JPH0585208B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of separating a mixed liquid by a pervaporation method (hereinafter referred to as pervaporation method) using a membrane formed by a wet method. Distillation has been conventionally known as a method for separating organic mixed liquids. However, it is extremely difficult to separate azeotropic mixtures, solvents with similar boiling points, isomers (ortho and para, cis and trans), etc. by distillation. For example, when separating a water-alcohol mixed liquid by distillation, a solvent such as benzene is added to the mixed liquid as a third component in order to increase the specific volatility of the alcohol component in the mixed liquid. It is necessary to distill off the ternary alcohol-solvent mixture and remove the pure alcohol from the bottom of the column. However, the above distillation method requires the installation of a device to separate and recover the third component from the ternary mixture distilled from the top of the distillation column and a large distillation column to process the ternary mixture, resulting in an increase in the size of the device. In addition, there was a problem in that a large amount of thermal energy was consumed for distilling the ternary component mixture and separating and recovering the third component. In recent years, various separation techniques have been studied to solve the problems of the above-mentioned distillation method. Among them, the mixed liquid to be separated is supplied to the feed liquid side (primary side) of two chambers separated by a separation membrane, and the components with high affinity with the membrane are passed through the membrane to the permeate side (secondary side). ) The pervaporation method, which preferentially transmits vapor as vapor, can in principle separate near-boiling point mixtures and azeotropic mixtures that are difficult to separate using conventional methods such as filtration or distillation, making it possible to significantly save energy. Moreover, it is attracting attention as a new separation technology that can significantly reduce the size of equipment because it can perform the separation and concentration process in one or several steps, which conventionally required multiple steps. Various experimental examples have been reported in which mixed liquids were separated by such conventional pervaporation methods. For example, U.S. Patent No. 2953502 describes an experimental example in which a water-methanol mixture was separated using a cellulose acetate membrane, and page 33 of the December issue of Functional Materials (1981) describes an experiment in which a water-ethanol mixture was separated using a cellulose acetate membrane. An example of an experiment in which a water-isopropanol mixture was separated using a cellophane membrane, Journal of Applied Polymer
Science vol. 26 (1981), page 3223, describes an experimental example in which a water-methanol mixture was separated using a grafted polyvinyl alcohol membrane, and U.S. Patent No.
No. 3726934 contains an experimental example in which styrene was separated from a styrene-benzene mixture using an acrylonitrile polymer membrane, and J. Polymer Sci: Symposium
On page 145 of No. 41, an experimental example is reported in which the addition of salt improves the separation performance when separating alcohol and water using a cellophane membrane. However, methods for separating mixed liquids by pervaporation using these membranes cannot be implemented on a laboratory scale, but have the following problems when implemented on an industrial scale. In other words, (1) the separation rate when a mixed liquid passes through a polymer membrane once [generally, the weight ratio of component A to component B after passing through the membrane is divided by the weight ratio of component A to component B before passing through the membrane; The obtained value is displayed as a separation coefficient α. In other words, α = (W _A /W _B ) in the permeate / (W _A /W _B in the permeate)
) (In the formula, W _A and W _B are the A component and B component, respectively.
Indicates the weight of the ingredients. )] is small, so in order to separate or concentrate to the desired concentration, it must be passed through a very large number of membranes. (2) Amount of permeation through a polymer membrane [Generally, the amount of permeation per unit membrane surface area and unit time, that is, Q
(expressed in Kg/m ² hr)] is small, so it is necessary to increase the membrane surface area. There are problems such as these, and it is far from being implemented on an industrial scale. The present inventors solved the problems of various pervaporation methods and provided a method for pervaporation separation of mixed liquids with a high separation coefficient and high permeation rate. After examining the relationship between the structure and the permeation direction of the mixed liquid, we found that in the reverse osmosis method, for example, as described on page 26 of "Separation Method by Membrane" edited by Hagiwara and Hashimoto, Kodansha Scientific Publishing (1972). However, surprisingly, in the pervaporation method, a mixed liquid that must be separated is supplied to the front side (the side where a dense layer is formed) that comes into contact with an easily solidified medium. The present invention was achieved based on the discovery that if the mixed liquid to be separated is supplied to the