Article pubs.acs.org/IECR
Novel Sorption-Enhanced Methanation with Simultaneous CO2 Removal for the Production of Synthetic Natural Gas Soo Ik Im and Ki Bong Lee* Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-713, Republic of Korea ABSTRACT: With increasing consumption of natural gas as a clean energy source and demand for efficient use of cheap and abundant coal, the production of synthetic natural gas from coal has been receiving considerable interest. In this study, the methanation reaction of coal-derived syngas for the production of synthetic natural gas was investigated using numerical simulations. In particular, the concept of a sorption-enhanced reaction, in which CO2 removal by sorption is carried out simultaneously with the reaction, was newly applied to the methanation reaction. Effects of the operating parameters such as the fraction of catalyst and sorbent, temperature, pressure, and feed ratio (H2/CO, H2O/CO, and CO2/CO) on CO conversion and purity, selectivity, and productivity of CH4 were evaluated by computational studies. It was found that the performance of the sorption-enhanced methanation reaction is controlled by both thermodynamic equilibrium and reaction kinetics. Therefore, the reaction would require an optimal catalyst fraction, temperature, and pressure conditions for maximum efficiency. Optimal H2/CO and H2O/CO ratios exist considering reaction performance, and any CO2 content in the feed reduces CH4 productivity. Compared to the conventional methanation reaction, the sorption-enhanced methanation reaction produces CH4 in high purity (>95%), which can be directly used for synthetic natural gas without further separation processes.
1. INTRODUCTION Since the past few years, the demand for energy has been increasing incredibly due to the rapid development of energyintensive industries and increasing human activities. Currently, about 80% of the global energy is produced from conventional fossil fuels. This massive usage of fossil fuels is primarily responsible for serious environmental problems such as pollution and global warming.1 With increasing awareness of these issues, the global consumption of natural gas has been increasing because it is considered to be a clean energy source; burning of natural gas is a highly efficient combustion process with lower emissions of CO2 and pollutants than any other fossil fuel.2,3 In contrast, coal has long been known as the major factor contributing to environmental issues. However, coal is cheap and abundant, and therefore, numerous efforts have been made to develop clean coal technologies to reduce the environmental impacts of the usage of coal energy.4 One of the promising clean coal technologies is to produce synthetic natural gas (SNG) from coal-derived synthesis gas. Unlike direct coal combustion, coal-to-SNG can reduce and efficiently control the emissions of CO2 and pollutants caused by coal usage.4 Also, established infrastructures such as pipelines and distribution networks for natural gas can be used for the transportation of SNG.5−7 In the production of SNG from coal, the raw material is initially converted to gases such as methane, carbon monoxide, hydrogen, carbon dioxide, and water vapor at high temperatures (>1000 °C). Additionally, the carbon monoxide and carbon dioxide in the refined synthesis gas are converted to methane, the main component of SNG, based on a catalytic methanation reaction. Before the reaction, sulfur compounds are removed by commercial processes based on © XXXX American Chemical Society
absorption using solvent (e.g., Selexol) and adsorption using ZnO. SNG production processes have been developed since the 1960s, and since then, most of the research efforts have been directed toward the design of efficient reactors and improvement in reaction efficiency.8 Since the reactions involved in SNG production are exothermic, it is preferable to carry out the reactions at lower temperatures, but then, the kinetics of the catalytic reaction would be unfavorable at very low temperatures. In addition to methanation reactions, the separation of the remaining CO in the feed and byproduct components such as CO2 and H2O is important for the production of high-purity methane.8 New reaction schemes also have been studied to improve the conventional methanation reaction. Lim et al. invented a new SNG production process in which the methanation reaction is carried out in two steps with the water gas shift (WGS) reaction sandwiched between them. This method can control the heat generation from the exothermic methanation reaction and extend catalyst lifetimes.9 Duyar et al. proposed a new methanation reaction process in which a dualfunctional material is used to capture CO2 from a flue gas at 320 °C, and the captured gas is directly used for a CO2 methanation reaction at the same temperature. The new process can save thermal energy required for the reaction.10 Recently, a new sorption-enhanced reaction (SER) concept was applied to methanation reactions to enhance conversion and to Received: May 1, 2016 Revised: August 6, 2016 Accepted: August 6, 2016
