Method for analyzing performance changes of composite semipermeable membranes, program therefor, and recording medium.

Abstract

This provides a method for quantitatively determining the contribution of consolidation to the performance of altered composite semipermeable membranes. [Solution] A method for analyzing the performance change of a composite semipermeable membrane due to compaction, characterized by using a relationship formula between a compaction index obtained in advance and the performance of the composite semipermeable membrane, and the measurement results of the compaction index of the composite semipermeable membrane whose performance has changed.

Description

Technical Field

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[0001] The present invention relates to a method for analyzing the performance change of a composite semipermeable membrane used for the selective separation of liquid mixtures.

Background Art

[0002] Regarding the separation of liquid mixtures, there are various techniques for removing substances (such as salts) dissolved in a solvent (such as water). However, the use of membrane separation methods, which have the characteristics of energy saving, space saving, and high separation performance, is expanding. The membranes used in membrane separation methods include microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, reverse osmosis membranes, etc. Separation membrane elements using these membranes are applied to various uses such as desalination of seawater and brackish water, production of ultrapure water, reuse of wastewater, and recovery of valuable substances.

[0003] Most of the reverse osmosis membranes and nanofiltration membranes that have been put into practical use are composite semipermeable membranes, and there are two types: those having a separation functional layer formed by crosslinking a gel layer and a polymer on a support membrane, and those having a separation functional layer formed by polycondensing monomers on a support membrane. Among them, a composite semipermeable membrane obtained by coating a support membrane with a separation functional layer composed of a crosslinked polyamide obtained by a polycondensation reaction between a polyfunctional amine and a polyfunctional acid halide is widely used as a high-performance separation membrane with excellent solvent permeability and selective separation properties.

[0004] Although the performance of the separation membrane varies depending on the type of separation membrane, the composition of the liquid to be treated, and the operating conditions, the performance of the separation membrane changes during operation. Therefore, for the stable operation of a liquid treatment device, it is necessary to grasp the cause of the performance change of the separation membrane and perform operation management to reduce its influence.

[0005] The factors causing changes in the performance of the separation membrane are diverse, such as physical damage due to contact with foreign substances that have flowed in, chemical changes in the membrane structure due to contact with chemical substances such as chemicals, and adhesion of inflow substances called fouling. Among them, compaction is an irreversible change factor of the separation membrane caused by high-pressure operation.

[0006] Non-Patent Document 1 describes how the time-dependent change in separation membrane performance due to consolidation depends on the membrane material and specifications, operating time, the temperature of the liquid being treated, and the pressure of the liquid being treated. Non-Patent Document 2 describes a method for expressing the time-dependent change in separation membrane performance due to consolidation using a consolidation coefficient (m value). In the empirical formula described in Non-Patent Document 2, the m value is treated as constant if the operating conditions are constant, but in actual liquid treatment equipment, the water temperature of the liquid being treated changes seasonally, and the pressure of the liquid being treated changes accordingly, so the m value is not necessarily constant. Non-Patent Document 3 describes a method for more accurately estimating the m value after a predetermined time has elapsed by sequential calculations based on the time-dependent changes in the temperature and pressure of the liquid being treated.

[0007] All of these known techniques for consolidation estimate the change in separation membrane performance due to consolidation based on operating condition information, and therefore require the acquisition of the above operating condition information. However, liquid processing equipment that is not equipped with sufficient instruments may not be able to acquire the above operating condition information. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] Journal of Membrane Science, 2010, Vol.347, P.159-164 [Non-Patent Document 2] Kimura, Naofumi, Membrane, Vol. 6, No. 3, 83 (1981) [Non-Patent Document 3] Naohiko UKAWA, Ikuo NAKATANI, and Hideo IWAHASHI, Journal of the Japan Society for Marine Science, Vol. 43, No. 4, 218 (1989) [Overview of the project] [Problems that the invention aims to solve]

[0009] Conventional methods allow for understanding the state of the support membrane as it changes due to compaction during operation, but it is difficult to estimate the separation membrane performance as it changes due to compaction during operation based on this understanding.

