Performance of silicone-coated polymeric membrane in separation of hydrocarbons and nitrogen mixtures

Journal of Membrane Science 254 (2005) 179–188

Performance of silicone-coated polymeric membranein separation of hydrocarbons and nitrogen mixtures�

Xin Jiang, Ashwani Kumar∗

Institute for Chemical Process and Environmental Technology, National Research Council of Canada,M-12 Montreal Road Campus, Ottawa, Ont., Canada K1A 0R6

Received 22 October 2003; received in revised form 28 September 2004; accepted 13 December 2004Available online 11 February 2005

Abstract

This study reports selectivities and permeances of pure nitrogen, oxygen, ethylene, ethane, propylene and propane and their mixturesthrough composite poly(dimethylsiloxane) (PDMS)–polysulfone membrane at ambient temperature. It was observed that both propyleneand/or propane significantly plasticized PDMS coating in pure as well as mixed gas permeation experiments. Above the plasticizationpressure, the permeance order wasPC > PC= > PC > PC= > PO > PN , which matched the solubility order. However, permeance or-d erme-a opylene andpC

K

1

fgioDefbhmonM

on

is ava-

s glassrdedrnt at

ndSft per-

ofane,MS.

panetranterene-sure

0d

3 3 2 2 2 2

er was changed toPC=3

> PC3 > PC2 > PC=2

> PO2 > PN2 below the plasticization pressure, showing that propylene was more pble than propane. Furthermore, plasticization caused coupling effects for ethylene, ethane and nitrogen in the presence of prropane.rown Copyright © 2005 Published by Elsevier B.V. All rights reserved.

eywords:Polysulfone; PDMS; Permeance; Selectivity; Membrane; Hydrocarbons; Nitrogen

. Introduction

The applications of rubbery polymer-coated membranesor separating volatile organic components (VOCs) from aas stream have been reported. Various industries are us-

ng more than 100 units supplied by Membrane Technol-gy and Research Inc. (MTR), US; GKSS, Germany; andalian Institute of Chemical Physics (DICP), China. An av-rage market growth of 8–10% per year has been mentioned

or these applications. It has been realized that the mem-ranes are often key separation units resulting in superiorigh-value products, substantial savings in energy and rawaterials. There are possibilities of numerous applicationsf such processes for recovering hydrocarbons and recyclingitrogen in the petroleum and polymer synthesis industries.ore fundamental research has been done to understand

� NRCC No. 46482.∗ Corresponding author. Tel.: +1 613 998 0498; fax: +1 613 941 2529.E-mail address:[email protected] (A. Kumar).

the sorption, diffusion and permeation effects of VOCsthe membranes coated with rubbery polymers[1–6]. Amongthe rubbery polymers, poly(dimethylsiloxane) (PDMS)solubility-selective polymer that is more permeable topors (condensable) than to gases (non-condensable). Ittransition temperature is among the lowest values recofor polymers (−129◦C) indicating a very flexible polymebackbone with long-range segmental motion active evevery low temperatures[7,8]. Many studies of chlorinated aother VOCs/N2 as well as CO2/N2 mixtures through PDMmembranes have been performed[9–12], however, most othe researchers have concentrated on pure componenmeations. Merkel et al.[13] reported the permeabilitiespure hydrogen, oxygen, nitrogen, carbon dioxide, methethane, propane and their perflourocarbons through PDThey found that there was strong plasticization as propenetrated the PDMS membrane. An increase in penediffusivity was believed to occur from increased polymlocal segmental motion caused by the presence of ptrant molecules in the polymer matrix. As penetrant pres

376-7388/$ – see front matter. Crown Copyright © 2005 Published by Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2004.12.041

180 X. Jiang, A. Kumar / Journal of Membrane Science 254 (2005) 179–188

Table 1Some physical properties of gases and vapors[14]

Gas Boiling point (101,325 Pa (1 atm = 101,325 Pa)) Critical parameters

Temperature (◦C) Temperature (◦C) Volume (cm3/mol)

