ARTICLE pubs.acs.org/IECR
Polytetrafluoroethylene (PTFE)-Sputtered Polypropylene Membranes for Carbon Dioxide Separation in Membrane Gas Absorption: Hollow Fiber Configuration Julianna A. Franco,† David D. deMontigny,‡ Sandra E. Kentish,† Jilska M. Perera,† and Geoffrey W. Stevens*,† †
The Cooperative Research Centre for Greenhouse Gas Technologies, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria, 3010, Australia ‡ The International Test Centre for Carbon Dioxide Capture, Faculty of Engineering, University of Regina, Saskatchewan, S4S 0A2, Canada ABSTRACT: Polytetrafluoroethylene (PTFE) plasma sputtering of polypropylene (PP) membranes allows the formation of an ultrathin fluorinated hydrophobic surface that retains the microporous surface structure of the underlying membrane. The absorption results presented in this paper show that the novel PP membrane material performs well for the separation of CO2 from other gases, using an amine-based solvent. Hollow fiber membrane experiments show that the plasma-treated hollow fiber membrane has a superior CO2 mass transfer rate to untreated PP when coupled with monoethanolamine solvent flowing through the fiber lumen for at least 45 h of absorption time. The CO2 mass-transfer rate through the treated hollow fiber membrane is also comparable with that of other researchers who have absorbed CO2 into amine solvents using PTFE hollow fibers and higher than that measured using packed column technology with structured packing and amine solvents.
1. INTRODUCTION Membrane gas absorption is a novel method that will potentially reduce the cost required to separate CO2 from other gases. In this instance, a hollow fiber membrane replaces the traditional packed column as the mechanism for providing interfacial area between the gas stream and the CO2 selective solvent. The approach has particular advantages for offshore natural gas sweetening, where it can offer a significantly reduced footprint and equipment mass, and for post-combustion capture from flue gases, where it can significantly reduce the equipment sizes, which are anticipated to be extremely large. In both natural gas sweetening and post-combustion capture operations, amine solvents such as monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA) are favored, because they have a relatively high loading capacity, a rapid absorption rate, and low cost for regeneration.1 However, in order for conventional amine-based solvents to be effectively used in a membrane contactor, a robust membrane material that resists solvent-related degradation and wetting must be used.24 It has been shown that the membrane mass-transfer coefficient is appreciably higher for the case of nonwetted pores, relative to that of wetted pores,5 because of the 4-fold increase in the masstransfer coefficient through gas-filled pores, relative to liquidfilled pores. Only polytetrafluoroethylene (PTFE), which is very expensive, has been reported not to become wetted when used with amine solvents.68 Untreated polypropylene (PP) has been shown to be incompatible for use with an MEA solvent.9,10 However, PP coated with an ultrathin hydrophobic film will potentially provide performance similar to that of a PTFE membrane at a much lower cost. In our preceding paper,11 the technique used to plasmasputter a PP membrane was presented. The properties of the r 2011 American Chemical Society
treated membrane were also tested, and its absorption performance was measured using a flat sheet membrane configuration. In this paper, the CO2 absorption performance into MEA solvent using a hollow fiber membrane is assessed, relative to the benchmark provided by untreated PP material.