[0010] The present invention has been made in view of the above, and aims to provide an analytical method for quantitatively understanding the contribution of consolidation to the performance of a modified composite semipermeable membrane. [Means for solving the problem]

[0011] To solve the above problems, the present invention comprises the following configurations (1) to (6). (1) A method for analyzing changes in performance of a composite semipermeable membrane due to compaction, comprising the following steps A to C. Step A: A step of cutting the composite semipermeable membrane to obtain a composite semipermeable membrane sample. Step B: A step of analyzing the cross-sectional structure of the composite semipermeable membrane sample and measuring the compaction index of the composite semipermeable membrane. Step C: A step of calculating the performance change due to the compaction of the composite semipermeable membrane using the relationship formula between the compaction index of the composite semipermeable membrane obtained in advance and the performance ratio or performance difference before and after the performance change due to compaction of the composite semipermeable membrane, and the measurement results of the compaction index of the composite semipermeable membrane obtained in Step B. (2) The method for analyzing changes in the performance of a composite semipermeable membrane according to (1), characterized in that the composite semipermeable membrane has a porous support layer between the separation functional layer and the substrate, and the compaction index of the composite semipermeable membrane is the thickness of the porous support layer, the pore diameter of the porous support layer, or the porosity of the porous support layer. (3) The method for analyzing the performance change of a composite semipermeable membrane according to (2), characterized in that the performance ratio or performance difference before and after the performance change of the composite semipermeable membrane in step C is the ratio or difference of solute removal rate, solute permeability, solute permeability coefficient, membrane permeation flux, pure water permeability coefficient, water production amount, or pressure drop before and after the performance change of the composite semipermeable membrane. (4) A composite semipermeable membrane performance change analysis program that, in order to analyze the performance change due to the compaction of the composite semipermeable membrane, measures the compaction index of the composite semipermeable membrane and then controls the computer to function as a relational expression input means for inputting a relational expression that has been previously acquired between the compaction index of the composite semipermeable membrane and the performance ratio or performance difference before and after the performance change due to the compaction of the composite semipermeable membrane, a relational expression storage means for storing the relational expression, and a performance change calculation means for calculating the performance change due to the compaction of the composite semipermeable membrane before and after the performance change based on the input value of the measurement result of the compaction index and the relational expression. (5) A program for analyzing changes in performance of a composite semipermeable membrane according to (4), characterized in that the composite semipermeable membrane has a porous support layer between the separation functional layer and the substrate, and the consolidation index of the composite semipermeable membrane is the thickness of the porous support layer, the pore diameter of the porous support layer, or the porosity of the porous support layer. (6) A computer-readable recording medium on which a program for analyzing the performance changes of a composite semipermeable membrane described in either (4) or (5) is recorded. [Effects of the Invention]

[0012] According to the performance change analysis method of the present invention, it is possible to quantitatively grasp the contribution of consolidation to the changed performance of a composite semipermeable membrane. [Brief explanation of the drawing]

[0013] [Figure 1] This graph plots the relationship between the thickness of the porous support layer in the cross-section and the membrane permeation flux ratio before and after compaction treatment of the composite semipermeable membrane. [Modes for carrying out the invention]

[0014] The present invention will be described in detail below, but these are merely examples of preferred embodiments, and the present invention is not limited to these.

[0015] The present invention relates to a method for analyzing the cross-sectional structure of a composite semi-permeable membrane after performance changes and analyzing the performance changes due to the consolidation of the composite semi-permeable membrane. Different from the prior art, without measuring the change over time of the separation membrane performance that occurs during the operation of a water treatment plant, it is possible to quantitatively analyze the contribution of consolidation to the separation membrane performance that has changed during the operation of the water treatment plant. Also, when analyzing the contribution of consolidation to the performance changes of the composite semi-permeable membrane, it is possible to perform a highly accurate analysis that excludes the influence of other performance change factors. Note that the performance changes of the composite semi-permeable membrane in the present invention refer to changes in the performance of the composite semi-permeable membrane, such as solute removal rate, solute permeation rate, solute permeation coefficient, membrane permeation flux, pure water permeation coefficient, water production rate, pressure loss, etc.

