methane separation: Eight component mixed-gas permeation properties

Journal Pre-proofs Polyalkylmethylsiloxanes composite membranes for hydrocarbon/methane separation: Eight component mixed-gas permeation properties E.A. Grushevenko, I.L. Borisov, A.A. Knyazeva, V.V. Volkov, A.V. Volkov PII: DOI: Reference:

S1383-5866(19)35034-8 https://doi.org/10.1016/j.seppur.2020.116696 SEPPUR 116696

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

3 November 2019 8 February 2020 9 February 2020

Please cite this article as: E.A. Grushevenko, I.L. Borisov, A.A. Knyazeva, V.V. Volkov, A.V. Volkov, Polyalkylmethylsiloxanes composite membranes for hydrocarbon/methane separation: Eight component mixedgas permeation properties, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur. 2020.116696

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Polyalkylmethylsiloxanes composite membranes for hydrocarbon/methane separation: Eight component mixed-gas permeation properties E.A. Grushevenko*, I.L. Borisov, A.A.Knyazeva, V.V. Volkov, A.V. Volkov A.V.Topchiev Institute of Petrochemical Synthesis RAS, 119991, Leninsky pr., 29, Moscow, Russian Federation * E-mail: [email protected], T: +7 495 6475927, Leninsky pr., 29, Moscow Russia, 119991. Abstract The separation of an eight component mixture of saturated and unsaturated hydrocarbons С1-С4 was studied for the first time using dense and composite membranes based on polyhexylmethylsiloxane

(PHexMS),

polyoctylmethylsiloxane

(POMS),

and

polydecylmethylsiloxane (PDecMS). Dense and composite membranes based on PDMS were also prepared for comparison. Composite membranes were formed on two supports of ultra- and microfiltration type (UFFK and MFFK-1, respectively). Membranes with MFFK-1 support demonstrated a higher С2+/СН4 separation selectivity. An increase of the separation selectivity was observed for both dense and composite membranes with an increase in the length of the polyalkylmethylsiloxane side chain. The selectivity for n-butane/methane gas pair rises from 15.5 to 18.2 with the transition from hexyl to decyl substituent which is more than twice the separation selectivity for this gas pair for PDMS (7.6). An insignificant decrease of selectivity was observed with the transition from dense to composite membranes. For instance, n-butane/methane selectivity of the PDecMS/MFFK composite membrane didn’t exceed 17.0 and was reduced to 16.3 with the decrease of n-butane activity from 0.13 to 0.06 (a drop of the feed mixture pressure from 3 to 1 atm (gauge)). The separation of the gas mixture on MDK-3 (ZAO STC “Vladipor”, Russia) and POMS (HZG, Germany) commercial membranes was investigated. It was shown that PDecMS/MFFK exhibits a 40% higher selectivity for n-butane/methane gas pair (16.7) in comparison with MDK-3 (10.1) and a 20% higher selectivity in comparison with POMS–HZG (13.9). Keywords:

membrane

polyalkylmethylsiloxanes,

eight

gas

separation, component

hydrocarbon/methane mixed-gas,

composite

separation, membrane

List of symbols and abbreviations PDMS – polydimethylsiloxane; PHexMS – polyhexylmethylsiloxane; POMS – polyoctylmethylsiloxane; PDecMS – polydecylmethylsiloxane; MFFK-1 (MFFK) – microfiltration porous support (Vladipor, Russia); UFFK – ultrafiltration porous support (Vladipor, Russia); PTMSP – poly[1-(trimethylsilyl)-1-propyne]; PMHS – polymethylhydrosiloxane; θ – stage cut; 𝐽𝑝𝑒𝑟𝑚 – volume permeate flow, cm3/s; 𝐽𝑓𝑒𝑒𝑑 – volume feed flow, cm3/s; – concentration of i component in the permeate, % vol.; 𝐶𝑝𝑒𝑟𝑚 𝑖 – concentration of i component in the feed, % vol.; 𝐶𝑓𝑒𝑒𝑑 𝑖 𝑝𝑓𝑒𝑒𝑑 – feed pressure, cmHg; 𝑝𝑝𝑒𝑟𝑚 – permeate pressure, cmHg; 𝐽𝑝𝑒𝑟𝑚 – volume permeate flow, cm3/s; 𝑆 – membrane area (cm2); P/l – membrane permeance, GPU (1 GPU = 10-6 cm3(stp)·cm-2·s-1·cmHg-1 = 7.5·10-12 m3(stp)·m-2 ·s-1·Pa-1); P – permeability coefficient, Barrer (1 Barrer = 7.5·10−18 m3·m·m−2·s-1·Pa-1); 𝛼𝑖 – ideal selectivity; 𝛼𝑚𝑖𝑥 – mixed gas selectivity. Introduction A problem of the separation of hydrocarbon gas mixtures with a broad composition range emerges during the production and processing of the natural and associated gas, as well as in the processes of oil refining, petro-, gas, and coal fuel chemistry. In the industry, this task is conventionally solved by low-temperature rectification, which requires high energy consumption. Membrane gas separation is considered to be one of the alternative approaches. Due to a lack of phase transition membrane gas separation is characterized by low energy consumption while the modularity of membrane equipment allows varying its productivity based on the separation task [1-5].

