- Email: [email protected]
Membranes for separation of higher hydrocarbons from methane
journal of MEMBRANE SCIENCE E LS E V I E R
Journal of Membrane Science 110 (1996) 37-45
Membranes for separation of higher hydrocarbons from methane J. Schultz *, K.-V. Peinemann GKSS-Forschungszentrum Geesthacht GmbH, D-21502 Geesthacht, Germany
Accepted 7 August 1995
Abstract In the search for a membrane material capable of separating higher hydrocarbons from methane about 40 different polymers were tested. The most promising two, polyoctylmethylsiloxane (POMS) and polytrimethylsilylpropyne (PTMSP), are compared in this article to the standard material polydimethylsiloxane (PDMS). The transport properties were investigated dependent on the process parameters temperature, feed pressure and feed composition under mixed gas conditions. It was possible to improve the separation performance by means of the selectivity from 5 to > 12 at acceptable fluxes. Keywords: Gas separation; Natural gas; Dew pointing; Mixed gas selectivity;Polyoctylmethylsiloxane;Polytrimethylsilylpropyne
1. Introduction A typical step in natural or raw gas treatment is the separation of propane and butane from the main component methane. Usually the removal of the higher hydrocarbons is done by pressure distillation or chilling out the condensable parts. This dewpointing process could possibly be done by butane selective membranes. One can easily integrate a membrane module in a gas processing plant utilizing the pipeline pressure of about 70 bar. One advantage of a membrane module would be a reduced requirement of cryogenic energy. Another advantage is that membrane modules are small. On off shore platforms, where optimizing space is crucial, the use of membranes might be especially beneficial. Several polymers and blends were tested for this application. The butane / methane selectivity (a~M) was chosen as the efficiency criteria for the membranes. A polydimethyl-siloxane composite membrane was * Corresponding author. Elsevier Science B.V. SSD10376-7388 ( 95 ) 00214-6
declared as the reference. It is a standard GKSS membrane and already in commercial use for the removal of hydrocarbons from waste air at petrol stations. The aim was to improve the properties of this reference membrane. The primary objective was to elevate the mixed gas butane/methane selectivity from 5, as shown by PDMS, to 12. Fluxes are of secondary importance but should not be less than 1 m3/m 2 h bar for butane. The butane flux of the reference PDMS composite membrane with a thickness of 10/.tm was 1.4 m3/m 2 h bar for a mixed gas measurement. 1.1. Experimental
More than 40 materials were tested during the polymer screening. Among these were imides, amidimides, kaptons, silanes, siloxanes, kraton, fluoroelastomers, butylketones, carbonates, sulfocarbonates and copolymers of the listed. Most of the tested polymers exhibited lower butane/methane selectivities than the reference and therefore will not be discussed.
38
J. Schultz, K.- V. Peinemann / Journal of Membrane Science 110 (1996) 37-45
A selection of the investigated polymers is shown below. These either show a better performance than the reference or behave somehow unusual, thus supplying valuable information for the separation problem. PDMS poly (dimethylsiloxane), the reference PDMS* PDMS radiation crosslinked POMS poly (octylmethylsiloxane) PHDMS poly (hexadecylmethylsiloxane) Carbosil polydimethylsiloxane-polycarbonate block copolymer Lestosil poly (dimethylsiloxanedi(phenylsiloxane) ) Kraton ® styrene-isoprene-styrene block copolymer Seragel polysulfone-polybutadiene block copolymer CMS perfluorodimethyldioxole-terafluorethylene copolymer PTMSP poly (trimethylsilylropyne) Also several blends of these polymers were tested. Thick films were prepared to measure the intrinsic properties of the polymers. Composite membranes were made by dip coating a mesoporous support in 1 to 10% polymer solutions and drying in air at room temperature. The support used was a microfiltration membrane made of a mesoporous polyetherimide layer on top of a polyester fabric. The surface porosity was about 5%. The transport resistance of this support was negligible compared to the dense layer of all used polymers. Gas fluxes and permselectivities were calculated from single gas measurements using the pressure increase method [ 1 ], typically at ambient feed pressure and permeate pressures up to 100 mbar. In gas mixture measurements, the feed, retentate and permeate were analysed with a selective detector (i.e. a mass spectrometer or a gas chromatograph). These experiments gave the data for more realistic conditions in terms of a mixed feed gas. The principal set up is shown in Fig. 1. Gas composition, stage cut and total pressure up to 10 bar were varied. All measurements were carded out at 30°C unless otherwise stated. All composites were first measured with a standard set of pure gases at 1 bar feed pressure: helium, nitrogen, oxygen, methane, propane and n-butane. The permeabilities were calculated from the steady state pressure increase with time. Further measurements were only carded out with defect free stamps. This information was based on the intrinsic oxygen/nitrogen selectivity measured for a thick, dense film. A
....................................T feed mixture
_
_
~
~
permeat~ _...~pressure
__~ mass- 1 t spectrometer) . . . . . . . . . . . . . . . .
'
@ vacuum pump Fig. 1. Set-upfor single and mixed gas measurements. composite with no defects must reach the intrinsic selectivity. The thickness of the selective layer was determined by scanning electron microscopy [ 2]. This method is not always accurate, especially when the coating penetrates the porous structure of the support. Another method is to calculate the thickness from the permeability coefficient divided by the flux for a quasiideal gas, which should not swell the membrane, such as nitrogen. If one assumes that a thin layer should have the same permeability coefficient as a thick film of the same polymer, the permeability coefficient (P*) measured for a dense film divided by the pressure normalized flux ( P / d ) , gives the thickness (d) of the measured thin top layer (d = P* / ( P / d ) ). This assumption is not necessarily always true, as will be seen later. The selectivities, i.e. the ratio of measured permeabilities, are denoted by a 's'-subscript for single (or pure) gas measurements and a 'mix'-subscript indicates a mixed gas measurement. The standard conditions for mixed gas measurements were 10 bar and 30°C for a feed gas of 3% butane in methane (97%).
2. Results The detected ideal selectivity (a~M.s) measured with pure gases, seldom predicts the mixed gas behaviour of the polymer accurately. A material showing a high pure gas selectivity a,aM.smay have a lower selectivity in the mixture a~M.m~xthan a material with a lower pure gas selectivity. The variation between the pure and mixed gas selectivities for various polymers is shown
J. Schultz, K.-V. Peinemann / Journal of Membrane Science 110 (1996)37~15
39
50
210
40
s
'~
0
1
mix
tN,
O3
Fig. 2. Pure (s) and mixed (mix) gas butane/methane-selectivities of different polymers.
