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Performance assessment of packed bed reactor and catalytic membrane reactor for steam reforming of methane through metal foam catalyst support
Accepted Manuscript Performance assessment of packed bed reactor and catalytic membrane reactor for steam reforming of methane through metal foam catalyst support Amin Alamdari PII:
S1875-5100(15)30170-0
DOI:
10.1016/j.jngse.2015.09.037
Reference:
JNGSE 1019
To appear in:
Journal of Natural Gas Science and Engineering
Received Date: 3 June 2015 Revised Date:
13 September 2015
Accepted Date: 17 September 2015
Please cite this article as: Alamdari, A., Performance assessment of packed bed reactor and catalytic membrane reactor for steam reforming of methane through metal foam catalyst support, Journal of Natural Gas Science & Engineering (2015), doi: 10.1016/j.jngse.2015.09.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Performance assessment of packed bed reactor and catalytic membrane reactor for steam reforming of methane through metal foam catalyst support
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Amin Alamdari1
1- Young Researchers And Elite club, Robatkarim Branch, Islamic Azad University,
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Robatkarim, Iran Abstract
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A kinetic model for the methane steam reforming has been investigated. The performances are compared between the packed bed reactor and catalytic membrane reactor with metal foam catalyst support. The effects of various variables such as pressure, temperature of the reaction, ratio of methane to steam in feed, thickness of membrane and sweep gas on the total methane
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conversion and hydrogen production were investigated qualitatively. The comparison between these two types of reactors were carried out in the temperature range of 3500C-7500C and pressure range of 2-30 bar. Isothermal modeling has been performed and showed a higher
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performance of the membrane reactor rather than the packed bed reactor. Methane conversion can be reached 100% for lower temperatures than used with industrial PBR, and better
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performances are reached with an increase of the operating pressure. The maximum conversion of methane for the CMR is quickly reached while the conversion evolution observed for the PBR is smoother.
1
∗ Corresponding author. Tel.: +98 9177717995. E-mail address: [email protected] (amin alamdari).
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Keywords: Metal foam catalyst support, Methane steam reforming, Hydrogen production, Modeling, Membrane reactor, Packed bed reactor.
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1. Introduction
Hydrogen production with high quality is widely studied (Coronel et al., 2011). Among all the available resources for the production of hydrogen, natural gas that consists mainly of methane is
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checked as a clean and abundant good option since methane is easily converted into hydrogen (Perez-Moreno et al., 2013). Hydrogen was used as a raw material in a range of chemical,
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petrochemical and metallurgical processes, the production of ammonia and other chemical products such as aniline. Given the rapid advances in fuel cells, hydrogen has become as an alternative to clean energy. Steam reforming of methane is the most common and economical way to produce hydrogen (Li et al., 2008). Hydrogen is a major industrial gas for conversion of
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hydrocarbons, iron ore reduction and clean energy for various applications, such as fuel cells and rocket fuels (Grace et al., 2001). In recent years a significant amount of hydrogen is used as a feedstock in the chemical industry, oil refining and petrochemical industries (Avci et al., 2001).
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Hydrogen is used in hydro treating, hydro cracking process, and applications such as the production of ammonia, methanol synthesis and production of chemicals such as
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pharmaceuticals (Furimsky, 1998; Pena et al., 1996). In addition, due to economic and environmental benefits of hydrogen, hydrogen can be used in the future as an energy source (Jamal and Wyszynski, 1994). In recent years, the demand for hydrogen production is increased (Abashar, 2004 ), this is because of various applications of hydrogen for producing hydrogen and syngas for ammonia and methanol production, hydrocracking and hydrotreating, oxoalcohol and Fischer-Tropsch synthesis (Adris et al., 1997) oil refining, methanol, metallurgy, ammonia,
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space transportation (Abashar, 2004) hydrogenation of fats and oils as well as in hydrocracking and hydro-desulfurization processes, etc. (Yu et al., 2005). Current technologies for hydrogen production include: steam reforming of hydrocarbons, partial oxidation of heavy oil, auto-
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thermal reforming of hydrocarbons, as well as ammonia cracking and electrolysis of water (Koroneos et al., 2004; Chen et al., 2004; Scholz, 1993). Industrial process used to produce hydrogen on a large scale in the process of refining crude oil, is steam reforming of natural gas
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(Goud et al., 2007; Jarosch et al., 2002). Currently, about 50 percent of hydrogen in the world comes through the natural gas steam reforming (Scholz, 1993; Armor, 1999; Perez-Moreno et
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al., 2013). The benefits of steam reforming of hydrocarbons are extraction of hydrogen from water in addition to hydrocarbon and high rate of reaction (Chen et al., 2004). SOFCs (solid oxide fuel cells) work at 10000C by pure H2 fuel or H2–CO mixtures. To produce pure H2 fuel, steam reforming of methane, fossil fuels and biogases are good processes. For this
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reason, many studies have been conducted in order to improve the catalysts for hydrocarbon steam reforming (Dokiya, 2002; Liguras et al., 2004; Minh, 2004; Giroux et al., 2005). The catalyst that deposited on metal foam supports due to the 3D structured supports has many
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advantages such as high hydraulic permeability and porosity, high heat and electroconductivity,
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pore diameter control and structural stiffness (Giroux et al., 2005; Giani et al., 2005; Twigg and Richardson, 2007). The foam supported monolithic catalysts were investigated in various research area of different heterogeneous catalytic reactions such as catalytic combustion of hydrocarbons (Pestryakov et al., 1995), partial oxidation of methanol (Pestryakov et al., 2002), methane conversion (Richardson et al., 2003), exhaust pollution control (Leonov et al., 1998) and solar methane reforming (Kodama et al., 2003).
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These catalyst supports are increasing heat and mass transfer and favorable for heterogeneous catalytic processes carrying out at high flow rates (GHSV up to 105 1/h) (Smorygo et al., 2009). Studies have shown that if the process be done in catalytic membrane reactors, operating
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conditions such as reaction temperature and pressure are adjusted (Oklanay et al., 1998; Prokopiev et al., 1992; Lin et al., 2003; Siriwardane et al., 2000). Tong and Matsumura (2005) examined the effect of catalytic activity and the flow rate of the reactants on the conversion of
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methane and hydrogen separation in membrane reactors. Studies show that the passage of hydrogen through the membrane, the activity of the catalyst for the separation of hydrogen and
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its production rate are effective. Modeling of methane steam reforming process in a ceramic membrane reactor has done by Yu et al., (2008). They study the effects of temperature, pressure and flow rate of gas reactants and methane conversion and hydrogen production. Fernandes and Soares (2006) modelled methane steam reforming process in the palladium membrane reactor
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and concluded that the methane conversion in membrane reactors was greater than fixed bed reactors and operating conditions are more balanced. Palladium membrane is a membrane which has high selectivity and permeability to hydrogen.
