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Separation of hydrogen from thermochemical processes using zirconia-silica composite membrane
journal of MEMBRANE SCIENCE
ELSEVIER
Journal of Membrane Science 97 ( 1994) 9 l-98
Separation of hydrogen from thermochemical processes using zirconia-silica composite membrane Haruhiko Ohya*, Hidehiko Nakajima, Naoki Togami, Masahiko Aihara, Youichi Negishi Department ofMaterial Science and Chemical Engineering, Yokohama National University, Hodogaya-ku Yokohama 240, Japan Received 24 September 1993; accepted in revised form 15June 1994
Abstract For the purpose of purifying HZ from gaseous mixtures from thermochemical water decomposition processes, the characteristics of zirconia-silica composite membranes coated on porous ceramic tubing were investigated up to 773 K. The maximum content of the Zr(OC3H7)4 in the metal-alkoxide solution used successfully in the coating process was 60 molO/o. The separation experiments for the gaseous mixtures of H,-H,O-HBr (for the UT-3 process) and of HI-HZ0 (for the IS process) were carried out at 423-773 K, and it was found that hydrogen can be purified as reject stream and that HBr and HI can be recovered from the permeation side of the membranes. Keywords: Hydrogen; Thermochemical
water decomposition;
1. Introduction
Hydrogen is the ideal fuel for the future as it is a clean, renewable and flexible carrier of energy. One of the most promising methods to produce hydrogen industrially is the thermochemical water decomposition process which uses high heat energy from the sun, geothermal, nuclear sources and so on. The UT-3 process consisting of bromine-calcium-iron compounds [ 1 ] and the IS process consisting of iodine-sulfur compounds [ 21 are the most likely candidates for the thermochemical water decomposition process, but the conversion efficiency of water into hydrogen is hoped to be greater [ 11. Therefore, a huge volume of water should be recycled from the gaseous mixture, for example, Hz, Hz0 and * Corresponding author.
Metal-alkoxide;
Zirconia-silica;
Composite membrane
HBr for the UT-3 cycle or Hz, Hz0 and HI for the IS cycle, to separate the produced hydrogen by condensing and evaporating. If hydrogen can be separated from the product gases without phase change, the thermal efficiency of the process would increase and the cost of hydrogen would decrease. In Japan, a few papers have been reported on hydrogen separation by inorganic membranes, such as the membranes developed in the C1 Chemistry Project sponsored by the Ministry of International Trade and Industry of Japan for the separation of hydrogen from the shift reaction gaseous mixtures [ 3,4], microporous glass membranes for the production of hydrogen from hydrogen sulfide [ $61, and inorganic membrane reactors for the hydrogenation of cyclohexane [ 7 1. The so-called Knudsen gas diffusion occurred in microporous membranes used to separate mixtures of different molecular
0376-7388/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZO376-7388(94)00150-2
H. Ohya et al. /Journal of Membrane Science 97 (I 994) 91-98
92
weight gases. In the studies mentioned above, the lightest molecular weight gas, Hz, permeated selectively through the inorganic membranes in accordance with Knudsen diffusion. Some silica-alumina gel membranes can separate condensable gases such as water vapor effectively from a mixture of condensable and non-condensable gases, by the mechanism of capillary condensation [ 8-111. Lee and Hwang studied the condensable vapor transport through microporous media experimentally as well as theoretically [ 121. In this study, the zirconia-silica composite microporous membranes, which can withstand a highly corrosive gas containing HBr or HI from thermochemical water decomposition processes, were prepared in order to investigate the possibility of separating hydrogen from the gas mixture produced in the thermochemical processes.
in N2 atmosphere
Mixing
I
Coating Solution
I
Dip Coating ( 1 min immersion, pulling up rate 4 mndmin in N2 atmosphere )
I Drying ( 15O”C,15 min ) Heating ( 550°C. 30 min Memb. A ) ( 8OO’C, 30 min - Memb. B ) (lOC@C,;O min - Memb. C ) ZrO2 - SiO2
Fig. 1. Preparation
Y2O3 Composite Membrane
procedure of the composite membranes.
2. Experimental 2.1. Membrane
Via a metal-alkoxide method [ 131, a thin microporous Zr02-Si02-Y203 layer was coated on the surface of a porous ceramic tubing, the dimensions ofwhich were 5.5 mm o.d. and 3.5 mm i.d., and the average pore diameter was 0.5 pm (supplied by Toto Co. Ltd.). The composition of the coating solutions are listed in Table 1 and the preparing procedure of the composite membrane is outlined in Fig. 1. The average micropore diameters of the prepared membranes A, B and C are 1.50,4.50 and 5.25 nm, respectively. Table I Composition
HB, SOLunON MF.AS”RlNG PlJMP H* GAS CYLlNDER EVAFORATOR
6 7 8 9
HEATER THERM0 COUPLE MEMBRANE COLDTRAP
II 12 13 14
CORRCISIONRESISTANCE PRESSURE GAUGE
IO
GAS WASHlNG BOlTLE
IS
FLEXIBLE HEATER FLOWMETER GLASSCELL PFzEsS”RE CONI-ROL VALVE MEASuRING CYLmDEFc
Fig. 2. Schematic diagram of apparatus.