2. EXPERIMENTAL PROCEDURES 2.1. Membrane Characterization Apparatus. Several membrane analysis techniques were used before and after absorption experiments. Scanning electron microscopy (SEM) images were captured using a JEOL Model JSM-5600 SEM system (Akishima-shi, Japan). X-ray photoelectron spectroscopy (XPS) was also used to characterize the chemical composition of the membrane surfaces. XPS was performed using an Axis Ultra spectrometer (Kratos Analytical, U.K.) that was equipped with a monochromatized X-ray source operating at 150 W. 2.2. Hollow Fiber Membrane Absorption Apparatus. A series of hollow fiber membrane cartridges, which were constructed in the laboratory, were separately tested in a countercurrent flow membrane module (see Figure 1). Membrane cartridges were constructed in house, and their specifications are listed in Table 1. Industrial-grade CO2 (Praxair, >99.9% purity, Mississauga, Ontario, Canada) was mixed with air in a 14:86 proportion (by volume) to simulate a flue gas stream. Gas flow controllers (Aalborg, Model GFC-17A, Orangeburg, NY) were used to Special Issue: Alternative Energy Systems: Nuclear Energy Received: February 17, 2011 Accepted: August 21, 2011 Revised: August 1, 2011 Published: August 21, 2011 1376
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obtained, the liquid flow was started (1885 mL/min). A ball valve was adjusted to increase the outlet liquid pressure to ∼0.1 bar higher than the gas-phase inlet pressure to prevent gas bubbling through the liquid. The liquid flow rate was measured at the outlet using a measuring cylinder. The CO2 gas-phase concentrations at equilibrium at the contactor inlet, outlet, and (for shell-side gas flow) midpoint were measured and used to calculate the overall mass-transfer coefficient. Liquid CO2 loading was also measured to complete a mass balance for CO2 absorption across the hollow fiber unit to verify the quality of each experiment. A tolerance of (5% error for the mass balance was employed. The CO 2 flux (N) through the membrane contactor is given by N ¼ Figure 1. Schematic diagram of hollow fiber (liquid flow through lumen) membrane gas absorption apparatus. (MFC = mass flow controller.)
Table 1. Properties of the Hollow Fiber Membrane Modules parameter polymer type
ð1Þ
where Y is the mole ratio of CO2 in the gas phase and G is the inert gas flow rate. The Reynolds number on the lumen side is calculated using the internal fiber diameter, whereas, on the shell side, the hydraulic mean diameter (dh) is given by
value
dh ¼
polypropylene (PP)
4 ðcross-sectional area of flowÞ dc;in 2 ndout 2 ¼ wetted perimeter dc;in þ ndout
fiber manufacturer
Memtec, Australia
average membrane pore size
0.2 μm
membrane porosity
50%
contact angle with 20 wt % MEA
117 ( 4a
3. RESULTS AND DISCUSSION
fiber ID fiber OD
0.3 mm 0.67 mm
number of fibers
500
contactor length
12.7 cm
3.1. Membrane Characterization. Figures 2 and 3 show SEM images of the outside and inside of each hollow fiber membrane material. Panels (a) and (c) in each figure are taken before MEA exposure, whereas panels (b) and (d) are after use for CO2 absorption, using 20 wt % MEA solution for 212 h (165 h with MEA on the shell side and 46 h on the lumen side). Profiles of the outside surface of both the untreated and treated PP membranes show that the surfaces change in morphology by becoming less textured after extended contact with MEA. Both surfaces seem to have lost some porosity. Profiles of the inside surface (Figures 3a and 3b) of the PP membrane show similar degradation to that observed for the outside PP surface (Figures 2a and 2b). However, no changes are evident after MEA exposure for the inside surface of the plasma-treated PP (see Figures 3c and 3d). Chemical changes also occur upon contact with MEA solvent during this absorption process. XPS analysis of the inside surface of both unused PP hollow fiber and PP that has been used for CO2 absorption experiments show an increase in the elemental oxygen on the surface, from 0.8% to 1.5% (see Table 2). This could be due to oxidation of the PP membrane. The membrane was in direct contact with a CO2/air stream for 165 h, and the thermal oxidation of PP is documented to occur at slow rates in air at atmospheric pressure and relatively low temperatures.12 Another possible cause for an increase in oxygen on the surface is chemical reactions between the membrane and MEA. This was also suggested by Wang et al. for the contact of PP membrane with DEA solution.4 The incorporation of additional oxygen into the PP surface will reduce its hydrophobicity and increase the likelihood of pore wetting in the presence of MEA and other lowsurface-tension solvents. No significant change in the chemical composition of the plasma-treated PP membrane occurs.
