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Poly(tetrafluoroethylene) Sputtered Polypropylene Membranes for Carbon Dioxide Separation in Membrane Gas Absorption Julianna A. Franco, Sandra E. Kentish, Jilska M. Perera, and Geoff W. Stevens* The Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Particulate Fluids Processing Centre (PFPC), and Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria, 3010, Australia ABSTRACT: This work presents a study of the use of polypropylene (PP) membranes that have been sputtered with poly(tetrafluoroethylene) (PTFE) in a radio frequency plasma reactor. Plasma treatment results in the deposition of an ultrathin microporous coating which has a high degree of fluorination and nano- and microscale roughness. The treated membrane maintains similar pore characteristics and a similar thickness to untreated PP, and therefore, it is likely that the treatment film provides a negligible resistance to CO2 mass transfer relative to that of the bulk PP membrane. The absorption results presented in this paper show that the plasma-treated membrane has a comparable or superior performance to that of PTFE for CO2 mass transfer into monoethanolamine solvent. A flat sheet membrane configuration is used to test the properties of the material and to show that the treated membrane has a comparable CO2 mass-transfer rate to PTFE after 25 days of solvent exposure. Results suggest that all membrane materials are at least partially wetted but that the extent of wetting is lower in the PTFE and plasma-sputtered membrane, relative to untreatred polypropylene.
1. INTRODUCTION The current technology which exists to separate CO2 from flue gases involves chemical absorption in a packed column using an amine-based solvent. This mature technology has been used in industry for over 50 years but is considered expensive for CO2 separation from flue gas streams.1 In particular, very large columns, approaching 20 m in diameter, would be necessary to process the large volumes of gas. Membrane gas absorption (MGA) offers some advantage, as much greater interfacial areas are available per unit volume (1500-3000 m2/m3) relative to classical contactors (100-800 m2/m3).2 There is thus the potential to significantly reduce construction costs and the physical footprint of the operation, to make the process of CO2 capture and storage more economically acceptable. MGA involves the transfer of CO2 through a nonselective hollow fiber membrane before it is chemically absorbed into a solvent (see Figure 1). The use of solvents and membranes is integrated in order to exploit the benefits of both technologies.3 To enhance the performance of MGA systems, it is important to ensure that the membrane is not wet by the liquid solvent. If the pores of the membrane are wetted by the solvent, the membrane can introduce a significant resistance to CO2 mass transfer. It has been shown that if the pores of the membrane become even marginally wetted (99.999% purity, Sydney, Australia) was administered to the reaction chamber at 5 cm3/min using a highprecision needle valve before plasma was sparked using the radio frequency generator. A power of 200 W was administered for 30 min, unless otherwise stated. Subsequently, the chamber was brought to atmospheric pressure and the samples could be removed. 3.2. Membrane Characterization Apparatus. 3.2.1. XPS. X-ray photoelectron spectroscopy (XPS) was used in order to characterize the chemical composition of the membrane surfaces.
Figure 2. Schematic diagram of plasma reactor: C = ethylene glycol coolant; rf = radio frequency.
