INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET)

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1 INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET) International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN (Print), ISSN (Print) ISSN (Online) Volume 5, Issue 5, May (2014), pp IAEME: Journal Impact Factor (2014): (Calculated by GISI) IJARET I A E M E GAS PERMEATION PROPERTIES OF HYDROGEN PERMEABLE MACROPOROUS ALUMINA CERAMIC MEMBRANES AT HIGH TEMPERATURE Abubakar Alkali 1*, Edward Gobina 1 Robert Gordon University, Riverside East, Garthdee Road, Aberdeen, AB10 7GJ, United Kingdom ABSTRACT The main purpose of this work is to investigate the hydrogen permeation behavior and selectivity of a commercial ceramic alumina membrane and compare same with that of a γ-alumina membrane graded with AlOOH sol using the dip coating method. The permeance of hydrogen and 5 other single gases (He, N 2, CH 4, CO 2 and Ar) were investigated at high temperature and results show that the permeance of increased with increasing temperature for the graded γ- Al 2 O 3 membrane while it decreased for the α-al 2 O 3 support. For the α-alumina membrane, a hydrogen permeance of up to mol m -2 s -1 Pa -1 was observed which increased by about four-fold to mol m -2 s -1 Pa -1 when the membrane was graded with Boehmite sol. However, the graded membrane was permeable to only hydrogen at fifth coating. The selectivity of the membranes for hydrogen with respect to the other 5 single gases decreased with increasing temperature and the order of gas permeation didn t exactly follow the order of gas kinetic diameter and molecular weight. Key words: Hydrogen, Porous Alumina Membranes, Dip Coating, Gas Permeance, Selectivity. INTRODUCTION Membrane technology for hydrogen separation, purification and production processes is becoming an important and enabling technology in the current global decarbonisation efforts aimed at combating climate change and ensuring energy security 1. Ultra pure hydrogen (99.99%) is required for use in chemical industries, domestic power and clean energy applications 2. More recently, the dawn of a new energy epoch in polymer electrolyte fuel cells (PEMFC) has motivated continued research in membrane technology for hydrogen processes 2. It is still work in progress before hydrogen is fully adopted as the global energy carrier to replace fossil fuels. During this 40

2 transition period from fossil fuels to, research interest into several production, separation and purification processes is expected to rise. Recently, inorganic membranes have assumed wider applications in hydrogen separation processes as a result of their ability to withstand harsh operating conditions and they can also be permeable to specific molecules 3. Inorganic membranes can be divided into 2: Dense (metallic) membranes and porous ceramic membranes 4. Palladium membranes are mostly used in hydrogen processes due to their infinite selectivity to hydrogen when defect free 5. However, due to the high cost of palladium membranes, there is great motivation for research into comparably cheaper membranes for hydrogen processes. Porous alumina ceramic membranes with different pore sizes can be considered for several gas separation applications including hydrogen by reducing the pore sizes or by improving specific surface properties 6. Interest is growing in porous ceramic alumina membranes for hydrogen processes due to their ability to withstand harsh operating conditions, thermal and mechanical stability as well as their resistance to corrosion 4,7. The gas transport mechanisms through inorganic membranes generally involves Knudsen diffusion, Hagen Poiseulles s flow, surface diffusion and viscous flow but molecular sieving also occurs in porous membranes with small pore sizes less than 1 nm 4.8. In Knudsen diffusion gas molecules collide with the pore wall and diffuse into the pores of the membrane 6. It occurs when the pore radius is less than the mean free path of the gas molecules and is based on the ratio of inverse square root of the molecular weights of the 2 gases A and B as follows 1 : α AB = M M B A 1/ 2 (1) The Knudsen diffusion is usually accompanied by high permeance but with a low selectivity while surface diffusion involves adsorption of the gas molecules that takes place on the membrane surface and it favors gases with high adsorption capacity while limiting the diffusion of less adsorbing gases 6. Molecular sieving occurs when the membrane pore diameter is close to that of the diffusing species. However, the permeance is low in this mechanism but an infinite separation factor can be achieved 6. Porous ceramic membranes have several layers with different pore sizes hence the mechanisms can occur simultaneously. Thus, gas transport mechanisms in a porous ceramic membrane can be evaluated by taking into account the separate transport mechanisms based on the properties of the membrane and the diffusing gases 8. The hydrogen permeation and transport behavior in porous alumina membranes has been reported by several authors. Li et al 4 (2012) investigated the permeance of hydrogen across porous alumina ceramic support which was graded with a top γ Al 2 O 3 layer of pore diameter 4 nm using the sol-gel technique. permeation tests were conducted at temperature from 25 0 C to C and the group reported a drastic decrease in permeance with increasing temperature from 25 0 C to C but for temperatures from C to C, the hydrogen permeance was more stable. The /N 2 selectivity reported by the group was between which was lower than the theoretical Knudsen value of The lower selectivity value below the theoretical Knudsen value indicates that there was a contribution of viscous flow since hydrogen and nitrogen cannot transport through the membrane by surface diffusion 4. Y.S Cheng et al 7 (2002) also investigated the hydrogen permeance of a mesoporous commercial alumina support with a nominal pore size of 5 nm. Although a hydrogen permeance of over 700 cm 2 cm -3 min -1 bar -1 was achieved, the alumina support could not separate hydrogen from town gas mixture. For /He selectivity, the transport mechanism was Knudsen and a separation factor of 1.5 was achieved which was above the theoretical Knudsen value of

