JP7595022B2 - Method for producing aqueous polymer dispersion - Google Patents

Description

The present invention relates to a method for producing an aqueous polymer dispersion having a concentration step by membrane filtration. More specifically, the present invention relates to a method for producing an aqueous polymer dispersion having a concentration step by membrane filtration with reduced fouling (membrane blockage).

Conventionally, solid-liquid separation has been performed by membrane filtration. For example, Patent Document 1 describes the clarification of suspended water by external pressure filtration using a porous hollow fiber membrane with an outer surface opening rate of 20% or more and a minimum pore size layer pore size of 0.03 μm to 1 μm. Patent Document 2 describes the separation of a fluid to be treated by forming a flow path with a flow path diameter of 1.5 mm or less inside a separation membrane for solid-liquid separation, and passing the fluid to be treated through the flow path under conditions of a membrane surface linear velocity of 6 m/s or more.

International Publication No. 2001/053213 JP 2017-94310 A

However, the membrane filtration method described in Patent Document 1 had the problem that suspended matter entered the inside of the filtration membrane, causing fouling inside the filtration membrane, or that components dissolved in the suspension were adsorbed to the inside of the filtration membrane, causing fouling inside the filtration membrane. Also, the separation treatment method described in Patent Document 2 had the problem that components dissolved in the treated fluid were adsorbed to the inside of the filtration membrane, causing fouling inside the filtration membrane.

To solve the above problems, the present invention provides a method for producing an aqueous polymer dispersion by membrane filtration in which fouling on the inner surface and inside the filtration membrane is suppressed.

In one or more embodiments, the present invention relates to a method for producing an aqueous polymer dispersion, the method including a membrane filtration step of filtering an aqueous polymer dispersion containing a polymer and water using a filtration membrane to obtain a concentrated aqueous polymer dispersion and a filtrate not containing a polymer, the filtration membrane having pores on its inner surface, and when the total number of pores is taken as 100%, 50% or more of the pores have a pore size smaller than the minimum particle size of the polymer, and the variance of the surface pore size distribution is 0.2 or less, and in the membrane filtration step, an initial SUVA ratio represented by the following mathematical formula (1) is 0.8 to 1.2, and the absolute value of the difference between the initial SUVA ratio represented by the following mathematical formula (1) and the secondary SUVA ratio represented by the following mathematical formula (2) is 0.2 or less.

According to the present invention, in the filtration membrane used in the production method of polymer dispersion, by reducing fouling on the inner surface and inside of the filtration membrane, it is possible to extend the life of the membrane and increase the productivity of the polymer dispersion. In addition, by reducing fouling, it is possible to reduce the frequency of use of chemicals to clean the filtration membrane, and membrane deterioration due to the chemicals can be prevented, so that the life of the membrane can be extended and the productivity of the polymer dispersion can be increased. In addition, it is possible to reduce the frequency of use of chemicals to clean the filtration membrane, so that the cost of using the chemicals can be reduced and the productivity of the polymer dispersion can be increased.

FIG. 1 is a graph showing the solid concentration of PHBH aqueous dispersions measured in Example 1 and Comparative Example 1, and the ratio of the permeation flux (J) at each solid concentration to the initial permeation flux (J0). FIG. 2 is a graph showing the permeation flow rate per membrane area of the PHBH aqueous dispersion measured in Example 1 and Comparative Example 1, and the difference in SUVA ratio (the absolute value of the difference between the initial SUVA ratio and the SUVA ratio at the permeation flow rate per given membrane area). FIG. 3 is a graph showing the solid concentration and the difference in SUVA ratio (the absolute value of the difference between the initial SUVA ratio and the SUVA ratio at each solid concentration) of the PHBH aqueous dispersion measured in Example 1 and Comparative Example 1. FIG. 4A is an SEM photograph of the membrane cross section after concentrating the polymer dispersion of the filtration membrane used in Example 1, and FIG. 4B is an SEM photograph of the membrane cross section after concentrating the polymer dispersion of the filtration membrane used in Comparative Example 1. FIG. 5A is an SEM photograph of a membrane cross section before concentrating the polymer dispersion of the filtration membrane used in Example 1, and FIG. 5B is an SEM photograph of a membrane cross section before concentrating the polymer dispersion of the filtration membrane used in Comparative Example 1. FIG. 6 is a graph showing the total permeation flux per membrane area measured in Example 2 and Comparative Example 2, and the ratio of the permeation flux (J) at a predetermined permeation flux to the initial permeation flux (J0). FIG. 7 is a schematic diagram of a filtration device used for concentrating the PHBH aqueous dispersion in Example 1 and Comparative Example 1. FIG. 8 is a schematic diagram of a filtration device used for circulating the PHBH aqueous dispersion in Example 2 and Comparative Example 2.

