Polyelectrolyte-salt blend membranes for acid gas separations

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,~IE'I~It2E Journal of Membrane Science 131 (1997) 61~59

Polyelectrolyte-salt blend membranes for acid gas separations R. Quinn*, D.V. Laciak, G.P. Pez Corporate Science and Technology Center, Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, PA 18195-1501, USA Received 15 October 1996; received in revised form 15 January 1997; accepted 16 January 1997

Abstract

The CO2/CH 4 and CO2/H2 permselectivity of poly(vinylbenzyltrimethylammonium fluoride), PVBTAF, polyelectrolyte membranes can be significantly improved by blending in certain fluoride-containing organic and inorganic salts. For example, the CO2 permeance of a PVBTAF--4CsF (4 mol CsF/mol repeat unit) composite membrane was more than four times that of a simple PVBTAF composite membrane while CO2/CH4 and CO2/H2 selectivities were comparable. Surprisingly, the blends are at least macroscopically homogeneous even with as much a 6 mol salt/mol PVBTAF repeat unit. The optimal salt loading appears to be approximately 4 mol CsF/mol polyelectrolyte repeat unit. Membrane performance is strongly dependent on the relative humidity of the gas streams and is maximized in the range of 30-50% relative humidity. Membranes containing choline fluoride exhibited improved membrane performance at relative humidities below 30%. Permselective data suggests that CO2 transport is kinetically limited in 10 ~tm thick films. The blends are stable in CO2/CH4/H2 streams for more than 30 days of continuous operation, however, the membranes suffer an irreversible degradation due to reaction with trace level sulfur-containing contaminants common to cylinder H2S.

Keywords: Gas separations; Facilitated transport; Polyelectrolyte membranes; Carbon dioxide

1. Introduction

We have previously reported on a new type of facilitated transport membrane which selectively permeates acid gases such as carbon dioxide or hydrogen sulfide while rejecting permanent gases, particularly hydrogen and methane [1]. Composite membranes consisting of an active separating layer of the polyelectrolyte poly(vinylbenzyltrimethylammonium fluoride) (PVBTAF) coated onto a microporous support exhibited large C O 2 / C H 4 and *Corresponding author. Tel.: (610) 481-4306; fax: (610) 4816517. 0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0376-7388(97)00025-2

unprecedented CO2/H 2 selectivity. Such membranes might be of use in, for example, a hydrogen synthesis plant and onboard reforming for H2 powered vehicles where the removal of CO2 from H2-containing process streams is required. However, a process analysis showed that although PVBTAF membranes were highly selective, higher CO2 permeance would be required to achieve cost effective separations [2,3]. One method for increasing permeance is to increase gas solubility in the membrane. For a facilitated transport membrane such a solubility increase can be achieved by increasing the concentration of the carrier species. In a PVBTAF membrane, hydrated fluoride ion reacts with CO2 as in Eq. (1) to form

62

R. Quinn et al./Journal of Membrane Science 131 (1997) 61~9

HCO~ which diffuses across the membrane [1]. 2F- • nH20 + CO2 : HCO3- + HF~- • (2n - 1)H20

(1) Thus, an increase in fluoride ion concentration should lead to increased CO2 permeance by increasing the concentration of HCO 3 in the membrane. We have previously reported that a variety of fluoride-containing salt hydrates exhibit large and reversible CO2 absorption capacities [4,5]. The incorporation of such salts into PVBTAF membranes could increase the chemical solubility of CO2 in the membrane and, hence, increase CO2 permeances. The notion that gas permeances can be increased by addition of a salt to a polymeric membrane has been applied to other gas separations. For example, the addition of ammonium thiocyanate to a crosslinked methacrylic acid modified poly(vinyl alcohol) membrane led to NH3/N 2 selectivities of 1000 or greater compared to 3 for a membrane containing no salt [6]. Similarly, selective permeation of unsaturated over saturated hydrocarbons (e.g. 1-butene over n-butane) was demonstrated for a crosslinked poly(vinyl alcohol) membrane containing AgNO3 [7]. Addition of various hygroscopic salts to cellulose or poly(vinyl alcohol) membranes led to improved permselective properties for water vapor separations [8-10]. Poly (vinyl alcohol) membranes impregnated with cesium fluoride (CsF) exhibited both improved water permeances and water/alcohol selectivities as a result of increased sorption of water [9,10]. We have found that apparently homogeneous membranes consisting of blends of PVBTAF with various inorganic and organic fluoride salts can be prepared. Furthermore, membranes with salt contents as large as 6 mol of CsF per mol of polyelectrolyte repeat unit remained homogeneous. The preparation and permselective properties of such PVBTAF-salt blend membranes is the subject of this paper.