2.2. Apparatus
A schematic flow diagram of the apparatus used for the gas separation experiments is shown in Fig. 2. All tubings were made of Teflon@’and heated above 423 K by the flexible heaters to avoid condensation of water in the tubings. The cell containing the composite membrane was made of quartz glass.
of coating solutions (mole ratio) A
Zr(OWGh Si(WH5), Y(CH3C00)9.4HZ0 i-&H,OH
0.4
0.6 0.024 10.02
B
C
0.5
0.6 0.4 0.036 11.00
0.5 0.030 9.01
2.3. Procedures The whole system was evacuated over 10 h, and then hydrogen gas was introduced from the Hz cylinder 3 into the evaporator 4 where aqueous HBr was fed by the measuring pump 2. The con-
H. Ohya et al. /Journal of Membrane Science 97 (1994) 91-98
centrations of HBr and H,O in the gaseous mixture were controlled by adjusting the feed rate and the HBr concentration of the aqueous solution. The permeated gases were introduced to the cold trap cooled by liquid Nz to remove HZ0 and HBr, and then washed in the gas washing bottles 10 containing aqueous NaOH to remove residual HBr. The pressure in the quartz glass cell was controlled by the pressure control valve 14 through which the rejected gaseous mixture was introduced to the gas washing bottles 10 containing aqueous NaOH. 2.4. Analysis and measurement The permeation rate of water was determined by the weight of liquid trapped in the cold trap 9 and the HBr concentration by the titration method. The permeation rate of HBr was determined as the sum of the amount of HBr trapped in the cold trap 9 and the amount of HBr reacted with NaOH in the gas washing bottles. The permeation rate of H2 was measured by the volumetric method.
93
8
“0.00
0.05
0.10
0.15
0.20
AP [ MPa ]
0.25
0.30
Fig. 3. Relationships between the transmembrane pressure and the pure gas permeation fluxes of Hz and H,O through membrane B at 423,473 and 773 K.
3. Results and discussion l/T
3.1. Flux Fig. 3 shows typical relationships between the transmembrane pressure (TMP), AP, and the pure gas permeation fluxes of H2 and Hz0 through membrane B at 423, 473 and 773 K. With an increase in temperature from 423 to 773 K, the HZ0 flux decreased drastically by - 80%, but the H2 flux increased slightly by - 13%. 3.2. Permeability Fig. 4 shows the relationships between the reciprocal of the absolute temperature and the permeabilities of Hz and Hz0 for the three membranes. (The calculated lines shown in Fig. 4 are discussed later. ) As shown in Fig. 4, membranes B and C have similar characteristics, that is, with an increase in temperature from 423 to 773 K, the Hz permeability increased slightly by - 13%,
x103 [
K-’ 1
Fig. 4. Relationships between the reciprocal of the absolute temperature and the permeabilities of H2 and H20 through membrane A, B and C. Solid lines are fitted by Eq. (3) using the parameters listed in Table 2.
but the H20 permeability decreased drastically by - 70-80%. At the same temperature, both the permeabilities of H2 and HZ0 decreased with the decrease of the micropore diameter, d,. Fig. 5 shows the linear relation at each temperature between d, and the permeability of HZ0 (The calculated lines shown in Fig. 5 are discussed later.) If the permeation mechanism is mainly based on Knudsen diffusion, the permeability should decrease with an increase in temperature, however, the observed permeability of Hz in Fig. 4 was contrary to the expected one. This fact suggests that Knudsen diffusion is not the predominant mechanism of the permeation. For the pure silica microporous membrane, the permeation
H. Ohya et al. /Journal of Membrane Science 97 (1994) 91-98
94
e 0 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00
dp I nm 1 Fig. 5. The linear relation between pore diameter and permeability of HzO. Solid lines are calculated by Eq. (4).
mechanism is based on Knudsen diffusion, as reported before [ 111. But for the membrane made by the addition of 40-60% zirconia to silica, the permeation behavior cannot be explained by only the Knudsen diffusion mechanism. 3.3. Permeation mechanism The total flux, JT, through the membrane is defined as the sum of the flux due to surface diffusion, Js [ 3 1, Knudsen diffusion, JK [ 3 3, and activated diffusion, JA [ 141: JT=Js+JK+JA =a*AP(MT)-‘/‘*[
exp( -E*/RT)
P=J,/A,
3
(1)
exp(b/T)-l]+K q(mol/s) =r.
*AP-(MT)-“2/(1+c/T)+P,*AP -
using Eq. (3) and the constants in Table 2, and those in Fig. 5 were calculated using Eq. (4). The agreement between the measured and calculated permeabilities is very good for membranes B and C, but not so good for membrane A, particularly at the temperature range of 523 to 673 K. The linear relation between the permeability of H20 and the average micropore diameter shown in Fig. 5 suggests that the number of micropores on the surface area is almost the same because in the surface diffusion mechanism the permeability is proportional to the sum of the periphery of micropores, C( x-d,). Assuming that the major permeation mechanism in membrane A with the smallest micropore diameter of 1.5 nm is the activated diffusion, the activation energy of the activated diffusion, obtained from the data at higher temperatures (Fig. 4), is 6.68 kJ/mol. In Fig. 6, AP, the differences between the measured permeabilities for membranes A, B and C and the permeability of the activated diffusion for membrane A, which is shown by dotted line in Fig. 4, are plotted against d,,3. The permeation rate of molecules of molecular weight M through a micropore of dp and length L at T by the Knudsen mechanism can be expressed by:
(2) (3)
The measured fluxes were analyzed according to Eq. (2) and the fitting parameters are listed in Table 2. Considering the trends shown in Fig. 4, the permeation mechanism of H20 might be governed by surface diffusion, and Fig. 5 shows that the permeability of H20 is proportional to d,. Therefore, the permeability of H20, PHzO,can be expressed by the following experimental equation derived from the first term of Eq. (2 ) :
The solid lines shown in Fig. 4 were calculated
2n -SAP MRT J
(5)
the linear relation between AP and dp3 proves that the mechanism of H2 permeability is mainly governed by the activated and Knudsen diffusion. As discussed for the water vapor permeation mechanism, the total number of micropores in membranes A, B and C is almost the same. Therefore the permeabilities of H2 through membranes A, B and C by the Knudsen mechanism will be proportional to dp3. The observed thickness of the Zr02-SO2 layer, 15 coatings, on the ceramic tubing by scanning electron micrographs shown in Fig. 7 is 10.8, 10.5 and 10.0 pm for membranes A, B and C, respectively. No separation of a CH,-CO, gas mixture was observed for membranes with less than 10 coatings.