back side in contact with the medium, a considerably larger separation coefficient is achieved than when the mixed liquid is supplied to the front side of the membrane. That is, in the present invention, a mixed liquid is supplied to the surface of the membrane formed by a wet method on the side that is in contact with the hard-to-solidify medium, and the components that have permeated through the membrane are transferred to the surface of the membrane that has come into contact with the easy-to-solidify medium. This is a method for separating a mixed liquid, which is characterized in that it is extracted in gaseous form from the membrane surface. The membranes used in the method of the present invention include cupric ammonia, a regenerated cellulose membrane wet-cast by a viscose method, a polyvinyl alcohol polymer membrane wet-cast a polyvinyl alcohol composition, dimethyl sulfoxide, pyrrolidone, dimethylaratamide alcohol and water. Ethylene-vinyl alcohol copolymer film formed by wet coagulation using a mixed solvent etc., polyamide film formed by wet coagulation using solvents such as sulfuric acid, dimethylacetamide (including salt-added systems), etc. , a polyimide film, etc. Both the wet-formed hollow fiber membranes and the wet-formed membranes after casting a polymer solution on a support have different structures between the front side in contact with the easy-to-solidify medium and the back side in contact with the difficult-to-solidify medium. have. For example, in an anisotropic membrane, a thin dense layer is formed on the front side that is in contact with an easy-to-solidify medium, and a porous structure is formed on the back side that is in contact with a difficult-to-solidify medium, and it is clear that the structures on the front and back sides of the membrane are different. Even in a homogeneous membrane, which is said to be substantially uniform in the cross section of the membrane, the front side in contact with the easy-to-solidify medium (especially its outermost layer) is different from the back side in contact with the difficult-to-solidify medium (especially its outermost layer). It has a relatively dense structure, and the structures on the front and back sides of the membrane are different. The hard-to-solidify medium and the easy-to-solidify medium in the present invention refer to media in which the solidifying ability of the solidifying medium is relatively low and relatively high, respectively. For example, in the case of a film-forming stock solution in which an ethylene-vinyl alcohol copolymer is dissolved in dimethylacetamide, a typical easy-to-solidify medium is acetone, which easily solidifies the copolymer when it comes into contact with the stock solution. Air that does not solidify easily even if it is allowed to solidify is a difficult-to-solidify medium. In addition, substances that are not compatible with the copolymer solvent (dimethylacetamide), such as hexane and benzene, and supports for spreading the membrane forming solution, such as glass plates, stainless steel plates, and Teflon plates, are also difficult-to-coagulate media. On the other hand, water is a typical example of an easily solidifying medium, but the distinction between difficult and easy solidifying media is relative, and when water is used as an easy solidifying medium, a 30% aqueous solution of dimethylacetamide is a difficult medium. When air is used as the hard-to-solidify medium, the dimethylacetamide aqueous solution becomes the easy-to-solidify medium. In the case of a wet coagulation film of an ethylene-vinyl alcohol copolymer film, an aqueous medium is usually used as the easy coagulation medium. Such an aqueous medium may be water alone, a mixture of water with a water-miscible organic solvent, usually the same solvent as the polymer solution, within a range of 70% by weight, or a system in which an inorganic salt such as Glauber's salt is further dissolved in water. etc. may also be used. On the other hand, there is almost no difference in the microstructure between the front and back sides of the membrane formed by the dry method. In addition, even in the case of wet-formed membranes, there are differences in the micropore structure on the front and back sides of the membrane in cases where the front and back sides of the membrane are brought into contact with the same coagulation liquid, such as flat cuprammonium regenerated cellulose membranes. is not recognized. Even if pervaporation separation is performed by supplying the mixed liquid to the front side or the back side of the above-mentioned membrane, the separation coefficient shows almost the same value. Therefore, the method of the present invention cannot be applied to a membrane in which there is no difference in the micropore structure between the front and back sides of the membrane. The tube-shaped cuprammonium regenerated cellulose membrane used as a dialysis membrane for coil artificial kidneys is produced by contacting the inside of the tube with a medium that is difficult to solidify (air) and the outside of the tube being in contact with a medium that is easy to solidify. Because of this, there are differences in the structure of the inside and outside of the tube. When performing pervaporation separation using such a tube-shaped cuprammonium regenerated cellulose membrane, it is necessary to supply the mixed liquid to be separated to the inside of the tube where it comes into contact with the hard-to-coagulate medium. In the case of an ethylene-vinyl alcohol copolymer film, for example, if a dimethyl sulfoxide solution of ethylene-vinyl alcohol (ethylene content 18 mol%) is formed into a film in water at 3°C using air as the medium inside the hollow fiber, it will solidify. A substantially homogeneous membrane can be obtained in which the outer surface (particularly the outermost surface) in direct contact with the liquid has a denser micropore structure than the inner surface. On the other hand, ethylene-vinyl alcohol (ethylene content 18 mol%) was mixed with propanol/water (50/50wt).
