- Email: [email protected]
Accurate measuring of membrane pore size distributions
14
Technology focus
Filtration+Separation November/December 2015
Liquid-Liquid Porometer
Accurate measuring of membrane pore size distributions G
as-liquid porometers are used to characterize a variety of porous media, but they are not suited to measure submicron and nanometric pore size distributions (PSD). To allow membrane manufacturers, users and scientists to accurately measure the actual PSD of their products, the Liquid-Liquid Porometer from IFTS could offer a solution.
Porous, semi-permeable and dense membranes are widely used for microfiltration, ultrafiltration, nanofiltration and reverse osmosis. They form the 'barrier' and ensure the retention of solids, colloids, molecules to be separated from initial fluid when a pressure difference is applied to the filter or the module.
Membranes also provide gas separation (gas permeation), pervaporation (extraction of a compound in steam form through a membrane under reduced pressure) and membrane distillation (transport of water vapour through a hydrophobic microporous membrane to an external compartment under vacuum where it is condensed).
The membranes have a common characteristic that describes their application domain and the more or less calibrated character of the separation: the pore size, which is expressed through multiple values: • •
The pore size distribution The sizes of the biggest pore, the average one and the smallest one
This intrinsic characteristic of a new media can also be observed at various times of its use to study and appreciate its aging. In industrial applications, the membrane aging can be normal, more intense, or sometimes even abnormal. This is determined by porometry using a porometer. The principle of measurement uses the surface interaction between two fluids and a solid (Figure 1), defined by 2 parameters: • Interfacial tension γ (N/m) • Contact angle θ (°) Wetting liquids naturally fill the pores of a porous medium in which they are retained by capillary forces depending upon γ, Θ and the pore diameter, D: the smaller the pore the larger the capillary force. Thus, for displacing the wetting liquid in a pore of diameter D, a pressure ΔP has to be applied, equal to:
οܲ ൌ రംೞഇ ವ Figure 1. Principle of porometry. (D: pore diameter, ∆P applied pressure, θ contact angle which tends to zero).
www.filtsep.com
(Equation 1 : Jurin's law) 0262 1762/15 © 2015 Elsevier Ltd. All rights reserved
Technology focus
Filtration+Separation November/December 2015
2 1 3
4
Figure 2. Variation of flow with applied pressure.
2
4
3
1 Figure 4. Porometer with (1) management measurement center, (2) testing cell for hollow fiber membranes (other cells are available for flat or tubular membranes) (3) fluid weighing for measurement (4) fluid reservoir.
Figure 3. Pore size distribution deducted from Figure 2.
The pressure ΔP can be applied by using either a gas or an immiscible liquid. In the gas-liquid porometer mode the pressure is applied on a gas, which pushes the liquid present in the pore and then flows through the pores.
the largest pore, the flow rate through the media is zero. When reaching this pressure, a first tiny flow is detected across the membrane. This first pore opening pressure is called the ‘bubble point’ of the membrane/module.
In the liquid-liquid porometer mode the pressure is applied on a first liquid, which pushes a second liquid present in the pore and then flows through the pores.
The pressure is then increased step by step in the most repeatable conditions and at each step the flow rate is recorded, its increase corresponding to the opening of smaller and smaller pores of diameters given by Equation 1. When all pores have been emptied, the second liquid flow rate becomes proportional to the applied pressure (Darcy Law).
The test membrane sample (flat sheet, disc, fiber, tube, etc) is first carefully wetted by the wetting liquid chosen from its contact angle and interfacial tension. Then a second liquid is deposited on the wet sample. A pressure is applied step by step onto the second liquid and the flow rate of either liquid is accurately measured. As long as the applied pressure is less than the capillary pressure in
Liquid-Liquid Porometer Gas-liquid porometers are broadly used to daily characterize a wide variety of porous media. But they are not suited to measure submicron and nanometric pore size distributions, (PSD). Indeed, the
very high pressure required to displace the wetting liquid from smallest pores necessarily deforms the media. Then, the obtained PSD is not the actual PSD of the original sample of the membrane.
opening of the smallest pore down to 2 nm using low pressures, compatible with the mechanical resistance of the membrane sample which is not deformed.
To overcome this problem and to allow membrane manufacturers, users and scientists to accurately measure the actual PSD of their products, IFTS, Technological Resources Centre, has designed, built and validated a new equipment: the Liquid-Liquid Porometer.