PTMSP Poly (trimethyl-silyl-propyne)
POMS Poly (octyl-methyl-siloxane)
CHs
CH 3
-(-C -- CI~
-(-Si-O
HJC --sSi--CH ~
} CH3 PDMS Poly (dimethyl-siloxane)
CHs -(-Si--O -~ I CH3 Fig. 3. Chemical structures of PTMSP, POMS and PDMS.
in Fig. 2. Usually the mixed gas selectivity is lower than the pure gas selectivity. For PTMSP, however, the butane/methane selectivity was found to be higher in mixed gas experiments. This confirms recent results [ 3 ] discovering PTMSP as an advanced alternative to rubbery polymers for C3 + separation. From pure to mixed gas performance, CMS reverses its separation behaviour, so that methane turns into the faster component in a mixed gas application. Since the performance of CMS was poor, these effects were not
investigated further. It was taken as an indication of transport interference by different species. Consequently, mixed gas measurements were necessary for adequate characterisation of membrane materials for the current application. The most promising polymer, PHDMS, with a pure gas selectivity of a~. s > 200 showed a poor selectivity of 2 in mixed gas tests. It was interpreted to be the result of swelling of the polymer by butane. Silicones are known to swell due to the sorption of vapours. When vapours are sorbed, the
40
J. Schultz, K.- V. Peinemann / Journal of Membrane Science 110 (1996)37-45
attractive chain-to-chain forces decrease and thus the free volume increases• This causes lower transport resistance for any species diffusing through the swollen polymer. Hence, the sorption of butane facilitates the methane transport in rubbery polymers in general. Therefore, the butane/methane selectivity is lower during mixed gas measurements than pure gas measurements. Thus, the solubility coefficient is the determining factor of the separation properties during mixed gas performance. This is the general behaviour for rubbery and most of the glassy polymers• An exception is the glassy PTMSP because of a different transport mechanism• Because of the previously mentioned difference between pure and mixed gas selectivities, the following discussion will be limited to mixed gas butane/methane selectivity unless otherwise stated. Based on these first results we chose two promising polymers for further investigation. POMS and PTMSP showed the highest mixed gas selectivities. Their chemical structure in comparison with PDMS is shown in Fig. 3. POMS is a silicone with a glass transition temperature far below the experimental temperatures• It is a derivative of the well known PDMS where some of the methyl-groups have been substituted by Cs-sidechains• PTMSP is a unique glassy polymer with a glass transition temperature above 540 K. Because of its stiff backbone, in combination with the bulky S i ( C H 3 ) 3 side-group, PTMSP is known to have a fairly high fractional free volume > 25%. The gas permeability coefficients were one order of magnitude higher than the ones of PDMS. Its butane/methane selectivity was higher in mixed gas measurements than the ideal selectivity measured with single gases (see Fig. 2). Since only composites were used for further experiments, mass transfer will only be discussed in terms of pressure normalized fluxes, which will simply be called fluxes• The thickness is limited by the preparation procedure anyway• However, determination of the thickness problems will be discussed with POMS.
of the flux on the total feed pressure. Usually, increased fluxes would be expected at higher partial pressures of swelling vapours. This is due to a higher sorption and therefore reduced transport resistance in the polymer. POMS, however, showed decreased fluxes at higher pressures, and therefore, higher butane partial pressures. This was interpreted as two interfering effects caused by swelling. POMS is a very soft material. It is not possible, for example, to prepare a dense thick film from POMS. For such kind of a polymer, swelling lowers the transport resistance as a function of butane partial pressure. Simultaneously, the POMS layer is softened, and therefore, densified by compression as a function of the increased total feed pressure. For the soft POMS layer, the compressibility was the dominant factor of the two and thus the flux decreased as a function of the total feed pressure as shown in Fig. 4. Even though the fluxes of POMS are reduced at higher pressures it is still one of the most selective rubbery poly12
, 15
total feed pressure
different POMS blends 13•
POMS-SSM O'
12
.0.
10
3. POMS
ongm
9
; Varying the mixed gas process parameters (i.e. total feed pressure p, molar ratio of the feed gas components xi and temperature O), it was found that the flux L is not constant• POMS [4] showed a strong dependence
[ bar]
Fig. 4. Pressure behaviour of the original POMS, where the decrease of fluxes is due to compression: ~ , methane; 0 , butane; × , selectivity.
;butane flux
m''l
[m z-h-bar|
;
Fig. 5. Improvement of the performance of POMS composites leading to higher selectivities and fluxes. The permeability coefficient could not be calculated because of inaccurate thickness determination.
J. Schultz, K. - V. Peinemann / Journal of Membrane Science 110 (1996) 37-45
41
lxr
Fig. 6. (Left) Cross section/original POMS The polymer forms a dense layer above the mesoporoussupport and penetrates the pores. The top layer is mechanicallysoft, thus compressible. Fig. 7. (Right) Cross section/(POMS-SSM) The POMS layer is entirely encapsulated in the pores of the support. Thereby the polymer is stabilized and does not exhibit compressibility. mers performing a butane/methane selectivity of 10. An indication for the theory of simultaneous swelling and compression is the different behaviour of an advanced type of POMS membrane. By using different crosslinking agents, varying the amount of pre-crosslinking before coating the mesoporous support and adding certain silicones as blend materials, it was possible to simultaneously increase the selectivity (from 10 to 12) and the permeability. The development of optimizing the membrane is shown in Fig. 5. The coating parameters had a strong influence on the formation of the top layer. As can be seen from a scanning electron microscopic picture (Fig. 6), the original POMS formed a dense layer of approximately 3/xm on top of the mesoporous support. In between the support layer and the dense top layer is a third layer, where the polymer penetrated the pores of the support. In the case of the advanced POMS composites, with a selectivity of 12, the top layer disappeared totally (Fig. 7). The selective layer was now entirely located inside the upper pores of the support.