%) A solution dissolved in a mixed solution to a concentration of 30wt% was heated at 3℃ using air as the medium in the hollow fiber.
When the membrane is formed in water, an anisotropic membrane having a support layer on the inner surface side of the hollow fiber and a dense layer on the outer surface side can be obtained. Therefore, when using ethylene-vinyl alcohol hollow fibers spun using air as the medium in the hollow fibers, it is necessary to supply a mixed liquid to the inside of the hollow fibers to perform pervaporation. The above-mentioned ethylene-vinyl alcohol copolymer membrane has a hollow fiber shape, a flat membrane shape depending on the mode of use,
It can be formed into a tube-shaped film. These membranes may be used alone or stacked on a sheet, film, or tubular porous body. The mixed liquids to be separated in the present invention include azeotropic mixed liquids, close boiling point mixed liquids, etc., and are particularly effective in separating organic mixed liquids. Among organic mixed liquids, azeotropic mixed liquids include water/ethanol and water/
Examples include water/alcohol such as isopropanol, and methyl acetate/methyl alcohol, ethyl acetate/ethyl alcohol, benzene/cyclohexane, methanol/acetone, benzene/methanol, benzene/ethanol, acetone/chloroform, methanol/acetone, and the like. Examples of the close boiling point mixed liquid include ethylbenzene/styrene, parachloroethylbenzene/parachlorostyrene, toluene/methylcyclohexane, butadiene/butenes, butadiene/butanes, and the like. Also included are mixed liquids that can be separated by ordinary distillation, such as water/acetone, water/ethylene glycol, water/glycerin, and water/methanol. The pervaporation device used in the present invention is not particularly limited, and any conventionally known device can be used, and such a device can be operated under conventional conditions to separate the mixed liquid. When performing pervaporation, the larger the pressure difference between the feed liquid side and the permeate side, the more effective it is, but for industrial implementation, it is preferable to provide a pressure difference of 0.5 to 1 atmosphere. It is. In addition, the pressure on the supply liquid side is preferably atmospheric pressure or a pressure close to it.
The pressure on the permeate side is preferably maintained at a reduced pressure below the vapor pressure of the permeate component. Methods for maintaining the permeate side at reduced pressure include drawing a vacuum to reduce the pressure, or flowing a gas that does not react with the constituent components to maintain a low vapor pressure. The appropriate separation temperature is 40°C or higher and lower than the azeotropic temperature of the organic liquid mixture to be separated. When separating a mixed liquid, if the desired concentration cannot be obtained by passing it through the membrane once,
It is possible to concentrate and separate the target concentration by installing similar equipment in series and passing it through multiple times, or by combining it with distillation. According to the present invention, higher separation coefficients and higher permeation rates are achieved than with conventional membrane separation methods.
Therefore, according to the method of the present invention, the separation system can be made more compact, the processing capacity can be increased, and the cost can be reduced. It is effective for the production of chemical substances, and has extremely great industrial utility. Next, the method of the present invention will be explained in more detail with reference to Examples. Example 1 An ethylene-vinyl alcohol copolymer with an ethylene content of 33 mol% was dissolved in a water-containing organic solvent containing dimethyl sulfoxide (9:1) to prepare a spinning stock solution with a concentration of 35% by weight.
0.7 mm), extrusion spinning was carried out in a coagulating liquid of water at 1° C. while blowing nitrogen into the tube of the nozzle. After washing with water, immerse in acetone and dry at room temperature.
A hollow fiber membrane with a thickness of 280 μm and 40 μm was obtained.
When a cross section of this hollow fiber membrane was observed using an electron microscope at a magnification of 50,400 times, it was observed that there was a difference in the microstructure between the inner and outer surfaces. Note that FIG. 1 shows the vicinity of the outer surface of the hollow fiber membrane, FIG. 2 shows the inside, and FIG. 3 shows the vicinity of the inner surface. The hollow fiber membrane has a membrane area of 8 cm ²
After bundling and making into modules, they are attached to a stainless steel pervaporation device to produce an azeotropic liquid mixture of ethanol/water (95/5% by weight).
The table below shows the results of measuring separation characteristics at 25℃ and 7mmHg.
Shown in 1.