The inventors of the present invention have intensively studied the suppression of fouling on the inner surface and inside of a filtration membrane used in solid-liquid separation during the production of a polymer dispersion. As a result, they have found that fouling on the inner surface and/or inside of a filtration membrane can be effectively reduced by using a filtration membrane in which, when the total number of pores present on the inner surface of the filtration membrane is taken as 100%, 50% or more of the pores have a pore size smaller than the minimum particle size of the polymer, and the variance of the surface pore size distribution is 0.2 or less, and by setting the initial SUVA ratio (represented by the mathematical formula (1) described later) to 0.8 or more and 1.2 or less, and setting the absolute value of the difference between the initial SUVA ratio and the secondary SUVA ratio (represented by the mathematical formula (2) described later) to 0.2 or less in the membrane filtration process. In one or more aspects of the present invention, the "inner surface" of the filtration membrane means the filtration surface in contact with the polymer dispersion.

The present invention is described in detail below, but is not limited to the following aspects.

In one or more embodiments of the present invention, the method for producing a polymer aqueous dispersion includes the following step (a) of concentrating the polymer aqueous dispersion as an essential step. Step (a) is a membrane filtration step in which the polymer aqueous dispersion is filtered through a filtration membrane to obtain a concentrated polymer aqueous dispersion and a polymer-free filtrate.

In step (a), the polymer aqueous dispersion is fed into the inside of the filtration membrane, whereby the polymer aqueous dispersion is filtered through the filtration membrane. The filtration method is not particularly limited, and examples thereof include a cross-flow method (a method in which the flow directions of the liquid to be filtered and the filtrate are perpendicular to each other) and a dead-end method (a method in which the flow directions of the liquid to be filtered and the filtrate are the same). Among these, the cross-flow method is preferred from the viewpoint of suppressing fouling of the filtration membrane surface by polymer particles.

In one or more aspects of the present invention, the polymer aqueous dispersion contains water and a polymer. Specifically, it is an aqueous dispersion in which polymer particles are dispersed in water. The polymer is not particularly limited, but polyhydroxyalkanoate (PHA), polyvinyl chloride, etc. can be preferably used.

In one or more aspects of the present invention, PHA is a general term for polymers having 3-hydroxyalkanoic acid as a monomer unit. The 3-hydroxyalkanoic acid is not particularly limited, but examples thereof include 3-hydroxypropionate, 3-hydroxybutyrate, 3-hydroxyvalerate, 3-hydroxyhexanoate, 3-hydroxyheptanoate, and 3-hydroxyoctanoate. The PHA may be a homopolymer having one type of 3-hydroxyalkanoic acid as a monomer unit, or a copolymer having two or more types of 3-hydroxyalkanoic acid as monomer units. Specific examples include copolymers of 3-hydroxybutyrate (3HB) and other 3-hydroxyalkanoic acids, and copolymers of 3-hydroxyalkanoic acids containing at least 3-hydroxyhexanoate (3HH) as a monomer unit. Among them, copolymers containing 3HH as a monomer unit, for example, a two-component copolymer (PHBH) of 3HB and 3HH (Macromolecules, 28, 4822-4828 (1995)), a three-component copolymer (PHBVH) of 3HB, 3-hydroxyvalerate (3HV) and 3HH (JP Patent No. 2777757, JP-A-08-289797), etc. are preferred in terms of physical properties. The composition ratio of each monomer unit constituting the two-component copolymer PHBH of 3HB and 3HH is not particularly limited, but when the total of all monomer units is taken as 100 mol%, the 3HH unit is preferably 1 mol% or more and 99 mol% or less, more preferably 1 mol% or more and 50 mol% or less, and even more preferably 1 mol% or more and 25 mol% or less. The composition ratio of each monomer unit constituting the ternary copolymer PHBVH of 3HB, 3HV and 3HH is not particularly limited. When the sum of all monomer units is taken as 100 mol%, for example, the 3HB unit is preferably in a range of 1 mol% or more and 95 mol% or less, the 3HV unit is preferably in a range of 1 mol% or more and 96 mol% or less, and the 3HH unit is preferably in a range of 1 mol% or more and 30 mol% or less.