2. Experimental 2.1. Materials PVBTAF was prepared as described previously [1]. Potassium fluoride, cesium fluoride, and tetramethylammonium fluoride tetrahydrate were obtained

commercially and were used as received. Tetrabutylammonium fluoride trihydrate was prepared from a clathrate of the salt [(C4H9)4N]F.32.8H20 obtained by literature methods [11]. The clathrate was dried under vacuum for three days at room temperature yielding a solid containing 2.8 mol water/mol salt as determined by Karl Fischer analysis. Choline fluoride was prepared by literature methods [12]. The NMR spectra of a D20 solution were consistent with those expected: IH NMR: 4.6 ppm, 1.5H, 0.7H20; 3.15ppm, 1.9H, HOCH2; 2.7ppm, 2.0H, CHEN; 2.45 ppm, 9.0H, CH3; 19F NMR: - 1 2 0 ppm, F-; 13C NMR: 67 ppm, 1.0C, HOCH2; 55 ppm, 1.1C, CHEN; 53.5 ppm, 3.0C, CH3. Karl Fischer water analysis, 0.5 mol HEO/mol salt. A 99.998% grade H2S was obtained from Scott Specialty Gas, Inc. Its purity was confirmed by GCMS [1]. 2.2. Membrane preparation and evaluation Membranes were prepared using methods described in detail previously [1]. Casting solutions consisting of PVBTAF and a fluoride salt were prepared using the same general procedure, which is illustrated by the following example. A solution of composition PVBTAF-4CsF was prepared by addition of 86.47 g of a 41.3 wt% CsF in water solution to 242.34 g of 4.69 wt% aqueous PVBTAE The CsF solution was added dropwise and slowly while the PVBTAF was stirred vigorously. Rapid addition of the salt solution to the polymer solution resulted in precipitation or coagulation of the polymer. After one hour of stirring, the solution was filtered through a coarse filter. The methods used to determine membrane permselective properties have been described in detail previously [1]. Pre-mixed feed gases were used except where noted. Typical feed flow rates were 20 sccm and sweep flow rates were 10 sccm. For determination of water permeances, a feed flow rate of 40 sccm was used. Water vapor concentrations were determined using a Carle gas chromatograph fitted with an 8 ft Porapak R column operating at 80°C. Feed gas containing high purity H2S was prepared by blending 99.998% HES, 4.7 sccm, with premixed 25.0% CO2 in CH4, 16.2 sccm. Gas streams with dew point less than 1°C were obtained by the blending of dry and humidified gases.

63

R. Quinn et al./Journal of Membrane Science 131 (1997) 61-69

The sources of the feed and sweep gases were split into two streams. One stream was passed through a series of water bubblers while the other was left dry. The two streams were blended prior to contacting the membrane. The relative humidity of the resulting stream was calculated based on the water bubbler temperature and the flow rates of the blended gas streams.