3 One of the ways of enhancing hydrogen permeation in alumina supports is through surface modification with AlOOH Boehmite sol to convert the topmost α- alumina layer to the more stable γ- alumina layer. In this work, the hydrogen permeation behavior of a commercial alumina support was investigated and the effect of temperature, pressure on gas permeance through the membrane and the selectivity were investigated. The hydrogen permeation behavior and selectivity of the modified γ-alumina membrane was also investigated and compared to that of the unmodified commercial alumina support. EXPERIMENTAL The ceramic alumina support is a macroporous membrane supplied by CTI (Ceramiques Techniques Industrielles SA) France with 6000 nm average pore size, I.D = mm, O.D = 26 mm and effective length = 0.32 m. A second alumina support with same specifications was graded with AlOOH sol using the sequential dip coating method. Both tubes were dried in an oven at 65 0 C for 2 hours to remove any water vapor prior to permeation test for the commercial alumina support and also before the support was modified. In the dip coating method for preparation of γ-alumina membrane, the Boehmite sol was prepared into which the support was inserted for 30 minutes for each dipping under continuous stirring. To prepare the Boehmite sol, 46.1 g of the AlOOH powder was weighed and diluted in 1000 ml of distilled water under constant stirring. A fresh Boehmite sol was used for each dipping in order to obtain a uniformly coated membrane. Prior to dipping, both ends of the support were sealed with plastic seals to avoid inner deposition. 5 sequential dippings were conducted and after each dipping, the modified support was dried for 10 hours at 65 0 C and calcined at 873 K for 24 hours and a permeation test for 6 single gases carried out. These gases are:, He, CH 4, CO 2, N 2 and Ar. The experimental set up for gas permeation consists of a shell and tube membrane reactor module, a gas flow system comprising of 3 connected parts: The feed, permeate and retentate. The third part of the experimental set up consists of the mass flow meter which monitors the gas flow rate. Gas permeation tests were conducted for both the modified and unmodified membranes at 298, 323, 373, 473 and. Membrane characterization was conducted using a scanning electron microscopy (SEM) and the elemental composition of the membrane was analyzed using energy dispersive x-ray analysis (EDXA). Figure 1: A Picture of the commercial alumina membrane 42

