Synthesis, modification and gas permeation properties of DD3R zeolite membrane for separation of natural gas impurities (N2 and CO2)

Accepted Manuscript Synthesis, modification and gas permeation properties of DD3R zeolite membrane for separation of natural gas impurities (N2 and CO2) Mohammad Javad Vaezi, Ali Akbar Babaluo, Hafez Maghsoudi PII:

S1875-5100(18)30031-3

DOI:

10.1016/j.jngse.2018.01.018

Reference:

JNGSE 2425

To appear in:

Journal of Natural Gas Science and Engineering

Received Date: 28 August 2017 Revised Date:

9 January 2018

Accepted Date: 10 January 2018

Please cite this article as: Vaezi, M.J., Babaluo, A.A., Maghsoudi, H., Synthesis, modification and gas permeation properties of DD3R zeolite membrane for separation of natural gas impurities (N2 and CO2), Journal of Natural Gas Science & Engineering (2018), doi: 10.1016/j.jngse.2018.01.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Synthesis, Modification and Gas Permeation Properties of DD3R Zeolite Membrane for Separation of Natural Gas Impurities (N2 and CO2)

Nanostructure Material Research Center (NMRC), Sahand University of Technology, P.O.Box 51335-1996, Tabriz,

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Iran

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Mohammad Javad Vaezi1, Ali Akbar Babaluo1,* Hafez Maghsoudi1

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Corresponding author. Tel.: +984133458084; Fax: +984133458084. E-mail address: [email protected] (a.a. babaluo)

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Abstract A good DD3R zeolite membrane layer was synthesized via secondary growth hydrothermal method, using a different procedure for preparing the membrane synthesis solution. X-ray

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diffraction (XRD) and scanning electron microscopy (SEM) indicated the formation of DD3R layer on the support. The defects formed in calcination step were blocked by surface treatment of the synthesized membrane via a simple coating technique using polydimethyl siloxane (PDMS)

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solution. SEM showed a good PDMS polymer coating on the surface of DD3R membrane. The gas separation properties of the membranes were extensively studied for gas permeation of CO2,

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N2 and CH4 and their mixtures with composition closed to natural gas composition. After applying PDMS, a considerable improvement of perm-selectivity for CO2/CH4 (0.6 to 220), CO2/N2 (0.8 to 28) and N2/CH4 (0.75 to 8) was observed. Furthermore, comparing these permselectivities with other modified DD3R membranes in literature showed better separation

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performance for our membrane both in the separation of CO2 and N2. The membrane selectivity for CO2/CH4 and N2/CH4 mixtures was 80-120 and 7-9 at room temperature respectively. The presence of different species in the binary and ternary mixtures have sufficient effects on the

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performance of modified membrane in the separation of natural gas impurities.

Keywords: DD3R; Zeolite Membrane; PDMS; Gas Separation; Modified Membrane;

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1.1 Introduction Separation and removal of natural gas impurities such as CO2 and N2 is of great interest to

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utilize upgraded CH4 as feed or a fuel. Because CO2 and N2 provides no heating value and CO2 as well as other acid gases, can form acids in the presence of water that corrode pipelines and other equipment. Furthermore, considerable amount of energy is wasted for cooling the N2 to low temperatures in some operations such as LNG. Conventional separation approaches of CO2

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and N2 are amine scrubbers, solexol process (physical dissolves at high pressure) and cryogenic

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distillation, but at high pressures of natural gas wells, operation of these units are expensive (Wu et al., 2015; Rufford et al., 2012). In the past decade, literatures have been surveyed to confirm the feasibility of membrane technology for removal of CO2 and N2 from natural gas (Rufford et al., 2012). Polymeric membranes are typically used for CO2 removal and among these, cellulose acetate is the most commonly applied polymeric membrane for this target. But it is not

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sufficiently selective for N2/CH4 (Rufford et al., 2012; Wu et al., 2015). Zeolite membrane as new type of membranes showed good performance on the separation of these impurities (Kosinov et al., 2016). Among zeolite membranes, DD3R zeolite membrane is more interest due

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to its high potential in the separation of both CO2 and N2 from natural gas. This potential is associated with the both high adsorption and diffusion parameter of CO2 and the diffusion

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parameter of N2 related to CH4 in DD3R zeolite membrane (Bergh et al., 2008; Himeno et al., 2007b; Tomita et al., 2004; Yang et al. 2016). Because of the good characteristics and potential applications of DD3R zeolite membrane in gas separation, various studies have attempted to synthesize defect-free layer of this kind of zeolite membrane. As the best of our knowledge Tomita et al. (2004) and Himeno et al. (2007a) and the other researchers at NGK insulator could be successful in synthesis of DD3R membrane 3

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with good performance. Except them, Zhou et al. (2013) were able to obtain a selective membrane (performance lower than Tomita et al. (2004) and Himeno et al. (2007a)) with modifying the synthesized membrane by PDMS coating, and the other studies (Nakayama et al.,

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2009; Uchikawa et al., 2013; Yajima and Nakayama, 2009; Yang et al., 2016; Zhou et al., 2013) could be successful in the formation of DD3R zeolite layer without acceptable separation

performance of CO2 and N2 from CH4. So, it was found that it is difficult to synthesize defect-

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free DD3R zeolite membrane with the optimal permeance and selectivity, especially on a large

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scale.