A
DOI: 10.1021/acs.iecr.6b01681 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research simultaneously remove the byproducts.2,11−13 This simultaneous byproduct removal by sorption, which occurs in the same reactor, enhances the reaction efficiency based on Le Chatelier’s principle. The SER process circumvents the thermodynamic limitation in the reaction: The conversion of reactants to products can be enhanced through the byproduct sorption for the same equilibrium constant, compared to the catalyst-alone case. It enables direct production of high-purity product without further purification process via a compact reaction process.14,15 The SER process has been mainly applied to the WGS and steam methane reforming reactions for high-purity hydrogen production.16−20 In this study, the methanation reaction, the WGS reaction, and CO2 sorption were integrated together to develop a novel sorption-enhanced methanation (SE-methanation) process (Figure 1). In this process, CO2 sorption is introduced for
CO2 methanation reaction ΔH298K = −165 kJ/mol
CO2 + 4H 2 ↔ CH4 + 2H 2O
(3)
2.1. Methanation and WGS Reactions. The empirical expressions by Xu and Froment (1989) were used for evaluating the reaction rates as follows:22 CO methanation reaction R1 =
k1 pH
2.5
(pCH pH O − pH 3 pCO K1) 4
2
2
2
1 DEN2
(4)
WGS reaction R2 =
pH pCO ⎞ 1 k2 ⎛ 2 2 ⎟ ⎜pCO pH O − 2 pH ⎝ K 2 ⎠ DEN2
(5)
2
CO2 methanation reaction ⎛ pH 4 pCO K1 ⎞ 1 k3 ⎜ 2 2 2 ⎟ p p R3 = − 3.5 ⎜ CH4 H 2O ⎟ DEN2 K pH ⎝ 2 ⎠ 2
DEN = 1 + K COpCO + K H2pH + K CH4pCH 2
+ K H2OpH O /pH 2
Figure 1. Scheme of sorption-enhanced methanation process.
* (P , T ) = nads
2. NUMERICAL MODELING SNG production normally involves multiple catalytic reactions. Methanation reactions are used to produce CH4, the main component of SNG, from CO and CO2 in the feed stream, while the WGS reaction simultaneously proceeds in the reactor as a side reaction in the presence of Ni-based catalysts.21 CO2 methanation reaction is a linear combination of CO methanation and WGS reactions and therefore the CO2 methanation reaction rate is dependent on the rates of CO methanation and WGS reactions. CO methanation reaction
mK C[1 + (a + 1)KR P a] 1 + K CP + K CKRP a + 1
(1)
⎛q ⎞ K C = K C0 exp⎜ c ⎟ ⎝ RT ⎠
(2)
⎛ ΔΗR ⎞ ⎟ KR = KR0 exp⎜ ⎝ RT ⎠
ΔH298K = − 206 kJ/mol
ΔH298K = −41 kJ/mol
(7)
2
(8)
where nads * denotes the specific amount of CO2 sorbed on the sorbent at an equilibrium state as a function of pressure, P, and temperature, T. m is the maximum sorption capacity of CO2 and a is the stoichiometric ratio of the reaction between gaseous CO2 and chemisorbed CO2. KC and KR are the equilibrium constants for the chemisorption and the additional reaction between gaseous CO2 and chemisorbed CO2, respectively. These equilibrium constants are dependent on the temperature as follows:
WGS reaction CO + H 2O ↔ CO2 + H 2
4
where ki and Ki are the rate and equilibrium constants, respectively, of reaction i. These values are given as a function of temperature. pj is the partial pressure of component j. DEN represents the denominator and is a term consisting of KCO, KH2, KCH4, and KH2O, which are the surface adsorption parameters at equilibrium.22 2.2. CO2 Sorption on K2CO3-Promoted Hydrotalcite. K2CO3-promoted hydrotalcite was used as a CO2 sorbent in this study because of its high sorption stability at the operating temperature (300−350 °C) of the methanation reaction.23,24 Lee et al. developed a CO2 sorption model for K2CO3promoted hydrotalcite as a function of temperature.23 The model assumes a two-step CO2 sorption mechanism: (i) reversible chemisorption of CO2 on the sorbent and (ii) reversible chemical reaction between gaseous CO2 and chemisorbed CO2 to produce a chemical complex.
promoting the WGS reaction to produce more H2, which can then be used for the CO methanation reaction, consequently leading to more CH4 production. The SE-methanation process was investigated using numerical simulations and the effects of operating parameters such as the catalyst fraction, temperature, pressure, and feed composition on reaction performance were examined. The performance of the SE-methanation process was compared with that of the conventional methanation reaction. From numerical simulation results, suitable operational conditions were obtained for an efficient SE-methanation process. The feasibility of this process was studied to ensure that the product composition could meet the natural gas grid specification for direct use of the product as SNG.