XPS was performed using an Axis Ultra spectrometer (Kratos Analytical, Manchester, U.K.) equipped with a monochromatized X-ray source operating at 150 W. XPS has a 2-10 nm sampling depth, a detection limit of 0.5%, and an error limit of (10% relative to the percentage quantity of each element detected on the surface. Peaks were identified using their characteristic binding energies27,28 and calibrated to the C1s peak at 285.0 eV. 3.2.2. AFM. Atomic force microscopy (AFM) images were taken on air-dried films with a Nanoscope IIIa microscope (Digital Instruments Inc., Santa Barbara, CA, USA) in tapping mode using silicon cantilevers with a resonance frequency of constant amplitude of 290 kHz (MikroMasch, San Jose, CA, USA). Several images were taken on macroscopically separated areas of the films to ensure representative AFM images of the samples. Image processing (first-order flattening and plane fitting) was carried out with Nanoscope 4.43r8 software. AFM was used to find the root mean square roughness (Rms) of the membranes to gauge the extent of their surface roughness. The Rms is the standard deviation of the height (z) values with the specific area and is calculated as vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u u ðzi - zav Þ t ð14Þ Rms ¼ i ¼ 1 NP
∑
where z is taken from the initial point (i) to the final point measured on the membrane surface (n) and zav and NP are the average of the z values and the number of points within a given area, respectively. 3.2.3. SEM. Scanning electron microscopy (SEM) images of flat sheet membranes were captured using a Philips XL30 FEG field emission SEM (FEI Co., Hillsboro, OR, USA). Images were used to observe the morphology of the surface and to calculate the surface porosity, average pore size, and thickness. A public 4013
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Industrial & Engineering Chemistry Research domain image analysis program (Version 1.24 ImageJ) was used to calculate the surface porosity. 3.2.4. Contact Angle. A contact angle goniometer (equipped with FTÅ200 analysis software) was used to measure the contact angle of both distilled water and 20 wt % MEA solution on flat sheet membranes. Water contact angles were tested on fresh membranes and membranes that had been exposed to 20 wt % MEA solution for 2 days. The same instrument was used to determine the interfacial tension of the MEA solutions. 3.2.5. Breakthrough Pressure. Breakthrough pressure measurements were conducted to find the liquid entry pressure of the membranes by pressuring a solution of 20 wt % 2-propanol (Sigma Aldrich, Sydney, Australia) which has a surface tension of 34 mN/m on supported porous flat sheet membranes using nitrogen gas. A digital pressure gauge was used to monitor the stepwise increase in gas pressure by 1 kPa/min. The breakthrough of the solution was visually observed, and averages of 10 measurements were calculated per membrane type to allow for variability between membrane sheets. 2-Propanol was used for these experiments because the nonideal wetting behavior of MEA solutions with the membrane materials (particularly PP) was time-dependent, which made it impractical to obtain reproducible results. Conversely, the use of pure water led to breakthrough pressures that were sometimes beyond the mechanical strength of the membrane, leading to membrane failure during the test. 3.3. Flat Sheet Membrane Absorption Apparatus. A cylinder of 14 vol % CO2 in N2 (BOC Gases) was used to simulate a flue gas stream. Industrial grade MEA (Orica Chemicals, 99.82% purity, East Melbourne, Australia) was diluted with distilled water to 20 wt %. The MEA solution was not preloaded with carbon dioxide, but a typical loading of 2 mol % was determined analytically due to exposure to the ambient atmosphere. The membrane cell was a vertical cylinder symmetrical about a flat horizontal membrane at the gas-liquid interface to ensure a uniform pressure profile across the membrane (Figure 3). Magnetically driven agitators were connected to a stirrer motor (Heidolph Instruments, Model RZR 2020, Schwabach, Germany) to avoid leakage from the cell. To ensure efficient mixing, the gas and liquid chambers were fitted with stainless steel baffles one-tenth of the size of the chamber’s inner diameter (0.41 cm). Both phases were agitated in the turbulent regime (at 385 and 585 rpm for the liquid and gas phase, respectively) to create homogeneity in the system and to reduce the gas- and liquid-phase mass-transfer resistances by decreasing the thickness of the gas and liquid films. A needle valve and gas flow meter (Aalborg, Model GFM17, Orangeburg, NY, USA) were used to regulate the gas flow into the membrane cell, and a gear pump (Link Pumps, Model 180 micropump, Williamstown North, Australia) was used to pump liquid through a rotameter and into the membrane cell. The membranes were secured between two silicon O-ring seals and supported on the low-pressure (gas) side of the cell. The support on the left was an exposed membrane area of 7.26 cm2 and an effective membrane area that varies with the porosity of each membrane. Pressure transducers (Davidson Measurement Pty. Ltd., Models PMP4070 and PMP1400, Scoresby, Australia) were used to monitor the transmembrane pressure difference using Labview data acquisition software (National Instruments, Version 7.3, Austin, TX, USA). The CO2 gas-phase concentration was measured using a Shimadzu 8A GC with a thermal conductivity detector, using helium carrier gas and a Poropak-Q packed column. CO2 liquid
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Figure 3. Schematic diagram of flat sheet fiber membrane gas absorption apparatus: FI = flow indicator, G = gas, L = liquid, PI = pressure indicator, and PIC = pressure indicator controller.