Such difference in the separation performance of membranes could be attributed to the defects induced in the calcination step which is carried out to remove template and activate the pores of zeolite (Hong et al., 2011). These defects are caused by removing the template molecules trapped in the different layers of the membrane structure outside the pores and/or by

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the thermal expansion and contraction during the calcination step. It is very useful to use a suitable method that can prevent defects during synthesis, on the other hand, reducing or even eliminating the flow contribution of possible defects within polycrystalline zeolite membranes by

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post-synthetic treatments could be a practical choice to improve the performance of membranes (Zhang et al., 2008). One of these suitable choices is surface coating method. In this method, a

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thin layer covers completely the surface of membrane or blocks the defects. Different kind of precursors such as polymer or prepared sol-gel solution can be used in surface coating. By membrane surface treatment, the flow contribution of defects decreases, while the total resistance against mass transfer remains constant (Maghsoudi, 2015). However, still a few works were carried out on the surface treatment of DD3R zeolite membrane for defect abatement.

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Table (1) shows a summary of the different methods conditions used for both synthesis and modification of DD3R zeolite membrane. Zheng et al. (2008) have synthesized the DD3R membrane and modified the surface of the membrane by chemical vapor deposition at 550 ℃ for

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6 days. They found that the modified DD3R membrane is capable to separate H2/CO2 with good perm-selectivity (32.7) at 550 ℃. This is in contrast with the result obtained by Tomita et al. (2004). Recently, Zhou et al. (2013) have synthesized the DD3R membrane and blocked the

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defects that formed during the calcination step by dip-coating the membrane in a 5 wt. % PDMS in n-heptane. After drying in an oven at 80 ℃ for 4 hours, the permeation tests showed that the

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surface coating with polymer solution could block the defects leading to the improvement of the performance of membrane. PDMS polymer with high absorption and flux of CO2 over CH4 (Berean et al., 2014) not only could block the defects but also improve the CO2 flux through plugged pores. Therefore, more studies on the surface modification of DD3R zeolite membrane

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to improve the separation performance are indispensable.

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Table 1. A summary of the different methods conditions used for synthesis and modification of DD3R zeolite membrane. Zhou et al. (2013) This Work Zheng et al. (2008) TMOS Ludox AS-30 TMOS Silica source Mineralizing agent Ethylenediamine Ethylenediamine Potassium Floride (KF) source Molar composition of 100:11240:47:404 100:4000:6:60-100 100:11240:47:404 solution (SiO2: H2O: ADA: ED/KF) Synthesis 160 160 160 temperature/℃ 36 hours 4 days Synthesis time/h Micron Seed crystals used for 300 nm Micron size size/pulverized seeding method 0.3 wt. %, DipDip-coating method Seed amount and coating method with vacuum method for seeding without vacuum Shake for 1 h Refluxed for 4 hours Reflux/shake of solution Surface coating with 5 Surface coating with Chemical Vapor Surface modification wt. % PDMS in n15 wt. % PDMS in Deposition method heptane solution n-hexane solution Contact time for 6 days 5-20 min 30 s without vacuum surface modification Surface modification Room temperature Room temperature 550 ℃ temperature Disk Shaped αDisk Shaped and Tubular α-alumina Support shape alumina Tublar α-alumina

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As main object of this study, DD3R zeolite membrane was synthesized via the secondary growth approach, using a different procedure for preparing the membrane synthesis solution to

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obtain a good zeolite membrane layer on the support. Then, the synthesized DD3R membrane was modified by a surface coating method using PDMS polymer solution to reduce or even eliminate the negative effects of defects induced in the calcination step at high temperatures. Surface modification of the DD3R membrane was carried out by a simple coating technique to block the defects. The main differences in the preparation of the zeolite membrane synthesis solution and treatment method compared to the literature were presented with more details in table (1). In order to study the ability of the obtained membrane in the separation of CO2 and N2 6

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from natural gas, the separation properties of the DD3R membranes were investigated for gas permeation of pure H2, CO2, N2 and CH4 and their different composition mixtures as a function of pressure. The effects of feed pressure and composition on the membrane permeance and

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selectivity were examined. The effects of N2 and CO2 impurities on both CO2/CH4 and N2/CH4 separations in a ternary mixture (closed to the composition of natural gas) were also studied.

