- Email: [email protected]
Al2O3 catalyst in fluidized bed reactor
Accepted Manuscript Production of synthetic natural gas by CO methanation over Ni/ Al2O3 catalyst in fluidized bed reactor
Liyan Sun, Kun Luo, Jianren Fan PII: DOI: Reference:
S1566-7367(17)30451-X doi:10.1016/j.catcom.2017.11.003 CATCOM 5243
To appear in:
Catalysis Communications
Received date: Revised date: Accepted date:
16 August 2017 28 October 2017 6 November 2017
Please cite this article as: Liyan Sun, Kun Luo, Jianren Fan , Production of synthetic natural gas by CO methanation over Ni/Al2O3 catalyst in fluidized bed reactor. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi:10.1016/j.catcom.2017.11.003
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.
ACCEPTED MANUSCRIPT Production of synthetic natural gas by CO methanation over Ni/Al2O3 catalyst in fluidized bed reactor Liyan Sun, Kun Luo*, Jianren Fan
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State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, PR China
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* Corresponding author. fax: +86 057187991863
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E-mail address: [email protected]
ACCEPTED MANUSCRIPT
Abstract In order to obtain the reaction characteristics, CO methanation process is numerically investigated in fluidized bed catalytic reactor over
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Ni/Al2O3 catalyst. The catalysts flow behavior is analyzed, which has
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significant effect on the activity of catalysts. The influences of operation
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parameter on CO conversion and CH4 yield are evaluated. The inventory of catalysts and superficial gas velocity will not influence the output of
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methane but the reaction rates. Meanwhile, the performance of water-gas
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shift reaction on CH4 yield is taken consideration during reactions.
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Keyword: catalytic reaction; CO methanation; fluidized bed; simulation
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1. Introduction The CO methanation has been widely investigated for converting coal or biomass to synthetic natural gas, which is considered as a
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reasonable way to overcome the shortage of natural gas [1]. Methanation
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reaction was first discovered by Sabatier and Senderens [2] in 1902. The
H 298K = -206.28 kJ / mol
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catalyst CO + 3H2 CH4 + H2O
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main reactions during CO methanation are following:
catalyst CO + H2O CO2 + H2
H 298K = -41.16 kJ / mol
(1) (2)
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Methanation reaction in both fixed bed and fluidized bed reactor have
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been proposed from 1970s and today sustained efforts are made to convert syngas to synthetic natural gas [3]. For this exothermic process, it
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is harmful for the safety of reactors since it emits massive heat during reaction. The fluidized bed is an ideal reactor for the highly exothermic
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reactions due to the high heat and mass transfer coefficient ensuring effective heat removal [4,5]. But the complex flow behavior influences much the activity of catalysts, which causes difficulty for reactor design and scale-up. Comparing with the well developed fixed bed reactor, there is no industry-scale commercial application until now and more efforts are needed for the development of this process.
ACCEPTED MANUSCRIPT Experimental study was conducted by Kopyscinski et al. [6] using spatially resolved measurements for first time and one-dimensional reaction rate was developed and validated in their work. Zhou et al. [7] investigated the methanation reaction using Ni/TiO 2 catalyst prepared by
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the dielectric barrier discharge. Gao et al. [8] conducted the experiments
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of methanation over Ni/Al2O3 catalyst and analyzed the effects of catalyst
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structure and reaction mechanism. Obtaining complete knowledge of gas and solid dynamics is a tough task, especially for the situation coupling
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with chemical reaction [9]. To overcome these challenges, the numerical
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simulation can complement as a beneficial tool to elaborate the complex multiphase gas-solid flow for better design and operation. Liu et al. [10]
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conducted the simulations to investigated optimal reactor for methanation
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process by Aspen Plus. Chein et al. [11] numerically investigated the production of synthetic natural gas by fixed bed reactor. Their results
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showed that CH4 yield and CO conversion were enhanced by increasing the H2/CO ratio and operating pressure. Hinrichsen et al. [12] carried out the simulation of methanation reaction and the movements of particles are well analyzed. However, the influences of fluidization behavior and chemical boundary conditions on catalytic process and the performance of fluidized bed were neglected. The transformation of energy is not taken consideration in above works. The important parameter, temperature, is not paid enough attention, which is the judgment basis for
ACCEPTED MANUSCRIPT the deactivation of catalyst particles. Hence, there are not enough studies for economical two-fluid model. The effects of operating parameters are needed to be analyzed carefully. Research on simulation of methanation by fluidized bed reactor has still plenty of spaces for improvement.
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In this paper, numerical simulations based on open-source CFD
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package MFIX are carried out for better understanding the CO
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methanation characteristics and activity of catalysts in fluidized bed. Simulation results are compared with experimental data from a bench-
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scale reactor for validation of the model. Interaction between
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hydrodynamic and reaction kinetic and the effect of fluidization behavior are analyzed. The distributions of temperature are analyzed numerically
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which is the most important for the catalysts. Influence of operating
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parameters, including inlet rates and catalyst inventory, are investigated numerically under different conditions.