loading and MEA concentration were measured using a Chittick carbon dioxide analyzer (VWR International, AB, Mississauga, Canada) according to the procedure outlined by the Association of Official Analytical Chemists.29 A known volume of loaded MEA was placed into a volumetric flask with methyl orange indicator and connected to the gastight titration apparatus. A 1 M amount of HCl (supplied by Sigma Aldrich) was added to the flask to free bound CO2 from the solution. A calibrated metering tube filled with standard solution was displaced by the freed CO2 and which could be used to back-calculate the CO2 loading of the MEA. For a typical absorption experiment, the gas and liquid chamber stirrer speeds were adjusted and data acquisition was commenced. The liquid flow was started first at a constant flow rate of approximately 70 mL/min followed by the gas which flowed cocurrently to the liquid at 0.3 L/min. A ball valve was adjusted to increase the liquid pressure to approximately 0.1 bar higher than the gas-phase pressure. The CO2 gas-phase concentrations at equilibrium at the membrane cell inlet and outlet were measured and used to calculate the overall mass-transfer coefficient. Liquid samples from the inlet and outlet of the membrane cell were analyzed to determine their CO2 loading which was used to complete a mass balance for CO2 across the membrane unit and verify the precision of each experiment. Due to the relatively small amount of CO2 absorption into the liquid, a tolerance of (30% error for the mass balance was employed. The CO2 mass-transfer rate was averaged from 10 experimental results to account for the small membrane area available for gas-liquid contact and the large differences in morphology that were encountered between membrane discs (despite all membranes originating from a single batch). The overall mass-transfer coefficient K was calculated from K ¼
Qg;in Cg;in - Qg;out Cg;out ACg;out
ð15Þ
where Qg,in and Qg,out are the volumetric flow rates of gas entering or exiting the membrane unit in the gas phase, Cg,in and Cg,out are the concentrations of CO2 entering and exiting the membrane unit in the gas phase, and A is the mass-transfer area. Of importance to the interpretation of the data is the fact that the enhancement factor for mass transfer in the liquid film, as used in eq 2, will differ from that in the membrane pore, used in eq 11. This is because the liquid-phase mass-transfer coefficient, kl, is significantly higher within this liquid film, leading to a 4014
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Figure 4. Regional XPS spectra of carbon 1s orbitals taken with six sweeps at a resolution pass energy of 20 for the flat sheet (a) untreated PP surface and the (b) optimized plasma treated surface.
smaller Hatta number and hence a smaller enhancement factor. Equation 3 was used to calculate the MH in the liquid film, giving a value of E = 62 for the experimental conditions of 2% liquid loading and 385 rpm liquid-phase stirrer speed, consistent with a fast reaction. Conversely, in the pore kl = Dl/δW which leads to an enhancement factor of 113 for the thicker PP membrane and 99 for the thinner PTFE system, for full wetting.
4. RESULTS AND DISCUSSION 4.1. Optimization of Plasma Treatment Conditions. The argon flow rate, treatment time, and rf power were varied sequentially to establish optimum conditions for PTFE sputtering. No correlation between the argon flow rate and the membrane contact angle could be detected within the testing range of 0.5-6 cm3/min at any power level. Treatment time was then optimized using an argon flow rate of 5 cm3/min and powers of both 50 and 200 W. While the fluorine content, determined by XPS analysis, increased with treatment time up to 60 min, the contact angle fell slightly for treatment times in excess of 30 min. This may be due to changes in surface morphology offsetting the benefit of having a higher surface concentrations of fluorine. The surface porosities of the membranes plasma-treated at 50 W were also examined, and these showed a small drop in porosity and change in surface morphology with an increase in treatment time. Consequently, the treatment time was restricted to 30 min for future experiments. The rf power was varied between 10 and 250 W using an argon flow rate of 5 cm3/min for 30 min. As power was increased, the contact angle trended upward until it peaks at a power of 200 W. Above 200 W, this angle dropped dramatically. XPS results showed a continuous increase in fluorine across this power range. SEM images showed that as as rf power increased, the surface became rougher or more ablated. There appears to be a correlation between surface roughening and contact angle which was also proposed by Veeramasuneni et al.30 for the surface