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3. Results and discussion

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Clear, apparently homogeneous films could be cast from solutions of PVBTAF containing large quantities of various fluoride salts, particularly CsE The quantities of CsF examined ranged from 0.25 to 6 mol CsF per mol of polymer repeat unit and are represented by the notation PVBTAF-nCsF (n--0.25-6). On a weight basis this represents 0.2-4.6 g CsF/g PVBTAF. It is rare that apparently homogeneous films can be obtained from solutions containing equimolar quantifies of a salt and a polymer, much less 4-6 fold molar excesses [13]. Membranes examined visually after permeation testing generally showed no sign of salt crystallization. Occasionally, some salt crystals formed upon storage (,~20-40% relative humidity) of the membranes prior to testing, but these 'disappeared' upon exposure to more humid room air. Optical microscopy revealed the surface to be homogeneous. A scanning electron micrograph (SEM) of a PVBTAF-CsF membrane revealed that the membrane surface was largely homogeneous with some areas of widely scattered crystals and a few regions of clusters of crystals. It is likely that some or all of these crystals formed in the high vacuum of the SEM. Fig. 1 illustrates the pronounced effect of adding CsF to PVBTAF membranes on the CO2 permeance. The graph shows how the CO2 permeance varies with the partial pressure of CO2 in the feed gas for both PVBTAF and PVBTAF-nCsF (n--0.5, 1, 4) composite membranes. All membranes display the pressuredependent CO2 permeability characteristic of a facilitated transport system. Moreover, CO2 permeances increased with increasing CsF concentration, up to 4 mol of salt per mol of repeat unit. The CO2/H2 and C O 2 / C H 4 selectivities of PVBTAF-CsF membranes were comparable to or greater than those of simple PVBTAF membranes. Membranes containing greater than 4 mol CsF/mol of repeat unit appeared to offer no

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Fig. 1. CO 2 permeances of PVBTAF-nCsF composite membranes as a function of feed partial pressure. Values of n: 0, circles; 0.5 squares; 1, diamonds; 4, triangles. Test conditions: feed: 30.6% CO2, 34.4% CH4, 35.0% H2; sweep: N2; RH=0.31 based on 5°C feed and sweep water bubblers; membrane temperature: 23°C.

performance advantage. For example, a PVBTAF6CsF membrane gave about the same permselective properties as a PVBTAF-4CsF membrane (Table 1) but the membrane itself had almost liquified in the presence of the humidified test gas streams. Thus, the best overall properties were obtained using PVBTAF4CsF membranes (see Table 1); the CO2 permeance of which was 4-6 fold greater than those obtained for a PVBTAF membrane under similar conditions. PVBTAF-nCsF membranes also exhibited improved HES permselective properties over PVBTAF membranes. As shown in Fig. 2, HES permeances increased with increasing concentration of CsF in the membrane. Again, optimal properties were achieved for PVBTAF--4CsF membranes (see Table 1) for which H2S permeances were about four-fold greater than those of PVBTAF membranes (Fig. 2). As expected, the H2S permeance was pressure-dependent indicating a facilitated transport mechanism. The C O 2 / C H 4 and C O 2 / n 2 selectivities of CsF-containing membranes were comparable to or greater than those of PVBTAF membranes. The H2S/ CO2 selectivities ranged from 10 to 4. Whereas the measured H2S/CH4 selectivities ranged from 500 to 220, in many instances CH4 was not observed to permeate the membrane. In these cases, the H2S/ CH4 selectivity was estimated using a maximum CH4 permeance based on the CH4 limit of detection and was greater than 500. As reported previously for

64

R. Quinn et al./Journal of Membrane Science 131 (1997) 6 1 ~ 9

Table 1 Permselective properties of PVBTAF--4CsF composite membranes

(Po/1) × 106 (cm3/cm2 s c mHg)

Feed composition a

Feed P(psig)

CO 2 P(cmHg)

C02

H2

CH4

CO2/H 2

CO21CH4

33.1% CO2, 33.1% CH4 in H2

3.2 15.9 26.0 40.0 54.9 94.6

30.6 52.4 69.7 93.8 119.3 187.4

25.7 24.1 21.9 21.5 19.1 13.0

0.203 0.228 0.218 0.238 0.248 0.299

0.0438 0.0535 0.0469 0.0511 0.0425 0.0350

127 106 100 90 77 43

588 450 466 420 449 370

Feed composition b

Feed P(psig)

