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Water vapor permeation in polyimide membranes
Journal of Membrane Science 379 (2011) 479–487
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Water vapor permeation in polyimide membranes George Q. Chen, Colin A. Scholes, Greg G. Qiao, Sandra E. Kentish ∗ Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville, VIC 3010 Australia
a r t i c l e
i n f o
Article history: Received 24 February 2011 Received in revised form 13 June 2011 Accepted 14 June 2011 Available online 21 June 2011 Keywords: Water vapor Permeation Concentration polarization Plasticization Clustering Carbon dioxide Polyimide
a b s t r a c t The study of the influence of water vapor in the feed stream of a mixed gas membrane separation system is of considerable practical importance. Water vapor may plasticize the membrane, it may undergo competitive sorption with other gas species and it can form clusters as it permeates. In this work, a modified mixed gas permeation system was employed to accurately measure the permeation properties of the polyimides 2,2 -bis(3,4 -dicarboxyphenyl) hexafluoropropane dianhydrid-2,3,5,6-tetramethyl1,4-phenylenediamine (6FDA-TMPDA) and poly(3,3 -4,4 -benzophenone tetracarboxylic – dianhydride diaminophenylindane) (Matrimid® 5218) under exposure to humidified methane at 35 ◦ C. This approach was then applied to further evaluate the permeation properties of the polymers in humidified mixtures of carbon dioxide and methane at 35 ◦ C at atmospheric and elevated feed pressures. Water vapor permeabilities obtained at 2 and 7.5 bar total feed pressure, increased from 3200 to 3900 Barrer and from 20000 to 27000 Barrer as the water level increased for Matrimid and 6FDA-TMPDA respectively, reflecting increases in water vapor solubility and possibly plasticization and clustering effects due to the presence of water vapor. Conversely, the permeabilities of CH4 and CO2 declined due to competitive sorption of water. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Gas separation membranes have been used commercially to separate gas mixtures. One important application is the capture of CO2 from methane in natural gas sweetening. Membranes with high CO2 permeabilities and selectivities also have great potential to be adopted in carbon dioxide capture from both pre and post combustion flue gases to mitigate against climate change. The implementation of such gas separation systems poses significant challenges. One of them is the presence of water vapor, a known plasticizer in membrane separation [1–3]. Water vapor is usually considered as a minor component of the system in such industrial carbon capture applications [4,5]. However, this species often exhibits very different permeation behavior compared to other gas species because of its very small size and its high hydrogen bonding affinity [6–8]. To synthesize efficient membranes that perform well over many years of service, it is necessary to study and understand the permeation behavior of water vapor in polymeric membranes. The transport of water vapor through a polymeric material has been studied in a number of areas: membrane dehumidification
∗ Corresponding author. Tel.: +61 3 8344 6682, fax: +61 3 8344 4153. E-mail address: [email protected] (S.E. Kentish). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.06.023
and dehydration [9–12], protective apparel [13], and packaging and clothing materials [14]. Water vapor permeabilities of various polymer materials have been measured using a variety of techniques, but only a few of these studies have been extended to determine the water vapor sorption and transport properties and its effects on the permeation of other gas species [15–17]. Further, very few results have been obtained under elevated gas pressure. It is particularly difficult to obtain reliable permeability values under such conditions [18]. The combined effects of a high feed pressure and the very high permeability of water causes concentration polarization at the membrane surface and this can introduce a significant error into the measured values of the membrane gas permeability [18–20]. However, the ability to acquire water vapor and gas permeabilities under mixed gas conditions at such high pressures is essential for improving the design and operation of commercial scale natural gas sweetening and carbon capture facilities. This paper applies an approach for simultaneously determining water vapor and gas permeabilities under atmosphere and elevated feed pressures, based on a modified mixed gas permeation apparatus in addition to minimizing concentration polarization effects. In particular, it is the first time that these permeabilities have been evaluated at the same time under elevated feed pressures in a laboratory scale flat sheet membrane system. Two polyimides, typical of those used in natural gas separation [21], are used to understand the effect upon membrane performance of exposure to humidified methane and wetted carbon dioxide and methane mixtures.
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2. Theory
and molecular distribution functions for a mixture. The value of the cluster function can be calculated as follows:
2.1. Water vapor in polymeric membranes Glassy polymeric membranes such as those that are the focus of this study can generally be described by a dual mode sorption model [22]. In this case, the polymer can be considered as comprising of a ‘matrix’ of polymer chains that behave in a manner comparable to a rubbery polymer and a series of larger microvoids within which gas sorption occurs in a manner similar to that of a porous solid. These microvoids are often referred to as Langmuir microvoids, because sorption in this region generally follows a Langmuir isotherm [22,23]. When non-plasticizing gas molecules, such as CH4 , diffuse through such polymeric films, the interaction between gas molecules and the polymer or between gas molecules themselves does not influence the diffusion behavior of other gas species. However, it is well known that the presence of more than one gas species can influence gas solubility in glassy polymers through competitive sorption, particularly in the microvoids or Langmuir component of the material. This competitive sorption effect is strongest for penetrants which are more condensable, that is, those with a higher critical temperature [23]. Conversely, when plasticizer gas molecules like CO2 are diffusing through such a membrane, the interaction between CO2 molecules and the polymer can also swell the rubbery matrix and thus increases the free volume within this region [7]. Simultaneously, the microvoids decrease in volume, reflected in a reduction in the glass transition temperature [15,22,24,25]. For CO2 sorption into polyimides, such as those under study here, this behavior generally causes a net increase in free volume and in the diffusivity of all gas species [7,26]. The addition of water as a penetrant can cause both competitive sorption and plasticization as described above. Indeed, as water has a very high critical temperature, it competes very strongly with other penetrants for absorption sites in the Langmuir voids of the polymer [21]. In addition, if the polymer is hydrophobic or weakly hydrophilic, polar species such as water may interact with each other, preferentially to the polymer, resulting in the formation of water clusters [27]. In this case the water molecules are not in mobile monomeric form but clusters of multiple water molecules held by hydrogen bonding. Clustering results in an increase in the average size of a water molecule but usually these clusters are not big enough to have the properties of bulk water [6]. Clustering in the polymer will significantly hinder the transport of other gas species, because the diffusing water clusters are larger species that provide a sizeable obstruction. Water clusters will also be more effective in swelling the polymeric matrix, enhancing plasticization effects. Conversely, some authors report that water clusters can hinder the diffusion of other water molecules, resulting in a diminished increase [17] or a decrease [28] in the water diffusivity as the water vapour activity increases.
G = (1 − ϕ)
∂ a/ϕ ∂a
−1
(1)
p,T
where G is the cluster integral, the partial molecular volume of the penetrant, ϕ the volume fraction of the penetrant in the polymeric film and a the activity of penetrant. If the cluster function G/ is greater than −1, it means that the concentration of water molecules is higher than average in the neighborhood of a given water molecule. This will be an indication of cluster formation (or non-random mixing) of the penetrant molecules. The larger the value, the stronger the tendency of the penetrant molecules to associate. If the clustering integral is less than −1, the molecules will tend to isolate from each other. 2.3. Concentration polarization Concentration polarization occurs when there is a concentration gradient formed at the surface of the membrane. It is a well known effect in membrane processes, particularly in liquid separation systems such as pervaporation, ultra-filtration, nanofiltration and reverse osmosis. In gas separation units, concentration polarization is usually neglected at the laboratory scale as the small stage cuts used ensures that these effects are minimal [19]. The stage cut is defined as the fraction of the feed components that permeates through the membrane. When a small stage cut is employed, only a small portion of any feed component permeates the membrane. Therefore, the change in gas composition on the surface of the membrane is insignificant and can thus be ignored. However, when water vapor is present in the gas streams, concentration polarization effects must be considered for the typical membrane cell/unit because of the extremely high permeability of this species. Fig. 1 represents the concentration profile of water vapor across a membrane film. If the gas–vapor mixture is not well mixed on the surfaces of the membrane, a concentration gradient will be generated on each side due to the high transport rate of water vapor through the membrane. These boundary layers can result in significant errors in the estimation of the permeability and selectivity values if this effect is not reduced [19].
2.2. Clustering analysis At low vapor activities, the interaction between each water molecule and the polymer is usually stronger than the interaction between water molecules themselves and water behaves as a common plasticizing species. As vapor activity increases, more water molecules will be absorbed, increasing the tendency to form water aggregates, provided that the membrane matrix is able to swell sufficiently to accommodate the clusters [6,29]. Zimm and Lundberg [30] have developed an approach to characterize the tendency of water/solvent clustering in two components systems. Their relation was derived based on statistical mechanical techniques, to simplify the relationship between thermodynamics
Fig. 1. Schematic representation for the change of concentration of water vapor across the boundary layers and the membrane due to concentration polarization.
