CH4 Separation

Article pubs.acs.org/IECR

Highly Permeable AlPO-18 Membranes for N2/CH4 Separation Zhaowang Zong,† Sameh K. Elsaidi,‡ Praveen K. Thallapally,‡ and Moises A. Carreon*,† †

Chemical and Biological Engineering Department, Colorado School of Mines, Golden, Colorado 80401, United States Pacific Northwest National Laboratory, Richland, Washington 99352, United States

‡

S Supporting Information *

ABSTRACT: Herein we demonstrate that AlPO-18 membranes can separate N2/CH4 gas mixtures at unprecedented N2 permeances. The best membranes separated N2/CH4 mixtures with N2 permeances as high as 3076 GPU and separation selectivities as high as 4.6. Gas mixture separation data, N2 and CH4 adsorption isotherms, ideal adsorbed solution theory (IAST), and breakthrough experiments were collected to understand the separation mechanisms. Competitive adsorption and differences in diffusivities were identified as the prevailing separation mechanisms. Differences in diffusivity played a more dominant role than the competitive adsorption, and led to nitrogen selective membranes.

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INTRODUCTION Nitrogen represents a significant impurity in natural gas wells and has to be removed because it decreases the energy content of the gas. Approximately 14% U.S. natural gas wells contain >4% N2.1 The U.S. natural gas pipeline specification requires inert gases, such as N2, to be less than 4%.1 Therefore, natural gas processing requires a cost-effective technology to reject or separate nitrogen.2 The benchmark technology to separate nitrogen from natural gas is cryogenic distillation, which is an energy intensive and expensive process. Membrane separation technology 3,4 is an energy-efficient alternative for N 2 purification from natural gas.5 The challenge becomes to find membranes displaying enhanced N2/CH4 separation performance. Current selective nitrogen polymeric membranes exhibit moderate N2/CH4 selectivities, but very low N2 permeances as shown in the Robeson plot.6 Several inorganic membranes displaying molecular sized pores have surpassed the separation performance of polymeric membranes. Examples of these membranes include carbon molecular sieve (CMS) membranes, reported by Koros group.7 These CMS membranes displayed a moderate N2/CH 4 selectivity of 7.7, but low N2 permeability of 6.8 barrers (1 barrer = 10−10 cm3 (STP)·cm/(cm2·s·cmHg)). Our group has reported the successful synthesis of SAPO-34 membranes for N2/CH4 separation.5,8,9 Through optimization of membrane synthesis parameters, we have developed membranes displaying permeances as high as ∼2600 GPU with N2/CH4 separation selectivities of 7.4.9 To our best knowledge this is so far the highest reported N2 permeance for this binary gas mixture. Huang et al.10 have reported highly selective N2/CH4 SAPO-34 membranes displaying moderate N2 permeances. These SAPO34 membranes displayed N2/CH4 selectivities as high as 11.2 with N2 permeance of ∼860 GPU.10 © XXXX American Chemical Society

AlPO-18, a member of microporous aluminophosphates (AlPOs) consisting of AlPO4− and PO4− tetrahedral units,11−16 represents a suitable candidate to separate N2 from CH4. The AEI topology of AlPO-18 possesses a three-dimensional framework with a crystallographic pore size of ∼0.38 nm.11 Figure S1 shows a schematic of the framework structure of AlPO-18. Based on the kinetic diameter of N2 (∼0.36 nm) and CH4 (∼0.38 nm), and the micropore size of AlPO-18, this aluminophosphate could potentially molecular sieve N2 over CH4 or at least make that N2 molecules could diffuse rapidly through the pores, whereas CH4 at most will diffuse slowly meaning that high N2 selectivities could be potentially achieved based on molecular diffusion differences. Our group reported the first example of continuous AlPO-18 membranes displaying molecular sieving properties for CO2/CH4 gas mixtures.16 Later reports by Zhou group17,18 confirmed the molecular sieving properties of AlPO-18 membranes for CO2/CH4, CO2/N2 and H2/CH4 mixtures. Table 1 summarizes the separation performance of existing AlPO-18 membranes over several industrial relevant gas mixtures. In this report, we demonstrate the separation ability of AlPO18 membranes for N2/CH4 gas mixtures. These AlPO-18 membranes displayed unprecedented N2 permeances at moderate N2/CH4 separation selectivities.

