- Email: [email protected]
Filtration characteristics of polysulfone membrane filters
Aerosol Science 37 (2006) 1198 – 1208 www.elsevier.com/locate/jaerosci
Filtration characteristics of polysulfone membrane filters Hsiao-Lin Huanga,∗,1 , Shinhao Yangb a Department of Occupational Safety and Health, Chia Nan University of Pharmacy & Science, 60, Erh-Jen Rd., Sec. 1,
Jen-Te, Tainan 717, Taiwan, ROC b Department of Leisure and Recreation Management, Toko University, Chia Yi County 613, Taiwan, ROC
Received 5 June 2005; received in revised form 22 November 2005; accepted 22 November 2005
Abstract Polysulfone (PSF) membrane filters are employed as thermoplastic material in fabricating membranes for use in water ultrafiltration systems and gas separation. However, PSF membrane filters have seldom been used in aerosol filtration. Therefore, this work clarifies the effects of PSF membrane filters on aerosol penetration. Three concentrations of casting solutions (10%, 15%, and 20%) were used to form membranes with the variously sized pores and porosities. The PSL monodisperse aerosols were applied as the challenged aerosols. The aerosol penetration was measured using a condensation particle counter. The effects of various factors, including particle size (0.038.0.81 m), flow rate (5.2, 10.4, and 15.6 l/ min) and relative humidity (RH 30% and RH 70%) on the aerosol collection characteristics were evaluated. The experimental results implied that 10%, 15%, and 20% PSF membrane filters have different porosities and pressure drops across them were about 4.51, 5.81, and 7.31 kPa. The data obtained herein reveal that the aerosol penetrations of the 0.3 m PSL aerosol through the 10%, 15%, and 20% PSF membrane filters were around 7.5%, 3.8%, and 0.9%, respectively. The size of the most penetrating aerosol through the PSF membrane was approximately 0.05 m. These results demonstrate that the aerosol penetration through PSF membrane filters declines as the concentration of the casting solution increases, because the pore size inside the PSF membrane filter decreases as the concentration of the casting solution rises. Moreover, aerosol penetration through the PSF membrane filters increases obviously with the flow rate when aerosol is less smaller than 0.21 m. When aerosol is greater than 0.3 m, variations of penetration through the PSF membrane filters become smaller at different flow rates. The relative humidity does not affect the aerosol penetration through the PSF membrane filters. The 20% PSF membrane filter has a quality factor larger than the 10% and 15% PSF membrane filter over the entire range of aerosol sizes studies. 䉷 2005 Elsevier Ltd. All rights reserved. Keywords: Aerosol penetration; Polysulfone membrane filters; Flow rate; Relative humidity; Quality factor
1. Introduction Filtration is the most extensively employed technology for sampling or collecting aerosol, primarily because it is low cost and simple. Generally, the filters used to sample or collect aerosols can be classified as fibrous filters, fabric filters, membrane filters or granular filters. The purpose of these various kind of filters would be governed by their ∗ Corresponding author. Tel.: +886 6 2664911x265; fax: +886 6 2667320.
E-mail addresses: [email protected] (H.-L. Huang), [email protected] (S. Yang). 1 Assistant Professor, Department of Occupational Safety and Health, Chia Nan University of Pharmacy & Science, 60, Erh-Jen Rd., Sec. 1,
Jen-Te, Tainan 717, Taiwan. 0021-8502/$ - see front matter 䉷 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaerosci.2005.11.010
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
1199
structure, pore size, packing fraction, fiber diameter, thickness, felting methods and the air resistance across the filter. In the aspect of particle sampling, the fibrous filters are applied first in sampling particles. Recently, the membrane filters have been widely used to sample particles in the fields of environmental hygiene and occupational hygiene (Carvacho et al., 2004; Hoek et al., 1997; Mader et al., 2003; Saitoh, Serab, Hirano, & Shirai, 2002), since they can sustain higher pressure and provide high-speed filtration performance. Moreover, the particles 2002 on the membrane filters can be weighed and chemically analyzed (Cohen & Hering, 1995). The membrane filters were grouped into two types. The first group is the capillary pore membrane filters, including the polycarbonate Nuclepore filter. The pores in such filters are cylindrical, with an almost uniform diameter, and predominantly perpendicular to the surface of the filter. The other type of membrane filter has variously sized pores and paths through the pores inside the filter are crooked. Previous investigations of the filtration characteristics of membrane filters have concentrated on the performance of Nuclepore membrane filters and membrane filters with uniformly sized pores. Spurny, Lodge, Frank, and Sheesley (1969a,b) studied the relationship between the filtration performance and particle and pore sizes, pressure drop, porosity of filters and temperature. Spurny and Pich (1964) determined that diffusion, and deposition and interception force are the main aerosol-capturing forces in the membrane filters. Manton (1978, 1979) characterized various particle collection mechanisms, including Brownian diffusion and impaction in the structures of Nuclepore filters. Caroff, Choudhary, and Gentry (1973) theoretically examined relationship between the sizes of the capillary pores and particle sizes and the filtration efficiencies. Smutek and Pich (1974) proposed a model of low-speed fluid flow in the capillary membrane filter. Montassier, Dupin, and Boulaud (1996) experimentally investigated the collection efficiency of the Isopore membrane filter, which has the same structure as the Nuclepore membrane, in relation to its pressure drop using the radioactive aerosol. Additionally, Sioutas, Koutrakis, Wang, Babich, and Wolfson (1999) examined the loading behavior of the Nuclepore membrane filters. Yamamoto, Fujii, Kumagai, and Yanagisawa (2004) also characterized the time course shift in penetration through capillary polycarbonate pore membrane filters for relatively short filtration periods (200 min). The polymer membrane filters made from organic material have the widest available range of pore sizes, supporting high pressure and filtering at high face velocity. Hence, the polymer membrane filters can be usefully and economically applied to aerosol sampling and air purification. Polysulfone (PSF) is a tough, rigid, high-strength and transparent thermoplastic, which maintains its characteristics over a wide range of temperatures, from −100 ◦ C to over 160 ◦ C. The chemical and physical properties of PSF, including its good thermal and chemical stability, mechanical strength and excellent oxidative resistance make the preferred material for use in a membrane substrate. PSF is extensively employed as an thermoplastic material in fabricating membranes for use in water ultrafiltration systems (Kaeselev, Pieracci, & Belfort, 2001; Kilduff, Mattaraj, Pieracci, & Belfort, 2000) and gas separation, such as in stripping carbon dioxide from natural gas streams and producing highly pure nitrogen from air (Liu, Chakma, & Feng, 2004). In particular, PSF is also used in ion exchange membranes in electro-membrane processes such as electrodialysis and polymer electrolyte membrane electrolysis (Yoshikawa, Hara, Tanigaki, Guiver, & Matsuura, 1992; Guiver et al., 2001). However, PSF membrane filter has very been used less in aerosol filtration, such as in aerosol sampling (Cullen, Field, & Sherrell, 2001). Therefore, this goal of this work is to generate PSF membrane filters in various concentrations of PSF and elucidate the aerosol penetration through the PSF membrane filters. Moreover, the PSF membrane filters have pores in different sizes (Summers, Ndawuni, & Summers, 2003). Previous studies always investigated the filtration characteristics of the capillary pore membrane filters. Thus, the work could also explore the filtration performance of various cylindrical filters. The effects of the face velocity and relative humidity on penetration of aerosol through PSF membrane filters were also examined.
