A method for measuring the pore size of a nanofiltration membrane

Abstract

The application discloses a method for measuring the pore size of a nanofiltration membrane, which comprises the following steps: measuring the flux of pure water, different concentrations of methanol or ethanol under a series of pressures, and obtaining the mass transfer coefficient K under each condition; according to the correlation between the molecular weight cut-off and the stokes radius, the membrane pore radius r is calculated; the power equation of the correlation between K and r under pure water of a standard membrane is fitted, K under different conditions is substituted into the fitting equation, and the effective r is obtained; according to the steric effect of different concentrations of methanol and ethanol relative to pure water, and the pore size reduction ratio of the to-be-measured membrane relative to the standard membrane, the real filtration pore size of the nanofiltration membrane is determined. The application reflects the pore characteristics and performance characterization of the nanofiltration membrane by calculating the real situation of the pore size of the nanofiltration membrane in the use process and the difference between the original pore size, and solves the problem that the pore sizes of the nanofiltration membranes produced by different manufacturers and in different batches cannot be quantitatively compared due to the quality difference of the nanofiltration membranes caused by the membrane preparation process.

Description

TECHNICAL FIELD

[0001] The present application relates to a membrane pore size determination method, in particular to a nanofiltration membrane pore size determination method. BACKGROUND

[0002] Nanofiltration (NF) is a membrane separation technology between ultrafiltration and reverse osmosis, with a molecular weight cut-off (MWCO) of about 200-1000 Da, a pore size of generally 1-2 nm, and a particle size of about 1 nm. Nanofiltration has the characteristics of green and efficient membrane technology, and is one of the membrane technologies that have been developed by countries around the world in recent years. It has the advantages of normal temperature, low pressure operation, no heat effect, high separation effect, low energy consumption, and no secondary pollution. At present, nanofiltration has been widely used in the fields of domestic water, industrial water supply and wastewater treatment, food, biochemical pharmaceuticals, etc. (Qiu S, Wu L, Zhang L, et al. Nanofiltration separation mechanism [J]. Water Treatment Technology, 2009, 35(1): 15-19.)

[0003] Nanofiltration separation is mainly based on pore size screening effect and charge effect. Among them, the pore size screening effect plays a major role most of the time. Therefore, the size and pore size distribution of nanofiltration membrane pores are very important for nanofiltration separation. However, the nanofiltration pore size is in the nanometer range, which cannot be directly observed, and is usually unevenly distributed and not completely round. At the same time, the solvent occupation effect and membrane pore pollution of nanofiltration will affect the real effective pore size of the nanofiltration membrane in the separation process. Therefore, with the increase of time and use frequency, or the change of solution environment, the pore size of the nanofiltration membrane will change. Therefore, it is of great significance for the industrial application of nanofiltration to use a reasonable method to indicate the pore size of the nanofiltration membrane and the change compared with the original pore size.

[0004] At present, the commonly used methods for measuring membrane pore size include electron microscope method, bubble point method, liquid-liquid displacement method, mercury intrusion method, solute rejection method, etc. (Li J, Wang N, Shuai S, et al. Research progress of detection methods for pore size of alumina ceramic membranes [J]. China Ceramics, 2022, 58(09): 9-14.) See Table 1. However, these methods are not completely suitable for measuring the small pore size of nanofiltration. Therefore, the present application provides a simple method for measuring the pore size of nanofiltration membrane, which establishes a convenient and feasible method based on the relationship between the mass transfer behavior in a confined space and the effective membrane pore radius r in different solvent systems, and provides a new solution for the pore size evaluation of nanofiltration membranes of different manufacturers, batches, and use cycles.

[0005] Table 1

[0006]

[0007] SUMMARY

[0008] The application aims to provide a nanofiltration membrane pore size determination method, which can be used to evaluate the differences in pore size of nanofiltration membranes from different manufacturers, different batches and different use periods.

[0009] Another object of the application is to analyze the integrity of the nanofiltration membrane according to the space occupation effect of water, different concentrations of methanol and different concentrations of ethanol in the nanofiltration membrane pores.