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In the membrane reactors, hydrogen production is greater than conventional reactors due to the
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membrane throughput of hydrogen resulted in changes in the balance and displacement reaction products at lower temperatures (Le Chatelier's Principle) (Barbieri et al., 1997). The use of steam as sweep gas improve the performance of the process (Madia et al., 1999; Kikuchi et al., 1991). Although the membranes have not been very widespread in recent years they have found wide use in oil and gas industry. Oertel et al. (1987) examined hydrogen production in a palladium membrane reactor with a thickness of 100 micrometers. This work is one of the first tasks in the field of natural gas steam reforming. The high thickness of the membrane can reduce the
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diffusion of hydrogen and enhance the temperature of reaction (700-800 0C). Uemiya et al. 1991 investigated hydrogen production using Pd membrane reactor based on porous ceramic pipes at a suitable temperature rather than thicker membranes. They used a sweep gas to reduce pressure of
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hydrogen side and enhancement the rates of production. Shu et al. (1995) by using stainless steel pipes with palladium and palladium-silver membrane were reached to 63% total methane conversion at temperature of 500°C. Nam et al. (2000) increased methane conversion to 80% by
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using a palladium-ruthenium membrane at a temperature of 500°C. They modeled the effects of different variables such as reaction temperature and thickness of the membrane and results
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showed that the full conversion of methane is possible at 600°C. Tong et al. (2005) investigated the diffusion flux in a laboratory research for methane steam reforming in Palladium- stainless steel membrane (6 micro-meters thickness) and concluded that the diffusion flux of hydrogen achieved 0.26 mole/(m2s) at a temperature of 500°C and a pressure of 100 kPa. Gallucci et al.
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2006 by using silver- palladium membrane with a thickness of 50 micrometers examined the effect of temperature and steam to methane ratio on the methane conversion. They also investigated countercurrent and cocurrent flow of the reactants and sweep gas on methane
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conversion and hydrogen production and concluded that the reactor type of countercurrent flow was better (Gallucci et al., 2004). They also studied mathematical modeling for steam reforming
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process in the Pd membrane reactor. In this study, the effect of different variables such as temperature, pressure and shell membrane thickness on methane conversion was investigated (Gallucci et al., 2004). Hacarlioglu et al. 2006 were studied the rate of reaction in steam reforming of natural gas at high pressures in the ceramic membrane reactor. The aim of this work is utilization of metal foams of Ni, Fe-Cr steel and Ni-Al intermetallic as a catalyst support for methane steam reforming in a packed bed reactor and the catalytic membrane
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reactor in order to methane processing and hydrogen production due to advantages of metal foam catalyst support.
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2. Process description Reforming reaction is endothermic and the equilibrium is limited. The main feed for the process of reforming is natural gas, which is usually mixed with steam and placed in contact with the
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catalyst. If the temperature is high enough, hydrocarbon reacts with steam and produce a mixture of partially reacted hydrocarbons, carbon monoxide and hydrogen. When the water-gas shift
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reaction is done at the same time, carbon dioxide is also present in the product mixture (Jarosch et al., 2002). On an industrial scale, process of steam reforming done on fixed bed catalyst packed into the tube reactor. With replacement pipes in a furnace high quantity heat for reforming reaction can be provided (Jarosch et al., 2002). Since the reaction is endothermic, the
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reaction is done at high temperature (Gallucci et al., 2004). Operational temperature and pressure are in the range from 700–900◦C and 2–3 MPa, respectively, and steam to carbon ratio is in the range 3:1–5:1 (Jarosch et al., 2002). Steam reforming includes reforming and water gas shift
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reactions that both reactions are reversible (Jarosch et al., 2002) which were investigated by Xu and Froment (1989):
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CH 4 + H 2O ⇔ CO + 3H 2 CO + H 2O ⇔ CO2 + H 2
∆H 298
∆H 298
CH 4 + 2 H 2O ⇔ CO2 + 4 H 2
K
K
= 206 kj / mol
= −41 kj / mol
∆H 298
K
= 165 kj / mol
(1) (2) (3)
Reforming process is endothermic and the thermodynamic equilibrium is limited. Therefore, developing a separation process based a membrane can increase conversion of the process. When
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hydrogen is removed selectively from the reactor, the chemical balance goes to the product side and makes more amount of methane convert to hydrogen and carbon monoxide (Jarosch et al.,
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2002). The combination of selective membrane increases efficiency of hydrogen production at low temperatures. Selective removal of hydrogen causes the equilibrium conditions do not reach and create higher conversion. The main advantage of using membranes is a sharp drop in the reaction
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temperature from 900oC to 500oC. Recently, several articles in the field of membrane reactor
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performance were evaluated in comparison with conventional reactors. Membranes based on platinum have high selectivity to hydrogen. Now the technology on an industrial scale using a membrane reactor for the steam reforming process is not used (De Falco, 2008). The conventional steam reforming process has disadvantages such as low effectiveness factor of
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the catalyst, a high temperature gradient, low efficiency of the catalyst and catalyst coking (Adris
(Grace, 2001).
3. Reaction model
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et al., 1996). Various modifications to improve the performance of this process is presented
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Fig 1 shows schematically the membrane reactor for steam reforming. Various reaction rates suggested for the methane steam reforming (Jarosch, 2002). Elnashaie and Elshishini (1993) have been reviewed some of the kinetics for steam reforming process over nickel catalysts in the literature, and concluded that kinetic equations developed by Xu and Froment (1989) are more suitable for this process. Kinetic relationships for reactions that have been used by Xu and Froment (1989) are as follows:
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Fig.1.
r3 =
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PH PCO k2 ( PCO PH 2O − 2 2 ) PH 2 K eq 2
(5)
DEN 2 PH42 PCO2 k3 2 ( P P − ) CH 4 H 2 O PH3.52 K eq 3
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r2 =
(4)
DEN 2
DEN 2
where,
(6)
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r1 =
PH32 PCO k1 ( PCH 4 PH 2O − ) PH2.52 K eq1
DEN = 1+ KCO PCO + KH2 PH2 + KCH4 + KH2O PH2O / PH2 k j = Aj e
− E j / RT
j = 1, 2,3
(8)
H 2O
(9)
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Ki = Bi e−∆Hi / RT i = CO, H 2 , CH 4 ,
(7)
In the above equations, ki is the coefficient rate of reaction i; Ki is the equilibrium constant of the reaction i or adsorption coefficient of component i; Pi is the partial pressure (atm); and ri is the
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rate of reaction i (mol/h gcat) (Fernandes and Soares, 2006). Table 1 shows the constants of
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reactions, pre-exponential and activation energy of reactions (Hara et al., 2008).
Table 1.
The conversion of component i on the reaction side and the dimensionless flow rate of hydrogen on the permeate side relative to initial flow rate of methane are given by:
Xi = 1−
YH 2 =
FH 2 0 FCH 4
(11)
Fi Fi 0
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where, Fi0 is the flow rate of component i in the feedstock (mol/h). The partial pressures of each species were calculated with the following equations (Fernandes and Soares, 2006): PCH 4 = (1 − X CH ) / σ
(12)
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4
PH 2O = (θ H 2O − X CH 4 − X CO2 ) / σ
(13)
PCO = (θCO + X CH 4 − X CO2 ) / σ
(14)
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PCO2 = (θCO2 + X CO2 ) / σ PH 2 = (θ H 2 + 3 X CH 4 − X CO2 − YH 2 ) / σ
with,
σ=
1 + θ H 2O + θCO + θCO2 + θ H 2 PT
, θj =
Fj0
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(15)
0 FCH 4
(16)
(17)
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where, PT is the total pressure, Pi is the partial pressure of component i. A series of independent and steady-state mass balances to calculate the methane conversion, carbon dioxide conversion and hydrogen production in a tubular membrane reactor is used. The changes of conversion of
dz dX H 2O dz dX H 2 dz
=
ρb A
(r1 + r2 )
(18)
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dX CH 4
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each component along the reactor length are obtained by Fernandes and Soares (2006):
(r1 + r2 + 2r3 )
(19)
=
(3r1 + r2 + 4r3 )
(20)
=
0 FCH 4
ρb A
0 FCH 4
ρb A
0 FCH 4
dX CO ρb A = 0 (r1 − r2 ) dz FCH 4
(21)
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dz dYH 2 dz
=
=
ρb A
(r2 + r3 )
(22)
2π Rm β 0.5 ( PH 2 − PP0.5 ) 0 δ GCH 4
(23)
0 FCH 4
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dX CO2
The appropriate boundary conditions to solve the above equations are:
at z = 0 : X CH 4 = X CO2 = YH 2 = 0
(24)
kg ); GCH40 is the methane flow rate 3 m
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where A is the area of the tube (m2); ρb is the bed density (
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at the inlet (mole/h m2); Pp is the partial pressure of hydrogen in the permeate zone (atm); Rm is the radius of the membrane (m); β is the permeance of the membrane (m3/m h atm 0.5); δ is the membrane thickness (m). The basic assumptions for equation 18 to 24 are steady state process, plug flow, isothermal process and the pressure drop in the reactor is negligible. Table 2 shows
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the operating conditions and reactor parameters;
Table 2.