2. Experimental materials and methods 2.1. Filter media The PSF used in this study was obtained from AMOCO Performance Products Inc. (Ridgefield, CT) under the trade name Udel P-3500. The solvent N -methylpyrrolidinone (NMP) was of reagent grade and used without further purification. Distilled water was the nonsolvent. PSF was dissolved in the above solvent mixture to form a casting solution at a predetermined temperature. The degassed casting solution was cast onto a preheated glass plate using a preheated Gardner knife. The thickness of the PSF membrane filters was around 0.03 mm. The nascent membrane
1200
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
Table 1 Characteristics of PSF membrane filters Type
Material
Measured weight of filter (g/m2 )
Filter thickness (mm)
Porosity (%)
10% PSF 15% PSF 20% PSF Isopore membrane filter
Polysulfone Polysulfone Polysulfone Polycarbonate
13.4 16.4 19.2 8.9
0.03 0.03 0.03 0.001
63.8 55.9 48.4 5–20
was immediately immersed in a distilled water coagulation bath at the same temperature, which was maintained for at least 1 day. The obtained membranes were peeled off and completely air dried at ambient temperature. Three casting solutions with different concentrations (10%, 15%, and 20%) were used to yield the PSF membrane filters. In an earlier work (Xu, Zhu, & Xu, 2004), they determined that the size of the pores in the PSF film declines as the concentration of the casting solution increased. Accordingly these three PSF membrane filters can be employed to compare the effect of the pore size on aerosol penetration through the PSF membrane filters. Table 1 provides the basic characteristics of the PSF membrane filters. A manufactured capillary pore membrane filter with 0.6 m pore size (Isopore membrane filter, Millipore Ltd.) was also used for comparison, to elucidate for the performance of the PSF membrane filters. The pore size of this Isopore membrane filter (0.6 m) is the moderate pore size of the available capillary pore membrane filters. The porosity of the selected membrane was in the range 5–20%. And the porosity of the selected membrane and the other Isopore membranes were also generated in the same range. Table 1 also presents the properties of the capillary pore membrane filter. In this work, the diameter of each filter was controlled at 47 mm. 2.2. Overall porosity The porosity of the PSF membrane filters was given by the following equation: Porosity =
V m − Vp × 100%, Vm
(1)
where Vm is the bulk volume of the membrane and Vp represents the polymer volume. Vm is determined by multiplying the area of the membrane by its thickness. A thickness gauge (Teclock Corp., Japan) was used to measure the thickness of the membrane. The volume occupied by the polymer (Vp ) is Wm /p , where Wm is the weight of the membrane and p is the density of the polymer and has a value of 1.24 g/cm3 for PSF (measured by Micromeritics, Accupyc Co. Model 1330). The porosities of the PSF membrane filters are shown in Table 1. 2.3. Aerosol generation and test system Fig. 1 schematically depicts the experimental setup for the aerosol penetration test of the PSF membrane filters. It comprises an aerosol generator, a neutralizer, a mixing column, a filter holder, a tested filter, an aerosol electrometer, a condensation particle counter (CPC) used to sample the aerosol concentrations upstream and downstream of the filter, and a pressure gauge to measure the pressure drop across the filter and a flow meter. Monodisperse polystyrene latex (PSL, Duke Inc.) aerosols were atomized using a Collison atomizer (model 3076, TSI Inc.) for testing. However, during the atomized process, some small sized mists were also generated in the aerosol flow. For avoiding these mists to interfere the experimental results, a Differential Mobility Analyzer (DMA, model 3080, TSI Inc.) was applied to electrically classify the aerosols and obtain the really monodisperse PSL aerosols. PSL aerosol sizes ranging from 0.038 to 0.81m were used in evaluating the aerosol penetration through the tested filters. The monodispersity of the generated aerosols was confirmed using a Scanning Mobility Particle Sizer (SMPS, model 3936, TSI Inc.) and an Aerodynamic Particle Sizer (APS, model 3310A, TSI Inc.). The geometric standard deviation (GSD) of these generated aerosols, based on particle counts varied from 1.04 to 1.05. Then, the latex monodisperse aerosol was passed through a Kr-85 radioactive source (model 3077, TSI Inc.), which neutralized the aerosol to the Boltzmann charge equilibrium. After it had passed through the neutralizer, the tested aerosol was flown into the mixing column, in which it was mixed with the diluted clean air. An aerosol electrometer (model 3068, TSI Inc., MN) was
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
1201
HEPA High-Volt Power Supply (-)
Compressed Air
Kr85
Air Supply System Diffusion Dryer
Compressed Air
DMA
Compressed Air Compressed
HEPA
Air
HEPA
Air Supply System
Excess Air
Collison Atomizer
Flow Meter Saturator Kr85 Flow Meter
Aerosol Electrometer
Pressure Gage Mixing Column
Flow Meter
Excess Air
CPC
Filter Holder
Fig. 1. Schematic diagram of the experimental system.
applied to detect the aerosol charge state in the mixing column. The diluted aerosol flow was drawn through the filter holder by using the flow meter and the pump. The penetration through the PSF membrane filters was measured using a CPC, (model 3022A, TSI Inc.), which measured the aerosol concentrations upstream and downstream of the tested filter. The pressure drop across the tested filter was measured using a pressure gauge (model 2000-50CM, Dwyer Instruments Inc.) during the period of testing. The flow rates across the tested filter were controlled using a flow meter and a pump. The testing flow rates were varied from 5.2 to 15.6 l/ min. Aerosol penetrations of 0.03.0.81 m using PSL monodisperse aerosols without any filter were measured to ensure that aerosol removal occurs only on the filter surface and not on the holder or any part of the test chamber. Aerosol concentrations upstream and downstream of the filter holder were immediately sampled using a CPC. Penetration tests were undertaken at flow rates of 5.2, 10.4, and 15.6 l/ min. Results displayed in Fig. 2, clearly display that losses of submicron aerosols through the holder are very small (< 5%), verifying that aerosol removal could only occur across the tested filter.
1202
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
100
Aerosoldownstream/Aerosolupstream (%)
98 96 94 92 90 88 86 84 82
5.2 l/min 10.4 l/min 15.6 l/min
80 10-2
10-1 Aerosol Size (µm)
100
Fig. 2. Evaluation of aerosol losses through the test chamber and filter holder.
2.4. Relative humidity control system This work considered the effect of relative humidity (RH) on the filtration characteristics. Two RHs were used in this work. The RH of the aerosol-flow stream was modified by changing the ratio of the flow rate of the dry gas stream to that of the humidified gas stream generated by the water vapor saturator. The final RH of the aerosol-flow stream was measured using a Q-Trak Plus (model 8552, TSI Inc.). Two relative humidity conditions used in experiments in this study are 30% and 85% and they correspond to dry and humid conditions. 2.5. Quality factor A good filter provides the highest collection efficiency with the lowest pressure drop. A useful index of quality for various filters is the quality factor, qF , defined by (Hind, 1982) qF =
ln (1/P ) , p
(2)
where P represents the particle penetration, and p is the pressure drop across the filter. A better filter has a larger qF . Values of qF can be compared only when the face velocity and size of the test particle are held constant.
3. Results and discussion 3.1. Pressure drop across the PSF membrane filters Fig. 3 plots the pressure drop across the two PSF membrane filters (10%, 15%, and 20% PSF) and manufactured polycarbonate Isopore membrane filter against the sampled flow rate. The pressure drop increases with the face velocity, suggesting that the airflow through the tested filters is laminar. The pressure drops across the 10%, 15%, and 20% PSF membrane filters are around 4.51, 5.81, and 7.31 kPa, respectively, at a flow rate of 10.4 l/min. The pressure drop across the 20% PSF membrane filter exceeds that across the 10% and 15% PSF membrane filter, because the porosity of the 20% PSF membrane filter (48.4%) is less than that of the 10% and 15% PSF membrane filters (63.8% and 55.9%). According to the comparison of the pressure drops across the three PSF membrane filters and manufactured capillarypore membrane filter, the pressure drops across the PSF membrane filters are higher than that across the manufactured
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
1203
20 PSF 10% Membrane Filter PSF 15% Membrane Filter PSF 20% Membrane Filter Isopore Membrane Filter
18
Pressure Drop (kPa)
16 14 12 10 8 6 4 2 0 0
5.2
10.4 Flow Rate (l/min)
15.6
20.8
Fig. 3. Pressure drop across the PSF membrane filters.