Oxygen −183.1 −118.7 73.0Nitrogen −195.9 −147.1 90.0Ethylene −103.9 9.5 131.1Ethane −88.7 32.0 145.5Propylene −47.8 91.6 184.6Propane −42.2 96.5 200.0

and, therefore, the penetrant concentration in the polymer in-creases, the tendency to plasticize a polymer matrix increasesparticularly for strongly sorbing penetrants. For designing amembrane process to separate a gas mixture, the fundamentalpermeation parameters of the gases present in the mixture willprovide more meaningful data. However, there is a generallack of such data in literature on separation of hydrocarbonsfrom nitrogen. The present work reports the permeation oflower hydrocarbons such as ethylene, ethane, propylene andpropane from lean binary, ternary and quaternary mixtures innitrogen through composite PDMS–polysulfone membraneat an ambient temperature of 22◦C and a total pressure of650 kPa (g).

2. Experimental apparatus

The gas permeation apparatus fabricated in our labora-tory is a standard constant-pressure permeation design. Itcomprised of three sections namely feed preparation, mem-brane cell and data collection. A desired composition andflow rate of a hydrocarbons and nitrogen mixture were pro-duced in the feed preparation section before the membranecell. There were four mass flow controllers and a read-out, which had controls to set a fixed ratio of hydrocar-b erer nef reo parec teringa ranew ssS po-

ration (Minnetonka, MN). Pure gases or mixtures penetratedthe membrane in the cell at a desired pressure. The reject gasstream was maintained at nearly the same pressure as feed gasin the cell and was expelled from another end of the SEPA®

CF Membrane Cell through a solenoid valve. The data collec-tion section had devices to measure pressures, temperaturesand compositions of gas mixtures for feed, reject as well aspermeate gas streams. Hewlett-Packard 6890 gas chromato-graph equipped with a thermal conductivity detector, a sam-ple injector (six-port valve) and microcapillary column wasused to determine the compositions of both permeate and re-ject streams. The precise thermocouple (type C) and pressuretransducer were installed in the feed line. Similarly, temper-ature, pressure and flow rate in the reject and permeate lineswere also recorded. All pressure transducers, mass flow me-ters and controllers with associated readouts were suppliedby MKS Instruments Corporation (Methuen, MA).

3. Materials

Two filler-free composite poly(dimethylsiloxane) mem-branes labeled as A and B, supplied by different laboratorieswere used for pure and mixed gas permeation experiments.Membrane A consisted of a highly microporous polysulfones eB oatedw ap a-t opy-l d int listedi

TC ses an

M 700M 218[

N

embra

on/nitrogen mixtures. Three mass flow controllers wated for 5000 cm3 (STP)/min of nitrogen equivalent and oor 2000 cm3 (STP)/min of nitrogen equivalent. The mixtur pure gas was allowed in the ballast volume to preonstant feed pressure and/or concentration before ent one end of the membrane cell. The flat sheet membith an effective area of 135 cm2 was evaluated in a stainleEPA® CF Membrane Cell, supplied by Osmonics Cor

able 2omparison of membranes’ properties with reported data for pure ga

Permeance (GPU)a

N2 O2 C=2 C2 C=

3

embrane A 117 217 727 840 1embrane B 25 40 94 105

13]b 11 23 NA 102 NA

A: not available.a 1 GPU = 10−6 cm3 (STP)/(cm2 s cmHg).b A filler-free film (35�m thick) on a highly microporous support by M

upport coated with a 0.20�m thick PDMS layer. Membranconsisted of dense homogenous polysulfone support cith a 0.45�m thick PDMS layer. They were cast fromolysulfoneN-methyl pyrrolidone (NMP) solution by gel

ion in cold water. Nitrogen, oxygen, ethylene, ethane, prene and propane with at least 99.8% purity were utilizehis work. Some physical properties of these gases aren Table 1.

d hydrocarbons at 275 kPa (g)

Selectivity (gas/nitrogen)

C3 O2 C=2 C2 C=

3 C3

1970 1.9 6.3 7.3 16.2 16.8143 1.6 3.9 4.3 8.9 5.9

252 2.0 NA 9.0 NA 22.3

ne Technology and Research Inc. at 35◦C.