[0010] Technical scheme: A nanofiltration membrane pore size determination method comprises the following steps: (1) by measuring the flux of pure water, different concentrations of methanol or ethanol under a series of pressures of the nanofiltration membrane, the mass transfer coefficient K under each condition is calculated, the unit is m·s -1 , collect Jv under different operating pressures of the series of solutions, the unit is m·s -1 , according to formula (1), the linear equation of ln[(1-R)·Jv / R] and Jv is fitted, R2 is greater than 0.9, the slope is 1 / K, the intercept is In[DK / δ], Jv is the membrane flux, the unit is m·s -1 , R is the retention rate, δ is the membrane thickness, the unit is cm, DK / δ is used to characterize the membrane mass transfer performance, the unit is m·s-1,

[0011] ln[(1-R)·Jv / R]=ln[DK / δ]+Jv / K (1);

[0012] (2) according to the correlation equation MWCO=1798.3r s of the molecular weight cut-off MWCO and the stokes radius r s 2.3168 , the stokes radius of the standard nanofiltration membrane is calculated; (3) the correlation equation of the mass transfer coefficient K of the standard nanofiltration membrane under the condition of pure water and the stokes radius r s of the membrane pore is fitted, the correlation equation of the mass transfer coefficient K and the stokes radius r s of the membrane pore is a power function, the correlation coefficient is greater than 0.9, the mass transfer coefficient K of the standard nanofiltration membrane and the membrane to be measured under the conditions of pure water, different concentrations of methanol or ethanol is substituted into the correlation equation of the mass transfer coefficient K and the stokes radius r s of the membrane pore, and the effective membrane pore radius r is obtained;

[0013] (5) according to the space occupation ratio=(r 2 醇 -r 2 水 ) / r 2 水 ·100%, the space occupation effect of different concentrations of methanol and ethanol relative to pure water is obtained, r 醇r is the effective radius in methanol or ethanol solution 水 r is the effective radius in pure water condition, and the reduction ratio of the membrane to be tested relative to the standard membrane pore size.

[0014] The method for determining the pore size of a nanofiltration membrane, wherein the series of pressures in step (1) are at least six different conditions and are equidistant.

[0015] The method for determining the pore size of a nanofiltration membrane, wherein the flux measurement for different solution conditions in step (1) uses pure water to wash to the original flux each time the solution is changed.

[0016] The method for determining the pore size of a nanofiltration membrane, wherein the mass transfer coefficient K in step (1) is obtained by fitting the linear equation of ln[(1-R)·Jv / R] and Jv, and the slope is 1 / K, Jv is the membrane flux, and the unit is m·s -1 , and R is the rejection rate.

[0017] The method for determining the pore size of a nanofiltration membrane, wherein the effective membrane pore radius r in step (3) is obtained by substituting the average rejection molecular weight of the standard membrane into the correlation equation of rejection molecular weight and stokes radius, the average rejection molecular weight of the membrane pore is 225 Da for 150-300 Da, and the average rejection molecular weight is 400 Da for 300-500 Da.

[0018] The method for determining the pore size of a nanofiltration membrane, wherein the correlation equation of the mass transfer coefficient K and the stokes radius r s in step (3) should be a power function, and the correlation regression coefficient is greater than 0.9.

[0019] The method for determining the pore size of a nanofiltration membrane, wherein when calculating the organic solvent occupation ratio in step (5), the occupation ratio = (r 2 醇 -r 2 水 ) / r 2 水 ·100% in r 水 r is the effective radius in pure water condition, r 醇 is the effective radius in methanol or ethanol solution.

[0020] The method for determining the pore size of a nanofiltration membrane, wherein when calculating the reduction ratio of the membrane to be tested in step (5), the reduction ratio = (r 2 醇 -r 2 水 ) / r 2 水 ·100% in r 水 r is the effective radius in pure water condition, r醇 Effective radius in methanol or ethanol solution.

[0021] Beneficial effects: (1) The present application solves the technical defects that the actual size of the membrane pore diameter changes due to the blockage and pollution of the membrane pores, and the real pore diameter of the nanofiltration membrane cannot be known. (2) The present application solves the problem that the quality of the nanofiltration membrane caused by the membrane preparation process leads to the problem that the pore diameters of the nanofiltration membranes produced by different manufacturers and different batches cannot be quantitatively compared. (3) The present application provides a technical solution for the integrity evaluation method of the nanofiltration membrane. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 The figure is the correlation diagram of the occupancy ratio of each pore diameter of the present application example 1 with the ethanol concentration.

[0023] Figure 2 The figure is the occupancy ratio trend diagram of the test membrane (150-300Da) and the standard membrane under pure water conditions in the present application example 2.