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2. Mathematical models
In fluidized bed reactor, the mass conservation equations for both phases are given as: g g t
g g u g Rg
s s s s u s 0 t
(3)
(4)
ACCEPTED MANUSCRIPT where ρ, α, and u are density, volume fraction and velocity, respectively. Rg is the source term due to methanation reaction for gas phase [13]. For catalyst the mass keep constant and the source term is set as 0. The momentum conservation equations are given as: g g u g
(5)
s s u s s s u s u s sp ps s s s s g + (u g - u s ) t
(6)
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g g u g u g g p g g g g g + (u s - u g )
t
β is the momentum transfer coefficient between gas phase and solid phase
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and is calculated as following:
3 CD g s g u g u s g2.65 dp 4 2 g s s g 1.75 u g u s 150 2 gd p dp
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s 0.2
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0.687 24 Re 1 0.15 Re g CD g 0.44
Re
(7) s 0.2
Re 1000
(8)
Re 1000
g d p u g us
(9)
g
is stress tensor and given as: T 2 g g u g u g g u g I
3
2 T s s u s u s s s u s I
3
(10) (11)
s and s are the solid shear viscosity and bulk viscosity, respectively.
ACCEPTED MANUSCRIPT For two-fluid model, the stress and solid pressure are calculated according to kinetic theory of granular flow (KTGF) to close the momentum conservation equation. The basic variable of KTGF is the granular temperature and the transport equation can be expressed as:
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3 g g s s u s ps I s : u s s s J s 2 t
(12)
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is the granular temperature, s represents the conductivity of granular
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temperature, s and J s are dissipation rate from collision and fluctuating
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velocity of particle.
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The transport equation of gas species is solved for methanation reaction and it can be written as follow: g gYg ,i
g g u gYg ,i g J g ,i Rg ,i
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t
(13)
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where Yg,i represents the mass fraction gas species i. J is the species diffusion flux. R represents the reaction rate.
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Methanation reaction is an exothermic process and energy conservation equation is solved: g g c pgTg t
s s c psTs t
6 g g u g c pgTg g k g c pg st Tg g hgs (Ts Tg ) S g ,i c pg ,iTg Prt ds
(14)
6 s s u s c psTs s ks c ps st Ts s hgs (Tg Ts ) Ss ,i c ps ,iTs Prt ds
(15)
ACCEPTED MANUSCRIPT k is the thermal conductivity. Pr is the Prandtl number. c is the specific heat capacity which is a function of temperature. h represents the heat transfer coefficient between phases. Two main reactions are concerned in this work, CO methanation and
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water-gas shift reaction. The composition of the gas mixture changes due
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to the reaction and the density of gas is calculated following the ideal gas
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law. Kopyscinski et al. [6, 14 ] developed the reaction rate for the methanation reaction. Xu et al. [11,15] derived the intrinsic rate equations
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accompanied by water-gas shift on nickel catalyst. According to previous
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results, the reaction rates over Ni/Al2O3 catalyst adopted in this work can be calculated as:
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pH3 2 pCO k1 2 rm 2.5 pCH 4 pH 2O DEN pH 2 K eq1 k2 pH 2
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pCO2 pH 2 pCO pH 2O K eq 2
2 DEN
DEN 1 KCH 4 pCH 4 KCO pCO K H 2 pH 2
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(16)
(17)
K H 2 O pH 2 O pH 2
(18)
In Eqs. (16) - (17), ki represents the kinetic rate constant: 240100 k1 3.7111017 exp RT 67130 k2 5.431exp RT
Keq, represents equilibrium constant:
[mol Pa 0.5 kgcat s]
[mol Pa 0.5 kgcat s]
(19)
(20)
ACCEPTED MANUSCRIPT 26830 Keq ,1 1.198 1023 exp T
[Pa 2 ]
4400 Keq ,2 1.767 102 exp T
(21)
(22)
Kj represents adsorption constant: [Pa 1 ]
(23)
70650 KCO 8.23 1010 exp RT
[Pa 1 ]
(24)
[Pa 1 ]
(25)
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38280 KCH 4 6.65 109 exp RT
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82900 K H 2 6.12 1014 exp RT
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3. Results and discussion
(26)
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88680 K H 2O 1.77 105 exp RT
A laboratory-scale methanation fluidized bed reactor is adopted
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according to the experimental setup of Kopyscinski et al. [16]. The inner diameter and height of fluidized bed reactor are set as 0.0052m and 0.2m,
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respectively. The superficial gas velocity ranges from 0.063 to 0.126 m/s. The initial bed material height ranges from 0.033-0.094m. Diameter and density of catalyst powder are 0.1mm and 2000 kg/m3, respectively. The time-averaged mass fractions of each species are presented in Fig. 1. H2 and CO reduce rapidly near the bottom of bed due the high reaction rates. In the upper region CO concentration is nearly zero but a little H2 can be found because the water-gas shift reaction will consume CO and
ACCEPTED MANUSCRIPT produce additional H2. The production of CH4 and vapor are very fast and it fills the upper region. The distributions of gas species are similar to
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Kopyscinski’s [16] results.
