A study on carbon dioxide removal by blending the ionic liquid in membrane synthesis

Accepted Manuscript A study on carbon dioxide removal by blending the ionic liquid in membrane synthesis Dzeti Farhah Mohshim, Hilmi Mukhtar, Zakaria Man PII: DOI: Reference:

S1383-5866(17)30814-6 http://dx.doi.org/10.1016/j.seppur.2017.06.034 SEPPUR 13812

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

15 March 2017 9 June 2017 12 June 2017

Please cite this article as: D. Farhah Mohshim, H. Mukhtar, Z. Man, A study on carbon dioxide removal by blending the ionic liquid in membrane synthesis, Separation and Purification Technology (2017), doi: http://dx.doi.org/ 10.1016/j.seppur.2017.06.034

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Title: A study on carbon dioxide removal by blending the ionic liquid in membrane synthesis Authors & affiliations: Dzeti Farhah Mohshim*1, Hilmi Mukhtar2, Zakaria Man 2 1 Petroleum Engineering Department, Universiti Teknologi PETRONAS, 31260 Bandar Seri Iskandar, Perak. 2 Chemical Engineering Department, Universiti Teknologi PETRONAS, 31260 Bandar Seri Iskandar, Perak. *[email protected] Abstract: The attention in ionic liquids (IL) is driven by its distinctive properties, such as negligible vapour pressure, thermal stability, and tunability of properties. To further grow its application in the separation field, the ionic liquid membranes (ILMs) and its separation technology have been proposed and developed rapidly. This paper presents details of recent research involving the blending of ionic liquid in membrane synthesis for Carbon Dioxide (CO2) removal from Methane (CH4). The low CO2/CH4 separation factor through commercialized membranes is due to the lack of CO2 affinity towards the polymeric membranes and the presence of interfacial voids in the mixed matrix membranes (MMM). The main objective is to study the influence of ionic liquid (IL) addition into the polymer and polymer-inorganic filler towards the membrane properties and performances. This was achieved by blending different concentrations of ILs at a fixed polyethersulfone (PES) and PES-SAPO-34 composition. The membranes were synthesized using dry-phase inversion technique to prevent the voids formation. The synthesized membranes were physico-chemically characterized and the performances of these membranes were evaluated in term of permeance and selectivity. The synthesized membranes were found to have dense and voids-free structure with lower polymer decomposition temperature (~410oC) as compared with pure PES membrane. The presence of ILs in the membranes had significantly increased the permeance of CO2 due to enhanced affinity effect. However, the permeance of CH4 is at the reverse trend due to less CH4 pathway within the membranes. Experimental results showed that both ILs have significant improvement on the ideal selectivity of the synthesized membranes. This significant improvement indicated that ionic liquid is worth to be explored as an alternative material to enhance the CO2 affinity in membranes for CO2 separation from CO2/CH4 mixture. Keyword: ionic liquid membrane blending, carbon dioxide removal, membrane separations, CO2 affinity