This technique allows the measurement of pore size distribution of most membranes and most small modules, from the bottom of ultrafiltration field to the top of microfiltration. It covers, particularly, the application domain of hollow fiber membranes with pores of 10 to 200 nm.
The IFTS Liquid-Liquid Porometer uses a couple of carefully chosen wetting liquids specific to the membrane material to be analyzed. These two liquids are not miscible and their low interfacial tension and contact angle with the membrane material are accurately measured. The use of low interfacial tension liquids allows the
The IFTS Porometer is composed of 3 sets: • • •
The Medium filter device The implementation centre for Measurement The Windows Software via USB plug driver, which conducts the characterization session and writes the measurement report www.filtsep.com
15
Relative Distribution of pores (%/nm)
Technology focus
Filtration+Separation November/December 2015
ized by liquid-liquid porometry without requiring implementation under high pressure (Figure 6). In liquid-liquid porometry, applied pressure does not reach 2 bar, it would be theoretically 21 bar for analysis with gas-liquid porometry; this pressure is greater than the fiber can withstand. This membrane has pores from 20 to 150 nm (average pore: 30 nm) and presents a 50 kDa molecular weight cut-off.
80 70 60 50 40 30 20 10 0 0
1
2 3 Pore size (nm)
4
5
Characterization of a flat ultrafiltration membrane for the retention of smaller molecules. Flat organic membrane, 10 kD cut-off, reveals by liquid-liquid porometry, its pores from 5 to 35 nm with an average pore of 8 nm (Figure 7).
Relative distribution of pores (%/nm)
Figure 5. Pore size distribution of a mineral membrane.
10 8 6 4 2 0 0
50
100 Pore size (nm)
150
200
The fluid-fluid porometer is a liquid-liquid porometer and gasliquid porometer. The Fluid-Fluid Porometer developed by IFTS, features 2 measuring modes – liquid-liquid and gas-liquid (table 1); it has the larger field of applications because it can determine: •
Figure 6. Pore size distribution of a hollow fiber membrane.
Reltive distribution of pores (%/nm)
16
8 7 6 5 4 3 2 1 0
•
0
10
20 30 Pore size (nm)
40
50
Figure 7. Pore size distribution of a flat organic membrane.
Using the Liquid-Liquid Porometer. Below, different graphs show examples of measurement results for several types of membranes. Characterization of pores down to 2 nanometers for an ultrafiltration membrane which can retain very small molecules. A mineral www.filtsep.com
membrane with molecular weight cut-off 3 kD, also containing an active layer of metal oxide is developed for the retention of very small molecules (Figure 5). Liquidliquid porometry allows to characterize its smallest pores down to 2 nm (pore distribution 2-3 nm with an average pore of 2.2 nm.)
Characterization of hollow fibers membrane representing the largest majority of ultrafiltration membranes. Ultrafiltration membranes widely used for water treatment are hollow fibers. An hydrophilic organic hollow fiber membrane active on the top of ultrafiltration domain is character-
The pore size distribution of submicron and nanometer pores of all types of membranes until the smallest pore of 2 nm by liquid-liquid porometry. The pore size distribution of micrometric pore from 1 to 300 μm of structured filter media (non woven, paper, felt, sintered, etc) by gas-liquid porometry.
There is measurement continuity from gas-liquid porometry mode to liquid-liquid porometry mode. An organic membrane with calibrated pore 0.2 μm in diameter measured by SEM, scanning electron microscopy, is characterized using the IFTS Porometer according each of two modes (Figure 8). The liquid-liquid characterization follows well those in gas-liquid mode; a very low difference (10%) is observed between the results of average diameter: 0.23 μm (gas-liquid mode) and 0.21 μm (liquid-liquid mode). This differ-
Technology focus
Filtration+Separation November/December 2015
ence is of the same magnitude of common variations encountered for pore sizes measured on several samples from a same membrane.
Relative distribution of pores (%/nm)
70
Characterization of membrane or filter media with pores greater than 0.1 μm by gas-liquid porometry. Flat hydrophobic microporous membrane can be characterized by gas-liquid porometry (Figure 9). A new developed membrane (electrospun membrane) is compared to an existing membrane or reference one for the intended application.
60
Gaz-Liquid Mode
50
Liquid-Liquid Mode
40 30 20 10 0 0
0.1
0.2 0.3 Pore size (μm)
0.4
0.5
Figure 8. Pore size distribution of a membrane with calibrated pore 0.2 μm.
