RU2140812C1 - Method of formation of asymmetric gas-separating and pervaporating membranes in form of hollow fibers - Google Patents

Abstract

FIELD: methods of production of asymmetric gas-separating and pervaporating membranes suitable for separation of mixtures containing acid components; applicable in gas, oil and chemical industries, and for cleaning of effluents to atmospheres. SUBSTANCE: method includes formation of solution of polysulfone or polyestersulfone in dimethylacetamide or N-methylpyrrolidone through spinneret accommodating mild precipitant. Time of fiber staying in precipitant is regulated by varying distance between spinneret and surface of hard precipitant in which fiber enters after precipitating in mild solvent. Mild solvent is removed by creation of opposite turbulent flow of hard precipitant. EFFECT: increased productivity of membranes without impairing their selectivity. 2 cl, 3 dwg, 1 tbl

Description

 The invention relates to the field of membrane formation and can be used for the manufacture of asymmetric membranes for the separation of gases and liquids.

 Known methods for the manufacture of membranes in the form of hollow fibers from melts of thermoplastic polymers [Scott J. Hollow fibers: Manufacture and Application. New Jersy. Noyes Data Corp. Park ridge 1981.]. The disadvantages of this molding method is that the wall of the hollow fiber has a homogeneous dense structure, and the performance of such a membrane is determined (inversely) by the thickness of the fiber wall. An increase in membrane productivity is achieved only by reducing the wall thickness of the hollow fiber, which is limited by the strength characteristics of the latter.

 A known method of forming composite membranes [Byrnes D. // Chem / proc. 1980 V. 43, N1, p. 32-33.], When a thin layer of another polymer is applied to the porous substrate, which ensures separation of the mixture, and the porous layer (substrate) is responsible only for the transport of matter and the mechanical stability (strength) of the entire membrane. The disadvantage of this method is the multi-stage manufacturing process: in fact, the manufacturing operation is carried out in two stages - first a porous membrane is prepared, and then a composite one.

 The closest in technical essence and the achieved result to the invention is a method of manufacturing an asymmetric membrane, according to which the asymmetric structure of the membrane is molded in a one-step process according to the method using a double precipitation bath [van't Hoff J.A. Wet spinning of asymmetric hollow fiber membranes for gas separation. Ph.D.-Thesis. University of Twente. Enschede, the Netherlands. 1988. - prototype.] - FIG. 1.

 The polymer solution 1 from the container 2, passing through the filter 3, is squeezed through the die 4 into the first external precipitation bath 5, which is a funnel with a thin tube. The first precipitation bath is filled with a weak precipitator 6, which causes phase decomposition of the polymer solution in the direction from the outer surface of the membrane fiber to the inner one by a retarded mechanism, that is, the fiber is not cured immediately in the entire mass, but only along the contact surface of the polymer solution and the soft precipitator, where a thin liquid layer with an increased concentration of polymer. Next, the fiber enters the second external precipitation bath 7 with a hard precipitant 8, where the polymer solution decomposes by the instantaneous mechanism, that is, it precipitates very quickly and rejects, which leads to the fixation of the formed asymmetric structure, i.e. a porous layer - substrate, with side of the inner surface of the hollow fiber and a dense selective layer, on the side of the outer surface. Next, the fiber through a low-speed engine through the roller 9 is fed into the washing bath and then into the fiber collector (not shown in the diagram). The thickness of the selective membrane layer, ceteris paribus, is determined by the residence time of the fiber in the medium of the first precipitation bath.

The disadvantages of this method are:
1. The difficulty of starting the installation, since at the initial moment it is necessary to thread a semi-liquid fiber into the narrow neck of the funnel 5.

 2. The need to maintain a certain liquid level in the first precipitation bath, since the fiber should not be in contact with air until it is completely cured.

 3. If a precipitator with a high viscosity (for example, glycerin) is used in the first precipitation bath, the first precipitant does not separate from the fiber immediately at the moment of contact with the second precipitator, but moves with the fiber up to roller 9, which leads to a significant increase in the residence time of the fiber in the first precipitant and, accordingly, to an unjustified increase in the thickness of the selective layer, which leads to a decrease in membrane performance.

 4. The difficulty of controlling the residence time of the fiber in the first precipitation bath, since in fact the fiber is in the liquid of the first precipitation bath and after leaving it, which leads to low reproducibility of the separation properties of the obtained membranes.

