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Permeation efficiency of Pd–Ag membrane modules with porous stainless steel substrates
Separation and Purification Technology 89 (2012) 189–192
Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Short Communication
Permeation efficiency of Pd–Ag membrane modules with porous stainless steel substrates Donglai Xie ⇑, Fang Wang, Kui Wu, Ertao Zhang, Yajun Zhang MOE Key Laboratory of Enhanced Heat Transfer & Energy Conservation, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 4 April 2011 Received in revised form 12 January 2012 Accepted 12 January 2012 Available online 21 January 2012 Keywords: Hydrogen Palladium membrane Porous stainless steel
a b s t r a c t It is generally regarded that the permeation of hydrogen through palladium or palladium alloy membranes follows the ‘‘solution–diffusion’’ mechanism, which can be described by Sieverts’ Law. The hydrogen permeability of actual membrane modules usually differ from those predicted from Sieverts’ Law due to the influence of substrate and some other factors. An efficiency (or efficiency factor) has been introduced by many researchers to denote the difference between the actual permeability and those predicted from Sieverts’ Law. Extensive experiments were carried out in an electrically heated vessel to study the hydrogen permeation flux and permeation efficiency of Pd–Ag membrane modules with porous stainless steel substrates. The influence of operation conditions on the membrane permeation flux and efficiency was examined. It was observed that the hydrogen permeation flux through the module increased by increasing the temperature and hydrogen pressure in the vessel, while the permeation efficiency increased by increasing the hydrogen pressure in the vessel side and decreasing the membrane temperature. The permeation efficiency was correlated to the vessel temperature (T) and hydrogen pressure difference (PH0.5 PL0.5) for the conditions studied. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is a valuable chemical material, widely used in processes like petroleum refining, petrochemical production, metal processing, semiconductor manufacturing, float glass processing, etc [1]. In recent years, with the development of fuel cell technologies, the demand of high-purity hydrogen is also growing for new energy related applications, such as transportation, power generation and combined micro heat and power cogeneration [2]. Large-scale hydrogen production is a relatively mature technology. Most of the world’s hydrogen is generated at large petroleum and chemical plants by steam reforming or partial oxidation of natural gas in parallel fixed bed reactors within huge top-fired or side-fired furnaces [3]. Purification is a key step in the hydrogen production process. It accounts for a large fraction of energy expenditure and capital investment. Currently Pressure Swing Adsorption (PSA) is widely used for industrial hydrogen separation. The PSA process with complicated devices features high hydrogen product purity (99%+) and moderate hydrogen recovery (65–90%) depending on the tail gas pressure [4]. Palladium (Pd) membranes have also been used for many years for the production of extremely pure hydrogen (99.999%+) for specific purposes. It is regarded as a potential method to produce ultra-pure hydrogen as compared ⇑ Corresponding author. Tel./fax: +86 20 22236985. E-mail addresses: [email protected], [email protected] (D. Xie). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2012.01.030
with conventional processes. It can also be integrated with chemical reactors where chemical reaction and hydrogen separation occur simultaneously to simplify the hydrogen production process. The fabrication and performance of palladium membranes have been investigated intensively. It is generally regarded that the permeation of hydrogen through palladium or its alloy membranes follows the ‘‘solution–diffusion’’ mechanism, which can be described by the Sieverts’ Law [5,6]:
S Ep Ms ¼ K eRT ðPnH PnL Þ t
ð1Þ
where MS is the hydrogen permeation rate, K is the pre-exponential factor, S is the effective area of membrane surface for hydrogen permeation, t is the thickness of palladium or palladium alloy membrane, Ep is the activation energy for permeation, R is the gas constant, T is the temperature, PH is the hydrogen partial pressure in vessel side, PL is the average hydrogen partial pressures in the membrane permeate side, and n is the parameter whose value depends on the limiting transport mechanism of hydrogen permeation through palladium or its alloy membrane. The value of n is decided by the thickness of the membrane. Ward and Dao [7] showed that for temperatures above 673 K, n was equal to 0.5 for membranes thicker than 10 lm. Usually to use Sieverts’ Law correctly with an exponent of 0.5, the thickness of membrane should be larger than 10 lm [8].