roughening of PTFE-coated surfaces. When a power of 250 W is used, the surface becomes smoother and less porous. This suggests that the deposited film has increased in thickness and covered some of the membrane’s pores. As a result of this optimization, the plasma treatment conditions selected for the remainder of the work were set at an rf power of 200 W for a treatment time of 30 min and an argon flow rate of of 5 cm3/min 4.2. Membrane Characterization. 4.2.1. Chemical Composition. XPS spectra of the carbon 1s orbitals (C1s) for both the flat sheet untreated and plasma-treated PP surfaces show that the treated surface is highly fluorinated (Figure 4). There is a large change in the C1s envelope going from untreated to treated PP, which can be attributed to the formation of fluorinated carbon centers C-F at 288.0 eV, C-F2 at 290.0 eV, and C-F3 at 292.0 eV. The predominant functionality of plasma-treated PP is the C-F2 group which contributes an atomic concentration of 13.0%. This differs only slightly from a spectrum reported for PTFE sputtering in an Ar atmosphere by Golub et al.16 who found a peak C1s intensity at 292.0 eV which was identified as C-F2. The F/C ratio for plasma treated PP that was calculated by deconvolution of the regional C1s spectra is 1.61.31 The F/C ratio for pristine PTFE is 2.0. This shows that PTFE sputtering is capable of achieving similarly fluorinated surfaces to PTFE. The F/C ratio achieved in this work is comparable to those reported in the literature for PTFE sputtering of PTFE or plasma polymerization of tetrafluoroethylene which mostly lie in the range from 1.2 to 1.914,16,31-42 but are also reported to be much lower (0.78-1.02) for select studies.43-45 4.2.2. Surface Roughness. To determine whether fluorination is solely responsible for the hydrophobicity of the treated flat sheet membranes, AFM was used to measure the surface roughness. The treated surface has an average root mean square roughness of 456 nm (compared with 149 nm for the untreated surface). It is likely that this roughness contributes to the hydrophobicity of the deposited films.46 4015
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Table 2. Membrane Material Wettabilities As Measured Experimentally polymer type
PP
PTFE
plasma-treated PP
water contact angle
127 ( 1.9
139 ( 3.6
151 ( 1.1
20 wt % MEA contact angle
117 ( 4
132 ( 2
138 ( 2
water contact angle after MEA exposure
110 ( 5.0
138 ( 3
149 ( 3.3
2-propanol breakthrough pressure (kPa)
29 ( 1.9
91 ( 8.3
71 ( 6.4
Figure 5. SEM images depicting the change in surface morphology of different membranes with 20 wt % MEA contact for 25 days (at magnification of 5000): (a) PP, 0 days; (b) PP, 25 days; (c) PTFE, 0 days; (d) PTFE, 25 days; (e) plasma-treated PP, 0 days; and (f) plasma-treated PP, 25 days.
4.2.3. Wettability. Membrane-solvent wettability can be experimentally determined using contact angle and breakthrough pressure. Table 2 displays wettability data for the three membrane materials. The water contact angle for the plasma-treated PP surpasses that of PTFE and is in the superhydrophobic regime (above 150°). This very high contact angle probably arises from a combination of the fluorine chemistry on the surface and the increased roughness, as described above. This trend is not reflected for the breakthrough pressure data. The highest breakthrough pressure occurs for PTFE membrane followed by the plasma-treated PP membrane and the PP membrane which is significantly lower. Unlike contact angle, bulk membrane properties may also affect breakthrough pressure That is, during the breakthrough pressure measurement, the wetting liquid may penetrate beyond the nanometer thick surface coating, causing failure earlier than with the full thickness PTFE membrane. After contact with 20 wt % MEA for 2 days, the plasma-
treated PP and PTFE membranes maintain their hydrophobicity, while the hydrophobicity of the PP membrane declines. 4.2.4. Surface Morphology. A comparison of the surface morphology of the fresh membrane surfaces and those that have been exposed to 20 wt % MEA for 25 days appears in Figure 5. The PP surface morphology appears to have changed with enlarged and disrupted pores and a lower surface porosity (which drops from 61 to 52%; Figure 5a,b). This behavior was also observed by Wang et al.11 and Barbe et al.47 for PP membrane who found an increase in the average pore diameter and a change in the shape of the pores due to amine solvent exposure. This may also explain the drop in contact angle observed for PP after MEA exposure (Table 2). A change in the PTFE membrane surface morphology is less obvious but also evident (Figure 5c,d). The pore size of the PTFE appears enlarged, and the membrane may have shrunk. However, little change in the morphology of the plasma-treated surface can be observed (Figure 5e,f). 4016
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Industrial & Engineering Chemistry Research Table 3. Overall Mass-Transfer Coefficients (K) for the Transfer of CO2 into 20 wt % MEA through Fresh and Presoaked Membrane Materials Based on Total Membrane Areaa
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Table 5. Theoretical Membrane Mass-Transfer Coefficient for Nonwetted (km,nw) and Fully Wetted (mEkm,w) Membrane Pores on a Gas Basis Where the Partition Coefficient (m) Is Equal to 0.7619 (Liquid Loading, 2%) membrane
K 104 (m/s) membrane
fresh membranes
PP
presoaked membranes
PP
1.13 ( 0.33
0.70 ( 0.16
PTFE
1.93 ( 0.30
1.47 ( 0.37
PT PP
3.18 ( 0.21
2.56 ( 0.46
a
Values have been evaluated at a liquid stirrer speed of 385 rpm and an initial CO2 loading of 2%. Error values indicate 90% confidence intervals. PP = polypropylene; PTFE = poly(tetrafluoroethylene); PT PP = plasma-treated polypropylene.