4 (a) (b) Figure 2: SEM Micrographs of the α-alumina membrane: (a) cross section (b) outer section RESULTS AND DISCUSSION The Dip Coating Method After each dipping, the weight of the membrane was calculated using the weight gain technique in order to monitor and make an accurate estimate of the thickness of the γ-alumina layer. The amount of AlOOH deposited was recorded after each dipping. The amount of deposited AlOOH W was measured by subtracting the weight of the alumina support before dipping from the weight after dipping. The average layer thickness L was calculated using the equation 9. L = ρ W (2) ρ is the density. The estimated thickness of the deposited layer after each dipping in this work using equation (2) is shown in Table 1. Table 1: Estimated layer thickness after each dip Dip Estimated Thickness (µm) It is necessary to mention that the thickness was not factored into the calculation for gas permeance. Unit thickness is used to standardize permeability measurements but not for permeance measurements. Nonetheless, layer thickness affects gas permeation across the membrane and the high the thickness the high the membrane resistance to gas permeation. 43

5 Gas Permeance and Selectivity The permeation behavior of a gas through a membrane is denoted as permeance or permeability 4. Permeance in mol m -2 s -1 Pa -1 is defined as the quantity of gas crossing a unit area in unit time or flux per unit pressure difference between the higher pressure and the lower pressure sides of the membrane 10. To obtain the permeability of the membrane, the unit thickness of the membrane is used to normalize the permeance 10. Permeance is represented in the equation 4 : J = Q / 22.4 A P (3) Where J is the gas permeance (mol/m 2. s. Pa), Q is the permeate gas flow rate (L/min) in permeate side, A is the membrane area for permeation (m 2 ) and P is the pressure difference across the membrane (Pa). Selectivity denotes the relationship between gas permeance and the permeance of another gas 4. If J H2 is the permeance of Hydrogen and J N2 is the permeance of Nitrogen through the alumina membrane, then the selectivity of hydrogen relative to nitrogen can be represented as follows 4 : α H2/N2 = J J N 2 (4) Figure 3: SEM micrograph for inner section of the α-alumina membrane Figure 4: EDXA of inner section of the α- alumina membrane SEM Micrographs and EDXA Results The SEM micrographs for the cross section of the α-alumina membrane shown in Fig. 2 (a) shows the morphology of the cross section with the pores becoming open and flowery but more closely knitted and finer at the surface. The outer section in Fig. 2 (b) shows a more homogeneous pore structure with a smaller grain size compared to both the inner and cross sections. Smaller grain sizes usually lead to better gas permeation through the membrane due the relatively smaller boundaries associated with small grains

6 Table 2: EDXA Elemental composition of the α-alumina membrane Element Weight% Atomic % O Al P Ti (mol m -2 s -1 Pa -1 ) Permeance 1.40E E E E E E E E E Average Pressure (Bar) (a) (mol m -2 s -1 Pa -1 ) Permeance 2.50E E-06 1st Coating 1.50E-06 2nd Coating 3rd Coating 1.00E-06 4th Coating 5th Coating 5.00E E Average Pressure (Bar) (c) (mol m -2 s -1 Pa -1 ) Perm e a nce 5.00E E E E E E E E E E E Average Pressure (Bar) (b) (mol m -2 s -1 Pa -1 ) Permeance 9.00E E E E E E E E E E Average Pressure (Bar) (d) Figure 5: Permeance of hydrogen through (a) commercial alumina at different temperature (b) modified γ-alumina at different temperature for 1 st coating, (c) modified γ-alumina at different coatings, (d) N 2 permeance for the commercial alumina at different temperature Fig.5 (a) shows hydrogen permeance of up to mol m -2 s -1 Pa -1 at which decreased with increasing temperature for the commercial unmodified alumina membrane. Impliedly, there is a reverse temperature dependence on gas permeation. The same trend was also observed for the 5 other single gases i.e. He, CO 2, CH 4, Ar and N 2. Table 3 shows the gas permeance and permselectivity for both the α alumina and γ-alumina membranes including the theoretical Knudsen selectivity. 45