The polymer aqueous dispersion may contain one type of polymer or two or more types of polymers.

The polymer aqueous dispersion to be concentrated in step (a) may contain other components in addition to water and polymer (particles). Examples of other components include solvents other than water, dispersants, surfactants, preservatives, etc. The content of other components can be appropriately selected depending on the purpose, etc.

The aqueous polymer dispersion to be concentrated in step (a) may be a PHA suspension derived from a disrupted product of a microorganism containing PHA. The PHA suspension can be obtained, for example, as described in WO 2010/116681.

The concentration of the polymer (solid content) in the polymer aqueous dispersion before filtration in step (a), i.e., the membrane filtration step, is not particularly limited, but may be, for example, 5% by weight (w/v%) or more and 40% by weight or less, or 10% by weight or more and 35% by weight or less. Step (a) can effectively concentrate the polymer aqueous dispersion.

The concentration of the polymer (solid content) in the concentrated aqueous polymer dispersion after filtration in step (a), i.e., the membrane filtration step, is not particularly limited, but may be, for example, 30% by weight (w/v%) or more and 65% by weight or less, or 35% by weight or more and 60% by weight or less. This makes it easier to remove water from the concentrated aqueous polymer dispersion.

In one or more aspects of the present invention, when the total number of pores present on the inner surface of the filtration membrane is taken as 100%, pores having a pore size smaller than the minimum particle size of the polymer are 50% or more, and the variance of the surface pore size distribution is 0.2 or less (hereinafter, also referred to as the surface pore size requirement). This effectively prevents polymer particles from entering the inside of the filtration membrane from the surface of the filtration membrane, and suppresses the closure of pores inside the filtration membrane. Preferably, when the total number of pores present on the inner surface of the filtration membrane is taken as 100%, pores having a pore size smaller than the minimum particle size of the polymer are preferably 55% or more, more preferably 60% or more, and even more preferably 65% or more. In addition, although not particularly limited, for example, from the viewpoint of further enhancing the effect of suppressing fouling on the surface and inside of the filtration membrane, when the total number of pores present on the inner surface of the filtration membrane is taken as 100%, it is ideal that pores having a pore size smaller than the minimum particle size of the polymer are 100%, but it may be 85% or less. Specifically, it may be within any of the above lower and upper limits. In addition, the filtration membrane preferably has a surface pore size distribution variance of 0.15 or less, more preferably 0.10 or less, and even more preferably 0.05 or less. In addition, from the viewpoint of further enhancing the effect of suppressing fouling on the surface and inside of the filtration membrane, it is ideal that the surface pore size distribution variance of the filtration membrane is 0, but it may be 0.02 or more. Specifically, it may be within any of the above lower and upper limits.

In one or more aspects of the present invention, the minimum particle size of the polymer refers to the minimum particle size Dmin in a volume-based particle size distribution curve measured by a laser diffraction scattering method using the polymer aqueous dispersion before concentration in step (a) as a sample. In one or more aspects of the present invention, the particle size distribution curve can be measured, for example, using a laser diffraction scattering type particle size distribution measuring device "Microtrac MT3300EX II" manufactured by Microtrac Bell. The measurement sample is a polymer aqueous dispersion that has not been concentrated even once by step (a).