Table 4. Effective Overall Mass-Transfer Coefficients (Keff) for the Transfer of CO2 into 20 wt % MEA through Fresh and Presoaked Membrane Materials Based on Membrane Porosity (ε)a K 10 (m/s)
km,nw 104 (m/s)
mEkm,w 104 (m/s)
162
3.6
PTFE
22
0.53
PT PP
55
2.2
has the best performance on this basis. Porosity can only be increased by amending the membrane manufacturing or treatment processes, which is difficult. However, a comparison of membrane performance based on the membrane porosity will give a direct measure of membrane wettability. Table 4 compares the performance of the three membrane types using an effective overall mass-transfer coefficient (Keff) which is based on the effective membrane area. Specifically Keff ¼
K ε
ð16Þ
4
membrane
fresh membranes
presoaked membranes
PP PTFE
1.84 ( 0.33 7.33 ( 0.30
1.14 ( 0.16 5.61 ( 0.37
PT PP
6.18 ( 0.21
4.97 ( 0.46
a
Error values indicate 90% confidence intervals. PP = polypropylene; PTFE = poly(tetrafluoroethylene); PT PP = plasma-treated polypropylene.
4.3. CO2 Absorption. A comparison of membrane absorption performance (Table 3) shows that plasma-treated PP has a masstransfer rate which is approximately 70% higher than PP and 40% higher than PTFE, which is in agreement with contact angle results (see Table 2). All membranes experience a drop in performance after MEA exposure. With the exception of the plasma-treated PP membrane, this drop in absorption performance is supported by SEM images which show extensive fiber degradation after MEA exposure (see Figure 5). Even so, the drop in performance using PTFE is quite surprising since most prior work suggests that PTFE is compatible with aqueous MEA solutions.7-9,13,48 The few works that have found that PTFE becomes semiwetted on contact with aqueous MEA10,49 have used a larger pore size (1 μm). Discrepancies between this study and the literature could arise due to differences in operating conditions such as the applied transmembrane pressure difference or differences in the properties of the solvent or membrane. The mass-transfer coefficient for a fully wetted PP membrane (0.10 10-4 m/s) was also measured by forcefully filtering the solvent through the membrane prior to the absorption experiment. For the wetted membrane, the overall mass-transfer coefficient does not vary with liquid flow rate, which confirms that the membrane resistance is dominant. The porosities of the membrane materials differ significantly (see Table 1), and it has been shown in the literature that, for rapidly reacting systems, a reduction in porosity is equivalent to a reduction in mass-transfer area.50 The PP membrane is significantly more porous than the PTFE, and this increases the overall mass-transfer coefficient. The combination of high porosity and low wetting ability means that the plasma-treated PP membrane
On this basis, the PTFE membrane performs at the highest level and PP is clearly the lowest performing membrane. The theoretical membrane mass-transfer coefficient for dry and fully wetted membrane pores can be calculated using eq 2. The values for the membrane porosity, ε, thickness, δ, tortuosity, τ, and the CO2 diffusivity, D, have either been experimentally determined (for ε and δ which appear in Table 1) or theoretically estimated (for D 49,51 as 1.49 10-5 m2/s (dry) or 2.00 10-9 m2/s (fully wet) and τ51 as 3.2 (PP), 3.8 (PTFE), and 4.3 (plasma-treated PP)). The diffusivity of CO2 through gas-filled pores was estimated by accounting for the combined effects of bulk and Knudsen diffusion. The tortuosity factor has been estimated using either an empirical correlation proposed by Mackie and Meares52 (eq 17) or the theoretical model for random clusters (eq 18): τ¼
ð2 - εÞ2 ε
ð17Þ
1 ε
ð18Þ
τ¼
The specific porosity-tortuosity relationship depends on the pore geometry which is in turn dictated by the method used to manufacture the membrane. Equation 17 has been most successfully applied to membranes manufactured using the phase inversion method.53-55 Equation 18 is better suited for use with polymer structures with randomly clustered pores. Iversen et al.53 predicted the tortuosity of polymeric membranes both experimentally and empirically and viewed their surface morphology using SEM. They found that eq 17 better predicted the characteristics of their PP (Celgard 2400 and 2500, Membrana GmbH, Obernburg, Germany) membrane whose surface pore structure resembled closely packed spheres, while eq 18 better predicted those of PTFE (Gore-Tex, Elkton, MD, USA) whose pore structure better resembled loosely packed spheres. The difference between the PP and PTFE membrane structure lies in their different manufacturing methods. The tortuosity for both PP membranes was therefore predicted using eq 17, and the tortuosity for PTFE was predicted using eq 18. The theoretically calculated membrane mass-transfer coefficients for nonwetted and fully wetted membranes are listed in Table 5. 4017