Thus the swelling was hindered and therefore the increase of diffusivity for methane, compared to pure gas measurements, was lower than for the original POMS composites. The swelling effect was reduced by framing the polymer inside the pores of the substructure. The resulting membrane was not a typical composite membrane. One might call it a sub surface membrane (SSM). The pressure dependency of the original POMS composite, shown in Fig. 4, which resulted in a decrease of fluxes at higher pressures, was not detected for the POMS-SSM. The flux of gases through POMS-SSM as a function of feed pressure is plotted in Fig. 8. It is impossible to determine an effective thickness of the selective layer of a SSM. Once the polymer was inside the pores, it changed its properties. The butane/ methane selectivity was higher (see Fig. 4) and the decreasing flux at increased feed pressures as measured for original POMS was not found for POMS-SSM (see Fig. 8). Hence, it is a different material in terms of transport properties. Therefore it is not accurate to nor-
42
J. Schultz, K.-V. Peinemann / Journal of Membrane Science 110 (1996) 37-45
14 P 12 10 . ~
~6
8 .~
.~ 4 4
i
2 2
4
6
feed pressure
8
10
12
[ bar ]
Fig. 8. POMS-SSM flux versus pressure. No compression effects were detected; polymer stabilized by encapsulation (compare Fig. 4).
malize the flux of POMS-SSM on the permeability coefficient of 'free' POMS because of the different permeability coefficients of one polymer in two different states. And it is obviously impossible to prepare a dense, homogeneous film of an encapsulated polymer. Using the second possible determination method, i.e. calculating an effective thickness from a SEM picture, would require deriving the thickness of a layer of fluids inside a porous matrix. That method is not accurate either, because the polymer only fills part of the visible layer. Tortuosity and porosity of the penetrated layer were unknown, since pore determining measurements could never give proper information about only one layer of an entire, asymmetric structure. In conclusion the experimental results showed no compressibility of the POMS-SSM membrane. The polymer seemed to be mechanically stabilized and supported by the pore system. With the invention of the POMS-SSM, the but~ine/methane selectivity from mixed gas tests was enhanced to a value of 12. The butane flux for this advanced type of POMS membrane was approximately 4.5 m3/m 2 h bar. The POMS-SSM was produced on a coating machine in technical scale, thus it is available in large amounts at reasonable pricing.
4. P T M S P
As an alternative to rubbery polymers PTMSP was found. This glassy polymer, with a glass transition temperature of Tg > 250°C and an extraordinary high free volume, showed unusual properties. A mixed gas selectivity, higher than the ideal selectivity, was measured
in pure gas tests. Additional experiments showed an increase of the selectivity with decreasing temperature. The transport behaviour of PTMSP can be represented either by an extended dual-mode mechanism introduced by Koros et al. [ 5 ] or by selective surface sorption. The extended dual mode model would explain the flux depression for methane in the presence of butane by competitive sorption in the Langmuir domains. Since the dual mode parameters have not been measured, the approach could not be verified. Explaining this flux depression by pore blocking, would lead to effects near to capillary condensation, such as competitive surface adsorption. A population of the more condensable species adsorbed on the inner surface of microcavities was assumed to be responsible for the reduced methane flux in the presence of butane. The reason for hypothesizing a kind of a capillary condensation, were results of pure gas measurements carried out for PTMSP. The flux of butane was measured versus feed pressure in the subambient range, as shown in Fig. 9a. The exhibited maximum is comparable to the behaviour of a typical porous material in which capillary condensation occurs, e.g. Vycore glass. The similarly shaped curves, as can be seen in Fig. 9a + b, were taken as evidence for capillary condensation. Unlike the original POMS, the total feed pressure had no influence on the transport properties of PTMSP, but the butane percentage in the feed caused a significant change in the mixed gas behaviour of PTMSP. This correlation is presented in Fig. 10. Experiments elevating the total feed pressure incrementally from 2-10 bar at a given fraction of butane, thus increasing the partial pressure of butane as well, showed almost no influence of the total feed pressure on the fluxes. The butane flux remained constant during these measurements. Usually greater capillary condensation at higher partial pressures of the condensable gas is expected. To elevate the partial pressure, one can either increase the total pressure of a constant composition feed mixture, or increase the butane fraction at a fixed total feed pressure. In the case where the higher partial pressure was achieved by higher total pressure of the same feed mixture, the effect on the selectivity was not significant. In the case where the fraction of butane was increased at constant total pressure (see Fig. 10), the butane/methane selectivity increased simultaneously. Thus, the selectivity was more a function of the butane fraction rather than of butane partial
43
J. Schultz, K.-V. Peinemann/Journal of Membrane Science 110 (1996)37-45
o.I3
..5
8
(b)
t
E
200 t::
i
100 (/3 v
0 0
1000
feed pressure
O0
2000
I
I
I
t
0.2
0.4
0.6
0 8
Relofive Meon Pressure
Pm/Po
[ mbar ]
Fig. 9. Butane permeability as a function of feed pressure, exhibited maximum is a result of superposition of transport mechanism in capillary condensation, which is similar for both. (a) PTMSP, (b) Vycore glass (quoted from [ 11 ] ).
pressure. Further experiments should be carried out to give a better understanding of this behaviour. Some points should be remarked concerning Fig. 10. At standard conditions for the mixed gas measurements (10 bar, 30°C, 3% C4) the butane flux was 35 m3/m 2 h bar and the selectivity was 27. The thickness of the selective layer of the measured composite membrane was 3 to 5 /xm. The butane/methane selectivity decreased from 35 to 18 as the butane concentration decreased from 9% to 0.5%. While removing the butane, the selectivity decreased. For the application i.e. the removal of higher hydrocarbons, which includes butane - this is undesirable. But a selectivity of 18 at a concentration of 0.5% butane in the feed is still a remarkable performance. 0.5% butane is equal to a pressure dewpoint below - 20°C, which is a required pipeline standard. m' ~J30
140
The decrease of selectivity with decreasing butane amount in the feed, support the theory of a 'pre-existing free volume' as a kind of micro-cavities in the material [ 6]. Other groups measured the fractional free volume of PTMSP to be higher than 25% [7,8], which is also the reason for the extraordinary high fluxes. These micro-cavities might allow capillary condensation of the more condensable parts in the gas mixture according to the Kelvin equation. Transport inside a porous matrix where capillary condensation occurs, is a superposition of a four domain mechanism: • diffusion in the dense polymer regions (absorbed species) • diffusion in empty caverns (gaseous species) • surface diffusion (adsorbed species) • diffusion in a condensed phase (solved species)
• 40
~
0=293K x B = 0.5%
•
je
10
10
0
~ 0 50 100 150 200 stored at ambient conditions [d] Fig. 11. Aging effects of PTMSP during storage at ambient conditions. Fluxes decreased by one order of magnitude after 3 months while the butane/methane selectivity remained stable: O, butane flux; O, selectivity. 0
• 0
•
30 ee
0 =303K
10
p'T=4 I ~ 0 1
butane fraction in the feed [ % ] Fig. 10. Mixed gas selectivity of PTMSP as a function of the butane fraction in the feed. The mixed gas selectivity decreased with the fraction of butane in the feed. The marginal zero-butane value for the selectivity was 18.