In one or more embodiments of the present invention, the surface pore size of a filtration membrane can be measured, for example, as follows.
(1) The inner surface of the filtration membrane is observed with a scanning electron microscope (SEM), and the image is captured by a computer. The scanning electron microscope is not particularly limited, but for example, a JSM-7001FA manufactured by JEOL Ltd. can be used, and the observation can be performed at 3500 times magnification. The type and magnification of the microscope may be appropriately changed depending on the size of the hole.
(2) All pores present within a range of 5 μm × 5 μm arbitrarily selected from the obtained image are analyzed using image processing software (ImageJ, developed by the National Institutes of Health, USA). Specifically, the SEM image is first binarized to obtain an image in which the pores are black and the structure is white. If the pores and structure cannot be neatly binarized due to the difference in contrast within the analysis image, the pores are painted black before image processing is performed to obtain the Feret diameter of the pores within the obtained analysis range. At this time, pores with an area of 0.0001 μm2 ^{or less} are excluded from the data to cut noise.
In addition, when the filtration membrane is a tubular membrane, the tubular membrane is cut into a half cylinder to expose the inner surface, and the inner surface of the filtration membrane is observed with a scanning electron microscope. In addition, when the filtration membrane is a flat membrane, the surface on the side with the smaller pore size is observed with a scanning electron microscope.

In one or more aspects of the present invention, the filtration membrane satisfies the requirement that, in the membrane filtration process, the initial SUVA ratio (expressed by the mathematical formula (1) described below) is 0.8 or more and 1.2 or less, and the absolute value of the difference between the initial SUVA ratio and the secondary SUVA ratio (expressed by the mathematical formula (2) described below) is 0.2 or less (hereinafter also referred to as the SUVA ratio requirement).

The initial SUVA ratio is 0.8 or more and 1.2 or less, i.e., the initial SUVA ratio is close to 1, and the absolute value of the difference between the initial SUVA ratio and the secondary SUVA ratio is 0.2 or less, i.e., the secondary SUVA ratio is close to the initial SUVA ratio, means that the water quality of the concentrated polymer dispersion and the filtrate is the same. In other words, it means that the organic impurities contained in the concentrated polymer dispersion and the filtrate are of the same label. If the organic impurities contained in the concentrated polymer dispersion and the filtrate are of the same label, the adsorption of the organic impurities on the inner surface and pores of the filtration membrane is reduced.

In one or more embodiments of the present invention, "DOC" is measured, for example, using a total organic carbon meter (TOC-L) manufactured by Shimadzu Corporation. "UV" is measured, for example, using a spectrophotometer (U-3900) manufactured by Hitachi High-Tech Science Corporation.

In one or more aspects of the present invention, the filtration membrane may be made of any material as long as it satisfies the above-mentioned surface pore size requirements. It may be an organic material or an inorganic material. Examples of organic materials include polypropylene, fluorine-based resins, cellulose esters, polysulfone-based resins, polyacrylonitrile, polyimide, and other resins. Examples of fluorine-based resins include polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, tetrafluoroethylene-hexafluoropropylene copolymers, and ethylene-tetrafluoroethylene copolymers. Examples of cellulose esters include cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate. Examples of polysulfone-based resins include polysulfone and polyethersulfone. Examples of inorganic materials include porous ceramics such as alumina, mullite, zirconia, and cordierite, and porous bodies made of porous sintered metals such as stainless steel. Among these, inorganic materials are preferred from the viewpoint of high roundness of the surface pores and easy satisfaction of the above-mentioned SUVA ratio requirements. The shape of the filtration membrane is not particularly limited, and may be any of a tubular membrane, a flat membrane, a hollow fiber membrane, etc. Among them, a tubular membrane is preferred from the viewpoint of preventing the passage from being blocked by the polymer and of easily satisfying the above-mentioned SUVA ratio requirement.

In one embodiment of the present invention, the polymer aqueous dispersion can be produced by the filtration apparatus shown in Figure 7. As shown in Figure 7, the polymer aqueous dispersion a is stirred by the stirrer 4 in the tank 3 and supplied to the filtration membrane 1 via the pump 2. By filtering through the filtration membrane 1, the polymer aqueous dispersion a is separated into a concentrated polymer aqueous dispersion b and a polymer-free filtrate c. The concentrated polymer aqueous dispersion b is returned to the tank 3, and the filtrate c is removed from the system, thereby concentrating the polymer aqueous dispersion in the tank 3.