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2.2 Experimental 2.2.1 Membrane synthesis and treatment

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The molar ratio of the both powder and membrane synthesis solution was 100 SiO2: 47 1ADA: 404 EN: 11240 H2O. The solution was prepared by dissolving 1-adamantane amine (1.754 g, C10H17N, Sigma-Aldrich, > 99 %) in ethylenediamine (6.0 g, C2H8N2, Merck, > 99 %). Then deionized water (50 g) was added and the resulting solution was stirred for 1 h. Thereupon the

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resulting solution was under reflux for 1 h at 95℃. The resulting clear solution was cooled down in ice and the ice-cooled tetramethyl orthosilicate (3.758 g, SiC4H12O4, Merck, > 99 %) was added drop wise while the solution was stirred vigorously. The resulting solution was kept under

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reflux for another 3 h at 95 ℃ to obtain a uniform clear solution. For the synthesis of DD3R zeolite powder for the first time, the final solution poured in an autoclave and the autoclave was

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placed in oven under rotating at 160 °C for 25 days. The synthesized powder was washed with deionized water until the pH of the washing water decreased to neutral. For the next time, synthesis of powder was carried out at 160 °C for 4 days with secondary growth method. In the following, the synthesis of the DD3R zeolite membrane was carried out via the procedure described below: Membrane was prepared by seeding the outside of the homemade tubular α-alumina support, which was prepared by gel casting method (6 mm inner diameter, 12 mm outer diameter, 3 mm 7

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in thickness) with an average pore size of 570 nm and a porosity of (42 to 47) % (Babaluo and Kokabi, 2002). The support seeding process was carried out by dip-coating the dried support into a 0.3 wt. % DD3R seeds suspension for 3 min (average particle size of (2 to 5) µm). In order to

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synthesize a zeolite layer on the outer surface of the support, two ends of the seeded support were closed by Teflon tape and placed in autoclave which was filled with synthesis solution, where the synthesis was carried out at 160 °C for 4 days. The synthesized membrane was washed

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with deionized water until the pH of the washing water decreased to neutral. Then, the synthesized DD3R zeolite layer was dried at 45℃ for 48 h. To study the quality of the

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synthesized layer, the nitrogen permeation test was carried out. The synthesis procedure of membrane was continued while the nitrogen permeation value downs zero. It should be noted that the seeding step is not required after the synthesis of the first zeolite layer. Finally, the obtained DD3R zeolite membrane calcined at 500 ℃ for 50 h.

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For coating the polymer layer on the membrane surface by dip-coating method, simply two ends of the calcined DD3R membrane were closed by Teflon and was immersed vertically once in a 15 wt. % polydimethylsiloxane solution in n-hexane at room temperature for 30s of contact

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time. Then, the membrane was dried at 60℃ for 6 h.

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The summary of conditions applied in the synthesis and modification of DD3R zeolite membrane are presented in table (1). 2.2.2 Morphology and Crystallinity The crystalline structure of the synthesized DD3R zeolite membrane were determined by XRD patterns. XRD analysis was carried out on a Bruker D8 ADVANCE X’Pert diffractometer using CuKa (l=1.54 Å) radiation operating at 40 kV and 40 mA (Step Size = 0.05[°2Th]). The morphology and thickness of the synthesized membrane were observed by scanning electron 8

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microscopy (SEM, VEGA\\, 3 nm, TESCAN, Check and SEM, Cam Scan MV2300, LEO 440I, UK).

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2.2.3 Single gas permeation test Single gas permeation experiments of H2, CO2, N2 and CH4 were carried out as the primary method to describe the quality of the membrane. The membrane was placed inside a stainless

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steel permeation cell and sealed with O-rings. Single gas permeation was measured using a soapfilm flow meter under different pressures. The perm-selectivity was defined as the ratio of the

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permeance of two gases measured at the same temperature: /(

Perm-selectivity=

.

/(

.

. .

Perm-selectivities are compared with the Knudsen selectivity

. .

) )

(1)

(i / j); The Knudsen selectivity

is pressure independent and is proportional to the inverse square root of the molecular weight: ( /

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( )=

( /

)

)

(2)

The perm-selectivity higher than the Knudsen selectivity may indicate that the mean size of

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inter-crystalline pores of the synthesized zeolite membrane is comparable to the molecular dimensions of the species (Lee et al., 2006).

