KR20020015749A - Membrane Formation by dual quenched method - Google Patents

Abstract

PURPOSE: Provided is a production process of a gas separation membrane with dual phase separation induction method based which keeps the temperature of a non-solvent bath to be low state so that it promotes the speed of phase separation. CONSTITUTION: In producing the gas separation membrane which is produced by heating high molecular compound at the lower temperature than the boiling point of solvent or additive to 50-70deg.C and then spinning, the production process of the gas separation membrane with dual phase separation induction method based comprises the steps of: (i) preparing 20-40wt% of high molecular substance with selective permeability; (ii) preparing 35-80wt% of solvent of the high molecular substance; and (iii) precipitating 3- or 4-component high molecular compound of additive as non-solvent with less than 100deg.C at boiling point in a non-solvent bath of low temperature with -10-15deg.C/

Description

Membrane Formation by dual quenched method

The process of separating a particular gas component from the various gas components in the gas mixture is very important. Processes used to perform such gas separation include a cryogenic method, a pressure swing adsorption method, and a membrane separation method.

The gas separation membrane used in the membrane separation method is used to separate and concentrate various gases such as hydrogen, helium, oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapor, ammonia, sulfur compounds, light hydrocarbon gases, and the like. Areas where gas separation can be applied include separation of oxygen or nitrogen in air, hydrogen separation from syngas in petrochemical processes, and carbon dioxide separation from natural gas. Separation of oxygen and nitrogen from the air is used to increase the efficiency of the fermentation process, increase the combustion efficiency, etc. In addition, the concentrated nitrogen can be used in the process of explosion protection of the ignition fluid tank, food storage and the like. Another important application of the gas separation membrane is to separate hydrogen from a mixture with various gases such as nitrogen and carbon dioxide, and the hydrogen separated therefrom is used in various processes such as hydrogen decomposition and catalytic cracking. The principle of gas separation using a membrane is based on the difference in permeability of each component in two or more gas mixtures permeating through the membrane. This is a dissolution-diffusion process in which the gas mixture is in contact with one side of the membrane to selectively dissolve at least one of the gas components. In the interior of the membrane a selective diffusion process is carried out through which the gaseous components are passed faster than at least one gaseous component of the gas mixture. Relatively low permeability gas components penetrate the membrane more slowly than at least one or more components of the gas mixture. By this principle, the gas mixture is separated into two streams, which are selectively permeate gas streams and those which are not permeate gas streams. Therefore, in order to properly separate the gas mixture, there is a need for a technique of controlling a structure capable of showing sufficient permeability by selecting a film-forming material having high permeability for a specific gas component.

This membrane structure control technique has been published in US Pat. No. 3,133,132 by Loeb and Sourirajan. Since then, the technology has evolved into a film-making technique, commonly called a phase inversion method, until commercialization of various organic synthetic polymers. The process of preparing the asymmetrical phase separation membrane from the polymer solution by the phase inversion method can be summarized in the following steps:

(1) casting or hollow fiber spinning of polymer dope solution, (2) evaporation of volatile components by contact of dope solution with air, (3) precipitation of solution into coagulation bath, (4) washing, drying, etc. It can be divided into four stages of post-processing step.

In order for the membrane to selectively separate and concentrate gas, the structure of the membrane should generally have an asymmetric structure consisting of a dense selective separation layer on the membrane surface and a porous support having a minimum permeation resistance at the bottom of the membrane. The selective separation ability of the gas (hereinafter referred to as selectivity or α), which is a characteristic of the membrane, depends on the structure of the separation layer, and the permeation amount of the selectively separated gas (hereinafter referred to as permeability or P) is the thickness of the separation layer. And the degree of porosity of the porous support, which is the substructure of the asymmetric membrane. To selectively separate the mixed gas, the surface of the separation layer must be free of defects and have a pore size of 5 mm or less. In addition, the separation layer should be as thin as possible in order to obtain high gas permeability because the gas permeability is inversely proportional to the effective membrane thickness. In order to minimize resistance to gas flow selectively passing through the separation layer, it is advantageous for the understructure of the asymmetric membrane to have a porous structure if possible.

The production of phase inversion membranes for gas separation has more difficulty than the production of separation membranes for water treatment. In the reverse osmosis process, it is necessary to prepare a membrane having pores smaller than the spherical radius of ions, but in gas separation, there should be no pores of nm size due to the low cohesion and small size of the gas. Although a method of preparing a gas separation membrane having such a high permeability selectivity without such defects is disclosed in US Pat. Nos. 4,902,442 and 4,772,392, it has been difficult to easily prepare a high performance gas separation membrane due to a complicated process.

