membrane Processes: AIChE Symposium Series. American Institute of Chemical Engineers, 1986, 82, No. 248, pp. 98-108, 255-262).
As can be seen from the above, that only a part of the membrane volume is active, and the smaller the dimensions of pores, the smaller is the active volume of the membrane. This is because the L-S method has limitations with regard to distribution of pore density, and the pores themselves have arbitrary distribution. More specifically, the structure of polymer membranes consists of crystalline and amorphous zones, and structural stability of such membranes depends on links between molecular chains inside the crystalline areas, whereas filtration of a substance occurs mainly through more porous amorphous zones. In microporous membranes filtration occurs through the interlinked pores inside the membrane. Thus, if efficiency of a membrane is determined in terms of its porosity, the arbitrary nature of porosity distribution can be considered as an additional contribution to decrease in membrane's efficiency because of twisting and elongation of the diffusion path. In actual membranes, the diffusion path is much longer than the membrane thickness. The L-S method in principle does not allow to increase the amorphous part in the volume of the active zone of the membrane or to make it comparable with the membrane thickness and thus to improve permeability. This is an essential disadvantage of the L-S method.
Another disadvantage of double-layered L-S membranes consists in that the porous substrate shields the active zone of the membrane and reduces its working area. Even though in many cases porosity of the permeable substrate is much higher than that of the active zone of the membrane, as one of the separable components is removed from the system, a relatively thick substrate presents a significant resistance to the transfer process.
Let us consider some particularities inherent in the manufacture of membrane filters by he L-S method.
AM and CM for Reverse Osmosis—Reverse osmosis can be carried out with both AM (on the basis of cellulose ethers, polyamides, and polysulfones) and CM (on the basis various polymers on substrates from polysulfone or polyethyleneterephalate). An AM membrane of this type consists of a substrate having a thickness of about 100 microns and an active layer having a thickness of about 0.2 microns. The membrane is produced by a dry, spontaneous, or a wet coagulation method (see E. Drioli, M. Nakagani. Membranes and Membrane Processes. N-Y.; Plenum Press, 1986, p. 115-187). In the dry method, a polymer, e.g., a cellulose ether or an ether mixture, is dissolved in a solvent, such as acetone. The solution is combined with pore-forming agents, water, and glycerol.
The solution is poured onto a substrate, and the solvent is gradually evaporated. In the wet method, the solution, which contains polymer such as a cellulose acetate, a pore-forming agent (magnesium perchrorate), water, and an organic solvent (acetone, methylethylketone, and methyl or ethyl alcohol), is applied in the form of a thin layer onto a glass or metal plate. Prior to application, the solution and the plate are cooled in a cooling chamber to a temperature from −8° C. to −16° C. After application of the solution, the plate is dried for several minutes and is immersed in cold water (0° C.). Following 1 hour retention in cold water, the solution is washed out from the coating and the coating is gelatinized. The film with an anizotropic structure, i.e., a thin surface layer on a microporous substra

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