mass-transfer area based on inner siameter (ID)
598 cm2
based on outer diameter (OD)
1337 cm2
membrane contactor packing fraction a
ðYin Yout ÞG A
29%
Measured on an equivalent flat sheet membrane.11
regulate the gas flows before they entered a pipe mixer and filter. Analytical-grade MEA (Fisher Scientific, Pittsburgh, PA) was diluted with deionized water to a concentration of 1020 wt % (1.6 to 3.3M) and preloaded to 0.270.30 mol CO2/mol MEA by bubbling CO2 through the solution, using a sintered glass sparger to simulate a regenerated solution. A magnetic-drive gear pump (Cole Parmer, Vernon Hills, IL) that used to pump MEA solution through a liquid filter and rotameter before being passed through the membrane contactor. The liquid passed through a liquid trap before entering the outlet tank, to create a positive liquid head and prevent gas entrainment. Analog pressure readings were available for the differential pressure across both the gas and liquid phases, as well as for the liquid inlet and outlet (Ashcroft, Stratford, CT). The CO2 gas-phase concentration was measured using an online infrared gas analyzer from Nova Analytical Systems, Inc. (Model 302WP, Hamilton, Ontario, Canada). For a typical absorption experiment, the gas flow was started first (1 L/min CO2 and 6.1 L/min air). After consistency in the CO2 concentration at the contactor inlet and outlet was
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Figure 2. SEM images of the hollow fiber, showing a side profile of the outside of an (a) unused PP hollow fiber, (b) PP hollow fiber used for absorption experiments, (c) unused treated PP hollow fiber, and (d) treated PP hollow fiber used for absorption experiments. (Scale bar = 10 μm for each panel.) CO2 absorption was observed using a 20 wt % (3.3 M) MEA solution for 212 h (165 h with MEA on the shell side and 46 h on the lumen side).
Figure 3. SEM images of the hollow fiber, showing a side profile of the inside of an (a) unused PP hollow fiber, (b) PP hollow fiber used for absorption experiments, (c) unused treated PP hollow fiber, and (d) treated PP hollow fiber used for absorption experiments. (Scale bar = 5 μm for panels a and b, 10 μm for panels c and d.) CO2 absorption was observed using a 20 wt % (3.3 M) MEA solution for 212 h (165 h with MEA on the shell side and 46 h on the lumen side).
It is also interesting to observe that the inside surface of the plasma-treated PP membrane has been plasma-treated more efficiently than the outside surface (see Table 2). The inside surface has a lower oxygen concentration and slightly higher fluorine concentration, compared to the outside surface. This may explain why the outside membrane surface undergoes morphological degradation
upon contact with MEA solvent (shown by SEM analysis), while the inside surface does not. CO2 Absorption with Gas Flow in Lumen. The performance of the PP membrane cartridges were first tested with liquid flow through the contactor shell. A direct comparison of the untreated PP membrane to plasma-treated PP is possible, since 1378
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these cartridges have the same mass-transfer area; ∼1337 cm2, based on the fiber outer diameter (Table 1) and the same fiber dimensions. The liquid mass-transfer resistance is significant in CO2 MEA systems that use such a hollow fiber membrane configuration. As seen from Figure 4, as the Reynolds number on the shell side of either contactor increases, the CO2 flux increases. This is a consequence of an increase in the turbulence in the liquid-side boundary layer. In packed columns, increasing the MEA concentration will also increase the rate of absorption, because of an increased presence of free amine to bond with CO2 molecules until the viscosity of the solution becomes high enough to offset this benefit.6 A similar trend is observed in the present case, with the greatest CO2 flux at the highest MEA concentration (20 wt %). However, the increase is greater in moving from 10 wt % to 15 wt % than from 15 wt % to 20 wt %. As the concentration of MEA increases, its solution viscosity increases13 and its surface tension decreases.14 An increase in solvent viscosity will lead to a higher pressure drop across the length of the membrane contactor. Similarly, a decrease in solvent surface tension results in a decrease in the breakthrough pressure required for liquid to enter into the membrane pores. These effects will combine to bring about wetting of the membrane pores as the MEA concentration is increased. Table 2. XPS Analysis of the Outside and Inside Surfaces of Untreated and Plasma-Treated (PT) Polypropylene (PP)a Mass Concentration (%) surface description fresh untreated PP outside fresh PT PP outside
oxygen
nitrogen
carbon
0
9.7 ( 1
1.6 ( 0.2 89 ( 9
2.0 ( 0.2
16 ( 2
2.0 ( 0.2 80 ( 8
fresh untreated PP inside
0
0.8 ( 0.08
0
99 ( 10
used untreated PP inside
0
1.5 ( 0.2
0
99 ( 10
2.4 ( 0.2 2.3 ( 0.2
10 ( 1 8 ( 0.8
fresh PT PP inside used PT PP inside a
fluorine
2.2 ( 0.2 85 ( 9 0 90 ( 9
The composition of the inside surfaces are shown before and after use for CO2 absorption, using 20 wt % (3.3 M) MEA solution for 212 h (165 h with MEA on the shell side and 46 h on the lumen side).