Many researchers have studied a mathematical expression for hydrogen permeability. The
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hydrogen permeability of the membrane depends on the quality and composition that can vary according to the type of alloy. An Arrhenius expression can be used to explain this variable.
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Ayturk et al. (2009) presented an Arrhenius expression for β :
β = Q0 exp(−
E0 ) RT
(25)
The pre exponential factor (Q0) and activation energy (E0) are 6322.7 m3 µ m.m −2 .hr.bar 0.5 and 15.6 kJ / mol , respectively. 4. Numerical solution
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At first, kinetic values (adsorption constants, reaction rate constants, equilibrium constants and permeability of hydrogen) were calculated. These parameters depend on temperature. The system is nodulation and differential equations were solved with MATLAB functions.
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Conversion of methane, carbon dioxide and hydrogen stored in the matrix. A 4th order Runge Kutta algorithm was used for the numerical solution of differential equations. The step size of the
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4th order Runge Kutta is 0.001.
5.1. Model validation
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5. Results and discussion
Figure 2 shows the experimental results of Smorygo et al. (2009) and results obtained from the model presented in this study.
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Fig. 2.
Numerical results of experimental work of Smorygo et al. (2009) and modeling of this work and also numerical values of error between them are provided in Table 3. Results obtained from the
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model with an acceptable error comply with the results of laboratory. Errors in Table 3 were calculated using the following formula: ( X CH 4 mod − X CH 4 exp )
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Error =
(26)
X CH 4 exp
5.2. Effect of temperature and pressure
Table. 3.
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In the mathematical modeling, the temperature remains constant. At first, the effect of temperature on conversion of methane is investigated. Figure 3 shows the total conversion of methane along the length of the reactor for CMR and PBR at T=5500C-6500C, P=1 bar kg . The total methane conversion in the reactor length for CMR is greater than m3
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and ρ = 2230
PBR. As shown in this figure, with increasing the temperature, the conversion of methane is
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increasing. The methane steam reforming is endothermic and with the increasing in temperature, the reaction moves to the product side and the consumption of methane is increasing (according
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to Le Chatelier's principle). So we conclude the maximum conversion will be obtained in the temperature that is tolerable for the reactor.
Fig.3.
Performance analysis was performed by simulating the reactor model equations for parameters
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such as temperature, pressure and its effect on methane conversion in both reactors. One of the major advantages of membrane reactor is a greater conversion of methane by removing hydrogen at medium temperatures (example 5000C). Effects of temperature and pressure of reaction on
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conversion of methane and carbon dioxide around the various catalysts are shown in Fig.4 and Fig.5. At constant temperature, with increasing the pressure the methane conversion is
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increasing, but in PBR with increasing the pressure the methane conversion is decreasing. In CMR the maximum conversion is obtained in 30 bars and 5300C and with decreasing pressure the maximum conversion is obtaining in temperature higher than 5300C. However, in PBR in all temperature the maximum conversion obtained in minimum pressure. The methane conversion is increasing with increasing the temperature in packed bed reactor and membrane reactor.
Fig.4.
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Fig.5. Since methane steam reforming reaction is strongly endothermic in thermodynamically scene, it is better to be done at high temperatures and low pressures. The results shown in Figure 4 imply
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that the overall conversion of methane to PBR increased by reducing pressure for all temperature ranges, since high pressure not only increases forward reaction but also effectively increases the rate of backward reaction in methane steam reforming.
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The high conversions resulted for PBR in temperature assessment (%95 for case a, %96 for b, %97 for c, %97.5 for d and %98 for e, at 7500C) due to low pressure gradient (one bar) that is
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used and it is consistent with the work of Oertel et al. (1987). Low partial pressure in the reaction may lead to the decomposition of methane to carbon and hydrogen and the conversion of methane is increasing. The overall conversion of methane in pressures 2,5,10, 20 and 30 bars, respectively 95%, 85%, %73, %59 and 50% for a, 96%, 88%, %75, %60 and 52% for b, 97%,
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88.5%, %76, %61 and 52.5% for c, 97.5%, 89%, %76.5, %61.2 and 53% for d and 98%, 89.5%, %77, %61.9 and 53.2% for e, respectively. Typically, industrial reformers operate at high
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pressures 30-40 bars.
When the driving force for hydrogen permeation increases through higher pressures in the side
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reaction, the partial pressure of hydrogen lead to enhance the methane conversion of CMR on all ranges of temperature, as shown in Figure 4. While the overall conversion of methane for PBR in 500 0C was below 20% above 10 bar, in CMR full conversion was achieved on various catalysts which can be seen from Figure 4.
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In PBR with increasing in temperature, the conversion of methane is increasing and the highest conversion is achieved in the high temperature, but in CMR the highest conversion is resulted in
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medium temperature. The ∆ index shows the difference between the overall conversion of methane in the CMR and PBR. A function of operating parameters and process parameters are defined as follows: ∆ = X CH 4 , MR − X CH 4 , PBR
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(27)
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Figure 6 shows the effects of temperature and pressure on the index. Based on the definition, the optimum conditions were obtained for the high pressures of 30 bar and a low temperature range as in Fig. 6. The temperature for the various catalysts was 600 0C, 575 0C, 560 0C, 550 0C and 545 0C, and with increasing the density of catalyst the temperature of reaction is reduced.
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Fig.6.
Fig 7 shows the hydrogen flux production (kmol/h) in a CMR reactor with various catalysts. For 1 kmol/h methane in the entrance of the reactor, nearly 4 kmol/hr hydrogen is produced. The
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maximum of hydrogen production has resulted in the highest pressure (30 bar). The temperature that is required for obtaining the maximum production of hydrogen is 630 0C, 575 0C, 573 0C,
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570 0C and 568 0C, for cases a, b, c, d and e, respectively.
Fig.7.
5.3. Effect of steam to methane ratio In order to decrease coke formation and deactivation of catalyst and enhance the total methane conversion, the process of methane steam reforming is done with excess steam. Fig 8 shows the
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effect of steam to methane ratio (m) on the total methane conversion in the range of 1 to 10, at 5000C and a pressure range of 5-30 bar. As shown in Fig 8, the maximum of total methane conversion was resulted in a ratio of 2-3. The plot of ∆ -index indicated that for m>3, the
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performance is decreased. The high ratio of steam to methane (m>3) need to excess amount of
of operating conditions in Fig 8.
5.4. Effect of membrane thickness
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Fig. 8.
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energy for production of steam in order to develop the yield of the process. We can use the range
The effect of membrane thickness on the total methane conversion is shown in Fig 9 at 5000C, a pressure range of 5-30 bar and steam to methane ratio of 3. As shown in Fig 9, with decreasing in thickness of membrane because of the enhancement in hydrogen removal, the total methane
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conversion was increased.