[0024] Figure 3 The figure is the occupancy ratio trend diagram of the test membrane (600-800Da) and the standard membrane under pure water conditions in the present application example 2. DETAILED DESCRIPTION

[0025] The above content of the present application will be further described in the form of examples, but this should not be understood as the scope of the above subject matter of the present application is limited to the following examples. Any technology realized based on the above content of the present application belongs to the scope of the present application.

[0026] Example 1: Establishing a standard nanofiltration membrane pore diameter determination method

[0027] Assemble the nanofiltration separation device, and place the nanofiltration membranes with standard pore diameters in the membrane tubes of the nanofiltration separation device, and clean them with purified water. The specific pore diameters of the nanofiltration membranes are mainly divided into four types, 150-300Da, 200-300Da, 300-500Da, and 600-800Da.

[0028] Measure the membrane flux Jv(m·s-1) of pure water, 20%, 50%, and 70% methanol, and 20%, 50%, and 70% ethanol (pure water and different concentrations of methanol and ethanol) under pressures of 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 MPa, respectively. -1 During the measurement, the pressure is adjusted by the rotation speed and the cutoff valve. The solution is first balanced under different pressure conditions, and then the membrane flux under the corresponding conditions is measured after the membrane flux is stable. When changing the solution environment conditions, the residual solution in the nanofiltration separation device is emptied, and then the purified water is cleaned until the pure water flux returns to the original state.

[0029] Fit the nanofiltration mass transfer coefficient K(m·s-1) under different solution conditions.-1 ), respectively, to obtain the mass transfer coefficient K 纯水 , K 20％乙醇 , K 50％乙醇 , K 70％乙醇 , K 20％甲醇 , K 50％甲醇 , K 70％甲醇 The membrane flux Jv(m·s -1 ) under different operating pressures of the series of solutions was collected.

[0030] ln[(1-R)·Jv / R] = ln[DK / δ] + Jv / K (1)

[0031] According to the linear equation of ln[(1-R)·Jv / R] and membrane flux Jv (R 2 >0.9) fitted according to formula (1), the slope is 1 / K, and the intercept is ln[DK / δ]. Jv is the membrane flux, m·s -1 ; R is the retention rate, and since the solution is water, methanol and ethanol, the permeability of nanofiltration is calculated as 99%; δ is the membrane thickness, cm; DK / δ is used to characterize the mass transfer performance of the membrane, m·s -1 The standard membrane mass transfer coefficient K under pure water is shown in Table 2.

[0032] Table 2

[0033]

[0034] Step 4: Fitting the correlation equation of the molecular weight cut-off and the equivalent pore radius of the membrane. The stokes radius rs of the standard nanofiltration membrane was calculated.

[0035] MWCO = 1798.3rs 2. 3168 (2)

[0036] According to the correlation equation of the molecular weight cut-off and the equivalent pore radius of the membrane in formula (2), the stokes radius r s of the membrane was calculated. Since the molecular weight cut-off of nanofiltration is usually within a certain range, the molecular weight cut-off MWCO is selected as the average molecular weight cut-off of the nanofiltration membrane, Da; r s is the stokes radius, nm. The results of the equivalent pore radius of the membrane are shown in Table 3.

[0037] Table 3

[0038]

[0039] The correlation between the mass transfer coefficient K and the stokes radius r s under pure water conditions was fitted to obtain the equation of formula (3). Since the mass transfer coefficient is similar to the mass transfer velocity, the size of the radius will affect the speed of mass transfer, and the mass transfer coefficient K is proportional to the stokes radius r spower correlation (R 2 > 0.9).

[0040] K = 134.49r s 2.926 (3)

[0041] The effective stokes radius of the standard membrane pore size under different solution conditions was calculated. The mass transfer coefficient K 20％乙醇 , K 50％乙醇 , K 70％乙醇 , K 20％甲醇 , K 50％甲醇 , K 70％甲醇 was substituted into equation (3) to obtain the real effective membrane pore radius r under the corresponding solution condition, and the effective membrane pore radius r was substituted into equation (2) to obtain the size of the effective molecular weight cut-off, and the results are shown in Table 4.

[0042] Table 4

[0043]

[0044] The solvent occupancy effect of the pore size of the standard membrane under different concentrations of ethanol relative to pure water conditions was calculated. First, the membrane flux of the standard membrane under different concentrations of ethanol was calculated, and the mass transfer coefficient K was obtained. See Table 5.