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Fig. 1 Time-averaged distribution of mass fractions of H2 , CO, CH4 and vapor
To validate the model used in this work, the simulated results are
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compared with the experimental data. Fig. 2 shows the quantitative analysis of volume-averaged gas species concentration in vol% along the
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height of bed under different operating conditions with 200g and 70g catalysts. For the first simulation(200g catalyst), H2 concentration
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decreases rapidly in first 15mm after distributor from 60 vol% to 10 vol%, and then keeps nearly constant to the end of bed. CH4 concentration increases to max value simultaneously. The CO concentration diminishes very quickly to zero in the first 10 mm. The N2 concentration increases simultaneously to the maximum nearly 45 vol% due to the volume reduction of gas phase. CO2 is formed in first 5 mm by water-gas shift reaction. The maximum value of standard deviation in the bottom for CO,
ACCEPTED MANUSCRIPT H2, CH4, N2 and CO2 are 0.15, 0.21, 0.38, 0.13 and 0.03, respectively. We think the main reason for this distribution is the complex reactions and the mass and heat transmission in this region. From 10mm, the rates of H2 consumption and CH4 formation keep constant due the end of catalyst
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bed and exhaustion of reactants. The simulation results agree well with
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the experimental data. The averaged deviation for CO, H2, CH4, N2 and
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CO2 are 0.014, 0.022, 0.05, 0.02 and 0.01, respectively. For second simulation, we get the similar trend of gas species concentrations and the
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deviation. The difference is the location of maximum concentration
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shifted by a few millimeters downwards. Since the experimental data were measured at points that would make difference for accident factors.
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Considering the total distribution of concentration of each species, the
90
200 gcat
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80 70 60
Height (mm)
100
50 40 30 20
90
70 gcat
80
Exp. CO H2
Exp. CO H2
70
CH4
CH4
H2O
H2O
60
CO2
CO2
50
Sim. CO H2
Sim. CO H2
40
CH4
CH4
H2O
H2O
CO2
CO2
30 20
10
10
0
0 0
10
20
30
Vol %
40
50
0
10
20
30
Height (mm)
100
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results are acceptable we think.
40
50
Vol %
Fig. 2 Comparison of the simulated species composition with experimental data
ACCEPTED MANUSCRIPT In the simulations, 70, 100, 140 and 200g of catalysts are used according to the experimental setup. Snapshots of instantaneous solid volume fraction are displayed in Fig. 3. The expanded height of bed materials increases with raising initial amount of inventories. The higher
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initial bed height leads to the longer interaction time of the reactants with
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catalysts. The results of H2 conversion and methane selectivity are shown
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in Fig. 4. The conversion of CO is not listed because it is consumed completely. For experiment, the methane production increases with
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catalyst inventory. The simulation results are in reasonable agreement
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with experimental data. However, the increase is quite minor, and we find a little decrease in simulation. Because of the fixed feedstock and the
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concentrations of catalysts for each cases are enough, further increase in
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the catalyst amount will not improve the production of methane. Another reason, we think, is the increase of inventory restrains the remove of
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reaction heat which will accelerate the water-gas reaction and reverse reaction.
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Fig. 3 Snapshots of instantaneous solid volume fraction for different catalyst
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inventories (70g, 100g, 140g, 200g)
ACCEPTED MANUSCRIPT
100
(a)
Experiment data
Simulation results
90
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XH2 (%)
80
70
50
70 g
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60
100 g
140 g
200 g
(b)
Experiment data
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100
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Catalyst inventory (g)
70
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SCH2 (%)
80
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90
Simulation results
60
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50
70 g
100 g
140 g
200 g
Catalyst inventory (g)
Fig. 4 Effect of catalyst inventory on conversion of H2 and production of CH4
Fig. 5 shows the distribution of concentration and temperature for two different cases (100g, 140g). Only the first 30mm are displayed to show the details of information since the reactions complete rapidly and important features of temperature profiles are within first few millimeters, while the upper parts of bed are quite same. In the simulation of 140g
ACCEPTED MANUSCRIPT catalyst, the H2 concentration decreases and CO diminishes very rapidly in first 5 mm. CO2 is generated in first 5 millimeters by water-gas shift reaction. The trend of gas concentration with 100g catalysts is similar with the case with 140g catalysts. But there are some differences need to
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be noticed. The locations of turning points of gas concentration curves are
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shifted by a few millimeters downwards compared with first simulation
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results (140g). The rate of conversion of H2 in simulation no.2 (100g) is faster than that in simulation no.1 (140g), as shown in the figure by angle
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a and b, also the production rate of CH4. The appropriate explanation can
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be found in the profile of temperature. According to the reaction kinetic theory, increase of temperature will accelerate both the methanation and
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reverse reaction rate, and the water-gas shift reaction plays an important
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role in this temperature range according to the experiment. We can distinguish more CO 2 in simulation no.1 (3%) than that in simulation no.2
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(1%). One would expect a high rate of H2 conversion and CH4 production, but the opposite is observed due to the high temperature.
ACCEPTED MANUSCRIPT 30
30
140g catalyst
25 20 15
25
CO H2
20
CH4
15
H2O CO2
5
5
0
0
5
10
15
20
25
30
35
40
45
50 328
332
30 25
CO H2
15
CH4 H 2O
10
CO2
5 0 0
5
10
15
20
25
30
35
40
45
50 328
332
344
30 25 20 15 10 5
336
340
0 344
Temperature
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Volume fraction (%)
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b
340
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100g catalyst
20
336
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0
Height of bed (mm)
a
10
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Height of bed (mm)
10
Fig. 5 Profile of simulated gas species concentration and bed temperature
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Fig. 6 illustrates the influence of gas velocity on the concentration of
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CO and CH4 along the bed height. The rates of CH4 production and CO conservation increase with raising gas velocity. The fluidization quality
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has significant impact on the reaction process. For methanation reaction, the remove of heat is the key for fluidized bed reactor. Under the high gas velocity, the reaction heat will be removed quickly and completely. However, for the case with low gas velocity, the methanation reaction will be restrained and reverse reaction will be accelerated due to high temperature. That is the reason we get the distribution in Fig. 6.
