zeolite 4A mixed matrix membrane

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 7 2 7 5 e1 7 2 8 4

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Sorption properties of hydrogen-selective PDMS/zeolite 4A mixed matrix membrane Mashallah Rezakazemi, Kazem Shahidi, Toraj Mohammadi* Research Centre for Membrane Separation Processes, Faculty of Chemical Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran

article info

abstract

Article history:

The present study explores the fundamental science of estimating sorption of gases in

Received 28 July 2012

membranes comprised of inorganic porous fillers within a polymer matrix with a novel

Accepted 24 August 2012

semi-empirical correlation. The sorption properties of H2, C3H8, CO2 and CH4 were deter-

Available online 23 September 2012

mined in polydimethylsiloxane (PDMS)/zeolite 4A mixed matrix membranes (MMMs) to assess the viability of these membranes for hydrogen purification and natural gas sweet-

Keywords:

ening. Zeolite filling in MMMs results an increase in solubility over neat PDMS membrane.

Sorption

In addition, incorporation of zeolite 4A to PDMS membrane improved H2 permeation and

Mixed matrix membrane

H2/CH4 selectivity. The results confirmed that zeolite 4A can significantly improve the

Hydrogen purification

separation properties of poorly H2-selective PDMS membrane from 0.7 up to 11 and this

PDMS

overcomes the Robeson upper-bound limitation. This improvement was explained referring the FloryeHuggins interaction parameter within MMMs. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Hydrogen is a valuable commodity in refining and petrochemical processes, which must be economically recovered from numerous process streams. For instance, refinery hydrogen requirements are growing fast due to increased use of hydrotreating and hydrocracking. Residual gases from these processes contain significant amounts of unused hydrogen at pressure, and membranes can provide an economical recovery method [1e9]. The characteristics of high permeabilities of rubbery polymers and high selectivities of zeolites and easy processability with good compatibility of rubbery polymers with zeolitic fillers have been brought researchers to work on zeolite-rubbery mixed matrix membranes (MMMs) [10e13]. Most of the MMMs studies employ zeolites as fillers due to their well-defined pore size and their ability to discriminate between molecules sizes and shapes [14e19]. Structure and

geometry of zeolites discriminate based on precise molecularsieving. Hence, incorporation of zeolites into polymers may increase separation performance of polymeric membranes due to their high selectivities in comparison with neat polymers. Additionally, there are many different types of zeolites and most of them have adjusted properties such as Si/Al ratio or ion-exchange, which make them more preferable fillers in MMMs preparation [20,21]. Predominantly of early working on filling MMMs with zeolites goes back to utilize elastomeric or rubbery polymers as continuous matrix phase. Zeolite incorporation into rubbery polymer membranes were first investigated by Hennepe et al. for pervaporation and gas separation purposes [22,23]. The results of pervaporation separation of ethanol/ water mixtures revealed that addition of silicalite-1, NaX and AgX type zeolites into PDMS matrix enhances both ethanol permeability and selectivity. Gas permeations of C2H4 and C2H6 through the same membranes also represented

* Corresponding author. Tel./fax: þ98 21 77240495. E-mail address: [email protected] (T. Mohammadi). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.08.109