Two methods have been developed to remove defects in the membrane separation layer. The first method is to coat silicon or heterogeneous polymer on the surface. The second method is to increase the concentration of polymer on the surface by forcing convection by mixing volatile solvent with dope solution. The foregoing prior art is described in the following patents. In US Pat. No. 4,230,463, a gas separation membrane was prepared using a coating on the surface of a porous membrane. US Pat. No. 4,484,935 describes a multicomponent composition comprising a coating layer consisting of a combination of a PDMS and a crosslinking agent with a modified silicon-containing monomer and a porous asymmetric membrane. A gas separation membrane is described. In US Pat. No. 4,728,346, the membrane surface is treated with an aromatic permeation modifier and coated to improve the selectivity of the gas separation membrane. Japanese Patent No. 61-107921 describes a gas separation membrane composed of a coating layer of a polymer of a catalyst and an acetylene monomer. However, the coating and curing process of the film surface is discontinuous and involves additional cost and complexity. The forced convection method described below is described in the following patents and articles. US Pat. No. 4,902,422 prepared a gas separation flat membrane cast from a four component polymer solution containing a volatile solvent and a nonsolvent using a convection evaporation process without additional coating process. The oxygen / nitrogen selectivity of this asymmetric flat membrane ranges from 5.0 to 6.5. See also, Pinnau et al., J. Memb. Sci., 88, 1-19 (1994)] applied a forced convection process to produce a hollow fiber membrane to prepare a hollow fiber membrane for gas separation having a selectivity of 6.5 without coating and curing process. However, manufacturing a hollow fiber membrane without a process of sealing such additional defects is quite difficult because of demanding manufacturing techniques such as a convection evaporation process. This requires a film production technique that does not have a defect without the above-described additional steps.

The present invention relates to a method for producing a gas separation membrane having a selectivity without additional processes such as coating or forced convection method.

Accordingly, the present inventors have repeatedly studied the gas separation membrane manufacturing method for suppressing the occurrence of defects in the gas separation membrane. As a result, the dope solution was deposited on the non-solvent to induce phase separation by infiltration of the solvent. The method of manufacturing the film | membrane in which the defect was suppressed by increasing the temperature difference was led.

Accordingly, it is an object of the present invention to provide a highly selective gas separation membrane having a manufacturing process and a method for producing the same, wherein the defect is suppressed and no coating or forced convection process is required to remove the defect.

1 is a schematic diagram of a hollow fiber gas separation membrane manufacturing apparatus used in the present invention.

* Explanation of symbols for main parts of the drawings

1: dope solution pump 2: double nozzle

3: internal coagulating liquid pump 4: primary coagulation bath

5: hollow fiber winding part 6: washing tank

In order to achieve the above object, in the present invention, a dope solution composition such as a polymer material for the selected gas separation membrane, a solvent, and an additive thereof is prepared in a state very close to a thermodynamic phase separation composition. However, the solution in this composition is very thermodynamically unstable, so even if the composition of the microstructure changes, phase separation proceeds. However, increasing the temperature of the dope solution increases the thermodynamic stability of the dope solution and stabilizes the mixed phase. This dope solution is dry-wet spinning. The process is as follows. In general, the casting or spinning process takes place at room temperature. In the present invention, the casting knife or spinnerette is heated at a temperature close to the boiling point of the volatile additive. The dope solution from the casting knife or spinnerette is brought into contact with the atmosphere and the hot dope solution evaporates the volatile solvent / nonsolvent additive. The conventional technique forms a dense layer on the surface by forced convection at this stage, but in the present invention, since the dope solution is exposed to the atmosphere at a temperature close to the boiling point of the solvent / non-solvent additive, evaporation of the solvent / non-solvent additive This is facilitated. In addition, in the next step, the dope solution is precipitated in the low-temperature nonsolvent coagulant liquid, which has a lower temperature than the normal temperature, thereby expanding the degree of rapid cooling. This increased degree of quenching accelerates the rate of phase separation and thereby inhibits the formation of interconnected pores by nucleation and growth mechanisms at the surface.

In the present invention, when the membrane is prepared using the phase inversion method, 1) phase separation by infiltration of a nonsolvent, and 2) phase separation by quenching temperature difference between a dope solution and a nonsolvent are increased to prepare a membrane with defects suppressed. That's how. Accordingly, the present invention provides a highly selective gas separation membrane and a method for manufacturing the same, which have a manufacturing process in which defects are suppressed and no coating or forced convection process is required to remove the defects.