 The objective of the invention is to improve the quality of the manufactured membranes, that is, the performance of the membranes without reducing selectivity, as well as increasing the stability of these qualities while simplifying the operation of the molding plant. This problem is achieved by the fact that in the method of forming asymmetric gas separation and pervaporation membranes in the form of hollow fibers by passing filtered amide solutions (in dimethylformamide, dimethylacetamide, N-methylpyrrolidone) of glassy polymers, mainly polysulfones and polyethersulfones, through the die plate, which consists in soft precipitator to obtain fiber, removal of soft precipitator, deposition of fiber in a hard precipitator and its washing, the operation of forming the fiber from the solution and precipitation using a precipitant soft performed simultaneously. In this case, the time is controlled from the moment the fiber is formed with the deposition of a soft precipitator until the fiber enters the hard precipitator by changing the distance between the die and the liquid surface of the hard precipitant and creating a counter (relative to the fiber motion) turbulent flow of the hard precipitant.

 The installation diagram is shown in FIG. 2. The structural difference is that the first external precipitation bath with a soft precipitator is located in the die itself (position 4), the flow of the first external precipitator (position 7) flows from the die into the air together with the fiber (position 5), and the residence time of the fiber in the first external precipitation bath, the height of the gap between the lower surface of the die and the liquid surface of the second precipitation bath is controlled (distance L in FIG. 2).

 To implement this method, a die is proposed containing three channels for different liquid flows (Fig. 3): a channel of a polymer solution 1, a channel of an internal precipitator 2, a channel of the first external precipitator 3. The jet of the first external precipitator, moving down with the fiber, gradually loses its continuity. In contact with the surface of the liquid of the second external coagulation bath, the flow of the external precipitant is dispersed and separated from the membrane. To facilitate the separation of the external precipitant, a device is provided (position 9 in Fig. 2), which generates a counter (relative to the direction of fiber movement) turbulent flow at the point of contact of the fiber with the liquid of the second precipitation bath. Water from the tank 8 (Fig. 2) under pressure is supplied to the device 9, which is a capillary with a tapering outlet.

 The foregoing is illustrated by the following examples.

Example 1. The membrane is formed from a 35% solution of polysulfone grade P-3500 (Union Carbide) in 1-methyl-2-pyrrolidone on a die with an external outlet diameter of 0.6 mm and a needle diameter of 0.3 mm. Molding is carried out in two ways - by the prototype method, that is, with a separate first external coagulation bath, and by the method where this bath is mounted in a die, that is, according to the claimed. All other parameters are the same. Molding Options:
The temperature of the polymer solution is 339 K;
The composition of the first precipitation bath is glycerin;
The temperature of the first precipitation bath is 294 K;
The composition of the internal precipitant is demineralized water;
The temperature of the internal precipitant is 294 K;
The composition of the second precipitation bath is demineralized water;
The temperature of the second precipitation bath is 294 K;
The spinning speed of the fiber is 5 m / min.

 The resulting hollow fibers are tested for permeability in the methane - carbon dioxide system. For the membranes of each series, 10 modules were produced that were tested for gas permeability. The concentration of the mixture is 75:25, respectively, the pressure of the mixture above the membrane is 0.6 MPa. For the selectivity was taken the ratio of the relative concentrations of the components on opposite sides of the membrane. Permeability was determined for a rapidly penetrating component - carbon dioxide. The test results are presented in the table.

 As can be seen from the above results, membranes manufactured by the claimed method have higher total selectivity and productivity, greater average productivity at the same selectivity, as well as better reproducibility of separation properties.

 The implementation of the inventive method of forming membranes in the form of a hollow fiber allows you to easily change such an important molding parameter as the residence time of the fiber in the first external precipitation bath. This distance is controlled by the height of the gap between the lower surface of the die and the liquid surface of the second precipitation bath (distance L in Fig. 2). This parameter can vary over a wide range and can be easily changed during the spinning process without stopping the process. In the prototype method, the residence time of the fiber in the first outer bath is determined by the length of the funnel through which the fiber is passed at the beginning of the molding process. Since at the beginning of the molding process it is necessary to stretch the semi-liquid fiber through the funnel, the length of the funnel is limited by the possibility of this procedure. Thus, the residence time of the fiber in the first outer bath is limited. In addition, changing the residence time during molding requires stopping the process to replace the funnel.

Claims (1)

 A method of forming asymmetric gas separation and pervaporation membranes in the form of hollow fibers, comprising passing a filtered solution of polysulfone or polyethersulfone in dimethylacetamide or N-methylpyrrolidone down through a die, depositing it in a soft precipitator to obtain a fiber, removing a soft precipitator, settling the fiber in a hard precipitant, and characterized in that the soft precipitant is placed in the die, the solution is precipitated in the soft precipitator while they flow out of the die, regulating the residence time of the obtained fiber in a soft precipitator by changing the distance between the die and the liquid surface of the hard precipitator, and the removal of the soft precipitator is carried out by creating a turbulent flow of a hard precipitator counter to the motion of the fiber.

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