7 Table 3: Gas Permeance and selectivity for the α-alumina membrane and the graded γ-alumina membrane after first dipping Gas Permeance (mol m -2 s -1 Pa -1 ) Selectivity Gas α-alumina γ-alumina α-alumina γ-alumina Knudsen S.F () () () () He CO 2 CH 4 Ar N /N 2 /He /CO 2 /CH 4 /Ar For the modified alumina membrane, a permeance of up to 4.61E 10-6 mol m -2 s -1 Pa -1 was observed at after the first dipping. The results indicate that there was about 4-fold increase in the hydrogen permeance of up to mol m -2 s -1 Pa -1 after the alumina support was modified with the Boehmite sol. Hence the modification with AlOOH sol reduced the hydrogen permeation resistance of the membrane. The modification enhanced surface diffusion of hydrogen molecules through the membrane pores leading to increased hydrogen permeance. However, the permeance was negated by increase in temperature. Surface adsorption is generally inversely proportional to temperature hence the low temperature enhances the surface diffusion of in α-alumina membrane 4. Molecular diffusion has been known to enhance the permeance of gases in nanoporous inorganic membranes hence those gas molecules with higher diffusivity permeate faster than those with lower diffusivity 12. Hydrogen is more diffusive than CO 2 hence hydrogen molecules enjoy more mobility and can retain in the permeate stream longer than CO 2 and by implication exhibit a higher permeance compared to CO The decrease in permeance at high temperature also indicates low mobility of the gas molecules in the alumina membrane pores. Li and Liang 4 (2012) reported similar results in their investigation of Hydrogen permeance across porous alumina ceramic membrane with an average pore size of 0.1 µm at temperature to 773 K and transmembrane pressure difference of Mpa. In this work, for the γ-alumina membrane, 5 successive coatings were carried out and a permeation test conducted after each dip. However, all the 6 gases permeated after the first 3 dippings but only hydrogen and CO 2 permeated after the 4 th dipping although the membrane was not permeable to CO 2 at room temperature at the 4 th dip. CO 2 permeated only at 323, and but hydrogen permeated at all the temperatures. After the fifth dipping, the membrane was permeable to only hydrogen. As shown in Fig. 5 (b) the hydrogen permeance increased with temperature up to mol m -2 s -1 Pa -1 at but reduced with the number of successive coatings as more layers of the AlOOH sol were deposit leading to increase in layer thickness of the membrane which increased the membrane resistance to permeation. As shown in Table 1, the layer thickness of the composite membrane increased with the number of dipping. Gu and Oyama 13 also reported an increase in hydrogen permeance of up to mol m -2 s -1 Pa -1 in their work on a macroporous alumina support modified with AlOOH sol. The gas permeation behavior observed in this work for the modified γ- alumina membrane was in contrast to that of the unmodified commercial alumina. This is because in the commercial alumina, 46

8 gas permeance decreased with temperature while for the modified membrane, it increased with temperature. The high increase in hydrogen permeance after modification suggests that there are cracks and defects in the commercial alumina membrane which were repaired by the deposition of the AlOOH layer. This led to a more uniform and finer surface which enhanced hydrogen permeation through the membrane 13. The apparent opposite trend in hydrogen permeation dependence on temperature for the modified membrane indicates that modifying the membrane with AlOOH sol has enhanced surface diffusion and mobility of the molecules. 14 (a) 4 (c) 12 /N 2 selectivity /C O 2 S elec tiv ity Feed Pressure (Bar) Feed Pressure (Bar) (b) 2.5 (d) /N 2 selectivity selectivity /CO Feed Pressure (Bar) Feed Pressure (Bar) Figure 6: (a) /N 2 selectivity commercial α-alumina support (b) /N 2 selectivity γ-alumina membrane (c) /CO 2 selectivity commercial α- alumina support (d) /CO 2 selectivity γ-alumina membrane The selectivity of the membrane for hydrogen in relation to the other gases was also investigated. In Fig. 6 (a) and Table 3, it can be observed that the highest /N 2 selectivity for the α- alumina support is at which was well above the theoretical Knudsen value of However, for /CO 2, the selectivity was 2.70 which is below the theoretical Knudsen selectivity of For the α-alumina support as shown in Fig. 6 (a) and (c) for /N 2 and /CO 2 selectivity respectively, the selectivity increased with increase in temperature. But a different trend was observed for the /N 2 and /CO 2 selectivity in the modified γ-alumina membrane as shown in Fig. 47