[Table] Example 2 An ethylene-vinyl alcohol copolymer with an ethylene content of 18 mol% was used, and it was dissolved in a water-containing organic solvent consisting of dimethyl sulfoxide and water at a mixing ratio of 9:1 to give a concentration of 33% by weight. After adjusting the solution, it is cast onto a glass plate and kept at 5℃ along with the glass plate.
After forming a film in water of
Table 2 shows the results of measuring the separation characteristics with Hg.

[Table] Example 3 A tube-shaped copper ammonia regenerated cellulose membrane for commercially available coil-type artificial kidneys with a membrane thickness of 25 μm (manufactured by Enca, Germany, trade name: 200 pM) was attached to a pervaporation device, and ethanol/water (50 μm) was attached to the pervaporation device. Table 3 shows the results of measuring the separation characteristics of a mixed liquid (20% by weight) at 60°C and 1 mmHg for 100 hours.

[Table] Example 4 The cuprammonium regenerated cellulose membrane used in Example 3 was attached to a pervaporation device, and an azeotropic liquid mixture of ethanol/water (95/5% by weight) was applied.
Table 4 shows the results of measuring the separation characteristics after 100 hours at 60°C and 1 mmHg.

[Table] Example 5 Ethylene-vinyl alcohol copolymer (ethylene content 18 mol%) was dissolved in a mixed solvent of propanol/water (50/50 wt%) to a concentration of 30 wt%, and the solution was placed on a glass plate at room temperature. and immersed together with a glass plate in water at 5° C. to obtain a wet coagulation film. As a result of observing the cross section of this film with an electron microscope at a magnification of 50,400 times, it was found that it was an anisotropic film with a degree of orientation of 70% and a dense layer on the side that was in contact with water (easily solidified medium) during film formation. observed. Table 5 shows the results of measuring the separation characteristics of an ethanol/water (95/5 wt%) mixed liquid at 25°C and 7 mmHg by attaching the above membrane to a pervaporation device.

【table】 [Brief explanation of the drawing]

Figures 1, 2, and 3 are magnifications of the cross section of the ethylene-vinyl alcohol copolymer hollow fiber membrane in the water-swollen state, respectively, obtained by the frozen replica method.
This is a transmission electron micrograph at 50400x magnification.

Claims (1)

[Claims] 1. A mixed liquid to be separated is supplied to the membrane surface of the membrane formed by a wet method on the side that came into contact with the hard-to-solidify medium, and the components that have permeated through the membrane are transferred to the membrane surface of the membrane on the side that came into contact with the easy-to-solidify medium. A method for separating a mixed liquid characterized by extracting it in a more gaseous state.