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D. Xie et al. / Separation and Purification Technology 89 (2012) 189–192
Nomenclature a, b, c Ep F K Ma MS N
PH
parameters in Eq. (4) activation energy for permeation, J mol1 hydrogen permeation flux, mol m2 s1 pre-exponential factor, mol m1 s1 MPan actual hydrogen permeation rate, mol s1 hydrogen permeation rate calculated from Sieverts’ Law, mol s1 parameter whose value depends on the limiting transport mechanism of hydrogen permeation through palladium or its alloy membrane, hydrogen partial pressure in vessel side, MPa
PL
hydrogen partial pressures in the membrane permeate side, MPa gas constant, J moll K1 effective area of membrane surface for hydrogen permeation, m2 temperature, K thickness of palladium or palladium alloy membrane, m
R S T t
Greek letter g permeation efficiency, –
Table 1 Permeation efficiencies reported in literature. Researchers
Operation temperature (°C)
Pressure (MPa)
Membrane module configuration
Efficiency
Adris et al. [12] Roy [13]
447–640 400–800
0.384 1.24
0.39 0.1 [14]
Mahecha-Botero et al. [15] Xie et al. [17]
545–550
0.65–0.90
Membrane tubes, 4.7 mm O.D., 0.20–0.28 mm thick Membrane tubes, 5.2 lm palladium coating on both outside and inside surfaces of the 3.175 mm O.D. substrate stainless steel tubes Membrane panels with Pd/Ag foil of thickness 25 lm on 316 stainless steel substrate
500–800
1.0–3.0
Membrane panel with Pd/Ag foil of thickness 50 lm on 316 stainless steel substrate
0.9
0.6–0.9 [16]
When Pd membrane is employed for hydrogen separation, it is usually supported on porous substrates, such as ceramic, porous glass and porous stainless steel [8,9] to withstand the pressure difference between the two surfaces of the palladium membrane. The substrate may affect the permeability of the membrane module. Other factors, such as the existence of gas species other than hydrogen, can also affect the membrane permeability [10]. An efficiency has been introduced by many researchers to denote the difference between the actual permeability (Ma) and those predicted from Sieverts’ Law (Ms) [11]:
Ma ¼ g Ms
ð2Þ
The values of g are reported in literature to be from 0.1 to approximately 0.9, and are listed in Table 1. The variation of these figures is in a wide range, and the factors affecting the permeation efficiencies are so far not clear, since all these reported data were obtained from experiments where membrane separation was coupled with chemical reactions for in situ hydrogen production. No work has been carried out to quantitatively study the influences of operation conditions and membrane module configuration on the membrane permeation efficiency. The purpose of the current study is to investigate the influence of operation conditions on the permeation efficiency for the widely used Pd–Ag membrane modules with porous stainless steel substrates, without coupling of any chemical reactions.
2. Experimental setup and procedure 2.1. The test membrane modules Three membrane modules as shown in Fig. 1 have been employed in the test. Each module consists of the following parts: frame, substrate, membrane foil, graphite gasket and flange. Pd–Ag membrane foils of 75% (wt) palladium and 25% (wt) silver with thicknesses of 25 and 50 lm were tested in the experiments. The membrane foils were supplied by Alfa-Aesar. The following performance data of such membrane foils was used: activation
Fig. 1. Structure of the Pd–Ag membrane module assembly.
Table 2 Configuration of the membrane modules employed in the experiments. Membrane module #
Membrane foil thickness (lm)
Effective membrane permeation area (mm2)
1 2 3
25 25 50
672 672 440
energy 9.18 kJ/mol, pressure order 0.5, pre-exponential factor 2.07 103 (mol m)/(m2 min bar0.5) [18]. The porous stainless steel material employed as substrate was supplied by Mott Corporation. It has a thickness of 1.2 mm and media grade of 0.5 lm. The configuration of these three modules is listed in Table 2.
2.2. Experimental procedure After the membrane module was assembled, it was installed inside an electrically heated pressure vessel, whose temperature was
D. Xie et al. / Separation and Purification Technology 89 (2012) 189–192
191
controlled by an electrical heater. Hydrogen was charged to the vessel after the air in the vessel was displaced by argon. The vessel pressure was controlled by a back pressure regulator in the off gas stream. Pure hydrogen from the permeate side of the membrane was metered by a bubble gas meter. The membrane module was heated in the vessel under hydrogen environment for approximately 40 h until a constant hydrogen permeation rate was observed. The operation conditions (pressure and temperature) were then changed. The hydrogen permeability (Ma) through the membrane module was measured. The corresponding hydrogen permeation flux F can then be calculated from
F¼
Ma S
ð3Þ
and the permeation efficiency can be calculated from Eq. (2). After the test was performed, the vessel was charged with pure argon, maintained at a pressure of 0.2 MPa and a temperature around 873 K. No flow was observed in the permeate side of the membrane module. Hence the membrane integrity was confirmed.