Relative distribution of pores (%/nm)
Conclusion The IFTS Porometer and its measuring mode liquid-liquid can characterize: •
•
•
organic or mineral membranes with very small pores down to 2 nm membranes in all forms : flat or tubular or hollow fibers (this last one are most commonly used in water ultrafiltration) according several measuring cells membranes for microfiltration, ultrafiltration to its lowest field
With its measuring gas-liquid mode, the IFTS Porometer is a 2 in 1 device for characterization of membranes and structured filter media eg non-woven, fabrics, papers, pens and sintered. It is useful for membrane production
40 35
Membrane under developpment
30
Membrane for reference
25 20 15 10 5 0 0
0.2
0.4
0.6
0.8
1
1.2
Pore size (μm) Figure 9. Pore size distribution of a classic membrane and under development new membrane.
routine control. It is used to characterize new membranes using new materials or other functionalized surface membrane. It can also
be used for membrane state diagnosis of used membrane, during their autopsy by revealing variations in their pore sizes distribution.
Characteristics of IFTS Porometer
Gas-liquid mode
Liquid-liquid mode
Filter media
non-woven with microfibers, nanofibers, papers, sintered, flat or tubular membranes, «electrospun»
flat, tubular, hollow fibers membranes
Pore size
from 300 to 1 micron
1.5 microns to 2 nanometres
Pressure
0 to 10 bar (1 MPa, 150 psi), ± 0.1%
Flow
0 to 100 NL/min ±0. 5%
0 to 2 kg/h ±0. 5%
Duration of the measure
10 min
20 min
Determination
• of the biggest pore: bubble point or droplet point • of the pore size distribution, • of the smallest pore size
Calibration of flow meters and pressure gauges using connected COFRAC standards sensors
The IFTS Porometer and in particular its liquid-liquid function is proposed to manufacturers of all types of membranes, and researchers from engineering schools and industry. It allows ever offered measures to connect to other membranes and properties.
•
Contact Marie Andrée Sirvain, Dr Ing. and Marion Dalex, sales manager IFTS Institute of Filtration and Separative Techniques Rue Marcel Pagnol - 47510 Foulayronnes France Tel: +33 5 53 95 83 94 Email: [email protected] www.ifts-sls.com
Table 1. Characteristics of the IFTS Porometer for each of its measurement modes.
www.filtsep.com
17
Technology focus
Filtration+Separation November/December 2015
Liquid-Liquid Porometer
Accurate measuring of membrane pore size distributions G
as-liquid porometers are used to characterize a variety of porous media, but they are not suited to measure submicron and nanometric pore size distributions (PSD). To allow membrane manufacturers, users and scientists to accurately measure the actual PSD of their products, the Liquid-Liquid Porometer from IFTS could offer a solution.
Porous, semi-permeable and dense membranes are widely used for microfiltration, ultrafiltration, nanofiltration and reverse osmosis. They form the 'barrier' and ensure the retention of solids, colloids, molecules to be separated from initial fluid when a pressure difference is applied to the filter or the module.
Membranes also provide gas separation (gas permeation), pervaporation (extraction of a compound in steam form through a membrane under reduced pressure) and membrane distillation (transport of water vapour through a hydrophobic microporous membrane to an external compartment under vacuum where it is condensed).
The membranes have a common characteristic that describes their application domain and the more or less calibrated character of the separation: the pore size, which is expressed through multiple values: • •
The pore size distribution The sizes of the biggest pore, the average one and the smallest one
This intrinsic characteristic of a new media can also be observed at various times of its use to study and appreciate its aging. In industrial applications, the membrane aging can be normal, more intense, or sometimes even abnormal. This is determined by porometry using a porometer. The principle of measurement uses the surface interaction between two fluids and a solid (Figure 1), defined by 2 parameters: • Interfacial tension γ (N/m) • Contact angle θ (°) Wetting liquids naturally fill the pores of a porous medium in which they are retained by capillary forces depending upon γ, Θ and the pore diameter, D: the smaller the pore the larger the capillary force. Thus, for displacing the wetting liquid in a pore of diameter D, a pressure ΔP has to be applied, equal to:
οܲ ൌ రംೞഇ ವ Figure 1. Principle of porometry. (D: pore diameter, ∆P applied pressure, θ contact angle which tends to zero).
www.filtsep.com
(Equation 1 : Jurin's law) 0262 1762/15 © 2015 Elsevier Ltd. All rights reserved
Technology focus
Filtration+Separation November/December 2015
2 1 3
4
Figure 2. Variation of flow with applied pressure.