9 6 (b) and (d) where there is a clustered selectivity. Fig. 6 (b) and (d) show the /N 2 and /CO 2 selectivity appearing closer to decreasing with temperature but it is generally clustered. It can be observed that hydrogen permeance for the α-alumina support decreased with temperature while the /N 2 and /CO 2 selectivity increased with temperature. For the modified γ-alumina membrane, the hydrogen permeance increased with temperature while a clustered /N 2 and /CO 2 selectivity that appears closer to decreasing with temperature was observed. Modification of the membrane decreased the hydrogen selectivity although the selectivity is generally low for both membranes. The lower selectivity below Knudsen denotes the contribution of another gas transport mechanism such as viscous flow. Gas Permeance and Kinetic Diameter (mol m -2 s -1 Pa -1 ) P e r m e a n c e 1.00E E E E E-03 He CO 2 Ar N 2 CH 4 P erm eance (mol m -2 s -1 Pa -1 ) 1.40E E E E E E E-07 He CH 4 N2 CO 2 Ar 0.00E Kinetic Diameter (A) (a) (b) Figure 7: Gas Permeance for α-alumina membrane as a function of (a) Kinetic diameter, (b) molecular weight The order of permeance of the single gases at different temperatures is as follows: Order of permeance: H2>He>CO2>CH4>Ar>N2 7.00E Molecular Weight Order of Kinetic Diameter: CH4 (3.8 Ǻ)>N2 (3.64 Ǻ)>Ar (3.4 Ǻ)>CO2 (3.3 Ǻ)>H2 (2.89 Ǻ)>He (2.65 Ǻ) Order of molecular weight: CO2 (44.01)>Ar (39.948)> N2 ( )>CO (28.011)>CH4 (16.044)>He (4.02)> H2 (2.016). Fig. 7 (a) shows the relationship between gas permeance and kinetic diameter for the α-alumina membrane. The permeance of the gases didn t exactly follow the order of their kinetic diameter. Although there is a trend such that the smaller molecules seem to permeate faster than the bigger molecules. He and have the lowest kinetic diameter but the highest permeance although (2.89 Ǻ) with a larger kinetic diameter permeated faster than He (2.65 Ǻ) which could be explained in terms of the higher sorptivity of compared to that of He. 48