H2S P(cmHg)

(Po/1) x 106 (cm3/cm2 s cmHg)

9.9% H2S , 10.0% CO2 in CH4

1.1 25.7 35.4 50.2

8.1 20.8 25.7 33.3

Selectivity

Selectivity

HzS

CO2

CH4

H2S/CO 2

H2S/CH4

91.6 73.6 57.3 42.0

8.91 8.83 8.30 9.03

nd nd nd nd

10.3 8.3 6.9 4.7

m

m

a Test conditions - N 2 sweep at ambient pressure; membrane temperature: 23°C; RH=0.31, based on 5°C feed and sweep water bubblers. b Test conditions - helium sweep at ambient pressure; membrane temperature: 30°C; RH=0.21, based on 5°C feed and sweep water bubblers. nd: none detected.

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50 100 150 CO 2 Feed Gas Pressure (cmHg)

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Fig. 2. H2S permeances of PVBTAF-nCsF composite membranes as a function of feed partial pressure. Values of n: 0, circles; 1, squares; 4, diamonds. Test conditions: feed: 9.9% HES, 10.0% CO2 in CH4; sweep: helium; RH=0.21, based on 5°C feed and sweep water bubblers; membrane temperature: 30°C.

PVBTAF membranes, the presence of H2S has a dramatic effect on permeation of C O 2. The C O 2 permeance of, for example, a PVBTAF--4CsF membrane obtained using an HzS-containing feed was less than half that for the same membrane in the absence of H2S. Even feed H2S concentrations as low as 760 ppm resulted in a 14% decrease in CO2 permeance when compared to a H2S-free feed stream.

PVBTAF membranes containing other fluoride salts were prepared and evaluated. These included tetramethylammonium fluoride tetrahydrate, [(CHa)4N]F.4H20, tetrabutylammonium fluoride trihydrate, [(CgH9)4N]F.3H20, and potassium fluoride, KF. Addition of the highly water soluble salt [(CHa)4N]F.4H20 gave PVBTAF-4[(CHa)4N]F.4H20 membranes with permselective properties comparable to those of PVBTAF-4CsE Membranes of the form PVBTAF.2.3 [(CgH9)4N]F.3H20 exhibited CO2 permeances comparable to those of a simple PVBTAF membrane. Potassium fluoride was considered as a low cost alternative to CsF but unfortunately membranes of the form PVBTAF-4KF exhibited large, non-selective gas fluxes indicative of defects. Such defects likely arise from crystallization of KF as a consequence of its lower water solubility [14]. Results obtained using one additional salt, choline fluoride, are described in Section 3.2.

3.1. Permselective properties of PVBTAF-4CsF membranes Of the various combinations of PVBTAF with fluoride containing salts, the composition of PVBTAF-4CsF appeared to have the most useful

R. Quinn et al./Journal of Membrane Science 131 (1997) 61~9 20.0

Table 2 Permselective properties of a PVBTAF-4CsF composite membrane as a function of membrane temperature at constant humidity a

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Membrane Bubbler T(°C) T(°C)

23 30 35 40

5.0 11.0 15.3 19.6

( P o/ l ) ×1 0 cm 2 s cmHg)