2
4
3
1 Figure 4. Porometer with (1) management measurement center, (2) testing cell for hollow fiber membranes (other cells are available for flat or tubular membranes) (3) fluid weighing for measurement (4) fluid reservoir.
Figure 3. Pore size distribution deducted from Figure 2.
The pressure ΔP can be applied by using either a gas or an immiscible liquid. In the gas-liquid porometer mode the pressure is applied on a gas, which pushes the liquid present in the pore and then flows through the pores.
the largest pore, the flow rate through the media is zero. When reaching this pressure, a first tiny flow is detected across the membrane. This first pore opening pressure is called the ‘bubble point’ of the membrane/module.
In the liquid-liquid porometer mode the pressure is applied on a first liquid, which pushes a second liquid present in the pore and then flows through the pores.
The pressure is then increased step by step in the most repeatable conditions and at each step the flow rate is recorded, its increase corresponding to the opening of smaller and smaller pores of diameters given by Equation 1. When all pores have been emptied, the second liquid flow rate becomes proportional to the applied pressure (Darcy Law).
The test membrane sample (flat sheet, disc, fiber, tube, etc) is first carefully wetted by the wetting liquid chosen from its contact angle and interfacial tension. Then a second liquid is deposited on the wet sample. A pressure is applied step by step onto the second liquid and the flow rate of either liquid is accurately measured. As long as the applied pressure is less than the capillary pressure in
Liquid-Liquid Porometer Gas-liquid porometers are broadly used to daily characterize a wide variety of porous media. But they are not suited to measure submicron and nanometric pore size distributions, (PSD). Indeed, the
very high pressure required to displace the wetting liquid from smallest pores necessarily deforms the media. Then, the obtained PSD is not the actual PSD of the original sample of the membrane.
opening of the smallest pore down to 2 nm using low pressures, compatible with the mechanical resistance of the membrane sample which is not deformed.
To overcome this problem and to allow membrane manufacturers, users and scientists to accurately measure the actual PSD of their products, IFTS, Technological Resources Centre, has designed, built and validated a new equipment: the Liquid-Liquid Porometer.
This technique allows the measurement of pore size distribution of most membranes and most small modules, from the bottom of ultrafiltration field to the top of microfiltration. It covers, particularly, the application domain of hollow fiber membranes with pores of 10 to 200 nm.
The IFTS Liquid-Liquid Porometer uses a couple of carefully chosen wetting liquids specific to the membrane material to be analyzed. These two liquids are not miscible and their low interfacial tension and contact angle with the membrane material are accurately measured. The use of low interfacial tension liquids allows the
The IFTS Porometer is composed of 3 sets: • • •
The Medium filter device The implementation centre for Measurement The Windows Software via USB plug driver, which conducts the characterization session and writes the measurement report www.filtsep.com
15
Relative Distribution of pores (%/nm)
Technology focus
Filtration+Separation November/December 2015
ized by liquid-liquid porometry without requiring implementation under high pressure (Figure 6). In liquid-liquid porometry, applied pressure does not reach 2 bar, it would be theoretically 21 bar for analysis with gas-liquid porometry; this pressure is greater than the fiber can withstand. This membrane has pores from 20 to 150 nm (average pore: 30 nm) and presents a 50 kDa molecular weight cut-off.
80 70 60 50 40 30 20 10 0 0
1
2 3 Pore size (nm)
4
5
Characterization of a flat ultrafiltration membrane for the retention of smaller molecules. Flat organic membrane, 10 kD cut-off, reveals by liquid-liquid porometry, its pores from 5 to 35 nm with an average pore of 8 nm (Figure 7).
Relative distribution of pores (%/nm)
Figure 5. Pore size distribution of a mineral membrane.
10 8 6 4 2 0 0
50
100 Pore size (nm)
150
200
The fluid-fluid porometer is a liquid-liquid porometer and gasliquid porometer. The Fluid-Fluid Porometer developed by IFTS, features 2 measuring modes – liquid-liquid and gas-liquid (table 1); it has the larger field of applications because it can determine: •
Figure 6. Pore size distribution of a hollow fiber membrane.
Reltive distribution of pores (%/nm)
16
8 7 6 5 4 3 2 1 0
•
0
10
20 30 Pore size (nm)
40
50
Figure 7. Pore size distribution of a flat organic membrane.