Ideal gas separation membranes have high permeability and do not lose their performance at high temperatures and pressures. In general, high permeability polymer materials have low selectivity and low permeability polymer materials have high selectivity. The selection of the film forming material of the gas separation membrane becomes very important.

The polymer material, which is an asymmetric gas separation membrane-forming material in general membrane production, is a natural or synthetic polymer material having useful gas separation performance. The polymer material is dissolved in a strong solvent to form a uniform solution. It forms in the form of a flat membrane or a hollow fiber membrane by immersing in the coagulant which is a nonsolvent for a substance.

The polymeric membrane material used in the asymmetric membrane of the present invention is selected in consideration of the thermal, mechanical and chemical durability of the constituent material, and in particular, it is preferable to have a permeability to at least one gas of the gas mixture. Or polyimides or polyamides and mixtures thereof.

In addition, as a solvent for the polymer material as the film forming material, for example, N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, N-dimethylsulfoxide and mixtures thereof Is included and is preferably N-methylpyrrolidone.

The additive of the present invention includes at least one organic solvent selected from the group consisting of tetrahydrofuran, ethanol, trichloroethane, 2-methyl-1-butanol, 2-methyl-2-butanol and 2-pentanol, The composition of the polymer mixture solution of the present invention is advantageously located at the boundary between the single phase and the separated phase. Therefore, the concentration of each component in the mixed solution should be chosen so that the mixed solution is close to the phase separation boundary. In addition, if the polymer concentration is too low, the size of the pores increases to decrease the selectivity.

The polymer dope solution for preparing the gas separation membrane of the present invention is composed of 30 to 40% by weight of the polymer material and 60 to 70% by weight of the solvent and the additive for high gas permeability and selectivity. In addition, the additive can greatly affect the film performance of the dope solution, it is possible to adjust the film-forming ability of the dope solution by adjusting the amount of the additive in the mixed solution. The weight ratio of the solvent and the additive is preferably 5 to 15% by weight of the total dope solution. If it is 5 wt% or less, the selectivity of the film is inferior. If it exceeds 15 wt%, it is difficult to obtain a uniform dope solution.

The temperature of the dope solution for hollow fiber spinning is about 50 ~ 60 ℃. The higher the temperature, the higher the separation layer, the higher the selectivity, but the lower the permeability. Because it changes.

The gas separation membrane manufacturing process of the present invention will be described with reference to FIGS. 1 and 2 by using hollow fiber membrane production as an example:

Each component of the mixed solution is made into a uniform solution using a closed reactor. The prepared homogeneous polymer mixed solution is left at room temperature and under reduced pressure for 24 hours to remove bubbles in the solution and then spun through a spinerrette (2). At this time, the spinnerette (2) structure is a double nozzle, the polymer mixture solution is released through the outer nozzle of the double nozzle, the inside of the double spinnerette (2) of 1 ~ 2ml / min The internal coagulant is discharged at the flow rate to emit hollow fiber. The inner diameter of the outer nozzle of the double nozzle is 0.5 to 0.8 mm, and the inner and outer diameters of the inner nozzle of the double nozzle are 0.2 to 0.3 mm and 0.4 to 0.6 mm, respectively.

The spinning solution first begins to solidify inside the hollow fiber before it is precipitated in the primary coagulant 4, which is a nonsolvent for the polymeric material. At this time, the internal coagulating liquid radiated through the double nozzle and the surface in contact with the solution is gelled by the interdiffusion action of the solvent and other additives in the solution and the internal coagulant to form a dense separation layer having relatively dense but very small pores. Under the separation layer, a porous lower structure layer 12 is formed due to the continuous phase separation between the internal coagulation solution and the polymer mixed solution, and then precipitates in the external coagulation solution 3.

At this time, the feature of the present invention is to promote the phase separation rate by setting the non-solvent temperature to a low temperature of -10 ° C to 5 ° C. This is to increase the driving force of the phase separation by making the instant quenching state at a temperature lower than 50 ℃ lower than the dope solution. Increasing the quench depth increases the selectivity of the membrane. This can be explained by both nucleation growth and spinodal decomposition, which are polymer phase separation mechanisms. According to the nuclear growth mechanism, if the phase separation rate is increased, the growth time of the surface of the polymer phase is shortened and the three-dimensional continuous path that acts as a defect on the surface of the selected layer cannot be formed. Because. In addition, spinoidal decomposition can be explained that the phase separation of a high concentration of the polymer solution occurs in a short time, when the instant diffusion of the non-solvent into the dope solution is a phenomenon that the defect is pressed by the osmotic pressure is suppressed.