1.0 INTRODUCTION In Malaysia, natural gas is produced from offshore fields located in Terengganu, Sabah and Sarawak. For gas utilisation in Peninsular Malaysia, natural gas from offshore Terengganu is piped to Kerteh, Terengganu where it is treated in the gas processing plants [1]. However, in some other Malaysian gas fields, the concentration of carbon dioxide can reach up to 70%, making it unable to be monetized. Thus, there is a great interest in technological innovation for exploration purposes. CO2 content in natural gas wells from several oil fields like Natuna (Indonesia), Gorgon (Australia) [2] and K5 (Sarawak, Malaysia) [3] could be as high as 80% [2]. In the presence of water, CO2 is highly corrosive and possible to rapidly destroy the equipment and pipelines unless it is partially removed [4]. Natural gas should be purified to meet the pipeline specification since CO2 reduces the energy content of the natural gas stream, thus limits its economical application for heating and chemical conversion. High CO2 content in gas reservoirs make most of gas field development uneconomical, and remained undeveloped [4]. Hence, the purification of natural gas remains as one of the challenging gas separation problems in process engineering and it is the objective of this study to further finding the feasible technology for it [5]. Membrane process is considered as an energy-saving alternative for gas separation since it does not involve any phase transformation. In addition, the greatest asset of membrane separation is its simplicity since it has almost no involvement of moving parts and the construction is quite simple [6]. Separation occurs when permeable gases start to dissolve into the membrane surface, which then diffuse through the membrane layer and finally desorb on the opposite side as the permeate gas. The residue gases or other residual will remain at high pressure as the non-permeable gases. Thus, the CO2 removal from natural gas strongly depends on how well the acid gas dissolves into the membrane surface and how good it diffuses through the membrane [7]. However, the commercialized membranes are yet to meet the requirement especially involving separation of high CO2 content from the natural gas. Currently, the research trend is to fill one of the gaps in the interphase morphology of membrane by using room temperature ionic liquids (RTILs) [8, 9]. RTILS are known as molten organic salts at ambient temperature and pressure which have unique

physicochemical properties such as negligible vapour pressure, non-flammable and high ionic conductivity [10]. The association of these features has brought opportunities in the application of IL-based membranes and its processes in CO2 separation applications, which are believed to be more energy efficient and environmentally friendly compared with other commercial separation technologies. Despite the appealing properties of ILs, their high viscosities and high production costs are two big challenges for its upscaling implementation. Therefore, in order to integrate ILs into existing membrane separation, the combination of ILs with one or more components may be attractive to promote an optimal and practical approach in membrane synthesis. Up till now, most of the studies have been reported were related to the ionic liquid supported membranes (ILSMs) [1113] and poly(ionic liquid) (PIL) membranes [14] which having the problem in liquid displacement upon applying at high pressure application particularly. In order to leverage on the characteristics of ionic liquid which having high CO2 absorptivity properties and further address the low CO2 affinity of membranes, it is the objective of this research to investigate the effect of ionic liquid addition on the membrane properties. It is expected that the physicochemical properties and separation ability of the membranes containing ionic liquid for CO2/CH4 separation will be enhanced with the presence of ionic liquid which is mainly due to the strong absorption selectivity towards CO2. 2.0 MATERIALS AND METHODS 2.1 Materials In this study, Polyethersulfone (PES) was chosen as the polymer support for this fabrication of enhanced mixed matrix membrane. The PES used in this study was a commercial PES Ultrason E 6020 P (industrial processing) purchased from BASF in flakes form. PES has a suitable Tg at around 225oC which is able to sustain high temperature during the mixed matrix membranes synthesis. The intrinsic properties such as chemically and thermally stable with good dimensional stability are the major advantages of using PES. High permeating gas diffusion rate and proper pore size of molecular sieves are two important properties in determining the filler selection. Commercial SAPO-34 has been purchased from ACS Material in powder form having the kinetic diameter of 0.38 nm. SAPO-34 (0.38 nm) has pores diameter which are

similar to the CH4 (0.38 nm) but however a bit larger than CO2 (0.33 nm) which allows SAPO-34 to separate CO2 easier if compared to CH4 [15]. Among

available

ionic

liquids,

1-ethyl-3-methyl

imidazolium

bis(trifluoromethanesulfonyl) amide (emim[Tf2N]) was selected in this study for the synthesis of PES-ionic liquid membranes (ILPMs) and PES-SAPO-34-ionic liquid MMM (IL3Ms). As far as the selection of ionic liquid was concerned, the CO2 solubility in the ionic liquid was a vital factor for consideration. The rate of CO2 solubility in an ionic liquid is governed by many factors, including the operating pressure and temperature, choice of anion, and the chain presence in the anion group [21]. Other than that, emim[Tf2N] contains a fluoromethyl chain that could promote a greater solubility of CO2. The ionic liquid was obtained from Sigma-Aldrich with 99.99% purity and was used without any purification. 2.2 Membrane Synthesis In this study, four (4) types of membranes were synthesized which were neat PES membrane as the benchmark (PES), enhanced polymer-ionic liquid membranes (ILPMs), neat mixed matrix membrane (PES-SAPO-34) and enhanced PES-SAPO-34ionic liquid mixed matrix membranes (IL3Ms). In total, there were six (6) membranes synthesized as tabulated in Table 1. All membranes were synthesized based on 20 wt/wt% PES concentration as mentioned in previous studies [16-18]. Table 1: Details Membrane Compositions PES (wt/wt%)