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Table 6. Estimated Resistances for PP, PTFE, and Plasma-Treated (PT) PP Flat Sheet Membranes Calculated Using MassTransfer Correlations resistance (s/m) membrane (days of MEA exposure)
overall
liquid
gas
contribution to overall resistance (%) membrane
liquid/overall
gas/overall
membrane/overall
PP (0)
8878
302
76
8503
3.4
0.9
95.8
PTFE (0)
5192
302
76
4815
5.8
1.5
92.7
PT PP (0) PP (25)
3143 14303
302 302
76 76
2765 13925
9.6 2.1
2.4 0.5
88.0 97.4
PTFE (25)
6786
302
76
6408
4.5
1.1
94.4
PT PP (25)
3908
302
76
3529
7.8
1.9
90.3
97660
302
76
97282
0.3
0.1
99.6
PP (fully wetted)
Table 7. Estimation of Degree of Wetting for Fresh and Presoaked Membrane Materials km 104 a (m/s)
pore wetting (%)
membrane
PP, fresh
1.18
>100
PTFE, presoaked
1.56
PTFE, fresh
2.08
24
PT PP, presoaked
2.83
78
PT PP, fresh
3.62
60
PP, fully wetted
0.10
>100
PP, presoaked
0.72
>100
membrane
a
km 104 a (m/s)
pore wetting (%) 31
km = membrane mass-transfer coefficient.
In all cases, the experimental mass-transfer coefficients (Table 3) lie closer to the values for a wetted membrane than those for a nonwetted membrane (Table 5). These values may reflect the pores being spontaneously partially wetted at the gasliquid transmembrane pressure applied. To explore these issues further, the membrane resistance has been isolated in order to determine the proportion of the membrane pores which have been wet by the liquid. Table 6 lists the magnitude of the liquid, gas, and membrane resistances and their contribution to the overall resistance, as determined from the experimental result (Table 3). As expected, membrane cell agitation has caused the liquid and gas resistances to be small. The proportion of the pores that were wetted (x) can be estimated from the theoretical coefficients provided in Table 5 and the calculated membrane resistance in Table 6 using eq 13 (see Table 7). The pores of the PTFE membrane are least wetted, followed by the plasma-treated PP membrane. This shows that, in terms of wettability, the PTFE and plasma-treated PP membrane materials are clearly superior to untreated PP. It is interesting to note that when 24% of the membrane pores for the PTFE membrane are wet, this degree of pore wetting results in a reduction in the theoretically attainable membrane mass-transfer coefficient by a factor of 10. Even a very small degree of pore wetting results in a significant reduction in the membrane mass-transfer coefficient. The pore wetting values calculated to be >100% indicate that the membrane is completely wetted under these conditions. The membrane mass-transfer coefficients are quite low in all cases but are comparable to the membrane mass-transfer coefficients for wetted membrane systems cited in the literature. Kreulen et al.56 absorbed CO2 into water using a flat sheet wetted membrane and obtained a membrane mass-transfer coefficient of 3 10-6 m/s using a mass-transfer area roughly 1.5 times smaller than that used in this study. Qi and Cussler57 absorbed CO2 into NaOH and recorded a mass-transfer coefficient as low as 2.5 10-5 m/s using a mass-transfer area roughly 32 times larger than that used in this study. The PP
membrane used in their study was wetted and gives a lower mass-transfer rate than the membranes in this study. A good comparison to Qi and Cussler’s57 research can be made to the work of Kreulen et al.58 who also absorbed CO2 into NaOH but using a nonwetted PP membrane. They achieved a CO2 flux of 2.6 10-3 mol/m2s which is a factor of 4 times higher than that achieved by Qi and Cussler.57 Therefore, the low membrane mass-transfer coefficients recorded in this study support the estimated pore wetting data listed in Table 7. It is this pore wetting which has increased the membrane resistance and caused a significant reduction in the membrane mass-transfer coefficient.