There are several groups of polymers capable of selective separation of hydrocarbon C2+/methane mixture [6; 7]. The most interesting materials for the researchers are the silicone rubbers [1; 8; 9] and high free volume glassy polymers, such as polyacetylenes [9-11], polybenzodioxanes [12-14] and polynorbornenes [15; 16]). An interesting feature of high free volume glassy polymers is the increased separation selectivity of hydrocarbon mixtures in comparison with the ideal selectivity calculated using individual gas permeances. For instance, for poly[1-(trimethylsilyl)-1-propyne] (PTMSP) the ideal n-butane/methane selectivity is 5, while selectivity increases to 28 in n-butane/methane binary mixture [9]. Drawbacks of highly permeable glassy polymers are related to aging problems, i.e. the decrease of their non-equilibrium free volume and subsequent deterioration in membrane permeability over time. Current research efforts are aimed at the stabilization of material transport properties [10; 17; 18] and thin-film composite membranes based on PTMSP [19]. Silicone rubber (PDMS in the first place) is by far the most widely used membrane material for C2+/methane separations [1; 8; 20]. These materials have earned a widespread use due to high permeance, chemical stability to the separated mixture, and relative ease of synthesis [21-24]. Also, PDMS is commonly applied to increase the selectivity of gas separation membranes by plugging the pin-holes of the selective layer [25-27]. However, with the transition from individual gases to mixtures PDMS selectivity significantly declines [9]. The silicone rubbers selectivity can be improved by increasing the solubility selectivity term. This can be achieved by physical modification of the polymer via mixed matrix membranes approach [28-30] and by chemical modification of the polymer chain [31-33]. Most of the works addressing the siloxanes chemical modification deal with dense membranes and single gas permeation. For example, a series of polysiloxanes with side and main chain modification was investigated and a significant increase of the propane/methane ideal selectivity was found [33]. At the same time, papers discussing the composite membrane preparation and mixed gas separation are significantly less common. Schultz J. and Peinemann K.V. [9] examined the separation of butane/methane mixture through composite membranes based on polyoctylmethylsiloxane (POMS). It was demonstrated that the separation selectivity of butane/methane mixture (3% of n-butane) for the POMS and PDMS composite membranes are 12 and 5 respectively. Thus, POMS membrane demonstrates 2.4 times higher selectivity comparing to PDMS membrane [9]. Earlier, we proposed a simplified one-stage synthesis of polyalkylmethylsiloxanes with the increased hydrocarbon ideal separation selectivity [34]. In this work, the separation of eight component mixture of saturated and unsaturated С1-С4 hydrocarbon through dense and composite membranes based on polyhexylmethylsiloxane (PHexMS), polyoctylmethylsiloxane (POMS),

polydecylmethylsiloxane (PDecMS) was studied for the first time. Dense and composite PDMS membranes synthesized in this work were used for comparison.

Experimental part 1. Materials The

following

reagents

were

used

to

prepare

the

polymer

solutions:

polymethylhydrosiloxane (PMHS) with number average molecular weight Mn 1700-3200 g/mol (Sigma-Aldrich), vinyl-terminated polydimethylsiloxane (PDMS, Mn 25000 g/mol (SigmaAldrich)), Karstedt’s catalyst (platinum (0) complex of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane in xylene, Aldrich), 1-hexene (97%, Sigma-Aldrich), 1-octene (98%, Sigma-Aldrich), 1-decene (94%, Sigma-Aldrich), 1,7-octadiene (98%, Sigma-Aldrich), n-hexane (99%, Chimmed), toluene (99.8%,

Chimmed).

All

reagents

were

used

without

additional

purification.