H. Ohya et al. /Journal of Membrane Science 97 (1994) 91-98
95
Table 2 Fitting parameters in Eq. (2) for membrane A, B and C A
B
H2
ax lo* (mol ‘I2 kg’/2 K’/2/mZ Pa s) bx 1OW (K) KX 10’ (mol ‘/’ kg’j2 K’/*/m* Pas) c (K) POX 10’ (mol/m2 Pa s) -% (kJ/mol)
H2O
_ 0.307 119 1.13 6.68
1.95 1.00 -
C
H2
H20
6.13 45.7 1.13 6.68
5.16 1.00 -
3.4. Separation of H,-H,O-HBr mixture
0-o
dp” [ nm’ ]
Fig. 6. Relationships between d,’ and the differences between the measured permeabilities for membrane A, B and C and the permeability of the activated diffusion for membrane A, AP.
Therefore, the thickness corresponding to the layer of 10 coatings is assumed to be a buffer layer, not an active layer, and the effective thickness corresponding to the active layer is estimated to be 3.60, 3.50 and 3.33 pm for membranes A, B and C, respectively. Using the estimated thickness, L, the pore diameter, dp, and the permeability based on Knudsen diffusion (Fig. 6)) AP, the numbers of pore can be calculated with Eq. ( 5 ). The numbers of micropore are 1.85x 1013, 1.61 x lOI and 1.29x 1013mW2 for membrane A, B and C, respectively, and decrease with the increase of the micropore diameter.
H2
Hz0
7.96 30.1 1.13 6.68
6.06 1.oo -
gaseous
The permeation characteristics of the Hz, H20 and HBr gaseous mixture (feed composition 0.52-0.54, 0.48-0.46 and (8-9)~10-~ mol%, respectively) for membrane B were measured. The analysis of the permeated gases showed that H2 was not detected and that the constituents of the permeated gases were Hz0 and HBr. In Fig. 8 the measured permeation flux of water, J,.,, is plotted against the partial TMP of H20, dP,, at each temperature. The solid lines shown in the Fig. 8 are redrawn from the pure H20 data shown in Fig. 3. The agreement between the solid lines and the measured data at higher partial TMP of water is quite good, but not good at lower partial TMP. In Fig. 9 the measured permeation flux of HBr, JHBr is plotted against the partial TMP of HBr, APuB,, at each temperature. It is interesting that JHBr does not depend on APHBr, but keeps almost a constant value of ( 1.21.5 ) x 1O- 5mol/m2 s. The reason why JHBr is almost constant over the wide range of the experimental conditions is not clear. Fig. 10 shows the relationship between the temperature and the separation factor, &rBr/H2o, defined as: ai/H20
A/ JH20 =~ Yi/YH20
(6)
The separation factors, aHBr/H20,6 to 36, for
96
H. Ohya et al. / JournalofMembraneScience
97 (1994) 91-98
0
10
20
40
30
A
P, [
60
50
70
80
kPa 1
Fig. 8. Relationships between partial transmembrane pressure of H,O, LIP,, and the measured permeation fluxes of HzO, Jw, for membrane B at transmembrane pressures of 0.15 and 0.20 MPa.
I
I
Membrane
0.1
0.2 APHBr
I
I
I
B
0.3 1 kPa
0.4
0.5
0.6
1
Fig. 9. Relationships between partial transmembrane pressure of HBr, dPnBr, and permeation fluxes of HBr, JHBr,for membrane B at transmembrane pressures of 0.15 and 0.20 MPa.
Fig. 7. Cross-sectional view of (a) membrane A; (b) membrane B; (c ) membrane C.
the range of the experimental conditions indicate that HBr permeated faster through the membrane than Hz0 and was concentrated in the permeated gases. All the measured separation factors increased with an increase in temperature. The permeation flux of water decreased with an increase in temperature as shown in Fig. 8; however, the permeation flux of HBr remained constant over the wide range of experimental conditions as shown in Fig. 9. The separation factor of the membrane with the smaller micropore is always larger than the membrane with the larger micropore. With the same membrane, the separation factor decreased with the increase of the total TMP, Al’-,-.
H. Ohya et al. /Journal of Membrane Science 97 (I 994) 91-98
97
flux of HI remained constant for the wide range of the experimental conditions. 50 45 40 y-
35
0
30
g
15
g
20
tf
4. Conclusions
15 10 5 0
400
500
600
700
800
TlKl
Fig. 10. Relationships between temperature and separation factor for HBr over Hz0 at transmembrane pressures of 0.15 and 0.20 MPa.
40 35
I -
Membrane
400
I
I
600
700
I’
A
500
800
TIKI
Fig. 11. Relationships between temperature and separation factor for HI over H,O at transmembrane pressures of 0.15 and 0.20 MPa.