6

( Cm 3 /

CO2

H2

CH 4

11.6 13.9 17.3 21.5

0.0543 0.0655 0.0792 0.1182

nd nd nd nd

Selectivity CO2/H2

(I

10.0

40

214 212 218 182

Test conditions - feed: 33.2% CO2, 33.2% CH4 in H2; feed pressure: 39.0 psig; CO2 partial pressure: 92.3 cmHg; sweep: N2 at ambient pressure; RH=0.32. nd: none detected. a

permeation properties and was therefore examined in more detail. The effects of feed partial pressures (Table 1), temperature, gas stream humidity, and membrane thickness were evaluated. Table 2 lists the permselective properties of PVBTAF-4CsF membranes as a function of temperature. Increasing the membrane temperature from 23 to 40°C resulted in a near doubling of CO2 permeance with little change in CO2/H2 selectivities. Activation energies for permeation were obtained from an Arrenhius plot as described previously [15]: CO2, 6.71 kcal/ mol; H2, 8.10 kcal/mol. It is worth noting that the activation energy of H2 is greater for PVBTAF-4CsF membranes than for PVBTAF membranes [1], 8.10 vs. 5.68 kcal/mol. This possibly reflects the greater ionic character of PVBTAF-4CsF vs. PVBTAF membranes. The permselective properties of PVBTAF-4CsF membranes are strongly dependent on feed and sweep stream relative humidity as shown in Fig. 3. Gas stream relative humidity, RH, is defined as the ratio of the water vapor pressure of the gas stream to the saturated vapor pressure of water at the membrane temperature. Permeances of all gases increased with increasing humidity. A gas stream relative humidity of 0.437 gave the highest CO2 permeance/selectivity combination. Higher relative humidities, 0.607 and 0.832, resulted in higher flux but lower selectivities which are attributable to a decrease in the ionic concentration of the membrane as more water is absorbed at higher humidity (i.e. an 'opening up' of the polyelectrolyte structure). For the lowest humidity examined (RH=0.251), selectivities were low due to higher than anticipated H2 and C H 4 permeances. This

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Fig. 3. Permselective properties of a PVBTAF-4CsF composite membrane as a function of gas stream relative humidity. CO2 permeance, circles; CO2/H2 selectivity, squares. Test conditions: feed: 33.2% CO2, 33.2% CI-I4 in H2; sweep: N2; membrane temperature: 23°C.

may result from crystallization of CsF and formation of defects. Similarly, exposure to anhydrous gas streams resulted in high and nonselective gas permeances indicative of large or numerous membrane defects under this condition. Not surprisingly, PVBTAF-4CsF membranes were also very effective at permeating H20. A quantitative determination of water vapor permeance was obtained using a humidified CO2 feed and a dry helium sweep. Water vapor permeance was independent of feed water vapor pressure over the relatively narrow range of partial pressures examined, 0.53-1.28 cmHg. An average value of 750>< 10 -6 cm3/cm 2 s cmHg was obtained. Although an experimental determination of the effects of membrane thickness on permeance would appear to be a straightforward procedure, significant difficulties were encountered in producing defect free membranes thinner than those prepared routinely (,~ 10-20 ~m). However, by fabricating thicker membranes, a reasonable relationship between membrane thickness and permeance was obtained. At least three, and often as many as six, membranes of each thickness were examined and average permeances are listed in Table 3. A useful representation of the data is shown in Fig. 4 where normalized permeance is plotted against the reciprocal of the relative membrane thickness; normalized permeance is defined as the permeance at a given thickness divided by the

66

R. Quinn et al./Journal of Membrane Science 131 (1997) 61--69

Table 3 Average Po/1 and selectivities for PVBTAF-4CsF composite membranes of various thicknesses a Average (Po/1)x 106 (cm3/cm2 s cmHg)

Relative thickness b

Casting solution (ml)

Average o~(CO2/I"I2)

2.0 2.5 3.0 3.5 4.0 5.0

1.00 1.25 1.50 1.75 2.0 2.5

CO2

HE

11.1 10.1 9.8 9.2 9.3 7.3

0.148 0.098 0.060 0.047 0.044 0.031

80 100 180 200 220 260

a Test conditions - feed: 30.8% CO2, 35.0% H 2 in CH4; feed pressure: 88.0 psig; CO 2 partial pressure: 163.8 cmHg; sweep: N 2 at ambient pressure; RH=0.31, based on 5°C feed and sweep water bubblers; membrane temperature: 23°C. b Based on volume of casting solution. 5,0