Polyalkylmethylsiloxanes were prepared via the hydrosilylation reaction of PMHS with alkenes in the presence of the Karstedt’s catalyst (Figure 1). The preparation of the dope solution and dense membranes was described in detail in our previous work [34]. A distinctive feature of the proposed technique is a simultaneous PMHS modification and cross-linking during membrane synthesis, leaving the stages of polymer extraction and additional reagents purification. Karstedt’s catalyst, 1-alkene, and 1,7-octadiene in the form of solutions in hexane were consequently added to a 3% wt. solution of PMHS in n-hexane. The ratio of PMHS:1-alkene:1,7-octadiene was respectively 1:0.95:0.05 [35]. The resulted mixture was stirred under the backflow condenser at 60°С until the desired viscosity for the membrane formation was achieved. Membranes based on polydimethylsiloxane (PDMS) were prepared for comparison. In separate flasks the following solutions in toluene were prepared: 3% wt. of vinyl-terminated PDMS with a number average molecular weight Mn 25000 g/mol (Sigma-Aldrich), 1,5% wt. of cross-linking agent PMHS with a number average molecular weight Mn 1700-3200 g/mol (Sigma-Aldrich) and Karstedt’s catalyst (Sigma-Aldrich). Solutions of PMHS and Karstedt’s catalyst were added to the PDMS solution, and the reaction mixture was vigorously stirred for 1 hour at 60°C (ratio of PDMS:PMHS:catalyst was 10:1:0.01 respectively). 2. Dense membranes preparation Dense membranes (films) were made by casting of the polymer solution in n-hexane (in case of polyalkylmethylsiloxanes) or toluene (in case of PDMS) on the stainless steel net (mesh size 40 μm) which was fixed on a fluoroplastic surface. It was shown elsewhere that the fabric reinforcement doesn’t affect the gas transport properties of the membranes based on polysiloxanes [34].

а)

+

CH3 Si

O

t, [Pt]

n

H

CH3

CH3 Si

Si

O

n-x

x

Si

b)

t, [Pt]

n

H

n

CH3

CH3

O

Si

O

CH3

+

CH3

O

O

Si

O

Si

n-x

x

Si

O n

CH3 +

CH3

c)

Si H

O n

CH3 t, [Pt]

Si

CH3 O

Si

O n-x

x

Si CH3

O n

Figure 1. Synthesis of polyalkylmethylsiloxanes: а) polyhexylmethylsiloxane – PHexMS, b) polyoctylmethylsiloxane – POMS, c) polydecylmethylsiloxane – PDecMS. 3. Composite membranes preparation

The composite membranes were prepared using the dip-coating technique, by drawing porous support over the surface of the polymer solution to form a meniscus which prevents soaking of the whole support [12; 36]. Two types of supports were used for the production of flat-sheet composite membranes: microfiltration and ultrafiltration membranes, MFFK-1 and UFFK (ZAO STC "Vladipor"), respectively, with a porous filtration layer based on fluoroplast F42L, coated on a nonwoven fabric from polyethylenterephthalat. The support was impregnated with water before the application of the selective layer. Commercial composite membranes MDK-3 (ZAO STC "Vladipor") and POMS (HZG, Germany) were studied for the comparison. POMS-HZG composite membrane was kindly supplied by Helmholtz-Zentrum Geesthacht Zentrum fur Material und Kustenforschung GmbH (Geesthacht, Germany). 4. Scanning electron microscopy The microstructure of the resulting films was studied on a TM-3000 scanning electron microscope (Hitachi, Japan) in a backscattered electron imaging mode. During microscopic measurements, the accelerating voltage was varied from 5 to 15 kV to maximize the quality and information content of the images. The composite membranes were subjected to the following pretreatment: the samples were cloven in a liquid nitrogen atmosphere; the resulting cleavages were coated with gold using a DSR-1 spraying gun (NSC Co., Iran). The gold film layer thickness was 50–100 Å. 5. Porosimetry The analysis of the support transport pores was performed by liquid-liquid displacement technique using the equipment «Poroliq 1000ML» (IB-FT GmbH, Germany) and isobutanol and water as wetting and non-wetting liquids, respectively. 6. Mixed gas permeability. The study of gas transport characteristics of dense and composite membranes during the separation of a multi-component mixture of С1-С4 hydrocarbons was carried out using the equipment presented in Figure 2. The composition of the separated mixture is presented in Table 1. The concentrations of components were chosen in accordance with the composition of associated petroleum gases of Pravdinskoe (RF) and Rechinskoe (Belarus) deposits [37]. The measurements were performed at a temperature of 25°С and feed membrane pressure from 1 to 3 atm (gauge). Table 1. The composition of the model hydrocarbon mixture.

Gas Concentration, % vol.