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EXPERIMENTAL SECTION 1. Synthesis of AlPO-18 Seeds. The gel composition employed to synthesize AlPO-18 seed crystals had a molar ratio Received: Revised: Accepted: Published: A

February 27, 2017 March 14, 2017 March 22, 2017 March 22, 2017 DOI: 10.1021/acs.iecr.7b00853 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

40 °C. The solution was transferred to a Teflon-lined autoclave (4744 General Acid Digestion Vessel, 45 mL, Parr Instrument) under autogenous pressure and heated at 215 °C for 10 h in a conventional oven. The membrane gel, and “AlPO-18 seeded” alumina supports (prepared by rubbing the inside of the support with AlPO-18 seeds) were placed inside the autoclave. The outside surface of the supports was wrapped with Teflon tape to avoid growth of membranes on the outside surface. The membranes were then taken out of the autoclave and washed with deonized water and dried overnight at 100 °C. Finally, the membranes were calcined at 450 °C for 10 h with heating and cooling rates of 0.6 °C/min in a conventional oven. 3. Structural and Morphological Characterization of AlPO-18 Crystals and Membranes. AlPO-18 crystals (seeds, and those collected from the bottom of the autoclaves after membrane synthesis) were analyzed by powder X-ray diffraction (PXRD) using a Siemens Kristalloflex 810 diffractometer operating at 30 kV and 25 mA with Cu Kα1 radiation (λ = 1.540 59 Å). AlPO-18 seeds and selected AlPO18 membranes were broken and analyzed by scanning electron microscopy (SEM, JEOL JSM-7000F). 4. Adsorption Isotherms and Breakthrough Experiments. CH4 and N2 adsorption were collected at 298 and 278 K using an automatic gas sorption analyzer (Quantachrome Autosorb IQ, Quantachrome Instruments, Boynton Beach, FL). In each experiment, about 100 mg of AlPO-18 crystals was loaded prior to activation. The sample was activated at 200 °C under dynamic pressure for 12 h and then brought to the adsorption temperature. Experimental column breakthrough measurements were conducted by packing 0.30 g of AlPO-18 sample in a 6.35 cm long and 0.5 cm diameter column. The sample was activated at a proper temperature. Pressurization of the columncontaining AlPO-18 sample was accomplished by syringe pump (Teledyne ISCO) directly connected to the system. An inline pressure transducer was used to verify column pressure. The column was cooled to room temperature and the pure He gas was initially flowed to a Stanford Research Residual Gas Analyzer (RGA) for first 3 min, after which the flow of He is stopped and flow of the CH4/N2 gas mixture is introduced to the fixed bed column containing AlPO-18 sample with flow rate of 5 mL/min and total pressure of 138 kPa at room temperature. Effluent gases were thereby tracked with the RGA, whereas the gases breaking through the column were indicated by an increase in the pressure. This ran for the next 2 h. 5. Gas Mixture Permeation Experiments. Gas mixture permeation experiments were carried out in a house-built flow

Table 1. Gas Separation Performance of Existing AlPO-18 Membranes mixtures

separation selectivity α

permeance (GPU)

reference

CO2/CH4 CO2/CH4 CO2/CH4 CO2/N2 H2/CH4

52−60 101 202 45 22

∼200 (CO2) 540 (CO2) ∼1940 (CO2) 1880 (CO2) 300 (H2)