SAPO-34 (wt/wt%)

Emim [Tf2N] (wt/wt%)

PES

20

-

-

PES-IL1_10

20

-

10

PES-IL1_20

20

-

20

Neat MMM

PES_S20

20

20

-

PES-SAPO-34 with Emim [Tf2N]

IL3M1_10

20

20

10

IL3M1_20

20

20

20

No.

Membranes

1

Neat PES

2 3

PES with Emim [Tf2N]

4 5 6

Membrane Abbreviation

PES polymer flakes were first dried to remove excess moisture trapped in the polymer upon membrane synthesis. For the pure PES polymeric membrane, the dried

PES polymer flakes were dissolved with N-methylpyrrolidone by 24 h of continuous stirring. For ILPM fabrication, the ionic liquid was mixed with N-methylpyrrolidone, and we observed that the solution was well-mixed because there was no phase separation. After a while, a proper amount of dried polymer was partially added, and stirring was also completed within 24 h. The dope solution preparation was conducted at room temperature. In the synthesis of PES-SAPO-34 mixed matrix membranes, SAPO-34 inorganic filler powder was dried and the weighed SAPO-34 powder was poured into the bottle containing NMP. The mixture was stirred for one hour to ensure good filler dispersion. After one hour, 1/5 of total weighed PES polymer was poured into the mixture and the following steps were repeated as described in PES polymeric membrane fabrication until whole weighed polymer was added. After an overnight stirring was completed, the solution was left to stand at 12 hours aimed for degassing the bubbles which might be formed during the solution stirring. The only difference in IL3Ms fabrications method is where the ionic liquid was blended first prior to PES and SAPO-34 addition. After the solutions are well mixed (no phase separation was observed), the dope solution was blade casted on a cleaned, dust-free, flat, dry and smooth glass plate at room temperature with a casting knife at 180µm gap. The casted membrane was preliminary dried in drying oven at 90oC for 8 hours and the drying was continued at 160 oC for 24 hours to further remove the residual solvent [18, 19]. The membranes were cooled down to room temperature and the membranes were found to be completely dried when they were self-detached from the glass plate. 2.3 Membrane Characterization The structures of morphology of fabricated flat sheet membranes were analyzed by using field emission scanning electron microscopy (FESEM). In this project, FESEM (ZEIS SUPRA TM 55VP) was used to determine the qualitative structural assessment of both surface and cross-section for the membranes. The dried membrane were cryogenically fractured in liquid nitrogen by immersing them for few seconds in order to obtain a smooth and clean cross-section image [20, 21]. Thermal gravimetric analyzed (TGA) was done to estimate the thermal stability, material composition and purity and the amount of remaining solvent left in the membranes [21, 22]. In this study, TGA was done to check the composition of ionic liquid left in the membranes after drying was completed. Membrane’s stability is also thermally measured in order to

analyze the suitable temperature that the membrane can withstand. In this study, membranes were thermally characterized by thermal gravimetric analyzed (TGA, Perkin Elmer, TGA 4000). The samples were cut into small pieces and heated from 25 oC to 800oC at 10oC/min heating rate with inert nitrogen (N2) flushed at 20 ml/min to avoid any interference of corrosive gas which may cause thermal oxidative degradation. 2.4 Membrane Performance Gas permeation experiment of the synthesized flat sheet membranes was conducted using a constant volume measurement where the upstream pressure was kept constant while measuring the flux across the membrane film. The gas permeation experiment of the synthesized flatsheet membranes was conducted according to previous studies, [16, 19, 23]and the steps were carried out as follows. The synthesized membranes were tested on pure CO2 and CH4 gas in the pressure range of 10–30 bars to investigate the effects of the pressure, and the tests were performed at room temperature (27–29 oC). The membrane was placed on a dead-end module, and the gas flowing into the membrane cell was perpendicular to the membrane position. The gas permeation system was vacuumed for about 20 min before we started to remove all of the residual gases trapped inside. The permeating gas flow rate was measured with a bubble flow meter, and the rate was recorded every 15–20 min three to five times [18, 24, 25]. To maintain a reliable result, the permeation rates were determined from the average of two membranes of the same composition. The individual gas permeance was calculated by the following equation: P J = l ∆p

(1)

where P/l is the permeance, which is expressed in gas permeance units;J is the flux of the gas passing through the membrane (cm3/cm2.s); l is the membrane thickness (cm); and ∆p is the pressure difference across the membrane. The ideal separation performance (αCO2/CH4) was defined as the ratio of CO2 permeance CH4 permeance (PCH4/l) and could be expressed as follows: PCO 2

α

CO

2

= CH

4

PCH 4

l l

(2)

The gas permeation results and the ideal separation performances were analysed with respect to the ionic liquid composition accordingly.

3.0 RESULTS AND DISCUSSIONS 3.1 Morphological Analysis Figure 1 shows the cross-sectional images of the synthesized membranes at 1.00 K magnification. The cross-sectional figure shows that the synthesized membranes have dense structure with no voids presence. It was noted that the addition of ionic liquid has no effect on the membrane morphology where the membranes are found to be dense. This finding also indicated that the ionic liquid is compatible with the polymer (PES) and the solvent used (NMP) as there was no phase separation detected through this analysis [19, 26] and observation in particle agglomeration as well.

(a) Neat PES Membrane

(b) PES-IL Membrane

(c) PES-SAPO_34 MMM

(d) IL3M Membrane

Figure 1: Cross-sectional Images of Synthesized Membranes

3.2 Thermal Analysis Table 2 shows the overall weight losses of polymeric based membrane across the temperature. Table 2: Weight Losses of Synthesized Membranes from TGA Analysis Description PES PM PES with emim[Tf2N] PES with SAPO-34 PES-SAPO-34 with emim[Tf2N]

Membranes Pure PES PES-IL1_10 PES-IL1_20 PES-SAPO-34 MMM IL3M1_10 IL3M1_20

Weight loss at 200-300oC, % 3 -

Weight loss at 400-600 oC, % 42 55 60

Residual amount, % 20 22 25

3

40

15

-

52

42

-

55

45

From the table, there were two (2) weight losses observed for neat PES membrane, where the first weight loss occurred at around 210oC. The weight loss continued until the temperature reached at 270oC. The weight loss calculated at this range was only 23% which was believed due to solvent. Another weight loss observed started from 470oC and ended at 590oC which indicates a total of 42% weight loss which was attributed to the PES polymer decomposition. The same observation of PES polymer degradation was also reported in [27] where they found PES started to decompose at 450oC onwards. Similar findings were also observed in PES with SAPO-34 MMM. It was noted that, all PES membranes containing emim[Tf2N] were seemed to be free from moisture as there is no weight loss up to 100 oC. However, the membranes decomposition temperature started earlier at ~410oC as compared to the neat PES polymeric membrane (~470 oC). Membranes with emim[Tf2N] started to decompose earlier might be due to the overlapping of ionic liquid bond on the PES groups [28]. Similar observations were found where IL3Ms seemed to be free of solvent and moisture as there is no weight loss at temperature range up to 200oC. However, the membranes started to experience an early polymer decomposition at 410oC for MMM with emim[TF2N] with approximately 45% weight loss for IL3M membranes.