CH4

CO2

C2H4

C2H6

C3H6

C3H8

i-C4H10 n-C4H10

69.2

3.3

0.1

3.4

0.6

11.8

0.8

Feed pressure gauge

Feed gas mixture

Retentate pressure gauge

Feed Feed on/off valve

10.6

Needle valve

Retentate

Pressure regulator

Membrane cell

Flowmeter - R to GS

Permeate pressure gauge

to GS Flowmeter - F

Vacuum pump

Figure 2. Scheme of the equipment employed for the measurement of membrane mixed gas transport characteristics. The pressure of the gas mixture fed to the membrane cell was set by the pressure regulator and detected by the pressure gauge. Permeate was degassed by a membrane vacuum pump Ilmvac 030 Z-EC and its flow rate was measured by a soap film meter Flowmeter-P. Partial component fluxes were calculated based on their concentrations in the feed, which was determined by the gas chromatography technique. Retentate flow was regulated by a needle valve and measured by a soap film meter Flowmeter-P. The pressure of feed, retentate, and permeate was determined using feed, retentate and permeate pressure gauges, respectively. Stage cut θ is an important parameter to characterize membrane properties in the separation of multicomponent mixtures. According to IUPAC terminology stage cut is a parameter defined as the fractional amount of the total feed entering a membrane module that passes through the membrane as permeate: 𝜃=

𝐽𝑝𝑒𝑟𝑚 𝐽𝑓𝑒𝑒𝑑

where 𝐽𝑝𝑒𝑟𝑚 – volume permeate flow (cm3/s), 𝐽𝑓𝑒𝑒𝑑 – volume feed flow (cm3/s).

In this work stage cut of 5% was adjusted by regulation of the retentate flow. As was noted in work [38], the low value of the stage cut allows neglecting the change in the concentration along the membrane cell. In current work, a simplification was assumed that the upstream concentrations of components are equal to their average concentrations between the inlet and the outlet of the membrane cell. The composition of feed, permeate and retentate was analyzed employing gas chromatograph “Gazochrom-2000” (Chromatek, Russia). The chromatograph was equipped with a thermal conductivity detector and a chromatography column packed with 20% wt. heptadecane on a diatomite carrier. The analysis of 0.5 ml gas sample was carried out under following conditions: the dosing valve’s thermostat temperature was 50 °C, the column’s thermostat temperature was 50 °C, the detector’s thermostat temperature was 160 °C, the flow rate of carrier gas (helium) was 30 ml/min. Membrane permeance was calculated using the formula: 𝐶𝑝𝑒𝑟𝑚 ∙ 𝐽𝑝𝑒𝑟𝑚 𝑖

𝑃/𝑙 = 𝑆 ∙ (𝐶𝑓𝑒𝑒𝑑 ∙ 𝑝𝑓𝑒𝑒𝑑 ― 𝐶𝑝𝑒𝑟𝑚 ∙ 𝑝𝑝𝑒𝑟𝑚) 𝑖

𝑖

(1)

– concentration of i component in the permeate (% vol.), 𝐶𝑓𝑒𝑒𝑑 - concentration of I 𝐶𝑝𝑒𝑟𝑚 𝑖 𝑖 component in the feed (% vol.), 𝑝𝑓𝑒𝑒𝑑 - feed pressure (cmHg), 𝑝𝑝𝑒𝑟𝑚 – permeate pressure (cmHg), 𝐽𝑝𝑒𝑟𝑚 – volume permeate flow (cm3/s), 𝑆 – membrane area (cm2). The separation selectivity between i and j components was calculated using the formula: (𝑃/𝑙)𝑖

𝛼𝑚𝑖𝑥 = (𝑃/𝑙)𝑗

(2)

2. Results and discussion 1. Dense membranes During the separation of a mixture of hydrocarbon gases, it is important to consider the interaction of its components with the membrane material. The transport properties of the membrane substantially depend on the composition of the mixture being separated. Therefore, the measurement of transport properties in a mixture of complex composition that simulates associated petroleum gas is an important step in assessing the applicability of the developed membrane materials and membranes for a specific separation task. Table 2 represents the comparison of gas transport characteristics for dense membranes from polyalkylmethylsiloxanes and PDMS synthesized in this work using eight component model mixture. All of the studied polyalkylmethylsiloxanes show lower permeabilities for methane, propane, and butane than PDMS. Moreover, gas permeability for all gases declines with the increase of the side-chain substituent length in the row PHexMS - POMS - PDecMS. At the same

time, the opposite trend is observed for the hydrocarbon separation selectivity. The selectivities С3H8/СH4 and С4H10/СH4 for all of the polyalkylmethylsiloxanes are higher than for PDMS, and their values rise with the increase of the side-chain substituent length in the row PHexMS - POMS - PDecMS. For the gas pair n-butane/methane the dense membrane separation selectivities PHexMS (αmix=15.5), POMS (αmix=16.5) and PDecMS (αmix=18.2) more than twice exceed the PDMS selectivity (αmix=7.6). Earlier we studied single gas permeabilities of methane, propane, and butane for membranes from PHexMS, POMS, PDecMS, and PDMS [34]. The values of ideal selectivities for С3H8/СH4 and С4H10/СH4 gas pairs, obtained in work [34], are also presented in table 2. It is clear that for all of the studied membrane materials a decrease of the separation selectivity is observed with the transition from ideal gases to the model mixture. Such reduction of gas transport characteristics (e.g. from 27 to 18.2 for PDecMS) suggests a significant swelling of the membrane material in the model mixture, first of all, due to the presence of butane. Table 2. Gas transport characteristics of dense membranes. Permeability coefficient Dense

in the gas mixture,

membrane

Barrer

Mixed gas selectivity

Ideal selectivity [34]