In one embodiment of the present invention, the polymer aqueous dispersion can be produced by the filtration apparatus shown in Figure 8. When the apparatus shown in Figure 8 is used, the concentration of the polymer aqueous dispersion can be kept constant by returning the filtrate c to tank 3, as shown in Figure 8.

［２］ 前記ポリマーは、ポリヒドロキシアルカノエートである、［１］に記載のポリマー水分散液の製造方法。
［３］ 前記ろ過膜は、無機材料で構成されている、［１］又は［２］に記載のポリマー水分散液の製造方法。
［４］ 膜ろ過工程でろ過される前のポリマー水分散液の固形分の濃度は１０重量％以上３５重量％以下である、［１］～［３］のいずれか１項に記載のポリマー水分散液の製造方法。
［５］ 膜ろ過工程でろ過された後の濃縮されたポリマー水分散液の固形分の濃度は３５重量％以上６０重量％以下である、［１］～［４］のいずれか１項に記載のポリマー水分散液の製造方法。
［６］ 膜ろ過工程でろ過するポリマー水分散液は、ポリヒドロキシアルカノエートを含有する微生物の破砕物由来のポリヒドロキシアルカノエートの懸濁液である、［１］～［５］のいずれか１項に記載のポリマー水分散液の製造方法。
［７］ 前記ろ過膜は、管状膜である、［１］～［６］のいずれか１項に記載のポリマー水分散液の製造方法。 The invention includes, for example, one or more of the following embodiments.
[1] A membrane filtration step of filtering a polymer aqueous dispersion containing a polymer and water using a filtration membrane to obtain a concentrated polymer aqueous dispersion and a filtrate not containing a polymer,
the filtration membrane has openings on its inner surface, and when the total number of openings is taken as 100%, 50% or more of the openings have a pore size smaller than the minimum particle size of the polymer, and the variance of the surface pore size distribution is 0.2 or less;
A method for producing an aqueous polymer dispersion, wherein in a membrane filtration step, an initial SUVA ratio represented by the following formula (1) is 0.8 or more and 1.2 or less, and an absolute value of a difference between the initial SUVA ratio represented by the following formula (1) and a secondary SUVA ratio represented by the following formula (2) is 0.2 or less.

[2] The method for producing a polymer aqueous dispersion according to [1], wherein the polymer is polyhydroxyalkanoate.
[3] The method for producing a polymer aqueous dispersion according to [1] or [2], wherein the filtration membrane is made of an inorganic material.
[4] The method for producing a polymer aqueous dispersion according to any one of [1] to [3], wherein the solid content concentration of the polymer aqueous dispersion before being filtered in the membrane filtration step is 10% by weight or more and 35% by weight or less.
[5] The method for producing a polymer aqueous dispersion according to any one of [1] to [4], wherein the solid content of the concentrated polymer aqueous dispersion after filtration in the membrane filtration step is 35% by weight or more and 60% by weight or less.
[6] The method for producing a polymer aqueous dispersion according to any one of [1] to [5], wherein the polymer aqueous dispersion to be filtered in the membrane filtration step is a suspension of polyhydroxyalkanoate derived from a disrupted product of a microorganism containing polyhydroxyalkanoate.
[7] The method for producing a polymer aqueous dispersion according to any one of [1] to [6], wherein the filtration membrane is a tubular membrane.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

(Measurement of polymer particle size)
A particle size distribution curve (volume basis) was measured using a laser diffraction scattering type particle size distribution measuring device "Microtrac MT3300EX II" manufactured by Microtrac Bell, and the minimum particle diameter Dmin and median diameter D50 were calculated.

(Measurement of surface pore size of filtration membrane)
(1) The tubular membrane was cut into a semi-cylindrical shape, and the inner surface of the filtration membrane was observed at 3,500 times magnification using a scanning electron microscope ("JSM-7001FA" manufactured by JEOL Ltd.) in a state where the inner surface was exposed, and the image was imported into a computer.
(2) All holes present within a range of 5 μm × 5 μm arbitrarily selected from the obtained image were analyzed using image processing software (ImageJ, developed by the National Institutes of Health, USA). Specifically, the SEM image was first binarized to obtain an image in which the pores were black and the structure was white. In addition, if the pores and the structure could not be neatly binarized due to the difference in contrast within the analysis image, the pores were painted black before image processing was performed to obtain the Feret diameter of the holes within the obtained analysis range. At this time, holes with an area of 0.0001 μm2 or ^less were excluded from the data in order to cut noise.