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Overall mass transfer coefficients (kov , cm s−1 ) are normally evaluated from the measured water vapor flux through the membrane and the bulk concentration difference across the membrane: JH2 o = kov (Cf − Cp )
(2)
where JH2 o is the flux of water vapor permeating through the membrane (cm3 (STP) cm−2 s−1 ), and Cf and Cp the bulk concentration of water vapor on the feed and permeate side of the membrane respectively (cm3 (STP) cm−3 ). If concentration polarisation is insignificant then the true permeability of the membrane can be evaluated from: Pi =
Ji .l (pi,feed − pi,permeate )
(3)
where l is the thickness of the membrane and pi,feed and pi,permeate the partial pressure (kPa) of component i on the feed and permeate side of the membrane respectively. To overcome or mitigate the effects of concentration polarization, the resistances of the boundary layers must be eliminated. Estimating these resistances is not always a simple task, so the enhancement of mixing within these layers to reduce their impact is an effective approach. A mesh spacer is one common method to enhance mixing on the surface of a membrane in liquid separation units [31], while increasing the crossflow stream flowrates can also be adopted for the same effect [18,19]. As the stream flowrates are increased or mixing is enhanced, the mass transfer rate will increase until a stable plateau is reached. The plateau value is indicative of the true permeability of the membrane, as it represents conditions under which concentration polarisation effects become insignificant. Tuning of the stream flowrates will usually be required to ensure this plateau has been reached. 3. Experimental 3.1. Membrane preparation Polysulfone (Aldrich), Matrimid 5218 (poly(3,3 -4,4 benzophenone tetracarboxylic – dianhydride diaminophenylindane, Huntsman Chemical CO.) and 6FDA-TMPDA (2,2 -bis(3,4 hexafluoropropane dianhydrid-2,3,5,6dicarboxyphenyl) tetramethyl-1,4-phenylenediamine) synthesized in house [32] were cast as dense flat sheets. The casting solution containing 2.5 (w/v%) of polymer in dichloromethane (Ajax Finechem, AR
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grade) was filtered in casting rings on glass plates. The membranes were dried at room temperature for at least 12 h. Using a small amount of distilled water, the homogeneous membranes were then removed from the glass plates and further dried for 15 h at 80 ◦ C under vacuum. These membranes were then annealed at 150 ◦ C for 48 h under vacuum. After the drying and annealing process, they were gradually cooled down to room temperature in the vacuum oven and then stored in desiccators. The membranes were stored for a week and then used within the following 7 days to minimize physical aging effects. The thickness of these membranes ranged from 40 to 60 m for Matrimid, 6FDA-TMPDA and polysulfone. Kapton® 100HN flat sheet films of 0.10 mil (25.4 m) were purchased from DuPontTM for permeability value validation. 3.2. Sorption measurement A Gravimetric Sorption Analyzer (GHP-FS, with a Cahn D-200 balance, VTI Scientific Instruments, Florida) was used to conduct sorption measurements and is described elsewhere [33]. A membrane sample was placed in a closed chamber with precise temperature and pressure control. Humidified helium of known relative humidity was passed through the chamber constantly. The change of the mass of the sample was recorded for each water activity tested, and was used to produce the sorption isotherm. 3.3. Water vapor and mixed gas permeation set-up A typical mixed gas permeation set-up (Fig. 2) was developed for measuring the permeabilities of water vapor and gas mixtures simultaneously over a wide temperature and pressure range (20–60 ◦ C and 1 – >10 bar). A pure gas (N2 or CH4 ) or a gas mixture (10% CO2 in CH4 ) was fed into a saturator vessel (filled with water) then a demister vessel to generate the humid gas stream. Both the saturator and demister were partially filled with stainless steel beads and immersed in a temperature controlled bath. The demister was used to capture any entrained liquid droplets from the saturator. The wet gas stream was then passed through a humidity and temperature transmitter (HMT, Probe type 334 Vaisala Oyj, Finland, measurement range: 0–100%RH, operating ranges: −70 to 180 ◦ C, 0–100 bar) which was fitted into the fan forced oven. The oven also contained the permeation cell, another HMT on the permeate side and the necessary stainless steel tubing. The permeation cell was modified from the
Fig. 2. Water vapor permeation apparatus.
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counter-current mixed gas permeation cell that has been used in our laboratory for many years [34]. Specifically, stainless steel wool packing was inserted on both sides of the membrane to enhance mixing and minimise concentration polarisation. The membrane, with an effective area of 10.75 cm2 , was supported by a circular porous stainless steel plate to withstand the feed side pressure. Both sides of the membrane were sealed with O rings. Before each series of experiment, the permeation cell was pre-heated to the operating temperature for at least an hour, with nitrogen and argon flowing through the top and bottom sides of the membrane, to avoid potential vapor condensation during the experiment. The humidity of the feed stream was controlled by the temperature of the saturator and the demister. The total pressure and flowrate (mass flow indicator MFI 1, Aalborg) of the feed and retentate streams were controlled by the back pressure controller (BPC, Cole-Parmer) in the retentate stream. The permeate side of the membrane used argon (UHP Ar, BOC Australia) as the sweep gas to remove any components permeated through the membrane. The permeate stream passed the HMT2 for water vapor content evaluation and then was chilled in an iced cold trap. The water vapor-free permeate stream was then send to the gas chromatography (450-GC, Varian, Inc.) for gas concentration analysis. The sweep gas flowrate was controlled by the mass flow controller (MFC 1, Aalborg) at 20 kPa g. The permeate flowrate before GC analysis was measured by MFI 2 (Aalborg). The temperature and relative humidity of HMT 1 and 2, as well as the gas composition of the permeate stream were recorded in 10 min intervals until the system equilibrium was reached (typically 2–3 h for each water activity). The stream temperature (T, ◦ C) and relative humidity (RH) was measured by the HMTs. At equilibrium conditions, relative humidity can be related to water activity (aw ): aw =
PH2 o RH = 100 Psat
The selectivity between two gas species (˛ij ) was determined by: ˛ij =
yi /xi yj /xj
(6)
where xi and xj are the mole fractions of the gas components in the feed, yi and yj the mole fractions of the gas components in the permeate. 4. Results and discussion 4.1. Water sorption The water sorption isotherms for Matrimid and 6FDA-TMPDA dense films are presented in Fig. 4(a). 6FDA-TMPDA exhibits higher water concentrations than Matrimid over the full range of vapor activities. Both sorption curves show no evidence of being concave to the pressure axis, which is usually indicative of the dual mode sorption model. This may suggest that the Langmuir microvoids are filled at very low water activities, so that water sorption into the rubbery matrix dominates across most of the water activity range. Rather, both curves are convex, consistent with the Flory-Huggins model [15] and indicative of plasticization by water [36]. Similar results have been observed by Lokhandwala et al. for 6FDA-based polyimides [36]. A clustering analysis as described by Zimm and Lundberg [30] was performed using this isotherm and Eq. (1). The cluster function
(4)
where PH2 o is the water vapor partial pressure, Psat the saturation water vapor pressure at the stream temperature and pressure. Water vapor pressure was calculated by the correlation from Hyland and Wexter [35] and further verified with conventional psychometric charts. 3.4. Permeability evaluation A fresh membrane was used for each experimental run. The mixture of water vapor and gases was fed into the membrane cell at 35 ◦ C with argon as the sweep gas on the permeate side. Each membrane was tested at increasing water vapor activity until saturation was approached and then discarded. The flux of each gas species passing through the membrane was evaluated by: Ji =
Qper yi i A
(5)
where Qper is the total permeate flowrate (cm3 (STP) s−1 ) after the iced cold trap, yi , the mole fraction of gas component i in the permeate stream and A the effective membrane area (cm2 ). The activity coefficient, i is taken as unity for all gas species, because the permeate stream contains mainly argon at atmospheric pressure and hence is considered as an ideal gas mixture. For nitrogen, methane and carbon dioxide, the permeate mol fraction (yi ) was determined directly by gas chromatography, whereas for water, this was determined from the water vapor partial pressure measured with the humidity probe. The permeability of the gas component was then determined by using Eq. (3), assuming concentration polarization effects are minimal [19].
Fig. 3. (a) Water vapor sorption isotherms for Matrimid () and 6FDA-TMPDA ( ) at 35 ◦ C. (b) Zimm–Lundberg analysis of clustering in Matrimid () and 6FDA-TMPDA ( ) at 35 ◦ C based on the water vapor sorption isotherms.