CH4

C3H8

nC4H10

C3/C1

C4/C1

C3/C1

C4/C1

PDMS

700

2440

5300

3.5

7.6

4.3

14.1

PHexMS

200

1050

3100

5.3

15.5

5.9

23.5

POMS

180

1020

2970

5.7

16.5

6.3

24.7

PDecMS

155

950

2850

6.1

18.2

6.7

26.7

Figure 3 shows permeabilities (a) and selectivities (b) for four polyalkylmethylsiloxanes (alkyl: С1, С6, С8, С10) during the separation of an eight component model mixture of hydrocarbons. The first homolog in the series, PDMS, possesses the highest permeability for all components of gas mixture (Figure 3a). Meanwhile, methane is the least permeable component of all four studied polyalkylmethylsiloxanes. The selectivity of СО2/СН4 separation is approximately equal for all polymers. At the same time, higher separation selectivities for dense PHexMS, POMS and PDecMS membranes compared to PDMS are observed for all saturated and unsaturated hydrocarbons. Moreover, selectivity values rise with the increase of the alkyl substituent length.

a)

b)

5000

P, Barrer

4000 3000 2000 1000 0

18 16 14 12 10 n-C4H10 8 i-C4H10 C3H8 6 C3H6 4 C2H6 C2H4 2 CO2 CH4 0

αmix (i/CH4)

6000

n-C4H10 i-C4H10 C3H8 C3H6 C2H6 C2H4 CO2 CH4

Figure 3. Permeability (a) and selectivity (b) of polyalkylmethylsiloxanes (PDMS, PHexMS, POMS, PDecMS) during the separation of eight component hydrocarbon model mixture. To understand the features of hydrocarbon transport through polyalkylmethylsiloxanes, it is important to understand the nature of hydrocarbon transport through PDMS. Many research groups observed a change in the transport characteristics of the silicone rubbers with the transition to mixed gas experiments [9; 38-41]. For instance, according to study [9], a decrease in the selectivity from 17 to 5 and from 26 to 12 was noticed for composite membranes with the selective layers from PDMS and POMS, respectively, with the transition from individual gas to the mixture, containing 3% vol. of n-butane in methane. The authors associated this selectivity decline with the swelling of the selective layer in n-butane, which in turn leads to the rise of methane transfer due to the diffusion component. The work of R.D.Raharjo et al. [39] has demonstrated that with the transition from individual gas to binary n-butane/methane mixture containing 6% mol. n-butane at 4.4 bar the PDMS selectivity reduced from 11.8 to 11.5. However, permeability increased from 1200 Barrer for pure methane to 1250 Barrer for the mixture containing 6% mol. of n-butane. The authors explained this difference in the same manner as I.Pinnau and Z.He [38], where it was noticed that not only the composition of the separated mixture but also the thickness of the membrane affects the values of permeability and selectivity. I.Pinnau and Z.He [38] noted that under the same experimental conditions the PDMS composite membrane n-butane permeability (5100 Barrer) is 2.5 times lower than the permeability of PDMS dense film with thickness 150 μm (13000 Barrer). As a result of nonuniformity in permeability decrease of mixture components (25% loss for methane), the selectivity of thick PDMS film is almost twice higher than for 10 μm PDMS composite membrane. Authors [38; 39] attributed the influence of thickness to the significant effect of the concentration polarization on the properties of the composite membrane.