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2.2.4 Mixture separations

CO2/CH4, CO2/N2, N2/CH4 and N2/CO2/CH4 mixtures selectivity with different compositions were measured at room temperature with feed pressures from (1.0 to 5.0) bar. The membrane is placed in a homemade membrane module, sealed with Viton O-rings. CO2, N2 and CH4 from gas cylinders are passed through mass flow controllers (MFC) and mixed together with desirable compositions. The mixed gas is passed through mass flow meter (MFM) and enter to the

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membrane module as feed. Argon was used as sweep gas with flow rate of 5 cm3/min. The inlet pressure of each gas at MFC is control by pressure regulator before the on-off valves. The pressure of gases before and after the collector were displayed by pressure indicators. The check

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valves ensure no back flow of mixed gas to mass flow controllers. Relief valves after gas

cylinders is used for safety. The back pressure regulator is used to adjust the applied pressure in feed side of the membrane module. The composition of permeate and retentate were analyzed by

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the gas chromatograph device (GC Chrom. Teif Gostar Faraz, Iran). The laboratory membrane testing unit was shown in Fig. 1, schematically. The separation factor is defined as

=

&1% &2

(3)

"1 %"2 is the composition ratio of gas 1 to gas 2 in the permeate side, and &1%&2 is

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that in the retentate side.

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Where

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,

"1 %"2

Fig. 1: Schematic of laboratory membrane test unit. 10

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3.3 Results and discussion 3.3.1 Morphology and Crystallinity

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The synthesized membrane was characterized by XRD, SEM and N2 permeation analyses. Fig. 2 depicts the XRD pattern of DD3R membrane synthesized by hydrothermal process as mentioned above. The XRD pattern of the synthesized membrane is in agreement with those

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reported in the reference for XRD pattern (Treacy and Higgins, 2001). Comparing the obtained XRD patterns confirmed that the DD3R zeolite membrane has been synthesized on the outer

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surface of -alumina support with high crystallinity.

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Fig. 2: XRD pattern of the synthesized DD3R zeolite membrane on the α-alumina support ((■): αalumina support and (▲): DD3R zeolite).

Fig. 3 indicates the nitrogen permeance for different layers of the as-synthesized DD3R membrane as a function of mean pressure (average pressure of the permeate and retentate sides). A significant permeance reduction was observed after the second layer formation. Increasing the number of synthesized layers up to 3 led to complete coverage of the support, and reduced the nitrogen permeance down to zero at room temperature before calcination. 11

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Fig. 3: Pure nitrogen permeance through the as-synthesized DD3R membrane before calcination as a function of mean pressure at room temperature.

Fig. 4 shows the surface SEM micrograph of DD3R zeolite membrane before modification.

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According to this, the zeolite crystals were found to be well inter-grown. Also, continuous, crack

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free and integrated DD3R zeolite membrane was formed on the surface of -alumina support.

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Fig. 4: The surface SEM image of the DD3R zeolite membrane before modification.

Fig. 5 shows cross section (right side) and top view (left side) of the modified DD3R zeolite membrane. According to the surface image, the modified membrane morphology was unchanged

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after the PDMS treatment. However, the appearance of modified membrane became blurred, showing that the PDMS polymer coating was formed on the outer surface of DD3R membrane. Regarding the cross section image, there was no significant PDMS polymer layer on the DD3R

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membrane layer. This simple one step modification method was desired and hypothesized to

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allow blockage of membrane defects with PDMS. Preparing another membrane with the same quality by the mentioned synthesis and modification procedure approved the repeatability of our method.

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Fig. 5: SEM images of the modified DD3R zeolite membrane: surface (left side) and cross section (right side).

3.3.2 Single gas permeation properties before and after surface coating treatment

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Fig. 6 shows the single gas permeances and perm-selectivities of CO2, CH4 and N2 as a function of pressure before surface coating treatment. As can be seen in Fig. 6d, the CO2 permeance decreased with pressure, but the CH4 and N2 permeance increased slightly.

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Decreasing the CO2 permeance is an indication of increasing the amount of CO2 adsorption with

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pressure. This can be because of CO2 diffusivity reduction as adsorption increases. This is not true for CH4 and N2 permeances, because these gases have weak adsorption on DD3R zeolite. Low performance of the synthesized membrane in the separation of these gases could be associated with the existence of some defects in the membrane layer. These defects have a significant influence on the separation performance of the synthesized DD3R membrane and lead to a higher permeance of CH4 over N2 instead of N2/CH4 selection. On the other hand, small kinetic diameter and strong adsorption of CO2 cause a higher permeance over to CH4 and N2. 14

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Comparing the perm-selectivities of gases with Knudsen selectivity in Fig. 6a b and c shows a good potential for the synthesized DD3R membrane in gas separation after surface treatment. Because, in all cases, the perm-selectivities are higher than Knudsen selectivity especially at low

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pressures. These results indicated that the DD3R zeolite pores were dominant path ways for gas permeation in the synthesized membrane but existence of some defects which are larger than

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0.44 × 0.36 nm, affects the separation performance of the synthesized membrane.

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Fig. 6: Comparison of perm-selectivity (symbols) and Knudsen selectivity (lines) for CO2/CH4 (a), N2/CH4 (b) and CO2/N2 (c) at room temperature. Perm-selectivity is based on single permeance data

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through the synthesized DD3R membrane after calcination (d).

