- Email: [email protected]
Ag membrane reactor
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 9 8 6 e1 2 9 9 2
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Autothermal reforming of methane for producing high-purity hydrogen in a Pd/Ag membrane reactor Hsin-Fu Chang a,*, Wen-Ju Pai a, Ying-Ju Chen a, Wen-Hsiung Lin b a b
Department of Chemical Engineering, Feng Chia University, Taichung 407, Taiwan Department of Beauty Science, Chienkuo Technology University, Changhua 500, Taiwan
article info
abstract
Article history:
Autothermal reforming of methane includes steam reforming and partial oxidizing
Received 23 December 2009
methane. Theoretically, the required endothermic heat of steam reforming of methane
Accepted 13 April 2010
could be provided by adding oxygen to partially oxidize the methane. Therefore, combining
Available online 14 May 2010
the steam reforming of methane with partial oxidation may help in achieving a heat balance that can obtain better heat efficacy. Membrane reactors offer the possibility of
Keywords:
overcoming the equilibrium conversion through selectively removing one of the products
Autothermal reforming
from the reaction zone. For instance, only can hydrogen products permeate through
Methane
a palladium membrane, which shifts the equilibrium toward conversions that are higher
Pd/Ag membrane
than the thermodynamic equilibrium. In this study, autothermal reforming of methane was carried out in a traditional reactor and a Pd/Ag membrane reactor, which were packed with an appropriate amount of commercial Ni/MgO/Al2O3 catalyst. A power analyzer was employed to measure the power consumption and to check the autothermicity. The average dense Pd/Ag membrane thickness is 24.3 mm, which was coated on a porous stainless steel tube via the electroless palladium/silver plating procedure. The experimental operating conditions had temperatures that were between 350 C and 470 C, pressures that were between 3 atm and 7 atm, and O2/CH4 ¼ 0e0.5. The effects of the operating conditions on methane conversion, permeance of hydrogen, H2/CO, selectivities of COx, amount of power supply, and the carbon deposition of the catalyst after the reaction is thoroughly discussed in this paper. The experimental results indicate that an optimum methane conversion of 95%, with a hydrogen production rate of 0.093 mol/m2. S, can be obtained from the autothermal reforming of methane at H2O/CH4 ¼ 1.3 and O2/CH4 near 0.4, at which the reaction does not consume power, and the catalysts are not subject to any carbon deposition. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The steam reforming of methane (SRM) has become the most important chemical process for hydrogen production in refining and fertilizer industries. Due to its highly endothermic nature, the process requires a high temperature in the range of 850e900 C and high pressures, 15e40 atm, in order to convert methane into hydrogen by using the following equations [1].
CH4 þH2 O/CO þ 3H2
DH0298 ¼ 206 KJ=mol
(1)
CO þ H2 O/CO2 þ H2
DH0298 ¼ 41 KJ=mol
(2)
Reaction (1) is the steam reforming reaction that is strongly endothermic and Reaction (2) is a moderately exothermic water-gas shift reaction. The SRM reaction is generally limited by equilibrium. Therefore, in order to achieve high methane
* Corresponding author. Fax: þ886 4 24510890. E-mail address: [email protected] (H.-F. Chang). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.04.060
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 9 8 6 e1 2 9 9 2
conversion, the SRM reaction must be carried out at a high temperature, which inevitably increases the fabrication cost for high temperature resistant facilities. In order to increase the conversion of the SRM reaction, but not to suffer its endothermicity, one possibility is to continuously remove the hydrogen that shifts the reaction to the products and breaks the equilibrium limit. The palladium membrane reactor, which incorporates the reaction zone and the separation zone in a single unit, uses this concept to improve the conversion [2e5]. Another possibility is to add oxygen that is co-fed with methane and steam in order to trigger the partial oxidation of methane and to give rise to a high amount of heat in order to supply the energy needed for the subsequent endothermic steam reforming reaction. This process can help in achieving autothermal reforming of methane ðDHy0Þ [6e8]. A novel concept has been recently proposed to perform the autothermal reforming of methane in a palladium membrane reactor. The combined effects of the conversion enhancement and the energy consumption reduction are very vital to mediate the currently stringent energy crisis situation [9e11]. The main objective of this study is to investigate the oxidative steam reforming of methane in a palladiumesilver membrane reactor that operates at near autothermal conditions. A delicate power analyzer was installed in the reaction module to measure the power consumption and to check the autothermicity of the reaction system. The experimental results were compared with the calculation results based on thermodynamic correlations.
2.
Experimental procedure [12]
2.1.
Preparation of Pd-base/PSS composite membranes
PdeAg/PSS (Porous stainless steel) membranes were prepared by using a conventional electroless plating technique. Prior to
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preparing the membrane, dirt and grease in the porous (average pore size ca. 4 mm) SS-316L tube support (effective area ca. 20 cm2; Poll Metallurgical Corporation) were removed by cleaning it successively with a dilute sodium hydroxide solution, a dilute phosphate acid solution, and deionized water. The activation procedure consisted of a two-step immersion sequence, which was first in an acidic SnCl2 solution (1 g/L), followed by an acidic PdCl2 solution (1 g/L). After surface activation, palladium deposition was performed first in a 4 g Pd(NH3)4Cl2/L plating bath that lasted for 1 h at 333 K with appropriate amount of N2H4 being added. Silver deposition in a 0.5 g AgNO3/L plating bath was then performed for 1 h under similar conditions. The plating step was repeated until an expected thickness of the membrane was reached. After each deposition, the membrane was rinsed with deionized water and then dried in an oven at 393 K for 2 h. Finally, the membrane was sintered and annealed in hydrogen at 450 C and was under atmospheric pressure for more than 18 h.
2.2.
Gas permeation measurement
Permeation measurements were conducted on the PdeAg/PSS membranes at elevated temperatures (350e600 C) and pressures (differences up to 1 MPa). The schematic diagram of the gas permeation apparatus is shown in Fig. 1. The assembly of permeability testing consisted of gas cylinders, a membrane reactor (no catalyst packed), a gas chromatograph, a backpressure regulator, and pressure gauges. The upstream pressure was controlled by the back-pressure regulator, whereas the downstream of gas was vented. Prior to the hydrogen permeation measurement, the flow system was preheated to the setting temperature in the nitrogen atmosphere. The permeation rates of the individual gases (hydrogen and nitrogen) were measured at atmospheric pressure and the ambient temperature was measured by a soap-bubble flowmeter.
Fig. 1 e A schematic diagram of the experimental apparatus: (1) mass flowmeter, (2) piston metering pump, (3) bubble flowmeter, (4) power analyzer, (5) temperature controller, (6) back-pressure regulator, (7) membrane module, (8) eight-pore valve, (9) gas chromatograph, (10) furnace.
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2.3. Methane autothermal reforming with PdeAg/PSS composite membrane reactor The autothermal reforming of methane, CH4 þ aH2 O þ ð1 a=2ÞO2 /CO2 þ ða þ 2ÞH2 ; occurred in the shell side of a palladium membrane reactor in which a commercial catalyst was packed. The product hydrogen that permeated through the membrane into the inner tube was collected. The reactor module was placed inside a thermostat that was connected to a power analyzer (IDRC CP-600) in order to measure the power parameter needed for determining the autothermicity. In between the stainless shell and the membrane tube, Ni/MgO/Al2O3 catalyst powders were uniformly mixed with silica powder. Water was fed to an evaporator with a high pressure syringe pump. Vaporized water, oxygen, and methane were sent to the catalyst bed of the reactor. The reaction pressure was controlled in the range of 3e7 atm with a back-pressure regulator. The catalytic activity that was measured in terms of methane conversion was defined as: X¼
out Fin methane Fmethane 100% in Fmethane
Fout i 100% Fout methane
Fin methane
The power parameter was evaluated QO2 ; Qlost and QwithoutO2 ; which was defined as: Power parameter ¼
QO2 Qlost Qwithout O2 Qlost
3.2.
(4) by
(5)
where Qlost is the power consumption that uses N2 as the feed and at the same volumetric flow rate as the reforming reaction, i.e., the power needed to maintain the reactor temperature from reducing due to natural or radiation cooling. QO2 and Qwithout O2 are the power consumptions for the reaction with and without O2 feed, respectively.
3.
Results and discussion
3.1.
Permeation test
could not be measured, which means that the H2/N2 permselectivity is infinite in the range of these conditions.
Autothermal reforming of methane
(3)
and the products distributions were evaluated through their selectivity and selectivity of species i, which was defined as: Si ¼
Fig. 2 e Hydrogen Flux through Pd/Ag alloy membrane at different temperatures.
Fig. 3 compares the typical CH4 conversion for a membrane reactor (MR) and a traditional reactor (TR) versus the pressure at 3e7 atm, at T ¼ 470 C and WHSV ¼ 0.3 h1, for CH4/H2O/ O2 ¼ 1/1.3/0.3. The CH4 conversion for the TR decreases when the pressure increases. This is because according to the Le Chatelier’s principle, when a chemical system equilibrium experiences a change, the equilibrium will shift in order to partially counter-act the imposed change. Thus, for the methane steam reforming reaction, the number of moles of reactants is less than that of the products, which means that the reaction will go backward when there is an increasing pressure in the reaction. This leads to a reduction in CH4 conversion. Membrane reactors offer the possibility of overcoming the equilibrium conversion through selectively removing the H2 of the products from the reaction zone. Thus, an increase in pressure leads to an increase in CH4 conversion. The conversion of CH4 in the MR is clearly higher than in the TR, especially at a high pressure. The CH4 conversion is nearly 37% in the TR and achieves 95% in the MR at 7 atm. The 100 Membrane Reactor Traditional Reactor
CH4 conversion (%)
80
Fig. 2 shows a plot of the hydrogen flux versus the difference 1/2 of the square roots of the hydrogen pressure (P1/2 H PL ) for the membrane at fixed temperature (350 C, 400 C and 450 C). A 0.5 confirms that the linear correlation JH ¼ FPL(P0.5 H PL ) hydrogen flux through the Pd membrane obeys Sievert’s law, indicating that a solution-diffusion mechanism is dominated.[2] The highest H2 permeance of 9238.7 mol/m2 h occurs at 450 C and the activation energy of 16.49 KJ/mole is obtained from plotting log(permeance) versus 1/T [13]. Permselectivity is defined as the ratio of hydrogen permeance to the permeances of the other gases. During the permselectivity test where the temperatures ranges from 350 C to 570 C, and the pressures ranged from 3 atm to 7 atm for 1 h, the nitrogen that was collected in the permeation side
60
40
20
0
2
3
4
5
6
7
8
Pressure (atm) Fig. 3 e CH4 conversion versus pressure for the MR and the TR at 470 C.