[0016] (Object of analysis) In the performance change analysis method of the present invention, the composite semi-permeable membrane to be analyzed is not particularly limited in its shape or material, but a composite material in which a separation functional layer is formed on a support membrane is exemplified. In particular, currently, crosslinked polyamide composite semi-permeable membranes that are widely used in various applications are the main objects of analysis.

[0017] The crosslinked polyamide composite semi-permeable membrane is a composite membrane composed of three layers: a base material, a porous support layer, and a separation functional layer made of crosslinked polyamide. The support membrane composed of the base material and the porous support layer does not substantially exhibit separation performance such as ions, and is for providing strength to the separation functional layer that controls the separation performance.

[0018] The material and shape of the base material are not particularly limited, but fabrics or non-woven fabrics mainly composed of at least one selected from polyester, polyamide, and polyolefin are exemplified. Polyester with high mechanical and thermal stability is preferably used. The thickness of the base material is generally within the range of 10 to 200 μm in order to ensure dimensional stability.

[0019] The material and shape of the porous support layer provided between the base material and the separation functional layer are not particularly limited. Generally, it has a porous structure with fine pores of about 0.1 nm to 100 nm on the surface on the side where the separation functional layer is formed. For example, it can be obtained by phase-separating a polymerized polymer cast on the base material. As the material of the porous support layer, various polymer materials such as polysulfone, polyethersulfone, polyphenylene sulfide sulfone, polyphenylene sulfone, and cellulose acetate are used alone or in combination. Polysulfone, which has high chemical, mechanical, and thermal stability and is easy to mold, is generally used.

[0020] As the separation functional layer that substantially exhibits separation performance such as ions in the composite semipermeable membrane, various materials and structures have been developed. Examples include thin films made of materials such as polyamide, cellulose acetate, graphene, polystyrene sulfonic acid, polyallylamine, and siloxane derivatives. Although not particularly limited, a crosslinked polyamide thin film with excellent water permeability and selective separation is preferably used. The crosslinked polyamide separation functional layer is formed by a polycondensation reaction between a polyfunctional amine and a polyfunctional acid halide and usually has a thickness of about 0.01 to 1 μm.

[0021] Here, densification means that the composite semipermeable membrane is irreversibly deformed by high-pressure operation.

[0022] Embodiments of the present invention include the following steps A to C in order to analyze the performance change due to densification of the composite semipermeable membrane.

[0023] (Step A) In the embodiments according to the present invention, the composite semipermeable membrane to be analyzed is cut, and a composite semipermeable membrane sample is obtained. The cutting direction is preferably perpendicular to the plane direction of the composite semipermeable membrane. The cutting method is not particularly limited, but in order to maintain the cross-sectional structure of the composite semipermeable membrane before and after cutting, it is preferable to cut in a frozen state. For example, a method of immersing the composite semipermeable membrane in liquid nitrogen to freeze it and then cutting it with a thin blade can be mentioned.

[0024] (Step B) In the method for analyzing performance changes of a composite semipermeable membrane according to the present invention, the densification index is measured by analyzing the cross-sectional structure of the fractured sample of the composite semipermeable membrane obtained in step A above. As the densification index in the cross-sectional structure, any structural value that indicates the morphology of the composite semipermeable membrane that changes with the operation of the composite semipermeable membrane can be used.

[0025] In the case of a composite semipermeable membrane having a porous support layer between the separation functional layer and the substrate, the densification index is preferably the morphology of the porous support layer. The morphology of the porous support layer that serves as the densification index is preferably the thickness of the porous support layer, the pore size of the porous support layer, or the porosity of the porous support layer. Since these densification indices decrease with increasing densification, having one of these as the densification index makes it possible to precisely analyze the relationship between densification and the change in performance of the composite semipermeable membrane.