CO + 3H 2 ↔ CH4 + H 2O
(6)
B
(9)
(10) DOI: 10.1021/acs.iecr.6b01681 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research where K0C and K0R are constants, qc is the molar isosteric heat of sorption and ΔHR is the reaction enthalpy of chemical complex formation. All the parameters related to CO2 sorption are summarized in Table 1. Other sorbent materials that can sorb high-temperature CO2 also can be applied in this process.
Energy balance ηρb Vstagec ps
dT = N inAc pg(T in − T ) dt
− ρb Vstage[∑ fcat ΔΗiR i +
∑ (1 − fcat )ΔHads, jR ads, j]
i
Table 1. Model Parameters for CO2 Sorption and Reaction parameter
value
m (mol/kg) A K0C (1/atm) K0R (1/atm) qc (kJ/mol) ΔHR (kJ/mol) ρb (g/cm3) Lc (cm) ε η dc (cm) no. of stages
0.25 0.285 exp(12.2/RT) 0.878 0.00134 21.0 42.2 0.824 50 0.75 1.2 1.73 30
j
+ πdcLstageU0(Tw − T )
(15)
where N is the total molar flux, A is the cross-sectional area of the reactor, ρb is the bulk density of solid consisting of catalyst and sorbent, and fcat is the fraction of the catalyst in the solid. yj is the mole fraction of component j and Ri is the rate of reaction i. η is a factor accounting for the heat capacity of the heatexchanger tube and body while nt is the total number of moles of gaseous components. cps and cpg are the heat capacities of the solid and gas phases, respectively. In the energy balance (eq 15), heat transfer between inside reactor and wall was considered, assuming a constant wall temperature (Tw). The parameters used in the numerical simulations are shown in Table 1. The equations were solved by Matlab software using ODE15s function, which is a solver for stiff differential equations with a variable order method. 2.4. Evaluation of Reaction Performance. To evaluate the performance of the SE-methanation process, CO and H2O conversions were used and they are defined as follows:
2.3. Mathematical Model. The well-known model of continuous stirred tank reactors (CSTRs) in series was adopted for our numerical simulations. For a same total reactor volume, a plug flow reactor can be described simply by CSTRs in a series model with a large number of CSTRs.25 The main assumptions for the mathematical model were (i) ideal gas behavior, (ii) instantaneous thermal equilibrium between the gas and solid inside the reactor, and (iii) no axial dispersion and pressure drop. The widely used linear driving force model was chosen for the mass transfer of CO2 sorption.25−28 From material and energy balances, the following governing equations were derived.
tR
CO conversion (%) =
in N inyCO t R − ∫ N outyCO dt 0 in N inyCO tR
(16)
H 2O conversion (%) tR
=
N inyHin O t R − ∫ N outyH O dt 0 2
2
N inyHin O t R
× 100 (17)
2
Ideal gas law
nt =
where N and N are the total molar fluxes of feed and product streams, respectively, and tR is the time on stream of the reaction. Since Nout and yH2O change with time, integration is needed to calculate the cumulative outlet concentration of each component. H2O conversion is used only when H2O exists in the feed stream. CH4 purity, CH4 selectivity, and CH4 productivity are also defined to assess the performance of the SE-methanation process. in
εPVstage (11)
RT
Molar balance in the solid phase (linear driving force model) R ads, j =
dnads, j dt
* j − nads, j) = kads, j(nads,
(12)
out
Overall molar balance in the gas phase
t
N A = N A − ρb Vstage[∑ ∑ fcat R i , j+ out
in
i
CH4 purity (%) =
j
dn ∑ (1 − fcat )R ads, j] − t dt j
dt
=
∫0 R N outyCH4 dt
CH4 selectivity (%) =
i
j
(18)
∫0 R N outyCH4 dt t
∫0 R N out(yCH4 + yCO2 ) dt
× 100
t
CH4 productivity =
+ (1 − fcat )R ads, j} − yj ρb Vstage{∑ ∑ fcat R i , j
∑ (1 − fcat )R ads, j}]
× 100
(19)
1 in in [N A(yj − yj ) + ρb Vstage{∑ fcat R i , j nt i
+
t
∫0 R N out dt t
(13)
Component molar balance in the gas phase dyj
× 100
j
∫0 R N outyCH4 dt ρb Lc
(20)
CH4 selectivity is calculated based on the CH4 fraction of the total amount of carbon compounds in the product stream. CH4 productivity is defined as the rate of CH4 production based on the total amount of solid in the reactor.