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100
80
CO selectivity (%)
80 70 60 Membrane Reactor P=7atm Membrane Reactor P=1atm Traditional Reactor P=1atm
50
60
60
40
40
20
20
0
40
0.0
0.1
0.2
0.3
0.4
0.5
behavior of high CH4 conversion in the MR at high pressure is probably due to the effects of H2 permeation. The increase of pressure results in an increase of H2 permeation through the membrane, which can cause CH4 conversion to rise. In order to discuss the results of adding O2 in the TR and in the MR at different pressures, it is useful to carry out the experiment with a fixed feed of CH4:H2O ¼ 1:1.3 and increasing the ratio of O2/CH4 from 0 to 0.5. Fig. 4 shows that at 1 atm, the CH4 conversion increases when the ratio of O2/CH4 increases and the CH4 conversion in the MR is always higher than that of the TR. Moreover, when the experiment was carried out at 7 atm in the MR, the CH4 conversion clearly increased when the ratio of O2/CH4 increased. The results clearly indicate that the CH4 conversion in the MR was greatly higher than that in the TR. This is because the MR offers the possibility to remove one of the products from the reaction zone, and to boost the reaction more. This is especially true in Pd membranes, where only H2 products can permeate through it. In this case, the maximum CH4 conversion is 98.54% at O2/CH4 ¼ 0.5 and 7 atm in the MR. The H2 permeation through a dense Pd/Ag membrane is strongly dependent on the temperature and pressure gradient over the membrane, as shown in Fig. 5. It is clear that the H2 flux
C C C C C
0.07 0.06 0.05 0.04
4
5
6
7
0 0.6
80
80
60
60
40
40
0
3
0.5
100
CO selectivity P=1atm CO2 selectivity P=1 atm CO selectivity P=7 atm CO2 selectivityP=7 atm
20
2
0.4
100
0.03 0.02
0.3
seems to increase when both the temperature and pressure increase since high pressures provide a thrust to H2 to permeate through the membrane. It is to be noted that an optimum value for H2 flux is 0.093 mol/m2 s at 470 C and 7 atm. Fig. 6 shows the CO and CO2 selectivities for the TR and the MR at various O2 addition (O2/CH4 ¼ 0e0.5) when the reaction pressure is at 1 atm, T at 470 C, and H2O/CH4 ¼ 1.3. The CO selectivities evidently decrease when O2 is added in both the TR and the MR, while the CO selectivity in the MR is always ca. 10% lower than in the TR. Concomitantly, the CO2 selectivities increase when there is an increase in adding O2, both in the TR and in the MR. The selectivity of CO2 in the TR is about 5% higher than that in the MR. These observations clearly indicate that the partial oxidation of CH4 reactions occurs simultaneously with the steam reforming of methane (SRM) and the water-gas shift reaction (WGS). The increase in selectivity of CO2 in the MR may suggest that the WGS reaction moves rapidly forward due to the product hydrogen that is consumed by the addition of O2. Moreover, Fig. 7 shows that in the MR the CO selectivity decreases while the CO2 increases with increasing pressure. This suggests that the MR could more easily avoid CO formation at a higher pressure.
CO selectivity (%)
2
H2 flux (mol/m *s)
0.08
0.2
O /CH ratio
0.10 T=470 T=440 T=410 T=380 T=350
0.1
Fig. 6 e Comparison of CO and CO2 selectivities for the MR and the TR at different O2/CH4 ratio at P [ 1 atm.
Fig. 4 e CH4 conversion versus O2/CH4 at different pressures.
0.09
0.0
0.6
O 2/CH 4 ratio
80
8
Pressure (atm)
Fig. 5 e The dependence of H2 flux on pressure at different temperatures. (CH4:H2O:O2 [ 1:1.3:0.3, WHSV [ 0.3).
0.0
0.1
0.2
CO2 selectivity (%)
CH4 conversion (%)
90
100 Traditional Reactor P=1atm Membrane Reactor P=1atm Traditional Reactor P=1atm Membrane Reactor P=1atm
CO selectivity (%)
100
20
0.3
0.4
0.5
0
0.6
O2/CH4 ratio
Fig. 7 e CO and CO2 selectivities versus O2/CH4 ratio in the MR at different pressures.
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8 Membrane reactor P=7atm Membrane reactor P=1atm Traditional reactor P=1atm
H2/CO ratio
7
6
5
4
0.0
0.1
0.2
0.3
0.4
0.5
0.6
O2/CH4 ratio Fig. 8 e The variation of H2/CO ratio at different O2/CH4 ratios. Fig. 8 presents the influence of the H2/CO ratio by adding O2 in the TR and the MR at 1 atm and 470 C. The H2/CO ratio increases when there is an increase in the addition of O2, especially in the MR. In particular, the H2/CO achieves a maximum value of 6.75 at O2/CH4 ¼ 0.5. This is because the addition of O2 not only increases the methane conversion, but also increases H2 production [6,7,13]. Moreover, the H2/CO ratio increases by increasing the pressure. This increase in the pressure could force H2 to permeate through the membrane, and to also increase the H2 flux so that the H2/CO ratio increases. According to the above discussion, carrying out the reaction in the MR has advantages in increasing H2 production and reducing CO formation. This suffices the crucial requirement that high-purity of hydrogen and minimum CO content (<30 ppm) should serve as a feed to the PEM fuel cell.
3.3.
Autothermicity
Upon addition of oxygen, the partial oxidations of methane can occur as:
Fig. 10 e Enthalpy of the reaction at different H2O and O2 contents as a function of temperature, where a is defined in the following equation: CH4 DaH2 ODð1La=2ÞO2 /CO2 DðaD2ÞH2 .
CH4 þ 0:5O2 /CO þ 2H2 CH4 þ O2 /CO2 þ 2H2
DHð743:15 KÞ ¼ 24:26 kJ=mol DHð743:15Þ ¼ 307:09 kJ=mol
(6) (7)
Fig. 9 shows the energy that is required for the reaction at H2O/CH4 ¼ 1.3 when the ratio of O2/CH4 varies. As shown in Fig. 9, in order to achieve the complete autothermicity, the autothermicity parameter, (QO2 Qlost)/(QwithoutO2 Qlost), is equal to zero, which means no external heating or cooling is required, the O2/CH4 feed ratio at for the MR is less than that for the TR. This is because in the membrane reactor Reaction (7) is prevailing and produces more heat, as well as CO2, for the endothermic steam reforming. Fig. 6 also confirms that the amount of CO2 is higher than that of CO in the MR. From Reactions (6) and (7), we can see that it is clear that heat release occurs 6 times more in Reaction (7) compared to Reaction (6) when 1 mole of O2 at 743.15 K is burned. As a consequence, the MR could conduct autothermal reaction with a lower amount of O2. Fig. 9 also shows that for the MR the O2/CH4 ratio that is required is higher at 7 atm than at
100
CH4 conversion (%)
80
60
40 O 2 /CH 4 =0.4 O 2 /CH 4 =0.3
20
0 340
360
380
400
420
440
460
480
o
Temperature ( C)
Fig. 9 e Comparison of power consumption for the MR and the TR at different O2/CH4 ratios.
Fig. 11 e CH4 conversion versus temperatures from 350 to 470 at O2/CH4 ratio [ 0.3 and 0.4.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 9 8 6 e1 2 9 9 2
100
CH4 conversion (%)
80
60
40
in exp eram ent 1 repeat in e xpe ram ent 1
20
0
2
3
4
5
6
7
8
P ressure (atm )
Fig. 12 e Stability test of Ni/MgO/Al2O3 catalyst.
proceed, i.e., DH ¼ 0. For comparison, a series of experiments were conducted to study the effect of O2/CH4 (0.3 and 0.4) on methane conversion. Fig. 11 shows that with the feed composition of O2/CH4 being 0.3, XCH4 increases with increasing temperatures due to its endothermic nature. Contrary to the above result, at O2/CH4 ¼ 0.4, XCH4 decreases slowly with the increasing temperature, indicating that an exothermic reaction is starting to happen. Conclusively, in order for an autothermal reaction to process, the CH4/O2 ratio must be 0.3e0.4 and near the ratio of 0.4. The results show almost the same results that were predicted by calculation. This strongly indicates that an optimum feeding gas condition is H2O/CH4 ¼ 1.3, O2/CH4, which is nearly 0.4 at 350 C to 470 C.
3.4. 1 atm. As observed from Fig. 7, the yield of CO at 7 atm is higher than that at 1 atm when the ratio of O2/CH4 ¼ 0.3. It can be seen with that Reaction (6) is more favorable. Thus, there is less energy released in Eq. (6), which leads to a larger amount of O2 that is needed to achieve the autothermal condition. From the above results and discussion, it is concluded that the MR is more easily achieved autothermally than the TR that has a lower amount of O2 at 1 atm. The enthalpy of reaction versus temperature at different H2O and O2 contents is calculated and depicted in Fig. 10. From this figure, we can see that at a feed H2O/CH4 ¼ 1.3, and over the temperature range 350e470 C, the ratio of O2/CH4 should be 0.365e0.37 in order for a complete autothermal reaction to
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Catalyst stability
In order to know whether the activity of catalyst decays or not after a series of reactions over a time span of 200 h, the first experiment was repeated to check the conversion of CH4. As shown in Fig. 12, the activity tests were performed at a pressure that ranged from 3 atm to 7 atm, WHSV ¼ 0.3 1/h and T ¼ 470 C. The methane conversions of both the first experiment and the repeated one followed a similar trend with respect to pressure and the difference between them was almost nil. Thus, the catalyst maintained the same activity as before and showed no sign of decay. Deposited carbon was not observed at low magnification (20.0 k), as shown in Fig. 13((a) and (b)), but some crystallite aggregates appeared after the reaction, as shown in Fig. 13((c) and (d)), at high magnification
Fig. 13 e SEM micrographs of the Ni catalyst before the reaction ((a), (c)) and after the reaction ((b), (d)), at different magnification.
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(150 k). It is believed that by reforming methane with steam and oxygen, there is no chance for carbon fiber formation, as was observed and reported by other researchers [14e16]. Moreover, the analysis with EDS showed no carbon filaments. This clearly proves that the catalyst almost had no carbon deposit after reactions [6].
4.
Conclusions
A dense palladiumesilver membrane reactor was used to perform oxidative steam reforming of methane in order to produce high-purity hydrogen. A delicate power analyzer was employed to measure the power consumption and determine the autothermicity (DH ¼ 0). With the feed of H2O/CH4 ¼ 1.3, and operating temperatures that ranged from 350 to 470 C, autothermicity was achieved at O2/CH4 near 0.4, at which the reactor did not consume power. The optimum methane conversion was 95% with a hydrogen production of 0.093 mol/ m2 s. When the oxygen and steam were co-fed, the catalyst became not susceptible to coke deposition after 200 h of operation.
Acknowledgements The authors like to thank the National Science Council of the Republic of China for the financial support under the Contract No. NSC95-2221-E-035-118.
references
[1] Barba D, Giacobbe F, De Cesaris A, Farace A, Iaquaniello G, Pipino A. Membrane reforming in converting natural gas to hydrogen (part one). International Journal of Hydrogen Energy 2008;33(14):3700e9. [2] Barbieri G, Di Maio FP. Simulation of the methane steam reforming process in a catalytic Pd-membrane reactor. Industrial and Engineering Chemistry Research 1997;36: 2121e7.
[3] Gallucci F, Paturzo L, Fama A, Basile A. Experimental study of the methane steam reforming reaction in a dense Pd/Ag membrane reactor. Industrial and Engineering Chemistry Research 2004;43(4):928e33. [4] Matsumura Y, Tong J. Methane steam reforming in hydrogen-permeable membrane reactor for pure hydrogen production. Topics in Catalysis 2008;51:123e32. [5] Uemiya S, Sato N, Ando H, Matsuda T, Kikuchi E. Steam reforming of methane in a hydrogen-permeable membrane reactor. Applied Catalysis 1990;67(1):223e30. [6] Ayabe S, Omoto H, Utaka T, Kikuchi R, Sasaki K, Teraoka Y, et al. Catalytic autothermal reforming of methane and propane over supported metal catalysts. Applied Catalysis A: General 2003;241:261e9. [7] Chen L, Hong Q, Lin J, Dautzenberg FM. Hydrogen production by coupled catalytic partial oxidation and steam methane reforming at elevated pressure and temperature. Journal of Power Sources 2007;164:803e8. [8] Takeguchi T, Furukawa SN, Inoue M, Koichi E. Autothermal reforming of methane over Ni catalysts supported over CaOeCeO2eZrO2 solid solution. Applied Catalysis A: General 2003;240(1e2):223e33. [9] Basile A, Paturzo L, Lagan F. The partial oxidation of methane to syngas in a palladium membrane reactor: simulation and experimental studies. Catalysis Today 2001;67(1e3):65e75. [10] Paturzo L, Basile A. Methane conversion to syngas in a composite palladium membrane reactor with increasing number of Pd layers. Industrial and Engineering Chemistry Research 2002;41(7):1703e10. [11] Shu J, Grandjean BPA, Van NA, Kaliaguine S. Catalytic palladium-based membrane reactors: a review. Journal of Chemical Engineering 1991;69(5):1036e60. [12] Lin WH, Chang HF. Characterizations of PdeAg membrane prepared by sequential electroless deposition. Surface and Coatings Technology 2005;194(1):157e66. [13] Pai WJ. The study of autothermal reforming of methane in the palladiumesilver membrane reactor. MS Thesis: Feng Chia University; 2007. [14] Mariana MVM, Martin S. Autothermal reforming of methane over Pt/ZrO2/Al2O3 catalysts. Applied Catalysis A: General 2005;281:19e24. [15] Rostrup-Nielsen J, Trimm LD. Mechanisms of carbon formation on nickel-containing catalysts. Journal of Catalysis 1977;48(1e3):155e65. [16] Alstrup I. A new model explaining carbon filament growth on nickel, iron, and NieCu alloy catalysts. Journal of Catalysis 1988;109(2):241e51.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Autothermal reforming of methane for producing high-purity hydrogen in a Pd/Ag membrane reactor Hsin-Fu Chang a,*, Wen-Ju Pai a, Ying-Ju Chen a, Wen-Hsiung Lin b a b
Department of Chemical Engineering, Feng Chia University, Taichung 407, Taiwan Department of Beauty Science, Chienkuo Technology University, Changhua 500, Taiwan
article info
abstract
Article history:
Autothermal reforming of methane includes steam reforming and partial oxidizing
Received 23 December 2009
methane. Theoretically, the required endothermic heat of steam reforming of methane
Accepted 13 April 2010
could be provided by adding oxygen to partially oxidize the methane. Therefore, combining
Available online 14 May 2010
the steam reforming of methane with partial oxidation may help in achieving a heat balance that can obtain better heat efficacy. Membrane reactors offer the possibility of
Keywords:
overcoming the equilibrium conversion through selectively removing one of the products
Autothermal reforming
from the reaction zone. For instance, only can hydrogen products permeate through
Methane
a palladium membrane, which shifts the equilibrium toward conversions that are higher
Pd/Ag membrane
than the thermodynamic equilibrium. In this study, autothermal reforming of methane was carried out in a traditional reactor and a Pd/Ag membrane reactor, which were packed with an appropriate amount of commercial Ni/MgO/Al2O3 catalyst. A power analyzer was employed to measure the power consumption and to check the autothermicity. The average dense Pd/Ag membrane thickness is 24.3 mm, which was coated on a porous stainless steel tube via the electroless palladium/silver plating procedure. The experimental operating conditions had temperatures that were between 350 C and 470 C, pressures that were between 3 atm and 7 atm, and O2/CH4 ¼ 0e0.5. The effects of the operating conditions on methane conversion, permeance of hydrogen, H2/CO, selectivities of COx, amount of power supply, and the carbon deposition of the catalyst after the reaction is thoroughly discussed in this paper. The experimental results indicate that an optimum methane conversion of 95%, with a hydrogen production rate of 0.093 mol/m2. S, can be obtained from the autothermal reforming of methane at H2O/CH4 ¼ 1.3 and O2/CH4 near 0.4, at which the reaction does not consume power, and the catalysts are not subject to any carbon deposition. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The steam reforming of methane (SRM) has become the most important chemical process for hydrogen production in refining and fertilizer industries. Due to its highly endothermic nature, the process requires a high temperature in the range of 850e900 C and high pressures, 15e40 atm, in order to convert methane into hydrogen by using the following equations [1].
CH4 þH2 O/CO þ 3H2
DH0298 ¼ 206 KJ=mol
(1)
CO þ H2 O/CO2 þ H2
DH0298 ¼ 41 KJ=mol
(2)
Reaction (1) is the steam reforming reaction that is strongly endothermic and Reaction (2) is a moderately exothermic water-gas shift reaction. The SRM reaction is generally limited by equilibrium. Therefore, in order to achieve high methane
* Corresponding author. Fax: þ886 4 24510890. E-mail address: [email protected] (H.-F. Chang). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.04.060
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 9 8 6 e1 2 9 9 2
conversion, the SRM reaction must be carried out at a high temperature, which inevitably increases the fabrication cost for high temperature resistant facilities. In order to increase the conversion of the SRM reaction, but not to suffer its endothermicity, one possibility is to continuously remove the hydrogen that shifts the reaction to the products and breaks the equilibrium limit. The palladium membrane reactor, which incorporates the reaction zone and the separation zone in a single unit, uses this concept to improve the conversion [2e5]. Another possibility is to add oxygen that is co-fed with methane and steam in order to trigger the partial oxidation of methane and to give rise to a high amount of heat in order to supply the energy needed for the subsequent endothermic steam reforming reaction. This process can help in achieving autothermal reforming of methane ðDHy0Þ [6e8]. A novel concept has been recently proposed to perform the autothermal reforming of methane in a palladium membrane reactor. The combined effects of the conversion enhancement and the energy consumption reduction are very vital to mediate the currently stringent energy crisis situation [9e11]. The main objective of this study is to investigate the oxidative steam reforming of methane in a palladiumesilver membrane reactor that operates at near autothermal conditions. A delicate power analyzer was installed in the reaction module to measure the power consumption and to check the autothermicity of the reaction system. The experimental results were compared with the calculation results based on thermodynamic correlations.
2.
Experimental procedure [12]
2.1.
Preparation of Pd-base/PSS composite membranes
PdeAg/PSS (Porous stainless steel) membranes were prepared by using a conventional electroless plating technique. Prior to
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preparing the membrane, dirt and grease in the porous (average pore size ca. 4 mm) SS-316L tube support (effective area ca. 20 cm2; Poll Metallurgical Corporation) were removed by cleaning it successively with a dilute sodium hydroxide solution, a dilute phosphate acid solution, and deionized water. The activation procedure consisted of a two-step immersion sequence, which was first in an acidic SnCl2 solution (1 g/L), followed by an acidic PdCl2 solution (1 g/L). After surface activation, palladium deposition was performed first in a 4 g Pd(NH3)4Cl2/L plating bath that lasted for 1 h at 333 K with appropriate amount of N2H4 being added. Silver deposition in a 0.5 g AgNO3/L plating bath was then performed for 1 h under similar conditions. The plating step was repeated until an expected thickness of the membrane was reached. After each deposition, the membrane was rinsed with deionized water and then dried in an oven at 393 K for 2 h. Finally, the membrane was sintered and annealed in hydrogen at 450 C and was under atmospheric pressure for more than 18 h.
2.2.
Gas permeation measurement
Permeation measurements were conducted on the PdeAg/PSS membranes at elevated temperatures (350e600 C) and pressures (differences up to 1 MPa). The schematic diagram of the gas permeation apparatus is shown in Fig. 1. The assembly of permeability testing consisted of gas cylinders, a membrane reactor (no catalyst packed), a gas chromatograph, a backpressure regulator, and pressure gauges. The upstream pressure was controlled by the back-pressure regulator, whereas the downstream of gas was vented. Prior to the hydrogen permeation measurement, the flow system was preheated to the setting temperature in the nitrogen atmosphere. The permeation rates of the individual gases (hydrogen and nitrogen) were measured at atmospheric pressure and the ambient temperature was measured by a soap-bubble flowmeter.
Fig. 1 e A schematic diagram of the experimental apparatus: (1) mass flowmeter, (2) piston metering pump, (3) bubble flowmeter, (4) power analyzer, (5) temperature controller, (6) back-pressure regulator, (7) membrane module, (8) eight-pore valve, (9) gas chromatograph, (10) furnace.
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2.3. Methane autothermal reforming with PdeAg/PSS composite membrane reactor The autothermal reforming of methane, CH4 þ aH2 O þ ð1 a=2ÞO2 /CO2 þ ða þ 2ÞH2 ; occurred in the shell side of a palladium membrane reactor in which a commercial catalyst was packed. The product hydrogen that permeated through the membrane into the inner tube was collected. The reactor module was placed inside a thermostat that was connected to a power analyzer (IDRC CP-600) in order to measure the power parameter needed for determining the autothermicity. In between the stainless shell and the membrane tube, Ni/MgO/Al2O3 catalyst powders were uniformly mixed with silica powder. Water was fed to an evaporator with a high pressure syringe pump. Vaporized water, oxygen, and methane were sent to the catalyst bed of the reactor. The reaction pressure was controlled in the range of 3e7 atm with a back-pressure regulator. The catalytic activity that was measured in terms of methane conversion was defined as: X¼
out Fin methane Fmethane 100% in Fmethane
Fout i 100% Fout methane
Fin methane
The power parameter was evaluated QO2 ; Qlost and QwithoutO2 ; which was defined as: Power parameter ¼
QO2 Qlost Qwithout O2 Qlost
3.2.
(4) by
(5)
where Qlost is the power consumption that uses N2 as the feed and at the same volumetric flow rate as the reforming reaction, i.e., the power needed to maintain the reactor temperature from reducing due to natural or radiation cooling. QO2 and Qwithout O2 are the power consumptions for the reaction with and without O2 feed, respectively.
3.
Results and discussion
3.1.
Permeation test
could not be measured, which means that the H2/N2 permselectivity is infinite in the range of these conditions.
Autothermal reforming of methane
(3)
and the products distributions were evaluated through their selectivity and selectivity of species i, which was defined as: Si ¼
Fig. 2 e Hydrogen Flux through Pd/Ag alloy membrane at different temperatures.
Fig. 3 compares the typical CH4 conversion for a membrane reactor (MR) and a traditional reactor (TR) versus the pressure at 3e7 atm, at T ¼ 470 C and WHSV ¼ 0.3 h1, for CH4/H2O/ O2 ¼ 1/1.3/0.3. The CH4 conversion for the TR decreases when the pressure increases. This is because according to the Le Chatelier’s principle, when a chemical system equilibrium experiences a change, the equilibrium will shift in order to partially counter-act the imposed change. Thus, for the methane steam reforming reaction, the number of moles of reactants is less than that of the products, which means that the reaction will go backward when there is an increasing pressure in the reaction. This leads to a reduction in CH4 conversion. Membrane reactors offer the possibility of overcoming the equilibrium conversion through selectively removing the H2 of the products from the reaction zone. Thus, an increase in pressure leads to an increase in CH4 conversion. The conversion of CH4 in the MR is clearly higher than in the TR, especially at a high pressure. The CH4 conversion is nearly 37% in the TR and achieves 95% in the MR at 7 atm. The 100 Membrane Reactor Traditional Reactor
CH4 conversion (%)
80
Fig. 2 shows a plot of the hydrogen flux versus the difference 1/2 of the square roots of the hydrogen pressure (P1/2 H PL ) for the membrane at fixed temperature (350 C, 400 C and 450 C). A 0.5 confirms that the linear correlation JH ¼ FPL(P0.5 H PL ) hydrogen flux through the Pd membrane obeys Sievert’s law, indicating that a solution-diffusion mechanism is dominated.[2] The highest H2 permeance of 9238.7 mol/m2 h occurs at 450 C and the activation energy of 16.49 KJ/mole is obtained from plotting log(permeance) versus 1/T [13]. Permselectivity is defined as the ratio of hydrogen permeance to the permeances of the other gases. During the permselectivity test where the temperatures ranges from 350 C to 570 C, and the pressures ranged from 3 atm to 7 atm for 1 h, the nitrogen that was collected in the permeation side
60
40
20
0
2
3
4
5
6
7
8
Pressure (atm) Fig. 3 e CH4 conversion versus pressure for the MR and the TR at 470 C.
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100
80
CO selectivity (%)
80 70 60 Membrane Reactor P=7atm Membrane Reactor P=1atm Traditional Reactor P=1atm
50
60
60
40
40
20
20
0
40
0.0
0.1
0.2
0.3
0.4
0.5
behavior of high CH4 conversion in the MR at high pressure is probably due to the effects of H2 permeation. The increase of pressure results in an increase of H2 permeation through the membrane, which can cause CH4 conversion to rise. In order to discuss the results of adding O2 in the TR and in the MR at different pressures, it is useful to carry out the experiment with a fixed feed of CH4:H2O ¼ 1:1.3 and increasing the ratio of O2/CH4 from 0 to 0.5. Fig. 4 shows that at 1 atm, the CH4 conversion increases when the ratio of O2/CH4 increases and the CH4 conversion in the MR is always higher than that of the TR. Moreover, when the experiment was carried out at 7 atm in the MR, the CH4 conversion clearly increased when the ratio of O2/CH4 increased. The results clearly indicate that the CH4 conversion in the MR was greatly higher than that in the TR. This is because the MR offers the possibility to remove one of the products from the reaction zone, and to boost the reaction more. This is especially true in Pd membranes, where only H2 products can permeate through it. In this case, the maximum CH4 conversion is 98.54% at O2/CH4 ¼ 0.5 and 7 atm in the MR. The H2 permeation through a dense Pd/Ag membrane is strongly dependent on the temperature and pressure gradient over the membrane, as shown in Fig. 5. It is clear that the H2 flux
C C C C C
0.07 0.06 0.05 0.04
4
5
6
7
0 0.6
80
80
60
60
40
40
0
3
0.5
100
CO selectivity P=1atm CO2 selectivity P=1 atm CO selectivity P=7 atm CO2 selectivityP=7 atm
20
2
0.4
100
0.03 0.02
0.3
seems to increase when both the temperature and pressure increase since high pressures provide a thrust to H2 to permeate through the membrane. It is to be noted that an optimum value for H2 flux is 0.093 mol/m2 s at 470 C and 7 atm. Fig. 6 shows the CO and CO2 selectivities for the TR and the MR at various O2 addition (O2/CH4 ¼ 0e0.5) when the reaction pressure is at 1 atm, T at 470 C, and H2O/CH4 ¼ 1.3. The CO selectivities evidently decrease when O2 is added in both the TR and the MR, while the CO selectivity in the MR is always ca. 10% lower than in the TR. Concomitantly, the CO2 selectivities increase when there is an increase in adding O2, both in the TR and in the MR. The selectivity of CO2 in the TR is about 5% higher than that in the MR. These observations clearly indicate that the partial oxidation of CH4 reactions occurs simultaneously with the steam reforming of methane (SRM) and the water-gas shift reaction (WGS). The increase in selectivity of CO2 in the MR may suggest that the WGS reaction moves rapidly forward due to the product hydrogen that is consumed by the addition of O2. Moreover, Fig. 7 shows that in the MR the CO selectivity decreases while the CO2 increases with increasing pressure. This suggests that the MR could more easily avoid CO formation at a higher pressure.
CO selectivity (%)
2
H2 flux (mol/m *s)
0.08
0.2
O /CH ratio
0.10 T=470 T=440 T=410 T=380 T=350
0.1
Fig. 6 e Comparison of CO and CO2 selectivities for the MR and the TR at different O2/CH4 ratio at P [ 1 atm.
Fig. 4 e CH4 conversion versus O2/CH4 at different pressures.
0.09
0.0
0.6
O 2/CH 4 ratio
80
8
Pressure (atm)
Fig. 5 e The dependence of H2 flux on pressure at different temperatures. (CH4:H2O:O2 [ 1:1.3:0.3, WHSV [ 0.3).
0.0
0.1
0.2
CO2 selectivity (%)
CH4 conversion (%)
90
100 Traditional Reactor P=1atm Membrane Reactor P=1atm Traditional Reactor P=1atm Membrane Reactor P=1atm
CO selectivity (%)
100
20
0.3
0.4
0.5
0
0.6
O2/CH4 ratio
Fig. 7 e CO and CO2 selectivities versus O2/CH4 ratio in the MR at different pressures.
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8 Membrane reactor P=7atm Membrane reactor P=1atm Traditional reactor P=1atm
H2/CO ratio
7
6
5
4
0.0
0.1
0.2
0.3
0.4
0.5
0.6
O2/CH4 ratio Fig. 8 e The variation of H2/CO ratio at different O2/CH4 ratios. Fig. 8 presents the influence of the H2/CO ratio by adding O2 in the TR and the MR at 1 atm and 470 C. The H2/CO ratio increases when there is an increase in the addition of O2, especially in the MR. In particular, the H2/CO achieves a maximum value of 6.75 at O2/CH4 ¼ 0.5. This is because the addition of O2 not only increases the methane conversion, but also increases H2 production [6,7,13]. Moreover, the H2/CO ratio increases by increasing the pressure. This increase in the pressure could force H2 to permeate through the membrane, and to also increase the H2 flux so that the H2/CO ratio increases. According to the above discussion, carrying out the reaction in the MR has advantages in increasing H2 production and reducing CO formation. This suffices the crucial requirement that high-purity of hydrogen and minimum CO content (<30 ppm) should serve as a feed to the PEM fuel cell.
3.3.
Autothermicity
Upon addition of oxygen, the partial oxidations of methane can occur as:
Fig. 10 e Enthalpy of the reaction at different H2O and O2 contents as a function of temperature, where a is defined in the following equation: CH4 DaH2 ODð1La=2ÞO2 /CO2 DðaD2ÞH2 .
CH4 þ 0:5O2 /CO þ 2H2 CH4 þ O2 /CO2 þ 2H2
DHð743:15 KÞ ¼ 24:26 kJ=mol DHð743:15Þ ¼ 307:09 kJ=mol
(6) (7)
Fig. 9 shows the energy that is required for the reaction at H2O/CH4 ¼ 1.3 when the ratio of O2/CH4 varies. As shown in Fig. 9, in order to achieve the complete autothermicity, the autothermicity parameter, (QO2 Qlost)/(QwithoutO2 Qlost), is equal to zero, which means no external heating or cooling is required, the O2/CH4 feed ratio at for the MR is less than that for the TR. This is because in the membrane reactor Reaction (7) is prevailing and produces more heat, as well as CO2, for the endothermic steam reforming. Fig. 6 also confirms that the amount of CO2 is higher than that of CO in the MR. From Reactions (6) and (7), we can see that it is clear that heat release occurs 6 times more in Reaction (7) compared to Reaction (6) when 1 mole of O2 at 743.15 K is burned. As a consequence, the MR could conduct autothermal reaction with a lower amount of O2. Fig. 9 also shows that for the MR the O2/CH4 ratio that is required is higher at 7 atm than at
100
CH4 conversion (%)
80
60
40 O 2 /CH 4 =0.4 O 2 /CH 4 =0.3
20
0 340
360
380
400
420
440
460
480
o
Temperature ( C)
Fig. 9 e Comparison of power consumption for the MR and the TR at different O2/CH4 ratios.
Fig. 11 e CH4 conversion versus temperatures from 350 to 470 at O2/CH4 ratio [ 0.3 and 0.4.
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100
CH4 conversion (%)
80
60
40
in exp eram ent 1 repeat in e xpe ram ent 1
20
0
2
3
4
5
6
7
8
P ressure (atm )
Fig. 12 e Stability test of Ni/MgO/Al2O3 catalyst.
proceed, i.e., DH ¼ 0. For comparison, a series of experiments were conducted to study the effect of O2/CH4 (0.3 and 0.4) on methane conversion. Fig. 11 shows that with the feed composition of O2/CH4 being 0.3, XCH4 increases with increasing temperatures due to its endothermic nature. Contrary to the above result, at O2/CH4 ¼ 0.4, XCH4 decreases slowly with the increasing temperature, indicating that an exothermic reaction is starting to happen. Conclusively, in order for an autothermal reaction to process, the CH4/O2 ratio must be 0.3e0.4 and near the ratio of 0.4. The results show almost the same results that were predicted by calculation. This strongly indicates that an optimum feeding gas condition is H2O/CH4 ¼ 1.3, O2/CH4, which is nearly 0.4 at 350 C to 470 C.
3.4. 1 atm. As observed from Fig. 7, the yield of CO at 7 atm is higher than that at 1 atm when the ratio of O2/CH4 ¼ 0.3. It can be seen with that Reaction (6) is more favorable. Thus, there is less energy released in Eq. (6), which leads to a larger amount of O2 that is needed to achieve the autothermal condition. From the above results and discussion, it is concluded that the MR is more easily achieved autothermally than the TR that has a lower amount of O2 at 1 atm. The enthalpy of reaction versus temperature at different H2O and O2 contents is calculated and depicted in Fig. 10. From this figure, we can see that at a feed H2O/CH4 ¼ 1.3, and over the temperature range 350e470 C, the ratio of O2/CH4 should be 0.365e0.37 in order for a complete autothermal reaction to
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Catalyst stability
In order to know whether the activity of catalyst decays or not after a series of reactions over a time span of 200 h, the first experiment was repeated to check the conversion of CH4. As shown in Fig. 12, the activity tests were performed at a pressure that ranged from 3 atm to 7 atm, WHSV ¼ 0.3 1/h and T ¼ 470 C. The methane conversions of both the first experiment and the repeated one followed a similar trend with respect to pressure and the difference between them was almost nil. Thus, the catalyst maintained the same activity as before and showed no sign of decay. Deposited carbon was not observed at low magnification (20.0 k), as shown in Fig. 13((a) and (b)), but some crystallite aggregates appeared after the reaction, as shown in Fig. 13((c) and (d)), at high magnification
Fig. 13 e SEM micrographs of the Ni catalyst before the reaction ((a), (c)) and after the reaction ((b), (d)), at different magnification.
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(150 k). It is believed that by reforming methane with steam and oxygen, there is no chance for carbon fiber formation, as was observed and reported by other researchers [14e16]. Moreover, the analysis with EDS showed no carbon filaments. This clearly proves that the catalyst almost had no carbon deposit after reactions [6].
4.
Conclusions
A dense palladiumesilver membrane reactor was used to perform oxidative steam reforming of methane in order to produce high-purity hydrogen. A delicate power analyzer was employed to measure the power consumption and determine the autothermicity (DH ¼ 0). With the feed of H2O/CH4 ¼ 1.3, and operating temperatures that ranged from 350 to 470 C, autothermicity was achieved at O2/CH4 near 0.4, at which the reactor did not consume power. The optimum methane conversion was 95% with a hydrogen production of 0.093 mol/ m2 s. When the oxygen and steam were co-fed, the catalyst became not susceptible to coke deposition after 200 h of operation.
Acknowledgements The authors like to thank the National Science Council of the Republic of China for the financial support under the Contract No. NSC95-2221-E-035-118.
references
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