Using the Liquid-Liquid Porometer. Below, different graphs show examples of measurement results for several types of membranes. Characterization of pores down to 2 nanometers for an ultrafiltration membrane which can retain very small molecules. A mineral www.filtsep.com
membrane with molecular weight cut-off 3 kD, also containing an active layer of metal oxide is developed for the retention of very small molecules (Figure 5). Liquidliquid porometry allows to characterize its smallest pores down to 2 nm (pore distribution 2-3 nm with an average pore of 2.2 nm.)
Characterization of hollow fibers membrane representing the largest majority of ultrafiltration membranes. Ultrafiltration membranes widely used for water treatment are hollow fibers. An hydrophilic organic hollow fiber membrane active on the top of ultrafiltration domain is character-
The pore size distribution of submicron and nanometer pores of all types of membranes until the smallest pore of 2 nm by liquid-liquid porometry. The pore size distribution of micrometric pore from 1 to 300 μm of structured filter media (non woven, paper, felt, sintered, etc) by gas-liquid porometry.
There is measurement continuity from gas-liquid porometry mode to liquid-liquid porometry mode. An organic membrane with calibrated pore 0.2 μm in diameter measured by SEM, scanning electron microscopy, is characterized using the IFTS Porometer according each of two modes (Figure 8). The liquid-liquid characterization follows well those in gas-liquid mode; a very low difference (10%) is observed between the results of average diameter: 0.23 μm (gas-liquid mode) and 0.21 μm (liquid-liquid mode). This differ-
Technology focus
Filtration+Separation November/December 2015
ence is of the same magnitude of common variations encountered for pore sizes measured on several samples from a same membrane.
Relative distribution of pores (%/nm)
70
Characterization of membrane or filter media with pores greater than 0.1 μm by gas-liquid porometry. Flat hydrophobic microporous membrane can be characterized by gas-liquid porometry (Figure 9). A new developed membrane (electrospun membrane) is compared to an existing membrane or reference one for the intended application.
60
Gaz-Liquid Mode
50
Liquid-Liquid Mode
40 30 20 10 0 0
0.1
0.2 0.3 Pore size (μm)
0.4
0.5
Figure 8. Pore size distribution of a membrane with calibrated pore 0.2 μm.
Relative distribution of pores (%/nm)
Conclusion The IFTS Porometer and its measuring mode liquid-liquid can characterize: •
•
•
organic or mineral membranes with very small pores down to 2 nm membranes in all forms : flat or tubular or hollow fibers (this last one are most commonly used in water ultrafiltration) according several measuring cells membranes for microfiltration, ultrafiltration to its lowest field
With its measuring gas-liquid mode, the IFTS Porometer is a 2 in 1 device for characterization of membranes and structured filter media eg non-woven, fabrics, papers, pens and sintered. It is useful for membrane production
40 35
Membrane under developpment
30
Membrane for reference
25 20 15 10 5 0 0
0.2
0.4
0.6
0.8
1
1.2
Pore size (μm) Figure 9. Pore size distribution of a classic membrane and under development new membrane.
routine control. It is used to characterize new membranes using new materials or other functionalized surface membrane. It can also
be used for membrane state diagnosis of used membrane, during their autopsy by revealing variations in their pore sizes distribution.
Characteristics of IFTS Porometer
Gas-liquid mode
Liquid-liquid mode
Filter media
non-woven with microfibers, nanofibers, papers, sintered, flat or tubular membranes, «electrospun»
flat, tubular, hollow fibers membranes
Pore size
from 300 to 1 micron
1.5 microns to 2 nanometres
Pressure
0 to 10 bar (1 MPa, 150 psi), ± 0.1%
Flow
0 to 100 NL/min ±0. 5%
0 to 2 kg/h ±0. 5%
Duration of the measure
10 min
20 min
Determination
• of the biggest pore: bubble point or droplet point • of the pore size distribution, • of the smallest pore size
Calibration of flow meters and pressure gauges using connected COFRAC standards sensors
The IFTS Porometer and in particular its liquid-liquid function is proposed to manufacturers of all types of membranes, and researchers from engineering schools and industry. It allows ever offered measures to connect to other membranes and properties.
•
Contact Marie Andrée Sirvain, Dr Ing. and Marion Dalex, sales manager IFTS Institute of Filtration and Separative Techniques Rue Marcel Pagnol - 47510 Foulayronnes France Tel: +33 5 53 95 83 94 Email: [email protected] www.ifts-sls.com
Table 1. Characteristics of the IFTS Porometer for each of its measurement modes.
www.filtsep.com
17