ACCEPTED MANUSCRIPT
20
XCO (Vol%)
16
Vg= 6.3 cm/s
12
Vg= 7.8 cm/s
8
Vg=12.6 cm/s
4 60
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40
Vg= 6.3 cm/s 30
Vg= 7.8 cm/s
20
Vg=12.6 cm/s
10
0
5
10
15
20
25
30
35
40
45
50
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Height of bed (mm)
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XCh4 (Vol%)
50
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Fig. 6 Profile of CO and CH4 concentration along the bed height
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4. Conclusion
The simulations of methanation reaction in fluidized bed over Ni/Al2O3
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catalyst reactor are carried out by CFD open-source software. The
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Eulerian-Eulerian model coupling with reaction kinetic is adopted for simulating the catalysts flow behavior and reaction characteristic. The
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results of species concentration are in reasonable agreement with experimental data. CO methanation reaction completes at first few millimeters and the reaction kinetic is dominated is this region. The CO and H2 are consumed rapidly and CH4 is produced in this region. However, the mixing is the main process in the upper parts. At the outlet we can find H2 due to water-gas shift reaction, which is beneficial for CO conversion. At current situation, the increase of catalyst inventory will not influence the total production of methane, but the conversion rate
ACCEPTED MANUSCRIPT decreases due to the change of distribution of temperature. The high fluidizing gas velocity is beneficial for the remove of reaction heat. At the current range of gas velocity, the CO conversion and methane production increase with the increase of gas velocity. More works for methanation
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reaction in scale-up reactor are in process in our group.
[J/(kg K)]
Specific heat
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c
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Nomenclature
drag coefficient
d
[m]
diameter
g
[m2 s-2 ]
gravitational acceleration
h
[w/(m K)]
heat transfer coefficient
J
[-]
diffusion flux
k
[W/(mk)]
p
[Pa]
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thermal conductivity pressure
Prandtl number
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Pr [-]
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Cd [-]
r
[mol/s kgcat ]
Reaction rate
R
[-]
Source term
Re [-]
Reynolds number
t
[s]
time
T
[K]
temperature
u
[m/s]
velocity
Y
[-]
mass fraction gas species
ACCEPTED MANUSCRIPT Greek letters [-]
volume fraction
[-]
drag coefficient
[Pas]
Shear viscosity
[Pas]
bulk viscosity
[kg/m3 ]
density
[m2 /s2 ]
granular temperature
[S/m]
conductivity of granular temperature
[-]
dissipation rate from collision
[Pa]
stress tensor
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SC
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Acknowledgement
This work is supported by China Postdoctoral Science Foundation funded
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project (509200-X91701), the Fundamental Research Funds for the
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Central Universities, for which we are grateful.
ACCEPTED MANUSCRIPT Reference
[1] Yue Yu, Guoqiang Jin, Yingyong Wang, Xiangyun Guo. Synthesis of natural gas from CO methanation over SiC supported Ni-Co bimetallic
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catalysts. Catalysis Communications. 2013,31: 5-10.
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[2] Sabatier P, Senderens J B. Comptes rendus des séances de l’academie
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des science, Section VI-Chimie. Paris: Imprimerie Gauthier-Villars, 1902. [ 3 ] Jan Kopyscinski, Tilman J Schildhauer, Serge M A Biollaz.
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Production of synthetic natural gas (SNG) from coal and dry biomass-A
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technology review from 1950 to 2009. Fuel, 2010,89: 1763-1783. [4] Wang Shuai, Lu Huilin, Zhao Feixiang, Liu Guodong. CFD studies of
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dual circulating fluidized bed reactors for chemical looping combustion processes. Chemical Engineering Journal. 2014,236: 121-130.
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[5] Wang Shuai, Wang Qi, Chen Juhui, Liu Guodong, Lu Huilin, Sun
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Liyan. Assessment of CO2 capture using potassium-based sorbents in circulating fluidized bed reactor by multiscale modeling. Fuel. 2016,164： 66-72.
[6] Jan Kopyscinki, Tilman J. Schildhauer, Frederic Vogel, Serge M. A. Biollaz, Alexander Wokaun. Applying spatially resolved concentration and temperature measurements in a catalytic plate reactor for the kinetic study of CO methanation. Journal of Catalysis. 2010,271: 262-279.
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[7] Rui Zhou, Ning Rui, Zhigang Fan, Changjun Liu. Effect of the structure of Ni/TiO2 catalyst on CO 2 methanation. International Journal of Hydrogen Energy. 2016,41: 22017-22025. [8] Zhiming Gao, Lin Gui, Hongwei Ma. Selective methanation of CO
PT
over Ni/Al2O3 catalyst: Effects of preparation method and Ru addition.
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International Journal of Hydrogen Energy 2016,41: 5484-5493.
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[ 9 ] Oscar Rabinovich, Alla Tsytsenka, Vladimir Kuznetsov, Sergei
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Moseenkov, Dmitry Krasnikov. A model for catalytic synthesis of carbon nanotubes in a fluidized-bed reactor: Effect of reaction heat. Chemical
MA
Engineering Journal. 2017,329: 305-311.