The untreated PP performed at a level comparable to that of the plasma-treated membrane at low liquid flow rates and low MEA concentrations (see Figure 4). Under these conditions, the resistance of the liquid phase to mass transfer is significant and, hence, differences in the membrane material have relatively little impact. However, at higher liquid flow rates and MEA concentrations, where the membrane resistance to mass transfer is more significant, the untreated PP performed better than the plasmatreated module. The reason for this enhanced performance is unknown. However, relative to the inside surface, the outside surface of the plasma-treated PP fibers were coated less effectively with fluorine (see Section 3.1), which makes this surface less inert and therefore more susceptible to wetting and degradation by MEA. This trend is maintained over longer periods of absorption (see Figure 5). Over 165 h of absorption, the untreated and plasma- treated PP cartridge performances decrease by an average of 19% and 12%, respectively, using 20 wt % MEA flowing at 1783 mL/min through the contactor shell. Modeling of the CO2 flux indicates that the degree of wetting generally increases as the exposure time increases,11 and this is reflected in the present results. Comparable drops in performance due to pore wetting have been reported in the literature.15,16 CO2 Absorption with Gas Flow in Shell. The CO2 flux through the membranes cartridges are shown in Figure 6 with the solvent flowing through the fiber lumen. It is immediately apparent that this flux is significantly higher than when the solvent is on the shell side (recall Figure 4). This increase is partly explained by the higher liquid-phase Reynolds number when the solvent flow is in the lumen. However, it is also likely that, when liquid flows through the shell, the flow distribution is not optimal (which has been shown for similar contactor designs6,17) and gasliquid contact is less efficient, relative to the case of liquid flow through the fiber lumen. This is suggested by the liquid (Figure 7a) and gas (Figure 7b) concentration profiles. Figure 7a shows that, when liquid flows through the shell side of the contactor, the liquid loading in the middle of the contactor is very high, relative to the outlet loading. This suggests that there is some dead volume in the contactor and/or eddies form that prevent efficient contact with the gas and liquid phases. Similarly,
Figure 4. Change in the CO2 flux for untreated (solid lines) and plasma-treated (dashed lines) PP membrane cartridges with liquid flow through the fiber shell, using 1020 wt % MEA solution. 1379
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Figure 5. Change in the CO2 flux for untreated (solid lines) and plasma-treated (dashed lines) PP membrane cartridges with liquid flow through the fiber shell and using 20 wt % (3.3 M) MEA solution.
Figure 6. Membrane comparison based on CO2 flux using 20 wt % (3.3 M) MEA solution after 0, 22, and 46 h of absorption time. Liquid flow is through the fiber lumen of the untreated (solid lines) and plasma-treated (dashed lines) PP membrane cartridges. Data for the PTFE contactor are also shown.
Figure 7b shows that this may occur for the gas phase when it flows through the contactor shell. Since the liquid resistance will be higher than the gas resistance,8 obtaining a good fluid distribution in the liquid phase will have a greater impact than achieving this for the gas phase. The untreated PP has a lower CO2 absorption flux than the plasma-treated module, and this performance decreases with exposure time. The plasma-treated PP maintains a superior performance, even after being used in absorption experiments for a period of 46 h. However, the performance of the treated fiber cartridge decreases over time, at a rate similar to that of the untreated PP (see Table 3). The drop in performance of the treated membrane over time is unexpected and cannot be explained by either morphological or chemical changes to the membrane surface (see Section 3.1). The CO2 flux does not decrease linearly with time and, indeed, seems to decrease at a
higher rate initially for both PP membrane cartridges. This poses a question as to whether the CO2 fluxes will asymptote to a lower limit or have no lower bound. The change in performance of the PP fibers over longer periods of time warrants further investigation and, in the case of the plasma-treated PP fibers, will dictate whether this material will remain competitive with PTFE. The CO2 mass-transfer rate through the treated hollow fiber membrane is also comparable to that of other researchers who have absorbed CO2 into MEA solvents using Teflon hollow fibers.6,18 deMontigny et al.6 reported a range of (2.53.3) 103 mol/(m2 s), while Hoff et al.18 reported a range of (0.44.0) 103 mol/(m2 s), using similar experimental conditions. Furthermore, it is higher than that measured using packed column technology with structured packing and amine solvents.6 This is a particularly promising result, because it is also well-known that such membrane contactors can accommodate a 1380
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Figure 7. (a) Liquid-phase CO2 concentration profile and (b) gas-phase CO2 concentration profile along the height of the membrane contactor. Experimental points were obtained using a 500-fiber PP membrane cartridge for the case of liquid flow through the shell, but a 275-fiber cartridge was used for the liquid flow through the lumen case. 1020 wt % MEA solution was used at a flow rate of 17 mL/min.
Table 3. Average Change in Performance of Untreated and Plasma-Treated PP Membrane Cartridges over Time, Using 20 wt % (3.3 M) MEA Solution Flowing through the Fiber Lumen Average Drop in Performance Relative to Fresh Statea (%)
PP plasma-treated PP
absorption time = 22 h
absorption time = 46 h
5.4 4.6
7.5 6.6
“Fresh” PP fibers have been used in CO2 absorption experiments for 165 h with 20 wt % MEA flowing through the fiber shell. Performance is represented by the term KGav (given in units of kmol/(m3 kPa h)). a
much greater interfacial area per unit volume than a packed column approach.19 This means that the CO2 separation can be completed with an equipment footprint and capital cost that are each significantly reduced.
4. CONCLUSION The polytetrafluoroethylene (PTFE)-sputtered polypropylene (PP) membrane maintains similar pore characteristics to untreated PP, and MEA contact does not seem to greatly affect its surface morphology or chemical composition. Upon exposure to MEA solvent, PP becomes degraded while the plasma-treated membranes are less susceptible to degradation and wetting on the inside fiber surface. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) on fresh and exposed membrane materials indicate that the efficiently coated inside surface of the plasma-treated membrane is unlikely to experience changes associated with its pore size, porosity, chemical composition, or wettability, following exposure to MEA. With respect to their CO2 absorption performance in monoethanolamine (MEA), the plasma-treated PP membranes perform better than the PP membranes when solvent flows through the hollow fiber lumen. The CO2 flux is ∼30% higher for the plasma-treated PP relative to the untreated PP after 46 h of
absorption. Ideally, a membrane cartridge would be used for a period of years before needing to be replaced due to solvent degradation/wetting and fouling. Although the plasma-treated PP performs well, this performance is not maintained over time. The reason for this decrease in performance is not likely to be due to morphological or chemical surface changes. This method for treating PP membrane is novel in that it has allowed for the creation of an ultrathin hydrophobic surface while retaining the microporous surface structure of the underlying membrane. Given further testing and validation over longer operating periods, this novel material may allow the commercialization of membrane contactors for CO2 separation using alkanolamine solvents.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +61 3 8344 6621. Fax: +61 3 8344 8824. E-mail: gstevens@ unimelb.edu.au.
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