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Fig. 4. The normalized permeances of PVBTAF-4CsF composite membranes as a function of membrane thickness. CO2 permeance, circles; 1-12permeance, squares. Test conditions, see Table 3.

permeance of the thickest membrane. For a so-called 'diffusion limited' membrane, diffusion of species across the membrane is fast relative to gas-membrane reactions and permeance increases proportionally with decreasing membrane thickness. A plot of normalized permeance vs. I/thickness is linear for a diffusion limited membrane as shown by the theoretical line labeled 'diffusion limited' in Fig. 4. In contrast, a 'kinetically limited' membrane exhibits a less than proportional increase in permeance with decreasing membrane thickness because gas-membrane reactions are slow relative to diffusion. The possibility also exists that diffusivities and reaction rates are comparable, as has been reported for other CO2 facilitated transport membranes [16]. Since permeation of H2 through a PVBTAF-4CsF membrane occurs by a solution diffusion mechanism, a normalized permeance vs. 1/thickness plot is expected to agree

well with that for a diffusion limited membrane. This is, in fact, true for thick membranes but, for thinner ones, agreement is poor because H2 permeances are 'too high'. This can be explained by the increased number and/or size of membrane defects for thinner membranes. Fig. 4 further shows that agreement between normalized CO2 permeances and those of a diffusion limited membrane are good for membranes of relative thicknesses greater than 2.0 (~30-40 ktm). Normalized permeances of thinner membranes are less than those predicted for a diffusion limited membrane implying that permeation of CO2 is at least partly limited by reaction kinetics; that is, the reaction of CO2 with PVBTAF-4CsF is slow compared with the rate of diffusion of species across the membrane. A major question relating to the practicality of any facilitated transport membrane is its stability over extended periods. PVBTAF--4CsF membranes were reasonably stable when exposed to CO2 containing feeds and over a 44-day period suffered no loss of CO2 permeance o r C O 2 / C H 4 and CO2/H 2 selectivity. The effects of H2S on membrane stability were more pronounced and, even at H 2 S concentrations as low as 4300 ppm, PVBTAF-4CsF membranes suffered permeance declines as great as 33% (CO2) after 32 days. Similar HES instability had been observed for PVBTAF membranes and it was shown that impurities present in n E S react irreversibly with the membrane most likely by crosslinking of the polymer [1]. Confirmation that impurities found in cylinder H2S are responsible for the observed permeance declines came from membrane testing using a high purity H2S. Employing a feed gas prepared by blending high p u r i t y H 2 S with a premixed C O 2 / C H 4 source, the

R. Quinn et al./Journal of Membrane Science 131 (1997) 61~59

permselective properties of a PVBTAF-4CsF membrane were obtained over a 30-day period. Although there was variation in permeance with time, there was no overall change in H2S or CO2 permeances. These results strongly imply that the presence of reactive contaminants in crude H2S are responsible of the flux declines observed. Such flux declines could be eliminated when the contaminants normally present in H2S are removed by, for example, passing the feed gas through a guard bed containing a solid absorbent, such as activated charcoal or powdered PVBTAF.

3.2. Membranes containing choline fluoride A process evaluation of a PVBTAF--4CsF membrane using a semi-empirical model of membrane performance [2] indicated that, as a result of its relatively large water permeance, depletion of water vapor would occur as the feed gas traveled down the length of the membrane element. Thus, a substantial portion of the membrane might be exposed to dry gas resulting in decreased performance and possibly membrane failure. Such shortcomings could be partly mitigated by various engineering controls such as the use of a humidified sweep gas but improvement of the permselective properties at low humidity would be valuable. We considered PVBTAF-salt blends incorporating fluoride salts which would have higher water affinities than CsE One possibility is choline fluoride, HOCH2CH2N(CH3)3+F -, (abbreviated ChF) for which the cation might form hydrogen bonds to water via its hydroxyl group. Membranes of the form PVBTAF-4ChF were found to exhibit much improved performance at lower humidities but at the same time exhibited lower CO2