44
J. Schultz, K.-V. Peinemann / Journal of Membrane Science 110 (1996) 37-45
If butane condensed in the pores, then the surface diffusion and diffusion of methane through pores filled with condensate, would have been hindered. A liquid butane barrier, however, is not much of a transport resistance to butane molecules. Thus the methane flux would be reduced by the presence of butane while the butane flux was almost not affected. The experimental data supported this hypothetical transport mechanism. These experiments showed that the increase in selectivity was mainly determined by a reduced methane flux due to pore blocking by the condensed butane. A further indication of capillary condensation in PTMSP was found in the literature about porous solids, as previously mentioned [ 11 ]. In that paper on the transport of capillary condensate, butane transport in vycore glass was studied and butane flux versus feed pressure was plotted for a pure gas measurement. The curve exhibited a maximum as a result of the combined transport mechanism (see Fig. 9b). In Fig. 9a the results of butane flux versus pressure are shown for PTMSP as carried out in the current work. The similar shape of the PTMSP curve is a strong indication of a similar transport mechanism and thus can be taken as evidence for the capillary condensation theory. Additionally, the aging behaviour of PTMSP was investigated. A thin film composite was stored in the dark for 6 months under ambient conditions. As shown in Fig. 11, the permeability decreased by one order of magnitude during the first three months, after which the permeability stabilized. The selectivity remained stable during this period. Different techniques of artificial aging are discussed in the literature [7,9,10]. Experiments inducing artificial aging using UV-light or heat were carried out. After exposing PTMSP to UV-light for about 30 min, the material became brittle. A one night exposure to 360 K caused a decrease of the fluxes, but the membrane was not brittle. The effects of different aging techniques on PTMSP were different. So it is hardly possible to make any assumptions on the aging effects of PTMSP under working conditions, based on the changes of the properties at artificial conditions.
5. Conclusions Two polymers showed a butane/methane selectivity of at least 12 during mixed gas measurements: PTMSP and POMS-SSM.
Compared to the reference polymer PDMS, the selectivities and the permeabilities were improved. The POMS-SSM membranes showed a mixed gas selectivity of a~M= 12 and no compression effects. This was an overall improvement from the original POMS composites. Swelling hindrance led to better membrane performance and should be tried out for other types of silicon. PTMSP showed the highest selectivities and fluxes among the polymers studied. Over a 3 month period, the butane flux decreased, but the aged PTMSP was still a viable alternative to rubbery polymers for the C3 + separation. The mixed gas selectivity of 27 at standard conditions and a butane flux of 4 m3/m 2 h bar through a 5 /zm thick layer, makes even the aged PTMSP a superior polymer for this application. The price, the availability and a varying quality of PTMSP, however, make current usage problematic. In addition, the aging should be observed under working conditions. Considerably more information must be collected about the reasons and mechanisms of the aging process of PTMSP. If this material could be stabilized, it will be an important alternative for gas separation membranes applied in natural gas treatment. For the POMS-SSM and PTMSP composites, further experiments should investigate temperature dependencies and long term, high pressure measurements under field conditions.
6. List of symbols selectivity (ratio of fluxes) ( - ) thickness (usually of selective layer) ( ~ m ) pressure normalized flux (simply called flux) (m3/m 2 h bar) pressure (bar) P p* permeability coefficient (barrer) T temperature (K) 0 temperature (°C) glass transition temperature (K) X molar ratio (%) Subscripts B butane i Gas i M methane mix result of a mixed gas measurement Ol
d P/d
J. Schultz, K.-V. Peinemann / Journal of Membrane Science 110 (1996) 3 7 4 5
s
T
result of a pure gas measurement total (usually Z B, M)
References [ 1] R.M. B arrer and H.T. Chio, Solution and diffusion of gases and vapors in silicone rubber membranes, J. Polym. Sci. C, 10 (1965) 111-138. [2] C. Fiedler and H. Hantsche, Genauigkeit und Einsatzbereich verschiedenerVerfahren zu Dickenbestimmung dtinner Schichten, untersucht an TiN-beschichteter Werkstoffe, in U. Ehrenwerth (Hrsg.), Beitr~ige zur elektronennfikroskopischen Direktabbildung von Oberfl~ichen, Bd. 25, Verlag R.A. Remy, 1SSN 0340-3815, 1993, pp. 78-82. [3] L.G. Toy, I. Pinnau and W. Baker, A gas separation process for the removal of condensable organic vapors from gas mixtures, US Pat. 5, 281,255 (1994). [4] D. Fritsch, K.-V. Peinemann, R.-D. Behling, R. Just, Membran auf Basis von Graft-copolymeren, DE Pat., 42 132 17 AI (1993).
45
[5] W.J. Koros, R.T. Chem, H.B. Hopfenberg, V.T. Stannett: A model for permeation of mixed gases and vapors in glassy polymers, J. Polym. Sci., Polym. Phys. Ed., 19 ( 1981 ) 1513. [6] R. Srinivasan, S.R. Auvil and P.M. Burban, Elucidating the mechanism ( s ) of gas transport in poly [ 1- (trimethylsilyl) - 1propyne], J. Membrane Sci., 86 (1994) 67-86. [7] Yu.P. Yampol'skii, S.M. Shishatskii, V.P. Shantorovich, E.M. Antipov, N.N. Kuzmin. S.V. Rykov, V.L. Khodjaeva and N.A. Plat6. Transport characteristics and other physiochemical properties of aged poly(l-(trimethylsilyl)-l-propyne), J. Appl. Polym. Sci., 48 (1993) 1935-1944. [8] N.A. Plat& A.K. Bokaraev, N.E. Kalluzhnyi, E.G. Litvinova, V.S. Khotimskii, V.V. Volkov and Yu.P. Yampol'ski, Gas and vapor permeation and sorption in poly (trimethysilylpropyne), J. Membrane Sci., 60 ( 1991 ) 13-24. [ 9 ] M. Langsam and L.M. Robeson, Substituted propyne polymers. Part II. Effects of aging on the gas permeability properties of poly [1-(trimethylsilyl) propyne] for gas separation membranes, Polym. Eng. Sci., 29( 1) (1989) 44-54. [ 10] A.C. Savoca, A.D. Surnamer and C.-F. Tien, Gas transport in poly(silylpropynes): The chemical structure point of view, Macromolecules, 26 (1993) 6211~5216. [ 11 ] H. Rhim and S.-T. Hwang, Transport of capillary condensate, J. Colloid Interface Sci., 52( 1) (1975) 174--181.