X. Jiang, A. Kumar / Journal of Membrane Science 254 (2005) 179–188 181

Fig. 1. Effects of pressure difference on permeance for pure gases and hydrocarbons.

4. Procedure

The membrane performances were characterized in termsof permeance (pressure normalized flux) and selectivity[15,16]. For pure gas experiments, both upstream and down-stream lines of the membrane cell were purged with pene-trant gas prior to a permeation experiment. A steady statewas deemed to be achieved when variation in the permeategas flow rate was less than 2%. At steady state, the perme-ance could be evaluated at a given feed pressure difference,which could be raised up to 730 kPa (g) for each of the pene-trant gases studied. Selectivity was calculated for pure hydro-carbon over nitrogen. For gas mixture experiments, desiredcompositions were prepared in the feed preparation section.After set pressure and temperature were achieved, these gas

mixtures were gradually transferred to membrane cell fromthe by-pass line. Subsequently, the solenoid valve on rejectline was adjusted to maintain the pressure at upstream ofmembrane cell and the ratio of permeate flow rate to feedflow rate (stage cut). In order to minimize concentration po-larization effects and the concentration differences betweenfeed and reject gases, higher flow rates of feed gases wereused for the mixed gas permeation experiments. The stagecut was maintained at less than 0.20 except in those experi-ments that contained C3 hydrocarbons and larger than 35%hydrocarbons. In those gas mixtures, the stage cut was alwayskept at less than 0.28. This was due to higher permeances forall gases (Table 2) and large membrane area (135 cm2). Flowrates, pressures and temperatures of feed, permeate and rejectstreams were determined. At steady state, the compositions

the mix
Fig. 2. Effects of total feed hydrocarbon concentration of ture on permeances of ethylene, propylene, propane and nitrogen.


Fig. 3. Effects of total feed hydrocarbon concentration of the mixture on permeance of ethane, propylene, propane and nitrogen.

of permeate and reject gases were analyzed by the gas chro-matograph. The steady state was assumed when the variationin the compositions and flow rates of permeate as well as re-ject gases was less than 3.0%. Permeance of each componentin the mixture was determined by utilizing an average par-tial pressure difference, which was calculated by subtractingpartial pressure of the permeating gases from the average par-tial pressure of feed and reject gases. These partial pressureswere changed by varying nitrogen concentration (52–98%)at a constant feed gas pressure. It is possible that gases on thepermeate side might not be immediately well mixed, whichcould lead to underestimation of the permeance values. In ourwork, selectivity was defined as the ratio of the permeancesof individual hydrocarbon over nitrogen measured in a mix-ture simultaneously. All concentration units were moleculepercent (mol.%). Variation in the mass balance of feed gas topermeate and reject gases for each experiment was less than10%.

5. Results and discussion

5.1. Physical properties of PDMS composite membrane

A variation in the morphology of composite membranesm tivityo rtiesa rel raneA tured /orh

d ins es ofo se ofm itro-g gher

resistance of the support layer and/or partial fill-in due to pen-etration of the support layer by the coatings. Membrane A hashigher selectivities for hydrocarbons over nitrogen, which arecomparable to reported data.