Fig. 3. Influence of vessel temperature and hydrogen pressure on hydrogen permeation flux and efficiency for membrane module #1 (scattered point: experimental; solid line: correlated by Eq. (4)).
3. Experimental results and discussion 3.1. Experimental results Figs. 2–4 show the variation of membrane permeation fluxes and efficiencies with the operation temperature and pressure for the membrane modules #1, #2 and #3, respectively. With similar operation conditions, the permeation fluxes of these three modules were close. Hence these experiments had a good repeatability. It can be seen that the hydrogen permeation flux through the module increased with increasing the temperature and hydrogen pressure in the vessel. It was also observed that the permeation efficiency increased by increasing the hydrogen pressure and decreasing the vessel temperature. The measured permeation efficiency varied from approximately 0.85 to 1.15, which means if Sieverts’ Law is used to predict the hydrogen permeability of these three membrane modules, the error range should be within ±15%. Also permeation efficiencies higher than 1.0 were observed in the tests under some conditions. One possible reason for this phenomenon could be the error in membrane foil thickness estimation. Alfa-Aesar, the supplier of the membrane foils, claimed an error of ±15% on these membrane foil thicknesses. Hence for an extreme case, if the membrane foil was 15% thinner than the claimed thickness, the calculated hydrogen permeation rate from Sieverts’ Law would be
Fig. 4. Influence of vessel temperature and hydrogen pressure on hydrogen permeation flux and efficiency for membrane module #1 (scattered point: experimental; solid line: correlated by Eq. (4)).
15% higher than the actual one. Another possible contribution could be the contact between the metal substrate and the membrane foil. The vessel was under pressure of 0.2–0.7 MPa during the tests. Some substance of the membrane material should be squeezed into the pores of the stainless steel substrate, leading to a thinner effective foil thickness than the original foil thickness. 3.2. Correlation of the permeation efficiency From the experimental results it can be concluded that the membrane permeation efficiency is influenced by the operation temperature and pressure for the membrane module configurations and operation conditions under study. Here the permeation efficiency is correlated to the temperature T and the hydrogen partial pressure difference of (PH0.5 PL0.5) by: b
0:5 c g ¼ aTeT ðP0:5 H PL Þ
Fig. 2. Influence of vessel temperature and hydrogen pressure on hydrogen permeation flux and efficiency for membrane module #1 (scattered point: experimental; solid line: correlated by Eq. (4)).
ð4Þ
where a, b and c are parameters that need to be correlated by experimental data. The least squares method is used to estimate the parameters a, b, and c. The correlated parameters, standard deviation and correlation coefficient of the measured permeation fluxes and the
192
D. Xie et al. / Separation and Purification Technology 89 (2012) 189–192
Table 3 Parameters correlated, standard deviation and correlation coefficient (hydrogen pressure 0.2–0.7 MPa, temperature 675–925 K). a
b
c
Standard deviation
Correlation coefficient
0.0003774
1037.20
0.10387
0.008397
0.9984
the vessel side and decreasing the membrane temperature. The permeation efficiency was correlated to the vessel temperature T and hydrogen pressure difference (PH0.5 PL0.5) for the conditions studied. Acknowledgment Financial support from the Guangzhou Scientific Research and Development Program (Project # 2010J-D00021) is gratefully acknowledged. References
Fig. 5. Comparison of the fitted permeation fluxes with experimental ones.
correlated fluxes from Eqs. (1), (2), and (4) are listed in Table 3. The correlated efficiencies and permeation fluxes for these three membrane modules are also plotted in the corresponding Figs. 2–4 as solid lines. It can be seen that Eq. (4) offers a very good correlation for the experimental data. The correlated permeation fluxes from Eqs. (1), (2), and (4) and the measured fluxes for all these three membrane modules under various operating conditions are also compared in Fig. 5. It can be observed that the tested values are uniformly distributed on either side of the diagonal line, which indicates that the correlation fits the experimental data well. 4. Concluding remarks Three membrane modules, consisting of Pd–Ag membrane foil with a thickness of 25 or 50 lm, porous stainless steel substrate of 0.5 lm, test frame and flange were assembled and tested in an electrically heated vessel. The influences of operating temperature and pressure on their hydrogen permeability and permeation efficiency were studied. It can be concluded from the experimental observations that the hydrogen permeation rate increased by increasing the temperature and hydrogen pressure in the vessel, while the permeation efficiency increased by increasing the hydrogen pressure in
[1] R. Ramachandran, R.K. Menon, An overview of industrial uses of hydrogen, Int. J. Hydrogen Energy 23 (1998) 593–598. [2] E. Calo, A. Giannini, G. Monteleone, Small stationary reformers for H2 production from hydrocarbons, Int. J. Hydrogen Energy 35 (2010) 9828–9835. [3] D. Xie, C.J. Lim, J.R. Grace, A.M. Adris, Gas and particle circulation in an internally circulating fluidized bed membrane reactor cold model, Chem. Eng. Sci. 64 (2009) 2599–2606. [4] Y. Lin, Separation of hydrogen form the gas mixture out of catalytic reformer by using supported palladium membrane, Sep. Purif. Technol. 25 (2001) 87– 95. [5] A. Sieverts, G. Zapf, Solubility of H and D in solid PD(I), Z. Phys. Chem. A174 (1935) 359–364. [6] G.C. Holleck, Diffusion and solubility of hydrogen in palladium and palladium– silver alloys, J. Phys. Chem. 74 (1970) 503–511. [7] T.L. Ward, T. Dao, Model of hydrogen permeation behavior in palladium membrane, J. Membr. Sci. 153 (1999) 211–231. [8] G. Federico, E.E. Erik, Y.H. Ma, Effects of surface activity, defects and mass transfer on hydrogen permeance and n-value in composite palladium-porous stainless steel membrane, Catal. Today 118 (2006) 24–31. [9] D. Pizzi, R. Worth, M.G. Baschetti, G. Sarti, K.I. Noda, Hydrogen permeability of 2.5 lm palladium–silver membranes deposited on ceramic supports, J. Membr. Sci. 325 (2008) 446–453. [10] A. Unemoto, K. Atsushi, S. Kazuhisa, O. Takanori, K. Yashiro, J. Mizusaki, et al., The effect of co-existing gases from the process of steam reforming reaction on hydrogen permeability of palladium alloy membrane at high temperatures, Int. J. Hydrogen Energy 32 (2007) 4023–4029. [11] D. Xie, J. Yu, F. Wang, N. Zhang, W. Wang, H. Yu, et al., Hydrogen permeability of Pd–Ag membrane modules with porous stainless steel substrates, Int. J. Hydrogen Energy 36 (2010) 1014–1026. [12] A.M. Adris, C.J. Lim, J.R. Grace, The fluidized-bed membrane reactor for steam methane reforming: model verification and parametric study, Chem. Eng. Sci. 52 (1997) 1609–1622. [13] S. Roy, FBMR with high-flux membrane and O2 input, PhD thesis, University of Calgary, Alberta, 1998. [14] D. Xie, W. Qiao, Z. Wang, W. Wang, H. Yu, F. Peng, Reaction/separation coupled equilibrium modeling of steam methane reforming in fluidized bed membrane reactors, Int. J. Hydrogen Energy 35 (2010) 11798–11809. [15] A. Mahecha-Botero, T. Boyd, A. Gulamhusein, N. Comyn, C.J. Lim, J.R. Grace, et al., Pure hydrogen generation in a fluidized-bed membrane reactor: experimental findings, Chem. Eng. Sci. 63 (2008) 2752–2762. [16] A. Mahecha-Botero, Z. Chen, J.R. Grace, S.S.E.H. Elnashaie, C.J. Lim, Comparison of fluidized bed flow regimes for steam methane reforming in membrane reactors: a simulation study, Chem. Eng. Sci. 64 (2009) 3598–3613. [17] D. Xie, A.M. Adris, C.J. Lim, J.R. Grace, Test on a modular fluidized bed membrane reactor for autothermal steam methane reforming, Acta Energiae Solaris Sinica 30 (2009) 704–707. [18] A. Li, C.J. Lim, J.R. Grace, Staged-separation membrane reactor for steam methane reforming, Chem. Eng. J. 138 (2008) 452–459.
Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Short Communication
Permeation efficiency of Pd–Ag membrane modules with porous stainless steel substrates Donglai Xie ⇑, Fang Wang, Kui Wu, Ertao Zhang, Yajun Zhang MOE Key Laboratory of Enhanced Heat Transfer & Energy Conservation, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 4 April 2011 Received in revised form 12 January 2012 Accepted 12 January 2012 Available online 21 January 2012 Keywords: Hydrogen Palladium membrane Porous stainless steel
a b s t r a c t It is generally regarded that the permeation of hydrogen through palladium or palladium alloy membranes follows the ‘‘solution–diffusion’’ mechanism, which can be described by Sieverts’ Law. The hydrogen permeability of actual membrane modules usually differ from those predicted from Sieverts’ Law due to the influence of substrate and some other factors. An efficiency (or efficiency factor) has been introduced by many researchers to denote the difference between the actual permeability and those predicted from Sieverts’ Law. Extensive experiments were carried out in an electrically heated vessel to study the hydrogen permeation flux and permeation efficiency of Pd–Ag membrane modules with porous stainless steel substrates. The influence of operation conditions on the membrane permeation flux and efficiency was examined. It was observed that the hydrogen permeation flux through the module increased by increasing the temperature and hydrogen pressure in the vessel, while the permeation efficiency increased by increasing the hydrogen pressure in the vessel side and decreasing the membrane temperature. The permeation efficiency was correlated to the vessel temperature (T) and hydrogen pressure difference (PH0.5 PL0.5) for the conditions studied. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is a valuable chemical material, widely used in processes like petroleum refining, petrochemical production, metal processing, semiconductor manufacturing, float glass processing, etc [1]. In recent years, with the development of fuel cell technologies, the demand of high-purity hydrogen is also growing for new energy related applications, such as transportation, power generation and combined micro heat and power cogeneration [2]. Large-scale hydrogen production is a relatively mature technology. Most of the world’s hydrogen is generated at large petroleum and chemical plants by steam reforming or partial oxidation of natural gas in parallel fixed bed reactors within huge top-fired or side-fired furnaces [3]. Purification is a key step in the hydrogen production process. It accounts for a large fraction of energy expenditure and capital investment. Currently Pressure Swing Adsorption (PSA) is widely used for industrial hydrogen separation. The PSA process with complicated devices features high hydrogen product purity (99%+) and moderate hydrogen recovery (65–90%) depending on the tail gas pressure [4]. Palladium (Pd) membranes have also been used for many years for the production of extremely pure hydrogen (99.999%+) for specific purposes. It is regarded as a potential method to produce ultra-pure hydrogen as compared ⇑ Corresponding author. Tel./fax: +86 20 22236985. E-mail addresses: [email protected], [email protected] (D. Xie). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2012.01.030
with conventional processes. It can also be integrated with chemical reactors where chemical reaction and hydrogen separation occur simultaneously to simplify the hydrogen production process. The fabrication and performance of palladium membranes have been investigated intensively. It is generally regarded that the permeation of hydrogen through palladium or its alloy membranes follows the ‘‘solution–diffusion’’ mechanism, which can be described by the Sieverts’ Law [5,6]:
S Ep Ms ¼ K eRT ðPnH PnL Þ t
ð1Þ
where MS is the hydrogen permeation rate, K is the pre-exponential factor, S is the effective area of membrane surface for hydrogen permeation, t is the thickness of palladium or palladium alloy membrane, Ep is the activation energy for permeation, R is the gas constant, T is the temperature, PH is the hydrogen partial pressure in vessel side, PL is the average hydrogen partial pressures in the membrane permeate side, and n is the parameter whose value depends on the limiting transport mechanism of hydrogen permeation through palladium or its alloy membrane. The value of n is decided by the thickness of the membrane. Ward and Dao [7] showed that for temperatures above 673 K, n was equal to 0.5 for membranes thicker than 10 lm. Usually to use Sieverts’ Law correctly with an exponent of 0.5, the thickness of membrane should be larger than 10 lm [8].