10 Moreover, CH 4 with a larger kinetic diameter (3.8 Ǻ) has a higher permeance compared to both N 2 (3.64 Ǻ) and Ar (3.4 Ǻ). This could also be explained in the same vein as for the and He permeance difference. As shown in Fig. 7 (b), the order of gas permeance didn t exactly follow the order of gas molecular weight for the α- alumina membrane., He, CH 4, N 2 and Ar belong to the group of nonadsorbed gases for alumina but CO 2 is a strongly adsorbing gas for alumina membranes 4. It can be observed from Fig. 7 (b) that the permeance of hydrogen was higher than that of all the other gases which was expected based on the fact that hydrogen has the lowest molecular weight. However, hydrogen is a nonadsorbed gas for alumina hence its higher permeance is explained in terms of surface diffusion. The same could be said of CO 2 which has the largest molecular weight; CO 2 is a strongly adsorbing gas for alumina which explains why CO 2 with a higher molecular weight has a higher permeance than N 2, Ar and CH 4. CONCLUSION The hydrogen permeation properties of a commercial α-alumina membrane including those of 5 other single gases: He, CH 4, CO 2, Ar, N 2 were investigated and compared with the permeation properties of γ-alumina membrane modified with Boehmite sol through the dip coating method. Results indicated that the modification of the membrane with Boehmite sol increased the hydrogen permeance by 4 fold. The gas permeance of the α-alumina membrane decreased with temperature and the highest hydrogen permeance of mol m -2 s -1 Pa -1 was observed at. For the modified γ-alumina membrane, the gas permeance increased with temperature and the highest hydrogen permeance of mol m -2 s -1 Pa -1 was achieved at. Conversely, the membrane selectivity for hydrogen in relation to the other 5 single gases increased with temperature for the α-alumina membrane while it decreased with temperature for the modified γ-alumina membrane. The modified γ-alumina membrane was permeable to all the gases at the first, second and third dips. However, after the 4 th dip, the membrane was permeable to only and CO 2 and after the fifth dip, the membrane was permeable to only. Both the commercial α-alumina membrane and the modified γ-alumina membrane show excellent thermal stability at higher temperature. REFERENCES 1. Lu, G. Q., Diniz da Costa, J.C., Duke, M., Giessler, S., Socolow, R., Williams, R.H. & Kreutz, T. (2007). Inorganic membranes for hydrogen production and purification: A critical review and perspective. Journal of colloid and interface science, 314: Meinema, H.A., Dirrix, R.W. J., Brinkman, H.W., Terspstra, R.A., Jekerle, J. & Kosters, P.H. (2005). Ceramic membranes for Gas separation- Recent developments and state of the Art. InterCeram, Vol. 54 (2): Tuznu, F.N., Kocdemir, E. & Uguz, G. (2012). Comparison of gas permeability and selectivity between alumina membranes and Vycor glass at high temperatures. Advances in Materials Physics and Chemistry, (2): Li, X. & Liang, B. (2012). Permeance of pure vapors in porous γ-al 2 O 3 /α-al 2 O 3 ceramic membrane, Journal of the Taiwan Institute of Chemical Engineering, (43): Yun, S. & Oyama, T.S. (2011). Correlations in palladium membranes for hydrogen separation: A review. Journal of membrane science, 375 (1-2): Lee, H-J., Yamuchi, H., Suda, H. & Huraya, K. (2006) Influence of adsorption on the gas permeation performance in the mesoporous alumina ceramic membrane. Separation and Purification Technology, (49):

11 7. Cheng, Y. S., Pena, M.A., Fierro, J.L., Hui, D.C.W. & Yeung, K.L. (2002). Performance of alumina, zeolite, palladium, Pd-Ag alloy membranes for hydrogen separation from towngas mixture. Journal of membrane science, 204: Wall, Y., Mudimu, O-A., Braun, G. & Brunner, G. (2010). Gas transport through ceramic membranes under super-critical conditions. Desalination, 250: Bientinesi, M. & Petarca, L. separation from gas mixtures through palladium membranes on metallic porous supports. 10. Lee, D. & Oyama, S.T. (2002). Gas permeation characteristics of a hydrogen selective supported silica membrane. Journal of Membrane Science. 210: Basko, M, L., Lombardo, E.A., Cornaglia, L.M. (2011). The effect of electroless plating time on the morphology, alloy formation and transport properties of Pd-Ag composite membranes. International journal of hydrogen energy 36: Othman, M.B., Mukhtar H. & Ahmad, A.L. (2004). Gas permeation characteristics across nanoporous inorganic membranes. IIUM Engineering Journal, Vol. 5. No. 2: Gu, Y. & Oyama, S.T. (2007). Ultra-thin, hydrogen-selective silica membranes deposited on alumina-graded structures prepared from size-controlled Boehmite sols. Journal of Membrane science, 306: B. Chirsabesan and M.Vijay, Membrane Assisted Electro Chemical Degradation for Quinoline Yellow, Eosin B and Rose Bengal Dyes Degradation, International Journal of Design and Manufacturing Technology (IJDMT), Volume 4, Issue 2, 2013, pp , ISSN Print: , ISSN Online: Abubakar Alkali and Edward Gobina, Hydrogen Permeation Behavior and Annealing in Composite Palladium Membranes at High Temperature, International Journal of Mechanical Engineering & Technology (IJMET), Volume 5, Issue 4, 2014, pp , ISSN Print: , ISSN Online: