JP2022111756A - Method and device for measuring impedance of filter film - Google Patents

Abstract

To accurately measure the impedance of a separation functional layer even when a filter film is pressurized into a water-permeable state.SOLUTION: A method is to measure impedance of a separation functional layer 6a by electrochemical impedance spectroscopy. As an electrolyte 5 on a support layer 6b side, used is an electrolyte including ions of higher valency than that of ions included in an electrolyte 4 on the separation functional layer 6a side. An RO film 6 is put into a water-permeable state to measure the impedance of the separation functional layer 6a.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a method for measuring the impedance of a filter membrane for water treatment, such as a reverse osmosis membrane, and an apparatus for carrying out the method.

Conventionally, as shown in Patent Documents 1 and 2, a filter membrane (filtration membrane) such as a reverse osmosis membrane or a nanofiltration membrane is used as one of the water treatment apparatuses for removing specific components contained in the water to be treated. Membrane filtration devices are widely put into practical use. Filter membranes such as reverse osmosis membranes basically have a separation function layer supported on one side of a support layer, and the specific components to be removed in the electrolytic solution as the water to be treated permeate through the separation function layer. It has the function of blocking and separating from the solvent water.

For this type of filter membrane, there is a demand for grasping the blocking ability of the components to be removed in the electrolytic solution. That is, the filter membrane inevitably deteriorates in water permeability due to fouling (dirt) during continued use, and also deteriorates in ability to block components to be removed. In order to suppress the deterioration, a small amount of chemical agent is added to the raw water during operation to suppress the progress of fouling, and when the deterioration becomes noticeable over time, the operation is temporarily stopped and the cleaning agent is used. However, in the case of a reverse osmosis membrane, since it is a membrane with a very fine nanostructure, depending on the conditions, the drug itself may cause deterioration of the ability to block the components to be removed, and the ability of the filter membrane to block the components to be removed may be reduced. There is a demand for a detailed understanding of

That is, when using this type of filter membrane, a technique for measuring and analyzing the ability to block the components to be removed as much as possible in a permeable operating state in real time, on-line or off-line is required. By utilizing this measurement and analysis technology, it is possible to improve the operating efficiency of the filter membrane, optimize the amount and timing of cleaning chemical usage, and make efficient use of the filter membrane.

Conventionally, as one method for evaluating the progress of fouling of a filter membrane, for example, as shown in Patent Document 2, an alternating current is applied to the electrolytic solution in which the filter membrane is immersed, and under that state, A method of measuring the impedance between the separation function layer and the support layer of the filter membrane and evaluating the progress of fouling based on the measured impedance is known. A method for measuring the impedance of the filter membrane is known as Electrochemical Impedance Spectroscopy (hereinafter referred to as EIS).

JP-A-1-228509 Japanese Patent Publication No. 2018-513383

In the above-described method of measuring the impedance of a filter membrane by EIS, pressure is applied from the separation function layer to the support layer side of the filter membrane to make the filter membrane water permeable and the impedance is measured. As a result, water flows into the support layer side, and the ion concentration on the support layer side decreases. As a result, the impedance attributed to the support layer becomes higher than the impedance attributed to the separation functional layer by about two orders of magnitude, making it difficult to accurately measure the impedance of the separation functional layer. This problem is particularly conspicuous in filter membranes that have a high ability to block components to be removed.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method capable of accurately measuring the impedance of a separation functional layer even when the filter membrane is pressurized to be in a permeable state. and Another object of the present invention is to provide an apparatus capable of implementing such a filter membrane impedance measurement method.

A method for measuring the impedance of a filter membrane according to the present invention comprises:
A filter membrane impedance measuring method comprising: immersing a filter membrane in which a separation functional layer is supported on a support layer in an electrolytic solution; and measuring the impedance of the separation functional layer by electrochemical impedance spectroscopy.
As the electrolytic solution on the support layer side, an electrolytic solution containing ions with a higher valence than the ions contained in the electrolytic solution on the separation functional layer side is used,
It is characterized by measuring the impedance of the separation functional layer with the filter membrane in a water-permeable state.

The above-mentioned "permeable state" means that the osmotic pressure on the separation function layer side (the primary side containing high-concentration ions in the electrolyte) is P1, and the osmotic pressure on the support layer side (contains low-concentration ions in the electrolyte) When the osmotic pressure on the secondary side) is P2, a pressure exceeding the osmotic pressure difference (P1-P2) is applied to the separation function layer side (primary side), and as a result, the osmotic pressure is overcome and separation occurs. It is a state in which water, which is the solvent in the electrolyte solution on the functional layer side, flows into the support layer through the separation functional layer. At this time, ions also slightly flow into the support layer through the separation function layer due to the difference in concentration between the primary side and the secondary side. As a result, a concentration distribution of ions (eg, K ⁺ , Cl ⁻ , etc.) of the electrolytic solution placed on the separation functional layer side occurs in the ultrathin separation functional layer under the water permeable state.

The filter membrane impedance measurement method according to the present invention can be applied when the filter membrane is, for example, a reverse osmosis membrane.

Further, the method for measuring the impedance of a filter membrane according to the present invention can be carried out by suitably using an electrolytic solution containing monatomic ions with a valence of 1 as the electrolytic solution on the side of the separation functional layer. Specific examples of such monatomic ions include lithium ions, sodium ions, potassium ions, fluoride ions, chloride ions, bromide ions, and the like. These ions are particularly suitable when the filter membrane is a reverse osmosis membrane.

In that case, the electrolytic solution containing ^divalent or higher ^{valent ions} is used as the electrolytic solution on the support layer side ^. valent monoatomic ions. In addition to these, trivalent ions such as [Fe(CN) ₆ ] ^3- (hexacyanide iron (III) acid ion) and the like can also be used as divalent or more valent ions.

On the other hand, the filter membrane impedance measuring device according to the present invention is
An impedance measuring device for a filter membrane, in which a filter membrane comprising a separation functional layer supported on a support layer is immersed in an electrolytic solution and the impedance of the separation functional layer is measured by electrochemical impedance spectroscopy,
It is characterized by having an electrolytic solution injection system for injecting an electrolytic solution containing ions having a higher valence than the ions contained in the electrolytic solution on the separation functional layer side into the support layer side.

According to the filter membrane impedance measuring method of the present invention, the electrolytic solution containing ions with a higher valence than the ions contained in the electrolytic solution on the separation functional layer side is injected into the support layer side. Therefore, the impedance caused by the support layer can be kept low. Furthermore, since ions in the electrolytic solution on the support layer side hardly diffuse into the separation functional layer, the distribution of ions in the electrolytic solution on the separation functional layer side in the separation functional layer is hardly affected. As a result, it is possible to accurately measure the impedance of the separation functional layer.

A filter membrane impedance measuring apparatus according to the present invention is a filter membrane in which a separation functional layer is supported on a support layer, and the impedance of the separation functional layer is measured by electrochemical impedance spectroscopy by immersing the filter membrane in an electrolytic solution. The impedance measuring device of (1) has an electrolytic solution injection system for injecting an electrolytic solution containing ions having a higher valence than the ions contained in the electrolytic solution on the separation functional layer side into the support layer side. An effective method for measuring the impedance of a filter film can be implemented.

1 is a schematic configuration diagram showing a filter membrane impedance measuring device according to an embodiment of the present invention; FIG. Diagram showing the equivalent circuit of the filter membrane whose impedance is to be measured Nyquist Plot diagram of the equivalent circuit in Figure 2 (schematic diagram) Nyquist plot of filter membrane measured by the method of the present invention A diagram showing the Nyquist plot of the filter membrane of FIG. 4 together with a curve fitting this Nyquist plot to the characteristics of an equivalent circuit. Nyquist plot of filter membrane measured after immersion in NaClO aqueous solution A diagram showing the Nyquist plot of the filter membrane of FIG. 6 together with a curve fitting this Nyquist plot to the characteristics of an equivalent circuit. Nyquist Plot diagram showing the impedance measurement result by the method of the comparative example for the present invention

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic configuration of a filter membrane impedance measuring apparatus according to an embodiment of the present invention. First, the apparatus shown in FIG. 1 will be described. The device has a pressure vessel 1 containing an electrolyte. This pressure vessel 1 is, for example, a vessel made of acrylic, resin, or steel plate, and comprises a first tank 2 and a second tank 3 which are separated from each other by a filter membrane, which will be described later. Electrolytic solutions 4 and 5 are stored in the first tank 2 and the second tank 3, respectively. In this embodiment, as an example, the electrolytic solution 4 is a KCl (potassium chloride) aqueous solution, and the electrolytic solution 5 is an MgCl ₂ (magnesium chloride) aqueous solution.

The pressure vessel 1 may be formed exclusively for the impedance measurement of the filter membrane, or may be a vessel constituting a filtering device that is actually working for filtering (filtration). Between the first tank 2 and the second tank 3 in the pressure vessel 1, a filter membrane 6 is held as an object whose impedance is to be measured. The filter membrane 6 of this example is a reverse osmosis membrane (RO membrane). Therefore, this filter film is hereinafter referred to as RO film 6 .

The RO membrane 6 has a composite membrane structure consisting of a separation functional layer 6a, a support layer 6b, and a non-woven fabric base material. formed. For example, the support layer 6b is made of polysulfone or cellulose acetate, and the separation function layer 6a is made of crosslinked aromatic polyamide. The nonwoven fabric substrate is made of polyester and has a thickness of about 100 μm. In addition, in FIG. 1, illustration of the nonwoven fabric substrate is omitted. The RO membrane 6 held as described above is in a state of being immersed in the electrolytic solutions 4 and 5 at the time of impedance measurement.

A first electrode 7 and a second electrode 8 are arranged in the first tank 2 and the second tank 3, respectively, facing each other with the RO membrane 6 held therebetween. The electrical connection between the first electrode 7 and the second electrode 8 will be detailed later.

The electrolytic solution 4 is stored in the first tank 2 under pressure, and the membrane filtration is performed by the cross-flow filtration method. A configuration for that purpose will be described below. One end of a raw water pipe 12 is connected to the inside of the first tank 2 , and the other end is connected to a raw water tank 13 that stores the electrolytic solution 4 . A high-pressure water pump 14 for pumping the electrolytic solution 4 is interposed in the raw water pipe 12 . When the electrolytic solution 4 in the raw water tank 13 is pumped to the first tank 2 by the high-pressure water pump 14, pressure is applied from the separation function layer 6a side (primary side) to the support layer 6b side (secondary side). As a result, the water in the electrolytic solution 4 flows into the support layer 6b through the separation functional layer 6a, thereby creating a water-permeable state. Under this water-permeable state, a concentration distribution of ions in the electrolytic solution 4 occurs in the separation function layer 6a.

When the RO membrane 6 is used for filtering the electrolytic solution 4, the water in the electrolytic solution 4 that permeates the RO membrane 6 flows into the second tank 3 as permeated water, and passes through the permeated water pipe 15 to the second tank. It flows out from 3. This permeated water is normally returned to the stock solution tank 13 through a pipe or the like (not shown), but if it contains MgCl ₂ or the like, which will be described later, it is discharged to another tank (not shown). On the other hand, the electrolytic solution 4 that has not permeated the RO membrane 6 in the first tank 2 is returned to the raw solution tank 13 through the raw water reflux pipe 16 .

In this apparatus, the second tank 3 is provided with an electrolytic solution injection system for injecting an electrolytic solution for measuring the impedance of the RO membrane 6 . This electrolyte injection system is interposed between a high-pressure gas cylinder 21, a gas pipe 22 having one end connected to the high-pressure gas cylinder 21, an electrolyte storage tank 23 having the other end connected to the gas pipe 22, and the gas pipe 22. an electrolytic solution pipe 25 for guiding the electrolytic solution 5 stored in the electrolytic solution storage tank 23 into the second tank 3; and a valve 26 interposed in the electrolytic solution pipe 25. .

The first electrode 7 in the first tank 2 and the second electrode 8 in the second tank 3 described above are connected to an impedance meter 33 via connection lines 31 and 32, respectively. For the electrodes 7 and 8, for example, a platinum black electrode (precipitated platinum fine particles on platinum) can be used in order to secure a sufficient effective area of the electrodes. As the electrode 7 and the electrode 8, for example, a platinum electrode having a platinum-blackened surface can be used. As will be described later, the impedance meter 33 constitutes AC applying means for applying AC to the electrolytic solutions 4 and 5 via the first electrode 7 and the second electrode 8 . At the same time, the impedance meter 33 measures the impedance between the first electrode 7 and the second electrode 8, that is, between one side and the other side of the RO membrane 6 in the water permeation direction. The impedance meter 33 is connected to an analysis section 34 such as a computer system, and the analysis section 34 is connected to a display section 35 such as a liquid crystal display device.

In the apparatus shown in FIG. 1, the impedance meter 33, the analysis section 34, and the display section 35 are shown separately from each other, but a single measuring instrument in which these are collectively provided may be used.

Next, an example of the impedance measuring method according to the present embodiment using the apparatus shown in FIG. 1 will be described. In this example, when measuring the impedance of the separation function layer 6a, the electrolyte 4 in which the separation function layer 6a side (primary side) of the RO membrane 6 is immersed is, for example, a 10 mMol/L KCl aqueous solution, and the support layer 6b is As the electrolytic solution 5 in which the side (secondary side) is immersed, for example, a 5 mMol/L MgCl ₂ aqueous solution is applied.

[Step 1]
After setting the RO membrane 6 whose impedance is to be measured in the pressure vessel 1, both the separation function layer 6a side (primary side) and the support layer 6b side (secondary side) were filled with an electrolytic solution 4, that is, a KCl aqueous solution. fill with The inner diameter of the pressure vessel is, for example, 20 mm, and the distance between the electrode and the RO membrane is, for example, 8 mm.
[Step 2]
The high-pressure water pump 14 is driven to pressurize the electrolytic solution 4 on the side of the separation functional layer 6a by the cross-flow filtration method, thereby making the RO membrane 6 water-permeable. Here, the pressurization pressure is, for example, about 0.3 to 0.7 MPa (mega-pascal), and the water permeation rate is, for example, about 200 cm ³ /min.

[Step 3]
Under the water permeable state described above, impedance measurement of the separation functional layer 6a by EIS is performed at regular intervals of, for example, about 30 minutes to 2 hours. In this EIS impedance measurement, an AC voltage of 100 mV is applied from the impedance meter 33 to the electrolytic solution 4 via the first electrode 7 and the second electrode 8 . In this example, this AC voltage is generated by sweeping the frequency, for example, within the range of 4.0 Hz to 4.0 MHz. While AC is being applied in this manner, the impedance meter 33 measures the impedance between the first electrode 7 and the second electrode 8, that is, between one side and the other side of the RO membrane 6 in the water permeation direction. This impedance measurement result is input to the analysis unit 34 .

This impedance measurement is performed by continuing the water permeation state, for example, for at least 10 hours until the measured impedance spectrum is stabilized and the desalination rate of the KCl aqueous solution by the RO membrane 6 is stabilized. Also, the water permeation rate is measured over a period of about 2 hours to confirm that it is stable.

[Step 4]
After confirming that the spectrum has stabilized, the valve 24 of the gas pipe 22 is opened, and the high-pressure gas in the high-pressure gas cylinder 21 is sent to the electrolyte storage tank 23 and pressurized. Thereby, the electrolytic solution 5 stored in the electrolytic solution storage tank 23 , that is, the MgCl ₂ aqueous solution is injected into the second tank 3 through the electrolytic solution pipe 25 . At this time, the opening degree of the valve 26 is adjusted so that the electrolytic solution 5 is slowly injected. As an example, the volume of the second tank 3 is approximately 2.5 cm ³ and the injection amount of the electrolytic solution 5 is approximately 15 cm ³ .
[Step 5]
It is confirmed that the electrolyte 5 has been sufficiently injected into the second tank 3 . This confirmation is performed by monitoring an ion concentration meter (not shown) arranged in the second tank 3, for example.
[Step 6]
After confirming that the electrolytic solution 5 has been sufficiently injected, the impedance is measured by EIS in the same manner as in Procedure 4. The AC voltage is applied by sweeping the frequency, for example, within the range of 4.0 Hz to 4.0 MHz. The obtained impedance measurement result is input to the analysis unit 34 .