Fig. 7 shows the single gas permeances and perm-selectivities of CO2, CH4, N2 and H2 as a function of pressure after surface treatment. Comparing the permeance of gases before and after modification reveals that the modification reduced the permeance of CO2, N2 and CH4 about 2.5, 75 and 800 times respectively. These results show the high CH4 permeance dependence to the existence of non zeolitic pores before the surface modification. So that, plugging the non zeolitic pores with PDMS polymer has the highest effect on the permeance of gases with large molecular 15

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size. Low reduction ratio of CO2 permeance could be attributed to the high adsorption/absorption amount of CO2 both on the zeolite pores and into the polymer. Another observation is that the obtained order in permeated amount of gases for modified DD3R membrane is: CO2 > H2 > N2 >

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CH4 while the order of permeation amount before modification was CO2 > CH4 > N2. These results indicate that the modified DD3R membrane controls the permeation by considering the molecular size. This performance improvement is due to plugging non zeolitic pores with PDMS

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polymer. In spite of the large kinetic diameter of CO2 over H2, permeance of CO2 was greater

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than H2. This behavior is related to the high CO2 adsorption amount on DD3R zeolite.

Fig. 7: Permeance (a) and perm-selectivity (b) of the CO2, N2, CH4 and H2 as single component through the modified DD3R membrane at room temperature.

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Fig. 8 shows the permeance of each gas against the kinetic diameter through the synthesized DD3R membrane before and after modification at the feed pressure of 2 bar. Comparing the permeances shows the significant effect of surface treatment on the performance of membrane.

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As can be seen, the permeance of gases after modification decreased by increasing the kinetic diameter of molecule due to the molecular sieving effect of the zeolite micro pores. As

mentioned earlier, mechanism of permeation by DD3R zeolite membrane is based on adsorption-

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diffusion mechanism and hence the high permeance of CO2 can be explained by the higher

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adsorption amounts of CO2 on the DD3R zeolite as well as its higher diffusivity.

Fig. 8: Permeance of the CO2, H2, N2 and CH4 through the synthesized DD3R membrane before (■) and

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after (●) modification vs the kinetic diameter of molecules at room temperature and feed pressure of 2 bar.

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As mentioned in the literature, Zhou et al. (2013) synthesized and modified the DD3R zeolite membrane. Table (2) shows the difference between the results of Zhou et al. (2013) and our work. The modified DD3R zeolite membrane by Zhou et al. (2013) exhibited a CO2/CH4 perm-selectivity of about 130 and a CO2 permeance of about 2.815×10-8 mol.m-2.Pa-1.s-1 at 298 K and feed pressure of 2 bar, which represents a great difference in the perm-selectivity (270) and CO2 permeance (2.565×10-7 mol.m-2.Pa-1.s-1) of the modified DD3R membrane at this work. Also the N2/CH4 perm-selectivity and the permeance of N2 obtained by our synthesis and 17

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modification procedure is higher than their results. These can be in the reason of synthesis procedure and treatment conditions, especially treatment without vacuum applying. Without vacuum, the low amount of polymer was diffused in non zeolitic pores of layer and blocked this

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type of pores at surface which had the highest effect on the permeance of CH4. So the CO2 permeance reduced lower and lead to high perm-selectivity.

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Table 2. Comparison the results of Zhou et al. (2013) and this work before and after modification at 2 bar as feed pressure and room temperature. Flux after modification

Perm-selectivity after

(mmol.m-2.s-1)

(mmol.m-2.s-1)

modification

CO2

N2

CH4

Zhou et al.

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2.68

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123.7

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Flux before modification

CO2

N2

CH4

CO2/CH4

N2/CH4

5.63

0.29

0.044

130

6.7

51.3

1.69

0.19

270

8.85

The obtained results lead to the conclusion that the modified DD3R zeolite membranes in

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our work have good potential for CO2 and N2 separation from CH4. However, binary and ternary permeation experiments are essential to study the effects of different gases on the separation

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performance of the modified membrane in gas mixtures. 3.3.3 Mixture permeation properties of modified DD3R membrane

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Fig. 9 shows comparisons of CO2/CH4 and N2/CH4 separation factor at different feed pressures with 50/50, 10/90 and 5/95 mixtures as feed. As can be seen in all cases, separation factor related to CO2/CH4, increases with increasing feed pressure, while separation factor of N2/CH4 decreases. The increasing trend of CO2/CH4 separation factor attributes to strongly selective adsorption behavior of CO2 over CH4. Strongly adsorption of CO2 affects the CH4 adsorption and reduces it. So that, CO2 permeation increases more with increasing pressure compared to the CH4 permeation. Another reason for this trend is the high absorption affinity of 18

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CO2 into PDMS which plugged the non zeolitic pores. In addition, a simple comparison between the CO2/CH4 perm-selectivity (270 to 190) and separation factor (80 to 120) reveals that, faster species such as CO2 can lead to ‘speeding up’ of slower species such as CH4 and conversely in

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mixture.