5. CONCLUSION The PTFE-sputtered PP membrane has a contact angle which is 26° higher than untreated PP and 12° higher than PTFE. In addition, due to its high degree of fluorination and nanoscale roughness, MEA contact does not appear to greatly affect the morphology or hydrophobicity of the sputtered surface. The PTFE and plasma-treated PP membranes have a comparable absorption performance using aqueous MEA solvent which is higher than that of untreated PP membrane. However, a drop in the performance of all of the membrane materials occurs following MEA exposure. After 25 days exposure to MEA, a drop in the mass-transfer rate by 18 and 20% was observed for the PTFE and plasma-treated membranes, respectively. Furthermore, all of the membranes were partially wetted, even when they had no previous exposure to MEA solution. The pores were estimated to be 100, 24, and 60% wet for the fresh PP, PTFE, and plasma-treated PP membranes, respectively. This degree of wetting causes a significant reduction in the theoretically attainable membrane mass-transfer coefficient. At this stage, it can be confirmed that the plasma-treated membrane is likely to perform at a comparable level to PTFE. However, this study has shown that even the PTFE membrane used here undergoes some pore wetting and consequent changes in surface morphology, so its performance should not be viewed as ideal. A better membrane 4018
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Industrial & Engineering Chemistry Research might be obtained by a reduction in membrane pore size, in addition to the hydrophobic surface, to further reduce wetting. This work used flat sheet membranes because of their simplicity and the ability to readily test surface properties such as contact angle and breakthrough pressures. In a commercial operation, a hollow fiber membrane contactor is more likely to be used due to the high gas-liquid interfacial area that this configuration can provide. It is therefore recommended to test the absorption performance of the treated membranes using a hollow fiber membrane configuration. This topic will be addressed by a later paper.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ NOMENCLATURE A = mass-transfer area (m2) av = specific surface area (m2/m3) b = stoichiometric coefficient C = concentration of CO2 (mol/m3) C’MEA = free concentration of MEA (mol/m3) dmax = maximum membrane pore diameter (m) ds = stirrer diameter (m) D = diffusivity of CO2 (m2/s) DMEA = diffusivity of MEA (m2/s) E = enhancement due to reaction Ei = E for an infinitely fast reaction H = Henry’s law constant (Pa 3 m3/mol) k = individual-phase mass-transfer coefficient (m/s) K = overall mass-transfer coefficient (m/s) Keff = effective overall mass-transfer coefficient (m/s) m = partition coefficient MH = modified Hatta number n = number of observations in data set NP = average number of points within a given membrane area Ns = stirrer speed (s-1) PBP = breakthrough pressure (Pa) PCO2* = equilibrium partial pressure of CO2 (Pa) Q = volumetric flow rate of CO2 (m3/s) Rms = root mean square roughness (m) Re = Reynolds number Sc = Schmidt number Sh = Sherwood number z = height in an AFM scan (m) Greek Letters
R = loading (moles of CO2 per mole of MEA) γlg = surface tension of interface between liquid and gas (mN/m) γ90 L = surface tension of liquid mixture which has a contact angle (mN/m) of 90° on a surface δ = thickness of membrane or membrane wall (m) ε = porosity of membrane θ = contact angle (deg) μ = viscosity (kg/m 3 s) F = density (kg/m3) τ = tortuosity of membrane Subscripts
g = gas in = entering membrane contactor
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l = liquid m = membrane out = exiting membrane contactor
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