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cluster within the membrane when vapor activity is above these values. 4.2. Flowrate tuning The calculation of permeability using Eq. (3) represents the intrinsic properties of the material only if the resistance of the permeation of the components is solely the membrane resistance. Careful study is required for water vapor mass transport resistances because its saturation partial pressure at 35 ◦ C is only 5.64 kPa (equivalent to 2.82 mol% at 2 bar and 0.75 mol% at 7.5 bar). If the gas vapor mixture is not well mixed, one would expect that the change of the gas vapor/membrane interface partial pressure will become much more significant due to the predicted high flux of water vapor compared to other gas components and the resulting concentration polarization. Therefore, the observed calculated permeability will be temporarily termed as an ‘apparent permeability’ before it can be confirmed that the mass transport resistances in the two boundary layers are not significant. To study the contribution of these mass transport resistances to the overall transport, a humidified stream of methane was fed to the membrane cell with the feed and permeate flowrates being varied. The resulting apparent water vapor permeabilities are plotted in Fig. 5. Relatively low water activities were chosen for both low and high total feed pressures (0.23 for Matrimid and 0.36 for 6FDA-TMPDA at 2 bar, 0.31 for both polymers at 7.5 bar). Low activity means low water vapor concentration in the bulk of the feed and thus concentration polarization tends to be more severe. The apparent water vapor permeability increases with increasing feed flowrate initially then approaches to constant values at flowrates greater than 800 cm3 min−1 and 1300 cm3 min−1 for Matrimid and 6FDA-TMPDA respectively (Fig. 5(a)). If these flowrates could be named “breaking flowrates”, they indicate the ‘well-mixed point’ for the two types of membranes. These breaking flowrates were determined by evaluating the percentage change in permeability per 100 ml/min increase of feed gas flowrate. A percentage change of less than 0.5% in permeability with respect to 100 ml/min increase in feed gas flowrate indicated the breaking flowrate. The constant apparent permeability for increasing feed flowrates illustrates the effect of concentration polarization is no longer significant at these feed flowrates. Operating under such conditions will ensure a well mixed condition on the feed side so that the concentration of water vapor on the membrane surface is equal to its
Fig. 4. (a) Apparent water vapor permeability of Matrimid (vapor activity 0.23, ) and 6FDA-TMPDA (vapor activity 0.36, ) with stainless steel wool packing in both compartments of the membrane cell, and Matrimid (vapor activity 0.19, ) with the packing only in the permeate compartment, for various CH4 feed flowrates at 35 ◦ C and 2 bar total pressure. (b) Apparent water vapor permeability of Matrimid () and 6FDA-TMPDA ( ) for various CH4 feed flowrates at 35 ◦ C, vapor activity 0.31 and 7.5 bar total pressure. (c) Apparent water vapor permeability of Matrimid (800 cm3 min−1 CH4 feed, vapor activity 0.23, ) and 6FDA-TMPDA (1300 cm3 min−1 CH4 feed, vapor activity 0.36, ) for various sweep flowrates at 35 ◦ C and 2 bar total pressure.
G/ versus water vapor activity is presented in Fig. 4(b) for both polyimides. The value of the cluster function is above −1 for vapor activities greater than ∼0.45 and ∼0.25 for Matrimid and 6FDATMPDA respectively, meaning that water molecules will tend to
Fig. 5. Water vapor permeabilities of polysulfone at 40 ◦ C ( ) and Kapton films ( ) at 30 ◦ C. Literature data: polysulfone 40 ◦ C from ( ) Swinyard et al. [53], () ) Metz et al. [18]; Kapton 30 ◦ C from ( ) Hubbell et al. Schult and Paul [29], ( [38], and 50 ◦ C from (
) Okamoto et al. [39].
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bulk concentration. The use of stainless steel wool packing can be seen to effectively eliminate concentration polarization (Fig. 5(a)), as the apparent permeability of Matrimid at 2 bar continues to increase after 1300 ml/min without placing the packing on the feed side of the membrane. A much higher breaking flowrate is observed for 6FDA-TMPDA because the mass transport rate of water vapor is much faster. A faster water vapor transport rate means a faster depletion of water vapor from the surface of the membrane, hence a higher flowrate is required to replenish the concentration level (i.e. provide a well mixed condition). When the total feed pressure is increased to 7.5 bar (Fig. 5(b)), the breaking flowrates are increased to 1200 cm3 min−1 and 1700 cm3 min−1 for Matrimid and 6FDA-TMPDA respectively. The water vapor transport rate should not be affected by the total pressure of the feed at the same temperature, because the only driving force should be the partial vapor pressure difference between the feed and the permeate side. Water vapor partial pressure is much less sensitive to system pressure than temperature [37]. Therefore, the increase in breaking flowrates is expected because increasing the total feed pressure by a factor of 3.75 will decrease the water vapor concentration by the same amount at any given water vapor activity. Subsequently, higher feed flowrates are required to maintain the well mixed condition in the feed side compartment. A similar approach was attempted for the permeate side. Fig. 5(c) shows the apparent water vapor permeability at various permeate flowrates under 2 bar total feed pressure. Unlike the feed side flowrate, the permeate side flowrate cannot be increased indefinitely otherwise the concentration of the gas components in the stream will be too low to be analyzed by any kind of analytical instrument. The configuration of our membrane cell results in a sufficiently low permeate breaking flowrate for both of Matrimid and 6FDA-TMPDA (15 and 20 cm3 min−1 respectively) that allows the measurement of gas component concentrations in the gas chromatograph. The flux of water vapor permeating to the permeate side of the membrane will not change significantly due to the minor increase of water vapor partial pressure at elevated pressure. Thus varying the permeate flowrate under elevated total feed pressure is not necessary. If the membrane cell operates at flowrates equal to or higher than the determined ‘breaking flowrates’, reliable water vapor permeabilities can be obtained. The feed flowrates used in later experiments were thus chosen to be at least 100 ml/min above these breaking flowrates (e.g. 900 and 1300 cm3 min−1 for Matrimid at 2 and 7.5 bar total feed pressure respectively). However, the process of determining the breaking flowrates for both side of the membrane under the desired operating pressures must be performed for each type of polymer material used. 4.3. Validation of permeability values Polysulfone films and Kapton® 100HN membranes were tested over a wide range of water activities at 40 ◦ C and 30 ◦ C respectively to validate the permeability values produced using the method developed above. Fig. 6 compares the experimental results from this study with the available literature values. The water vapor permeabilities of polysulfone films are observed to be in perfect agreement with the literature values. Those of Kapton films are not perfectly matched to the literature values but still very convincing, because Hubbell et al. [38] in 1975 used a different experimental set-up with only water vapor present on feed side and argon as sweep gas on the permeate side. The determination of water vapor concentration relied on an electrical hygrometer. With the more modern water content measuring technology used in this work and the enhanced mixing conditions on the feed side of the film, the small difference in the permeation values between the two is reasonable and acceptable. The lowest literature value was obtained
Fig. 6. Water vapor permeability of Matrimid and 6FDA-TMPDA under humidified gas conditions at 35 ◦ C. (2 bar CH4 (), 2 bar 10% CO2 in CH4 ( ), 7.5 bar CH4 ( ) and 7.5 bar 10% CO2 in CH4 ( ).)
at 50 ◦ C by Okamoto et al. [39] using their measured sorption data of Kapton (permeability coefficient equals solubility coefficient multiplies diffusivity coefficient). Using such an approach for determining permeability is less reliable, also the higher measuring temperature (50 ◦ C) attributes to the slight disagreement. Natural gas separation operates at elevated feed pressure. Metz et al. [18] reported a 40% decrease in water vapor permeability of poly(ethylene oxide) poly(butylenes terephthalate) multi-block copolymer (PEO-PBT) when the feed pressure was increased from 3.5 to 10 bar. They concluded that the trend was caused by an increasing resistance in the feed boundary layer with higher pressures, rather than a true change in permeability. Following the methodology developed in this work, the permeabilities of both polyimides under humidified 2 and 7.5 bar CH4 at 35 ◦ C were determined and plotted in Fig. 7. The water vapor permeabilities do not decrease with increased pressure (7.5 bar) and in fact, are very similar to those recorded at 2 bar. These results confirm that measuring water vapor permeability at elevated feed pressures can be achieved at laboratory scale, provided that the feed flowrates can be increased to a sensible range. 4.4. Water vapor permeability Fig. 7 summarizes the vapor activity dependence of water vapor permeabilities for Matrimid and 6FDA-TMPDA under the following humidified feed conditions: CH4 at 2 and 7.5 bar, and 10% CO2 in CH4 at 2 and 7.5 bar. Generally, all water vapor permeabilities increase with vapor activity. This is consistent with the
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the difference in feed conditions (single-component versus multicomponent system). The trend is significantly different when 10% CO2 is added at 7.5 bar feed pressure. The infinite dilution water permeability increases from 3200 Barrer to around 3650 Barrer for Matrimid and from 20,000 Barrer to around 23,000 Barrer for 6FDA-TMPDA. These small increases are most likely related to CO2 inducing a plasticization effect as the CO2 partial pressure increases. There is evidence that increases in the diffusion coefficient can still occur in glassy polyimides even at these low CO2 partial pressures [7]. These results are contrary to that reported by Reijerkerk et al. [16] for a poly(ethylene oxide) based block copolymer who conclude that the presence of 2.5 bar CO2 had no effect on the water permeability. However, this result is for a rubbery material that may be more resistant to CO2 plasticization. Water vapor has a minimal effect on this value up to ∼0.5 and ∼0.3 vapor activity for Matrimid and 6FDA-TMPDA respectively. This may be because the water vapor is unable to induce further plasticization under these conditions, or that competitive sorption from the CO2 limits the increases in water solubility. At higher water vapor activities the data for both polyimides merges with that recorded at lower pressures and appears to increase again in line with these results. Thus, at these higher vapor activities, the presence of the water vapor appears to dominate over any plasticization or competitive sorption effects induced by CO2 . 4.5. Membrane performance
Fig. 7. Percentage of the initial gas permeability of Matrimid under 2 and 7.5 bar humidified feed ((a) and (b) respectively). (CH4 () in humidified CH4 feed; CH4 ( ) and CO2 ( ) in humidified 10% CO2 in CH4 feed.) Insert: CO2 /CH4 selectivity () under humidified 10% CO2 in CH4 feed.