190
D. Xie et al. / Separation and Purification Technology 89 (2012) 189–192
Nomenclature a, b, c Ep F K Ma MS N
PH
parameters in Eq. (4) activation energy for permeation, J mol1 hydrogen permeation flux, mol m2 s1 pre-exponential factor, mol m1 s1 MPan actual hydrogen permeation rate, mol s1 hydrogen permeation rate calculated from Sieverts’ Law, mol s1 parameter whose value depends on the limiting transport mechanism of hydrogen permeation through palladium or its alloy membrane, hydrogen partial pressure in vessel side, MPa
PL
hydrogen partial pressures in the membrane permeate side, MPa gas constant, J moll K1 effective area of membrane surface for hydrogen permeation, m2 temperature, K thickness of palladium or palladium alloy membrane, m
R S T t
Greek letter g permeation efficiency, –
Table 1 Permeation efficiencies reported in literature. Researchers
Operation temperature (°C)
Pressure (MPa)
Membrane module configuration
Efficiency
Adris et al. [12] Roy [13]
447–640 400–800
0.384 1.24
0.39 0.1 [14]
Mahecha-Botero et al. [15] Xie et al. [17]
545–550
0.65–0.90
Membrane tubes, 4.7 mm O.D., 0.20–0.28 mm thick Membrane tubes, 5.2 lm palladium coating on both outside and inside surfaces of the 3.175 mm O.D. substrate stainless steel tubes Membrane panels with Pd/Ag foil of thickness 25 lm on 316 stainless steel substrate
500–800
1.0–3.0
Membrane panel with Pd/Ag foil of thickness 50 lm on 316 stainless steel substrate
0.9
0.6–0.9 [16]
When Pd membrane is employed for hydrogen separation, it is usually supported on porous substrates, such as ceramic, porous glass and porous stainless steel [8,9] to withstand the pressure difference between the two surfaces of the palladium membrane. The substrate may affect the permeability of the membrane module. Other factors, such as the existence of gas species other than hydrogen, can also affect the membrane permeability [10]. An efficiency has been introduced by many researchers to denote the difference between the actual permeability (Ma) and those predicted from Sieverts’ Law (Ms) [11]:
Ma ¼ g Ms
ð2Þ
The values of g are reported in literature to be from 0.1 to approximately 0.9, and are listed in Table 1. The variation of these figures is in a wide range, and the factors affecting the permeation efficiencies are so far not clear, since all these reported data were obtained from experiments where membrane separation was coupled with chemical reactions for in situ hydrogen production. No work has been carried out to quantitatively study the influences of operation conditions and membrane module configuration on the membrane permeation efficiency. The purpose of the current study is to investigate the influence of operation conditions on the permeation efficiency for the widely used Pd–Ag membrane modules with porous stainless steel substrates, without coupling of any chemical reactions.
2. Experimental setup and procedure 2.1. The test membrane modules Three membrane modules as shown in Fig. 1 have been employed in the test. Each module consists of the following parts: frame, substrate, membrane foil, graphite gasket and flange. Pd–Ag membrane foils of 75% (wt) palladium and 25% (wt) silver with thicknesses of 25 and 50 lm were tested in the experiments. The membrane foils were supplied by Alfa-Aesar. The following performance data of such membrane foils was used: activation
Fig. 1. Structure of the Pd–Ag membrane module assembly.
Table 2 Configuration of the membrane modules employed in the experiments. Membrane module #
Membrane foil thickness (lm)
Effective membrane permeation area (mm2)
1 2 3
25 25 50
672 672 440
energy 9.18 kJ/mol, pressure order 0.5, pre-exponential factor 2.07 103 (mol m)/(m2 min bar0.5) [18]. The porous stainless steel material employed as substrate was supplied by Mott Corporation. It has a thickness of 1.2 mm and media grade of 0.5 lm. The configuration of these three modules is listed in Table 2.
2.2. Experimental procedure After the membrane module was assembled, it was installed inside an electrically heated pressure vessel, whose temperature was
D. Xie et al. / Separation and Purification Technology 89 (2012) 189–192
191
controlled by an electrical heater. Hydrogen was charged to the vessel after the air in the vessel was displaced by argon. The vessel pressure was controlled by a back pressure regulator in the off gas stream. Pure hydrogen from the permeate side of the membrane was metered by a bubble gas meter. The membrane module was heated in the vessel under hydrogen environment for approximately 40 h until a constant hydrogen permeation rate was observed. The operation conditions (pressure and temperature) were then changed. The hydrogen permeability (Ma) through the membrane module was measured. The corresponding hydrogen permeation flux F can then be calculated from
F¼
Ma S
ð3Þ
and the permeation efficiency can be calculated from Eq. (2). After the test was performed, the vessel was charged with pure argon, maintained at a pressure of 0.2 MPa and a temperature around 873 K. No flow was observed in the permeate side of the membrane module. Hence the membrane integrity was confirmed.