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increased permeability and selectivity of the membranes. They demonstrated that the selectivity enhancement comes from a longer pathway for the larger component around the zeolite particle and a shorter pathway for the smaller component through the zeolite pores. Jia et al. [24] studied PDMS membranes filled with silicalite1. Permeabilities of gases such as N2, CH4 and C4H10 decreased, while those of He, H2, O2 and CO2 increased. Silicalite-1 in PDMS facilitated permeation of smaller molecules and behaved as a molecular sieve. Addition of 50% (w/w) silicalite1 increased selectivities of O2/N2 and CO2/CH4 from 2.15 to 2.50 and from 3.45 to 5.67, respectively. Duval et al. [25] investigated effects of adding various types of zeolites including, 5A, silicalite, 13X and KY and commercial carbon molecular sieves (CMSs) on membranes performance of a range of rubbery polymers such as PDMS, ethylene-propylene rubber (EPDM), and nitrile-butadiene rubber (NBR). The permeation results of prepared MMM with NBR and 46 vol% zeolite KY showed significant enhancement of CO2/CH4 selectivity from 13.5 to 35, while the selectivity results of prepared MMM for O2/N2 separation showed slight enhancement, for example, the O2/N2 selectivity of the EPDM rubber MMM with 53 vol% silicalite, increased from 3.0 to 4.7. On the other hand, no improvement obtained by zeolite 5A filled MMMs owing to either adsorbed water in the pores of zeolite 5A or strong adsorption in 5A that makes permeation very slow. Besides, the CMSs filled MMMs also indicated no improvement in performance of rubbery polymeric membranes due to the dead end porous nature of CMSs. Tantekin-Ersolmaz et al. [21] prepared PDMS/silicalite MMMs and studied permeabilities of different gases including, O2, N2 and CO2, and investigated the effect of zeolite particle size on performance of the polymer-zeolite MMMs. The results showed slight improvement in the CO2/N2, O2/N2 and CO2/O2 ideal selectivities with incorporation of silicalite particles into PDMS matrix. As they observed, the permeability of gases correspond to the MMMs exceed those pertaining to the neat PDMS membrane and the permeabilities increase with increasing particle size of the silicalite crystallites. In our previous study [26], it was attempted to investigate the incorporation of zeolite 4A in PDMS membranes from the viewpoints of MMM preparation methodology and gas permeation. As a part of that work, the knowledge regarding the zeoliteepolymer interactions was applied to describe hydrogen separation and purification and natural gas sweetening results using those zeolite-loaded membranes. The experiments were carried out to investigate whether zeolite 4A is beneficial to the relatively poorly PDMS membrane for CO2/CH4 and H2/CH4 separation applications. In the current work, however, to fulfill complete research on this type of membranes, sorption aspects of synthesized membranes were investigated, first on the neat PDMS membrane and then on the MMMs to study the influence of the different zeolite contents incorporated and justify an improved H2/CH4 separation performance surpassing the Robeson’s trade-off curve, confirming industrial importance of these membranes. To best knowledge of authors, so far there is no academic literature available on sorption of gases in inorganic porous filler filled polymeric membranes.

2.

Experimental

2.1.

Materials

Dehesive 944 silicone, crosslinker and catalyst (RTV 615A en B, density 1.02 g/mL) were supplied from Wacker Silicones Corporation, Adrian; MI. Toluene was purchased from Merck and used as received. Zeolite 4A nanoparticles with particle size 80e250 nm and Si/Al molar ratio ¼ 1 were dedicated from Research Institute of Petroleum Industries. H2, CO2, and CH4 gases with purity of 99.5% were supplied by Technical Gas Services, Inc., and C3H8 gas with purity of 99.9% was supplied by Air Products and Chemicals, Inc.

2.2.

Mixed matrix membrane preparation

The zeolite-loaded PDMS membranes were synthesized by first uniform dispersing the zeolite 4A nanoparticles in the polymer solution followed by a new procedure which was discussed in details in our previous study [26]. In MMMs preparation, toluene was used as solvent and evaporation was performed at 80  C under vacuum condition.

2.3.

Sorption measurement

Pure gas sorption measurements were performed using a sorption apparatus. The entire system was evacuated and then filled with the feed gas. The module was leaded to reach equilibrium using water bath. The gas pressure was monitored using sensitive pressure transducers and recorded automatically by a data acquisition system employing LabTech software. A temperature control system was scheduled to set the gas temperature at 35  C and the module temperature was adjusted by the water bath during the sorption process. A vacuum pump was connected to the module to degas the module, whenever required. The gas concentrations within the membranes were calculated using mole balance. In any gas sorption experiment of each MMM, five samples were placed into the module, and their totally sorption values were measured, simultaneously. Whereby, in this manner, impacts of various surface morphology of the MMMs with the same content of zeolite nanoparticles on sorption of the gas were considered. However, isotherms sorption experiments were conducted via continually increasing pressure throughout the experiment. Once sorption and desorption of gases in the membranes became equal, the system reached to equilibrium and pressure reduction stopped. The differences between initial and final moles of gases in the module were considered as the moles of gas sorbed into the polymer. Moreover, the gases were then injected into the module and the same procedure was repeated again. By this incremental way, the gases uptake could be measured as a function of pressure. Concentration of the dissolved gases, C [cm3(STP)/cm3 polymer], at equilibrium state in the membranes at specified temperature and pressure were measured using the following equation [27]:

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C¼

 V 22; 414
m pi  pf Vp RT

(1)

where pi and pf are the initial and the final pressure in the module, respectively. Vm and Vp refer to the volumes of the module and the membrane sample [cm3], respectively and 22,414 is the number of cm3 (STP) of gases per mole.