result of many other workers [16,17,40,41]. As discussed in this body of prior work, the observed increasing trend in permeability may relate to both plasticization of the polymer matrix by water vapor [17,42,43] and to the increasing water vapor solubility [16,17,40,44], as shown in Fig. 3. Conversely, the increase may be restricted somewhat by the formation of water clusters, which will diffuse more slowly than individual water molecules [17,28]. The cluster function depicted in Fig. 4(b) indicates that these will tend to occur above 0.45 and 0.25 vapor activity for Matrimid and 6FDA-TMPDA respectively. As mentioned above, similar vapor permeabilities are observed while increasing the total feed pressure from 2 to 7.5 bar. At 2 bar total feed pressure, the addition of CO2 from the humidified CH4 to the 10% CO2 in CH4 mixture is not observed to significantly influence the water vapor permeabilities for either polyimide. Within experimental error, these three data sets all converge to an infinite dilution permeability of around 3200 and 20,000 Barrer, and to around 3900 and 27,000 Barrer at water saturation for Matrimid and 6FDA-TMPDA respectively. A slightly lower infinite dilution permeability (14000 Barrer) was observed by Sato et al. [41] for 6FDA-TMPDA using pure water vapor in a constant volumevariable pressure apparatus, with a greater increasing trend in vapor permeabilities. Such differences are somewhat expected as the absolute value of CO2 permeability for 6FDA-TMPDA shows similar variation, with values reported from 440 to 612 Barrer [45–48]. The discrepancies can be attributed to the different drying and annealing protocols used (hence different thermal history) and
The permeability of CH4 and CO2 in the two polyimide films under the four different feed conditions described above was also acquired. The addition of 10% CO2 to dry methane under both 2 and 7.5 bar feed pressure causes a slight decrease in the absolute value of the CH4 permeability (from 0.234 to 0.224 ± 0.005 Barrer for Matrimid and from 34.2 to 30.5 ± 0.7 Barrer for 6FDA-TMPDA), consistent with the results of other workers [26,49,50]. This is in contrast to the impact of CO2 on water permeability, when a clear increase in water permeability was observed at lower water activities and higher feed pressures. The differences may relate to the differences in critical temperature of methane and water. That is, methane cannot compete successfully with CO2 for sorption sites in the Langmuir voids and hence any membrane plasticization is offset by a loss of solubility. Conversely, due to its higher critical temperature, water is able to resist displacement by CO2 in these microvoids and so the impact of plasticization is dominant. The change in the gas permeabilities under humid conditions, relative to that tested under dry feed conditions, is plotted in Figs. 7 and 8 respectively. The small inserts at the bottom left corner of both figures also show the observed changes in CO2 /CH4 selectivity. For both polyimides, a decrease in CH4 and CO2 permeability is observed as water vapor activity increases under both total feed pressures, which suggests that the presence of water vapor in the feed depresses the membrane performance. Such depression, caused by competition of the penetrating species has been observed elsewhere [2,44,51,52]. In the present case, these competitive sorption effects dominate over any plasticizing effects observed with water vapor permeation. For Matrimid, the CH4 permeability declines to 75% of its original value at saturation, upon addition of water at either total feed pressure. This reflects competitive sorption from the water molecules, in addition to that of the CO2 . The decline in CO2 permeability with increasing water content is more dramatic for both feed pressures. This may relate again to competition sorption effects. That is, as the high initial CO2 permeability is related to its high solubility in the Langmuir voids, the displacement by water has a greater effect. It is noted that the decline at 2 bar total feed pressure is larger than in the 7.5 bar case – a decrease of 45% and 35% of the
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5. Conclusion The study of the effect of water vapor in the feed stream of a mixed gas membrane separation system on the performance of industrial separation units is of considerable practical importance. Plasticization and clustering are possibly induced by water vapor as it permeates through a polymeric membrane. A water vapor permeation measuring rig was modified from a conventional mixed gas permeation system in this work for the measurement of these effects. The experimental set-up was validated by comparison with available literature data and this analysis showed that the flowrates required to eliminate concentration polarization effects were higher for membranes with higher permeabilities and for higher operating feed pressures. Gas permeabilities for Matrimid 5218 and 6FDA-TMPDA obtained at 2 and 7.5 bar total feed pressure in humidified streams underlined the water vapor-induced plasticization and clustering phenomena. The water permeabilities increased from 3200 Barrer at indefinite dilution through to a maximum of around 3900 Barrer in a water saturated gas stream for Matrimid, and from 20,000 to 27,000 Barrer for 6FDA-TMPDA. However, a decrease in permeabilities of both CH4 and CO2 for both pressures was also observed as water vapor was added for both polyimides, with water saturated values between 30 and 75% of the dry gas values. This loss in permeability can be related to competitive sorption effects. For Matrimid, CO2 is more significantly influenced by competitive sorption than methane, causing the CO2 /CH4 selectivity to decrease. Conversely, in 6FDA-TMPDA, a slight increase in CO2 /CH4 selectivity was observed as water saturation increases, which is likely related to diffusional pathways being blocked by the presence of water clusters. Acknowledgements Fig. 8. Percentage of the initial gas permeability of 6FDA-TMPDA under 2 and 7.5 bar humidified feed ((a) and (b) respectively). (CH4 () in humidified CH4 feed; CH4 ( ) and CO2 ( ) in humidified 10% CO2 in CH4 feed.) Insert: CO2 /CH4 selectivity (♦) under humidified 10% CO2 in CH4 feed.
initial value respectively at water saturation. This can be explained by the increase (3.75 times) of the CO2 partial pressure in the feed when the total pressure is increased from 2 to 7.5 bar. The larger quantities of CO2 are able to compete more effectively for sorption sites in the microvoids. The net effect is a loss in CO2 /CH4 selectivity of between 70 and 85% of its original value at water saturation (from 42 to around 30 and 42 to 35 at 2 and 7.5 bar respectively). Similarly, the CH4 permeability for 6FDA-TMPDA declines to around 30% of its original value at saturation, upon addition of water at both total feed pressures (Fig. 8). However in this case, the decline in CO2 permeability with increasing water content is slightly less than that of CH4 for both feed pressures. The net effect is a minor gain in CO2 /CH4 selectivity; from 22 under dry mixed gas condition to 25 at water saturation at 2 bar and similarly from 21 to 23 at 7.5 bar (Fig. 8, inserts). This is in contrast to what was observed in Matrimid and may be due to the higher solubility of H2 O in 6FDATMPDA (Fig. 3(a)). As observed by other authors, it is possible that diffusional pathways are being blocked by water and water clusters [17,28]. This decrease in diffusivity would impact CH4 more than CO2 as it is a larger molecule, leading to a net increase in selectivity. Similar to Matrimid, the decline in CO2 permeability at 2 bar total feed pressure is larger than in the 7.5 bar case – a decrease to 35% and 50% of the initial value respectively at water saturation. This again can be explained by the increase of CO2 partial pressure at the higher total feed pressure.
