ТЕОРЕТИЧЕСКИЕ ОСНОВЫ ХИМИЧЕСКОЙ ТЕХНОЛОГИИ, 2012, том 46, № 6, с. 680-688
УДК 661.961.62
ANALYSIS OF HYDROGEN PRODUCTION FROM METHANE AUTOTHERMAL REFORMER WITH A DUAL CATALYST-BED CONFIGURATION
© 2012 г. Y. Patcharavorachot", M. W&suleewan", S. Assabumrungrat", A. Arpornwichanop", b
a Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand b Computational Process Engineering, Chulalongkorn University, Bangkok 10330, Thailand
Amornchai.A@chula.ac.th Received 20.04.2012
This paper presents a performance analysis of a dual-bed autothermal reformer for hydrogen production from methane using a non-isothermal, one dimensional reactor model. The first section of Pt/Al2O3 catalyst is designed for oxidation reaction, whereas the second one based on Ni/MgAl2O4 catalyst involves steam reforming reaction. The simulation results show that the dual-bed autothermal reactor provides higher reactor temperature and methane conversion compared with a conventional fixed-bed reformer. The H2O/CH4 and O2/CH4 feed ratios affect the methane conversion and the H2/CO product ratio. The addition of steam at lower temperatures to the steam reforming section of the dual-bed reactor can produce the synthesis gas with a higher H2/CO product ratio.
INTRODUCTION
Hydrogen is considered an intermediate energy carrier that can be efficiently converted into many useful energy forms. In addition, it is a significant energy source along with the development of a fuel cell technology. To date, methane, a major component in natural gas, is a convenient feedstock for producing hydrogen [1, 2]. There are three major reforming techniques used to produce hydrogen from methane [3], i.e., steam reforming [4—7], partial oxidation [8—10] and autothermal reforming [1, 2, 11]. Among these techniques, the steam reforming is a well-established process and can provide the highest hydrogen yield; however, it involves a highly endothermic reaction, which requires high external heat source [5]. In a partial oxidation process, methane is combusted under the condition of limited oxygen. Although a large energy input is not needed for this process, hydrogen yield is a major issue [8]. Recently, a number of studies have been conducted to develop an autothermal reforming process by coupling an endothermic steam reforming with an exothermic partial oxidation. This process is perceived as a thermally neutral process because heat generated from the partial oxidation can be supplied to the steam reforming and thus, it has low energy requirement [12, 13].
In the autothermal reforming of methane, a consecutive reaction pathway that a total oxidation of part of methane is carried out to produce CO2 and H2O, followed by a steam reforming of the remaining methane with steam, is occurred [8, 14, 15]. Since the oxi-
dation reaction is faster than the reforming reaction, a large temperature gradient in the catalyst bed is generally observed. The presence of a hot spot in the autothermal reformer may affect the stability and safety of the process and severe catalyst deactivation by sintering and carbon formation [14—16]. Thus, many studies have been concentrated on the appropriate selection of operating conditions [13, 17], catalysts [18— 21], reactor configurations [22—26].
In general, Ni-based catalyst is normally employed to catalyze both the steam reforming and oxidation reactions [8, 13—15]. Scognamiglio et al. [16] proposed the mathematical model of a catalytic autothermal methane reformer based on Ni-catalyst. The numerical results indicated that the steam reforming reaction and the oxidation reaction are separately occurred. This is because Ni can easily be oxidized to ionic nickel in the presence of high oxygen, whereas Ni metallic exhibits the reforming activity. This results in a large temperature gradient in the catalyst bed. Considering the catalyst types for the autothermal reforming of methane, Pt catalyst exhibits the combustion activity [19] and provides a flat temperature profile due to the overlapping of the oxidation zone and the reforming zone [20]. However, Pt catalysts suffer from a carbon deposition in the reforming zone. Taking the advantages of both the catalysts, a dual catalyst-bed configuration of the autothermal reforming of methane has been studied. In the dual catalyst-bed reformer, Pt catalyst in the first catalyst bed is used to carry out a total oxidation, whereas Ni catalyst is used in the second bed to accomplish the steam reforming of unconverted
methane. Avci et al. [24] presented the numerical study of a catalyst fixed-bed reactor system in a dual-bed configuration and revealed that the large temperature gradient is still developed in the catalyst bed. Thus, in order to control the reaction temperature, the autothermal reformer should be operated under suitable conditions. Previous investigations showed that the addition of more steam to the autothermal reformer decreases the reformer temperature and increases hydrogen yield because the methane oxidation is less pronounced [14, 18, 19]. Consequently, the temperature and position of feeding steam should be carefully selected for the operation of the autothermal reformer.
In this work, a performance of the dual-bed autothermal reforming of methane for hydrogen production is analyzed. A non-isothermal, one-dimensional reactor model is used to investigate the reformer performance in terms of methane conversion and H2/CO product ratio. The effects of H2O/CH4 feed ratio, O2/CH4 feed ratio and temperature and location of steam feed are examined to determine an optimal condition for the dual-bed autothermal reactor. An understanding of the characteristics of the dual-bed autothermal reformer is necessary and the results obtained can be further used for design and experimental study of the reformer.
MODEL OF A DUAL-BED AUTOTHERMAL REACTOR
A dual-bed autothermal reactor is generally an adi-abatic fixed-bed reactor in which a catalyst bed is divided into two sections (Fig. 1). The first section involves the oxidation reaction based on Pt/Al2O3 catalyst, whereas the second one involves the steam reforming reaction based on Ni/MgAl2O4 catalyst. The reformer feeds consist of methane, steam and air. Methane is reacted with oxygen to produce carbon dioxide and steam (Eq. (1)). Then, the remaining methane and the other gaseous components enter the second catalytic zone, where hydrogen-rich gas is produced via steam reforming and water gas-shift reactions (Eqs. (2)—(4)). In this study, it was assumed that the oxidation section occupies the reactor volume of 10%:
(1)
CH4 + 2O2 ^ CO2 + 2H2O: АH0 = -802.3 kJ/mol,
CH4 + H2O о CO + 3H2: AH0 = 206.2 kJ/mol, (2)
CO + H2O o CO2 + H2: AH0 = -41.2 kJ/mol, (3)
CH4 + 2H2O o CO2 + 4H2: AH0 = 162.0 kJ/mol.
(4)
CH4 AIR -STEAM
^SYNTHESIS GAS
Fig. 1. Schematic of dual-bed autothermal reactor.
A one-dimensional model was employed to investigate the performance of an autothermal reforming of methane in the dual-bed reactor. The following assumptions have been made; steady-state and non-isothermal conditions; adiabatic and isobaric operations; ideal gas behavior; and no mixing in axial coordinate. Based on these assumptions, the governing equations are as follows:
molar balance equations for component i:
= (1 - sMPcat У »ft jJj\
dz
j=l
(5)
energy balance equations:
y Hfipi df = (1 - sMPcatXj (-AH) . (6)
dz , j
i j=1
The values of effectiveness factors (n j) of the reactions (Eqs. (1)—(4)) accounting for the intraparticle transport limitation are 0.05, 0.07, 0.70 and 0.06 [8]. The kinetic rate expressions and parameters for the total oxidation reaction (Pt/Al2O3 catalyst) and for the steam reforming and water gas-shift reactions (Ni/MgAl2O4 catalyst) are obtained from Halabi et al. [13]:
j, =
k1apCH4 pO2
1 /1 , Tsox , Tsox \2
(1 + KCH4 PCH4 + KO2 PO2 ) klbPCH4 PO2
(1 + KCH4 PCH4 + K°o, Po2 )
(7)
J2 =
_^2/PH'2 (PCH4PH2O - PH2PCO jKeq2 (B )
(1 + K CO pCO + KH2 pH2 + K CH4 pCH4 + K H2O pH2o/ pH2 )2
J3 =
_кз/PH2 (PCOP^O - PH2PooJKeq3 (9)
(1 + K CO pCO + KH 2 pH2 + K CH4 pCH4 + KH2O pH2O¡ pH2)2
J4 =
_fct/Ph (PCH4PH2O - pH2PaaJKeq,4) (10)
(1 + K CO pCO + KH2 pH2 + KCH4 pCH4 + KH2O pH2O ¡ pH2 )2
4
Table 1. Reaction equilibrium constants and Arrhenius kinetic parameters
Reaction koj, mol/(kgcat s) Ea j, kJ/mol Koj H, kJ/mol
1 ki, a 8.11 x 105 bar2 86.00 - -
kl, b 6.82 x 105 bar2 86.00 - -
2 k2 1.17 x 1015 bar0 5 240.10 5.75 x 1012 11,476
3 k3 2.83 x 1014 bar0.5 243.90 7.24 x 1010 -4,639
4 k4 5.43 x 105 bar-1 67.13 1.26 x 10-2 21,646
Tables 1 and 2 show the Arrhenius kinetic parameters of the reaction rate constants and the Van't Hoff parameters for species adsorption used for the calculation of the reaction rate and equilibrium constants.
MODEL VALIDATION
The fixed-bed reactor considered in this work is of cylindrical shape with 0.04 m in diameter and 1 m in length. To prove the validity of the proposed model, the simulation results of a conventional autothermal reactor are compared with industrial data published in the literature [8]. In an industrial operation, the autothermal reactor was operated at inlet temperature of 808 K, inlet pressure of 25.33 bar, inlet gas flow rate of 3483 Nm3/h, O2/CH4 ratio of0.598 and H2O/CH4 ratio of 1.4. For the conventional autothermal reactor, the reactor considered is an adiabatic fixed-bed reactor packed with Ni/MgAl2O4 catalysts. The values of the parameters used for the conventional reactor are listed in Table 3. The simulation results indicate that the model predictions in terms of the product yield
show a good agreement with industrial data as shown in Table 4.
PERFORMANCE OF CONVENTIONAL AUTOTHERMAL REFORMER
Simulations of a conventional autothermal reformer based on Ni/MgAl2O4 catalysts are performed and the temperature and methane conversion profiles along the reactor length are shown in Figs. 2 and 3. Figure 2 shows that the
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