3.

Theoretical background

3.1.

Calculating dispersed phase loading in MMMs

The existence models considering mass transfer through MMMs are typically functions of particle volume loadings, while the most of reports regarding the MMMs illustrate particle loadings in wt.% or simply % and this complicates understanding of mass transport. The “ion-containing” framework densities of molecular sieves should be employed in mixed matrix studies since it is assumed the polymer is excluded from the internal pores of the molecular sieves. The relationship between volume loadings and component masses in MMMs is as follows [28]: 4¼

yd md =rd
 ¼ yd þ yp ðmd =r Þ þ mp =r d p

(2)

where 4 is the particle volume fraction of the membrane, n, m, r are the volume, the mass, and the density, respectively. The subscripts d and p refer to the dispersed and polymer phases of the membrane, respectively. The “ion-containing” framework density of zeolite 4A is 1.52 g/cm3 [28], and Wacker reports 0.91 g/cm3 as the density of PDMS. Using the densities of PDMS and zeolite 4A, and solving Eq. (2) for 40 vol.% loading of zeolite 4A in PDMS, the required particle loading of zeolite is about w53 wt.%. This procedure was used to compute the required zeolite loading in the PDMS to produce MMMs.

3.2. A general semi-empirical correlation of sorption of light gases in MMMs

CF ¼

1 þ bp

CP ¼ kD p

(4)

where p is the gas pressure and kD is the equilibrium Henry’s law coefficient for the pure polymer and the gas. The present study explores the estimating sorption of gases in membranes comprised of porous fillers within rubbery polymer matrixes using a new semi-empirical correlation. The solubility of gases in membranes comprised of nonporous fumed silica particles as filler within glassy polymers was previously presented by Angelis and Sarti [32] based on NELF model. This model relates gas solubility in the composite membranes to the variations induced by addition of the fillers on the polymers density and swellability. However, the gas solubility in an MMM can be calculated in a general form through the number of gas moles (n) per unit volume of the membrane as follows: C¼

np þ nF ¼ ð1  yF ÞCP þ yF CF V

(5)

where yF is the volume fraction of the filler (yP ¼ 1  yF in which yp is the volume fraction of the polymer phase), C is the gas solubility per unit volume of the MMM, CF and CP are the gas solubilities in the filler and in the polymer, per unit volume of the filler and the polymer, respectively. It must be emphasized to this point that the units of all solubilities are in terms of “per unit of volume”. In MMMs, sorption of the zeolite depends on the polymer since some surface area and pores are blocked. Therefore, it is necessary to determine the sorption equilibrium of the mixed matrix rather than calculate it from experiments on the separate phases. The similarity of the sorption isotherm of MMMs to zeolite sorption behavior suggests that it is possible to utilize the dual-mode sorption theory to explain solubility of low-sorbed gases behavior within MMMs by a proposed semi-empirical equation. Hence, total concentration of a gas in an MMM can be written as follows:  C ¼ yp CP þ yF CF ¼ yp kD p þ yF

Diffusion of penetrants through polymeric filled membranes can be summarized by diffusion of sorbing gas molecules through: (1) polymer, (2) pores of filler, (3) polymer/filler interface and (4) surface diffusion on the filler [29,30]. Gas solubility of a polymer matrix depends drastically on the vicinity of the gas temperature to the glass transition temperature, Tg, of the polymer. The solubility of rubbery polymers is function of Henry’s law while glassy polymers follow a dual-mode sorption. The solubility coefficient is the ratio of the equilibrium sorbed concentration of a gas over the external gas pressure [31]. The isotherm sorption of gas in zeolites framework can be described by Langmuir-type sorption model: C0A bp

The gas concentration dissolved in polymer phase, CP, can be written by Henry’s law:

3.3.