Subsequently, the phase separation process that occurs due to contact with the coagulating liquid inside the hollow fiber also occurs on the outer surface of the hollow fiber precipitated in the primary coagulating liquid 4. The hollow fiber membrane subjected to the spinning and coagulation step is then made into a membrane consisting of a dense separation layer without defects on the outer surface and a porous substructure layer having pores through a washing step, a substitution step and a drying step.

In the case of the flat membrane, the polymer mixed solution is cast on the nonwoven fabric, and then precipitated in a coagulant solution, which is a non-solvent in the polymer material, to undergo a coagulation step.

The cross section of the hollow fiber membrane and the flat membrane for gas separation prepared according to the present invention is shown in FIG. In the case of the hollow fiber membrane, the outer diameter is about 0.3-0.5 mm, the inner diameter is about 0.025 mm, and the thickness of the dense separation layer having the resolution is about 0.1 μm.

The method of the present invention will be described in more detail by process. First, in the coagulation step, gelation occurs relatively quickly between the coagulation solution and the solvent / additive mixture at the surface where the coagulation solution and the polymer mixed solution contact each other during the preparation of the separator, in which phase separation occurs. Using a dope solution prepared to facilitate this generation and increasing the temperature difference between the dope solution and the coagulation bath proceeds with very rapid phase separation, thereby forming a separator consisting of a dense separation layer and a porous structure layer including pores.

The coagulating solution should be non-solvent for the polymer material and should be compatible with all components constituting the polymer mixed solution except for the polymer. The coagulating solution used in the present invention is alcohol, such as water, methanol, ethanol, isopropyl alcohol, a mixture thereof, and the like, and preferably water. In addition, in the present invention, the shipbuilding of the dope solution is used as an additive to approach the binodal line in which phase separation starts.

The gas separation membrane solidified by spinning or casting as described above is subjected to a washing step to easily remove a mixture of a solvent and an additive remaining in the inside and the surface thereof, a coagulating solution, and at least one day, preferably three days. Perform an abnormal wash step. As the washing liquid used in the present invention, water is preferable.

Therefore, in the substitution step, the membrane is precipitated in the substitution solution to remove the remaining mixture present in the small pores or defects on the surface of the membrane even after the washing process so that the substitution liquid is completely wetted in the small pores or defects. Substituents use less of the surface tension than the remaining mixture to remove the remaining mixture from the separation membrane and are completely wetted in small pores or defects. The remaining mixture refers to a mixture containing a solvent, an additive, a coagulating liquid or a washing liquid present in small pores or defects on the membrane surface. The substitution step is preferably carried out for 1 day or more.

Substituents used in the present invention include, for example, alcohols such as methanol, ethanol and 2-propanol, organic solvents including hexane, and mixtures thereof. Preferred are alcohols, in particular methanol.

In the drying step, since the substitution liquid completely wetted in the small pores or defects on the surface of the membrane has a surface tension equal to or less than that of the polymer material that is the film forming material, the substitution liquid is volatilized and separated by capillary pressure during drying. It will completely eliminate small pores or defects in the layer. The separation membrane of which the substitution process is completed is completed by drying under vacuum at 70 ° C. for 3 hours.

As described above, the present invention provides a coagulation step of controlling the pore or defect size in the dense separation layer by promoting phase separation of the polymer mixed solution by an additive, and the remaining mixture present in the dense separation layer as a substitute to the surface tension. Substitution step to remove and replace from the membrane by, and to make it possible to produce a separation-free membrane through drying.

The present invention will be described in more detail with reference to the following Examples, but the present invention is not limited to these Examples.

Example 1

Add 35% by weight of polyethersulfone (Sumitomo, sumikaexcel), a polymer, to 45% by weight of N-methylpyrrolidone solvent, based on the total amount of the mixture, using 5% by weight of tetrahydrofuran and ethanol as additives. Weight percent was added and mixed to make a uniform solution. Bubbles in the prepared uniform spinning solution were removed at room temperature and reduced pressure for 24 hours, and foreign substances were removed using a 60 μm filter. It was then spun using a cylinder pump at a temperature of 60 ℃. At this time, the air gap was 10cm, double spinnerettes were used, and the internal coagulating solution was spun with water at room temperature, and the internal coagulation bath was wound at a temperature of 5 and 15 ° C. After cutting, the mixture was washed for 2 days in running water to remove the remaining mixture of solvent and additives. Subsequently, it was immersed in methanol for 3 hours or more to replace the water present in the dense separation layer, and again immersed in hexane (n-hexane) for 3 hours. Subsequently, methanol was replaced with hexane. Desert was prepared.