16 17 18 18 18

of 1.0 Al2O3:3.16 P2O5:6.32 TEAOH:186 H2O, a similar composition to our previous report.16 In a typical synthesis, aluminum isopropoxide (Al(i-C3H7O)3, Aldrich 99.99%), deionized water and tetraethylammonium hydroxide (TEAOH, Aldrich 35 wt % aqueous solution) were mixed together for 2 h to form a homogeneous solution. Then phosphoric acid (H3PO4, Sigma-Aldrich 85 wt % aqueous solution) was added dropwise to this solution. The resultant solution was stirred for another 24 h at room temperature. The solution was transferred to a Teflon-lined autoclave (4744 General Acid Digestion Vessel, 45 mL, Parr Instrument) under autogenous pressure and heated at 200 °C for 72 h in a conventional oven. After the solution was cooled to room temperature, it was centrifuged at 3,300 rpm for 5 min to collect the seeds, which were washed with deionized water washing. The centrifugation-washing process was repeated three times, and the resulting seeds were dried overnight at 100 °C. 2. Synthesis of AlPO-18 Membranes. AlPO-18 membranes were prepared by secondary seeded growth method inside the surface of porous α-Al2O3 tubes. The 6 cm supports (Inopor GmbH) have an inside diameter of 0.7 cm and an outside diameter of 1.1 cm and are asymmetric within the inner layer, which has a pore size of 100 nm. The effective support area was 7.0−7.5 cm2. The synthesis gel used to grow the AlPO-18 membranes had a molar ratio of 1.0 Al2O3:1 P2O5:1.8 TEAOH:xH2O (x = 90, 120, 150, 200). Membranes prepared with x = 120 showed the best separation performance, and therefore we focused on this particular gel composition. Other gel compositions led to poor separation performance (x = 90 and 120) or the formation of discontinuous membranes (x = 200). To prepare the membrane gel, aluminum isopropoxide (Al(i-C3H7O)3, Aldrich 99.99%), deionized water and tetraethylammonium hydroxide (TEAOH, Aldrich 35 wt % aqueous solution) were mixed together for 1 h to form a homogeneous solution, followed by the addition of phosphoric acid (H3PO4, Sigma-Aldrich 85 wt % aqueous solution). The resultant solution was aged for 2 h at

Figure 1. XRD and SEM of AlPO-18 crystals employed as seeds for membrane preparation. B

DOI: 10.1021/acs.iecr.7b00853 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


All membranes were prepared independently using the same gel composition and same hydrothermal synthesis conditions detailed in the Experimental Section. The XRD pattern of the crystals collected from the bottom of the Teflon autoclaves after AlPO-18 membranes synthesis corresponded to AEI topology typical AlPO-18 (Figure S2). AlPO-18 membranes displayed N2 permeances as high as 3076 GPU, with N2/CH4 separation selectivity of 3.8. To our best knowledge, this is the highest N2 permeance reported for any membrane for this particular binary mixture. Although the separation selectivity is moderate, our preliminary economic evaluation for N2/CH4 separation5 suggests that high N2 permeances are essential to reduce the N2 rejection cost employing membrane technology. The separation index π was calculated to assess membrane reproducibility. This index [π = N2 permeance × (selectivity-1) × permeate pressure] has been used as a reliable parameter to predict porous crystalline membrane reproducibility.19 Separation index π was in the 0.11−0.25 mol/(m2·s) range indicating good membrane reproducibility. Figure 2 shows representative top and cross section view SEM images of AlPO-18 membranes. The top view SEM (Figure 2a) shows well intergrown rectangular AlPO-18 crystals on the surface of the alumina support. The size of the surface crystals of membrane is clearly larger than that of the seeds, suggesting that secondary seed growth led to a recrystallization process. Cross-sectional SEM imaging (Figure 2b) shows a ∼2.4 μm-thick dense aluminophosphate layer. The observed AlPO-18 thin layer may be in part responsible for the high observed N2 permeances. To have a better understanding on the potential separation mechanisms of the AlPO-18 membranes, adsorption isotherms, and column breakthrough experiments were collected for N2 and CH4. Figure 3 shows the adsorption isotherms for N2 and CH4 measured at 298 and 278 K for AlPO-18 crystals. The adsorption isotherms at 140 KPa (which corresponds ∼ to the trans-membrane pressure employed during the separation experiments) revealed CH4 uptakes of 12 and 15 cm3/g at 298 and 278 K, respectively and N2 uptakes of 4.5 and 10 cm3/ g, respectively. These results indicate that at the studied separation conditions of ∼140 kPa, and room temperature, AlPO-18 adsorbed ∼2.7 times more CH4 than N2. The preferential adsorption of CH4 over N2 in AlPO-18 may be

