Abstract: Removal of CO2 in natural gas sweetening is nowadays focusing on membrane based practice. An investigation on miscible blends of ionic liquids-polyethersulfone has been done for CO2/CH4 separation purpose. Different types of ionic liquids namely emim [Tf2N] and emim[CF3SO3] is blended separately with polyethersulfone (PES) as the membrane casting solution. Experimental results show that both ionic liquids are miscible with PES and create dense polymeric membranes through FESEM analysis. The presence of ionic liquids in both PES membranes has considerably improved the CO2 permeance without scarifying much on the CO2/CH4 ideal selectivity. At 10 barg pressure, the incorporation of emim [Tf2N] and emim [CF3SO3] has also increased the CO2/CH4 ideal selectivity for 47 and 55% accordingly as compared to the pure PES membrane. It was noticed that blending of emim [Tf2N] produced higher CO2 permeance as expected and the miscible blend of emim [CF3SO3] has high CO2/CH4 selectivity as emim [CF3SO3] is selective towards CO2/CH4. It can be concluded that the imidazolium based ionic liquids are good to be used in natural gas sweeting system.
INTRODUCTION
The issues on high natural gas demand have attracted ceaseless debates on undeveloped natural gas well, high carbon dioxide (CO2) content and its removal technology. Apart from the well-implemented absorption and adsorption process (Van Der Sluijs et al., 1992), membrane technology has been a looking forward method because of its advantages on energy and space saving, small footprint and easy to scale up (Koros and Fleming 1993; D’Alessandro et al., 2010; Merkel et al., 2010; Strathmann, 2011; Pan et al., 2013). For instance, membrane technology especially polymeric membrane has already occupied about 20% of the CO2/CH4 separation market back in 2009 (Ebner and Ritter, 2009; Mohshim et al., 2012).
In addition, Room Temperature Ionic Liquid (RTIL) have pinched considerable attention in CO2 capture since the RTIL can be tailored to possess high CO2 affinity (Zhao, 2006). RTIL which is organic salt at room temperature are stable at high temperature, has negligible vapor pressure and non-flammable makes it suit for CO2 separation in natural gas (Fredlake et al., 2004; Zhao, 2006). Recently, Universiti Teknologi PETRONAS has conducted the experiment of polymeric membrane fabrication blended with selected RTIL. Two different RTILs which are imidazolium based have been used and the performance of both RTILs is compared in this study.
MATERIALS AND METHODS
Materials and membrane preparation: Two Ionic Liquid Modidfied Polymeric Membranes (ILMPM) were prepared by blending two different ionic liquids which are 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, emim [Tf2N] and 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate, emim [CF3SO3] in the solvent which is 1-Methyl-2-pyrrolidinone, NMP. Then, the polymer, polyethersulfone (PES) which is in flakes form is dissolved in the solvent-ionic liquid mixture. Pure PES membrane was also prepared with the same method without ionic liquid addition. The composition of each material is presented in Table 1.
Membrane casting and drying: Membranes were casted by using the membrane casting machine. The casting solution was poured onto glass plate and the casting knife was adjusted at 180 μm. The membranes were dried in drying oven at 90°C for overnight. The membranes were found to be fully dried as they are peeled off from the glass plate.
RESULTS AND DISCUSSION
Membrane characterization
| • | Field emission scanning electron microscope (FESEM): FESEM was used to analyze the influence of the ionic liquid, emim [Tf2N] and emim [CF3SO3] blending with PES into the polymeric membrane fabrication. This test shows the characterization of the membrane surface morphology and the global chemical composition of the membranes and the distribution of the IL within the membranes |
| Figure 1 shows examples of FESEM images of pure PES membrane and ILMPM based on emim [Tf2N] and emim [CF3SO3]. It is noticed that the membranes are all dense in structure and this resembles that both ionic liquid emim [Tf2N] and emim [CF3SO3] is miscible with PES and NMP as no double layer of morphology shown in the images (Rahman et al., 2006; Zhang et al., 2008) | |
| • | Fourier transform infrared (FTIR): In this study, FTIR test was performed in order to determine the chemical structures changes with the addition of emim [CF3SO3]. All samples were mounted at the sample position with outer skin surface facing the IR beam and IR absorption spectra were obtained at room temperature from 4000-450 cm-1 and average of 16 scans |
| From Fig. 2, we can see some pronounced peaks at about 3097, 1590, 2880 and 3160 cm-1. The peaks at 3097 and 1590 cm-1 are attributed to the stretching vibration of C-H and aromatic ring accordingly in PES group. For a membrane consists of miscible blends, frequency shifts usually indicate specific interactions between the characteristic group of the mixture. The wavenumber of pronounced peaks are mostly shifted from 3097-3096 cm-1 and from 1589-1587 and 1586 cm-1 in membranes containing ionic liquid. This shifting indicates that the membranes are performing new O-H bond with the addition of ionic liquid (Rahman et al., 2006). On the other hand, the peaks at 2880 and 3160 cm-1 represent the CH3 group and C-H stretching in ionic liquids. In addition, the peak of 3448 cm-1 in pure PES membrane is missing in both ILMPM which indicate some other bonding are formed | |
| • | Thermogravimetric analysis (TGA): TGA was performed to measure the weight loss of each sample as a function of temperature. Figure 3 shows the TGA result for pure PES membrane and ILMPM1_10 and ILMPM2_10. Both analysis shows samples are still containing solvent which presents the weight loss occurred at 202°C, at the boiling point of NMP. However, there is a slight different of weight loss at high temperature. In Fig. 3a, a massive weight loss at temperature above 500°C while in Fig. 3b, the massive weight loss occurred at temperature around 400°C. This weight loss is due to the degradation of polymer PES containing ionic liquid which degrades at lower temperature as compared to pure PES membrane |
| Table 1: | Membrane composition |
| Fig. 1(a-c): | FESEM images of (a) Pure PES membrane, (b) ILMPM1_10 and (c) ILMPM2_10 |
| Fig. 2: | FTIR spectra for all fabricated membranes |
Membrane separation performance: Membrane gas permeation test was performed to check the membrane separation performance. The test was conducted using pure CO2 and pure CH4 gases and CO2 permeance and CO2/CH4 ideal selectivity were calculated from the results. Figure 4 shows the CO2 permeance and CO2/CH4 ideal selectivity across the pressure.
From Fig. 4a, it shows that the membranes exhibit the glassy polymeric membrane behaviour as the CO2 permeance is decreasing when the pressure increased. Overall, the addition of ionic liquid has also improved the CO2 permeance as compared to pure PES membrane. It is also noticed that, the CO2 permeance of ILMPM1 is slightly higher than ILMPM2. In contrast, the CO2/CH4 ideal selectivity of all membranes is increasing with increasing of pressure which indicates the generic glassy polymeric membrane behaviour. At 10 barg pressure, the incorporation of emim [Tf2N] and emim [CF3SO3] has also increased the CO2/CH4 ideal selectivity for 47 and 55% accordingly as compared to the pure PES membrane.
| Fig. 3(a-b): | TGA for (a) Pure PES membrane and (b) ILMPM1_10 and ILMPM2_10 |
However, the ILMPM2 is having higher CO2/CH4 ideal selectivity as compared to ILMPM1 as emim [CF3SO3] is more selective towards CO2/CH4.
| Fig. 4(a-b): | (a) CO2 permeance and (b) CO2/CH4 ideal selectivity across pressure |
CONCLUSION
The properties of PES membranes containing ionic liquid imidazolium based give good contact between the polymer and the ionic liquid. Pure CO2 and CH4 gas permeation data showed a substantial improvement for both blending. The CO2 permeation for eminTf2N-PES blending has been improved as expected while the CO2/CH4 ideal selectivity is improved with emim CF3SO3-PES blending as compared to the pure PES membrane separation performance.
ACKNOWLEDGMENTS
We would like to thank to the Universiti Teknologi PETRONAS for the facilities provided and to the ERGS Grant No. ERGS/1/2011/TK/UTP/02/46 for financial support.