3.5. Separation of H,O-HI
( 1) The permeation flux of water increased with a decrease in temperature, and was proportional to the mean diameter of the micropore of the membrane. These results suggest that the permeation mechanism is mainly due to surface diffusion in the pore. (2) The permeation flux of H2 increased with an increase in temperature. The permeation mechanism of H2 is mainly due to the mechanisms of activated diffusion and Knudsen diffusion. (3 ) The permeation flux of HBr was almost constant over the wide range of the experiments conditions. (4) In the separation experiments of Hz-H20HBr gaseous mixture, it was found that only H20 and HBr permeated through the membrane and that HBr was concentrated in the permeated gases. The separation factors were in the range of 6 to 36. ( 5 ) In the separation experiments of the H20HI gaseous mixture, it was found that HI was concentrated in the permeated gases. The separation factors were in the range of 5 to 26. (6) The zirconia composite membranes separated Hz from the thermochemical decomposed gaseous mixture successfully.
gaseous mixture
Fig. 11 shows the relationship between the temperature and the separation factor &n/H20 for membrane A (feed compositions, H20:HI=0.9987-0.9989:(1.3-1.1)~10-3; TMP, 0.15 and 0.20 MPa), The separation factors, a H1/HZo,5 to 26, for the range of the experimental conditions indicate that HI permeates faster through the membrane than HZ0 and was concentrated in the permeated gases. All the measured separation factors increased with an increase in temperature. The permeation flux of water vapor decreased drastically with an increase in temperature; however, the permeation
5. List of symbols A P T P Y L” R ‘yif Hz0 J
difference (-) pressure (Pa) absolute temperature (K) permeability (mol/m’ Pa s) molecular weight (kg/mol) micropore diameter (m) length (m) gas constant (J/mol K) separation factor for i over (i=HBr or HI) (-) permeation flux ( mol/m2 s)
HZ0
H. Ohya et al. /Journal
98
EA TMP Y
ofMembrane
activation energy (kJ/mol) transmembrane pressure (MPa or kPa) mole fraction in the feed (-)
5.1. Subscripts T s K A W
total surface diffusion Knudsen diffusion activated diffusion J-W
Acknowledgments This work was supported by Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, in Japan (02203 105, 03203106,04203106).
References [ I ] H. Kameyama and K. Yoshida, Br-Ca-Fe
water decomposition cycles for hydrogen production, Proc. 2nd World Hydrogen Energy Conf., Zurich, Oct. 1978, Pergamon, Oxford, 1978, p. 829. [2]K. Onuki, S. Shimizu, H. Nakajima, S. Fujita, Y. Ikezoe, S. Sato and S. Machi, Studies on an iodine-sulfur process for thermochemical hydrogen production, Proc. 8th World Hydrogen Energy Conf., Honolulu, July 1990, Pergamon, Oxford, 1990, p. 547. [ 3]Y. Shindo, T. Hakuta, H. Yoshitome and H. Inoue, Gas diffusion in microporous media in Knudsen’s regime, J. Chem. Eng. Jpn., 16 (1983) 120.
Science 97 (I 994) 91-98
[ 4]K. Itaya, S. Sugawara, K. Arai and S. Saito, Properties of porous anodic aluminum oxide films as membranes, J. Chem. Eng. Jpn., 17 (1984) 514. [ 5 ] T. Kameyama, K. Fukuda, M. Fujishige, H. Yokokawa and M. Dokiya, Production of hydrogen from hydrogen sulfide by means of selective diffusion membranes, Hydrogen Energy Prog., 2 ( 198 1) 569. [6]T. Kameyama, M. Dokiya, M. Fujishige, H. Yokokawa and K. Fukuda, Production of hydrogen from hydrogen sulfide by means of selective diffusion membranes, Int. J. Hydrogen Energy, 8 (1983) 5. [ 7]N. Itoh, Y. Shindo, K. Haraya and T. Hakuma, A membrane reactor for promoting a reversible reaction, Proc. Int. Congr. Membranes and Membrane Processes, Tokyo, June 8-12, 1987, p. 654. [ 8]M. Asaeda and L.D. Du, Separation of alcohol/water gaseous mixtures by thin ceramic membrane, J. Chem. Eng. Jpn., 19 (1986) 72. [9]M. Asaeda, L.D. Du and M. Fuji, Separation of alcohol/water gaseous mixtures by an improved ceramic membrane, J. Chem. Eng. Jpn., 19 ( 1986) 84. [ lO]M. Asaeda, L.D. Du and K. Ikeda, Experimental studies of dehumidification of air by an improved ceramic membrane, J. Chem. Eng. Jpn., 19 (1986) 238. [ 11 ]H. Ohya, S. Sato, A. Ishii, Y. Negishi and K. Matsumoto The separation of gaseous mixture with composite microporous glass membranes at high temperature, Maku, 15 (1990) 72. [ 121 K.H. Lee and S.T. Hwang, The transport of condensable vapors through a microporous vycor glass membrane, J. Colloid Interface Sci., 110 ( 1986) 544. [ 131 M. Niwa, H. Ohya, Y. Tanaka, N. Yoshikawa, K. Matsumoto and Y. Negishi, Separation of gaseous mixtures of CO2 and CH4 using a composite microporous glass membrane on ceramic tubing, J. Membrane Sci., 39 (1988) 301. [ 141S. Kitao, H. Kameda and M. Asaeda, Gas separation by thin porous silica membrane of ultra fine pores at high temperature, Maku, 15 ( 1990) 222.