(6)

FloryeHuggins theory

The FloryeHuggins theory is applied for more soluble vapors in rubbery polymers as the following expression [33]:

Table 1 e Penetrants properties including critical properties, saturation vapor pressure at 35  C, partial molar volume, molar volume at the normal boiling point, and kinetic diameter [33,35]. Penetrant

(3)

where CF, C0A , b, and p are the sorbed concentration (ccSTP gas sorbed/cc zeolite), the Langmuir capacity constant, the Langmuir affinity parameter, and the external gas pressure, respectively.

 C0A bp 1 þ bp

H2 CO2 CH4 C3H8

Tc [K]

pc [atm]

Vc [cm3 /mol]

psat [atm]

33.2 304.1 190.5 369.8

12.9 73.8 46.0 42.5

64.3 93.9 99.2 203.0

314.8 80.58 354.0 12.3

V2 V2 [cm3 [cm3 /mol] /mol] 40 45 46 80

28 46 38 76

d ˚] [A 2.89 3.30 3.80 4.30

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ln a ¼ ln 42 þ ð1  42 Þ þ cð1  42 Þ2

Table 2 e The model-based sorption parameters of MMMs. Membrane

kD

C0A

ðccðSTPÞ=ccbarÞ

ðccSTP =ccÞ



b bar1



CO2 PDMS PDMS/4A PDMS/4A PDMS/4A PDMS/4A PDMS/4A

(10 wt.%) (20 wt.%) (30 wt.%) (40 wt.%) (50 wt.%)

1.700 1.721 1.750 1.792 1.893 1.911

41.973 44.267 49.379 52.921 58.149

0.0501 0.0554 0.0611 0.0638 0.0687

CH4 PDMS PDMS/4A PDMS/4A PDMS/4A PDMS/4A PDMS/4A

(10 wt.%) (20 wt.%) (30 wt.%) (40 wt.%) (50 wt.%)

0.470 0.476 0.483 0.491 0.495 0.503

9.850 13.63 15.29 15.96 16.38

0.034 0.039 0.048 0.056 0.063

H2 PDMS PDMS/4A PDMS/4A PDMS/4A PDMS/4A PDMS/4A

(10 wt.%) (20 wt.%) (30 wt.%) (40 wt.%) (50 wt.%)

0.069 0.073 0.075 0.081 0.083 0.087

1.894 1.612 1.399 1.259 1.067

0.089 0.101 0.111 0.135 0.167

(7)

where a is the penetrant activity in the vapor phase, 42 is the volume fraction of the sorbed penetrant and c is the FloryeHuggins interaction parameter. The volume fraction of sorbed penetrant, 42 , is measured from the equilibrium penetrant concentration in the polymer as follows: 1  22; 414 42 ¼ 1 þ CV2

(8)

where V2 is the penetrant partial molar volume and is estimated as described by Merkel et al. [34]. In this expression, C and V2 have units of [cm3 (STP)/cm3 polymer] and [cm3/mol], respectively. 22414 is a conversion factor [cm3 (STP)/mol]. When these equations are used, some penetrant properties are required as provided in Table 1. As can be seen, the operational temperature (35  C) is higher than the critical temperature of all gases, except C3H8; hence, the saturation vapor pressure at the experimental temperature is unknown. They can be predicted by extrapolation of the vapor pressure curve to 35  C. The critical temperature of CO2 is slightly lower than 35  C; whereby, prediction of its vapor pressure is reasonable [33]. Indeed, at pressures lower than 25 bar, deviation from ideal behavior for given gases is negligible, so the activity expression, a ¼ f =f sat , can be written as a ¼ p=psat . Kamiya et al. [35] found that partial molar volumes of C1eC5 hydrocarbon dissolved in rubbery polymers are related to their Van der Waals volumes. However, the partial molar volumes used in this study were achieved from Kamiya et al. [35] studies.

Fig. 1 e Penetrants concentration in the synthesized PDMS/zeolite 4A MMMs as a function of upstream pressure.

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4.

Results and discussion

4.1. Model-based correlation and estimation of sorption in MMMs The model sorption parameters were correlated well with the zeolite content based on a general semi-empirical correlation presented for sorption of light gases in MMMs. As can be seen in Table 2, all model parameters increase with increasing the zeolite loading. The influence of zeolite content on the lowsorbed gases is also presented in Fig. 1, where it is observed that the higher zeolite loading, the more gas sorption in the MMMs. The results obtained here imply that addition of the zeolite improves significantly the gas sorption in the Langmuir and the Henry’s sites. This impact is particularly remarkable at the higher zeolite loading. On the other hand, indeed, in the MMMs structure, the accessibility to the most effective zeolite surface decreases significantly, whereby the sorptive capacity of the zeolite decreases, while the zeolite addition enhances the sorptive capacity of the MMMs. Such increment in solubility of the MMMs is originated from the polymer density reduction or the fraction free volume (FFV) enhancement [36].

4.2.

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Evaluation of gas sorption performance in MMMs

Penetrants concentration of CO2, CH4, H2, and C3H8 in the various synthesized PDMS/zeolite 4A MMMs with different zeolite loading at 35  C is presented in Fig. 1. The findings of the current study in results of the gas sorption isotherms in PDMS membranes, as shown in Fig. 2, are excellently consistent with those of previously reported by Merkel et al. [37], Raharjo et al. [38], Prabhakar et al. [29], Shah et al. [39] Fleming and Koros [40] and our previous study [33]. On the basis of these findings, the order of gas solubilities within the PDMS membranes (whether pure or mixed) is as follows: C3 H8 [CO2 > CH4 > H2 This is well in accordance with their critical temperatures, Tc. Gas solubility increases with penetrant condensability. Critical temperature increases smoothly as the diffusing molecule size increases, therefore, solubility coefficient often increases as diffusing penetrant size increases. This is the main reason that solubility selectivity is often larger for more condensable penetrants, while diffusivity selectivity is larger for smaller gas molecules.

Fig. 2 e Penetrants concentration in the synthesized PDMS/zeolite 4A (40 wt.%) MMM as a function of transmembrane pressure, data obtained in this study are compared with those of Merkel et al. [37], Raharjo et al. [38], Prabhakar et al. [29], Shah et al. [39], Fleming and Koros [40], and our previous study [33].

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Fig. 3 e The FloryeHuggins interaction parameter, c, as a function of pressure for (a) the neat PDMS membrane and (b) the PDMS/zeolite 4A (40 wt.%) MMM

In order to investigate the effect of zeolite loading on gas sorption properties of MMMs, the affinity of zeolite 4A towards each individual gas was evaluated. Zeolite 4A almost has no affinity towards C3H8 [41,42] and H2 [43,44], while zeolite 4A has strong affinity towards CO2 and CH4 [28,44]. Grande et al. [41] reported experimental data regarding the adsorption of pure C3H8 on zeolite 4A at 150  C and 200  C. As they reported, adsorption of C3H8 is extremely slow and sorption isotherms are temperature independent. For CO2 and CH4 gases, zeolite 4A exhibits relatively high gas solubility [28] compared to neat PDMS membrane [33]. From screening the results, it can be concluded that the gas solubility increases in the order of PDMS < PDMS/zeolite 4A (10 wt.% < 20 wt.% < 30 wt.% < 40 wt.% < 50 wt.%), which is well in accordance with their increasing zeolite loading, which is probably due to the gas solubility within the zeolite; whereby it can increase sorption sites within the membrane. In other words, the higher sorption levels are likely due to the extra gas sorption site provided by addition of the zeolite.

4.3. gases

FloryeHuggins interaction parameter of sorbed

On the basis of experimental results shown in Figs. 1 and 2, the isotherms are linear for H2 and CH4 and for CO2 almost linear and obey the Henry’s law. Fleming and Koros [40] realized that the Henry’s law for pressures up to 300 psia, can predict sorption data very well and the Henry’s law constant for CO2 was reported 1.385 [cm3 (STP)/cm3 polymer