The authors would like to thank the Particulate fluids Processing Centre (PFPC), a Special Research Centre of the Australian Research council and the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) for access to equipment and resources. Funding for this project is provided by the Australian Government through its CRC program. References [1] G.T. Paulson, A.B. Clinch, F.P. McCandless, The effects of water vapor on the separation of methane and carbon dioxide by gas permeation through polymeric membranes, J. Membr. Sci. 14 (1983) 129–137. [2] R.T. Chern, W.J. Koros, E.S. Sanders, R. Yui, “Second component” effects in sorption and permeation of gases in glassy polymers, J. Membr. Sci. 15 (1983) 157–169. [3] K. Tanaka, M.N. Islam, M. Kido, H. Kita, K.-i. Okamoto, Gas permeation and separation properties of sulfonated polyimide membranes, Polymer 47 (2006) 4370–4377. [4] C.A. Scholes, S.E. Kentish, G.W. Stevens, Carbon dioxide separation through polymeric membrane systems for flue gas applications, Recent Pat. Chem. Eng. (2007) 52–66. [5] C.A. Scholes, G.W. Stevens, S.E. Kentish, The effect of hydrogen sulfide, carbon monoxide and water on the performance of a PDMS membrane in carbon dioxide/nitrogen separation, J. Membr. Sci. 350 (2010) 189–199. [6] M. Modesti, C. Dall’Acqua, A. Lorenzetti, E. Florian, Mathematical model and experimental validation of water cluster influence upon vapour permeation through hydrophilic dense membrane, J. Membr. Sci. 229 (2004) 211–223. [7] X. Duthie, S. Kentish, C. Powell, K. Nagai, G. Qiao, G. Stevens, Operating temperature effects on the plasticization of polyimide gas separation membranes, J. Membr. Sci. 294 (2007) 40–49. [8] K.A. Schult, D.R. Paul, Water sorption and transport in a series of polysulfones, J. Polym. Sci. Part B: Polym. Phys. 34 (1996) 2805–2817. [9] L.-Z. Zhang, Investigation of moisture transfer effectiveness through a hydrophilic polymer membrane with a field and laboratory emission cell, Int. J. Heat Mass Transfer 49 (2006) 1176–1184. [10] J. Min, T. Hu, X. Liu, Evaluation of moisture diffusivities in various membranes, J. Membr. Sci. 357 (2010) 185–191. [11] R.W. Baker, Future directions of membrane gas separation technology, Ind. Eng. Chem. Res. 41 (2002) 1393–1411.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Water vapor permeation in polyimide membranes George Q. Chen, Colin A. Scholes, Greg G. Qiao, Sandra E. Kentish ∗ Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville, VIC 3010 Australia
a r t i c l e
i n f o
Article history: Received 24 February 2011 Received in revised form 13 June 2011 Accepted 14 June 2011 Available online 21 June 2011 Keywords: Water vapor Permeation Concentration polarization Plasticization Clustering Carbon dioxide Polyimide
a b s t r a c t The study of the influence of water vapor in the feed stream of a mixed gas membrane separation system is of considerable practical importance. Water vapor may plasticize the membrane, it may undergo competitive sorption with other gas species and it can form clusters as it permeates. In this work, a modified mixed gas permeation system was employed to accurately measure the permeation properties of the polyimides 2,2 -bis(3,4 -dicarboxyphenyl) hexafluoropropane dianhydrid-2,3,5,6-tetramethyl1,4-phenylenediamine (6FDA-TMPDA) and poly(3,3 -4,4 -benzophenone tetracarboxylic – dianhydride diaminophenylindane) (Matrimid® 5218) under exposure to humidified methane at 35 ◦ C. This approach was then applied to further evaluate the permeation properties of the polymers in humidified mixtures of carbon dioxide and methane at 35 ◦ C at atmospheric and elevated feed pressures. Water vapor permeabilities obtained at 2 and 7.5 bar total feed pressure, increased from 3200 to 3900 Barrer and from 20000 to 27000 Barrer as the water level increased for Matrimid and 6FDA-TMPDA respectively, reflecting increases in water vapor solubility and possibly plasticization and clustering effects due to the presence of water vapor. Conversely, the permeabilities of CH4 and CO2 declined due to competitive sorption of water. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Gas separation membranes have been used commercially to separate gas mixtures. One important application is the capture of CO2 from methane in natural gas sweetening. Membranes with high CO2 permeabilities and selectivities also have great potential to be adopted in carbon dioxide capture from both pre and post combustion flue gases to mitigate against climate change. The implementation of such gas separation systems poses significant challenges. One of them is the presence of water vapor, a known plasticizer in membrane separation [1–3]. Water vapor is usually considered as a minor component of the system in such industrial carbon capture applications [4,5]. However, this species often exhibits very different permeation behavior compared to other gas species because of its very small size and its high hydrogen bonding affinity [6–8]. To synthesize efficient membranes that perform well over many years of service, it is necessary to study and understand the permeation behavior of water vapor in polymeric membranes. The transport of water vapor through a polymeric material has been studied in a number of areas: membrane dehumidification
∗ Corresponding author. Tel.: +61 3 8344 6682, fax: +61 3 8344 4153. E-mail address: [email protected] (S.E. Kentish). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.06.023
and dehydration [9–12], protective apparel [13], and packaging and clothing materials [14]. Water vapor permeabilities of various polymer materials have been measured using a variety of techniques, but only a few of these studies have been extended to determine the water vapor sorption and transport properties and its effects on the permeation of other gas species [15–17]. Further, very few results have been obtained under elevated gas pressure. It is particularly difficult to obtain reliable permeability values under such conditions [18]. The combined effects of a high feed pressure and the very high permeability of water causes concentration polarization at the membrane surface and this can introduce a significant error into the measured values of the membrane gas permeability [18–20]. However, the ability to acquire water vapor and gas permeabilities under mixed gas conditions at such high pressures is essential for improving the design and operation of commercial scale natural gas sweetening and carbon capture facilities. This paper applies an approach for simultaneously determining water vapor and gas permeabilities under atmosphere and elevated feed pressures, based on a modified mixed gas permeation apparatus in addition to minimizing concentration polarization effects. In particular, it is the first time that these permeabilities have been evaluated at the same time under elevated feed pressures in a laboratory scale flat sheet membrane system. Two polyimides, typical of those used in natural gas separation [21], are used to understand the effect upon membrane performance of exposure to humidified methane and wetted carbon dioxide and methane mixtures.
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2. Theory
and molecular distribution functions for a mixture. The value of the cluster function can be calculated as follows:
2.1. Water vapor in polymeric membranes Glassy polymeric membranes such as those that are the focus of this study can generally be described by a dual mode sorption model [22]. In this case, the polymer can be considered as comprising of a ‘matrix’ of polymer chains that behave in a manner comparable to a rubbery polymer and a series of larger microvoids within which gas sorption occurs in a manner similar to that of a porous solid. These microvoids are often referred to as Langmuir microvoids, because sorption in this region generally follows a Langmuir isotherm [22,23]. When non-plasticizing gas molecules, such as CH4 , diffuse through such polymeric films, the interaction between gas molecules and the polymer or between gas molecules themselves does not influence the diffusion behavior of other gas species. However, it is well known that the presence of more than one gas species can influence gas solubility in glassy polymers through competitive sorption, particularly in the microvoids or Langmuir component of the material. This competitive sorption effect is strongest for penetrants which are more condensable, that is, those with a higher critical temperature [23]. Conversely, when plasticizer gas molecules like CO2 are diffusing through such a membrane, the interaction between CO2 molecules and the polymer can also swell the rubbery matrix and thus increases the free volume within this region [7]. Simultaneously, the microvoids decrease in volume, reflected in a reduction in the glass transition temperature [15,22,24,25]. For CO2 sorption into polyimides, such as those under study here, this behavior generally causes a net increase in free volume and in the diffusivity of all gas species [7,26]. The addition of water as a penetrant can cause both competitive sorption and plasticization as described above. Indeed, as water has a very high critical temperature, it competes very strongly with other penetrants for absorption sites in the Langmuir voids of the polymer [21]. In addition, if the polymer is hydrophobic or weakly hydrophilic, polar species such as water may interact with each other, preferentially to the polymer, resulting in the formation of water clusters [27]. In this case the water molecules are not in mobile monomeric form but clusters of multiple water molecules held by hydrogen bonding. Clustering results in an increase in the average size of a water molecule but usually these clusters are not big enough to have the properties of bulk water [6]. Clustering in the polymer will significantly hinder the transport of other gas species, because the diffusing water clusters are larger species that provide a sizeable obstruction. Water clusters will also be more effective in swelling the polymeric matrix, enhancing plasticization effects. Conversely, some authors report that water clusters can hinder the diffusion of other water molecules, resulting in a diminished increase [17] or a decrease [28] in the water diffusivity as the water vapour activity increases.
G = (1 − ϕ)
∂ a/ϕ ∂a
−1
(1)
p,T
where G is the cluster integral, the partial molecular volume of the penetrant, ϕ the volume fraction of the penetrant in the polymeric film and a the activity of penetrant. If the cluster function G/ is greater than −1, it means that the concentration of water molecules is higher than average in the neighborhood of a given water molecule. This will be an indication of cluster formation (or non-random mixing) of the penetrant molecules. The larger the value, the stronger the tendency of the penetrant molecules to associate. If the clustering integral is less than −1, the molecules will tend to isolate from each other. 2.3. Concentration polarization Concentration polarization occurs when there is a concentration gradient formed at the surface of the membrane. It is a well known effect in membrane processes, particularly in liquid separation systems such as pervaporation, ultra-filtration, nanofiltration and reverse osmosis. In gas separation units, concentration polarization is usually neglected at the laboratory scale as the small stage cuts used ensures that these effects are minimal [19]. The stage cut is defined as the fraction of the feed components that permeates through the membrane. When a small stage cut is employed, only a small portion of any feed component permeates the membrane. Therefore, the change in gas composition on the surface of the membrane is insignificant and can thus be ignored. However, when water vapor is present in the gas streams, concentration polarization effects must be considered for the typical membrane cell/unit because of the extremely high permeability of this species. Fig. 1 represents the concentration profile of water vapor across a membrane film. If the gas–vapor mixture is not well mixed on the surfaces of the membrane, a concentration gradient will be generated on each side due to the high transport rate of water vapor through the membrane. These boundary layers can result in significant errors in the estimation of the permeability and selectivity values if this effect is not reduced [19].