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Fig. 9.
5.5. Effect of sweep gas
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The effect of sweep gas ratio (s) was investigated at 5000C and a pressure of 15 and 30 bars. By increasing the sweep gas ratio to the permeate side of CMR, a significant effect was resulted in the total methane conversion due to enhance the driving force for the hydrogen permeation flux. The plot indicated that the effect of sweep gas on CMR was suitable to ratio of almost 10.
Fig. 10.
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6. Conclusion An isothermal mathematical model was provided to compare the performance of PBR and CMR reactors with metal foam catalyst. The highest performance was obtained for a membrane reactor
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with specific parameters. To analysis the process, the ∆ -index was presented. Based on the ∆ index analysis, the optimum conditions for CMR were obtained within the operating conditions ranges of temperature 565–600 0C, pressure >=20 bar, thickness <10 micron, steam to methane
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ratio 2< m < 3 and sweep factor s >= 10. The maximum of hydrogen production is resulting in
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the highest pressure (30 bar) and a temperature range that was required for obtaining the
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maximum production of hydrogen was 568 0C -630 0C through metal foam catalyst support.
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Nomenclature coefficient rate of reaction i
Ki
equilibrium constant of reaction i or adsorption coefficient of component i
ri
rate of reaction i (mol/h gcat)
Fi0
flow rate of component i in the feedstock (mol/h)
PT
total pressure
Pi
partial pressure of component i,
Xi
conversion of component i, and
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ki
A
area of the tube (m2)
ρb
bed density (
kg ) m3
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YH2 ratio of hydrogen flux to that of initial methane flow rate
GCH40 methane flow rate in the inlet (mole/h m2) partial pressure of hydrogen in the permeate zone (atm)
Rm
radius of the membrane (m)
β
permeance of the membrane (m3/m h atm 0.5)
δ
membrane thickness (m).
∆
difference between the overall conversion of methane in the CMR and PBR (-)
s
sweep gas ratio (-)
m
steam to methane ratio (-)
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Pp
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pre exponential factor ( m3 µ m.m −2 .hr.bar 0.5 )
E0
activation energy ( kJ / mol )
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Q0
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comparative simulation study of methane steam reforming in a porous ceramic membrane reactor using nitrogen and steam as sweep gases. International Journal of Hydrogen Energy. 33, 685-
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List of Figures:
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Fig.1. Scheme of catalytic membrane reactor. Fig.2. Comparison between results of experimental work of Smorygo et al. (2009) and this work. Fig.3. The total conversion of methane along the length of CMR and PBR at T=5500C- 6500C, kg . m3
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P=1 bar and ρ = 2230
various catalysts a) ρ = 0.4
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Fig.4. Effects of temperature and pressure on the methane conversion in PBR and CMR with g 6.5Ru/26(LaPrMnCrO/NiO/YSZ) cm3
g 3.0Ru/13.8(LaPrMnCrO/NiO/YSZ) cm3
c) ρ = 1.37
g 2.4Ru/11.6(LaPrMnCrO/NiO/YSZ) cm3
g 2.0Ru/7.3(LaPrMnCrO/NiO/YSZ) cm3
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e) ρ = 2.23
g 2.8Ru/10.7(LaPrMnCrO/NiO/YSZ) cm 3
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b) ρ = 1.17
Fig.5. Effects of temperature and pressure on the carbon dioxide conversion in PBR and CMR with various catalysts a) ρ = 0.4
b) ρ = 1.17
g 6.5Ru/26(LaPrMnCrO/NiO/YSZ) cm3
g 3.0Ru/13.8(LaPrMnCrO/NiO/YSZ) cm3
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g 2.4Ru/11.6(LaPrMnCrO/NiO/YSZ) cm3
d) ρ = 1.79
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e) ρ = 2.23
g 2.0Ru/7.3(LaPrMnCrO/NiO/YSZ) cm3
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c) ρ = 1.37
g 6.5Ru/26(LaPrMnCrO/NiO/YSZ) cm3
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various catalysts a) ρ = 0.4
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Fig.6. ∆ index for describing the impact of temperature and pressure in PBR and CMR with
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c) ρ = 1.37
g 2.4Ru/11.6(LaPrMnCrO/NiO/YSZ) cm3
d) ρ = 1.79
g 2.8Ru/10.7(LaPrMnCrO/NiO/YSZ) cm 3 g 2.0Ru/7.3(LaPrMnCrO/NiO/YSZ) cm3
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b) ρ = 1.17
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Fig.7. Hydrogen production in CMR with various catalysts a) ρ = 0.4
g 6.5Ru/26(LaPrMnCrO/NiO/YSZ) cm3
b) ρ = 1.17
g 3.0Ru/13.8(LaPrMnCrO/NiO/YSZ) cm3
c) ρ = 1.37
g 2.4Ru/11.6(LaPrMnCrO/NiO/YSZ) cm3
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g 2.8Ru/10.7(LaPrMnCrO/NiO/YSZ) cm3
e) ρ = 2.23
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d) ρ = 1.79
Fig. 8. The effect of steam to methane ratio (m) on performance of CMR (T=5000C, PT=30 bar , Pp=1 bar, δ =1 micron and L=5 m)
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Fig.9. The effect of membrane thickness ( δ ) on performance of CMR (T=5000C, PT=30 bar ,
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Fig. 10. The effect of sweep gas (s) on performance of CMR (T= 5000C, PT= 30 bar, Pp= 1 bar
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List of Tables: Table 1. Kinetic parameters for the reactions involved in the methane steam reforming.
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Table 2. Operating conditions and reactor parameters.
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Table 3. Model validation of this work with experimental results of Smorygo et al. (2009)
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Table 1. Kinetic parameters for the reactions involved in the methane steam reforming. Pre- exponential factor (Aj or Bj)
Ej or ∆H i
k1
3.711× 1011 mol MPa 0.5 / g − cat s
240.1
k2
5.431×103 mol / MPa g − cat s
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Constant of reactions
67.13
243.9 8.960 ×1010 mol MPa 0.5 / g − cat s
k3
KCO
8.23 × 10 −4 MPa −1
KH2
6.12 × 10 −8 MPa −1
KCH4
6.65 × 10 −3 MPa −1
-38.28
KH2O
1.77 × 105 MPa −1
88.68
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Reaction side total pressure PT (bar)
2-30
Permeate side total pressure Pp (bar)
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Methane flow rate G0(CH4) (kmol/hr) Methane/steam ratio Membrane thickness δ (m)
1 1/3 10-6
Catalyst density (gcat/cm3)
g cm3
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a) ρ = 0.4
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Table 2. Operating conditions and reactor parameters.
6.5Ru/26(LaPrMnCrO/NiO/YSZ) g cm3
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3.0Ru/13.8(LaPrMnCrO/NiO/YSZ) c) ρ = 1.37
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2.4Ru/11.6(LaPrMnCrO/NiO/YSZ) d) ρ = 1.79
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2.8Ru/10.7(LaPrMnCrO/NiO/YSZ) e) ρ = 2.23
g cm3
2.0Ru/7.3(LaPrMnCrO/NiO/YSZ)
Reactor length L (m)
5
Tube internal radius (m)
0.1016
Tube external radius (m)
0.1322
Membrane radius Rm (m)
0.0203
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Table. 3. Model validation of this work with experimental results of Smorygo et al.
450
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1- A kinetic model for the methane steam reforming has been investigated. 2- Performances are compared between the PBR and CMR with metal foam catalyst support.