[ 10 ] Jiao Liu, Dianmiao Cui, Changbin Yao, Jian Yu, Fabing Su,
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Guangwen Xu. Syngas methanation in fluidized bed for an advanced two-
130-137.
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stage process of SNG production. Fuel Processing Technology, 2016,141:
AC C
[11] Rei Yu Chein, Ching Tsung Yu, Chi Chang Wang. Numerical simulation on the effect of operating conditions and syngas compositions for synthetic natural gas production via methanation reaction. Fuel. 2016,185: 394-409. [12] Yefei Liu, Olaf Hinrichsen. CFD simulation of hydrodynamics and methanation reactions in a fluidized bed reactor for the production of
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synthetic natural gas. Industrial & Engineering Chemistry Research. 2014,53: 9348-9356. [13] Chen Juhui, Yin Weijie, Wang Shuai, Meng Cheng, Li Jiuru, Qin Bai, Yu Guangbin. Effect of reactions in small eddies on biomass
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model. Bioresource Technology 2016;211: 93-100.
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gasification with eddy dissipation concepet - Sub-grid scale reaction
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[ 14 ] Jan Kopyscinski, Tilman J. Schildhauer, Serge M. A. Biollaz.
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Fluidized-bed methanation: interaction between kinetics and mass transfer. Industrial & Engineering Chemistry Research. 2011,50: 2781-
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[ 16 ] Jan Kopyscinski, Tilman J. Schildhauer, Serge M. A. Biollaz.
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Methanation in a fluidized bed reactor with high initial CO partial pressure: Part I-Experimental investigation of hydrodynamics, mass transfer effects, and carbon deposition. Chemical Engineering Science. 2011,66: 924-934.
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Graphical abstract Highlights
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CO methanation is numerical studied considering the fluidization
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of catalysts.
The water-gas shift reaction is taken into consideration during
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process.
AC C
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Higher gas velocity is helpful to remove the reaction heat.
Liyan Sun, Kun Luo, Jianren Fan PII: DOI: Reference:
S1566-7367(17)30451-X doi:10.1016/j.catcom.2017.11.003 CATCOM 5243
To appear in:
Catalysis Communications
Received date: Revised date: Accepted date:
16 August 2017 28 October 2017 6 November 2017
Please cite this article as: Liyan Sun, Kun Luo, Jianren Fan , Production of synthetic natural gas by CO methanation over Ni/Al2O3 catalyst in fluidized bed reactor. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi:10.1016/j.catcom.2017.11.003
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.
ACCEPTED MANUSCRIPT Production of synthetic natural gas by CO methanation over Ni/Al2O3 catalyst in fluidized bed reactor Liyan Sun, Kun Luo*, Jianren Fan
PT
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, PR China
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* Corresponding author. fax: +86 057187991863
AC C
EP T
ED
MA
NU
SC
E-mail address: [email protected]
ACCEPTED MANUSCRIPT
Abstract In order to obtain the reaction characteristics, CO methanation process is numerically investigated in fluidized bed catalytic reactor over
PT
Ni/Al2O3 catalyst. The catalysts flow behavior is analyzed, which has
RI
significant effect on the activity of catalysts. The influences of operation
SC
parameter on CO conversion and CH4 yield are evaluated. The inventory of catalysts and superficial gas velocity will not influence the output of
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methane but the reaction rates. Meanwhile, the performance of water-gas
MA
shift reaction on CH4 yield is taken consideration during reactions.
AC C
EP T
ED
Keyword: catalytic reaction; CO methanation; fluidized bed; simulation
ACCEPTED MANUSCRIPT
1. Introduction The CO methanation has been widely investigated for converting coal or biomass to synthetic natural gas, which is considered as a
PT
reasonable way to overcome the shortage of natural gas [1]. Methanation
RI
reaction was first discovered by Sabatier and Senderens [2] in 1902. The
H 298K = -206.28 kJ / mol
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catalyst CO + 3H2 CH4 + H2O
SC
main reactions during CO methanation are following:
catalyst CO + H2O CO2 + H2
H 298K = -41.16 kJ / mol
(1) (2)
MA
Methanation reaction in both fixed bed and fluidized bed reactor have
ED
been proposed from 1970s and today sustained efforts are made to convert syngas to synthetic natural gas [3]. For this exothermic process, it
EP T
is harmful for the safety of reactors since it emits massive heat during reaction. The fluidized bed is an ideal reactor for the highly exothermic
AC C
reactions due to the high heat and mass transfer coefficient ensuring effective heat removal [4,5]. But the complex flow behavior influences much the activity of catalysts, which causes difficulty for reactor design and scale-up. Comparing with the well developed fixed bed reactor, there is no industry-scale commercial application until now and more efforts are needed for the development of this process.