The decreasing trend of N2/CH4 separation factor could be related to selective adsorption/absorption of CH4 from N2 both on DD3R and PDMS. CH4 adsorption/absorption

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increases on DD3R and PDMS as pressure increases and leads to increase the permeation of

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CH4. So, N2/CH4 separation factor decreases by increasing pressure.

Fig. 9: Separation factor of N2/CH4 and CO2/CH4 in the investigated binary mixtures 50/50 (a), 10/90 (b) and 5/95 (c) through the modified DD3R membrane as a function of feed pressure at room temperature.

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Considering the separation factor of CO2/CH4 at different feed compositions shows that CO2/CH4 separation factor decreases by decreasing CO2 concentration in mixture. Reducing CO2 concentration in the feed (50 to 10%) causes a sharp decrease in the separation factor, but the

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separation factor remains constant by decreasing the concentration from 10 to 5%. This reveals that concentration of mixture is more effective on the gas permeation of strongly adsorbed

molecules such as CO2. Also, it was found that even at low concentration, the adsorption of CO2

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is predominant and would effectively block the CH4 gas, resulting in higher CO2/CH4 separation factor. These trends are not the same for N2/CH4 separation factor, because both N2 and CH4

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molecules are weakly adsorbed molecules and higher or lower concentrations have no considerable effect on the separation factor of these molecules. So, N2/CH4 separation factor remains constant by decreasing concentration of N2.

Fig. 10 shows the permeance of CH4 in its binary mixtures (50/50, 10/90 and 5/95) with N2

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and CO2. As can be seen in Fig. 10a, CH4 permeance increases by increasing its composition and feed pressure. Comparing these results with pure gas permeance of CH4 confirms that lower permeance of CH4 at mixtures with N2 is due to feed composition effects and reveals that the

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presence of N2 reduced the permeation of CH4 and this reduction is proportional. Approximately constant values of N2/CH4 separation factor at different feed compositions in Fig. 9 confirm

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these points. These trends are not the same for CH4 permeance at its mixtures with CO2. As can be seen in Fig. 10b, the CH4 permeance in the 10/90 and 5/95 CO2/CH4 mixtures is lower than pure gas permeance (Fig. 7a) reveals that the permeance of CH4 at mixtures with CO2 is suppressed. This behavior can be clearly observed by comparing the CH4 permeance at low composition mixtures (10/90 and 5/95) of CO2 with that of N2. On the other hand, high

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permeance of CH4 at 50/50 mixture compared to the 10/90 and 5/95 mixtures can be related to

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‘speeding up’ phenomena which was caused by CO2 molecules.

Fig. 10: Permeance of CH4 in the 50/50, 10/90 and 5/95 binary mixtures with N2 (a) and CO2 (b) through

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the modified DD3R membrane as a function of feed pressure at room temperature.

At this point it is good to investigate the effects of different components on the membrane

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separation factor in ternary mixture containing N2, CO2 and CH4. But, before investigating this issue, it is better to study the separation factor of CO2/N2 and investigate the effects of these two components on the adsorption and diffusion properties of each other. Fig. 11 shows the separation factor of CO2/N2 at two different feed compositions (CO2:N2 = 50:50 and 30:70) as a function of feed pressure. As can be seen, separation factor at two compositions increases with increasing pressure and approximately remains constant. The increasing trend of CO2/N2 separation factor up to 3 bar was induced by increasing adsorption 21

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amount of CO2 with increasing pressure and increasing the pressure up to 5 bar induces an increased N2 diffusivity through the non zeolitic pores plugged with PDMS, leads to constant or

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slightly decreasing separation factor.

Fig. 11: Separation factor of CO2/N2 for binary mixtures of 30:70 and 50:50 through the modified DD3R membrane as a function of feed pressure at room temperature.

Fig. 12 shows the permeance of CO2 and N2 in the 50:50 and 30:70 binary mixtures. As can

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be seen, the permeance of both N2 and CO2 at 50:50 mixture is higher than that at 30:70 mixture and these permeances decrease by increasing pressure. But at 30:70 mixture the permeances have increasing trend by increasing pressure. The reason is the difference in the partial pressure of

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CO2 at mixtures. High partial pressure leads to high adsorption amount of CO2 and prevents N2 adsorption. So the permeance of N2 decreases by increasing feed pressure. On the other hand, the

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higher permeance of N2 at 50:50 mixture at pressures up to 3 bar may be related to ‘speeding up’ phenomena which was caused by CO2 molecules. But increasing the pressure at 30:70 mixture leads to the high partial pressure of N2 and then the high permeance of N2 compared to 50:50 mixture. Therefore, it can be concluded from Figs. 11 and 12 that the presence of CO2 and N2 could have sufficient effect on the permeance of each other and could affect the separation of N2 and CO2 from CH4 in the ternary mixtures.