3.3 Membrane Separation Performance 3.3.1 Neat PES Polymeric Membrane Figure 2 (a) shows the permeance of CO2 and CH4 across neat PES polymeric membrane at different operating pressures. This figure shows that the gas permeance of both CO2 and CH4 decreased when the feed pressure increased. This trending clearly showed the typical behavior of PES as a glassy polymer where the gas permeance is inversely proportional to the pressure. It can be seen that CO2 permeance are higher than CH4 permeance due to the ability of CO2 gas to be more soluble in PES glassy polymer. In addition, CO2 is more condensable as compared to the CH4. Figure 2 (b) shows the CO2/CH4 ideal selectivity of neat PES polymeric membrane recorded at the feed pressure of 10 to 30 bars. From this figure, it is observed that the CO2/CH4 ideal selectivity of based PES polymeric membrane is increasing with increasing pressure. Furthermore, larger gas molecular size restricted the interaction with polymer chain thus decreased the diffusion coefficient as compared to the small gas molecules [29]. As a consequence, a smaller kinetic diameter of molecules like CO2 (0.33 nm) are always favourable to pass through over larger molecules like CH4 (0.38 nm) [30]. This phenomenon is supporting the natural behavior of glassy polymer like PES where the gas selectivity is inversely proportional to the gas permeance when tested at the escalated pressure. [31]. However, reaching to 30 bar pressure, there was an increasing value in membrane CO2 permeance and a decreasing trend in the CO2/CH4 selectivity. The interference in the membrane packing density and the chain mobility during the plasticization effects will lead to a vice versa phenomenon where the gas permeance will increase while the selectivity will decrease. This is caused by the sorption of CO2 into the excess free volume of PES glassy polymer.

(a)

(b)

Figure 2: (a) CO2 and CH4 Permeance and (b) CO2/CH4 Selectivity of PES Membrane 3.3.2 PES-Ionic Liquid Polymeric Membranes Figure 3 (a) shows the CO2 permeance of PES membrane containing ionic liquid across the pressure of 10 to 30 bar. It was observed that the CO2 gas permeance is decreasing with increasing pressure. This finding is similar with the effects of pressure on the based PES polymeric membrane where the CO2 gas permeance was also decreased with increasing pressure. This trending was expected since the gas permeance follows the behavior of PES glassy polymer having an inverse relationship of CO2 permeance and pressure. At the highest tested pressure (30 bar), the increase in pressure has led to very small difference in CO2 permeance. This is due to the small increases of CO2 solubility at high pressure which is consistent with the reported literatures [32-34].

(a)

(b)

Figure 3: (a) CO2 Permeance and (b) CO2/CH4 Selectivity of PES-IL Membrane

Figure 3 (b) shows the CO2/CH4 ideal selectivity of PES membranes containing ionic liquid emim[Tf2N] across the pressure. It was observed that the CO2/CH4 ideal selectivity increased with increasing pressure. The permeation and selectivity data has shown that there is no inverse behavior of PES polymeric membranes as it still follows the characteristics of glassy polymer which have the inverse trend of their permeance and selectivity across the pressure. Furthermore, the ionic liquid concentration also has an increasing trending effect on the CO2/CH4 ideal selectivity. The addition of ionic liquid was believed to enhance the permeation of condensable gas like CO2 to condense and soluble in the PES matrix since the alkyl and fluoroalkyl chain in emim[Tf2N] attracted the CO2 to be more soluble [33]. 3.3.3 Mixed Matrix Membranes Figure 4 (a) shows that the permeation of CO2 across PES-SAPO-34 mixed matrix membrane and IL3Ms decreased with increasing operating pressure. The PES molecules pack more effectively with the effect of increasing pressure, so the free volume and transport mobility are decreased [35]. These outcomes also liaise with the natural behaviour of PES polymer having inverse relationship between its permeance and selectivity. Under the pressure difference, the penetrant molecules dissolve in the upper stream (high pressure) of a membrane and diffuse across the membrane and desorb from the downstream (low pressure). Diffusion is the rate-controlling step in penetrant permeation. The rate-controlling step in diffusion creates gaps in polymer matrix sufficiently large to accommodate the penetrant molecules to pass through the membrane [36]. In addition, glass polymer allows molecules to flow in the free volume between the monomer. As the pressure increases, the free volume is decreasing, hence reduces the permeability. However, CH4 have bigger kinematic diameter than CO2. It was expected that the rate of permeance for CH4 is much less at higher pressure as compared to CO2 and eventually exhibited in enhanced CO2/CH4 selectivity [37].