In this article, the thickness of the dense film membranes was around 85 μm, which is almost twice lower than in works [38; 39]. Therefore, the decrease of selectivity, as well as permeability with a transition from individual gases to hydrocarbon mixtures may be caused by the combination of the polymer swelling in the gas mixture components (propane, butane) and the concentration polarization. An eight component hydrocarbon mixture was studied in our work, unlike [9; 38; 39]. During the separation of that mixture, it is important to take into account the presence of several components with high solubility coefficients in the membrane material which leads to a mutual component influence on a transfer through the membrane. For instance, a relative drop of the separation selectivity significantly differs for n-butane and propane with close concentrations (n-butane 10.6% vol. and propane 11.8% vol.) in the mixture (Table 2) when mixture experiments are compared to individual gas tests. For the POMS (Table 2), the selectivity decrease is equal to 33% and 11% for n-butane/methane and propane/methane, respectively. In case of PDMS, a relative selectivity drop is even higher: 47 and 20% for n-butane/methane and propane/methane, respectively. From this point of view, the studied polyalkylmethylsiloxanes have an advantage against PDMS. The revealed decrease of polysiloxanes “plasticization” effect in the presence of long alkyl substituent in the side-chain may be related to the different macrochain packing in the polymer. In our recent work [34] it was noted that the side groups of polyalkylmethylsiloxanes may pack with the formation of a mesophase that is clearly visible on X-ray diffraction patterns at temperatures below the polymer glass transition temperatures. According to [34], such packing of side groups was the most probable reason for the n-butane/methane diffusion selectivity increase with the rise in length of the side substituent (from PDMS to PDecMS). Polyalkylmethylsiloxanes, in contrast to PDMS, are heterogeneous materials in terms of sorption and diffusion properties. They have areas formed by alkyl side substituents and areas formed by polysiloxane chains. Methane transport is difficult due to its poor solubility in hydrocarbon fragments. At the same time, C2+ hydrocarbons are freely transferred through the membrane. The combination of these factors leads to significantly greater selectivity for C3+ hydrocarbons for polyalkylmethylsiloxanes compared to PDMS. The growing trend of the С2+ separation selectivity from hydrocarbon mixture using polyalkylmethylsiloxanes is preserved with the increase of side substituent length. Thereby, the PDecMS membrane showed the highest selectivities for all of the mixture components: 3.0, 2.8, 7.0, 6.1, 7.6 and 18.2 for C2H4, C2H6, C3H6, C3H8 i-C4H10 and n-C4H10, respectively.

2. Composite membranes It is common practice in commercial membrane gas separation to employ composite membranes with the selective layer from silicone rubbers for the recovery of organic vapors and

hydrocarbons С2+ [1; 4; 42]. Undoubtedly the separation properties of the composite membrane are largely determined by the selective layer material and thickness, but the porous support also affects membrane transport characteristics. 2.1.

The influence of the porous support on the transport characteristics of the composite membrane.

Gas transport properties of composite membranes prepared using MFFK-1 and UFFK supports were compared. The supports are characterized by different pore sizes and total porosity which was measured using the liquid-liquid displacement porometer. The average pore size in the MFFK-1 support (250-410 nm) is twice bigger than in the UFFK (128-145 nm). For the MFFK-1 support, the largest pore size lies in the range of 400-480 nm, whereas, the size of the smallest pore is in the interval of 170-240 nm. For the UFFK support, these ranges are smaller: the largest pore is 251-260 nm, the smallest – 86-97 nm. Such a difference in the porous structure of supports certainly affects their permeance. The CO2 permeance of the MFFK-1 support is 420 m3/(m2·h·atm), for UFFK - 195 m3/(m2·h·atm). PHexMS was used as a selective layer material. SEM micrographs of the synthesized membranes are presented in Figure 4. The membranes’ selective layer thickness differs by a factor of two and is equal to 5.5 and 2.5 μm for PHexMS/MFFK and PHexMS/UFFK, respectively.

a)

b)

Figure 4. SEM micrographs of the composite membranes PHexMS/MFFK (a) and PHexMS/UFFK (b). Based on the selective layer thickness difference, as expected, mixture components permeance values are higher for PHexMS/MFFK, than for PHexMS/UFFK (Figure 5). However for the separation selectivity, the situation is the opposite, namely, PHexMS/MFFK selectivity is higher than PHexMS/UFFK selectivity (Figure 6).

1200 1000

P/l, GPU

800 600 400 200 1

2

0

Figure 5. The permeance of composite membranes PHexMS/MFFK (1) and PHexMS/UFFK (2). The reverse osmotic composite membranes on supports from Nylon 6,6 MF were assessed in work [43], and the same effect was observed: the screening of the selective layer was reduced with the increase of the surface porosity due to decline of the support mass transfer resistance. Thus, based on the research objective, namely the development of the composite membrane with increased hydrocarbon separation selectivity, MFFK-1 support was chosen for further investigation. 14.0

αmix (X/CH4)

12.0 10.0 8.0 6.0 4.0

1 2

2.0 0.0

Figure 6. The selectivity (Х/СН4) for composite membranes PHexMS/MFFK (1) and PHexMS/UFFK (2).

2.2.

The influence of the selective layer material on the transport properties of the composite membrane.

Composite membranes were formed on the porous MFFK-1 support in order to study the influence of polyalkylmethylsiloxane, serving as a selective layer material, on the transport properties. The SEM micrographs of the produced membranes are presented in Figures 4a and 7.