100
Aerosol Penetration (%)
PSF 10% Membrane Filter PSF 15% Membrane Filter PSF 20% Membrane Filter 10
1
0.1 0.01
0.1 Aerosol Size (µm)
1
Fig. 4. Aerosol penetration through the PSF membrane filters.
capillary-pore membrane filter. At a flow rate of 10.4 l/min, the pressure drop across the manufactured capillary-pore membrane filter is about 3.04 kPa. 3.2. Aerosol penetration through the PSF membrane filters This work applied three different concentration of casting solutions (10%, 15%, and 20%) were employed to generate the PSF membrane filters and the effects of the different concentration of the casting solutions on the aerosol penetration were investigated. Fig. 4 plots the experimental penetrations of “Boltzmann equilibrium” PSL aerosols through the 10%, 15%, and 20% PSF membrane filters. In each test, the flow rate was maintained at 10.4 l/min. The aerosol penetrations of the 0.3 m PSL aerosol through the 10%, 15%, and 20% PSF membrane filters were around 7.5%, 3.8%, and 0.9%, respectively. The data in Fig. 3 indicate that the most penetrating aerosol size through the PSF membrane filters was
1204
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
100
Aerosol Penetration (%)
Isopore Membrane Filter
10
1 0.01
0.1 Aerosol Size (µm)
1
Fig. 5. Aerosol penetration through the Isopore membrane filter.
approximately 0.05 m. Additionally, the penetration of aerosol through the 20% PSF membrane filter is lower than that through the 10% and 15% PSF membrane filters and the penetration through the 10% PSF membrane filter is the highest in all cases. It is mainly because the size of the pores in the PSF membrane filter decreases as the PSF concentration of the casting solution increases. The previous study (Xu et al., 2004) determined that the size of the pores in the PSF film declines as the concentration of the casting solution increased. The impaction and interception mechanic aerosol capturing forces increased as the size of the pores in the membrane filter dropped. Therefore, the aerosol penetration fell as the concentration of the casting solution increased. Fig. 5 plots the aerosol penetration through the manufactured capillary-pore membrane filter against the aerosol size at a face rate of 10.4 l/min. The results demonstrate that the penetration through the manufactured capillary-pore membrane filter in the range of 11.2–16.5%. According to the previous investigations (Liu, Pui, & Rubow, 1981; Spurny et al., 1969a), the penetration through manufactured capillarypore membrane filter is in the moderate penetration range. The penetration through the manufactured membrane is larger than that of these three PSF membrane filters. It is indicating that these three PSF filters have the better aerosol penetration than the manufactured capillary-pore membrane filter. However, the pressure drop across these three PSF membrane filters (4.51, 5.81, and 7.31 kPa) is larger than that across the manufactured capillary-pore membrane filter (3.04 kPa). A good filter has not only a high collection efficiency but also a low-pressure drop. Thus, this work used a quality factor (qF ), which is defined as Eq. (2), to further understand the performance of PSF membrane filters. The quality factor is a useful index of the filtration performance, which incorporates both the pressure drop and the aerosol penetration. A larger qF corresponds to a better filter. Fig. 6 plots the qF of the 10%, 15%, and 20% PSF membrane filters, and manufactured capillary-pore membrane filter versus the size of the aerosol at a flow rate of 10.4 l/min. The experimental data reveal that the 20% PSF membrane filter has larger qF than the manufactured capillary-pore membrane filters over the entire range of aerosol sizes examined. The qF of 10% PSF membrane filter is higher than that of manufactured capillary-pore membrane filter when aerosol size is larger than 0.45 m. It is suggesting that although the pressure drop across the 20% and 10% PSF membrane filters exceeds that across the manufactured capillary-pore membrane filter, the filtration performance of the PSF membrane filters is better. The pore size and penetration of the manufactured membrane filter belong to the moderate membrane filters. So we understood that some PSF membrane filters produced in laboratory could reach the moderate manufactured capillary-pore membrane filter. Furthermore, the experimental results demonstrate that the 20% PSF membrane filter has a larger qF than the 10% and 15% PSF membrane filters over the whole range of aerosol sizes. Comparing the qF ’s of 10% and 15% PSF membrane filters, we see 10% PSF membrane filter has the lager qF than 15% PSF membrane filter. And at the larger aerosol size, the qF of the 10% PSF membrane filter is close to that of 20% membrane filter. Although the penetration through the 10% PSF membrane filter is higher than that through 15% and 20% PSF membrane filters, the pressure drop across the 10% PSF
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
1205
1.0
Quality Factor (1/cm H2O)
0.8
0.6
0.4 PSF 10% Membrane Filter PSF 15% Membrane Filter PSF 20% Membrane Filter Polycarbonate Isopore Membrane Filter
0.2
0.0 0.01
0.1 Aerosol Size (µm)
1
Fig. 6. Quality factor of the PSF membrane filters versus aerosol size.
100
Aerosol Penetration (%)
15% PSF Membrane Filter 15.6 l/min 10.4 l/min 5.2 l/min
10
1
0.1 0.01
0.1 Aerosol Size (µm)
1
Fig. 7. Aerosol penetration through the 15% PSF membrane filter at different flow rates.
membrane filter is lower than that across 15% and 20% PSF membrane filters. That indicates that lower concentration of the casting solution could also get higher qF . 3.3. Effect of flow rate on aerosol penetration Fig. 7 plots aerosol penetration through the 15% PSF membrane filter versus the aerosol size of the “Boltzmann equilibrium” PSL aerosol at various flow rates (5.2, 10.4, and 15.6 l/ min). Fig. 7 shows an increase in the penetration of the 0.21-m aerosol through the 15% PSF membrane filter from around 2.4% to 9.4% as the flow rate rises from 5.2 to 15.6 l/ min. However, the 0.81-m aerosol penetrations through the 15% PSF membrane filter at flow rates of 5.2, 10.4, and 15.6 l/ min are 1.3, 2.1, and 2.5 l/ min, respectively. Similar results are obtained for the 10% and 20% PSF membrane filters. The 0.2-m-aerosol penetration through the 20% PSF membrane filter rises from 0.6% to 3.4%
1206
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
100
Aerosol Penetration (%)
RH 30% RH 85%
10
1
0.1 0.01
0.1 Aerosol Size (µm)
1
Fig. 8. Aerosol penetration through the 15% PSF membrane filter at different relative humidities.
as flow rate rises from 5.2 to 15.6 l/ min; the 0.81-m aerosol penetrations through the 20% PSF membrane filter at flow rates of 5.2, 10.4, and 15.6 l/ min are 0.3%, 0.4%, and 0.4%, respectively. This suggests that aerosol penetration through the PSF membrane filters increases obviously with the flow rate from 5.2 to 15.6 l/ min when aerosol size is less than 0.21 m. When aerosol is larger than 0.3 m, the variation of penetration through the PSF membrane filters at different flow rate become smaller. It is because diffusion effect works on smaller aerosols (< 0.5 m). A higher flow rate corresponds to a shorter residence time associated with aerosol deposition by diffusion. When aerosols are greater than 0.5 m, the impaction and interception effects become obvious. The data are consistent with the results obtained by Liu and Lee (1976). 3.4. Effect of relative humidity on aerosol penetration Fig. 8 plots the aerosol penetration through the 15% PSF membrane filter versus the aerosol size for two values of RH (30% and 85%). The experimental data show that the aerosol penetrations through the 15% PSF membrane filter at RH 30% and 85% are almost the same. For example, the 0.3-m-aerosol penetrations through the 15% PSF membrane at RH values of 30% and 85% are 4.8% and 5.2%. Similarly, the aerosol penetrations through the 10% and 20% PSF membrane filters at both RH values are almost identical, indicating that the RH affects the penetration of aerosol through the PSF membrane filters. This fact in turn reveals that the water molecules do not affect the structure of the membrane filter at higher RH. 3.5. Comparison of the effect of different parameters on filtration performance An regression equation was used to understand the effects of different parameters on filtration performance of PSF membrane filters. The aerosol penetration was almost not affected by the different RHs. Thus, the RH was not regressed in the equation. The regression equation considered the most relevant parameters, including aerosol size (0.038.0.81 m), flow rate (5.2.15.6 l/ min), and porosity of the membrane filter (63.8%, 55.9%, and 48.4%). The experimental results can be fitted to the following equation: QF = ad bp p c ud ,
(3)
where a, b, c, and d are constants, qF is quality factor of the membrane filter, dp is the aerosol size, u is the flow rate, and p is porosity of the membrane filter. According to the regression analysis, the regression equation is shown in the
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
1207
following: QF = 0.7dp0.124 p −0.132 u−0.029 .