2.2. Clustering analysis At low vapor activities, the interaction between each water molecule and the polymer is usually stronger than the interaction between water molecules themselves and water behaves as a common plasticizing species. As vapor activity increases, more water molecules will be absorbed, increasing the tendency to form water aggregates, provided that the membrane matrix is able to swell sufficiently to accommodate the clusters [6,29]. Zimm and Lundberg [30] have developed an approach to characterize the tendency of water/solvent clustering in two components systems. Their relation was derived based on statistical mechanical techniques, to simplify the relationship between thermodynamics
Fig. 1. Schematic representation for the change of concentration of water vapor across the boundary layers and the membrane due to concentration polarization.
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Overall mass transfer coefficients (kov , cm s−1 ) are normally evaluated from the measured water vapor flux through the membrane and the bulk concentration difference across the membrane: JH2 o = kov (Cf − Cp )
(2)
where JH2 o is the flux of water vapor permeating through the membrane (cm3 (STP) cm−2 s−1 ), and Cf and Cp the bulk concentration of water vapor on the feed and permeate side of the membrane respectively (cm3 (STP) cm−3 ). If concentration polarisation is insignificant then the true permeability of the membrane can be evaluated from: Pi =
Ji .l (pi,feed − pi,permeate )
(3)
where l is the thickness of the membrane and pi,feed and pi,permeate the partial pressure (kPa) of component i on the feed and permeate side of the membrane respectively. To overcome or mitigate the effects of concentration polarization, the resistances of the boundary layers must be eliminated. Estimating these resistances is not always a simple task, so the enhancement of mixing within these layers to reduce their impact is an effective approach. A mesh spacer is one common method to enhance mixing on the surface of a membrane in liquid separation units [31], while increasing the crossflow stream flowrates can also be adopted for the same effect [18,19]. As the stream flowrates are increased or mixing is enhanced, the mass transfer rate will increase until a stable plateau is reached. The plateau value is indicative of the true permeability of the membrane, as it represents conditions under which concentration polarisation effects become insignificant. Tuning of the stream flowrates will usually be required to ensure this plateau has been reached. 3. Experimental 3.1. Membrane preparation Polysulfone (Aldrich), Matrimid 5218 (poly(3,3 -4,4 benzophenone tetracarboxylic – dianhydride diaminophenylindane, Huntsman Chemical CO.) and 6FDA-TMPDA (2,2 -bis(3,4 hexafluoropropane dianhydrid-2,3,5,6dicarboxyphenyl) tetramethyl-1,4-phenylenediamine) synthesized in house [32] were cast as dense flat sheets. The casting solution containing 2.5 (w/v%) of polymer in dichloromethane (Ajax Finechem, AR
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grade) was filtered in casting rings on glass plates. The membranes were dried at room temperature for at least 12 h. Using a small amount of distilled water, the homogeneous membranes were then removed from the glass plates and further dried for 15 h at 80 ◦ C under vacuum. These membranes were then annealed at 150 ◦ C for 48 h under vacuum. After the drying and annealing process, they were gradually cooled down to room temperature in the vacuum oven and then stored in desiccators. The membranes were stored for a week and then used within the following 7 days to minimize physical aging effects. The thickness of these membranes ranged from 40 to 60 m for Matrimid, 6FDA-TMPDA and polysulfone. Kapton® 100HN flat sheet films of 0.10 mil (25.4 m) were purchased from DuPontTM for permeability value validation. 3.2. Sorption measurement A Gravimetric Sorption Analyzer (GHP-FS, with a Cahn D-200 balance, VTI Scientific Instruments, Florida) was used to conduct sorption measurements and is described elsewhere [33]. A membrane sample was placed in a closed chamber with precise temperature and pressure control. Humidified helium of known relative humidity was passed through the chamber constantly. The change of the mass of the sample was recorded for each water activity tested, and was used to produce the sorption isotherm. 3.3. Water vapor and mixed gas permeation set-up A typical mixed gas permeation set-up (Fig. 2) was developed for measuring the permeabilities of water vapor and gas mixtures simultaneously over a wide temperature and pressure range (20–60 ◦ C and 1 – >10 bar). A pure gas (N2 or CH4 ) or a gas mixture (10% CO2 in CH4 ) was fed into a saturator vessel (filled with water) then a demister vessel to generate the humid gas stream. Both the saturator and demister were partially filled with stainless steel beads and immersed in a temperature controlled bath. The demister was used to capture any entrained liquid droplets from the saturator. The wet gas stream was then passed through a humidity and temperature transmitter (HMT, Probe type 334 Vaisala Oyj, Finland, measurement range: 0–100%RH, operating ranges: −70 to 180 ◦ C, 0–100 bar) which was fitted into the fan forced oven. The oven also contained the permeation cell, another HMT on the permeate side and the necessary stainless steel tubing. The permeation cell was modified from the
Fig. 2. Water vapor permeation apparatus.
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counter-current mixed gas permeation cell that has been used in our laboratory for many years [34]. Specifically, stainless steel wool packing was inserted on both sides of the membrane to enhance mixing and minimise concentration polarisation. The membrane, with an effective area of 10.75 cm2 , was supported by a circular porous stainless steel plate to withstand the feed side pressure. Both sides of the membrane were sealed with O rings. Before each series of experiment, the permeation cell was pre-heated to the operating temperature for at least an hour, with nitrogen and argon flowing through the top and bottom sides of the membrane, to avoid potential vapor condensation during the experiment. The humidity of the feed stream was controlled by the temperature of the saturator and the demister. The total pressure and flowrate (mass flow indicator MFI 1, Aalborg) of the feed and retentate streams were controlled by the back pressure controller (BPC, Cole-Parmer) in the retentate stream. The permeate side of the membrane used argon (UHP Ar, BOC Australia) as the sweep gas to remove any components permeated through the membrane. The permeate stream passed the HMT2 for water vapor content evaluation and then was chilled in an iced cold trap. The water vapor-free permeate stream was then send to the gas chromatography (450-GC, Varian, Inc.) for gas concentration analysis. The sweep gas flowrate was controlled by the mass flow controller (MFC 1, Aalborg) at 20 kPa g. The permeate flowrate before GC analysis was measured by MFI 2 (Aalborg). The temperature and relative humidity of HMT 1 and 2, as well as the gas composition of the permeate stream were recorded in 10 min intervals until the system equilibrium was reached (typically 2–3 h for each water activity). The stream temperature (T, ◦ C) and relative humidity (RH) was measured by the HMTs. At equilibrium conditions, relative humidity can be related to water activity (aw ): aw =
PH2 o RH = 100 Psat
The selectivity between two gas species (˛ij ) was determined by: ˛ij =
yi /xi yj /xj
(6)
where xi and xj are the mole fractions of the gas components in the feed, yi and yj the mole fractions of the gas components in the permeate. 4. Results and discussion 4.1. Water sorption The water sorption isotherms for Matrimid and 6FDA-TMPDA dense films are presented in Fig. 4(a). 6FDA-TMPDA exhibits higher water concentrations than Matrimid over the full range of vapor activities. Both sorption curves show no evidence of being concave to the pressure axis, which is usually indicative of the dual mode sorption model. This may suggest that the Langmuir microvoids are filled at very low water activities, so that water sorption into the rubbery matrix dominates across most of the water activity range. Rather, both curves are convex, consistent with the Flory-Huggins model [15] and indicative of plasticization by water [36]. Similar results have been observed by Lokhandwala et al. for 6FDA-based polyimides [36]. A clustering analysis as described by Zimm and Lundberg [30] was performed using this isotherm and Eq. (1). The cluster function
(4)
where PH2 o is the water vapor partial pressure, Psat the saturation water vapor pressure at the stream temperature and pressure. Water vapor pressure was calculated by the correlation from Hyland and Wexter [35] and further verified with conventional psychometric charts. 3.4. Permeability evaluation A fresh membrane was used for each experimental run. The mixture of water vapor and gases was fed into the membrane cell at 35 ◦ C with argon as the sweep gas on the permeate side. Each membrane was tested at increasing water vapor activity until saturation was approached and then discarded. The flux of each gas species passing through the membrane was evaluated by: Ji =
Qper yi i A
(5)
where Qper is the total permeate flowrate (cm3 (STP) s−1 ) after the iced cold trap, yi , the mole fraction of gas component i in the permeate stream and A the effective membrane area (cm2 ). The activity coefficient, i is taken as unity for all gas species, because the permeate stream contains mainly argon at atmospheric pressure and hence is considered as an ideal gas mixture. For nitrogen, methane and carbon dioxide, the permeate mol fraction (yi ) was determined directly by gas chromatography, whereas for water, this was determined from the water vapor partial pressure measured with the humidity probe. The permeability of the gas component was then determined by using Eq. (3), assuming concentration polarization effects are minimal [19].