(14)
C
DOI: 10.1021/acs.iecr.6b01681 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 2. Change of mole fraction profile in the SE-methanation process: (a) output gas stream and (b) CO2 gas inside the reactor. T = 350 °C, P = 10 atm, H2/CO = 1, CO2/CO = 0.2.
Figure 3. Effect of catalyst and sorbent ratio on the performance of SE-methanation process: (a) CO conversion, (b) CH4 purity, (c) CH4 selectivity, and (d) CH4 productivity. T = 350 °C, P = 10 atm, H2/CO = 1, CO2/CO = 0.2.
3. RESULTS AND DISCUSSION To systematically investigate the effects of the parameters on the SE-methanation process, a base condition (as given below) was chosen and a single parameter was changed while maintaining other parameters constant. The reaction temperature was set at 350 °C because the reaction for bulk production of methane is usually carried out at 300−350 °C.8 A feed gas ratio was selected at CO2/H2/CO = 0.2/1/1 based on the syngas composition of the coal gasification process (CO2, 2.1−15.1 mol%; H2, 28.5−33.5 mol%; and CO, 34.4−57.2 mol %).4 The gas composition is over stoichiometric in terms of the methanation process. The total feed flow rate was fixed at 0.8 mmol/cm2/min, corresponding to weight hourly space velocity (WHSV) of 700 1/h. T = 350 °C,
P = 10 atm,
CO2 /CO = 0.2,
Figure 2a, it can be seen that the product gases start to come out of the reactor column at ∼20 min after the reaction begins. It is noteworthy that high-purity CH4 can be produced between ∼20 min and ∼60 min. During this period, CH4 is produced from the CO methanation reaction (eq 1) and the byproduct H2O further reacts with CO to produce CO2 and H2 via the WGS reaction (eq 2). The H2 produced from the WGS reaction can also be used for the CO methanation reaction. In the SE-methanation process, CO2 produced from the WGS reaction or initially contained in the feed stream can be removed in the reactor, facilitating the WGS reaction with more H2 production. Eventually, high-purity CH4 production can be increased. The sorption-enhanced reaction continues until the sorbent in the reactor is saturated with CO2. After the sorbent is saturated with CO2 (∼60 min in Figure 2a), CO2 and CO breakthroughs come out from the reactor, and the concentration of CH4 reduces. Finally, the effluent composition becomes the same as the equilibrium composition in the presence of the catalyst only (CH4, 41.1 mol%; CO2, 57.2 mol %; and CO, 1.1 mol%; H2, 0.2 mol%; H2O, 0.4 mol%). Therefore, to obtain high-purity CH4, the SE-methanation
H 2 /CO = 1,
fcat = 0.7
3.1. Sorption-Enhanced Methanation Process. Figure 2 shows the change in the mole fraction profile during the SEmethanation process operated in the base condition. From D
DOI: 10.1021/acs.iecr.6b01681 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 4. Effect of temperature on the performance of SE-methanation process and conventional methanation reaction: (a) CO conversion, (b) CH4 purity, (c) CH4 selectivity, and (d) CH4 productivity. P = 10 atm, H2/CO = 1, CO2/CO = 0.2.
catalyst, there is no noticeable change in performance with increasing the catalyst fraction any further. It is noteworthy that CH4 productivity first increases with increasing fcat, reaches a maximum, and then decreases when fcat is larger than 0.7. In the presence of a large catalyst fraction, CO2 sorption is reduced due to the small amount of sorbent present in the system, resulting in diminished effects of the sorption-enhanced reaction and lower CH4 productivity. From the two conflicting effects of catalyst and sorbent amounts, there is an optimal fcat (0.7) for maximum CH4 productivity in this condition. 3.3. Effect of Temperature. Since reaction rate is strongly dependent on temperature, it is one of the important operational parameters for the SE-methanation process. Figure 4 depicts the effect of temperature on the SE-methanation process and conventional methanation reaction. At low reaction temperatures of 275−300 °C, a very low CO conversion (95%) at 350−600 °C due to the effect of the sorption-enhanced reaction which compensates for the reduction of reaction performance at high temperatures: byproduct (CO2) removal can increase the conversion of CO to products based on the Le Chatelier’s principle. Since the reactor temperature can rise up to 600 °C during the methanation reaction,8,33 maintaining a high CO conversion over a wide temperature range is beneficial. Compared to the methanation and WGS reactions, the exothermic effect of CO2 sorption is not significant. Figure 4b shows that a low CH4 purity (