system. The membrane was mounted in a stainless steel module and sealed at each end with silicone O-rings. The pressure on permeate and rententate sides of the membrane was independently controlled by back pressure regulators. Fluxes were measured using a bubble flow meter. A premixed 50/50 N2/CH4 gas was used as feed gas. The total feed flow rate was 100 standard cubic centimeters per minute (sccm). The testing temperature was room temperature, the feed pressure was 223 kPa, and the trans-membrane pressure drop was 138 kPa. The compositions of the feed, retentate and permeate streams were measured, after reaching the steady state, using a gas chromatograph (SRI instruments, 8610C) equipped with a thermal conductivity detector and HAYESEPD packed column. The oven, injector and detector temperatures in the GC were kept at 40, 50 and 150 °C, respectively. Permeances and separation selectivities were calculated as described in the Supporting Information.

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RESULTS AND DISCUSSION The XRD pattern of the uncalcined AlPO-18 seeds is shown in Figure 1a, corresponding to AEI topology, of which is typical for structure of AlPO-18. The morphology of the AlPO-18 seeds was inspected by scanning electron microscopy (Figure 1b). AlPO-18 seeds consisted of thin sheet-like crystals displaying narrow size distribution with sizes below 1 μm. These seeds exhibiting small and narrow size distribution (highly desirable for membrane preparation) were employed to grown AlPO-18 membranes. The synthesized AlPO-18 membranes were used to separate premixed equimolar N2/CH4 mixtures. The feed pressure for the separation experiments was 223 kPa, and the pressure in the permeate side was 85 kPa. The separation results carried out at room temperature for these membranes are shown in Table 2. Table 2. AlPO-18 Membranes Separation Performance for Equimolar N2/CH4 Mixtures membrane

separation selectivity α

N2 permeance (GPU)

separation index π (mol/(m2·s))

M1 M2 M3 M4

4.6 3.8 4.4 3.0

1463 3076 1356 1867

0.15 0.25 0.13 0.11

Figure 2. Representative SEM images of AlPO-18 membrane: (a) top and (b) cross-sectional views. C


Article


Figure 5. Column breakthrough experiment for 50:50 CH4/N2 gas mixture at 298 K and 140 kPa for AlPO-18.

Figure 3. Single component CH4 and N2 adsorption isotherms for AlPO-18 collected at 298 and 278 K.

breakthrough is longer, N2 diffuses faster than CH4 (due to its the lower affinity to AlPO-18 as demonstrated by the single adsorption isotherms and IAST) as well as to its smaller kinetic diameter. Therefore, breakthrough column experiments suggest that differences in diffusivity favor the separation of N2 over CH4 in the gas mixture. Figure 6 compares our AlPO-18 membranes to polymer membranes presented in “Robeson” plot 6 for N 2 /CH 4

explained by differences in polarizabilities of these two molecules. CH4 has higher polarizability (25.9 × 1025/cm3) as compared to N2 (17.4 × 1025/cm3).20 Therefore, it is likely that stronger electrostatic interactions between the AlPO-18 surface carrying partial charges13 and the higher polarizability of CH4 promotes its preferential adsorption over N2. Ideal adsorbed solution theory (IAST) was used to predict the selectivity of CH4/N2 binary mixture based on the experimental single adsorption isotherms collected at 298 K. The adsorption selectivity for a 50:50 CH4/N2 binary gas mixture at 298 K, was ∼6.9 (Figure 4). These results indicate that at the employed separation conditions, AlPO-18 can adsorb more CH4 than N2, in agreement with the single adsorption isotherm results. To learn about the relative diffusivity differences between N2 and CH4, column breakthrough experiments were conducted at 298 K and 140 kPa, for a 50:50 N2/CH4 gas mixture over AlPO-18 (Figure 5). Because the retention time of CH4 in the