atm]. As observed, the solubility curve of C3H8 is concave to the vertical axis at low pressures and tends to approach a straight line at high pressures. The convex curvature of C3H8 sorption isotherms at elevated pressures is in good agreement with the behavior of highly sorbing penetrants in rubbery polymers, which obey the FloryeHuggins expression. It must be noted that the FloryeHuggins interaction parameter can also be used to calculate the degree of gas sorption into a polymer composite [45]. The FloryeHuggins interaction parameters, c, were evaluated for all gases sorbed into the synthesized neat PDMS membrane and PDMS/zeolite 4A MMMs as a function of transmembrane pressure as shown in Fig. 3. As observed, c slightly increases with pressure for all gases in the neat PDMS membrane. As observed, for C3H8, it increases little more. This means that average values of c for H2, CO2 and CH4 in a wide range of pressure can be used to solve the FloryeHuggins equation. For C3H8 in the neat PDMS membrane, a linear function of c with pressure can be used to solve the FloryeHuggins equation. High affinity between penetrant and polymer can be determined mainly by small values of c [33]. In other words, c is strongly influenced by similarity or dissimilarity of the gases with the membrane. CH4 and C3H8 are chemically more similar to PDMS polymer. Since PDMS with repeating unit of [eSi(CH3)2eOe], is a hydrocarbon-based polymer; whereby; low c value of CH4 is rationalized. On the other hands, the gas solubility in polymeric membranes is mainly determined by interplay between condensability and polymerepenetrant interaction [34]. Hence, even though CH4

Table 3 e Concentration-averaged FloryeHuggins interaction parameter, c. Penetrant Merkel et al. [34] Shah et al. [39] Prabhakar et al. [29] Fleming and Koros [40] Previousstudy [33] This study H2 CO2 CH4 C3H8

2.563 0.585 0.196 0.386

e 0.643 0.138 0.275

e 0.643 0.138 0.275

e 0.450 e e

2.043 0.293 0.067 0.262

2.196 0.418 0.139 0.247

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4.4.

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Enthalpy of gas sorption

Solubility of a gas can be typically attributed to the two different steps involving gas condensations from a gas-like density to a liquid-like density. Thereafter, the condensed gas molecule is solved (mixed) within the polymer chains corresponding to their interaction parameter. The enthalpy of sorption can be calculated as follows [29]: DHs ¼ DHcond þ DHmix

(10)

where DHcond is the enthalpy changes correspond to the gas condensation which only relates to the physical properties of gas and can be obtained at each temperature from the literature [46]. DHmix is the enthalpy changes of mixing the gas molecules within the polymer matrix which can be calculated as follows: DHmix ¼ RTcð1  42 Þ Fig. 4 e Effect of volume fraction of C3H8 on the FloryeHuggins interaction parameter in the synthesized PDMS membrane and the PDMS/4A (40 wt.%) MMM, data obtained in this study are compared with those of Prabhakar et al. [29], Shah et al. [39], and our previous study [33].

has lower c value than C3H8, its sorption in the neat PDMS membrane and the MMMs is less. Concentration-averaged FloryeHuggins interaction parameters, c, of all gases within the PDMS/4A MMMs were calculated by means of the following expression as reported in Table 3.

c¼

1 42;max

Z

42;max

cð42 Þd42 0

where 42;max is the maximum gas volume fraction.

(9)

(11)

where c and 42 are the FloryeHuggins interaction parameter and the partial molar volume of gas in the polymer, respectively. As can be seen in Eq. (11), the FloryeHuggins interaction parameter, c, is proportional to the gas concentration in the polymer as suggested by the FloryeHuggins equation. Fig. 4 represents the effect of volume fraction of C3H8 on the FloryeHuggins interaction parameter. Higher solubility of C3H8 in contrast with other tested gases is due to its higher condensability nature. This can be also concluded from the results that c of C3H8 is a function of penetrant concentration. The much more condensability of a gas can be reflected by its higher critical temperature and lower saturation vapor pressure. It must be emphasized that for the less soluble gases like CH4, c is independent of concentration [33]. The obtained results in this study, similar to the results of Prabhakar et al. [29], Shah et al. [39] and our previous work [33], confirm that increasing the C3H8 concentration enhances the c values. It means that interactions among C3H8 molecules are more favorable than those between the penetrant molecules and the polymer chains.

Fig. 5 e Gas solubility in the synthesized neat PDMS membrane and the PDMS/4A (40 wt.%) MMM as a function of pressure at 35  C.

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Table 4 e The infinite dilution solubility at 35  C in the neat PDMS [33]. Penetrant H2 CO2 CH4 C3H8

SN experimental [cm3 (STP)/cm3 bar]

SN FloryeHuggins [cm3 (STP)/cm3 bar]

n [103 cm3 (STP)/cm3 bar2]

0.0794 1.5821 0.4288 6.8093

0.0729 1.4970 0.4403 6.5457

0.477 6.377 2.057 542.517

Table 5 e The infinite dilution solubility at 35  C in the PDMS/4A (40 wt.%) MMM. Penetrant H2 CO2 CH4 C3H8

4.5.

SN experimental [cm3 (STP)/cm3 bar]

SN FloryeHuggins [cm3 (STP)/cm3 bar]

n [103 cm3 (STP)/cm3 bar2]

0.1090 2.4220 0.6410 6.2750

0.0869 1.7079 0.4708 6.4487

0.035 4.792 0.124 676.125

Infinite dilution solubility

SN psat ¼

The solubility of each gas was determined by dividing its concentration to pressure as presented in Fig. 5. As observed, the solubility coefficients of CO2, CH4 and H2 are almost constant in both the neat PDMS membrane and the PDMS/4A MMMs, while the C3H8 solubility in all the membranes increases strongly with pressure. The solubility of all penetrants can be related by a linear equation to pressure as follows: S ¼ SN þ np

22; 414 V2 expð1 þ cÞ

(14)

SN , n and solubility selectivity of C3H8 are presented in Tables 4 and 5.

(12)

In this equation, n specifies the pressure dependence of solubility and SN is the infinite dilution solubility which can be determined as follows: SN ¼ lim ðC=pÞ

(13)

p/0

Additionally, can be calculated using the FloryeHuggins expressions as follows:

Fig. 6 e The infinite dilution solubility, SN , as a function of gases’ critical temperature, Tc, at 35  C for the neat PDMS membrane (-), the PDMS/4A (40 wt.%) MMM (,).

Fig. 7 e C3H8/gas and H2/gas solubility selectivity of the synthesized pure PDMS membrane and the PDMS/4A (40 wt.%) MMM as a function of upstream pressure at 35  C.

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The solubilities of relatively low-sorbing penetrants (the sorptions of light gases), like H2 and CH4, show lower values of n, which are independent on pressure and can be explained by the Henry’s law, whereas for the high-sorbing penetrants such as C3H8 and CO2 with higher values of n, the solubilities are dependent on pressure [47]. Without any interaction between the penetrant and the polymer phase, gas solubility can be determined by critical temperature, Tc. As shown in Fig. 6, the logarithm of infinite dilution solubility, SN , increases linearly with Tc. The best-fit line in slope intercept form for the neat PDMS membrane and the PDMS/4A (40 wt.%) MMM using the sorption data are 0.012 and 0.011, respectively. The findings of current study are in an excellent consistent with those of Van Amerongen, Michaels and Bixler, and Merkel et al. (0.017, 0.016, and 0.014, respectively) [40].

4.6.

Solubility selectivity

Fig. 7 shows the solubility selectivities data of C3H8/gas and H2/gas, for the neat PDMS membrane and the MMMs. It is meaningfully obvious that the increased solubility selectivity of C3H8/H2 dominates the C3H8/CO2 and C3H8/CH4 separations. As discussed in our previous study [26], introducing the zeolite 4A to the PDMS membranes changes its application from C3H8/H2 separation to H2/CH4 separation. The zeolite affinity towards gases leads to their higher solubilities in the MMMs. As can be seen, the solubility selectivity value of H2/CH4 is higher than those of H2/CO2 and H2/C3H8.

5.

Conclusions

PDMS/4A MMMs were successfully prepared with various levels of zeolite 4A nanoparticles loadings to investigate their gas sorption properties. A novel general semi-empirical correlation for sorption of light gases in MMMs was introduced. The sorption behavior of the rubbery polymer filled with the inorganic porous fillers could be well described by the proposed model to assess the validity and the reliability of the model. The results demonstrated that with increasing the zeolite content, gas sorption of the PDMS membrane significantly enhances. Additionally, the results indicated that zeolite 4A loading can increase the H2/CH4 separation performance and overcome the Robeson upper-bound limitations. This great improvement was explained by the FloryeHuggins theory.

Acknowledgements The authors would like to appreciate Dr. M. Bruetsch, Wacker Silicones Corporation, for his efforts to supply Dehesive 944 package, consisting of silicone oil, crosslinker and catalyst. The authors also are grateful for the financial support of the Renewable Energy Organization of Iran [SUNA].

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