All of the developed composite membranes had comparable selective layer thickness – 5.5 μm, which allowed a comparison of the permeances of membranes without taking into account the effects of layer thickness.

a)

b)

Figure 7. SEM micrographs of POMS/MFFK (a) and PDecMS/MFFK (b) membranes.

a)

b)

16

1000

14

800 600 400 200 0

αmix (i/CH4)

P/l, GPU

18

1200

12 10

8 n-C4H10 i-C4H10 6 C3H8 4 C3H6 C2H6 2 C2H4 CO2 0 CH4

n-C4H10 i-C4H10 C3H8 C3H6 C2H6 C2H4 CO2 CH4

Figure 8. Permeance (a) and selectivity (b) of composite membranes at mixed-gas experiments. Figure 8 represents the comparison of transport characteristics for the produced composite membranes with different polyalkylmethylsiloxanes as a top layer. The general trend corresponds to the results obtained for the dense film membranes: the separation selectivity of hydrocarbons and methane rises for all mixture components with the increase of the hydrocarbon substituent length (Figure 8b) while permeance slightly declines (Figure 8a). In this work, gas permeance measurements were conducted at low pressures of the feed hydrocarbon mixture, and accordingly in the range of low partial pressures of the most plasticizing agents: propane, iso-butane, and n-butane. For example, only an insignificant rise of n-

butane/methane selectivity from 16.3 to 17 was observed for the composite membrane PDecMS/MFFK for the studied change of n-butane activity from 0.06 to 0.13, which amounts to the increase of pressure from 1 to 3 atm. As can be seen from Figure 9, the permeance of each component increases with an increase in pressure (Figure 9a), though methane permeance virtually doesn’t change, because methane sorption in the membrane material is minimal, compared to the other components of the gas mixture. The rise of hydrocarbon gas molecular weight boosts its permeance dependence from pressure. For ethane and ethylene, the permeance increase with the rise of pressure from 1 to 3 atm (gauge) is practically absent (Figure 9a); the selectivity with respect to methane also remains changeless (about 3.1-3.1 and 2.5-2.6 respectively, Figure 9b). For propane and propylene, the dependence from pressure is evident: permeance increases from 315 and 303 GPU to 345 and 335 GPU for propane and propylene with the increase in pressure from 1 to 3 atm (gauge) (Figure 9a). Meanwhile, propane/methane and propylene/methane selectivities rise from 6.2 to 6.6 and from 6.0 to 6.5 respectively (Figure 9b). Change is even more significant for iso-butane and especially for n-butane. With an increase of pressure from 1 to 3 atm (gauge) the n-butane permeance rises by 9.5% from 820 GPU to 895 GPU, separation selectivity grows from 16.3 to 17.4 (by 9.5%). As in work [44] for PDMS, the current study didn’t demonstrate considerable selectivity for ethane/ethylene and propane/propylene gas pairs for the composite membranes based on polyalkylmethylsiloxanes. The experimentally obtained results are in good agreement with previously published data. For example, an increase of permeance (from 2520 to 5480 GPU with the gain in pressure from 10 to 40 bar), as well as of selectivity (from 22 to 27 with the gain in pressure to 30 bar) were observed for membranes based on POMS in work [30]. However, a further increase of pressure led to a decline of selectivity (down to 25 [30]). The authors connected such behavior with a substantial swelling of the selective layer material in n-butane, which leads to significant growth of methane diffusion. For PDMS-based membranes, similar trends were observed for binary nbutane/methane and propane/methane mixtures in works [38; 39; 45]. This demonstrates that the increase of permeance and selectivity with pressure reported in this work is a characteristic property of silicone rubbers.

18

1000 nC4H10

nC4H10

15

800 600 iC4H10

400

C3H6 C3H8

200

C2H4 C3H6 CO2 CH4

0 2

a)

3 pfeed, bar

4

α (X/CH4)

P/l, GPU

12 9

iC4H10

6

C3H8 C3H6 CO2 C2H6 C2H4

3 0 2

b)

3 pfeed, bar

4

Figure 9. Feed pressure dependence of permeance and selectivity for PDecMS/MFFK membrane.

2.3.

A comparison of the transport properties of composite membranes and commercial analogs.