Figure 6. N2/CH4 separation performance of AlPO-18 membranes in the Robeson plot.

separation. Membrane thickness of 2.5 μm (estimated from SEM) was used in the calculation of N2 permeabilities in Barrer (permeance × membrane thickness). The data points for all studied AlPO-18 membranes are significantly above the upper bound. To our best knowledge, these AlPO-18 membranes display the highest reported N2 permeances for N2/CH4 gas mixtures. Depending on the nature of the porous crystalline membrane, the mixture system, and the operating separation conditions, gas mixtures can be separated by one of the

Figure 4. IAST calculated selectivity for 50:50 CH4/N2 gas mixture at 298 K for AlPO-18. D



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following three mechanisms:5 (a) molecular sieving (larger molecules do not fit into the pores, whereas the smaller molecules preferentially diffuse through the pore structure); (b) differences in diffusivity (the smaller, less hindered molecule in a mixture, diffuses faster than the larger molecules) and (c) competitive adsorption (one type of molecule is more strongly adsorbed on the porous structure and can inhibit permeation of the second molecule). Based on the moderate observed separation selectivities, as well to the fact that both molecules N2 and CH4 can fit within the pores of AlPO-18, it is unlikely that true molecular sieving could be a separation mechanism. Adsorption isotherms (Figure 3), and IAST (Figure 4) indicate that CH4 adsorbs more strongly than N2. Therefore, the preferential adsorption of CH4 would favor separating CH4 over N2 in the mixture. On the other hand, breakthrough experiments (Figure 5) confirm higher diffusivity of N2 over CH4 favoring N2 selectivity. Thus, the differences in diffusivities would favor separating N2 over CH4 in the mixture. These results suggest that differences in diffusivities between N2 and CH4 played a more dominant role than competitive adsorption. Realistic separation conditions may require the membrane to perform well at high pressures and in the presence of other natural gas impurities. The overall separation performance of the membranes under high pressure and in the presence of other impurities, will likely decrease due to concentration polarization and molecule competitive adsorption, respectively.

ACKNOWLEDGMENTS M. A. Carreon thanks the Renewable Energy Materials Research Science and Engineering Center (REMRSEC) under Award Number DMR-0820518 for financial support of this work.

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CONCLUSIONS In summary, we have successfully synthesized AlPO-18 membranes and demonstrated their separation ability for N2/ CH4 gas mixtures. AlPO-18 membranes separated N2/CH4 mixtures with unprecedented N2 permeances as high as 3076 GPU and separation selectivities as high as 4.6. Competitive adsorption and differences in diffusivities were identified as the prevailing separation mechanisms. Our experimental evidence indicates that AlPO-18 membranes were nitrogen selective because differences in diffusivities between N2 and CH4 played a more critical role than competitive adsorption. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00853. AlPO-18 structure, permeance and selectivity calculations, additional AlPO-18 XRD pattern, ideal adsorbed solution theory (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Moises A. Carreon. E-mail: [email protected]. ORCID

Moises A. Carreon: 0000-0001-6391-2478 Author Contributions

The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. Funding

Renewable Energy Materials Research Science and Engineering Center (REMRSEC) under Award Number DMR-0820518 Notes

The authors declare no competing financial interest. E


Article

Industrial & Engineering Chemistry Research (19) Carreon, M. A.; Li, S. G.; Falconer, J. L.; Noble, R. D. SAPO-34 seeds and membranes prepared using multiple structure directing agents. Adv. Mater. 2008, 20 (4), 729−732. (20) Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The Properties of Gases and Liquids, 5th ed.; McGraw Hill: New York, 2001.

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CH4 Separation

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