Fig. 3. Influence of vessel temperature and hydrogen pressure on hydrogen permeation flux and efficiency for membrane module #1 (scattered point: experimental; solid line: correlated by Eq. (4)).
3. Experimental results and discussion 3.1. Experimental results Figs. 2–4 show the variation of membrane permeation fluxes and efficiencies with the operation temperature and pressure for the membrane modules #1, #2 and #3, respectively. With similar operation conditions, the permeation fluxes of these three modules were close. Hence these experiments had a good repeatability. It can be seen that the hydrogen permeation flux through the module increased with increasing the temperature and hydrogen pressure in the vessel. It was also observed that the permeation efficiency increased by increasing the hydrogen pressure and decreasing the vessel temperature. The measured permeation efficiency varied from approximately 0.85 to 1.15, which means if Sieverts’ Law is used to predict the hydrogen permeability of these three membrane modules, the error range should be within ±15%. Also permeation efficiencies higher than 1.0 were observed in the tests under some conditions. One possible reason for this phenomenon could be the error in membrane foil thickness estimation. Alfa-Aesar, the supplier of the membrane foils, claimed an error of ±15% on these membrane foil thicknesses. Hence for an extreme case, if the membrane foil was 15% thinner than the claimed thickness, the calculated hydrogen permeation rate from Sieverts’ Law would be
Fig. 4. Influence of vessel temperature and hydrogen pressure on hydrogen permeation flux and efficiency for membrane module #1 (scattered point: experimental; solid line: correlated by Eq. (4)).
15% higher than the actual one. Another possible contribution could be the contact between the metal substrate and the membrane foil. The vessel was under pressure of 0.2–0.7 MPa during the tests. Some substance of the membrane material should be squeezed into the pores of the stainless steel substrate, leading to a thinner effective foil thickness than the original foil thickness. 3.2. Correlation of the permeation efficiency From the experimental results it can be concluded that the membrane permeation efficiency is influenced by the operation temperature and pressure for the membrane module configurations and operation conditions under study. Here the permeation efficiency is correlated to the temperature T and the hydrogen partial pressure difference of (PH0.5 PL0.5) by: b
0:5 c g ¼ aTeT ðP0:5 H PL Þ
Fig. 2. Influence of vessel temperature and hydrogen pressure on hydrogen permeation flux and efficiency for membrane module #1 (scattered point: experimental; solid line: correlated by Eq. (4)).
ð4Þ
where a, b and c are parameters that need to be correlated by experimental data. The least squares method is used to estimate the parameters a, b, and c. The correlated parameters, standard deviation and correlation coefficient of the measured permeation fluxes and the
192
D. Xie et al. / Separation and Purification Technology 89 (2012) 189–192
Table 3 Parameters correlated, standard deviation and correlation coefficient (hydrogen pressure 0.2–0.7 MPa, temperature 675–925 K). a
b
c
Standard deviation
Correlation coefficient
0.0003774
1037.20
0.10387
0.008397
0.9984
the vessel side and decreasing the membrane temperature. The permeation efficiency was correlated to the vessel temperature T and hydrogen pressure difference (PH0.5 PL0.5) for the conditions studied. Acknowledgment Financial support from the Guangzhou Scientific Research and Development Program (Project # 2010J-D00021) is gratefully acknowledged. References
Fig. 5. Comparison of the fitted permeation fluxes with experimental ones.
correlated fluxes from Eqs. (1), (2), and (4) are listed in Table 3. The correlated efficiencies and permeation fluxes for these three membrane modules are also plotted in the corresponding Figs. 2–4 as solid lines. It can be seen that Eq. (4) offers a very good correlation for the experimental data. The correlated permeation fluxes from Eqs. (1), (2), and (4) and the measured fluxes for all these three membrane modules under various operating conditions are also compared in Fig. 5. It can be observed that the tested values are uniformly distributed on either side of the diagonal line, which indicates that the correlation fits the experimental data well. 4. Concluding remarks Three membrane modules, consisting of Pd–Ag membrane foil with a thickness of 25 or 50 lm, porous stainless steel substrate of 0.5 lm, test frame and flange were assembled and tested in an electrically heated vessel. The influences of operating temperature and pressure on their hydrogen permeability and permeation efficiency were studied. It can be concluded from the experimental observations that the hydrogen permeation rate increased by increasing the temperature and hydrogen pressure in the vessel, while the permeation efficiency increased by increasing the hydrogen pressure in
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