The performance of the prepared hollow fiber membrane was 99.9% oxygen and nitrogen, respectively, and applied to three identical hollow fiber modules under 5 atm to measure the average oxygen and nitrogen gas permeability and selectivity. Each hollow fiber module included ten hollow fiber membranes and gas permeability was measured using a mass flow meter. Gas permeation unit was used (Gas Permeation Unit, 10-6 × cm3 / cm2 sec cmHg). The results are shown in Table 1.

TABLE 1

Non-solvent temperature (℃) Oxygen permeability (Q, GPU), 25 ℃ Oxygen / Nitrogen Permeation Selectivity (Q (O2) / Q (N2)) 5 16 5.0 15 18 3.8

Example 2

Add 37 parts by weight of polyethersulfone (Sumitomo, Sumikaexcel), a polymer, to 43% by weight of N-methylpyrrolidone solvent, based on the total amount of the mixture, using 5% by weight of tetrahydrofuran and ethanol as additives. The same process as in Example 1 was conducted except that a uniform solution was prepared by adding and mixing the wt%. Performance evaluation results of the prepared membrane are shown in Table 2 below.

TABLE 2

Non-solvent temperature (℃) Oxygen permeability (Q, GPU), 25 ℃ Oxygen / Nitrogen Permeation Selectivity (Q (O2) / Q (N2)) 5 12 5.4 15 14 4.0

Example 3

The external coagulation solution based on the total amount of the mixture was carried out in the same manner as in Example 1 except for using ethanol and the temperature of the coagulation bath at -5 and -15 ℃. Performance evaluation results of the prepared membrane are shown in Table 3 below.

TABLE 3

Non-solvent temperature (℃) Oxygen permeability (Q, GPU), 25 ℃ Oxygen / Nitrogen Permeation Selectivity (Q (O2) / Q (N2)) -15 5 3.5 -5 7 3.0

Comparative Example 1

The same procedure as in Example 1 was carried out except that the temperature of the external coagulation bath was 25 and 35 ° C. Performance evaluation results of the prepared membrane are shown in Table 4 below.

TABLE 4

Non-solvent temperature (℃) Oxygen permeability (Q, GPU), 25 ℃ Oxygen / Nitrogen Permeation Selectivity (Q (O2) / Q (N2)) 25 20 1.4 35 26 1.2

Comparative Example 2

Add 35% by weight of polyethersulfone (Sumitomo, sumikaexcel) as a polymer to 45% by weight of N-methylpyrrolidone solvent, and add 20% by weight of tetrahydrofuran as an additive based on the total amount of the mixture. The same procedure as in Example 1 was conducted except that a uniform solution was prepared. Performance evaluation results of the prepared membrane are shown in Table 5 below.

TABLE 5

Non-solvent temperature (℃) Oxygen permeability (Q, GPU), 25 ℃ Oxygen / Nitrogen Permeation Selectivity (Q (O2) / Q (N2)) 5 20 2.5 15 26 2.0

As described above, in the case of Comparative Example 2 using a non-solvent temperature range of 15 ° C. or more or three components as in Comparative Example 1, the gas separation membranes prepared in Examples 1 to 3 according to the present invention had oxygen / nitrogen selectivity. It can be seen that the lower than 2 cases.

As described above, if the non-solvent temperature is promoted to a low temperature of -10 ° C to 5 ° C to promote the phase separation rate, the momentary quenching state is made at a temperature of 50 ° C or lower than the dope solution, thereby increasing the driving force of the phase separation. This makes it possible to produce a film in which a selective layer is formed without using a secondary coating process.

Claims (5)

In the method of producing a gas separation membrane by heating the polymer mixed solution to a temperature lower than the boiling point of the solvent or additive to 50 ℃ ~ 70 ℃, (A) 20 to 40% by weight of a polymer material having a selective permeability (B) 35 to 80% by weight of a solvent of the polymer material, (c) a three or four-component polymer mixed solution of a poor solvent having a boiling point of 100 ° C. or lower or a non-solvent additive at a low temperature of −10 ° C. to 15 ° C. Method for producing a gas separation membrane, characterized in that the film is precipitated in the tank. The method of claim 1 wherein the polymer is polyethersulfone, polysulfone, polyetherimide, polyimide or mixtures thereof. The method of claim 1, wherein the solvent of the polymer material is N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, N-dimethylsulfoxide or a mixture thereof. . The method of claim 1, wherein the additive is at least one organic solvent selected from tetrahydrofuran, trichloroethane, 2-methyl-1-butanol, 2-methyl-2-butanol and 2-pentanol. . Gas separation membrane to module prepared by the method of claim 1.