(Measurement of DOC)
The measurements were made using a Shimadzu total organic carbon meter (TOC-L).

(UV Measurement)
The measurement was carried out using a spectrophotometer (U-3900) manufactured by Hitachi High-Tech Science.

Example 1
Ralstonia eutropha KNK-005 strain was cultured by the method described in Example 1 of WO 2010/116681 to prepare a bacterial cell culture solution containing poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH). Next, the bacterial cell culture solution obtained above was heated and stirred at an internal temperature of 60 to 80°C for 20 minutes to perform sterilization. 0.2% by weight of sodium dodecyl sulfate was added to the sterilized bacterial cell culture solution obtained above. Furthermore, an aqueous sodium hydroxide solution was added so that the pH was 11.0, and the mixture was kept at 50°C for 1 hour. Thereafter, high-pressure homogenization was performed at a pressure of 450 to 550 kgf/ ^cm2 using a high-pressure homogenizer (Nilosoavi "High-pressure homogenizer model PA2K type"). An equal amount of distilled water was added to the homogenized solution obtained above after high-pressure homogenization. This was centrifuged, and the supernatant was removed to concentrate it twice. To this concentrated PHA aqueous suspension, an aqueous sodium hydroxide solution (pH 11) was added in the same amount as the removed supernatant, centrifuged, the supernatant was removed, and water was added again to suspend the suspension. 0.2% by weight of sodium dodecyl sulfate and 1/100 weight of protease (Novozymes, Esperase) of PHA were added, and the suspension was stirred for 2 hours while maintaining pH 10 and 50°C. Then, the supernatant was removed by centrifugation to concentrate 4 times. Further water was added to obtain a PHBH aqueous dispersion having a median diameter (D50) of PHBH particles of 2 μm and a solid content concentration of 26 wt% (PHBH particle content: 260 g/L). The minimum particle diameter (Dmin) of the polymer particles in the PHBH aqueous dispersion was 0.75 μm. In a filtration apparatus shown in Fig. 7 using a tubular membrane (MEMBRALOX (registered trademark) 1T1-70, manufactured by PALL, material: alumina ceramic) as a filtration membrane, the PHBH aqueous dispersion was circulated and supplied to the tubular membrane for concentration. In the tubular membrane, when the total number of pores was taken as 100%, the proportion of the number of pores having a pore size smaller than the minimum particle size of the polymer particles in the PHBH aqueous dispersion was 67%, the variance of the surface pore size distribution was 0.02, the initial SUVA ratio was 0.83, and the absolute value of the difference between the initial SUA ratio and the secondary SUA ratio was 0.05 to 0.2. As shown in FIG. 7 , a polymer aqueous dispersion a is stirred in a tank 3 by a stirrer 4 and supplied to a filtration membrane 1 via a pump 2. By filtering through the filtration membrane 1, the polymer aqueous dispersion a is separated into a concentrated polymer aqueous dispersion b and a polymer-free filtrate c. The concentrated polymer aqueous dispersion b is returned to the tank 3, and the filtrate c is removed from the system, thereby concentrating the PHA aqueous dispersion in the tank 3.