[0045] Table 5

[0046]

[0047] The mass transfer coefficient K of the standard membrane under different concentrations of ethanol was substituted into equation (3) to obtain the effective membrane pore radius r of the standard membrane under different concentrations of ethanol. At the same time, the effective membrane pore radius r obtained was substituted into equation (2) to obtain the actual average molecular weight cut-off, and the results are shown in Table 6.

[0048] Table 6 Effective r

[0049]

[0050] The effective membrane pore radius r result was substituted into equation (2) to obtain the actual average molecular weight cut-off, and the results are shown in Table 7.

[0051] Table 7

[0052]

[0053]

[0054] The occupancy ratio = (r 2 醇 -r 2 水 ) / r 2 水 ·100% (4)

[0055] r 水 r is the effective radius of each membrane in pure water conditions 醇 r is the effective radius in methanol or ethanol solution. The solvent accessible effect of each membrane in different concentrations of ethanol relative to pure water conditions was calculated. The solvent accessible effect of each concentration condition was calculated according to equation (4) and the results are shown in Table 8.

[0056] Table 8

[0057]

[0058] The relationship between the pore size of each membrane and the concentration of each solvent was fitted to establish a standard nanofiltration membrane pore size determination method for analyzing the differences between the test nanofiltration membrane and the standard membrane, and determining the pore size of the test nanofiltration membrane. The results are shown in the attached Table 13. Figure 1 .

[0059] Example 2: Determination of the pore size of the test nanofiltration membrane

[0060] The change in the pore size of the nanofiltration membrane used for more than five years was measured, taking the pore sizes of 150-300 Da and 600-800 Da as examples.

[0061] The membrane flux of pure water and different concentrations of ethanol under a series of pressures of the test membrane was measured to obtain the mass transfer coefficient K. See Table 9.

[0062] Table 9

[0063]

[0064] The mass transfer coefficient K of the test membrane under different concentrations of ethanol was substituted into equation (3) to obtain the effective membrane pore radius r of the test membrane under different concentrations of ethanol. See Table 10.

[0065] Table 10

[0066]

[0067] At the same time, the effective membrane pore radius r obtained was substituted into equation (2) to obtain the actual average molecular weight cut-off. See Table 11.

[0068] Table 11

[0069]

[0070]

[0071] The solvent accessible effect of the test membrane in different concentrations of ethanol relative to pure water conditions was calculated according to equation (4). See Table 12.

[0072] Table 12

[0073]

[0074] Fitting the relationship between the concentration and the ratio of the tested membrane to the standard membrane, the trend line of the ratio of the tested membrane (150-300 Da) to the standard membrane under the condition of pure water, and the results are shown in the following table 8. Figure 2 It can be seen that the trend of the ratio of the tested membrane to the standard membrane is consistent, and the overall performance of the tested membrane is good, but the effective pore size changes. Figure 3 The trend of the ratio of the tested membrane to the standard membrane is relatively consistent, and the effective pore size also changes.

[0075] The solvent occupation effect of the tested membrane under different ethanol concentrations on the standard membrane under the condition of pure water is calculated. 水 The standard membrane is measured, that is, the ratio of the reduced pore size compared with the pore size of the standard membrane is obtained.

[0076] Table 13

[0077]

[0078] Based on the above analysis, with water solution as the reference, the effective pore size of the nanofiltration membrane used for five years (150-300 Da) is reduced by 25.95%, and the pore size ratio under different ethanol concentrations shows that the plasticity of the pore size of the nanofiltration membrane used for a long time is reduced. The effective pore size of the tested membrane (600-800 Da) is reduced by 49.00%, and compared with the data in table 8, it can be seen that the effective filtration pore size of the nanofiltration membrane is reduced due to membrane pollution after long-term use.

[0079] The present application can calculate the solvent occupation effect and the effective membrane pore radius under different organic solvent concentrations. Even if the nanofiltration membrane is used for many years, the effective pore size is reduced due to pollution and blockage of the membrane pore size, and the real effective pore size, the real molecular weight cut-off, and the reduction ratio compared with the initial state can be conveniently obtained by the method, and the effective correction of the nanofiltration membrane pore size is realized. Clear understanding of the related parameters of the membrane pore size can better guide the production and application of the nanofiltration membrane, and achieve the purpose of hitting the target.