Fig. 3. (a) Water vapor sorption isotherms for Matrimid () and 6FDA-TMPDA ( ) at 35 ◦ C. (b) Zimm–Lundberg analysis of clustering in Matrimid () and 6FDA-TMPDA ( ) at 35 ◦ C based on the water vapor sorption isotherms.
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cluster within the membrane when vapor activity is above these values. 4.2. Flowrate tuning The calculation of permeability using Eq. (3) represents the intrinsic properties of the material only if the resistance of the permeation of the components is solely the membrane resistance. Careful study is required for water vapor mass transport resistances because its saturation partial pressure at 35 ◦ C is only 5.64 kPa (equivalent to 2.82 mol% at 2 bar and 0.75 mol% at 7.5 bar). If the gas vapor mixture is not well mixed, one would expect that the change of the gas vapor/membrane interface partial pressure will become much more significant due to the predicted high flux of water vapor compared to other gas components and the resulting concentration polarization. Therefore, the observed calculated permeability will be temporarily termed as an ‘apparent permeability’ before it can be confirmed that the mass transport resistances in the two boundary layers are not significant. To study the contribution of these mass transport resistances to the overall transport, a humidified stream of methane was fed to the membrane cell with the feed and permeate flowrates being varied. The resulting apparent water vapor permeabilities are plotted in Fig. 5. Relatively low water activities were chosen for both low and high total feed pressures (0.23 for Matrimid and 0.36 for 6FDA-TMPDA at 2 bar, 0.31 for both polymers at 7.5 bar). Low activity means low water vapor concentration in the bulk of the feed and thus concentration polarization tends to be more severe. The apparent water vapor permeability increases with increasing feed flowrate initially then approaches to constant values at flowrates greater than 800 cm3 min−1 and 1300 cm3 min−1 for Matrimid and 6FDA-TMPDA respectively (Fig. 5(a)). If these flowrates could be named “breaking flowrates”, they indicate the ‘well-mixed point’ for the two types of membranes. These breaking flowrates were determined by evaluating the percentage change in permeability per 100 ml/min increase of feed gas flowrate. A percentage change of less than 0.5% in permeability with respect to 100 ml/min increase in feed gas flowrate indicated the breaking flowrate. The constant apparent permeability for increasing feed flowrates illustrates the effect of concentration polarization is no longer significant at these feed flowrates. Operating under such conditions will ensure a well mixed condition on the feed side so that the concentration of water vapor on the membrane surface is equal to its
Fig. 4. (a) Apparent water vapor permeability of Matrimid (vapor activity 0.23, ) and 6FDA-TMPDA (vapor activity 0.36, ) with stainless steel wool packing in both compartments of the membrane cell, and Matrimid (vapor activity 0.19, ) with the packing only in the permeate compartment, for various CH4 feed flowrates at 35 ◦ C and 2 bar total pressure. (b) Apparent water vapor permeability of Matrimid () and 6FDA-TMPDA ( ) for various CH4 feed flowrates at 35 ◦ C, vapor activity 0.31 and 7.5 bar total pressure. (c) Apparent water vapor permeability of Matrimid (800 cm3 min−1 CH4 feed, vapor activity 0.23, ) and 6FDA-TMPDA (1300 cm3 min−1 CH4 feed, vapor activity 0.36, ) for various sweep flowrates at 35 ◦ C and 2 bar total pressure.
G/ versus water vapor activity is presented in Fig. 4(b) for both polyimides. The value of the cluster function is above −1 for vapor activities greater than ∼0.45 and ∼0.25 for Matrimid and 6FDATMPDA respectively, meaning that water molecules will tend to
Fig. 5. Water vapor permeabilities of polysulfone at 40 ◦ C ( ) and Kapton films ( ) at 30 ◦ C. Literature data: polysulfone 40 ◦ C from ( ) Swinyard et al. [53], () ) Metz et al. [18]; Kapton 30 ◦ C from ( ) Hubbell et al. Schult and Paul [29], ( [38], and 50 ◦ C from (
) Okamoto et al. [39].
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bulk concentration. The use of stainless steel wool packing can be seen to effectively eliminate concentration polarization (Fig. 5(a)), as the apparent permeability of Matrimid at 2 bar continues to increase after 1300 ml/min without placing the packing on the feed side of the membrane. A much higher breaking flowrate is observed for 6FDA-TMPDA because the mass transport rate of water vapor is much faster. A faster water vapor transport rate means a faster depletion of water vapor from the surface of the membrane, hence a higher flowrate is required to replenish the concentration level (i.e. provide a well mixed condition). When the total feed pressure is increased to 7.5 bar (Fig. 5(b)), the breaking flowrates are increased to 1200 cm3 min−1 and 1700 cm3 min−1 for Matrimid and 6FDA-TMPDA respectively. The water vapor transport rate should not be affected by the total pressure of the feed at the same temperature, because the only driving force should be the partial vapor pressure difference between the feed and the permeate side. Water vapor partial pressure is much less sensitive to system pressure than temperature [37]. Therefore, the increase in breaking flowrates is expected because increasing the total feed pressure by a factor of 3.75 will decrease the water vapor concentration by the same amount at any given water vapor activity. Subsequently, higher feed flowrates are required to maintain the well mixed condition in the feed side compartment. A similar approach was attempted for the permeate side. Fig. 5(c) shows the apparent water vapor permeability at various permeate flowrates under 2 bar total feed pressure. Unlike the feed side flowrate, the permeate side flowrate cannot be increased indefinitely otherwise the concentration of the gas components in the stream will be too low to be analyzed by any kind of analytical instrument. The configuration of our membrane cell results in a sufficiently low permeate breaking flowrate for both of Matrimid and 6FDA-TMPDA (15 and 20 cm3 min−1 respectively) that allows the measurement of gas component concentrations in the gas chromatograph. The flux of water vapor permeating to the permeate side of the membrane will not change significantly due to the minor increase of water vapor partial pressure at elevated pressure. Thus varying the permeate flowrate under elevated total feed pressure is not necessary. If the membrane cell operates at flowrates equal to or higher than the determined ‘breaking flowrates’, reliable water vapor permeabilities can be obtained. The feed flowrates used in later experiments were thus chosen to be at least 100 ml/min above these breaking flowrates (e.g. 900 and 1300 cm3 min−1 for Matrimid at 2 and 7.5 bar total feed pressure respectively). However, the process of determining the breaking flowrates for both side of the membrane under the desired operating pressures must be performed for each type of polymer material used. 4.3. Validation of permeability values Polysulfone films and Kapton® 100HN membranes were tested over a wide range of water activities at 40 ◦ C and 30 ◦ C respectively to validate the permeability values produced using the method developed above. Fig. 6 compares the experimental results from this study with the available literature values. The water vapor permeabilities of polysulfone films are observed to be in perfect agreement with the literature values. Those of Kapton films are not perfectly matched to the literature values but still very convincing, because Hubbell et al. [38] in 1975 used a different experimental set-up with only water vapor present on feed side and argon as sweep gas on the permeate side. The determination of water vapor concentration relied on an electrical hygrometer. With the more modern water content measuring technology used in this work and the enhanced mixing conditions on the feed side of the film, the small difference in the permeation values between the two is reasonable and acceptable. The lowest literature value was obtained
Fig. 6. Water vapor permeability of Matrimid and 6FDA-TMPDA under humidified gas conditions at 35 ◦ C. (2 bar CH4 (), 2 bar 10% CO2 in CH4 ( ), 7.5 bar CH4 ( ) and 7.5 bar 10% CO2 in CH4 ( ).)