(Comparative Example 1)
A PHBH aqueous dispersion having a median diameter D50 of 2 μm and a solid content concentration of 23 wt % (PHBH particle content: 230 g/L) was obtained in the same manner as in Example 1, except that the amount of water added at the end was changed, using a bacterial cell culture obtained in the same manner as in Example 1. The minimum particle diameter (Dmin) of the polymer particles in the PHBH aqueous dispersion was 0.75 μm. The PHBH aqueous dispersion was circulated and supplied to a tubular membrane (MICRODYN (registered trademark) MD020TP2N, manufactured by Microdyn-Nadir GmbH, material: polypropylene) as a filtration membrane in the filtration apparatus shown in FIG. 7, and concentration was performed. In the tubular membrane, when the total number of pores is taken as 100%, the ratio of the number of pores having a pore size smaller than the minimum particle size of the polymer particles in the PHBH aqueous dispersion is 38%, the variance of the surface pore size distribution is 0.50, the initial SUVA ratio is 0.90, and the absolute value of the difference between the initial SUVA ratio and the secondary SUVA ratio is 0.3. As shown in Fig. 7, the polymer aqueous dispersion a stirred by the stirrer 4 in the tank 3 and supplied to the filtration membrane 1 via the pump 2 is filtered by the filtration membrane 1 to be separated into a concentrated polymer aqueous dispersion b and a filtrate c not containing a polymer, the concentrated polymer aqueous dispersion b is returned to the tank 3, and the filtrate c is removed from the system, thereby concentrating the PHA aqueous dispersion in the tank 3.

The results are shown in Figures 1 to 4. Figure 1 is a graph showing the solid concentration of the PHBH aqueous dispersion measured in Example 1 and Comparative Example 1, and the ratio of the permeation flux (J) at each solid concentration to the initial permeation flux (J0). Figure 2 is a graph showing the permeation flow rate per membrane area of the PHBH aqueous dispersion measured in Example 1 and Comparative Example 1, and the difference in SUVA ratio (the absolute value of the difference between the initial SUVA ratio and the SUVA ratio at the permeation flow rate per membrane area). Figure 3 is a graph showing the solid concentration of the PHBH aqueous dispersion measured in Example 1 and Comparative Example 1, and the difference in SUVA ratio (the absolute value of the difference between the initial SUVA ratio and the SUVA ratio at each solid concentration). Figure 4A is an SEM photograph of the membrane cross section after concentrating the polymer dispersion of the filtration membrane used in Example 1, and Figure 4B is an SEM photograph of the membrane cross section after concentrating the polymer dispersion of the filtration membrane used in Comparative Example 1.

From FIG. 1, in Example 1, even when the concentration was completed and the solid concentration of the aqueous dispersion of the polymer was about 40% by weight, the ratio of the permeation flux to the initial permeation flux was 0.5 or more, and fouling was effectively suppressed. From FIG. 2 and FIG. 3, in Example 1, in the membrane filtration process, the initial SUVA ratio was 0.8 to 1.2, and the absolute value of the difference between the initial SUVA ratio and the secondary SUVA ratio was 0.2 or less, so that the adhesion of impurities to the inside of the filtration membrane was suppressed and fouling inside the filtration membrane was reduced. FIG. 5A is an SEM photograph of the membrane cross section before concentrating the polymer dispersion of the filtration membrane used in Example 1. From the comparison of FIG. 4A and FIG. 5A, it was confirmed that in Example 1, the intrusion of PHBH particles into the inside of the filtration membrane was prevented, and therefore fouling inside the filtration membrane was reduced.

On the other hand, as can be seen from FIG. 1, when the concentration was completed and the solid concentration of the aqueous dispersion of the polymer was about 40% by weight, in Comparative Example 1, the ratio of the permeation flux to the initial permeation flux was less than 0.2, and fouling could not be suppressed. Also, as can be seen from FIGS. 2 and 3, in the membrane filtration process, the absolute value of the difference between the initial SUVA ratio and the secondary SUVA ratio was 0.3 or more, so that the adhesion of impurities to the inside of the filtration membrane could not be suppressed, and fouling inside the filtration membrane could not be suppressed. FIG. 5B is an SEM photograph of the membrane cross section before concentrating the polymer dispersion of the filtration membrane used in Comparative Example 1. As can be seen from the comparison between FIG. 4B and FIG. 5B, in Comparative Example 1, the intrusion of PHBH particles into the inside of the membrane could not be prevented, and therefore fouling inside the membrane could not be suppressed.

Example 2
A PHBH aqueous dispersion having a median diameter D50 of 2 μm and a solid content concentration of 25% by weight (PHBH particle content: 250 g/L) was obtained in the same manner as in Example 1, except that the amount of water added at the end was changed using a bacterial cell culture obtained in the same manner as in Example 1. The minimum particle diameter (Dmin) of the polymer particles in the PHBH aqueous dispersion was 0.75 μm. The same tubular membrane as in Example 1 was used, and the PHBH aqueous dispersion was circulated and supplied to the tubular membrane to perform circulation. As shown in FIG. 8, the concentration of the PHBH aqueous dispersion was kept constant by returning the filtrate c to the tank 3, and circulation was performed in a state where factors causing a decrease in the permeation flux other than fouling were eliminated, and fouling was evaluated.

(Comparative Example 2)
In the same manner as in Example 2, a PHBH aqueous dispersion was obtained in which the median diameter of the PHBH particles was 2 μm and the solid content concentration was 25% by weight (PHBH particle content: 250 g/L). The minimum particle diameter (Dmin) of the polymer particles in the PHBH aqueous dispersion was 0.75 μm. A tubular membrane similar to that used in Comparative Example 1 was used, and the PHBH aqueous dispersion was circulated and supplied to the tubular membrane, and circulation was performed in the same manner as in Example 2. As shown in FIG. 8, the filtrate c was returned to the tank 3 to keep the concentration of the PHBH aqueous dispersion constant, and circulation was performed in a state in which factors causing a decrease in the permeation flux other than fouling were eliminated, thereby evaluating fouling.

The results are shown in Figure 6. Figure 6 is a graph showing the permeation flow rate per membrane area measured in Example 2 and Comparative Example 2, and the ratio of the permeation flux (J) at a given permeation flow rate to the initial permeation flux (J0). As can be seen from Figure 6, in Example 1, the ratio of the permeation flux to the initial permeation flux was 0.8 or more at the stage where the amount of permeated liquid relative to the membrane area was 10,000 kg/m ² , and fouling could be suppressed. On the other hand, in Comparative Example 2, the ratio of the permeation flux to the initial permeation flux was less than 0.5 at the stage where the amount of permeated liquid relative to the membrane area was 10,000 kg/m ² , and fouling could not be suppressed.

1 Filtration membrane 2 Pump 3 Tank 4 Agitator

Claims (7)

The method includes a membrane filtration step of filtering a polymer aqueous dispersion containing a polymer and water using a filtration membrane to obtain a concentrated polymer aqueous dispersion and a filtrate not containing a polymer,
the polymer is a polyhydroxyalkanoate;
The filtration membrane is made of porous ceramics,
the filtration membrane has openings on its inner surface, and when the total number of openings is taken as 100%, 50% or more of the openings have a pore size smaller than the minimum particle size of the polymer, and the variance of the surface pore size distribution is 0.2 or less;
A method for producing an aqueous polymer dispersion, wherein in a membrane filtration step, an initial SUVA ratio represented by the following formula (1) is 0.8 or more and 1.2 or less, and an absolute value of a difference between the initial SUVA ratio represented by the following formula (1) and a secondary SUVA ratio represented by the following formula (2) is 0.2 or less.

The method for producing an aqueous polymer dispersion according to claim 1, wherein the polyhydroxyalkanoate is a copolymer of 3-hydroxybutyrate (3HB) and another 3-hydroxyalkanoic acid . 3. The method for producing an aqueous polymer dispersion according to claim 1, wherein the porous ceramic is at least one selected from the group consisting of alumina, mullite, zirconia, and cordierite . The method for producing a polymer aqueous dispersion according to any one of claims 1 to 3, wherein the solids concentration of the polymer aqueous dispersion before filtration in the membrane filtration step is 10% by weight or more and 35% by weight or less. The method for producing a polymer aqueous dispersion according to any one of claims 1 to 4, wherein the concentrated polymer aqueous dispersion after filtration in the membrane filtration step has a solids concentration of 35% by weight or more and 60% by weight or less. The method for producing a polymer aqueous dispersion according to any one of claims 1 to 5, wherein the polymer aqueous dispersion to be filtered in the membrane filtration step is a suspension of polyhydroxyalkanoate derived from disrupted microorganisms containing polyhydroxyalkanoate. The method for producing a polymer aqueous dispersion according to any one of claims 1 to 6, wherein the filtration membrane is a tubular membrane.

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