Journal of Membrane Science 110 (1996) 37-45
Membranes for separation of higher hydrocarbons from methane J. Schultz *, K.-V. Peinemann GKSS-Forschungszentrum Geesthacht GmbH, D-21502 Geesthacht, Germany
Accepted 7 August 1995
Abstract In the search for a membrane material capable of separating higher hydrocarbons from methane about 40 different polymers were tested. The most promising two, polyoctylmethylsiloxane (POMS) and polytrimethylsilylpropyne (PTMSP), are compared in this article to the standard material polydimethylsiloxane (PDMS). The transport properties were investigated dependent on the process parameters temperature, feed pressure and feed composition under mixed gas conditions. It was possible to improve the separation performance by means of the selectivity from 5 to > 12 at acceptable fluxes. Keywords: Gas separation; Natural gas; Dew pointing; Mixed gas selectivity;Polyoctylmethylsiloxane;Polytrimethylsilylpropyne
1. Introduction A typical step in natural or raw gas treatment is the separation of propane and butane from the main component methane. Usually the removal of the higher hydrocarbons is done by pressure distillation or chilling out the condensable parts. This dewpointing process could possibly be done by butane selective membranes. One can easily integrate a membrane module in a gas processing plant utilizing the pipeline pressure of about 70 bar. One advantage of a membrane module would be a reduced requirement of cryogenic energy. Another advantage is that membrane modules are small. On off shore platforms, where optimizing space is crucial, the use of membranes might be especially beneficial. Several polymers and blends were tested for this application. The butane / methane selectivity (a~M) was chosen as the efficiency criteria for the membranes. A polydimethyl-siloxane composite membrane was * Corresponding author. Elsevier Science B.V. SSD10376-7388 ( 95 ) 00214-6
declared as the reference. It is a standard GKSS membrane and already in commercial use for the removal of hydrocarbons from waste air at petrol stations. The aim was to improve the properties of this reference membrane. The primary objective was to elevate the mixed gas butane/methane selectivity from 5, as shown by PDMS, to 12. Fluxes are of secondary importance but should not be less than 1 m3/m 2 h bar for butane. The butane flux of the reference PDMS composite membrane with a thickness of 10/.tm was 1.4 m3/m 2 h bar for a mixed gas measurement. 1.1. Experimental
More than 40 materials were tested during the polymer screening. Among these were imides, amidimides, kaptons, silanes, siloxanes, kraton, fluoroelastomers, butylketones, carbonates, sulfocarbonates and copolymers of the listed. Most of the tested polymers exhibited lower butane/methane selectivities than the reference and therefore will not be discussed.
38
J. Schultz, K.- V. Peinemann / Journal of Membrane Science 110 (1996) 37-45
A selection of the investigated polymers is shown below. These either show a better performance than the reference or behave somehow unusual, thus supplying valuable information for the separation problem. PDMS poly (dimethylsiloxane), the reference PDMS* PDMS radiation crosslinked POMS poly (octylmethylsiloxane) PHDMS poly (hexadecylmethylsiloxane) Carbosil polydimethylsiloxane-polycarbonate block copolymer Lestosil poly (dimethylsiloxanedi(phenylsiloxane) ) Kraton ® styrene-isoprene-styrene block copolymer Seragel polysulfone-polybutadiene block copolymer CMS perfluorodimethyldioxole-terafluorethylene copolymer PTMSP poly (trimethylsilylropyne) Also several blends of these polymers were tested. Thick films were prepared to measure the intrinsic properties of the polymers. Composite membranes were made by dip coating a mesoporous support in 1 to 10% polymer solutions and drying in air at room temperature. The support used was a microfiltration membrane made of a mesoporous polyetherimide layer on top of a polyester fabric. The surface porosity was about 5%. The transport resistance of this support was negligible compared to the dense layer of all used polymers. Gas fluxes and permselectivities were calculated from single gas measurements using the pressure increase method [ 1 ], typically at ambient feed pressure and permeate pressures up to 100 mbar. In gas mixture measurements, the feed, retentate and permeate were analysed with a selective detector (i.e. a mass spectrometer or a gas chromatograph). These experiments gave the data for more realistic conditions in terms of a mixed feed gas. The principal set up is shown in Fig. 1. Gas composition, stage cut and total pressure up to 10 bar were varied. All measurements were carded out at 30°C unless otherwise stated. All composites were first measured with a standard set of pure gases at 1 bar feed pressure: helium, nitrogen, oxygen, methane, propane and n-butane. The permeabilities were calculated from the steady state pressure increase with time. Further measurements were only carded out with defect free stamps. This information was based on the intrinsic oxygen/nitrogen selectivity measured for a thick, dense film. A
....................................T feed mixture
_
_
~
~
permeat~ _...~pressure
__~ mass- 1 t spectrometer) . . . . . . . . . . . . . . . .
'
@ vacuum pump Fig. 1. Set-upfor single and mixed gas measurements. composite with no defects must reach the intrinsic selectivity. The thickness of the selective layer was determined by scanning electron microscopy [ 2]. This method is not always accurate, especially when the coating penetrates the porous structure of the support. Another method is to calculate the thickness from the permeability coefficient divided by the flux for a quasiideal gas, which should not swell the membrane, such as nitrogen. If one assumes that a thin layer should have the same permeability coefficient as a thick film of the same polymer, the permeability coefficient (P*) measured for a dense film divided by the pressure normalized flux ( P / d ) , gives the thickness (d) of the measured thin top layer (d = P* / ( P / d ) ). This assumption is not necessarily always true, as will be seen later. The selectivities, i.e. the ratio of measured permeabilities, are denoted by a 's'-subscript for single (or pure) gas measurements and a 'mix'-subscript indicates a mixed gas measurement. The standard conditions for mixed gas measurements were 10 bar and 30°C for a feed gas of 3% butane in methane (97%).
2. Results The detected ideal selectivity (a~M.s) measured with pure gases, seldom predicts the mixed gas behaviour of the polymer accurately. A material showing a high pure gas selectivity a,aM.smay have a lower selectivity in the mixture a~M.m~xthan a material with a lower pure gas selectivity. The variation between the pure and mixed gas selectivities for various polymers is shown
J. Schultz, K.-V. Peinemann / Journal of Membrane Science 110 (1996)37~15
39
50
210
40
s
'~
0
1
mix
tN,
O3
Fig. 2. Pure (s) and mixed (mix) gas butane/methane-selectivities of different polymers.
PTMSP Poly (trimethyl-silyl-propyne)
POMS Poly (octyl-methyl-siloxane)
CHs
CH 3
-(-C -- CI~
-(-Si-O
HJC --sSi--CH ~
} CH3 PDMS Poly (dimethyl-siloxane)
CHs -(-Si--O -~ I CH3 Fig. 3. Chemical structures of PTMSP, POMS and PDMS.
in Fig. 2. Usually the mixed gas selectivity is lower than the pure gas selectivity. For PTMSP, however, the butane/methane selectivity was found to be higher in mixed gas experiments. This confirms recent results [ 3 ] discovering PTMSP as an advanced alternative to rubbery polymers for C3 + separation. From pure to mixed gas performance, CMS reverses its separation behaviour, so that methane turns into the faster component in a mixed gas application. Since the performance of CMS was poor, these effects were not
investigated further. It was taken as an indication of transport interference by different species. Consequently, mixed gas measurements were necessary for adequate characterisation of membrane materials for the current application. The most promising polymer, PHDMS, with a pure gas selectivity of a~. s > 200 showed a poor selectivity of 2 in mixed gas tests. It was interpreted to be the result of swelling of the polymer by butane. Silicones are known to swell due to the sorption of vapours. When vapours are sorbed, the
40
J. Schultz, K.- V. Peinemann / Journal of Membrane Science 110 (1996)37-45
attractive chain-to-chain forces decrease and thus the free volume increases• This causes lower transport resistance for any species diffusing through the swollen polymer. Hence, the sorption of butane facilitates the methane transport in rubbery polymers in general. Therefore, the butane/methane selectivity is lower during mixed gas measurements than pure gas measurements. Thus, the solubility coefficient is the determining factor of the separation properties during mixed gas performance. This is the general behaviour for rubbery and most of the glassy polymers• An exception is the glassy PTMSP because of a different transport mechanism• Because of the previously mentioned difference between pure and mixed gas selectivities, the following discussion will be limited to mixed gas butane/methane selectivity unless otherwise stated. Based on these first results we chose two promising polymers for further investigation. POMS and PTMSP showed the highest mixed gas selectivities. Their chemical structure in comparison with PDMS is shown in Fig. 3. POMS is a silicone with a glass transition temperature far below the experimental temperatures• It is a derivative of the well known PDMS where some of the methyl-groups have been substituted by Cs-sidechains• PTMSP is a unique glassy polymer with a glass transition temperature above 540 K. Because of its stiff backbone, in combination with the bulky S i ( C H 3 ) 3 side-group, PTMSP is known to have a fairly high fractional free volume > 25%. The gas permeability coefficients were one order of magnitude higher than the ones of PDMS. Its butane/methane selectivity was higher in mixed gas measurements than the ideal selectivity measured with single gases (see Fig. 2). Since only composites were used for further experiments, mass transfer will only be discussed in terms of pressure normalized fluxes, which will simply be called fluxes• The thickness is limited by the preparation procedure anyway• However, determination of the thickness problems will be discussed with POMS.
of the flux on the total feed pressure. Usually, increased fluxes would be expected at higher partial pressures of swelling vapours. This is due to a higher sorption and therefore reduced transport resistance in the polymer. POMS, however, showed decreased fluxes at higher pressures, and therefore, higher butane partial pressures. This was interpreted as two interfering effects caused by swelling. POMS is a very soft material. It is not possible, for example, to prepare a dense thick film from POMS. For such kind of a polymer, swelling lowers the transport resistance as a function of butane partial pressure. Simultaneously, the POMS layer is softened, and therefore, densified by compression as a function of the increased total feed pressure. For the soft POMS layer, the compressibility was the dominant factor of the two and thus the flux decreased as a function of the total feed pressure as shown in Fig. 4. Even though the fluxes of POMS are reduced at higher pressures it is still one of the most selective rubbery poly12
, 15
total feed pressure
different POMS blends 13•
POMS-SSM O'
12
.0.
10
3. POMS
ongm
9
; Varying the mixed gas process parameters (i.e. total feed pressure p, molar ratio of the feed gas components xi and temperature O), it was found that the flux L is not constant• POMS [4] showed a strong dependence
[ bar]
Fig. 4. Pressure behaviour of the original POMS, where the decrease of fluxes is due to compression: ~ , methane; 0 , butane; × , selectivity.
;butane flux
m''l
[m z-h-bar|
;
Fig. 5. Improvement of the performance of POMS composites leading to higher selectivities and fluxes. The permeability coefficient could not be calculated because of inaccurate thickness determination.
J. Schultz, K. - V. Peinemann / Journal of Membrane Science 110 (1996) 37-45
41
lxr
Fig. 6. (Left) Cross section/original POMS The polymer forms a dense layer above the mesoporoussupport and penetrates the pores. The top layer is mechanicallysoft, thus compressible. Fig. 7. (Right) Cross section/(POMS-SSM) The POMS layer is entirely encapsulated in the pores of the support. Thereby the polymer is stabilized and does not exhibit compressibility. mers performing a butane/methane selectivity of 10. An indication for the theory of simultaneous swelling and compression is the different behaviour of an advanced type of POMS membrane. By using different crosslinking agents, varying the amount of pre-crosslinking before coating the mesoporous support and adding certain silicones as blend materials, it was possible to simultaneously increase the selectivity (from 10 to 12) and the permeability. The development of optimizing the membrane is shown in Fig. 5. The coating parameters had a strong influence on the formation of the top layer. As can be seen from a scanning electron microscopic picture (Fig. 6), the original POMS formed a dense layer of approximately 3/xm on top of the mesoporous support. In between the support layer and the dense top layer is a third layer, where the polymer penetrated the pores of the support. In the case of the advanced POMS composites, with a selectivity of 12, the top layer disappeared totally (Fig. 7). The selective layer was now entirely located inside the upper pores of the support.
Thus the swelling was hindered and therefore the increase of diffusivity for methane, compared to pure gas measurements, was lower than for the original POMS composites. The swelling effect was reduced by framing the polymer inside the pores of the substructure. The resulting membrane was not a typical composite membrane. One might call it a sub surface membrane (SSM). The pressure dependency of the original POMS composite, shown in Fig. 4, which resulted in a decrease of fluxes at higher pressures, was not detected for the POMS-SSM. The flux of gases through POMS-SSM as a function of feed pressure is plotted in Fig. 8. It is impossible to determine an effective thickness of the selective layer of a SSM. Once the polymer was inside the pores, it changed its properties. The butane/ methane selectivity was higher (see Fig. 4) and the decreasing flux at increased feed pressures as measured for original POMS was not found for POMS-SSM (see Fig. 8). Hence, it is a different material in terms of transport properties. Therefore it is not accurate to nor-
42
J. Schultz, K.-V. Peinemann / Journal of Membrane Science 110 (1996) 37-45
14 P 12 10 . ~
~6
8 .~
.~ 4 4
i
2 2
4
6
feed pressure
8
10
12
[ bar ]
Fig. 8. POMS-SSM flux versus pressure. No compression effects were detected; polymer stabilized by encapsulation (compare Fig. 4).
malize the flux of POMS-SSM on the permeability coefficient of 'free' POMS because of the different permeability coefficients of one polymer in two different states. And it is obviously impossible to prepare a dense, homogeneous film of an encapsulated polymer. Using the second possible determination method, i.e. calculating an effective thickness from a SEM picture, would require deriving the thickness of a layer of fluids inside a porous matrix. That method is not accurate either, because the polymer only fills part of the visible layer. Tortuosity and porosity of the penetrated layer were unknown, since pore determining measurements could never give proper information about only one layer of an entire, asymmetric structure. In conclusion the experimental results showed no compressibility of the POMS-SSM membrane. The polymer seemed to be mechanically stabilized and supported by the pore system. With the invention of the POMS-SSM, the but~ine/methane selectivity from mixed gas tests was enhanced to a value of 12. The butane flux for this advanced type of POMS membrane was approximately 4.5 m3/m 2 h bar. The POMS-SSM was produced on a coating machine in technical scale, thus it is available in large amounts at reasonable pricing.
4. P T M S P
As an alternative to rubbery polymers PTMSP was found. This glassy polymer, with a glass transition temperature of Tg > 250°C and an extraordinary high free volume, showed unusual properties. A mixed gas selectivity, higher than the ideal selectivity, was measured
in pure gas tests. Additional experiments showed an increase of the selectivity with decreasing temperature. The transport behaviour of PTMSP can be represented either by an extended dual-mode mechanism introduced by Koros et al. [ 5 ] or by selective surface sorption. The extended dual mode model would explain the flux depression for methane in the presence of butane by competitive sorption in the Langmuir domains. Since the dual mode parameters have not been measured, the approach could not be verified. Explaining this flux depression by pore blocking, would lead to effects near to capillary condensation, such as competitive surface adsorption. A population of the more condensable species adsorbed on the inner surface of microcavities was assumed to be responsible for the reduced methane flux in the presence of butane. The reason for hypothesizing a kind of a capillary condensation, were results of pure gas measurements carried out for PTMSP. The flux of butane was measured versus feed pressure in the subambient range, as shown in Fig. 9a. The exhibited maximum is comparable to the behaviour of a typical porous material in which capillary condensation occurs, e.g. Vycore glass. The similarly shaped curves, as can be seen in Fig. 9a + b, were taken as evidence for capillary condensation. Unlike the original POMS, the total feed pressure had no influence on the transport properties of PTMSP, but the butane percentage in the feed caused a significant change in the mixed gas behaviour of PTMSP. This correlation is presented in Fig. 10. Experiments elevating the total feed pressure incrementally from 2-10 bar at a given fraction of butane, thus increasing the partial pressure of butane as well, showed almost no influence of the total feed pressure on the fluxes. The butane flux remained constant during these measurements. Usually greater capillary condensation at higher partial pressures of the condensable gas is expected. To elevate the partial pressure, one can either increase the total pressure of a constant composition feed mixture, or increase the butane fraction at a fixed total feed pressure. In the case where the higher partial pressure was achieved by higher total pressure of the same feed mixture, the effect on the selectivity was not significant. In the case where the fraction of butane was increased at constant total pressure (see Fig. 10), the butane/methane selectivity increased simultaneously. Thus, the selectivity was more a function of the butane fraction rather than of butane partial
43
J. Schultz, K.-V. Peinemann/Journal of Membrane Science 110 (1996)37-45
o.I3
..5
8
(b)
t
E
200 t::
i
100 (/3 v
0 0
1000
feed pressure
O0
2000
I
I
I
t
0.2
0.4
0.6
0 8
Relofive Meon Pressure
Pm/Po
[ mbar ]
Fig. 9. Butane permeability as a function of feed pressure, exhibited maximum is a result of superposition of transport mechanism in capillary condensation, which is similar for both. (a) PTMSP, (b) Vycore glass (quoted from [ 11 ] ).
pressure. Further experiments should be carried out to give a better understanding of this behaviour. Some points should be remarked concerning Fig. 10. At standard conditions for the mixed gas measurements (10 bar, 30°C, 3% C4) the butane flux was 35 m3/m 2 h bar and the selectivity was 27. The thickness of the selective layer of the measured composite membrane was 3 to 5 /xm. The butane/methane selectivity decreased from 35 to 18 as the butane concentration decreased from 9% to 0.5%. While removing the butane, the selectivity decreased. For the application i.e. the removal of higher hydrocarbons, which includes butane - this is undesirable. But a selectivity of 18 at a concentration of 0.5% butane in the feed is still a remarkable performance. 0.5% butane is equal to a pressure dewpoint below - 20°C, which is a required pipeline standard. m' ~J30
140
The decrease of selectivity with decreasing butane amount in the feed, support the theory of a 'pre-existing free volume' as a kind of micro-cavities in the material [ 6]. Other groups measured the fractional free volume of PTMSP to be higher than 25% [7,8], which is also the reason for the extraordinary high fluxes. These micro-cavities might allow capillary condensation of the more condensable parts in the gas mixture according to the Kelvin equation. Transport inside a porous matrix where capillary condensation occurs, is a superposition of a four domain mechanism: • diffusion in the dense polymer regions (absorbed species) • diffusion in empty caverns (gaseous species) • surface diffusion (adsorbed species) • diffusion in a condensed phase (solved species)
• 40
~
0=293K x B = 0.5%
•
je
10
10
0
~ 0 50 100 150 200 stored at ambient conditions [d] Fig. 11. Aging effects of PTMSP during storage at ambient conditions. Fluxes decreased by one order of magnitude after 3 months while the butane/methane selectivity remained stable: O, butane flux; O, selectivity. 0
• 0
•
30 ee
0 =303K
10
p'T=4 I ~ 0 1
butane fraction in the feed [ % ] Fig. 10. Mixed gas selectivity of PTMSP as a function of the butane fraction in the feed. The mixed gas selectivity decreased with the fraction of butane in the feed. The marginal zero-butane value for the selectivity was 18.
44
J. Schultz, K.-V. Peinemann / Journal of Membrane Science 110 (1996) 37-45
If butane condensed in the pores, then the surface diffusion and diffusion of methane through pores filled with condensate, would have been hindered. A liquid butane barrier, however, is not much of a transport resistance to butane molecules. Thus the methane flux would be reduced by the presence of butane while the butane flux was almost not affected. The experimental data supported this hypothetical transport mechanism. These experiments showed that the increase in selectivity was mainly determined by a reduced methane flux due to pore blocking by the condensed butane. A further indication of capillary condensation in PTMSP was found in the literature about porous solids, as previously mentioned [ 11 ]. In that paper on the transport of capillary condensate, butane transport in vycore glass was studied and butane flux versus feed pressure was plotted for a pure gas measurement. The curve exhibited a maximum as a result of the combined transport mechanism (see Fig. 9b). In Fig. 9a the results of butane flux versus pressure are shown for PTMSP as carried out in the current work. The similar shape of the PTMSP curve is a strong indication of a similar transport mechanism and thus can be taken as evidence for the capillary condensation theory. Additionally, the aging behaviour of PTMSP was investigated. A thin film composite was stored in the dark for 6 months under ambient conditions. As shown in Fig. 11, the permeability decreased by one order of magnitude during the first three months, after which the permeability stabilized. The selectivity remained stable during this period. Different techniques of artificial aging are discussed in the literature [7,9,10]. Experiments inducing artificial aging using UV-light or heat were carried out. After exposing PTMSP to UV-light for about 30 min, the material became brittle. A one night exposure to 360 K caused a decrease of the fluxes, but the membrane was not brittle. The effects of different aging techniques on PTMSP were different. So it is hardly possible to make any assumptions on the aging effects of PTMSP under working conditions, based on the changes of the properties at artificial conditions.
5. Conclusions Two polymers showed a butane/methane selectivity of at least 12 during mixed gas measurements: PTMSP and POMS-SSM.
Compared to the reference polymer PDMS, the selectivities and the permeabilities were improved. The POMS-SSM membranes showed a mixed gas selectivity of a~M= 12 and no compression effects. This was an overall improvement from the original POMS composites. Swelling hindrance led to better membrane performance and should be tried out for other types of silicon. PTMSP showed the highest selectivities and fluxes among the polymers studied. Over a 3 month period, the butane flux decreased, but the aged PTMSP was still a viable alternative to rubbery polymers for the C3 + separation. The mixed gas selectivity of 27 at standard conditions and a butane flux of 4 m3/m 2 h bar through a 5 /zm thick layer, makes even the aged PTMSP a superior polymer for this application. The price, the availability and a varying quality of PTMSP, however, make current usage problematic. In addition, the aging should be observed under working conditions. Considerably more information must be collected about the reasons and mechanisms of the aging process of PTMSP. If this material could be stabilized, it will be an important alternative for gas separation membranes applied in natural gas treatment. For the POMS-SSM and PTMSP composites, further experiments should investigate temperature dependencies and long term, high pressure measurements under field conditions.
6. List of symbols selectivity (ratio of fluxes) ( - ) thickness (usually of selective layer) ( ~ m ) pressure normalized flux (simply called flux) (m3/m 2 h bar) pressure (bar) P p* permeability coefficient (barrer) T temperature (K) 0 temperature (°C) glass transition temperature (K) X molar ratio (%) Subscripts B butane i Gas i M methane mix result of a mixed gas measurement Ol
d P/d
J. Schultz, K.-V. Peinemann / Journal of Membrane Science 110 (1996) 3 7 4 5
s
T
result of a pure gas measurement total (usually Z B, M)
References [ 1] R.M. B arrer and H.T. Chio, Solution and diffusion of gases and vapors in silicone rubber membranes, J. Polym. Sci. C, 10 (1965) 111-138. [2] C. Fiedler and H. Hantsche, Genauigkeit und Einsatzbereich verschiedenerVerfahren zu Dickenbestimmung dtinner Schichten, untersucht an TiN-beschichteter Werkstoffe, in U. Ehrenwerth (Hrsg.), Beitr~ige zur elektronennfikroskopischen Direktabbildung von Oberfl~ichen, Bd. 25, Verlag R.A. Remy, 1SSN 0340-3815, 1993, pp. 78-82. [3] L.G. Toy, I. Pinnau and W. Baker, A gas separation process for the removal of condensable organic vapors from gas mixtures, US Pat. 5, 281,255 (1994). [4] D. Fritsch, K.-V. Peinemann, R.-D. Behling, R. Just, Membran auf Basis von Graft-copolymeren, DE Pat., 42 132 17 AI (1993).
45
[5] W.J. Koros, R.T. Chem, H.B. Hopfenberg, V.T. Stannett: A model for permeation of mixed gases and vapors in glassy polymers, J. Polym. Sci., Polym. Phys. Ed., 19 ( 1981 ) 1513. [6] R. Srinivasan, S.R. Auvil and P.M. Burban, Elucidating the mechanism ( s ) of gas transport in poly [ 1- (trimethylsilyl) - 1propyne], J. Membrane Sci., 86 (1994) 67-86. [7] Yu.P. Yampol'skii, S.M. Shishatskii, V.P. Shantorovich, E.M. Antipov, N.N. Kuzmin. S.V. Rykov, V.L. Khodjaeva and N.A. Plat6. Transport characteristics and other physiochemical properties of aged poly(l-(trimethylsilyl)-l-propyne), J. Appl. Polym. Sci., 48 (1993) 1935-1944. [8] N.A. Plat& A.K. Bokaraev, N.E. Kalluzhnyi, E.G. Litvinova, V.S. Khotimskii, V.V. Volkov and Yu.P. Yampol'ski, Gas and vapor permeation and sorption in poly (trimethysilylpropyne), J. Membrane Sci., 60 ( 1991 ) 13-24. [ 9 ] M. Langsam and L.M. Robeson, Substituted propyne polymers. Part II. Effects of aging on the gas permeability properties of poly [1-(trimethylsilyl) propyne] for gas separation membranes, Polym. Eng. Sci., 29( 1) (1989) 44-54. [ 10] A.C. Savoca, A.D. Surnamer and C.-F. Tien, Gas transport in poly(silylpropynes): The chemical structure point of view, Macromolecules, 26 (1993) 6211~5216. [ 11 ] H. Rhim and S.-T. Hwang, Transport of capillary condensate, J. Colloid Interface Sci., 52( 1) (1975) 174--181.