When this measurement is performed, the electrolytic solution 4 on the side of the separation functional layer 6a contains monoatomic ions with a valence of 1, such as K ⁺ and Cl ⁻ . On the other hand, the electrolytic solution 5 on the support layer 6b side contains ions Mg ²⁺ having a valence of 2 in addition to Cl ⁻ . Under this water-permeable state, a concentration distribution of ions of the electrolytic solution 4, that is, K ⁺ and Cl ⁻ occurs in the separation function layer 6a.

Next, the analysis performed by the analysis unit 34 that receives the output of the impedance meter 33 when the impedance measurement described above is performed will be described. In this impedance measurement, the resistance and capacitance of the support layer 6b are R0 and C0, respectively, the resistance of a part of the separation functional layer 6a is R1, the resistance of the remaining part is R2, and the resistance of the separation functional layer 6a is R2. This is equivalent to obtaining the impedance of the equivalent circuit shown in FIG. As an example of the impedance meter 33, an E4980A precision LCR meter manufactured by Keysight Technologies can be used.

The analysis unit 34 receives the output of the impedance meter 33 and draws a Nyquist plot in which the horizontal axis indicates the real part of the impedance and the vertical axis indicates the imaginary part. At the same time, the analysis unit 34 draws a Bode plot in which the above-described AC frequency f is plotted on the horizontal axis and the absolute value |Z| of the impedance Z and the phase angle θ are plotted on the horizontal axis. These two types of plots show the same phenomenon in different forms, and the high frequency end and low frequency end of the frequency f in the Bode plot respectively correspond to the minimum value and maximum value of the real part in the Nyquist plot. Each obtained Plot is displayed on the display unit 35 shown in FIG. In this embodiment, by observing the display on the display unit 35, it can be confirmed that the spectrum caused by the separation functional layer 6a can be measured. The analysis unit 34 can select the frequency region to be analyzed. An example of this frequency domain will be described later with reference to FIG. If a Nyquist plot is drawn for the equivalent circuit shown in FIG. 2, it will be roughly as shown in FIG.

FIG. 4 shows an example of Nyquist Plot of one embodiment of the present invention. In this figure, the real part of the impedance is Z' and the imaginary part is Z'' (both units are Ω). In FIG. 4, the frequency f corresponding to the real part of the impedance Z' It is indicated by the numerical value attached to the arrow indicating the position.If the prefix M (mega) or k (kilo) of the SI unit system is attached, the numerical value excluding it is the frequency f (unit: Hz) .

FIG. 5 shows part of the Nyquist plot of FIG. 4 together with a curve that fits the part of the Nyquist plot resulting from the separation function layer 6a to the characteristics of the equivalent circuit of FIG. Equivalent circuit analysis of impedance measurement by this fitting can be performed using, for example, ZView equivalent circuit analysis software (manufactured by AMETEK Scientific Instruments). In the figure, a plurality of dots are actually measured impedance values, and a single curve separate from the curve connecting them is a curve for fitting. The fitting is performed until the error between the Nyquist plot shown in FIG. 5 and the curve for fitting is generally within 5%. The frequency range for fitting in this case is 6.16 Hz to 616 Hz.

Then, if the fitting is performed within the above error, the resistances R0 to R2 and the capacitances C0 to C2 are obtained based on the fitting curve at that time. In this example, the resistance of the separation functional layer 6a is calculated as R1+R2=312Ω+146Ω=458Ω.

Here, a comparative example for comparison with the above embodiment will be described. In the above embodiment, an aqueous solution of MgCl ₂ is injected into the second tank 3 as the electrolytic solution 5 during the impedance measurement by EIS. For example, a 10 mMol/L KCl aqueous solution is injected as an electrolytic solution 4 into the . Except for this point, the impedance is measured in the same manner as in the above-described embodiment. The ion concentration on the 6b side decreases. As a result, the impedance of the support layer 6b significantly increases compared to before pressure application.