(4)
In the regression analysis, coefficient of determination R 2 is about 0.93. Comparing of coefficients of b, c, and d, we found effects of different parameters on the filtration performance. The result shows that the largest value is c, indicating the influence of the porosity of the membrane filter is the highest. The second highest value is of b, and the smallest value is d. Thus, the effect of aerosol size on filtration performance is the second in the tested aerosol sized range. The effect of the flow rate on filtration performance is the lowest. The influence degree of these three parameters only existed in the controlled condition of this work. 4. Conclusions The experimental results implied that the pressure drops across 10%, 15%, and 20% PSF membrane filters were about 4.51, 5.81, and 7.31 kPa, respectively. The pressure drop across the 20% PSF membrane filter exceeds that across the 10% and 15% PSF membrane filter, because the porosity of the 20% PSF membrane filter (48.4%) is lower than that of the 10% and 15% PSF membrane filter (63.8% and 55.9%). The aerosol penetrations of the 0.3 m PSL aerosol through the 10%, 15%, and 20% PSF membrane filters were approximately 7.5%, 3.8%, and 0.9%, respectively. The most penetrating aerosol size through the PSF membrane filters is about 0.05 m. The aerosol penetration through the 20% PSF membrane filter is lower than that through the 10% and 15% PSF membrane filters throughout the range of experimental aerosol sizes, probably because the size of the pores in the PSF membrane filter declines as the PSF concentration in the casting solution increases. The mechanical impaction and interception aerosol capturing forces increase as the size of the pores in the membrane filter falls. The aerosol penetration through the PSF membrane filters increases obviously with the flow rate when aerosol size is smaller than 0.21 m; when aerosol larger than 0.3 m, the change of penetration through PSF membrane filters becomes smaller at different flow rate. It is due to this that the diffusion effect works on the smaller aerosols (< 0.5 m) and impaction and interception effects become obvious when aerosols size is larger than 0.5 m. The relative humidity does not affect the performance of the PSF membrane filters. The experimental data demonstrate that the 20% PSF membrane filter has a larger qF than the 10% and 15% PSF membrane filters over the whole range of aerosol sizes. This work might also offer a new application for using PSF membrane filters in aerosol filtration. Acknowledgements The authors would like to thank the National Science Council of Republic of China for financially supporting this research under Contract No. NSC 92-2218-E-041-006. References Caroff, M., Choudhary, K. R., & Gentry, J. W. (1973). Effect of pore and particle size distribution on efficiencies of membrane filters. Journal of Aerosol Science, 4, 93–102. Carvacho, O. F., Krystyna, T. N., Ashbaugh, L. L., Flocchini, R. G., Melin, P., & Celisn, J. (2004). Elemental composition of springtime aerosol in Chillan, Chile. Atmospheric Environment, 38, 5349–5352. Cohen, B. S., & Hering, S. V. (1995). Air sampling instruments for evaluation of atmospheric contaminants (8th ed.), ACGIG: Cincinnati, OH. Cullen, J. T., Field, M. P., & Sherrell, R. M. (2001). Determination of trace elements in filtered suspended particulate material by sector field HR-ICP-MS. Journal of Analytical Atomic Spectrometry, 16, 1307–1312. Guiver, M. D., Robertson, G. P., Rowe, S., Foley, S., Kang, Y. S., Park, H. C. et al. (2001). Modified polysulfones. IV. Synthesis and characterization of polymers with silicon substitutents for a comparative study of gas-transport properties, Journal of polymer science. Polymer Chemistry Edition, 39, 2103. Hind, W. C. (1982). Aerosol technology. NewYork: Wiley. Hoek, G., Forsberg, B., Borowska, M., Hlawiczka, S., Vaskovi, E., Welinder, H. et al. (1997). Wintertime PM10 and black smoke concentrations across Europe: Results from the PEACE study. Atmospheric Environment, 31, 3341–3349. Kaeselev, B., Pieracci, J., & Belfort, G. (2001). Photoinduced grafting of ultrafiltration membranes: Comparison of poly(ether sulfone) and poly(sulfone). Journal of Membrane Science, 194, 245.
1208
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
Kilduff, J. E., Mattaraj, S., Pieracci, J. P., & Belfort, G. (2000). Photochemical modification of poly(ether sulfone) and sulfonated (polysulfone) nanofiltration membranes for control of fouling of natural organic matter. Desalination, 132, 133. Liu, B. H. Y., & Lee, K. W. (1976). Efficiency of membrane and nuclepore filters for submicronmeter aerosols. Environment Science and Technology, 10, 345–350. Liu, B. H.Y., Pui, D.Y. H., & Rubow, K. L. (1981). Characteristics of air sampling filter media. Aerosol in the Mining and Industrial Work Environment, III Instrumentation, 989–1038. Liu, L., Chakma, A., & Feng, X. (2004). Preparation of hollow fiber poly(ether block amide)/polysulfone composite membranes for separation of carbon dioxide from nitrogen. Chemical Engineering Journal, 105, 43–51. Mader, B. T., Schauer, J. J., Seinfeld, J. H., Flagan, R. C., Yu, J. Z., Yang, H. et al. (2003). Sampling methods used for the collection of particle-phase organic and elemental carbon during ACE-Asia. Atmospheric Environment, 37, 1435–1449. Manton, M. J. (1978). The impaction of aerosols on a nuclepore filter. Atmospheric Environment, 12, 1669–1675. Manton, M. J. (1979). Brownian diffusion of aerosols to the face of a nuclepore filter. Atmospheric Environment, 13, 525–531. Montassier, M., Dupin, L., & Boulaud, D. (1996). Experimental study on the collection efficiency of membrane filters. Journal of Aerosol Science, 27(Suppl. 1), S637–S638. Saitoh, K., Serab, K., Hirano, K., & Shirai, T. (2002). Chemical characterization of particles in winter-night smog in Tokyo. Atmospheric Environment, 36, 435–440. Sioutas, C., Koutrakis, P., Wang, P. Y., Babich, P., & Wolfson, M. (1999). Experimental investigation of pressure drop with particle loading in nuclepore filters. Aerosol Science and Technology, 30, 71–83. Smutek, M., & Pich, J. (1974). Impaction of particles on the surface of membrane filters. Journal of Aerosol Science, 5, 1724. Spurny, K., & Pich, J. (1964). The separation of aerosol particles by means of membrane filters by diffusion and inertial impaction. International Journal of Air and Water Pollution, 8, 193. Spurny, K. R., Lodge, J. P., Frank, E. R., & Sheesley, D. C. (1969a). Aerosol filtration by means of nuclepore filters structural and filtration properties. Environmental Science & Technology, 3, 453–464. Spurny, K. R., Lodge, J. P., Frank, E. R., & Sheesley, D. C. (1969b). Aerosol filtration by means of nuclepore filters aerosol sampling and measurement. Environmental Science and Technology, 3, 464–468. Summers, G. J., Ndawuni, M. P., & Summers, C. A. (2003). Dipyridyl functionalized polysulfones for membrane production. Journal of Membrane Science, 226, 21–33. Xu, Y., Zhu, B., & Xu, Y. (2004). A study on formation of regular honeycomb pattern in polysulfone film. Polymer, 46, 713–717. Yamamoto, N., Fujii, M., Kumagai, K., & Yanagisawa, Y. (2004). Time course shift in particle penetration characteristics through capillary pore membrane filters. Journal of Aerosol Science, 35, 731–741. Yoshikawa, M., Hara, H., Tanigaki, M., Guiver, M. D., & Matsuura, T. (1992). Modified polysulfone membranes. 1. Pervaporation of water/alcohol mixtures through modified polysulfone membranes having methyl ester moiety. Polymer, 33, 4805–4813.
Filtration characteristics of polysulfone membrane filters Hsiao-Lin Huanga,∗,1 , Shinhao Yangb a Department of Occupational Safety and Health, Chia Nan University of Pharmacy & Science, 60, Erh-Jen Rd., Sec. 1,
Jen-Te, Tainan 717, Taiwan, ROC b Department of Leisure and Recreation Management, Toko University, Chia Yi County 613, Taiwan, ROC
Received 5 June 2005; received in revised form 22 November 2005; accepted 22 November 2005
Abstract Polysulfone (PSF) membrane filters are employed as thermoplastic material in fabricating membranes for use in water ultrafiltration systems and gas separation. However, PSF membrane filters have seldom been used in aerosol filtration. Therefore, this work clarifies the effects of PSF membrane filters on aerosol penetration. Three concentrations of casting solutions (10%, 15%, and 20%) were used to form membranes with the variously sized pores and porosities. The PSL monodisperse aerosols were applied as the challenged aerosols. The aerosol penetration was measured using a condensation particle counter. The effects of various factors, including particle size (0.038.0.81 m), flow rate (5.2, 10.4, and 15.6 l/ min) and relative humidity (RH 30% and RH 70%) on the aerosol collection characteristics were evaluated. The experimental results implied that 10%, 15%, and 20% PSF membrane filters have different porosities and pressure drops across them were about 4.51, 5.81, and 7.31 kPa. The data obtained herein reveal that the aerosol penetrations of the 0.3 m PSL aerosol through the 10%, 15%, and 20% PSF membrane filters were around 7.5%, 3.8%, and 0.9%, respectively. The size of the most penetrating aerosol through the PSF membrane was approximately 0.05 m. These results demonstrate that the aerosol penetration through PSF membrane filters declines as the concentration of the casting solution increases, because the pore size inside the PSF membrane filter decreases as the concentration of the casting solution rises. Moreover, aerosol penetration through the PSF membrane filters increases obviously with the flow rate when aerosol is less smaller than 0.21 m. When aerosol is greater than 0.3 m, variations of penetration through the PSF membrane filters become smaller at different flow rates. The relative humidity does not affect the aerosol penetration through the PSF membrane filters. The 20% PSF membrane filter has a quality factor larger than the 10% and 15% PSF membrane filter over the entire range of aerosol sizes studies. 䉷 2005 Elsevier Ltd. All rights reserved. Keywords: Aerosol penetration; Polysulfone membrane filters; Flow rate; Relative humidity; Quality factor
1. Introduction Filtration is the most extensively employed technology for sampling or collecting aerosol, primarily because it is low cost and simple. Generally, the filters used to sample or collect aerosols can be classified as fibrous filters, fabric filters, membrane filters or granular filters. The purpose of these various kind of filters would be governed by their ∗ Corresponding author. Tel.: +886 6 2664911x265; fax: +886 6 2667320.
E-mail addresses: [email protected] (H.-L. Huang), [email protected] (S. Yang). 1 Assistant Professor, Department of Occupational Safety and Health, Chia Nan University of Pharmacy & Science, 60, Erh-Jen Rd., Sec. 1,
Jen-Te, Tainan 717, Taiwan. 0021-8502/$ - see front matter 䉷 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaerosci.2005.11.010
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
1199
structure, pore size, packing fraction, fiber diameter, thickness, felting methods and the air resistance across the filter. In the aspect of particle sampling, the fibrous filters are applied first in sampling particles. Recently, the membrane filters have been widely used to sample particles in the fields of environmental hygiene and occupational hygiene (Carvacho et al., 2004; Hoek et al., 1997; Mader et al., 2003; Saitoh, Serab, Hirano, & Shirai, 2002), since they can sustain higher pressure and provide high-speed filtration performance. Moreover, the particles 2002 on the membrane filters can be weighed and chemically analyzed (Cohen & Hering, 1995). The membrane filters were grouped into two types. The first group is the capillary pore membrane filters, including the polycarbonate Nuclepore filter. The pores in such filters are cylindrical, with an almost uniform diameter, and predominantly perpendicular to the surface of the filter. The other type of membrane filter has variously sized pores and paths through the pores inside the filter are crooked. Previous investigations of the filtration characteristics of membrane filters have concentrated on the performance of Nuclepore membrane filters and membrane filters with uniformly sized pores. Spurny, Lodge, Frank, and Sheesley (1969a,b) studied the relationship between the filtration performance and particle and pore sizes, pressure drop, porosity of filters and temperature. Spurny and Pich (1964) determined that diffusion, and deposition and interception force are the main aerosol-capturing forces in the membrane filters. Manton (1978, 1979) characterized various particle collection mechanisms, including Brownian diffusion and impaction in the structures of Nuclepore filters. Caroff, Choudhary, and Gentry (1973) theoretically examined relationship between the sizes of the capillary pores and particle sizes and the filtration efficiencies. Smutek and Pich (1974) proposed a model of low-speed fluid flow in the capillary membrane filter. Montassier, Dupin, and Boulaud (1996) experimentally investigated the collection efficiency of the Isopore membrane filter, which has the same structure as the Nuclepore membrane, in relation to its pressure drop using the radioactive aerosol. Additionally, Sioutas, Koutrakis, Wang, Babich, and Wolfson (1999) examined the loading behavior of the Nuclepore membrane filters. Yamamoto, Fujii, Kumagai, and Yanagisawa (2004) also characterized the time course shift in penetration through capillary polycarbonate pore membrane filters for relatively short filtration periods (200 min). The polymer membrane filters made from organic material have the widest available range of pore sizes, supporting high pressure and filtering at high face velocity. Hence, the polymer membrane filters can be usefully and economically applied to aerosol sampling and air purification. Polysulfone (PSF) is a tough, rigid, high-strength and transparent thermoplastic, which maintains its characteristics over a wide range of temperatures, from −100 ◦ C to over 160 ◦ C. The chemical and physical properties of PSF, including its good thermal and chemical stability, mechanical strength and excellent oxidative resistance make the preferred material for use in a membrane substrate. PSF is extensively employed as an thermoplastic material in fabricating membranes for use in water ultrafiltration systems (Kaeselev, Pieracci, & Belfort, 2001; Kilduff, Mattaraj, Pieracci, & Belfort, 2000) and gas separation, such as in stripping carbon dioxide from natural gas streams and producing highly pure nitrogen from air (Liu, Chakma, & Feng, 2004). In particular, PSF is also used in ion exchange membranes in electro-membrane processes such as electrodialysis and polymer electrolyte membrane electrolysis (Yoshikawa, Hara, Tanigaki, Guiver, & Matsuura, 1992; Guiver et al., 2001). However, PSF membrane filter has very been used less in aerosol filtration, such as in aerosol sampling (Cullen, Field, & Sherrell, 2001). Therefore, this goal of this work is to generate PSF membrane filters in various concentrations of PSF and elucidate the aerosol penetration through the PSF membrane filters. Moreover, the PSF membrane filters have pores in different sizes (Summers, Ndawuni, & Summers, 2003). Previous studies always investigated the filtration characteristics of the capillary pore membrane filters. Thus, the work could also explore the filtration performance of various cylindrical filters. The effects of the face velocity and relative humidity on penetration of aerosol through PSF membrane filters were also examined.
2. Experimental materials and methods 2.1. Filter media The PSF used in this study was obtained from AMOCO Performance Products Inc. (Ridgefield, CT) under the trade name Udel P-3500. The solvent N -methylpyrrolidinone (NMP) was of reagent grade and used without further purification. Distilled water was the nonsolvent. PSF was dissolved in the above solvent mixture to form a casting solution at a predetermined temperature. The degassed casting solution was cast onto a preheated glass plate using a preheated Gardner knife. The thickness of the PSF membrane filters was around 0.03 mm. The nascent membrane
1200
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
Table 1 Characteristics of PSF membrane filters Type
Material
Measured weight of filter (g/m2 )
Filter thickness (mm)
Porosity (%)
10% PSF 15% PSF 20% PSF Isopore membrane filter
Polysulfone Polysulfone Polysulfone Polycarbonate
13.4 16.4 19.2 8.9
0.03 0.03 0.03 0.001
63.8 55.9 48.4 5–20
was immediately immersed in a distilled water coagulation bath at the same temperature, which was maintained for at least 1 day. The obtained membranes were peeled off and completely air dried at ambient temperature. Three casting solutions with different concentrations (10%, 15%, and 20%) were used to yield the PSF membrane filters. In an earlier work (Xu, Zhu, & Xu, 2004), they determined that the size of the pores in the PSF film declines as the concentration of the casting solution increased. Accordingly these three PSF membrane filters can be employed to compare the effect of the pore size on aerosol penetration through the PSF membrane filters. Table 1 provides the basic characteristics of the PSF membrane filters. A manufactured capillary pore membrane filter with 0.6 m pore size (Isopore membrane filter, Millipore Ltd.) was also used for comparison, to elucidate for the performance of the PSF membrane filters. The pore size of this Isopore membrane filter (0.6 m) is the moderate pore size of the available capillary pore membrane filters. The porosity of the selected membrane was in the range 5–20%. And the porosity of the selected membrane and the other Isopore membranes were also generated in the same range. Table 1 also presents the properties of the capillary pore membrane filter. In this work, the diameter of each filter was controlled at 47 mm. 2.2. Overall porosity The porosity of the PSF membrane filters was given by the following equation: Porosity =
V m − Vp × 100%, Vm
(1)
where Vm is the bulk volume of the membrane and Vp represents the polymer volume. Vm is determined by multiplying the area of the membrane by its thickness. A thickness gauge (Teclock Corp., Japan) was used to measure the thickness of the membrane. The volume occupied by the polymer (Vp ) is Wm /p , where Wm is the weight of the membrane and p is the density of the polymer and has a value of 1.24 g/cm3 for PSF (measured by Micromeritics, Accupyc Co. Model 1330). The porosities of the PSF membrane filters are shown in Table 1. 2.3. Aerosol generation and test system Fig. 1 schematically depicts the experimental setup for the aerosol penetration test of the PSF membrane filters. It comprises an aerosol generator, a neutralizer, a mixing column, a filter holder, a tested filter, an aerosol electrometer, a condensation particle counter (CPC) used to sample the aerosol concentrations upstream and downstream of the filter, and a pressure gauge to measure the pressure drop across the filter and a flow meter. Monodisperse polystyrene latex (PSL, Duke Inc.) aerosols were atomized using a Collison atomizer (model 3076, TSI Inc.) for testing. However, during the atomized process, some small sized mists were also generated in the aerosol flow. For avoiding these mists to interfere the experimental results, a Differential Mobility Analyzer (DMA, model 3080, TSI Inc.) was applied to electrically classify the aerosols and obtain the really monodisperse PSL aerosols. PSL aerosol sizes ranging from 0.038 to 0.81m were used in evaluating the aerosol penetration through the tested filters. The monodispersity of the generated aerosols was confirmed using a Scanning Mobility Particle Sizer (SMPS, model 3936, TSI Inc.) and an Aerodynamic Particle Sizer (APS, model 3310A, TSI Inc.). The geometric standard deviation (GSD) of these generated aerosols, based on particle counts varied from 1.04 to 1.05. Then, the latex monodisperse aerosol was passed through a Kr-85 radioactive source (model 3077, TSI Inc.), which neutralized the aerosol to the Boltzmann charge equilibrium. After it had passed through the neutralizer, the tested aerosol was flown into the mixing column, in which it was mixed with the diluted clean air. An aerosol electrometer (model 3068, TSI Inc., MN) was
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
1201
HEPA High-Volt Power Supply (-)
Compressed Air
Kr85
Air Supply System Diffusion Dryer
Compressed Air
DMA
Compressed Air Compressed
HEPA
Air
HEPA
Air Supply System
Excess Air
Collison Atomizer
Flow Meter Saturator Kr85 Flow Meter
Aerosol Electrometer
Pressure Gage Mixing Column
Flow Meter
Excess Air
CPC
Filter Holder
Fig. 1. Schematic diagram of the experimental system.
applied to detect the aerosol charge state in the mixing column. The diluted aerosol flow was drawn through the filter holder by using the flow meter and the pump. The penetration through the PSF membrane filters was measured using a CPC, (model 3022A, TSI Inc.), which measured the aerosol concentrations upstream and downstream of the tested filter. The pressure drop across the tested filter was measured using a pressure gauge (model 2000-50CM, Dwyer Instruments Inc.) during the period of testing. The flow rates across the tested filter were controlled using a flow meter and a pump. The testing flow rates were varied from 5.2 to 15.6 l/ min. Aerosol penetrations of 0.03.0.81 m using PSL monodisperse aerosols without any filter were measured to ensure that aerosol removal occurs only on the filter surface and not on the holder or any part of the test chamber. Aerosol concentrations upstream and downstream of the filter holder were immediately sampled using a CPC. Penetration tests were undertaken at flow rates of 5.2, 10.4, and 15.6 l/ min. Results displayed in Fig. 2, clearly display that losses of submicron aerosols through the holder are very small (< 5%), verifying that aerosol removal could only occur across the tested filter.
1202
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
100
Aerosoldownstream/Aerosolupstream (%)
98 96 94 92 90 88 86 84 82
5.2 l/min 10.4 l/min 15.6 l/min
80 10-2
10-1 Aerosol Size (µm)
100
Fig. 2. Evaluation of aerosol losses through the test chamber and filter holder.
2.4. Relative humidity control system This work considered the effect of relative humidity (RH) on the filtration characteristics. Two RHs were used in this work. The RH of the aerosol-flow stream was modified by changing the ratio of the flow rate of the dry gas stream to that of the humidified gas stream generated by the water vapor saturator. The final RH of the aerosol-flow stream was measured using a Q-Trak Plus (model 8552, TSI Inc.). Two relative humidity conditions used in experiments in this study are 30% and 85% and they correspond to dry and humid conditions. 2.5. Quality factor A good filter provides the highest collection efficiency with the lowest pressure drop. A useful index of quality for various filters is the quality factor, qF , defined by (Hind, 1982) qF =
ln (1/P ) , p
(2)
where P represents the particle penetration, and p is the pressure drop across the filter. A better filter has a larger qF . Values of qF can be compared only when the face velocity and size of the test particle are held constant.
3. Results and discussion 3.1. Pressure drop across the PSF membrane filters Fig. 3 plots the pressure drop across the two PSF membrane filters (10%, 15%, and 20% PSF) and manufactured polycarbonate Isopore membrane filter against the sampled flow rate. The pressure drop increases with the face velocity, suggesting that the airflow through the tested filters is laminar. The pressure drops across the 10%, 15%, and 20% PSF membrane filters are around 4.51, 5.81, and 7.31 kPa, respectively, at a flow rate of 10.4 l/min. The pressure drop across the 20% PSF membrane filter exceeds that across the 10% and 15% PSF membrane filter, because the porosity of the 20% PSF membrane filter (48.4%) is less than that of the 10% and 15% PSF membrane filters (63.8% and 55.9%). According to the comparison of the pressure drops across the three PSF membrane filters and manufactured capillarypore membrane filter, the pressure drops across the PSF membrane filters are higher than that across the manufactured
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
1203
20 PSF 10% Membrane Filter PSF 15% Membrane Filter PSF 20% Membrane Filter Isopore Membrane Filter
18
Pressure Drop (kPa)
16 14 12 10 8 6 4 2 0 0
5.2
10.4 Flow Rate (l/min)
15.6
20.8
Fig. 3. Pressure drop across the PSF membrane filters.
100
Aerosol Penetration (%)
PSF 10% Membrane Filter PSF 15% Membrane Filter PSF 20% Membrane Filter 10
1
0.1 0.01
0.1 Aerosol Size (µm)
1
Fig. 4. Aerosol penetration through the PSF membrane filters.
capillary-pore membrane filter. At a flow rate of 10.4 l/min, the pressure drop across the manufactured capillary-pore membrane filter is about 3.04 kPa. 3.2. Aerosol penetration through the PSF membrane filters This work applied three different concentration of casting solutions (10%, 15%, and 20%) were employed to generate the PSF membrane filters and the effects of the different concentration of the casting solutions on the aerosol penetration were investigated. Fig. 4 plots the experimental penetrations of “Boltzmann equilibrium” PSL aerosols through the 10%, 15%, and 20% PSF membrane filters. In each test, the flow rate was maintained at 10.4 l/min. The aerosol penetrations of the 0.3 m PSL aerosol through the 10%, 15%, and 20% PSF membrane filters were around 7.5%, 3.8%, and 0.9%, respectively. The data in Fig. 3 indicate that the most penetrating aerosol size through the PSF membrane filters was
1204
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
100
Aerosol Penetration (%)
Isopore Membrane Filter
10
1 0.01
0.1 Aerosol Size (µm)
1
Fig. 5. Aerosol penetration through the Isopore membrane filter.
approximately 0.05 m. Additionally, the penetration of aerosol through the 20% PSF membrane filter is lower than that through the 10% and 15% PSF membrane filters and the penetration through the 10% PSF membrane filter is the highest in all cases. It is mainly because the size of the pores in the PSF membrane filter decreases as the PSF concentration of the casting solution increases. The previous study (Xu et al., 2004) determined that the size of the pores in the PSF film declines as the concentration of the casting solution increased. The impaction and interception mechanic aerosol capturing forces increased as the size of the pores in the membrane filter dropped. Therefore, the aerosol penetration fell as the concentration of the casting solution increased. Fig. 5 plots the aerosol penetration through the manufactured capillary-pore membrane filter against the aerosol size at a face rate of 10.4 l/min. The results demonstrate that the penetration through the manufactured capillary-pore membrane filter in the range of 11.2–16.5%. According to the previous investigations (Liu, Pui, & Rubow, 1981; Spurny et al., 1969a), the penetration through manufactured capillarypore membrane filter is in the moderate penetration range. The penetration through the manufactured membrane is larger than that of these three PSF membrane filters. It is indicating that these three PSF filters have the better aerosol penetration than the manufactured capillary-pore membrane filter. However, the pressure drop across these three PSF membrane filters (4.51, 5.81, and 7.31 kPa) is larger than that across the manufactured capillary-pore membrane filter (3.04 kPa). A good filter has not only a high collection efficiency but also a low-pressure drop. Thus, this work used a quality factor (qF ), which is defined as Eq. (2), to further understand the performance of PSF membrane filters. The quality factor is a useful index of the filtration performance, which incorporates both the pressure drop and the aerosol penetration. A larger qF corresponds to a better filter. Fig. 6 plots the qF of the 10%, 15%, and 20% PSF membrane filters, and manufactured capillary-pore membrane filter versus the size of the aerosol at a flow rate of 10.4 l/min. The experimental data reveal that the 20% PSF membrane filter has larger qF than the manufactured capillary-pore membrane filters over the entire range of aerosol sizes examined. The qF of 10% PSF membrane filter is higher than that of manufactured capillary-pore membrane filter when aerosol size is larger than 0.45 m. It is suggesting that although the pressure drop across the 20% and 10% PSF membrane filters exceeds that across the manufactured capillary-pore membrane filter, the filtration performance of the PSF membrane filters is better. The pore size and penetration of the manufactured membrane filter belong to the moderate membrane filters. So we understood that some PSF membrane filters produced in laboratory could reach the moderate manufactured capillary-pore membrane filter. Furthermore, the experimental results demonstrate that the 20% PSF membrane filter has a larger qF than the 10% and 15% PSF membrane filters over the whole range of aerosol sizes. Comparing the qF ’s of 10% and 15% PSF membrane filters, we see 10% PSF membrane filter has the lager qF than 15% PSF membrane filter. And at the larger aerosol size, the qF of the 10% PSF membrane filter is close to that of 20% membrane filter. Although the penetration through the 10% PSF membrane filter is higher than that through 15% and 20% PSF membrane filters, the pressure drop across the 10% PSF
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
1205
1.0
Quality Factor (1/cm H2O)
0.8
0.6
0.4 PSF 10% Membrane Filter PSF 15% Membrane Filter PSF 20% Membrane Filter Polycarbonate Isopore Membrane Filter
0.2
0.0 0.01
0.1 Aerosol Size (µm)
1
Fig. 6. Quality factor of the PSF membrane filters versus aerosol size.
100
Aerosol Penetration (%)
15% PSF Membrane Filter 15.6 l/min 10.4 l/min 5.2 l/min
10
1
0.1 0.01
0.1 Aerosol Size (µm)
1
Fig. 7. Aerosol penetration through the 15% PSF membrane filter at different flow rates.
membrane filter is lower than that across 15% and 20% PSF membrane filters. That indicates that lower concentration of the casting solution could also get higher qF . 3.3. Effect of flow rate on aerosol penetration Fig. 7 plots aerosol penetration through the 15% PSF membrane filter versus the aerosol size of the “Boltzmann equilibrium” PSL aerosol at various flow rates (5.2, 10.4, and 15.6 l/ min). Fig. 7 shows an increase in the penetration of the 0.21-m aerosol through the 15% PSF membrane filter from around 2.4% to 9.4% as the flow rate rises from 5.2 to 15.6 l/ min. However, the 0.81-m aerosol penetrations through the 15% PSF membrane filter at flow rates of 5.2, 10.4, and 15.6 l/ min are 1.3, 2.1, and 2.5 l/ min, respectively. Similar results are obtained for the 10% and 20% PSF membrane filters. The 0.2-m-aerosol penetration through the 20% PSF membrane filter rises from 0.6% to 3.4%
1206
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
100
Aerosol Penetration (%)
RH 30% RH 85%
10
1
0.1 0.01
0.1 Aerosol Size (µm)
1
Fig. 8. Aerosol penetration through the 15% PSF membrane filter at different relative humidities.
as flow rate rises from 5.2 to 15.6 l/ min; the 0.81-m aerosol penetrations through the 20% PSF membrane filter at flow rates of 5.2, 10.4, and 15.6 l/ min are 0.3%, 0.4%, and 0.4%, respectively. This suggests that aerosol penetration through the PSF membrane filters increases obviously with the flow rate from 5.2 to 15.6 l/ min when aerosol size is less than 0.21 m. When aerosol is larger than 0.3 m, the variation of penetration through the PSF membrane filters at different flow rate become smaller. It is because diffusion effect works on smaller aerosols (< 0.5 m). A higher flow rate corresponds to a shorter residence time associated with aerosol deposition by diffusion. When aerosols are greater than 0.5 m, the impaction and interception effects become obvious. The data are consistent with the results obtained by Liu and Lee (1976). 3.4. Effect of relative humidity on aerosol penetration Fig. 8 plots the aerosol penetration through the 15% PSF membrane filter versus the aerosol size for two values of RH (30% and 85%). The experimental data show that the aerosol penetrations through the 15% PSF membrane filter at RH 30% and 85% are almost the same. For example, the 0.3-m-aerosol penetrations through the 15% PSF membrane at RH values of 30% and 85% are 4.8% and 5.2%. Similarly, the aerosol penetrations through the 10% and 20% PSF membrane filters at both RH values are almost identical, indicating that the RH affects the penetration of aerosol through the PSF membrane filters. This fact in turn reveals that the water molecules do not affect the structure of the membrane filter at higher RH. 3.5. Comparison of the effect of different parameters on filtration performance An regression equation was used to understand the effects of different parameters on filtration performance of PSF membrane filters. The aerosol penetration was almost not affected by the different RHs. Thus, the RH was not regressed in the equation. The regression equation considered the most relevant parameters, including aerosol size (0.038.0.81 m), flow rate (5.2.15.6 l/ min), and porosity of the membrane filter (63.8%, 55.9%, and 48.4%). The experimental results can be fitted to the following equation: QF = ad bp p c ud ,
(3)
where a, b, c, and d are constants, qF is quality factor of the membrane filter, dp is the aerosol size, u is the flow rate, and p is porosity of the membrane filter. According to the regression analysis, the regression equation is shown in the
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
1207
following: QF = 0.7dp0.124 p −0.132 u−0.029 .