67

permeances than those of PVBTAF-4CsF membranes. Such membranes could be exposed to anhydrous gases for periods greater than 4 days with no sign of membrane failure, although both CO2 and H2 permeances were very low. A compromise between the 'high' CO2 permeances of PVBTAF--4CsF membranes and the improved low humidity performance of PVBTAF-4ChF membranes are membranes which contain both salts. Membranes containing a 1 to 1 and a 3 to 1 ratio of CsF to ChF were prepared. Evaluation of membranes of the form PVBTAF3CsF-ChF and PVBTAF-2CsF-2ChF gave CO2 permeances ranging from comparable to slightly lower than those of a typical PVBTAF-4CsF membrane at a fixed humidity. The overall performance and properties of PVBTAF-3CsF-ChF membranes were found to be more suitable. Gas permeances as a function of pressure for a PVBTAF-3CsF-ChF membrane are listed in Table 4. The permselective properties of a PVBTAF-3CsFChF membrane as a function of gas stream humidity are illustrated in Fig. 5. The CO2 permeance was relatively constant for relative humidities between 0.234 and 0.832. The H2 permeances, as expected, increased steadily with increasing humidity and as a consequence, CO2/H 2 selectivities decreased. At the highest relative humidity examined, 0.832, the H2 permeance was quite large and the gas permeation was nearly nonselective but the CO2 selective properties of the membrane could be restored by decreasing the gas stream humidity. Humidities lower than 0.234 resulted in significantly lower CO2 permeance. Thus, as shown in Fig. 5, CO2/H 2 selectivities are lower at the extremes of high and low humidity. An optimal combination of high CO2 permeance and high selec-

Table 4 Permselective properties of a PVBTAF-3CsF--ChF composite membrane a Feed P(psig) 22.9 43.7 55.8 110.3

CO2 P(cmHg) 48.7 75.6 91.3 161.9

(Poll) × 106, cm3/cm2 s cmHg

Selectivity

COJCn4 CO 2

CH4

24.39 21.81 22.24 19.31

0.0611 0.0917 0.0768 0.1085

399 238 289 178

Test conditions - feed: 25.0% CO2 in CH4; helium sweep at ambient pressure; membrane temperature: 23°C; RH=0.31, based on 5°C feed and sweep water bubblers. a

R. Quinn et al./Journal of Membrane Science 131 (1997) 61-69

68

20.0

300

200 10.0 100 "~

J

g.

ff 0.0

0.4

0.0

0.8

0

Gas Stream Relative Humidity

Table 5 Water permeances of a PVBTAF-3CsF--ChF composite membrane a Bubbler T (°C)

RH

(Po/1)x 106 (cma/cm2 s cmHg) H20

Selectivity H20/CO 2

20.0 18.0 15.0 12.0 10.0 8.0 5.0 3.0 1.0

0.832 0.735 0.607 0.499 0.437 0.382 0.311 0.270 0.234

398 429 424 366 422 350 367 278 314

52 54 67 116 160 b 170 414 876

Fig. 5. Permselective properties of a PVBTAF-3CsF-ChF composite membrane as a function of gas stream relative humidity. CO2 permeance, circles; CO2/I-I2 selectivity, squares. Test conditions: feed; 30.8% CO2, 35.0% HE in CH4; sweep: N2; membrane temperature: 23°C.

a Test conditions - feed: 30.6% CO 2 in CH4 at 37.74-0.4 psig; sweep: dry helium at ambient pressure; membrane temperautre: 23°C. b Unrealistic value for CO2 permeance obtained.

tivity can be obtained by regulating gas stream relative humidity between -,~0.3 and 0.45. Exposure of a PVBTAF-3CsF-ChF membrane to dry gases resulted in a very low CO2 permeance, 0.0089× 10 - 6 cma/cm2 s cmHg, and undetectable quantities of H2 and CH4. Even after four days of exposure, gas permeances remained very low, consistent with the absence of membrane defects. These observations suggest that, unlike PVBTAF or PVBTAF-4CsF membranes, the water affinity of a PVBTAF-3CsF-ChF membrane is sufficient to prevent formation of cracks and defects upon exposed to dry gases. Further, the effects of exposure to dry gas were reversible and rehumidification of feed and sweep gases led to a restoration of the permselective properties.

Water permeances of a PVBTAF-3CsF--ChF membrane for feed gas relative humidities of 0.832-0.234 are listed in Table 5. Except for the two lowest humidities, there was no clear change in water permeance with humidity. If water permeances are assumed to be constant for relative humidities greater than 0.270, an average value of 4 0 0 x l 0 - 6 c m 3 / cm2s cmHg is obtained, which is almost half that of a PVBTAF--4CsF membrane. As expected, CO2 permeances generally increased with increasing humidity and also, as expected, permeances were lower than those obtained when both feed and sweep gases were humidified. As for PVBTAF-4CsF membranes, an increase in membrane temperature from 27 to 50°C at constant humidity (~0.19) resulted in a 2.6-fold increase in

Table 6 Permselective properties of a PVBTAF-3CsF-ChF composite membrane as a function of membrane temperature at constant humidity a Membrane T(°C)

Bubbler T(°C)

Feed P(psig)

25.4 30.0 35.3 40.1 45.1 50.0

0.2 3.9 7.9 12.0 16.0 20.0

41.7 50.0 45.6 45.3 46.4 48.3

P(CO2) P(cmHg)

87.6 100.5 93.7 93.2 94.9 97.9

( P o / 1) x 1 0 6 cm 2 s cmHg)

( Cm 3/

CO 2

CH4

9.005 9.313 11.59 12.71 15.93 18.09

nd nd 0.0281 0.0476 0.0580 0.0668

Selectivity CO2/Cn 4

--413 267 275 271

a Test conditions - feed: 30.0% CO2 in CH4; helium sweep at ambient pressure; feed and sweep water bubblers at indicated temperatures;

RH=0.19.

R. Quinn et al./Journal of Membrane Science 131 (1997) 61-69

CO2 permeance for a P V B T A F - 3 C s F - C h F membrane (Table 6). An Arrenhius-type plot of ln(Po/1) vs. 1 / T gave an activation energy of CO2 permeation of 6.4 kcal/mol. This value is comparable to activation energies for P V B T A F - 4 C s F and PVBTAF membranes, 6.7 and 6.0 kcal/mol [1], respectively.

4. Conclusions (1) Incorporation of large molar quantities of CsF into PVBTAF membranes results in apparently homogeneous membranes with improved CO2 and H2S permeances without loss of selectivity. A P V B T A F 4CsF membrane exhibited CO2 permeances which are at least four-fold greater than the same membrane without CsE (2) Gas stream relative humidity is a critical variable impacting the permselective properties of PVBTAF--4CsF membranes. Optimal performance is obtained at a relative humidity of ~0.4. The poor low humidity performance of P V B T A F - 4 C s F membranes can be at least partially mitigated by incorporation of choline fluoride. Membranes of the form P V B T A F - 3 C s F - C h F exhibited an optimal performance at relative humidities between 0.3 and 0.45 and, in contrast to P V B T A F - 4 C s F membranes, exposure to dry gas streams did not result in membrane failure.

Acknowledgements The outstanding efforts of Greg Cooper in obtaining permselectivity data and Tony Zehnder for modeling membrane performance are gratefully acknowledged along with the efforts of Jane Van Horn and Paula McDaniel for NMR spectra and James R. Stets for microscopy. Partial funding from the U.S. Department of Energy, Office of Industrial Technologies (Contract Nos. DE-FC02-89ID12779 and DE-FC3694GO1004) is also gratefully acknowledged.

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