ELSEVIER
Journal of Membrane Science 97 ( 1994) 9 l-98
Separation of hydrogen from thermochemical processes using zirconia-silica composite membrane Haruhiko Ohya*, Hidehiko Nakajima, Naoki Togami, Masahiko Aihara, Youichi Negishi Department ofMaterial Science and Chemical Engineering, Yokohama National University, Hodogaya-ku Yokohama 240, Japan Received 24 September 1993; accepted in revised form 15June 1994
Abstract For the purpose of purifying HZ from gaseous mixtures from thermochemical water decomposition processes, the characteristics of zirconia-silica composite membranes coated on porous ceramic tubing were investigated up to 773 K. The maximum content of the Zr(OC3H7)4 in the metal-alkoxide solution used successfully in the coating process was 60 molO/o. The separation experiments for the gaseous mixtures of H,-H,O-HBr (for the UT-3 process) and of HI-HZ0 (for the IS process) were carried out at 423-773 K, and it was found that hydrogen can be purified as reject stream and that HBr and HI can be recovered from the permeation side of the membranes. Keywords: Hydrogen; Thermochemical
water decomposition;
1. Introduction
Hydrogen is the ideal fuel for the future as it is a clean, renewable and flexible carrier of energy. One of the most promising methods to produce hydrogen industrially is the thermochemical water decomposition process which uses high heat energy from the sun, geothermal, nuclear sources and so on. The UT-3 process consisting of bromine-calcium-iron compounds [ 1 ] and the IS process consisting of iodine-sulfur compounds [ 21 are the most likely candidates for the thermochemical water decomposition process, but the conversion efficiency of water into hydrogen is hoped to be greater [ 11. Therefore, a huge volume of water should be recycled from the gaseous mixture, for example, Hz, Hz0 and * Corresponding author.
Metal-alkoxide;
Zirconia-silica;
Composite membrane
HBr for the UT-3 cycle or Hz, Hz0 and HI for the IS cycle, to separate the produced hydrogen by condensing and evaporating. If hydrogen can be separated from the product gases without phase change, the thermal efficiency of the process would increase and the cost of hydrogen would decrease. In Japan, a few papers have been reported on hydrogen separation by inorganic membranes, such as the membranes developed in the C1 Chemistry Project sponsored by the Ministry of International Trade and Industry of Japan for the separation of hydrogen from the shift reaction gaseous mixtures [ 3,4], microporous glass membranes for the production of hydrogen from hydrogen sulfide [ $61, and inorganic membrane reactors for the hydrogenation of cyclohexane [ 7 1. The so-called Knudsen gas diffusion occurred in microporous membranes used to separate mixtures of different molecular
0376-7388/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZO376-7388(94)00150-2
H. Ohya et al. /Journal of Membrane Science 97 (I 994) 91-98
92
weight gases. In the studies mentioned above, the lightest molecular weight gas, Hz, permeated selectively through the inorganic membranes in accordance with Knudsen diffusion. Some silica-alumina gel membranes can separate condensable gases such as water vapor effectively from a mixture of condensable and non-condensable gases, by the mechanism of capillary condensation [ 8-111. Lee and Hwang studied the condensable vapor transport through microporous media experimentally as well as theoretically [ 121. In this study, the zirconia-silica composite microporous membranes, which can withstand a highly corrosive gas containing HBr or HI from thermochemical water decomposition processes, were prepared in order to investigate the possibility of separating hydrogen from the gas mixture produced in the thermochemical processes.
in N2 atmosphere
Mixing
I
Coating Solution
I
Dip Coating ( 1 min immersion, pulling up rate 4 mndmin in N2 atmosphere )
I Drying ( 15O”C,15 min ) Heating ( 550°C. 30 min Memb. A ) ( 8OO’C, 30 min - Memb. B ) (lOC@C,;O min - Memb. C ) ZrO2 - SiO2
Fig. 1. Preparation
Y2O3 Composite Membrane
procedure of the composite membranes.
2. Experimental 2.1. Membrane
Via a metal-alkoxide method [ 131, a thin microporous Zr02-Si02-Y203 layer was coated on the surface of a porous ceramic tubing, the dimensions ofwhich were 5.5 mm o.d. and 3.5 mm i.d., and the average pore diameter was 0.5 pm (supplied by Toto Co. Ltd.). The composition of the coating solutions are listed in Table 1 and the preparing procedure of the composite membrane is outlined in Fig. 1. The average micropore diameters of the prepared membranes A, B and C are 1.50,4.50 and 5.25 nm, respectively. Table I Composition
HB, SOLunON MF.AS”RlNG PlJMP H* GAS CYLlNDER EVAFORATOR
6 7 8 9
HEATER THERM0 COUPLE MEMBRANE COLDTRAP
II 12 13 14
CORRCISIONRESISTANCE PRESSURE GAUGE
IO
GAS WASHlNG BOlTLE
IS
FLEXIBLE HEATER FLOWMETER GLASSCELL PFzEsS”RE CONI-ROL VALVE MEASuRING CYLmDEFc
Fig. 2. Schematic diagram of apparatus.
2.2. Apparatus
A schematic flow diagram of the apparatus used for the gas separation experiments is shown in Fig. 2. All tubings were made of Teflon@’and heated above 423 K by the flexible heaters to avoid condensation of water in the tubings. The cell containing the composite membrane was made of quartz glass.
of coating solutions (mole ratio) A
Zr(OWGh Si(WH5), Y(CH3C00)9.4HZ0 i-&H,OH
0.4
0.6 0.024 10.02
B
C
0.5
0.6 0.4 0.036 11.00
0.5 0.030 9.01
2.3. Procedures The whole system was evacuated over 10 h, and then hydrogen gas was introduced from the Hz cylinder 3 into the evaporator 4 where aqueous HBr was fed by the measuring pump 2. The con-
H. Ohya et al. /Journal of Membrane Science 97 (1994) 91-98
centrations of HBr and H,O in the gaseous mixture were controlled by adjusting the feed rate and the HBr concentration of the aqueous solution. The permeated gases were introduced to the cold trap cooled by liquid Nz to remove HZ0 and HBr, and then washed in the gas washing bottles 10 containing aqueous NaOH to remove residual HBr. The pressure in the quartz glass cell was controlled by the pressure control valve 14 through which the rejected gaseous mixture was introduced to the gas washing bottles 10 containing aqueous NaOH. 2.4. Analysis and measurement The permeation rate of water was determined by the weight of liquid trapped in the cold trap 9 and the HBr concentration by the titration method. The permeation rate of HBr was determined as the sum of the amount of HBr trapped in the cold trap 9 and the amount of HBr reacted with NaOH in the gas washing bottles. The permeation rate of H2 was measured by the volumetric method.
93
8
“0.00
0.05
0.10
0.15
0.20
AP [ MPa ]
0.25
0.30
Fig. 3. Relationships between the transmembrane pressure and the pure gas permeation fluxes of Hz and H,O through membrane B at 423,473 and 773 K.
3. Results and discussion l/T
3.1. Flux Fig. 3 shows typical relationships between the transmembrane pressure (TMP), AP, and the pure gas permeation fluxes of H2 and Hz0 through membrane B at 423, 473 and 773 K. With an increase in temperature from 423 to 773 K, the HZ0 flux decreased drastically by - 80%, but the H2 flux increased slightly by - 13%. 3.2. Permeability Fig. 4 shows the relationships between the reciprocal of the absolute temperature and the permeabilities of Hz and Hz0 for the three membranes. (The calculated lines shown in Fig. 4 are discussed later. ) As shown in Fig. 4, membranes B and C have similar characteristics, that is, with an increase in temperature from 423 to 773 K, the Hz permeability increased slightly by - 13%,
x103 [
K-’ 1
Fig. 4. Relationships between the reciprocal of the absolute temperature and the permeabilities of H2 and H20 through membrane A, B and C. Solid lines are fitted by Eq. (3) using the parameters listed in Table 2.
but the H20 permeability decreased drastically by - 70-80%. At the same temperature, both the permeabilities of H2 and HZ0 decreased with the decrease of the micropore diameter, d,. Fig. 5 shows the linear relation at each temperature between d, and the permeability of HZ0 (The calculated lines shown in Fig. 5 are discussed later.) If the permeation mechanism is mainly based on Knudsen diffusion, the permeability should decrease with an increase in temperature, however, the observed permeability of Hz in Fig. 4 was contrary to the expected one. This fact suggests that Knudsen diffusion is not the predominant mechanism of the permeation. For the pure silica microporous membrane, the permeation
H. Ohya et al. /Journal of Membrane Science 97 (1994) 91-98
94
e 0 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00
dp I nm 1 Fig. 5. The linear relation between pore diameter and permeability of HzO. Solid lines are calculated by Eq. (4).
mechanism is based on Knudsen diffusion, as reported before [ 111. But for the membrane made by the addition of 40-60% zirconia to silica, the permeation behavior cannot be explained by only the Knudsen diffusion mechanism. 3.3. Permeation mechanism The total flux, JT, through the membrane is defined as the sum of the flux due to surface diffusion, Js [ 3 1, Knudsen diffusion, JK [ 3 3, and activated diffusion, JA [ 141: JT=Js+JK+JA =a*AP(MT)-‘/‘*[
exp( -E*/RT)
P=J,/A,
3
(1)
exp(b/T)-l]+K q(mol/s) =r.
*AP-(MT)-“2/(1+c/T)+P,*AP -
using Eq. (3) and the constants in Table 2, and those in Fig. 5 were calculated using Eq. (4). The agreement between the measured and calculated permeabilities is very good for membranes B and C, but not so good for membrane A, particularly at the temperature range of 523 to 673 K. The linear relation between the permeability of H20 and the average micropore diameter shown in Fig. 5 suggests that the number of micropores on the surface area is almost the same because in the surface diffusion mechanism the permeability is proportional to the sum of the periphery of micropores, C( x-d,). Assuming that the major permeation mechanism in membrane A with the smallest micropore diameter of 1.5 nm is the activated diffusion, the activation energy of the activated diffusion, obtained from the data at higher temperatures (Fig. 4), is 6.68 kJ/mol. In Fig. 6, AP, the differences between the measured permeabilities for membranes A, B and C and the permeability of the activated diffusion for membrane A, which is shown by dotted line in Fig. 4, are plotted against d,,3. The permeation rate of molecules of molecular weight M through a micropore of dp and length L at T by the Knudsen mechanism can be expressed by:
(2) (3)
The measured fluxes were analyzed according to Eq. (2) and the fitting parameters are listed in Table 2. Considering the trends shown in Fig. 4, the permeation mechanism of H20 might be governed by surface diffusion, and Fig. 5 shows that the permeability of H20 is proportional to d,. Therefore, the permeability of H20, PHzO,can be expressed by the following experimental equation derived from the first term of Eq. (2 ) :
The solid lines shown in Fig. 4 were calculated
2n -SAP MRT J
(5)
the linear relation between AP and dp3 proves that the mechanism of H2 permeability is mainly governed by the activated and Knudsen diffusion. As discussed for the water vapor permeation mechanism, the total number of micropores in membranes A, B and C is almost the same. Therefore the permeabilities of H2 through membranes A, B and C by the Knudsen mechanism will be proportional to dp3. The observed thickness of the Zr02-SO2 layer, 15 coatings, on the ceramic tubing by scanning electron micrographs shown in Fig. 7 is 10.8, 10.5 and 10.0 pm for membranes A, B and C, respectively. No separation of a CH,-CO, gas mixture was observed for membranes with less than 10 coatings.
H. Ohya et al. /Journal of Membrane Science 97 (1994) 91-98
95
Table 2 Fitting parameters in Eq. (2) for membrane A, B and C A
B
H2
ax lo* (mol ‘I2 kg’/2 K’/2/mZ Pa s) bx 1OW (K) KX 10’ (mol ‘/’ kg’j2 K’/*/m* Pas) c (K) POX 10’ (mol/m2 Pa s) -% (kJ/mol)
H2O
_ 0.307 119 1.13 6.68
1.95 1.00 -
C
H2
H20
6.13 45.7 1.13 6.68
5.16 1.00 -
3.4. Separation of H,-H,O-HBr mixture
0-o
dp” [ nm’ ]
Fig. 6. Relationships between d,’ and the differences between the measured permeabilities for membrane A, B and C and the permeability of the activated diffusion for membrane A, AP.
Therefore, the thickness corresponding to the layer of 10 coatings is assumed to be a buffer layer, not an active layer, and the effective thickness corresponding to the active layer is estimated to be 3.60, 3.50 and 3.33 pm for membranes A, B and C, respectively. Using the estimated thickness, L, the pore diameter, dp, and the permeability based on Knudsen diffusion (Fig. 6)) AP, the numbers of pore can be calculated with Eq. ( 5 ). The numbers of micropore are 1.85x 1013, 1.61 x lOI and 1.29x 1013mW2 for membrane A, B and C, respectively, and decrease with the increase of the micropore diameter.
H2
Hz0
7.96 30.1 1.13 6.68
6.06 1.oo -
gaseous
The permeation characteristics of the Hz, H20 and HBr gaseous mixture (feed composition 0.52-0.54, 0.48-0.46 and (8-9)~10-~ mol%, respectively) for membrane B were measured. The analysis of the permeated gases showed that H2 was not detected and that the constituents of the permeated gases were Hz0 and HBr. In Fig. 8 the measured permeation flux of water, J,.,, is plotted against the partial TMP of H20, dP,, at each temperature. The solid lines shown in the Fig. 8 are redrawn from the pure H20 data shown in Fig. 3. The agreement between the solid lines and the measured data at higher partial TMP of water is quite good, but not good at lower partial TMP. In Fig. 9 the measured permeation flux of HBr, JHBr is plotted against the partial TMP of HBr, APuB,, at each temperature. It is interesting that JHBr does not depend on APHBr, but keeps almost a constant value of ( 1.21.5 ) x 1O- 5mol/m2 s. The reason why JHBr is almost constant over the wide range of the experimental conditions is not clear. Fig. 10 shows the relationship between the temperature and the separation factor, &rBr/H2o, defined as: ai/H20
A/ JH20 =~ Yi/YH20
(6)
The separation factors, aHBr/H20,6 to 36, for
96
H. Ohya et al. / JournalofMembraneScience
97 (1994) 91-98
0
10
20
40
30
A
P, [
60
50
70
80
kPa 1
Fig. 8. Relationships between partial transmembrane pressure of H,O, LIP,, and the measured permeation fluxes of HzO, Jw, for membrane B at transmembrane pressures of 0.15 and 0.20 MPa.
I
I
Membrane
0.1
0.2 APHBr
I
I
I
B
0.3 1 kPa
0.4
0.5
0.6
1
Fig. 9. Relationships between partial transmembrane pressure of HBr, dPnBr, and permeation fluxes of HBr, JHBr,for membrane B at transmembrane pressures of 0.15 and 0.20 MPa.
Fig. 7. Cross-sectional view of (a) membrane A; (b) membrane B; (c ) membrane C.
the range of the experimental conditions indicate that HBr permeated faster through the membrane than Hz0 and was concentrated in the permeated gases. All the measured separation factors increased with an increase in temperature. The permeation flux of water decreased with an increase in temperature as shown in Fig. 8; however, the permeation flux of HBr remained constant over the wide range of experimental conditions as shown in Fig. 9. The separation factor of the membrane with the smaller micropore is always larger than the membrane with the larger micropore. With the same membrane, the separation factor decreased with the increase of the total TMP, Al’-,-.
H. Ohya et al. /Journal of Membrane Science 97 (I 994) 91-98
97
flux of HI remained constant for the wide range of the experimental conditions. 50 45 40 y-
35
0
30
g
15
g
20
tf
4. Conclusions
15 10 5 0
400
500
600
700
800
TlKl
Fig. 10. Relationships between temperature and separation factor for HBr over Hz0 at transmembrane pressures of 0.15 and 0.20 MPa.
40 35
I -
Membrane
400
I
I
600
700
I’
A
500
800
TIKI
Fig. 11. Relationships between temperature and separation factor for HI over H,O at transmembrane pressures of 0.15 and 0.20 MPa.
3.5. Separation of H,O-HI
( 1) The permeation flux of water increased with a decrease in temperature, and was proportional to the mean diameter of the micropore of the membrane. These results suggest that the permeation mechanism is mainly due to surface diffusion in the pore. (2) The permeation flux of H2 increased with an increase in temperature. The permeation mechanism of H2 is mainly due to the mechanisms of activated diffusion and Knudsen diffusion. (3 ) The permeation flux of HBr was almost constant over the wide range of the experiments conditions. (4) In the separation experiments of Hz-H20HBr gaseous mixture, it was found that only H20 and HBr permeated through the membrane and that HBr was concentrated in the permeated gases. The separation factors were in the range of 6 to 36. ( 5 ) In the separation experiments of the H20HI gaseous mixture, it was found that HI was concentrated in the permeated gases. The separation factors were in the range of 5 to 26. (6) The zirconia composite membranes separated Hz from the thermochemical decomposed gaseous mixture successfully.
gaseous mixture
Fig. 11 shows the relationship between the temperature and the separation factor &n/H20 for membrane A (feed compositions, H20:HI=0.9987-0.9989:(1.3-1.1)~10-3; TMP, 0.15 and 0.20 MPa), The separation factors, a H1/HZo,5 to 26, for the range of the experimental conditions indicate that HI permeates faster through the membrane than HZ0 and was concentrated in the permeated gases. All the measured separation factors increased with an increase in temperature. The permeation flux of water vapor decreased drastically with an increase in temperature; however, the permeation
5. List of symbols A P T P Y L” R ‘yif Hz0 J
difference (-) pressure (Pa) absolute temperature (K) permeability (mol/m’ Pa s) molecular weight (kg/mol) micropore diameter (m) length (m) gas constant (J/mol K) separation factor for i over (i=HBr or HI) (-) permeation flux ( mol/m2 s)
HZ0
H. Ohya et al. /Journal
98
EA TMP Y
ofMembrane
activation energy (kJ/mol) transmembrane pressure (MPa or kPa) mole fraction in the feed (-)
5.1. Subscripts T s K A W
total surface diffusion Knudsen diffusion activated diffusion J-W
Acknowledgments This work was supported by Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, in Japan (02203 105, 03203106,04203106).
References [ I ] H. Kameyama and K. Yoshida, Br-Ca-Fe
water decomposition cycles for hydrogen production, Proc. 2nd World Hydrogen Energy Conf., Zurich, Oct. 1978, Pergamon, Oxford, 1978, p. 829. [2]K. Onuki, S. Shimizu, H. Nakajima, S. Fujita, Y. Ikezoe, S. Sato and S. Machi, Studies on an iodine-sulfur process for thermochemical hydrogen production, Proc. 8th World Hydrogen Energy Conf., Honolulu, July 1990, Pergamon, Oxford, 1990, p. 547. [ 3]Y. Shindo, T. Hakuta, H. Yoshitome and H. Inoue, Gas diffusion in microporous media in Knudsen’s regime, J. Chem. Eng. Jpn., 16 (1983) 120.
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[ 4]K. Itaya, S. Sugawara, K. Arai and S. Saito, Properties of porous anodic aluminum oxide films as membranes, J. Chem. Eng. Jpn., 17 (1984) 514. [ 5 ] T. Kameyama, K. Fukuda, M. Fujishige, H. Yokokawa and M. Dokiya, Production of hydrogen from hydrogen sulfide by means of selective diffusion membranes, Hydrogen Energy Prog., 2 ( 198 1) 569. [6]T. Kameyama, M. Dokiya, M. Fujishige, H. Yokokawa and K. Fukuda, Production of hydrogen from hydrogen sulfide by means of selective diffusion membranes, Int. J. Hydrogen Energy, 8 (1983) 5. [ 7]N. Itoh, Y. Shindo, K. Haraya and T. Hakuma, A membrane reactor for promoting a reversible reaction, Proc. Int. Congr. Membranes and Membrane Processes, Tokyo, June 8-12, 1987, p. 654. [ 8]M. Asaeda and L.D. Du, Separation of alcohol/water gaseous mixtures by thin ceramic membrane, J. Chem. Eng. Jpn., 19 (1986) 72. [9]M. Asaeda, L.D. Du and M. Fuji, Separation of alcohol/water gaseous mixtures by an improved ceramic membrane, J. Chem. Eng. Jpn., 19 ( 1986) 84. [ lO]M. Asaeda, L.D. Du and K. Ikeda, Experimental studies of dehumidification of air by an improved ceramic membrane, J. Chem. Eng. Jpn., 19 (1986) 238. [ 11 ]H. Ohya, S. Sato, A. Ishii, Y. Negishi and K. Matsumoto The separation of gaseous mixture with composite microporous glass membranes at high temperature, Maku, 15 (1990) 72. [ 121 K.H. Lee and S.T. Hwang, The transport of condensable vapors through a microporous vycor glass membrane, J. Colloid Interface Sci., 110 ( 1986) 544. [ 131 M. Niwa, H. Ohya, Y. Tanaka, N. Yoshikawa, K. Matsumoto and Y. Negishi, Separation of gaseous mixtures of CO2 and CH4 using a composite microporous glass membrane on ceramic tubing, J. Membrane Sci., 39 (1988) 301. [ 141S. Kitao, H. Kameda and M. Asaeda, Gas separation by thin porous silica membrane of ultra fine pores at high temperature, Maku, 15 ( 1990) 222.