Claims

1. A method for determining the pore size of a nanofiltration membrane, characterized in that, The method comprises the following steps: (1) By measuring the flux of pure water, different concentrations of methanol or ethanol under the pressure of nanofiltration membrane series, the mass transfer coefficient K under each condition is calculated, with the unit of m·s -1 , collect Jv of series solution under different operating pressure, with the unit of m·s -1 , according to formula (1) to fit the linear equation of ln[(1-R)·Jv / R] and Jv, R 2 greater than 0.9, the slope is 1 / K, the intercept is In[DK / δ], Jv is the membrane flux, with the unit of m·s -1 , R is the rejection rate, δ is the membrane thickness, with the unit of cm, DK / δ is used to characterize the mass transfer performance of the membrane, with the unit of m·s -1 , ln[(1-R)Jv / R] = ln[DK / δ] + Jv / K (1); (2) According to the correlation equation of the molecular weight cut-off MWCO and the stokes radius rs, MWCO = 1798.3r s 2.3168 , the stokes radius r of the membrane pore of the standard nanofiltration membrane is calculated s ; (3) fitting the correlation equation of standard nanofiltration membrane mass transfer coefficient K and membrane pore stokes radius r s The correlation equation of mass transfer coefficient K and membrane pore stokes radius r s is a power function, and the correlation regression coefficient is greater than 0.9; the mass transfer coefficient K of the standard nanofiltration membrane and the membrane to be measured under the conditions of pure water, different concentrations of methanol or ethanol is substituted into the correlation equation of mass transfer coefficient K and membrane pore stokes radius r to obtain the effective membrane pore radius r; (4) The effective membrane pore radius r is substituted into the correlation equation for molecular weight cut-off versus stokes radius r s MWCO = 1798.3 r s 2.3168 to obtain the actual molecular weight cut-off. (5) According to the occupation ratio = (r 2 醇 -r 2 水 ) / r 2 水 ·100% to find the different concentrations of methanol, ethanol, and pure water relative to the occupation effect, r 醇 for methanol or ethanol solution effective radius, r 水 for pure water conditions effective radius, and the measured film relative to the standard film aperture reduction ratio; (6) According to the difference ratio of the pore size of the measured film and the standard film, the pore size of the measured film is calculated.

2. The method of claim 1, wherein the membrane pore size is determined by measuring the rejection of the solute. The series of pressures in step (1) are at least six different conditions and are equidistant.

3. The method of claim 1, wherein the membrane pore size is determined by measuring the rejection of the solute. In step (1), for the flux measurement of different solution conditions, the original flux is washed with pure water every time the solution is replaced.

4. The method of claim 1, wherein the membrane is a nanofiltration membrane. The mass transfer coefficient K described in step (1) is obtained from the slope 1 / K of the linear equation of ln[(1-R)Jv / R] versus Jv, Jv is the membrane flux, unit: m-s -1 , R is the rejection rate.

5. The method of claim 1, wherein the membrane pore size is determined by measuring the rejection of the solute. The effective membrane pore radius r in step (2) is obtained from the correlation equation of the average molecular weight cut-off and the stokes radius r of the standard membrane by substituting the average molecular weight cut-off of the membrane s The average molecular weight cut-off of the membrane is 225 Da for the pore size of 150-300 Da and 400 Da for the pore size of 300-500 Da.

6. The method of claim 1, wherein the membrane pore size is determined by measuring the rejection of a solute with a known molecular weight. In step (3), the correlation equation between the mass transfer coefficient K and the stokes radius rs is a power function, and the correlation regression coefficient is greater than 0.

9.

7. The method of claim 1, wherein the membrane pore size is determined by measuring the rejection of a solute with a known molecular weight. The organic solvent occupation ratio is calculated in step (5) as follows: occupation ratio = (r 2 醇 -r 2 水 ) / r 2 水 • 100% in r 水 is the effective radius of each membrane in pure water, r 醇 is the effective radius in methanol or ethanol solution.

8. The method of claim 1, wherein the membrane pore size is determined by measuring the rejection of a solute with a known molecular weight. The reduction ratio of the film to be measured is calculated in step (5) as reduction ratio = (r 2 醇 -r 2 水 ) / r 2 水 r 水 is the effective radius of the standard film in pure water, and r 醇 is the effective radius in methanol or ethanol solution.

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