(4)
In the regression analysis, coefficient of determination R 2 is about 0.93. Comparing of coefficients of b, c, and d, we found effects of different parameters on the filtration performance. The result shows that the largest value is c, indicating the influence of the porosity of the membrane filter is the highest. The second highest value is of b, and the smallest value is d. Thus, the effect of aerosol size on filtration performance is the second in the tested aerosol sized range. The effect of the flow rate on filtration performance is the lowest. The influence degree of these three parameters only existed in the controlled condition of this work. 4. Conclusions The experimental results implied that the pressure drops across 10%, 15%, and 20% PSF membrane filters were about 4.51, 5.81, and 7.31 kPa, respectively. The pressure drop across the 20% PSF membrane filter exceeds that across the 10% and 15% PSF membrane filter, because the porosity of the 20% PSF membrane filter (48.4%) is lower than that of the 10% and 15% PSF membrane filter (63.8% and 55.9%). The aerosol penetrations of the 0.3 m PSL aerosol through the 10%, 15%, and 20% PSF membrane filters were approximately 7.5%, 3.8%, and 0.9%, respectively. The most penetrating aerosol size through the PSF membrane filters is about 0.05 m. The aerosol penetration through the 20% PSF membrane filter is lower than that through the 10% and 15% PSF membrane filters throughout the range of experimental aerosol sizes, probably because the size of the pores in the PSF membrane filter declines as the PSF concentration in the casting solution increases. The mechanical impaction and interception aerosol capturing forces increase as the size of the pores in the membrane filter falls. The aerosol penetration through the PSF membrane filters increases obviously with the flow rate when aerosol size is smaller than 0.21 m; when aerosol larger than 0.3 m, the change of penetration through PSF membrane filters becomes smaller at different flow rate. It is due to this that the diffusion effect works on the smaller aerosols (< 0.5 m) and impaction and interception effects become obvious when aerosols size is larger than 0.5 m. The relative humidity does not affect the performance of the PSF membrane filters. The experimental data demonstrate that the 20% PSF membrane filter has a larger qF than the 10% and 15% PSF membrane filters over the whole range of aerosol sizes. This work might also offer a new application for using PSF membrane filters in aerosol filtration. Acknowledgements The authors would like to thank the National Science Council of Republic of China for financially supporting this research under Contract No. NSC 92-2218-E-041-006. References Caroff, M., Choudhary, K. R., & Gentry, J. W. (1973). Effect of pore and particle size distribution on efficiencies of membrane filters. Journal of Aerosol Science, 4, 93–102. Carvacho, O. F., Krystyna, T. N., Ashbaugh, L. L., Flocchini, R. G., Melin, P., & Celisn, J. (2004). Elemental composition of springtime aerosol in Chillan, Chile. Atmospheric Environment, 38, 5349–5352. Cohen, B. S., & Hering, S. V. (1995). Air sampling instruments for evaluation of atmospheric contaminants (8th ed.), ACGIG: Cincinnati, OH. Cullen, J. T., Field, M. P., & Sherrell, R. M. (2001). Determination of trace elements in filtered suspended particulate material by sector field HR-ICP-MS. Journal of Analytical Atomic Spectrometry, 16, 1307–1312. Guiver, M. D., Robertson, G. P., Rowe, S., Foley, S., Kang, Y. S., Park, H. C. et al. (2001). Modified polysulfones. IV. Synthesis and characterization of polymers with silicon substitutents for a comparative study of gas-transport properties, Journal of polymer science. Polymer Chemistry Edition, 39, 2103. Hind, W. C. (1982). Aerosol technology. NewYork: Wiley. Hoek, G., Forsberg, B., Borowska, M., Hlawiczka, S., Vaskovi, E., Welinder, H. et al. (1997). Wintertime PM10 and black smoke concentrations across Europe: Results from the PEACE study. Atmospheric Environment, 31, 3341–3349. Kaeselev, B., Pieracci, J., & Belfort, G. (2001). Photoinduced grafting of ultrafiltration membranes: Comparison of poly(ether sulfone) and poly(sulfone). Journal of Membrane Science, 194, 245.
1208
H.-L. Huang, S. Yang / Aerosol Science 37 (2006) 1198 – 1208
Kilduff, J. E., Mattaraj, S., Pieracci, J. P., & Belfort, G. (2000). Photochemical modification of poly(ether sulfone) and sulfonated (polysulfone) nanofiltration membranes for control of fouling of natural organic matter. Desalination, 132, 133. Liu, B. H. Y., & Lee, K. W. (1976). Efficiency of membrane and nuclepore filters for submicronmeter aerosols. Environment Science and Technology, 10, 345–350. Liu, B. H.Y., Pui, D.Y. H., & Rubow, K. L. (1981). Characteristics of air sampling filter media. Aerosol in the Mining and Industrial Work Environment, III Instrumentation, 989–1038. Liu, L., Chakma, A., & Feng, X. (2004). Preparation of hollow fiber poly(ether block amide)/polysulfone composite membranes for separation of carbon dioxide from nitrogen. Chemical Engineering Journal, 105, 43–51. Mader, B. T., Schauer, J. J., Seinfeld, J. H., Flagan, R. C., Yu, J. Z., Yang, H. et al. (2003). Sampling methods used for the collection of particle-phase organic and elemental carbon during ACE-Asia. Atmospheric Environment, 37, 1435–1449. Manton, M. J. (1978). The impaction of aerosols on a nuclepore filter. Atmospheric Environment, 12, 1669–1675. Manton, M. J. (1979). Brownian diffusion of aerosols to the face of a nuclepore filter. Atmospheric Environment, 13, 525–531. Montassier, M., Dupin, L., & Boulaud, D. (1996). Experimental study on the collection efficiency of membrane filters. Journal of Aerosol Science, 27(Suppl. 1), S637–S638. Saitoh, K., Serab, K., Hirano, K., & Shirai, T. (2002). Chemical characterization of particles in winter-night smog in Tokyo. Atmospheric Environment, 36, 435–440. Sioutas, C., Koutrakis, P., Wang, P. Y., Babich, P., & Wolfson, M. (1999). Experimental investigation of pressure drop with particle loading in nuclepore filters. Aerosol Science and Technology, 30, 71–83. Smutek, M., & Pich, J. (1974). Impaction of particles on the surface of membrane filters. Journal of Aerosol Science, 5, 1724. Spurny, K., & Pich, J. (1964). The separation of aerosol particles by means of membrane filters by diffusion and inertial impaction. International Journal of Air and Water Pollution, 8, 193. Spurny, K. R., Lodge, J. P., Frank, E. R., & Sheesley, D. C. (1969a). Aerosol filtration by means of nuclepore filters structural and filtration properties. Environmental Science & Technology, 3, 453–464. Spurny, K. R., Lodge, J. P., Frank, E. R., & Sheesley, D. C. (1969b). Aerosol filtration by means of nuclepore filters aerosol sampling and measurement. Environmental Science and Technology, 3, 464–468. Summers, G. J., Ndawuni, M. P., & Summers, C. A. (2003). Dipyridyl functionalized polysulfones for membrane production. Journal of Membrane Science, 226, 21–33. Xu, Y., Zhu, B., & Xu, Y. (2004). A study on formation of regular honeycomb pattern in polysulfone film. Polymer, 46, 713–717. Yamamoto, N., Fujii, M., Kumagai, K., & Yanagisawa, Y. (2004). Time course shift in particle penetration characteristics through capillary pore membrane filters. Journal of Aerosol Science, 35, 731–741. Yoshikawa, M., Hara, H., Tanigaki, M., Guiver, M. D., & Matsuura, T. (1992). Modified polysulfone membranes. 1. Pervaporation of water/alcohol mixtures through modified polysulfone membranes having methyl ester moiety. Polymer, 33, 4805–4813.