[0026] The compaction index in the cross-sectional structure of a fractured sample can be measured by microscopic observation of the porous support layer of the frozen fractured sample. For example, the morphology of the porous support layer can be observed using instruments such as a scanning electron microscope or a transmission electron microscope. When observing with a scanning electron microscope, the sample prepared in step A for cross-sectional observation is thinly coated with platinum, platinum-palladium, or ruthenium tetrachloride, preferably ruthenium tetrachloride, and observed with a high-resolution field emission scanning electron microscope (UHRFE-SEM) at an accelerating voltage of 3 to 15 kV. For example, the S-900 electron microscope from Hitachi, Ltd. can be used as the high-resolution field emission scanning electron microscope. From the obtained electron microscope images, the compaction index indicating the morphological structure of the porous support layer is determined. The thickness of the porous support layer, one of the compaction indices, can be obtained, for example, by photographing a cross-section of the composite semipermeable membrane at 1,000x magnification and measuring the distance from the point where the porous support layer contacts the separation functional layer to the point where it contacts the substrate in a direction perpendicular to the membrane surface. The pore diameter of the porous support layer can be determined, for example, by photographing a cross-section of the composite semipermeable membrane at 10,000x magnification, loading the image into image analysis software such as image-J, performing black and white binarization, and then analyzing the particles of the black area within a 5 μm square range in the depth direction from the membrane surface. The porosity of the porous support layer can be determined, for example, by photographing a cross-section of the composite semipermeable membrane at 10,000x magnification, loading the image into image analysis software such as image-J, performing black and white binarization, and then calculating the area ratio of the black area within a 5 μm square range in the depth direction from the membrane surface. Note that the compaction index shown in this invention is the average value of 20 or more points.

[0027] (Process C) In the present invention's method for analyzing performance changes of a composite semipermeable membrane, the performance change due to compaction of the composite semipermeable membrane, which is the subject of analysis, is calculated from the measurement results of the compaction index obtained in step B, using a previously acquired relationship formula between the compaction index of the composite semipermeable membrane and the performance ratio or performance difference before and after compaction of the composite semipermeable membrane.

[0028] The method for creating the above relational equation is explained below. First, multiple composite semipermeable membrane samples are obtained from composite semipermeable membranes manufactured using the same method as the composite semipermeable membrane to be analyzed, and compacted composite semipermeable membrane samples are prepared by compacting each sample under different conditions. Here, the composite semipermeable membrane samples may be composite semipermeable membrane fragments obtained by disassembling the separation membrane element and cutting it into a size that is easy to handle. Next, the separation membrane performance is evaluated for each compacted composite semipermeable membrane sample and the uncompacted composite semipermeable membrane sample. From these evaluation results, the performance ratio or performance difference of the composite semipermeable membrane before and after the performance change due to compaction is determined for each compaction condition. At this time, the form of the composite semipermeable membrane to be evaluated is not limited to flat membranes, but may also be elements processed in combination with components such as flow channel materials and permeable water pipes. The compaction index is measured for each composite semipermeable membrane sample after the performance evaluation. A relational equation is created based on the correlation between the measurement results of the compaction index of each sample and the performance ratio or performance difference before and after the performance change due to each compaction condition. For example, one can create an approximation formula that approximates the relationship between the measured consolidation index and the performance ratio as a function in a planar coordinate system plotted with the consolidation index measurement results on the x-axis and the performance ratio on the y-axis. The method for obtaining the approximation line can be the least squares method or multiple regression analysis. Furthermore, there are no restrictions on the shape of the approximation line.

[0029] A pre-compacted composite semipermeable membrane can be prepared, for example, by applying water pressure to the composite semipermeable membrane or a separation membrane element equipped with a composite semipermeable membrane under arbitrary conditions (pressure, temperature, time, etc.) in a pressure vessel.

[0030] Furthermore, in the performance change analysis method of the present invention, in step C, instead of "a relationship formula between the densification index of the composite semipermeable membrane obtained in advance and the performance ratio or performance difference before and after the performance change due to densification of the composite semipermeable membrane," a relationship formula between the ratio or difference of the densification index before and after the performance change due to densification of the composite semipermeable membrane obtained in advance and the performance ratio or performance difference before and after the performance change due to densification of the composite semipermeable membrane is used. In addition, instead of the measured value of the densification index obtained in step B, the ratio or difference of the densification index before and after the performance change obtained in step B is used to calculate the performance change due to densification of the composite semipermeable membrane. That is, the performance change due to densification of the composite semipermeable membrane can be calculated using the relationship formula between the ratio or difference of the densification index before and after the performance change due to densification obtained in advance and the performance ratio or performance difference before and after the performance change due to densification, and the ratio or difference of the densification index before and after the performance change obtained in step B.