(a)

(b)

Figure 4: (a) CO2 and CH4 Permeance and (b) CO2/CH4 Selectivity of PES-SAPO34-IL Membranes

Figure 4 (b) shows the CO2/CH4 ideal selectivity behaviour of PES-SAPO-34 mixed matrix membranes containing emim[Tf2N] at different operating pressure. It was found that the CO2/CH4 selectivity increased with increasing pressure for all membranes. For IL3M1_20%, the increasing in selectivity is from 48 to 62 at these operating pressures ranges from 10 to 30 bars. From this figure, the ideal gas selectivity of IL3M1_20% is also increased by 3 folds as compared to based PES-SAPO membrane at 30 bar. The influence of ionic liquid in the membrane for CO2/CH4 selectivity is 62 at 30 bars for PES-SAPO-34 MMM containing 20 wt/wt% emim[Tf2N]. Other than that, it was noticed that the CO2/CH4 ideal selectivity increased as the emim[Tf2N] concentration increased for all pressure. In addition, there is no CO2 permeance increase and selectivity decreased for these membranes when they were tested up to 30 bar pressure. Hence, it can be concluded that ionic liquid has acted as the anti-plasticizer since neat PES membrane showed the plasticization effects at pressure more than 27 bars [30, 38].

4.0 CONCLUSION Six (6) membranes have been successfully synthesized through this study including one neat PES polymeric membrane (PES), two PES polymeric membranes containing ionic liquids (PES-IL), one PES-SAPO-34 mixed matrix membrane (MMMs) and two

PES-SAPO-34 mixed matrix membranes containing ionic liquids (IL3Ms). The effects of different ionic liquid concentration loadings were investigated on the physical and thermal properties of membranes as well as the carbon dioxide and methane separation performance. The cross-sectional views from FESEM analysis of the synthesized membranes were found to be dense, homogenous and voids-free membranes. The addition of the imidazolium based ionic liquid also produced homogenous and dense membrane structure which showed the miscibility among PES, solvent and ionic liquid. From TGA analysis, the PES membrane and PES-SAPO-34 MMM were found to be stabled up to ~ 470 oC. The addition of ionic liquid into the PES matrix has also proved that the membranes were thermally stable up to ~ 410 oC. The performance of the synthesized membranes was evaluated in term of CO2 and CH4 permeance across a pressure range of 10-30 bar. All of synthesized membranes showed a similar trending where the gas permeance decreased and CO2/CH4 selectivity increased with increased in pressure. This is due to the intrinsic behavior of glassy polymer like PES. The incorporation ionic liquid into the PES membrane (PES-IL) was found to enhance the CO2 permeance while the CH4 permeance was found at the reverse trend. Otherwise, the addition of SAPO-34 fillers was found to increase the CO2 permeance. Although the addition of SAPO-34 was also found to increase the CH4 permeance, the increase in CO2 permeance is more significant. These results demonstrate that the imidazoliumbased ionic liquids were fast CO2 adsorbents, and this method could be used as a preliminary study for the advancement of materials in membrane fabrication.

ACKNOWLEDGMENTS The authors thank the Ministry of Education of Malaysia for providing the funding (contract grant number ERGS-0153ABC67) and the Universiti Teknologi Petronas for supporting this research work.

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Highlights • Enhanced mixed matrix membranes are prepared by embedding ionic liquid. • The synthesized membranes are homogenous in structure and thermally stable up to 410oC. • The membranes show high gas separation performance with αCO2/CH4=60 at 30 bar pressure.