FIG. 8 shows how this impedance changes as a Nyquist plot. Here, changes in the Nyquist Plot from before water permeation to the RO membrane 6 to when water is permeated are shown, and the curve of the Nyquist Plot changes so that the curve gradually increases as the water permeation rate increases. Specifically, the real part of the Nyquist Plot increases by two orders of magnitude from approximately 700Ω to approximately 70000Ω, and almost all of the Nyquist Plot (the maximum curve) in the permeable state is due to the support layer 6b. To be more precise, in the Nyquist plot in the permeable state, what is caused by the separation functional layer 6a slightly appears at the end of the curve on the low frequency side (the portion surrounded by the triangle in FIG. 8). It's nothing more than With such a Nyquist Plot, it is extremely difficult to measure the impedance (about several hundred Ω) of the separation functional layer 6a.

On the other hand, in the above-described embodiment, the impedance of the support layer 6b is lowered to about 600 Ω by injecting the MgCl ₂ aqueous solution as the electrolytic solution 5 into the second tank 3. It is well measurable.

Furthermore, FIG. 6 is obtained by immersing the same RO membrane as the RO membrane 6 measured in FIG. This is a Nyquist Plot. By being immersed in the aqueous NaClO solution, the surface of the separation functional layer 6a made of polyamide is oxidized by NaClO, the amide bonds are decomposed, and the crosslinked state of the polymer is loosened. As in FIG. 4, the frequency f corresponding to the real part Z' of the impedance is indicated by the numerical value attached to the arrow indicating the position on the horizontal axis.

FIG. 7 shows part of the Nyquist plot of FIG. 6, together with the curve for fitting. The manner of display in FIG. 7 is the same as in FIG. The fitting is performed until the error between the Nyquist plot shown in FIG. 7 and the curve for fitting is generally within 5%. The frequency range for fitting in this case is 30.8 Hz to 632 Hz. At this time, the resistance of the separation functional layer 6a is calculated as R1+R2=103.4Ω+29.4Ω=132.8Ω. This result shows that the resistance (458Ω) of the separation functional layer 6a obtained in FIG. This is probably because the amide bond of the separation functional layer 6a was decomposed by NaClO, and the crosslinked state of the polymer became loose.

That is, in the above embodiment, the impedance of the separation functional layer 6a is measured with and without the change in the separation functional layer 6a due to the NaClO aqueous solution, and the resistance R1+R2 of the separation functional layer 6a in both cases is examined. , indicates that the deterioration state of the separation functional layer 6a can be evaluated from this resistance.

1 Pressure Vessel 2 First Tank of Pressure Vessel 3 Second Tank of Pressure Vessel 4 Electrolyte 5 Electrolyte 6 RO Membrane 6a Separation Function Layer of RO Membrane 6b Supporting Layer of RO Membrane 7 First Electrode 8 Second Electrode 12 Raw Water Piping 13 Raw water tank 14 High pressure water pump 15 Permeated water pipe 16 Raw water reflux pipe 21 High pressure gas cylinder 22 Gas pipe 23 Electrolyte storage tank 24, 26 Valve 25 Electrolyte pipe 31, 32 Connection line 33 Impedance meter 34 Analysis unit 35 Display unit

Claims (4)

A filter membrane impedance measuring method comprising: immersing a filter membrane in which a separation functional layer is supported on a support layer in an electrolytic solution to measure the impedance of the separation functional layer by electrochemical impedance spectroscopy;
Using an electrolytic solution containing ions with a higher valence than the ions contained in the electrolytic solution on the separation functional layer side as the electrolytic solution on the support layer side,
A method for measuring the impedance of a filter membrane, wherein the impedance of the separation function layer is measured while the filter membrane is in a water-permeable state. 2. The filter membrane impedance measuring method according to claim 1, wherein the filter membrane is a reverse osmosis membrane. 3. The filter membrane impedance measuring method according to claim 1, wherein the ions contained in the electrolytic solution on the separation functional layer side are monoatomic ions having a valence of 1. A filter membrane impedance measuring device for measuring the impedance of a separation functional layer by electrochemical impedance spectroscopy by immersing a filter membrane in which a separation functional layer is supported on a support layer in an electrolytic solution,
Impedance measurement of a filter membrane, characterized in that it has an electrolytic solution injection system for injecting an electrolytic solution containing ions with a higher valence than the ions contained in the electrolytic solution on the separation functional layer side into the support layer side. Device.

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