[0031] The performance ratio or difference before and after the performance change of the composite semipermeable membrane is preferably a ratio or difference of solute removal rate, solute permeability, solute permeability coefficient, membrane permeation flux, pure water permeability coefficient, water production rate, or pressure drop, which are indicators of the performance of the composite semipermeable membrane. Among these, an index related to water permeability, which is easy to measure and analyze, is particularly preferred.

[0032] (Program for analyzing changes in the performance of composite semipermeable membranes, recording medium) Another embodiment of the present invention is a performance change analysis program for a composite semipermeable membrane, which causes a computer to function as a relational expression input means for inputting a relational expression between a previously acquired compaction index of the composite semipermeable membrane and the performance ratio or performance difference before and after compaction; a relational expression storage means for storing the relational expression; and a performance change calculation means for calculating the performance change due to compaction of the composite semipermeable membrane before and after compaction based on the relational expression in response to the input of a measured value of the compaction index. In this embodiment, the computer having each of these means is made to function for diagnosing the performance change state of the separation membrane. In this embodiment, the computer can be recorded on a recording device such as the computer's memory or hard disk, and the form of recording is not particularly limited. Another embodiment of the present invention is a computer-readable recording medium on which the above-mentioned performance change analysis program for the composite semipermeable membrane is stored.

[0033] The consolidation index entered into the computer is not particularly limited as long as it is a value of the structure or performance that changes along with the performance of the composite semipermeable membrane, but as mentioned above, it is preferable to use the thickness of the porous support layer, the pore size of the porous support layer, or the porosity of the porous support layer. [Examples]

[0034] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way by these examples.

[0035] (Method for evaluating water permeability) A reverse osmosis membrane element used for seawater desalination was disassembled, and multiple composite semipermeable membrane fragments were cut out and used as samples. A flat membrane testing apparatus was used to evaluate the water permeability. Under the measurement conditions, a sodium chloride aqueous solution with a concentration of 32,000 mg / L, pH 6.5, and temperature of 25°C was supplied at a pressure of 5.5 MPa and a concentrated water flow rate of 3.5 L / min, and the membrane permeation flux of each sample was measured.

[0036] (Method for measuring the compaction index) A composite semipermeable membrane fragment was immersed in liquid nitrogen for 30 seconds. After freezing, the fragment was immediately cut perpendicular to the planar direction with a thin-bladed knife to prepare a sample for cross-sectional observation. This sample was coated with ruthenium tetrachloride and observed with a high-resolution field emission scanning electron microscope (Hitachi S-900) to obtain cross-sectional images. In the cross-sectional images, the distance from the point where the porous support layer contacts the separation functional layer to the point where it contacts the substrate was measured at 20 arbitrary points perpendicular to the membrane surface, and the thickness of the porous support layer was determined by taking the average value.

[0037] <Reference example 1> Unused reverse osmosis membrane elements for seawater desalination were dismantled, and multiple composite semipermeable membrane fragments were cut out. These fragments were then treated with a sodium chloride aqueous solution at a concentration of 32,000 mg / L, pH 6.5, and temperature 35°C, under conditions of 7.0 MPa pressure and a concentrated water flow rate of 3.5 L / min. The treatment time was varied to 5 minutes, 15 minutes, 1 hour, and 6 hours, respectively, to obtain multiple composite semipermeable membrane fragments treated under different conditions. Subsequently, the membrane permeation flux was measured for both the untreated composite semipermeable membrane fragments and each of the fragments treated under different conditions using a flat membrane testing apparatus. From the measurement results, the membrane permeation flux ratio before and after consolidation was determined by dividing the membrane permeation flux after consolidation by the membrane permeation flux before consolidation. Following the permeability evaluation, the thickness of the porous support layer was measured as a consolidation index for each composite semipermeable membrane fragment.

[0038] The thickness of the porous support layer in each composite semipermeable membrane fragment obtained above and the membrane permeation flux ratio before and after compaction treatment of the composite semipermeable membrane were plotted as shown in Figure 1, and the relationship was formulated mathematically to obtain the following equation.