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Fig. 12: Permeance of CO2 (a) and N2 (b) in the 50:50 and 30:70 binary mixtures through the modified

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DD3R membrane as a function of feed pressure at room temperature.

Fig. 13 shows the separation factor of CO2/CH4 and N2/CH4 at ternary mixture of 10:5:85 CO2:N2:CH4 as a function of feed pressure. As can be seen, the changing trend of CO2/CH4 and

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N2/CH4 separation factors are the same in the ternary mixture compared to that in the binary mixtures of components. Separation factor values at ternary mixture for CO2/CH4 is higher than

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5:95 binary mixture and for N2/CH4 remains approximately constant. The higher separation factor values of CO2/CH4 in ternary mixture was related to the presence of N2. So that, N2 may reduce the ‘speeding down’ effect of CH4 on the CO2 and leads to have higher CO2 permeance compared to 5/95 CO2/CH4 mixture. Comparing the permeances of N2 and CH4 in Fig. 14 shows that the permeances of these gases in ternary mixture are affected in the presence of CO2 and decreased by about the same factor compared to that in 10:90 binary mixture of N2 and CH4.

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Fig. 13: Separation factor of the N2/CH4 and CO2/CH4 for ternary mixture of 10:5:85 CO2/N2/CH4

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through the modified DD3R membrane as a function of feed pressure at room temperature.

Fig. 14: Permeance of N2, CH4 and CO2 in the ternary mixture of 10:5:85 CO2/N2/CH4 through the

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modified DD3R membrane as a function of feed pressure at room temperature.

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4.4 Conclusion In this work, it is concluded that continuous, crack free and integrated DD3R zeolite layer

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can be grown on the outer surface of α-alumina support using the presented new procedure. Calcination of this type of zeolite layer at high temperatures induced formation of defects in the membrane layer. The negative effects of these defects on the ability of the membrane in the separation of gases could be effectively eliminated by the simple coating technique, which just

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coat the DD3R membrane layer with the PDMS polymer solution. The single gas permeation

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results of the DD3R membranes before and after PDMS applying show a great increasing trend of selectivities from Knudsen to high values for CO2/CH4, CO2/N2 and N2/CH4. This synthesis procedure and modification method could have very promising results for obtaining the selective DD3R zeolite membrane for natural gas purification. The results of extensive studies on the binary and ternary mixture permeation experiments showed that composition of species could

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impurities.

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have sufficient effect on the performance of modified membrane in separation of natural gas

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Acknowledgements The authors wish to thank Sahand University of Technology (SUT) for the support of this work.

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Also, thank co-workers and technical staff in the chemical engineering department, Institute of

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nanostructure materials research center of SUT for their help during various stages of this work.

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References: Babaluo, A.A., Kokabi, M., 2002. Manufacture of porous support systems of membranes by in-

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situ polymerization. Iran Polym. J. 15, No.3. Berean, K., Zhen, J., Nour, M., Latham, K., Mcsweeney, C., Paull, D., Halim, A., Kentish, S., Doherty, C.M., Hill, A.J., Kalantarzadeh, K., 2014. The effect of crosslinking temperature on the permeability of PDMS membranes : Evidence of extraordinary CO2 and CH4 gas permeation.

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Sep. Purif. Technol. 122, 96–104.

Bergh, J.V.D., Zhu, W., Gascon, J., Moulijn, J. A., Kapteijn, F., 2008. Separation and

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permeation characteristics of a DD3R zeolite membrane. J. Memb. Sci. 316, 35–45. Himeno, Sh., Tomita, T., Suzuki, K., Nakayama, K., Yajima, K., Yoshida, S., 2007a. Synthesis and permeation properties of a DDR-type zeolite membrane for separation of CO2/CH4 gaseous mixtures. Ind. Eng. Chem. Res. 46, 6989–6997.

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Himeno, Sh., Tomita, T., Suzuki, K., Yoshida, S., 2007b. Characterization and selectivity for methane and carbon dioxide adsorption on the all-silica DD3R zeolite. Microporous Mesoporous Mater. 98, 62–69.

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Hong, Z., Zhang, C., Gu, X., Jin, W., Xu, N., 2011. A simple method for healing nonzeolitic

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pores of MFI membranes by hydrolysis of silanes. J. Memb. Sci. 366, 427–435. Kosinov, N., Gascon, J., Kapteijn, F., Hensen, E.J.M., 2016. Recent developments in zeolite membranes for gas separation. J. Memb. Sci. 499, 65–79. Lee, S.R., Son, Y.H., Julbe, A., Choy, J.H., 2006. Vacuum seeding and secondary growth route to sodalite membrane. Thin Solid Films. 495, 92–96.