In composite membranes the support is usually kept at10–100 times more permeable than the selective film to en-sure that the separation properties are determined by the rub-bery layer and not by the support[17,18]. Our experimentaldata support this observation. It is obvious fromTable 2thatmembrane A gives superior performance. Consequently, thismembrane was selected for detailed investigation. Permeanceversus applied pressure difference for pure gases and hydro-carbons are plotted inFig. 1. It is clear that hydrocarbonpermeances increase with increasing pressure, whereas per-meances of non-condensable gases were pressure indepen-dent. This appears to be a typical solubility-selective mem-brane gas separation in rubbery polymer. Above an appliedpressure of 114.6 kPa (g), the permeance of propane washigher than that of propylene. The permeances of propyleneand propane increased significantly, indicating that propy-lene and propane sorbed in PDMS might loosen its poly-mer matrix, and cause plasticization continuously abovethis pressure[19,20]. Permeance order wasPC3 > PC=

3>

PC2 > PC=2

> PO2 > PN2. This matches the order of solu-bility for hydrocarbons and gases in the polymer accordingto critical temperatures (Table 1) that are indicators for sol-

TS

N

CCCCCCCC

ade from same materials could impact the permselecf gas and hydrocarbons remarkably. Membranes’ propend selected reported data[13] for PDMS membranes a

isted inTable 2. It is seen that the permeances of membare much larger than those of membrane B and litera

ata[13] owing to either extremely thin coating layer andighly microporous support.

Considering that same polymer materials were utilizeeparating layers of these membranes, their selectivitixygen over nitrogen are comparable except in the caembrane B. The selectivities of hydrocarbons over nen for membrane B are lower obviously due to the hi

able 3ummary of names and compositions of various feed gas mixtures

ame Feed composition (mol.%)

3 ternary plus C=2 N2 (61–95), C=3 (2–14), C3 (1–15), C=2 (2–10)

3 ternary plus C2 N2 (60–93), C=3 (2–14), C3 (1–15), C2 (2–10)

3 ternary N2 (66–96), C=3 (2–18), C3 (2–16)

3 binary N2 (74–98), C3 (2–26)=3 binary N2 (83–98), C=3 (2–17)

2 ternary N2 (52–97), C=2 (2–27), C2 (1–21)

2 binary N2 (68–97), C2 (3–32)=2 binary N2 (62–98), C=2 (2–38)


Fig. 4. Effects of pressure differences on propylene selectivities for various feed compositions (Table 3).

ubilities at any temperature and pressure[21,22]. A simi-lar trend for nitrogen, ethane and propane had been alsoreported[13]. However, at an applied pressure of lowerthan 114.6 kPa (g), the permeance order was found to bePC=

3> PC3 > PC2 > PC=

2> PO2 > PN2. Nitrogen, oxygen

and hydrocarbons except propylene still followed the order ofsolubilities and propylene was more permeable than propane.This observation would suggest that the diffusivity of propy-lene was dominant and larger than that of propane due to itssmaller molecular size. Tanaka et al.[23] reported similarresult for pure propylene and propane in dense PDMS mem-brane at 196.1 kPa and 50◦C, where plasticization effectscan be assumed to be minimal or non-existent. Therefore,the pressure where permeance order reverses for C3 com-ponents is termed as plasticization pressure[20,24]. This

behavior was observed in the permeation of gas mixturealso.

5.2. Hydrocarbon permselectivity in gas mixtures

Two sets of several gas mixtures composed of equal con-centrations of individual hydrocarbons were prepared for thepermeation experiments. The total feed hydrocarbon concen-trations were in the range of 3–33%.Fig. 2 displays per-meance versus the total hydrocarbon concentration in feedgas for first set containing a mixture of ethylene, propylene,propane and nitrogen. The permeance order was observedto bePC=

3> PC3 > PC=

2> PN2. Hydrocarbon permeances

increased with increasing hydrocarbon concentrations (par-tial pressures), however, the permeances of propylene and

pane s
Fig. 5. Effects of pressure difference on pro electivities of various feed compositions (Table 3).


Fig. 6. Effects of pressure difference on ethylene selectivities of various feed compositions (Table 3).

propane were comparable once the concentration was largerthan 27%. Nitrogen permeance followed a linear relationshipwith concentration and it only rose to 19.0% during this con-centration increase. Each gas showed similar tendency andappeared to behave like pure gases at applied pressures thatwere lower than plasticization pressure. A similar trend wasobserved in variation of permeance with increase in total hy-drocarbon concentration (Fig. 3) for the second set compris-ing of a mixture of ethane, propylene, propane and nitrogen.The permeances of propylene and propane were higher thanthose observed for the gas mixture of ethylene, propylene,propane and nitrogen, and also permeance order changed at13.3% of total hydrocarbon concentration for propane andpropylene, indicating that permeation behavior in the mix-tures was same as in pure gases. The permeance of nitro-gen increased up to 33.9% with increasing total hydrocar-bon concentration in feed gas. This could be due to the factthat ethane is more condensable than ethylene. Hydrocarbonsorption into PDMS coating layer would cause significantswelling in the chain packing and enhance penetrant mobil-ity. Moreover, based on a report of coupling effect by Yeomet al. [12] in CO2/N2 gas mixture through a PDMS mem-brane, the presence of large hydrocarbons may influence thepermeation of other gases in a mixture by plasticization andsorption non-ideality in our work. In addition, the permeancesof hydrocarbons in gas mixtures were significantly lower thanc com-

petition amongst hydrocarbons present in the gas mixture forthe limited number of active sites in the polymer, which de-creased their permeance values[12].

In order to investigate plasticization and coupling ef-fects on permeation, systematic experiments with a seriesof gaseous mixtures were conducted as outlined inTable 3.

Effects of selectivities of propylene and propane on theirpartial pressure differences or pressure differences for a se-ries of gas mixtures with different compositions and purehydrocarbons are shown inFigs. 4 and 5, respectively. Itis clear from these figures that selectivities for both propy-lene and propane in gas mixtures were significantly lower(36–39%) than their pure hydrocarbon selectivities. The se-lectivity values were comparable and would not be obviouslydistinguished whether C2 components were present or not,which indicated that C2 components could have not con-tributed to the plasticization once the PDMS had been plasti-cized enough by C3 components as mentioned earlier. Similarresults were reported by Chan et al.[25] for co-polyimidessynthesized from 6-FDA and 1,5-NDA (naphthalene)/Durenediamines. These authors reported no significant plasticizationby C2 and C=

2 even at an applied pressure of up to 1621 kPa,whereas significant plasticization was observed with C3 andC=

3 . Figs. 6 and 7show the comparison of selectivities forethylene and ethane in several gas mixtures with respec-tive pure hydrocarbons as their partial pressure differenceso d that

TS ses for

N

M e C=3 and

M ease C=3 an ations

M .9%, reM 17.0%,

orresponding pure gas values. This could be due to the

able 4ummary of names and compositions for mixtures of various feed ga

ame Conditions

ixture 1 Fix C=2 between 4.7and 5.9%, increas

ixture 2 Fix C=2 between 11.0 and 18.0%, incr

ixture 3 Fix both C=3 and C3 between 4.6 and 5

ixture 4 Fix both C=3 and C3 between 13.0 and

r pressure differences were increased. It was observe

swelling studies

C3 each in the range of 2–17% and maintaining same concentrationsd C3 each in the range of 1.5–19.5% and maintaining same concentrspectively, increase C=

2 in the range of 2.0–16.0%respectively, increase C=

2 in the range of 2.0–16.0%


Fig. 7. Effects of pressure difference on ethane selectivities of various feed compositions (Table 3).

C2 selectivities in the gas mixtures containing C3 componentswere much larger than those in the gas mixtures without C3components and they increased obviously with the increaseof partial pressure difference. The selectivity of ethylene inC3 ternary plus C=2 was approximately 69% of pure hydrocar-bon selectivity, whereas the selectivity of ethane in C3 ternaryplus C2 was approximately 55%. However, the selectivitiesof ethylene in C=2 binary and C2 ternary were comparableand there were no obvious increases as their partial pressuredifferences increased. Their average values were 46% of purehydrocarbon selectivity. The selectivity of ethane in C2 bi-nary was stable as partial pressure difference increased. Itsaverage was 42% of the pure hydrocarbon selectivity. The se-lectivity of ethane in C2 ternary increased considerably withthe increase of the partial pressure difference, indicating itsintrinsic characteristics of condensation. Its average value

was 48% of pure hydrocarbon selectivity. This is clear ev-idence of the positive coupling effect in multi-componentmixed gas transport though a rubbery membrane: in gas mix-tures without C3 hydrocarbons (C=2 binary, C2 binary and C2ternary) the selectivity of ethylene or ethane was based on itsintrinsic properties, indicating that there was no plasticiza-tion. However, in gas mixtures containing C3 hydrocarbons(C3 ternary plus C=2 and C3 ternary plus C2), the selectivi-ties of ethylene or ethane by means of flexible-chain poly-mer membrane obviously corresponded to the enhancementof their permselectivities. It should, therefore, be favorablyaffected by the coupling in terms of permselectivity, indicat-ing that there were plasticization and sorption non-ideality inPDMS coating layer due to the interaction of C3 hydrocar-bons. Coupling effect was analyzed theoretically in a binarygas mixture for the rubber membrane by Petropoulos[26],

ion in f
Fig. 8. Effects of total hydrocarbon concentrat eed gas on permeance for mixtures 1 and 2 (Table 4).


Fig. 9. Effects of total hydrocarbon concentration in feed gas on selectivity for mixtures 1 and 2 (Table 4).

and studied experimentally by Yeom et al.[12] in CO2/N2gas mixture through PDMS membrane. Barbari et al.[27] hadalso reported that plasticization effects due to the presenceof the methane had enhanced the carbon dioxide mobility inCO2/CH4 gas mixture through bisphenol A-based polymers.

As mentioned in the preceding section, sorbed C3 hydro-carbons swell the polymer matrix that causes plasticizationand coupling effect. The degree of swelling for polymer ma-trix could vary as hydrocarbon concentration in a mixture thatinteracts with the membrane. Mixtures of ethylene, propy-lene, propane and nitrogen with four concentration levels(seeTable 4) were prepared to study the effects of their in-dividual concentrations or partial pressures on the degree ofswelling.

Fig. 8shows the effects of C3 concentrations increase withincreasing degrees of swelling on the permeances as C=

2 con-centrations were fixed at two levels (mixtures 1 and 2). Itcan be seen that the permeances of propylene and propaneincreased rapidly with an increase of total hydrocarbon con-

centration in feed gas, indicating that every molecule sorbedin the polymer contributed to the continued swelling of thePDMS coating layer. Therefore, plasticization pressures wereobserved at 121 and 133 kPa partial pressure differences ofC3 hydrocarbons for mixtures 1 and 2, respectively. It wasfound that these are slightly higher than the plasticizationpressure of pure gases (114.6 kPa (g)). Spontaneous increasesof ethylene permeances were observed even though their par-tial pressures were almost constant. It would suggest that theincrease of the degree of swelling caused a positive couplingeffect. This also influenced permeance of nitrogen. In mixture1, as nitrogen concentration decreased from 88.3 to 55.1%, itspermeance increased by 23.8%; in mixture 2, as nitrogen con-centration decreased from 78.4 to 51.2%, its permeance in-creased by 15.0%. It appears that a higher degree of swellinghad loosened the polymer matrix that had allowed the diffu-sion of additional nitrogen through the membrane. However,each hydrocarbon and nitrogen with a lower concentrationof ethylene (mixture 1) was more permeable than those with

tion in
Fig. 10. Effects of total hydrocarbon concentra feed gas on permeance for mixtures 3 and 4 (Table 4).


Fig. 11. Effects of total hydrocarbon concentration in feed gas on selectivity for mixtures 3 and 4 (Table 4).

higher concentration of ethylene (mixture 2). One explana-tion could be that the large amount of ethylene (two or threetimes higher) sorbed into the membrane might have causedpartial blocking of some free volume elements and therebyreduced the gas permeation. The gas permeation propertiesof this membrane material also follow an immutable trade-off relationship: polymers that are more permeable tend tobe less selective and vice versa[8]. Fig. 9 shows that theselectivities with higher concentration of ethylene (mixture2) were all larger than those with lower concentrations ofethylene (mixture 1).Figs. 10 and 11show the permeancesand selectivities with increasing ethylene concentrations asC3 concentrations were fixed at two levels (mixtures 3 and4), which meant that two degrees of swelling were set up.In mixture 3 with lower C3 concentration, the permeances ofpropylene and propane were almost constant with increasingC=

2 partial pressure, whereas the C=2 permeance tended to

increase slightly. As the partial pressures of C3 componentswere lower than the plasticization pressure, the permeanceorder (PC=

3> PC3 > PC=

2> PN2) illustrated that the diffu-

sivity of propylene was dominant, indicating that the degreeof swelling was low. In mixture 4, the partial pressures ofC3 components were higher than the plasticization pressure,hence the higher amount of propylene and propane swelledthe PDMS to such an extent that chain segments might havebecome more flexible due to their presence and might havea newc rder( e-g wasm itivec aisedi ylenefi se ofi ropy-l t that

the solubility of ethylene also depended on its partial pres-sure and could reach a saturated state for a gas mixture inPDMS with a higher degree of swelling. However, the perme-ances of nitrogen in these two mixtures gradually decreased.In mixture 3, as nitrogen concentration decreased from 87.4to 73.6%, its permeance decreased 1.8%; in mixture 4, as ni-trogen concentration decreased from 65.4 to 58.4%, its per-meance decreased 6.4%. According to Yeom et al.[9] as thedegree of swelling was kept constant by maintaining a cer-tain fixed amount of C3 components, an increasing amountof ethylene sorbed into the membrane, which led to partiallyblocking of the small free volume elements. This would re-sult in decreased nitrogen permeation as lower amount ofnitrogen would be present in the mixture. Consequently, se-lectivities increased enormously with increasing C=

2 partialpressure difference mainly due to the additional depressionof nitrogen permeances as seen inFig. 11.

6. Conclusions

It was observed that propane and propylene significantlyplasticized the PDMS coating layer both as pure hydrocar-bons and as a component of gas mixtures. At applied pres-sures that were lower than the plasticization pressures, theoP er-m ed toh ngedt st d se-l redt g ef-f panea h ani e and

dapted to this new condition by rearranging towards aonfiguration. It can be confirmed by the permeation oPC3 > PC=

3> PC=

2> PN2), which demonstrated that the d

ree of swelling was high. The permeance of ethyleneuch larger than the value in mixture 3 due to the pos

oupling effect as the partial pressure of ethylene was rn the same ranges. Moreover, the permeance of ethrst increased and then became steady with the increats partial pressure, whereas the permeances of both pene and propane remained constant. It would sugges

bserved permeance order wasPC3> PC3 > PC2 > PC2

>

O2 > PN2, which concluded that propylene was more peable than propane. As the pressure was increasigher than the plasticization pressure, the order was cha

o PC3 > PC3> PC2 > PC2

> PO2 > PN2, which matchehe solubility order. It was observed that permeances anectivities were significantly lower in gas mixtures compao pure gases. Furthermore, there were positive couplinects for ethane and/or ethylene in the presence of prond propylene in gas mixtures. It was observed that wit

ncreased degree of swelling the permeances of ethylen


nitrogen (both non-plasticizing components) increased spon-taneously although their partial pressures were either constantor decreasing.

Acknowledgements

Authors are thankful to Mr. Brad Stimson for his help withthe experimental system and Dr. Jamal Kurdi for helpful dis-cussions. Financial support from Natural Resources Canada,under PERD project number 11402 is gratefully acknowl-edged.

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Performance of silicone-coated polymeric membrane in separation of hydrocarbons and nitrogen mixtures