[0039] The membrane permeation flux ratio of a composite semipermeable membrane = 0.0534 × (thickness of the porous support layer) - 1.07 <Example 1> A reverse osmosis membrane element for seawater desalination, identical to that used in Reference Example 1 for one year, was disassembled, and multiple composite semipermeable membrane fragments were cut out. The permeability of the composite semipermeable membrane fragments was evaluated using a flat membrane testing apparatus, and the membrane permeation flux ratio after one year of use compared to the permeability at the time of production was calculated to be 0.86.

[0040] After the water permeability evaluation, the thickness of the porous support layer of the composite semipermeable membrane was measured in the same manner as in Reference Example 1, and a value of 38 μm was obtained.

[0041] Using the thickness of the porous support layer, measured as a consolidation index, and the relationship between the consolidation index and the membrane permeation flux ratio obtained in Reference Example 1, the performance change due solely to consolidation in a composite semipermeable membrane used for seawater desalination for one year was calculated to be 0.96. That is, the decrease in the membrane permeation flux ratio due to consolidation was estimated to be 0.04. On the other hand, the membrane permeation flux ratio was 0.86 from the measured water permeability of an unused composite semipermeable membrane and the measured water permeability of a composite semipermeable membrane after one year of use. That is, from the measured water permeability, the membrane permeation flux ratio of the composite semipermeable membrane after one year of use decreased by 0.14. From this, it was determined that in composite separation membranes used for one year in a seawater desalination plant, performance degradation occurred not only due to consolidation but also due to factors other than consolidation. [Industrial applicability]

[0042] This invention makes it possible to analyze the contribution of consolidation to performance changes in composite semipermeable membranes separately from the influence of other degradation factors, and is therefore useful for elucidating the causes of performance degradation and improving operation in water treatment plants.

Claims

1. A method for analyzing performance changes of a composite semipermeable membrane due to compaction, characterized by comprising the following steps A to C. Step A: A step of cutting the composite semipermeable membrane to obtain a composite semipermeable membrane sample. Step B: A step of analyzing the cross-sectional structure of the composite semipermeable membrane sample and measuring the compaction index of the composite semipermeable membrane. Step C: A step of calculating the performance change due to the compaction of the composite semipermeable membrane using the relationship formula between the compaction index of the composite semipermeable membrane obtained in advance and the performance ratio or performance difference before and after the performance change due to compaction of the composite semipermeable membrane, and the measurement results of the compaction index of the composite semipermeable membrane obtained in Step B.

2. The composite semipermeable membrane has a porous support layer between the separation functional layer and the substrate. The method for analyzing changes in the performance of a composite semipermeable membrane according to claim 1, characterized in that the consolidation index of the composite semipermeable membrane is the thickness of the porous support layer, the pore diameter of the porous support layer, or the porosity of the porous support layer.

3. The method for analyzing the performance change of a composite semipermeable membrane according to claim 2, characterized in that the performance ratio or performance difference before and after the performance change of the composite semipermeable membrane in step C is the ratio or difference of solute removal rate, solute permeability, solute permeation coefficient, membrane permeation flux, pure water permeation coefficient, water production amount, or pressure drop before and after the performance change of the composite semipermeable membrane.

4. To analyze the performance changes due to compaction of the composite semipermeable membrane, after measuring the compaction index of the composite semipermeable membrane, a computer is used. A relational expression input means for inputting a relational expression between the densification index of the composite semipermeable membrane, which has been acquired in advance, and the performance ratio or performance difference before and after the performance change due to the densification of the composite semipermeable membrane. A relational expression storage means for storing the aforementioned relational expression, A performance change calculation means that calculates the performance change due to consolidation before and after the performance change of the composite semipermeable membrane, based on the input value of the measurement result of the consolidation index and the relational formula. A program for analyzing changes in the performance of composite semipermeable membranes to enable them to function as such.

5. The composite semipermeable membrane has a porous support layer between the separation functional layer and the substrate. A program for analyzing changes in the performance of a composite semipermeable membrane according to claim 4, characterized in that the consolidation index of the composite semipermeable membrane is the thickness of the porous support layer, the pore diameter of the porous support layer, or the porosity of the porous support layer.

6. A computer-readable recording medium that records a program for analyzing the performance change of a composite semipermeable membrane according to claim 4 or claim 5.

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