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Magsoudi, H., 2015. Defects of zeolite membranes: characterization, modification and posttreatment techniques. Sep. Purif. Rev. 45, 169-192. Nakayama, K., Suzuki, K., Yoshida, M., Yajima, K., Tomita, T., 2006. Method for preparing

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DDR type zeolite membrane, DDR type zeolite membrane, and composite DDR type zeolite membrane, and method for preparation thereof, Patent No.: US 7,014,680 B2.

Rufford, T.E., Smart, S., Watson, G.C.Y., Graham, B.F., Boxall, J., Diniz da Costa, J.C., May,

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E.F., 2012. The removal of CO2 and N2 from natural gas: A review of conventional and emerging process technologies. J. Pet. Sci. Eng. 94-95, 123–154.

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Tomita, T., Nakayama, K., Sakai, H., 2004. Gas separation characteristics of DDR type zeolite membrane. Microporous Mesoporous Mater. 68, 71–75.

Treacy, M.M.J., Higgins, J.B., 2001. Collection of simulated XRD powder patterns for zeolites, fourth ed., Elsevier: New York.

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Uchikawa, T., Yajima, K., Nonaka, H., Tomita, T., 2013. Method for production of DDR type zeolite membrane, Patent No.: US 8,377,838 B2. Wu, T., Diaz, M.C., Zheng, Y., Zhou, R., Funke, H.H., Falconer, J.L., Noble, R.D., 2015.

Sci. 473, 201–209.

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Influence of propane on CO2/CH4 and N2/CH4 separations in CHA zeolite membranes. J. Memb.

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Yajima, K., Nakayama, K., 2009. Process for producing DDR type zeolite membrane, Patent No.: US 2009/0011926 A1.

Yang, Sh., Cao, Z., Arvanitis, A., Sun, X., Xu, Zh., Dong, J., 2016. DDR-type zeolite membrane synthesis, modification and gas permeation studies. J. Memb. Sci. 505, 194–204.

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Zhang, B., Wang, C., Lang, L., Cui, R., Liu, X., 2008. Selective defect-patching of zeolite membranes using chemical liquid deposition at organic/aqueous interfaces. Adv. Funct. Mater. 18, 3434–3443.

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Zheng, Zh., Hall, A.S., Guliants, V.V., 2008. Synthesis, characterization and modification of DDR membranes grown on α-alumina supports. J. Mater. Sci. 43, 2499–2502.

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Zhou, Zh., Nair, S., 2013. Zeolite DDR membranes, Patent No.: US 2013/0064747 A1.

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Table 1. A summary of the different methods conditions used for synthesis and modification of DD3R zeolite membrane. Zheng et al. (2008) Zhou et al. (2013) This Work TMOS Ludox AS-30 TMOS Silica source Mineralizing agent Ethylenediamine Ethylenediamine Potassium Floride (KF) source Molar composition of 100:11240:47:404 100:4000:6:60-100 100:11240:47:404 solution (SiO2: H2O: ADA: ED/KF) Synthesis 160 160 160 temperature/℃ 36 hours 4 days Synthesis time/h Micron Seed crystals used for 300 nm Micron size size/pulverized seeding method 0.3 wt. %, DipDip-coating method Seed amount and coating method with vacuum method for seeding without vacuum Shake for 1 h Refluxed for 4 hours Reflux/shake of solution Surface coating with 5 Surface coating with Chemical Vapor Surface modification wt. % PDMS in n15 wt. % PDMS in Deposition method heptane solution n-hexane solution Contact time for 6 days 5-20 min 30 s without vacuum surface modification Surface modification Room temperature Room temperature 550 ℃ temperature Disk Shaped αDisk Shaped and Tubular α-alumina Support shape alumina Tublar α-alumina

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Table 2. Comparison the results of Zhou et al. (2013) and this work before and after modification at 2 bar as feed pressure and room temperature. Flux before modification

Flux after modification

Perm-selectivity after

(mmol.m-2.s-1)

(mmol.m-2.s-1)

modification

CH4

CO2

N2

CH4

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16.1

2.95

2.68

5.63

0.29

0.044

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123.4

123.7

154.2

51.3

1.69

0.19

CO2/CH4

N2/CH4

130

6.7

270

8.85

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N2

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CO2

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Highlights Different procedure was applied for preparing the DD3R membrane synthesis solution.

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Surface treatment of the DD3R membrane by a simple coating technique.

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Blockage of DD3R membrane defects without the formation of polymer layer.

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Highest effect of the surface modification was on the CH4 permeance reduction.

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The DD3R membrane exhibited good performance in natural gas impurities separation.

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