at 50 ◦ C by Okamoto et al. [39] using their measured sorption data of Kapton (permeability coefficient equals solubility coefficient multiplies diffusivity coefficient). Using such an approach for determining permeability is less reliable, also the higher measuring temperature (50 ◦ C) attributes to the slight disagreement. Natural gas separation operates at elevated feed pressure. Metz et al. [18] reported a 40% decrease in water vapor permeability of poly(ethylene oxide) poly(butylenes terephthalate) multi-block copolymer (PEO-PBT) when the feed pressure was increased from 3.5 to 10 bar. They concluded that the trend was caused by an increasing resistance in the feed boundary layer with higher pressures, rather than a true change in permeability. Following the methodology developed in this work, the permeabilities of both polyimides under humidified 2 and 7.5 bar CH4 at 35 ◦ C were determined and plotted in Fig. 7. The water vapor permeabilities do not decrease with increased pressure (7.5 bar) and in fact, are very similar to those recorded at 2 bar. These results confirm that measuring water vapor permeability at elevated feed pressures can be achieved at laboratory scale, provided that the feed flowrates can be increased to a sensible range. 4.4. Water vapor permeability Fig. 7 summarizes the vapor activity dependence of water vapor permeabilities for Matrimid and 6FDA-TMPDA under the following humidified feed conditions: CH4 at 2 and 7.5 bar, and 10% CO2 in CH4 at 2 and 7.5 bar. Generally, all water vapor permeabilities increase with vapor activity. This is consistent with the
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the difference in feed conditions (single-component versus multicomponent system). The trend is significantly different when 10% CO2 is added at 7.5 bar feed pressure. The infinite dilution water permeability increases from 3200 Barrer to around 3650 Barrer for Matrimid and from 20,000 Barrer to around 23,000 Barrer for 6FDA-TMPDA. These small increases are most likely related to CO2 inducing a plasticization effect as the CO2 partial pressure increases. There is evidence that increases in the diffusion coefficient can still occur in glassy polyimides even at these low CO2 partial pressures [7]. These results are contrary to that reported by Reijerkerk et al. [16] for a poly(ethylene oxide) based block copolymer who conclude that the presence of 2.5 bar CO2 had no effect on the water permeability. However, this result is for a rubbery material that may be more resistant to CO2 plasticization. Water vapor has a minimal effect on this value up to ∼0.5 and ∼0.3 vapor activity for Matrimid and 6FDA-TMPDA respectively. This may be because the water vapor is unable to induce further plasticization under these conditions, or that competitive sorption from the CO2 limits the increases in water solubility. At higher water vapor activities the data for both polyimides merges with that recorded at lower pressures and appears to increase again in line with these results. Thus, at these higher vapor activities, the presence of the water vapor appears to dominate over any plasticization or competitive sorption effects induced by CO2 . 4.5. Membrane performance
Fig. 7. Percentage of the initial gas permeability of Matrimid under 2 and 7.5 bar humidified feed ((a) and (b) respectively). (CH4 () in humidified CH4 feed; CH4 ( ) and CO2 ( ) in humidified 10% CO2 in CH4 feed.) Insert: CO2 /CH4 selectivity () under humidified 10% CO2 in CH4 feed.
result of many other workers [16,17,40,41]. As discussed in this body of prior work, the observed increasing trend in permeability may relate to both plasticization of the polymer matrix by water vapor [17,42,43] and to the increasing water vapor solubility [16,17,40,44], as shown in Fig. 3. Conversely, the increase may be restricted somewhat by the formation of water clusters, which will diffuse more slowly than individual water molecules [17,28]. The cluster function depicted in Fig. 4(b) indicates that these will tend to occur above 0.45 and 0.25 vapor activity for Matrimid and 6FDA-TMPDA respectively. As mentioned above, similar vapor permeabilities are observed while increasing the total feed pressure from 2 to 7.5 bar. At 2 bar total feed pressure, the addition of CO2 from the humidified CH4 to the 10% CO2 in CH4 mixture is not observed to significantly influence the water vapor permeabilities for either polyimide. Within experimental error, these three data sets all converge to an infinite dilution permeability of around 3200 and 20,000 Barrer, and to around 3900 and 27,000 Barrer at water saturation for Matrimid and 6FDA-TMPDA respectively. A slightly lower infinite dilution permeability (14000 Barrer) was observed by Sato et al. [41] for 6FDA-TMPDA using pure water vapor in a constant volumevariable pressure apparatus, with a greater increasing trend in vapor permeabilities. Such differences are somewhat expected as the absolute value of CO2 permeability for 6FDA-TMPDA shows similar variation, with values reported from 440 to 612 Barrer [45–48]. The discrepancies can be attributed to the different drying and annealing protocols used (hence different thermal history) and
The permeability of CH4 and CO2 in the two polyimide films under the four different feed conditions described above was also acquired. The addition of 10% CO2 to dry methane under both 2 and 7.5 bar feed pressure causes a slight decrease in the absolute value of the CH4 permeability (from 0.234 to 0.224 ± 0.005 Barrer for Matrimid and from 34.2 to 30.5 ± 0.7 Barrer for 6FDA-TMPDA), consistent with the results of other workers [26,49,50]. This is in contrast to the impact of CO2 on water permeability, when a clear increase in water permeability was observed at lower water activities and higher feed pressures. The differences may relate to the differences in critical temperature of methane and water. That is, methane cannot compete successfully with CO2 for sorption sites in the Langmuir voids and hence any membrane plasticization is offset by a loss of solubility. Conversely, due to its higher critical temperature, water is able to resist displacement by CO2 in these microvoids and so the impact of plasticization is dominant. The change in the gas permeabilities under humid conditions, relative to that tested under dry feed conditions, is plotted in Figs. 7 and 8 respectively. The small inserts at the bottom left corner of both figures also show the observed changes in CO2 /CH4 selectivity. For both polyimides, a decrease in CH4 and CO2 permeability is observed as water vapor activity increases under both total feed pressures, which suggests that the presence of water vapor in the feed depresses the membrane performance. Such depression, caused by competition of the penetrating species has been observed elsewhere [2,44,51,52]. In the present case, these competitive sorption effects dominate over any plasticizing effects observed with water vapor permeation. For Matrimid, the CH4 permeability declines to 75% of its original value at saturation, upon addition of water at either total feed pressure. This reflects competitive sorption from the water molecules, in addition to that of the CO2 . The decline in CO2 permeability with increasing water content is more dramatic for both feed pressures. This may relate again to competition sorption effects. That is, as the high initial CO2 permeability is related to its high solubility in the Langmuir voids, the displacement by water has a greater effect. It is noted that the decline at 2 bar total feed pressure is larger than in the 7.5 bar case – a decrease of 45% and 35% of the
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5. Conclusion The study of the effect of water vapor in the feed stream of a mixed gas membrane separation system on the performance of industrial separation units is of considerable practical importance. Plasticization and clustering are possibly induced by water vapor as it permeates through a polymeric membrane. A water vapor permeation measuring rig was modified from a conventional mixed gas permeation system in this work for the measurement of these effects. The experimental set-up was validated by comparison with available literature data and this analysis showed that the flowrates required to eliminate concentration polarization effects were higher for membranes with higher permeabilities and for higher operating feed pressures. Gas permeabilities for Matrimid 5218 and 6FDA-TMPDA obtained at 2 and 7.5 bar total feed pressure in humidified streams underlined the water vapor-induced plasticization and clustering phenomena. The water permeabilities increased from 3200 Barrer at indefinite dilution through to a maximum of around 3900 Barrer in a water saturated gas stream for Matrimid, and from 20,000 to 27,000 Barrer for 6FDA-TMPDA. However, a decrease in permeabilities of both CH4 and CO2 for both pressures was also observed as water vapor was added for both polyimides, with water saturated values between 30 and 75% of the dry gas values. This loss in permeability can be related to competitive sorption effects. For Matrimid, CO2 is more significantly influenced by competitive sorption than methane, causing the CO2 /CH4 selectivity to decrease. Conversely, in 6FDA-TMPDA, a slight increase in CO2 /CH4 selectivity was observed as water saturation increases, which is likely related to diffusional pathways being blocked by the presence of water clusters. Acknowledgements Fig. 8. Percentage of the initial gas permeability of 6FDA-TMPDA under 2 and 7.5 bar humidified feed ((a) and (b) respectively). (CH4 () in humidified CH4 feed; CH4 ( ) and CO2 ( ) in humidified 10% CO2 in CH4 feed.) Insert: CO2 /CH4 selectivity (♦) under humidified 10% CO2 in CH4 feed.
initial value respectively at water saturation. This can be explained by the increase (3.75 times) of the CO2 partial pressure in the feed when the total pressure is increased from 2 to 7.5 bar. The larger quantities of CO2 are able to compete more effectively for sorption sites in the microvoids. The net effect is a loss in CO2 /CH4 selectivity of between 70 and 85% of its original value at water saturation (from 42 to around 30 and 42 to 35 at 2 and 7.5 bar respectively). Similarly, the CH4 permeability for 6FDA-TMPDA declines to around 30% of its original value at saturation, upon addition of water at both total feed pressures (Fig. 8). However in this case, the decline in CO2 permeability with increasing water content is slightly less than that of CH4 for both feed pressures. The net effect is a minor gain in CO2 /CH4 selectivity; from 22 under dry mixed gas condition to 25 at water saturation at 2 bar and similarly from 21 to 23 at 7.5 bar (Fig. 8, inserts). This is in contrast to what was observed in Matrimid and may be due to the higher solubility of H2 O in 6FDATMPDA (Fig. 3(a)). As observed by other authors, it is possible that diffusional pathways are being blocked by water and water clusters [17,28]. This decrease in diffusivity would impact CH4 more than CO2 as it is a larger molecule, leading to a net increase in selectivity. Similar to Matrimid, the decline in CO2 permeability at 2 bar total feed pressure is larger than in the 7.5 bar case – a decrease to 35% and 50% of the initial value respectively at water saturation. This again can be explained by the increase of CO2 partial pressure at the higher total feed pressure.
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