ACCEPTED MANUSCRIPT Experimental study was conducted by Kopyscinski et al. [6] using spatially resolved measurements for first time and one-dimensional reaction rate was developed and validated in their work. Zhou et al. [7] investigated the methanation reaction using Ni/TiO 2 catalyst prepared by
PT
the dielectric barrier discharge. Gao et al. [8] conducted the experiments
RI
of methanation over Ni/Al2O3 catalyst and analyzed the effects of catalyst
SC
structure and reaction mechanism. Obtaining complete knowledge of gas and solid dynamics is a tough task, especially for the situation coupling
NU
with chemical reaction [9]. To overcome these challenges, the numerical
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simulation can complement as a beneficial tool to elaborate the complex multiphase gas-solid flow for better design and operation. Liu et al. [10]
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conducted the simulations to investigated optimal reactor for methanation
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process by Aspen Plus. Chein et al. [11] numerically investigated the production of synthetic natural gas by fixed bed reactor. Their results
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showed that CH4 yield and CO conversion were enhanced by increasing the H2/CO ratio and operating pressure. Hinrichsen et al. [12] carried out the simulation of methanation reaction and the movements of particles are well analyzed. However, the influences of fluidization behavior and chemical boundary conditions on catalytic process and the performance of fluidized bed were neglected. The transformation of energy is not taken consideration in above works. The important parameter, temperature, is not paid enough attention, which is the judgment basis for
ACCEPTED MANUSCRIPT the deactivation of catalyst particles. Hence, there are not enough studies for economical two-fluid model. The effects of operating parameters are needed to be analyzed carefully. Research on simulation of methanation by fluidized bed reactor has still plenty of spaces for improvement.
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In this paper, numerical simulations based on open-source CFD
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package MFIX are carried out for better understanding the CO
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methanation characteristics and activity of catalysts in fluidized bed. Simulation results are compared with experimental data from a bench-
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scale reactor for validation of the model. Interaction between
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hydrodynamic and reaction kinetic and the effect of fluidization behavior are analyzed. The distributions of temperature are analyzed numerically
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which is the most important for the catalysts. Influence of operating
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parameters, including inlet rates and catalyst inventory, are investigated numerically under different conditions.
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2. Mathematical models
In fluidized bed reactor, the mass conservation equations for both phases are given as: g g t
g g u g Rg
s s s s u s 0 t
(3)
(4)
ACCEPTED MANUSCRIPT where ρ, α, and u are density, volume fraction and velocity, respectively. Rg is the source term due to methanation reaction for gas phase [13]. For catalyst the mass keep constant and the source term is set as 0. The momentum conservation equations are given as: g g u g
(5)
s s u s s s u s u s sp ps s s s s g + (u g - u s ) t
(6)
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g g u g u g g p g g g g g + (u s - u g )
t
β is the momentum transfer coefficient between gas phase and solid phase
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and is calculated as following:
3 CD g s g u g u s g2.65 dp 4 2 g s s g 1.75 u g u s 150 2 gd p dp
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s 0.2
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0.687 24 Re 1 0.15 Re g CD g 0.44
Re
(7) s 0.2
Re 1000
(8)
Re 1000
g d p u g us
(9)
g
is stress tensor and given as: T 2 g g u g u g g u g I
3
2 T s s u s u s s s u s I
3
(10) (11)
s and s are the solid shear viscosity and bulk viscosity, respectively.
ACCEPTED MANUSCRIPT For two-fluid model, the stress and solid pressure are calculated according to kinetic theory of granular flow (KTGF) to close the momentum conservation equation. The basic variable of KTGF is the granular temperature and the transport equation can be expressed as:
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3 g g s s u s ps I s : u s s s J s 2 t
(12)
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is the granular temperature, s represents the conductivity of granular
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temperature, s and J s are dissipation rate from collision and fluctuating
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velocity of particle.
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The transport equation of gas species is solved for methanation reaction and it can be written as follow: g gYg ,i
g g u gYg ,i g J g ,i Rg ,i
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t
(13)
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where Yg,i represents the mass fraction gas species i. J is the species diffusion flux. R represents the reaction rate.
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Methanation reaction is an exothermic process and energy conservation equation is solved: g g c pgTg t
s s c psTs t
6 g g u g c pgTg g k g c pg st Tg g hgs (Ts Tg ) S g ,i c pg ,iTg Prt ds
(14)
6 s s u s c psTs s ks c ps st Ts s hgs (Tg Ts ) Ss ,i c ps ,iTs Prt ds
(15)
ACCEPTED MANUSCRIPT k is the thermal conductivity. Pr is the Prandtl number. c is the specific heat capacity which is a function of temperature. h represents the heat transfer coefficient between phases. Two main reactions are concerned in this work, CO methanation and
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water-gas shift reaction. The composition of the gas mixture changes due
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to the reaction and the density of gas is calculated following the ideal gas
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law. Kopyscinski et al. [6, 14 ] developed the reaction rate for the methanation reaction. Xu et al. [11,15] derived the intrinsic rate equations
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accompanied by water-gas shift on nickel catalyst. According to previous
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results, the reaction rates over Ni/Al2O3 catalyst adopted in this work can be calculated as:
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pH3 2 pCO k1 2 rm 2.5 pCH 4 pH 2O DEN pH 2 K eq1 k2 pH 2
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pCO2 pH 2 pCO pH 2O K eq 2
2 DEN
DEN 1 KCH 4 pCH 4 KCO pCO K H 2 pH 2
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(16)
(17)
K H 2 O pH 2 O pH 2
(18)
In Eqs. (16) - (17), ki represents the kinetic rate constant: 240100 k1 3.7111017 exp RT 67130 k2 5.431exp RT
Keq, represents equilibrium constant:
[mol Pa 0.5 kgcat s]
[mol Pa 0.5 kgcat s]
(19)
(20)
ACCEPTED MANUSCRIPT 26830 Keq ,1 1.198 1023 exp T
[Pa 2 ]
4400 Keq ,2 1.767 102 exp T
(21)
(22)
Kj represents adsorption constant: [Pa 1 ]
(23)
70650 KCO 8.23 1010 exp RT
[Pa 1 ]
(24)
[Pa 1 ]
(25)
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38280 KCH 4 6.65 109 exp RT
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82900 K H 2 6.12 1014 exp RT
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3. Results and discussion
(26)
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88680 K H 2O 1.77 105 exp RT
A laboratory-scale methanation fluidized bed reactor is adopted
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according to the experimental setup of Kopyscinski et al. [16]. The inner diameter and height of fluidized bed reactor are set as 0.0052m and 0.2m,
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respectively. The superficial gas velocity ranges from 0.063 to 0.126 m/s. The initial bed material height ranges from 0.033-0.094m. Diameter and density of catalyst powder are 0.1mm and 2000 kg/m3, respectively. The time-averaged mass fractions of each species are presented in Fig. 1. H2 and CO reduce rapidly near the bottom of bed due the high reaction rates. In the upper region CO concentration is nearly zero but a little H2 can be found because the water-gas shift reaction will consume CO and
ACCEPTED MANUSCRIPT produce additional H2. The production of CH4 and vapor are very fast and it fills the upper region. The distributions of gas species are similar to
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Kopyscinski’s [16] results.
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Fig. 1 Time-averaged distribution of mass fractions of H2 , CO, CH4 and vapor
To validate the model used in this work, the simulated results are
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compared with the experimental data. Fig. 2 shows the quantitative analysis of volume-averaged gas species concentration in vol% along the
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height of bed under different operating conditions with 200g and 70g catalysts. For the first simulation(200g catalyst), H2 concentration
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decreases rapidly in first 15mm after distributor from 60 vol% to 10 vol%, and then keeps nearly constant to the end of bed. CH4 concentration increases to max value simultaneously. The CO concentration diminishes very quickly to zero in the first 10 mm. The N2 concentration increases simultaneously to the maximum nearly 45 vol% due to the volume reduction of gas phase. CO2 is formed in first 5 mm by water-gas shift reaction. The maximum value of standard deviation in the bottom for CO,
ACCEPTED MANUSCRIPT H2, CH4, N2 and CO2 are 0.15, 0.21, 0.38, 0.13 and 0.03, respectively. We think the main reason for this distribution is the complex reactions and the mass and heat transmission in this region. From 10mm, the rates of H2 consumption and CH4 formation keep constant due the end of catalyst
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bed and exhaustion of reactants. The simulation results agree well with
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the experimental data. The averaged deviation for CO, H2, CH4, N2 and
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CO2 are 0.014, 0.022, 0.05, 0.02 and 0.01, respectively. For second simulation, we get the similar trend of gas species concentrations and the
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deviation. The difference is the location of maximum concentration
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shifted by a few millimeters downwards. Since the experimental data were measured at points that would make difference for accident factors.
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Considering the total distribution of concentration of each species, the
90
200 gcat
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80 70 60
Height (mm)
100
50 40 30 20
90
70 gcat
80
Exp. CO H2
Exp. CO H2
70
CH4
CH4
H2O
H2O
60
CO2
CO2
50
Sim. CO H2
Sim. CO H2
40
CH4
CH4
H2O
H2O
CO2
CO2
30 20
10
10
0
0 0
10
20
30
Vol %
40
50
0
10
20
30
Height (mm)
100
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results are acceptable we think.
40
50
Vol %
Fig. 2 Comparison of the simulated species composition with experimental data
ACCEPTED MANUSCRIPT In the simulations, 70, 100, 140 and 200g of catalysts are used according to the experimental setup. Snapshots of instantaneous solid volume fraction are displayed in Fig. 3. The expanded height of bed materials increases with raising initial amount of inventories. The higher
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initial bed height leads to the longer interaction time of the reactants with
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catalysts. The results of H2 conversion and methane selectivity are shown
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in Fig. 4. The conversion of CO is not listed because it is consumed completely. For experiment, the methane production increases with
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catalyst inventory. The simulation results are in reasonable agreement
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with experimental data. However, the increase is quite minor, and we find a little decrease in simulation. Because of the fixed feedstock and the
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concentrations of catalysts for each cases are enough, further increase in
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the catalyst amount will not improve the production of methane. Another reason, we think, is the increase of inventory restrains the remove of
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reaction heat which will accelerate the water-gas reaction and reverse reaction.
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Fig. 3 Snapshots of instantaneous solid volume fraction for different catalyst
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inventories (70g, 100g, 140g, 200g)
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100
(a)
Experiment data
Simulation results
90
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XH2 (%)
80
70
50
70 g
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60
100 g
140 g
200 g
(b)
Experiment data
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100
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Catalyst inventory (g)
70
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SCH2 (%)
80
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90
Simulation results
60
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50
70 g
100 g
140 g
200 g
Catalyst inventory (g)
Fig. 4 Effect of catalyst inventory on conversion of H2 and production of CH4
Fig. 5 shows the distribution of concentration and temperature for two different cases (100g, 140g). Only the first 30mm are displayed to show the details of information since the reactions complete rapidly and important features of temperature profiles are within first few millimeters, while the upper parts of bed are quite same. In the simulation of 140g
ACCEPTED MANUSCRIPT catalyst, the H2 concentration decreases and CO diminishes very rapidly in first 5 mm. CO2 is generated in first 5 millimeters by water-gas shift reaction. The trend of gas concentration with 100g catalysts is similar with the case with 140g catalysts. But there are some differences need to
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be noticed. The locations of turning points of gas concentration curves are
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shifted by a few millimeters downwards compared with first simulation
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results (140g). The rate of conversion of H2 in simulation no.2 (100g) is faster than that in simulation no.1 (140g), as shown in the figure by angle
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a and b, also the production rate of CH4. The appropriate explanation can
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be found in the profile of temperature. According to the reaction kinetic theory, increase of temperature will accelerate both the methanation and
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reverse reaction rate, and the water-gas shift reaction plays an important
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role in this temperature range according to the experiment. We can distinguish more CO 2 in simulation no.1 (3%) than that in simulation no.2
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(1%). One would expect a high rate of H2 conversion and CH4 production, but the opposite is observed due to the high temperature.
ACCEPTED MANUSCRIPT 30
30
140g catalyst
25 20 15
25
CO H2
20
CH4
15
H2O CO2
5
5
0
0
5
10
15
20
25
30
35
40
45
50 328
332
30 25
CO H2
15
CH4 H 2O
10
CO2
5 0 0
5
10
15
20
25
30
35
40
45
50 328
332
344
30 25 20 15 10 5
336
340
0 344
Temperature
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Volume fraction (%)
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b
340
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100g catalyst
20
336
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0
Height of bed (mm)
a
10
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Height of bed (mm)
10
Fig. 5 Profile of simulated gas species concentration and bed temperature
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Fig. 6 illustrates the influence of gas velocity on the concentration of
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CO and CH4 along the bed height. The rates of CH4 production and CO conservation increase with raising gas velocity. The fluidization quality
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has significant impact on the reaction process. For methanation reaction, the remove of heat is the key for fluidized bed reactor. Under the high gas velocity, the reaction heat will be removed quickly and completely. However, for the case with low gas velocity, the methanation reaction will be restrained and reverse reaction will be accelerated due to high temperature. That is the reason we get the distribution in Fig. 6.
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20
XCO (Vol%)
16
Vg= 6.3 cm/s
12
Vg= 7.8 cm/s
8
Vg=12.6 cm/s
4 60
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40
Vg= 6.3 cm/s 30
Vg= 7.8 cm/s
20
Vg=12.6 cm/s
10
0
5
10
15
20
25
30
35
40
45
50
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Height of bed (mm)
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XCh4 (Vol%)
50
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Fig. 6 Profile of CO and CH4 concentration along the bed height
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4. Conclusion
The simulations of methanation reaction in fluidized bed over Ni/Al2O3
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catalyst reactor are carried out by CFD open-source software. The
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Eulerian-Eulerian model coupling with reaction kinetic is adopted for simulating the catalysts flow behavior and reaction characteristic. The
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results of species concentration are in reasonable agreement with experimental data. CO methanation reaction completes at first few millimeters and the reaction kinetic is dominated is this region. The CO and H2 are consumed rapidly and CH4 is produced in this region. However, the mixing is the main process in the upper parts. At the outlet we can find H2 due to water-gas shift reaction, which is beneficial for CO conversion. At current situation, the increase of catalyst inventory will not influence the total production of methane, but the conversion rate
ACCEPTED MANUSCRIPT decreases due to the change of distribution of temperature. The high fluidizing gas velocity is beneficial for the remove of reaction heat. At the current range of gas velocity, the CO conversion and methane production increase with the increase of gas velocity. More works for methanation
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reaction in scale-up reactor are in process in our group.
[J/(kg K)]
Specific heat
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c
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Nomenclature
drag coefficient
d
[m]
diameter
g
[m2 s-2 ]
gravitational acceleration
h
[w/(m K)]
heat transfer coefficient
J
[-]
diffusion flux
k
[W/(mk)]
p
[Pa]
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thermal conductivity pressure
Prandtl number
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Pr [-]
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Cd [-]
r
[mol/s kgcat ]
Reaction rate
R
[-]
Source term
Re [-]
Reynolds number
t
[s]
time
T
[K]
temperature
u
[m/s]
velocity
Y
[-]
mass fraction gas species
ACCEPTED MANUSCRIPT Greek letters [-]
volume fraction
[-]
drag coefficient
[Pas]
Shear viscosity
[Pas]
bulk viscosity
[kg/m3 ]
density
[m2 /s2 ]
granular temperature
[S/m]
conductivity of granular temperature
[-]
dissipation rate from collision
[Pa]
stress tensor
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Acknowledgement
This work is supported by China Postdoctoral Science Foundation funded
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project (509200-X91701), the Fundamental Research Funds for the
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Central Universities, for which we are grateful.
ACCEPTED MANUSCRIPT Reference
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catalysts. Catalysis Communications. 2013,31: 5-10.
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[2] Sabatier P, Senderens J B. Comptes rendus des séances de l’academie
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[5] Wang Shuai, Wang Qi, Chen Juhui, Liu Guodong, Lu Huilin, Sun
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Methanation in a fluidized bed reactor with high initial CO partial pressure: Part I-Experimental investigation of hydrodynamics, mass transfer effects, and carbon deposition. Chemical Engineering Science. 2011,66: 924-934.
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Graphical abstract Highlights
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CO methanation is numerical studied considering the fluidization
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of catalysts.
The water-gas shift reaction is taken into consideration during
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process.
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Higher gas velocity is helpful to remove the reaction heat.