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3- Modelling showed a higher performance for CMR rather than PBR.
S1875-5100(15)30170-0
DOI:
10.1016/j.jngse.2015.09.037
Reference:
JNGSE 1019
To appear in:
Journal of Natural Gas Science and Engineering
Received Date: 3 June 2015 Revised Date:
13 September 2015
Accepted Date: 17 September 2015
Please cite this article as: Alamdari, A., Performance assessment of packed bed reactor and catalytic membrane reactor for steam reforming of methane through metal foam catalyst support, Journal of Natural Gas Science & Engineering (2015), doi: 10.1016/j.jngse.2015.09.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Performance assessment of packed bed reactor and catalytic membrane reactor for steam reforming of methane through metal foam catalyst support
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Amin Alamdari1
1- Young Researchers And Elite club, Robatkarim Branch, Islamic Azad University,
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Robatkarim, Iran Abstract
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A kinetic model for the methane steam reforming has been investigated. The performances are compared between the packed bed reactor and catalytic membrane reactor with metal foam catalyst support. The effects of various variables such as pressure, temperature of the reaction, ratio of methane to steam in feed, thickness of membrane and sweep gas on the total methane
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conversion and hydrogen production were investigated qualitatively. The comparison between these two types of reactors were carried out in the temperature range of 3500C-7500C and pressure range of 2-30 bar. Isothermal modeling has been performed and showed a higher
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performance of the membrane reactor rather than the packed bed reactor. Methane conversion can be reached 100% for lower temperatures than used with industrial PBR, and better
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performances are reached with an increase of the operating pressure. The maximum conversion of methane for the CMR is quickly reached while the conversion evolution observed for the PBR is smoother.
1
∗ Corresponding author. Tel.: +98 9177717995. E-mail address: [email protected] (amin alamdari).
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Keywords: Metal foam catalyst support, Methane steam reforming, Hydrogen production, Modeling, Membrane reactor, Packed bed reactor.
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1. Introduction
Hydrogen production with high quality is widely studied (Coronel et al., 2011). Among all the available resources for the production of hydrogen, natural gas that consists mainly of methane is
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checked as a clean and abundant good option since methane is easily converted into hydrogen (Perez-Moreno et al., 2013). Hydrogen was used as a raw material in a range of chemical,
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petrochemical and metallurgical processes, the production of ammonia and other chemical products such as aniline. Given the rapid advances in fuel cells, hydrogen has become as an alternative to clean energy. Steam reforming of methane is the most common and economical way to produce hydrogen (Li et al., 2008). Hydrogen is a major industrial gas for conversion of
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hydrocarbons, iron ore reduction and clean energy for various applications, such as fuel cells and rocket fuels (Grace et al., 2001). In recent years a significant amount of hydrogen is used as a feedstock in the chemical industry, oil refining and petrochemical industries (Avci et al., 2001).
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Hydrogen is used in hydro treating, hydro cracking process, and applications such as the production of ammonia, methanol synthesis and production of chemicals such as
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pharmaceuticals (Furimsky, 1998; Pena et al., 1996). In addition, due to economic and environmental benefits of hydrogen, hydrogen can be used in the future as an energy source (Jamal and Wyszynski, 1994). In recent years, the demand for hydrogen production is increased (Abashar, 2004 ), this is because of various applications of hydrogen for producing hydrogen and syngas for ammonia and methanol production, hydrocracking and hydrotreating, oxoalcohol and Fischer-Tropsch synthesis (Adris et al., 1997) oil refining, methanol, metallurgy, ammonia,
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space transportation (Abashar, 2004) hydrogenation of fats and oils as well as in hydrocracking and hydro-desulfurization processes, etc. (Yu et al., 2005). Current technologies for hydrogen production include: steam reforming of hydrocarbons, partial oxidation of heavy oil, auto-
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thermal reforming of hydrocarbons, as well as ammonia cracking and electrolysis of water (Koroneos et al., 2004; Chen et al., 2004; Scholz, 1993). Industrial process used to produce hydrogen on a large scale in the process of refining crude oil, is steam reforming of natural gas
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(Goud et al., 2007; Jarosch et al., 2002). Currently, about 50 percent of hydrogen in the world comes through the natural gas steam reforming (Scholz, 1993; Armor, 1999; Perez-Moreno et
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al., 2013). The benefits of steam reforming of hydrocarbons are extraction of hydrogen from water in addition to hydrocarbon and high rate of reaction (Chen et al., 2004). SOFCs (solid oxide fuel cells) work at 10000C by pure H2 fuel or H2–CO mixtures. To produce pure H2 fuel, steam reforming of methane, fossil fuels and biogases are good processes. For this
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reason, many studies have been conducted in order to improve the catalysts for hydrocarbon steam reforming (Dokiya, 2002; Liguras et al., 2004; Minh, 2004; Giroux et al., 2005). The catalyst that deposited on metal foam supports due to the 3D structured supports has many
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advantages such as high hydraulic permeability and porosity, high heat and electroconductivity,
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pore diameter control and structural stiffness (Giroux et al., 2005; Giani et al., 2005; Twigg and Richardson, 2007). The foam supported monolithic catalysts were investigated in various research area of different heterogeneous catalytic reactions such as catalytic combustion of hydrocarbons (Pestryakov et al., 1995), partial oxidation of methanol (Pestryakov et al., 2002), methane conversion (Richardson et al., 2003), exhaust pollution control (Leonov et al., 1998) and solar methane reforming (Kodama et al., 2003).
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These catalyst supports are increasing heat and mass transfer and favorable for heterogeneous catalytic processes carrying out at high flow rates (GHSV up to 105 1/h) (Smorygo et al., 2009). Studies have shown that if the process be done in catalytic membrane reactors, operating
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conditions such as reaction temperature and pressure are adjusted (Oklanay et al., 1998; Prokopiev et al., 1992; Lin et al., 2003; Siriwardane et al., 2000). Tong and Matsumura (2005) examined the effect of catalytic activity and the flow rate of the reactants on the conversion of
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methane and hydrogen separation in membrane reactors. Studies show that the passage of hydrogen through the membrane, the activity of the catalyst for the separation of hydrogen and
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its production rate are effective. Modeling of methane steam reforming process in a ceramic membrane reactor has done by Yu et al., (2008). They study the effects of temperature, pressure and flow rate of gas reactants and methane conversion and hydrogen production. Fernandes and Soares (2006) modelled methane steam reforming process in the palladium membrane reactor
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and concluded that the methane conversion in membrane reactors was greater than fixed bed reactors and operating conditions are more balanced. Palladium membrane is a membrane which has high selectivity and permeability to hydrogen.
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In the membrane reactors, hydrogen production is greater than conventional reactors due to the
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membrane throughput of hydrogen resulted in changes in the balance and displacement reaction products at lower temperatures (Le Chatelier's Principle) (Barbieri et al., 1997). The use of steam as sweep gas improve the performance of the process (Madia et al., 1999; Kikuchi et al., 1991). Although the membranes have not been very widespread in recent years they have found wide use in oil and gas industry. Oertel et al. (1987) examined hydrogen production in a palladium membrane reactor with a thickness of 100 micrometers. This work is one of the first tasks in the field of natural gas steam reforming. The high thickness of the membrane can reduce the
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diffusion of hydrogen and enhance the temperature of reaction (700-800 0C). Uemiya et al. 1991 investigated hydrogen production using Pd membrane reactor based on porous ceramic pipes at a suitable temperature rather than thicker membranes. They used a sweep gas to reduce pressure of
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hydrogen side and enhancement the rates of production. Shu et al. (1995) by using stainless steel pipes with palladium and palladium-silver membrane were reached to 63% total methane conversion at temperature of 500°C. Nam et al. (2000) increased methane conversion to 80% by
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using a palladium-ruthenium membrane at a temperature of 500°C. They modeled the effects of different variables such as reaction temperature and thickness of the membrane and results
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showed that the full conversion of methane is possible at 600°C. Tong et al. (2005) investigated the diffusion flux in a laboratory research for methane steam reforming in Palladium- stainless steel membrane (6 micro-meters thickness) and concluded that the diffusion flux of hydrogen achieved 0.26 mole/(m2s) at a temperature of 500°C and a pressure of 100 kPa. Gallucci et al.
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2006 by using silver- palladium membrane with a thickness of 50 micrometers examined the effect of temperature and steam to methane ratio on the methane conversion. They also investigated countercurrent and cocurrent flow of the reactants and sweep gas on methane
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conversion and hydrogen production and concluded that the reactor type of countercurrent flow was better (Gallucci et al., 2004). They also studied mathematical modeling for steam reforming
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process in the Pd membrane reactor. In this study, the effect of different variables such as temperature, pressure and shell membrane thickness on methane conversion was investigated (Gallucci et al., 2004). Hacarlioglu et al. 2006 were studied the rate of reaction in steam reforming of natural gas at high pressures in the ceramic membrane reactor. The aim of this work is utilization of metal foams of Ni, Fe-Cr steel and Ni-Al intermetallic as a catalyst support for methane steam reforming in a packed bed reactor and the catalytic membrane
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reactor in order to methane processing and hydrogen production due to advantages of metal foam catalyst support.
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2. Process description Reforming reaction is endothermic and the equilibrium is limited. The main feed for the process of reforming is natural gas, which is usually mixed with steam and placed in contact with the
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catalyst. If the temperature is high enough, hydrocarbon reacts with steam and produce a mixture of partially reacted hydrocarbons, carbon monoxide and hydrogen. When the water-gas shift
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reaction is done at the same time, carbon dioxide is also present in the product mixture (Jarosch et al., 2002). On an industrial scale, process of steam reforming done on fixed bed catalyst packed into the tube reactor. With replacement pipes in a furnace high quantity heat for reforming reaction can be provided (Jarosch et al., 2002). Since the reaction is endothermic, the
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reaction is done at high temperature (Gallucci et al., 2004). Operational temperature and pressure are in the range from 700–900◦C and 2–3 MPa, respectively, and steam to carbon ratio is in the range 3:1–5:1 (Jarosch et al., 2002). Steam reforming includes reforming and water gas shift
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reactions that both reactions are reversible (Jarosch et al., 2002) which were investigated by Xu and Froment (1989):
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CH 4 + H 2O ⇔ CO + 3H 2 CO + H 2O ⇔ CO2 + H 2
∆H 298
∆H 298
CH 4 + 2 H 2O ⇔ CO2 + 4 H 2
K
K
= 206 kj / mol
= −41 kj / mol
∆H 298
K
= 165 kj / mol
(1) (2) (3)
Reforming process is endothermic and the thermodynamic equilibrium is limited. Therefore, developing a separation process based a membrane can increase conversion of the process. When
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hydrogen is removed selectively from the reactor, the chemical balance goes to the product side and makes more amount of methane convert to hydrogen and carbon monoxide (Jarosch et al.,
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2002). The combination of selective membrane increases efficiency of hydrogen production at low temperatures. Selective removal of hydrogen causes the equilibrium conditions do not reach and create higher conversion. The main advantage of using membranes is a sharp drop in the reaction
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temperature from 900oC to 500oC. Recently, several articles in the field of membrane reactor
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performance were evaluated in comparison with conventional reactors. Membranes based on platinum have high selectivity to hydrogen. Now the technology on an industrial scale using a membrane reactor for the steam reforming process is not used (De Falco, 2008). The conventional steam reforming process has disadvantages such as low effectiveness factor of
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the catalyst, a high temperature gradient, low efficiency of the catalyst and catalyst coking (Adris
(Grace, 2001).
3. Reaction model
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et al., 1996). Various modifications to improve the performance of this process is presented
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Fig 1 shows schematically the membrane reactor for steam reforming. Various reaction rates suggested for the methane steam reforming (Jarosch, 2002). Elnashaie and Elshishini (1993) have been reviewed some of the kinetics for steam reforming process over nickel catalysts in the literature, and concluded that kinetic equations developed by Xu and Froment (1989) are more suitable for this process. Kinetic relationships for reactions that have been used by Xu and Froment (1989) are as follows:
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Fig.1.
r3 =
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PH PCO k2 ( PCO PH 2O − 2 2 ) PH 2 K eq 2
(5)
DEN 2 PH42 PCO2 k3 2 ( P P − ) CH 4 H 2 O PH3.52 K eq 3
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r2 =
(4)
DEN 2
DEN 2
where,
(6)
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r1 =
PH32 PCO k1 ( PCH 4 PH 2O − ) PH2.52 K eq1
DEN = 1+ KCO PCO + KH2 PH2 + KCH4 + KH2O PH2O / PH2 k j = Aj e
− E j / RT
j = 1, 2,3
(8)
H 2O
(9)
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Ki = Bi e−∆Hi / RT i = CO, H 2 , CH 4 ,
(7)
In the above equations, ki is the coefficient rate of reaction i; Ki is the equilibrium constant of the reaction i or adsorption coefficient of component i; Pi is the partial pressure (atm); and ri is the
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rate of reaction i (mol/h gcat) (Fernandes and Soares, 2006). Table 1 shows the constants of
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reactions, pre-exponential and activation energy of reactions (Hara et al., 2008).
Table 1.
The conversion of component i on the reaction side and the dimensionless flow rate of hydrogen on the permeate side relative to initial flow rate of methane are given by:
Xi = 1−
YH 2 =
FH 2 0 FCH 4
(11)
Fi Fi 0
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where, Fi0 is the flow rate of component i in the feedstock (mol/h). The partial pressures of each species were calculated with the following equations (Fernandes and Soares, 2006): PCH 4 = (1 − X CH ) / σ
(12)
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4
PH 2O = (θ H 2O − X CH 4 − X CO2 ) / σ
(13)
PCO = (θCO + X CH 4 − X CO2 ) / σ
(14)
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PCO2 = (θCO2 + X CO2 ) / σ PH 2 = (θ H 2 + 3 X CH 4 − X CO2 − YH 2 ) / σ
with,
σ=
1 + θ H 2O + θCO + θCO2 + θ H 2 PT
, θj =
Fj0
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(15)
0 FCH 4
(16)
(17)
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where, PT is the total pressure, Pi is the partial pressure of component i. A series of independent and steady-state mass balances to calculate the methane conversion, carbon dioxide conversion and hydrogen production in a tubular membrane reactor is used. The changes of conversion of
dz dX H 2O dz dX H 2 dz
=
ρb A
(r1 + r2 )
(18)
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dX CH 4
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each component along the reactor length are obtained by Fernandes and Soares (2006):
(r1 + r2 + 2r3 )
(19)
=
(3r1 + r2 + 4r3 )
(20)
=
0 FCH 4
ρb A
0 FCH 4
ρb A
0 FCH 4
dX CO ρb A = 0 (r1 − r2 ) dz FCH 4
(21)
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dz dYH 2 dz
=
=
ρb A
(r2 + r3 )
(22)
2π Rm β 0.5 ( PH 2 − PP0.5 ) 0 δ GCH 4
(23)
0 FCH 4
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dX CO2
The appropriate boundary conditions to solve the above equations are:
at z = 0 : X CH 4 = X CO2 = YH 2 = 0
(24)
kg ); GCH40 is the methane flow rate 3 m
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where A is the area of the tube (m2); ρb is the bed density (
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at the inlet (mole/h m2); Pp is the partial pressure of hydrogen in the permeate zone (atm); Rm is the radius of the membrane (m); β is the permeance of the membrane (m3/m h atm 0.5); δ is the membrane thickness (m). The basic assumptions for equation 18 to 24 are steady state process, plug flow, isothermal process and the pressure drop in the reactor is negligible. Table 2 shows
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the operating conditions and reactor parameters;
Table 2.
Many researchers have studied a mathematical expression for hydrogen permeability. The
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hydrogen permeability of the membrane depends on the quality and composition that can vary according to the type of alloy. An Arrhenius expression can be used to explain this variable.
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Ayturk et al. (2009) presented an Arrhenius expression for β :
β = Q0 exp(−
E0 ) RT
(25)
The pre exponential factor (Q0) and activation energy (E0) are 6322.7 m3 µ m.m −2 .hr.bar 0.5 and 15.6 kJ / mol , respectively. 4. Numerical solution
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At first, kinetic values (adsorption constants, reaction rate constants, equilibrium constants and permeability of hydrogen) were calculated. These parameters depend on temperature. The system is nodulation and differential equations were solved with MATLAB functions.
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Conversion of methane, carbon dioxide and hydrogen stored in the matrix. A 4th order Runge Kutta algorithm was used for the numerical solution of differential equations. The step size of the
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4th order Runge Kutta is 0.001.
5.1. Model validation
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5. Results and discussion
Figure 2 shows the experimental results of Smorygo et al. (2009) and results obtained from the model presented in this study.
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Fig. 2.
Numerical results of experimental work of Smorygo et al. (2009) and modeling of this work and also numerical values of error between them are provided in Table 3. Results obtained from the
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model with an acceptable error comply with the results of laboratory. Errors in Table 3 were calculated using the following formula: ( X CH 4 mod − X CH 4 exp )
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Error =
(26)
X CH 4 exp
5.2. Effect of temperature and pressure
Table. 3.
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In the mathematical modeling, the temperature remains constant. At first, the effect of temperature on conversion of methane is investigated. Figure 3 shows the total conversion of methane along the length of the reactor for CMR and PBR at T=5500C-6500C, P=1 bar kg . The total methane conversion in the reactor length for CMR is greater than m3
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and ρ = 2230
PBR. As shown in this figure, with increasing the temperature, the conversion of methane is
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increasing. The methane steam reforming is endothermic and with the increasing in temperature, the reaction moves to the product side and the consumption of methane is increasing (according
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to Le Chatelier's principle). So we conclude the maximum conversion will be obtained in the temperature that is tolerable for the reactor.
Fig.3.
Performance analysis was performed by simulating the reactor model equations for parameters
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such as temperature, pressure and its effect on methane conversion in both reactors. One of the major advantages of membrane reactor is a greater conversion of methane by removing hydrogen at medium temperatures (example 5000C). Effects of temperature and pressure of reaction on
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conversion of methane and carbon dioxide around the various catalysts are shown in Fig.4 and Fig.5. At constant temperature, with increasing the pressure the methane conversion is
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increasing, but in PBR with increasing the pressure the methane conversion is decreasing. In CMR the maximum conversion is obtained in 30 bars and 5300C and with decreasing pressure the maximum conversion is obtaining in temperature higher than 5300C. However, in PBR in all temperature the maximum conversion obtained in minimum pressure. The methane conversion is increasing with increasing the temperature in packed bed reactor and membrane reactor.
Fig.4.
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Fig.5. Since methane steam reforming reaction is strongly endothermic in thermodynamically scene, it is better to be done at high temperatures and low pressures. The results shown in Figure 4 imply
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that the overall conversion of methane to PBR increased by reducing pressure for all temperature ranges, since high pressure not only increases forward reaction but also effectively increases the rate of backward reaction in methane steam reforming.
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The high conversions resulted for PBR in temperature assessment (%95 for case a, %96 for b, %97 for c, %97.5 for d and %98 for e, at 7500C) due to low pressure gradient (one bar) that is
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used and it is consistent with the work of Oertel et al. (1987). Low partial pressure in the reaction may lead to the decomposition of methane to carbon and hydrogen and the conversion of methane is increasing. The overall conversion of methane in pressures 2,5,10, 20 and 30 bars, respectively 95%, 85%, %73, %59 and 50% for a, 96%, 88%, %75, %60 and 52% for b, 97%,
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88.5%, %76, %61 and 52.5% for c, 97.5%, 89%, %76.5, %61.2 and 53% for d and 98%, 89.5%, %77, %61.9 and 53.2% for e, respectively. Typically, industrial reformers operate at high
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pressures 30-40 bars.
When the driving force for hydrogen permeation increases through higher pressures in the side
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reaction, the partial pressure of hydrogen lead to enhance the methane conversion of CMR on all ranges of temperature, as shown in Figure 4. While the overall conversion of methane for PBR in 500 0C was below 20% above 10 bar, in CMR full conversion was achieved on various catalysts which can be seen from Figure 4.
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In PBR with increasing in temperature, the conversion of methane is increasing and the highest conversion is achieved in the high temperature, but in CMR the highest conversion is resulted in
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medium temperature. The ∆ index shows the difference between the overall conversion of methane in the CMR and PBR. A function of operating parameters and process parameters are defined as follows: ∆ = X CH 4 , MR − X CH 4 , PBR
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(27)
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Figure 6 shows the effects of temperature and pressure on the index. Based on the definition, the optimum conditions were obtained for the high pressures of 30 bar and a low temperature range as in Fig. 6. The temperature for the various catalysts was 600 0C, 575 0C, 560 0C, 550 0C and 545 0C, and with increasing the density of catalyst the temperature of reaction is reduced.
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Fig.6.
Fig 7 shows the hydrogen flux production (kmol/h) in a CMR reactor with various catalysts. For 1 kmol/h methane in the entrance of the reactor, nearly 4 kmol/hr hydrogen is produced. The
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maximum of hydrogen production has resulted in the highest pressure (30 bar). The temperature that is required for obtaining the maximum production of hydrogen is 630 0C, 575 0C, 573 0C,
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570 0C and 568 0C, for cases a, b, c, d and e, respectively.
Fig.7.
5.3. Effect of steam to methane ratio In order to decrease coke formation and deactivation of catalyst and enhance the total methane conversion, the process of methane steam reforming is done with excess steam. Fig 8 shows the
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effect of steam to methane ratio (m) on the total methane conversion in the range of 1 to 10, at 5000C and a pressure range of 5-30 bar. As shown in Fig 8, the maximum of total methane conversion was resulted in a ratio of 2-3. The plot of ∆ -index indicated that for m>3, the
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performance is decreased. The high ratio of steam to methane (m>3) need to excess amount of
of operating conditions in Fig 8.
5.4. Effect of membrane thickness
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Fig. 8.
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energy for production of steam in order to develop the yield of the process. We can use the range
The effect of membrane thickness on the total methane conversion is shown in Fig 9 at 5000C, a pressure range of 5-30 bar and steam to methane ratio of 3. As shown in Fig 9, with decreasing in thickness of membrane because of the enhancement in hydrogen removal, the total methane
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conversion was increased.
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Fig. 9.
5.5. Effect of sweep gas
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The effect of sweep gas ratio (s) was investigated at 5000C and a pressure of 15 and 30 bars. By increasing the sweep gas ratio to the permeate side of CMR, a significant effect was resulted in the total methane conversion due to enhance the driving force for the hydrogen permeation flux. The plot indicated that the effect of sweep gas on CMR was suitable to ratio of almost 10.
Fig. 10.
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6. Conclusion An isothermal mathematical model was provided to compare the performance of PBR and CMR reactors with metal foam catalyst. The highest performance was obtained for a membrane reactor
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with specific parameters. To analysis the process, the ∆ -index was presented. Based on the ∆ index analysis, the optimum conditions for CMR were obtained within the operating conditions ranges of temperature 565–600 0C, pressure >=20 bar, thickness <10 micron, steam to methane
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ratio 2< m < 3 and sweep factor s >= 10. The maximum of hydrogen production is resulting in
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the highest pressure (30 bar) and a temperature range that was required for obtaining the
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maximum production of hydrogen was 568 0C -630 0C through metal foam catalyst support.
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Nomenclature coefficient rate of reaction i
Ki
equilibrium constant of reaction i or adsorption coefficient of component i
ri
rate of reaction i (mol/h gcat)
Fi0
flow rate of component i in the feedstock (mol/h)
PT
total pressure
Pi
partial pressure of component i,
Xi
conversion of component i, and
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ki
A
area of the tube (m2)
ρb
bed density (
kg ) m3
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YH2 ratio of hydrogen flux to that of initial methane flow rate
GCH40 methane flow rate in the inlet (mole/h m2) partial pressure of hydrogen in the permeate zone (atm)
Rm
radius of the membrane (m)
β
permeance of the membrane (m3/m h atm 0.5)
δ
membrane thickness (m).
∆
difference between the overall conversion of methane in the CMR and PBR (-)
s
sweep gas ratio (-)
m
steam to methane ratio (-)
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Pp
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pre exponential factor ( m3 µ m.m −2 .hr.bar 0.5 )
E0
activation energy ( kJ / mol )
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Q0
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comparative simulation study of methane steam reforming in a porous ceramic membrane reactor using nitrogen and steam as sweep gases. International Journal of Hydrogen Energy. 33, 685-
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List of Figures:
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Fig.1. Scheme of catalytic membrane reactor. Fig.2. Comparison between results of experimental work of Smorygo et al. (2009) and this work. Fig.3. The total conversion of methane along the length of CMR and PBR at T=5500C- 6500C, kg . m3
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P=1 bar and ρ = 2230
various catalysts a) ρ = 0.4
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Fig.4. Effects of temperature and pressure on the methane conversion in PBR and CMR with g 6.5Ru/26(LaPrMnCrO/NiO/YSZ) cm3
g 3.0Ru/13.8(LaPrMnCrO/NiO/YSZ) cm3
c) ρ = 1.37
g 2.4Ru/11.6(LaPrMnCrO/NiO/YSZ) cm3
g 2.0Ru/7.3(LaPrMnCrO/NiO/YSZ) cm3
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e) ρ = 2.23
g 2.8Ru/10.7(LaPrMnCrO/NiO/YSZ) cm 3
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b) ρ = 1.17
Fig.5. Effects of temperature and pressure on the carbon dioxide conversion in PBR and CMR with various catalysts a) ρ = 0.4
b) ρ = 1.17
g 6.5Ru/26(LaPrMnCrO/NiO/YSZ) cm3
g 3.0Ru/13.8(LaPrMnCrO/NiO/YSZ) cm3
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g 2.4Ru/11.6(LaPrMnCrO/NiO/YSZ) cm3
d) ρ = 1.79
g 2.8Ru/10.7(LaPrMnCrO/NiO/YSZ) cm3
e) ρ = 2.23
g 2.0Ru/7.3(LaPrMnCrO/NiO/YSZ) cm3
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c) ρ = 1.37
g 6.5Ru/26(LaPrMnCrO/NiO/YSZ) cm3
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various catalysts a) ρ = 0.4
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Fig.6. ∆ index for describing the impact of temperature and pressure in PBR and CMR with
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c) ρ = 1.37
g 2.4Ru/11.6(LaPrMnCrO/NiO/YSZ) cm3
d) ρ = 1.79
g 2.8Ru/10.7(LaPrMnCrO/NiO/YSZ) cm 3 g 2.0Ru/7.3(LaPrMnCrO/NiO/YSZ) cm3
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b) ρ = 1.17
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Fig.7. Hydrogen production in CMR with various catalysts a) ρ = 0.4
g 6.5Ru/26(LaPrMnCrO/NiO/YSZ) cm3
b) ρ = 1.17
g 3.0Ru/13.8(LaPrMnCrO/NiO/YSZ) cm3
c) ρ = 1.37
g 2.4Ru/11.6(LaPrMnCrO/NiO/YSZ) cm3
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g 2.8Ru/10.7(LaPrMnCrO/NiO/YSZ) cm3
e) ρ = 2.23
g 2.0Ru/7.3(LaPrMnCrO/NiO/YSZ) cm3
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d) ρ = 1.79
Fig. 8. The effect of steam to methane ratio (m) on performance of CMR (T=5000C, PT=30 bar , Pp=1 bar, δ =1 micron and L=5 m)
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Fig.9. The effect of membrane thickness ( δ ) on performance of CMR (T=5000C, PT=30 bar ,
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Fig. 10. The effect of sweep gas (s) on performance of CMR (T= 5000C, PT= 30 bar, Pp= 1 bar
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List of Tables: Table 1. Kinetic parameters for the reactions involved in the methane steam reforming.
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Table 2. Operating conditions and reactor parameters.
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Table 1. Kinetic parameters for the reactions involved in the methane steam reforming. Pre- exponential factor (Aj or Bj)
Ej or ∆H i
k1
3.711× 1011 mol MPa 0.5 / g − cat s
240.1
k2
5.431×103 mol / MPa g − cat s
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Constant of reactions
67.13
243.9 8.960 ×1010 mol MPa 0.5 / g − cat s
k3
KCO
8.23 × 10 −4 MPa −1
KH2
6.12 × 10 −8 MPa −1
KCH4
6.65 × 10 −3 MPa −1
-38.28
KH2O
1.77 × 105 MPa −1
88.68
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Reaction side total pressure PT (bar)
2-30
Permeate side total pressure Pp (bar)
1
Methane flow rate G0(CH4) (kmol/hr) Methane/steam ratio Membrane thickness δ (m)
1 1/3 10-6
Catalyst density (gcat/cm3)
g cm3
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Table 2. Operating conditions and reactor parameters.
6.5Ru/26(LaPrMnCrO/NiO/YSZ) g cm3
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3.0Ru/13.8(LaPrMnCrO/NiO/YSZ) c) ρ = 1.37
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2.4Ru/11.6(LaPrMnCrO/NiO/YSZ) d) ρ = 1.79
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2.8Ru/10.7(LaPrMnCrO/NiO/YSZ) e) ρ = 2.23
g cm3
2.0Ru/7.3(LaPrMnCrO/NiO/YSZ)
Reactor length L (m)
5
Tube internal radius (m)
0.1016
Tube external radius (m)
0.1322
Membrane radius Rm (m)
0.0203
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Table. 3. Model validation of this work with experimental results of Smorygo et al.
450
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85
88
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97
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Error
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1- A kinetic model for the methane steam reforming has been investigated. 2- Performances are compared between the PBR and CMR with metal foam catalyst support.
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3- Modelling showed a higher performance for CMR rather than PBR.