A comparison of the properties of the composite membranes synthesized in this work with commercially available analogs was carried out using two industrial membranes based on silicon rubber: MDK-3 (Vladipor, Russia) and POMS (HZG, Germany). The top layer of the POMS membrane (HZG, Germany) is composed of cross-linked polysiloxane, based on polyoctylmethylsiloxane [30], and Lestosil (co-polymer of polyphenyl silsesquioxane and polydimethylsiloxane) is the selective layer of MDK-3 [46; 47]. Figure 10 represents a comparison of transport properties of commercial membranes and composite membrane PDecMS/MFFK obtained in the current study utilizing the original method of one-stage polyalkylmethylsiloxanes synthesis [34; 48]. 2000

18 16 14 αmix (X/CH4)

P/l, GPU

1600 1200 800 400 0

a)

12 10 8 6 4

2 1 3

2 0

b)

12 3

Figure 10. A comparison of permeances (a) and selectivities (b) of a composite membrane PDecMS/MFFK produced in current study (1) with commercial analogs MDK-3 (2) and POMSHZG (3) (under equal experimental conditions: р = 2 atm, t = 25°C, θ = 1%). Analyzing the results of gas transport experiments for different membranes it should be noted that permeances of composite membranes based on polyalkylmethylsiloxanes (including POMS (HZG, Germany)) are substantially inferior to MDK-3 (Vladipor, Russia), however, POMS membrane selectivity is more than double that of MDK-3 and PDMS/MFFK membranes. The highest selectivity was demonstrated by PDecMS/MFFK membrane, with selectivity 16.7 for nbutane/methane gas pair. Hydrocarbon С2+ permeance of the PDecMS/MFFK membrane is comparable with the POMS-HZG membrane. An undoubtedly important result is that the developed membrane possesses selectivity for all of the components of the eight component mixture with respect to methane. Such effect shows promise for the use of membranes based on PDecMS for the extraction of hydrocarbon gases from gas mixtures.

Conclusions For the first time, the properties of polyalkylmethylsiloxanes (PHexMS, POMS, PDecMS) during the separation of eight component hydrocarbon mixture which models the associated petroleum gas were studied. С3H8/СH4 and С4H10/СH4 separation selectivities are higher for all polyalkylmethylsiloxanes than for PDMS, and their values rise with the increase of the side substituent length in the row PHexMS < POMS < PDecMS. Separation selectivities for the nbutane/methane gas pair on dense membranes made from PHexMS (αmix=15.5), POMS (αmix=16.5) and PDecMS (αmix=18.2) are more than two times greater than separation selectivity for the same gas pair on PDMS membrane (αmix=7.6). Also, a decrease of the polysiloxane “plasticization” effect in the presence of long alkyl substituent in the side chain was found, and this effect was attributed to different macrochain packing in the polymer. PDecMS membrane showed the highest selectivity for all of the gas mixture components with respect to methane: 3.0, 2.8, 7.0, 6.1, 7.6 and 18.2 for C2H4, C2H6, C3H6, C3H8, i-C4H10 and n-C4H10, respectively. A comparison of gas transport properties of composite membranes deposited on the MFFK-1 and UFFK (micro- and ultrafiltration type) supports was made in this study. It was shown that support with bigger pore size provides higher separation selectivity due to the decrease of support mass transfer resistance. Gas permeability of PDecMS/MFFK composite membrane with increasing from 0.06 to 0.13 n-butane activity (feed pressure change from 1 to 3 atm) was measured, and an increase in n-butane/methane selectivity from 16.3 to 17.0 was observed. Same

as for dense membranes, hydrocarbon С2+/methane separation selectivity increases with the growth of alkyl substituent length in the row PHexMS/MFFK < POMS/MFFK < PDecMS/MFFK. A comparison of PDecMS/MFFK membrane gas transport properties with that of commercial composite membranes with selective layers from silicone rubbers (MDK-3, ZAO STC “Vladipor”, Russia, and POMS, HZG, Germany) was conducted. The permeance of multicomponent mixture through the membranes PDecMS/MFFK, MDK-3, and POMS–HZG at similar experimental conditions was measured. The highest n-butane/methane selectivity value of 16.7 was demonstrated by PDecMS/MFFK membrane, which is 40% and 20% higher than for MDK-3 (10.1) and POMS– HZG (13.9) membranes, respectively. This indicates a way to utilize composite membranes with the selective layer from PDecMS for the separation of С2+ hydrocarbons from their mixtures with methane.

Acknowledgments This work was carried out in the A.V.Topchiev Institute of Petrochemical Synthesis (Russian Academy of Sciences) and was funded by the Russian Science Foundation, grant number 19-1900647. The research of porous support influence on composite membranes gas transport characteristics was performed within the frame of TIPS RAS State program. The authors acknowledge D.S.Bakhtin for SEM photos and A.A.Yuskin for porosimetry measurements.

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Membrane gas separation of eight component С1-С4 hydrocarbons mixture is studied.

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Composite membranes based on polyalkylmethylsiloxanes are obtained.

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The effect of the selective layer material on gas transport properties is shown.

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The effect of porosity of porous support on gas transport properties is presented.

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The resulting membranes are compared with commercial analogues.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: