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Experimental and process simulation of hydrate-based CO2 capture from biogas
Journal of Natural Gas Science and Engineering 72 (2019) 103008
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Journal of Natural Gas Science and Engineering journal homepage: http://www.elsevier.com/locate/jngse
Experimental and process simulation of hydrate-based CO2 capture from biogas Qi Li a, b, c, Shuanshi Fan a, *, Qiuxiong Chen b, Guang Yang b, Yunwen Chen b, Luling Li b, Gang Li a a
Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, PR China b Shenzhen Gas Corporation Ltd., Shenzhen, 518040, PR China c CAS Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Guangzhou, 510640, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Biogas Clathrate hydrates Process simulation Energy consumption Carbon dioxide capture
This study experimentally investigated the effects of operation conditions for the CO2 capture from biogas by clathrate hydrate formation. The experiments were tested at 3.0 MPa and 278 K in the present of 5.0 wt % tetran-butylammonium bromide (TBAB). In addition, the dissociation enthalpies of CH4/CO2 hydrates in TBAB so lution was calculated for energy consumption. The hydrate process simulation with Aspen Plus was carried out at the same experimental conditions to simulate the actual hydrate-based CO2 capture (HBCC), and the energy consumption of this process was analyzed. Experimental results show that the CH4 fraction in the residual gas phase increased with the decrease of gas liquid volume ratio (Rv). Rv decreased from 35.93 to 3.61 while the concentration of CH4 in residual gas increased from 80.5 mol % to 92.76 mol %. The CH4 concentration in re sidual gas phase was enriched from 67.00 mol% to 97.00 mol% by two stages of HBCC. Process simulation results showed that the energy cost was 1.17 kWh/kg CH4. According to the handing capacity of the feed gas, the energy cost was 0.390 kWh/kg biogas, which was lower than that by chemical absorption method. After adjusting the process parameters and increasing exhaust gas energy recovery, the energy consumption was reduced to 0.357 kWh/kg biogas. The energy consumption could be decreased to 0.204–0.223 kWh/kg biogas by using static hydration enhancement technology.
1. Introduction Because of its capability for continuous energy production, and very low grade feedstock, such as biomasses and wastes, along with energy crops, biogas becomes a kind of the most important renewable energy source (Khan et al., 2017). Raw biogas is mainly composed of methane (CH4, 40–75% v/v) and carbon dioxide (CO2, 15–60% v/v), along with trace amounts of hydrogen sulfide (H2S), oxygen (O2), carbon monoxide (CO) and nitrogen (N2) generally not greater than 2% v/v (Andriani et al., 2014). Biogas produced from organic materials can be used directly to generate power, but the large volume of CO2 reduce its heating value, and limiting economic feasibility to use. On average, the calorific value of natural gas is 35.8 MJ/m3, while that of biogas is 21.5 MJ/m3 (Kadam et al., 2017). Removing CO2 from the biogas increases the calorific value for the concentration of methane is increased. Pressure Swing
Adsorption (PSA) (Yin et al., 2012), Water Scrubbing (WS) (Kapoor et al., 2017), Chemical Absorption (CA) (Ma et al., 2014), Membrane Separation (MS) (Yan et al., 2014) and Cryogenic Upgrading (CU) sep aration (Grande et al., 2014) are effective techniques applied to capture CO2 and other small amount components from biogas, which have been studied for years. In addition, gas hydrate separation technology was a promising method for CO2 capture (Dashti et al., 2014; Zheng et al., 2016; Li et al., 2019) and attracted numerous attentions in recent years. The basis for the separation was the selective partition of CO2 between the hydrate phase and the gas phase. The HBCC can be of high capacity and simple to operate, for its unique physical properties such as guest substance selectivity, large gas storage capacity and large value of the enthalpy of formation/decomposition (Wang et al., 2014). Moreover, the using of promoters such as cyclopentane (Zhong et al., 2013, 2016; Brown et al., 2016), tetrahydrofuran (Linga et al., 2007; Mech et al., 2016; Liu et al., 2019), propane (Kumar et al., 2008) and TBAB (Li et al.,
* Corresponding author. Room 302, YiFu Engineering Building, School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou, 510640, PR China. E-mail address: [email protected] (S. Fan). https://doi.org/10.1016/j.jngse.2019.103008 Received 20 August 2019; Received in revised form 18 September 2019; Accepted 23 September 2019 Available online 25 September 2019 1875-5100/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. The schematic diagram of the devices for gas separation based on hydrate formation.
2011, 2015; Fan et al., 2013) moderated the conditions of hydrate for mation. Meanwhile, TBAB forms semiclathrate hydrates at ambient conditions of atmosphere pressure and 285.15 K (Dyadin et al., 1984; Oyama et al., 2005), which can trap small gas molecules like CH4 and CO2 in the small cavities (S-cage, 512) at favorable temperatures and pressures. The technology of HBCC from various types of gas mixture (e.g., shifted synthesis gas, flue gas, and the sour natural gas or biogas) have been studied and proved to be promising and economically feasible compared with traditional techniques (Wang et al., 2013; Xu et al., 2014). Recently, researchers put attentions to evaluate the feasibility of HBCC through gas hydrate formation by simulating and analyzing the energy costs. Tajima et al. (2004) analyzed the energy cost for CO2 capturing through the formation of CO2 hydrate for a practical appli cation to a flue gas (CO2/N2), and found the major contributors to the energy consumption can be attributed to the compression process, because high pressure conditions are necessary for CO2 hydrate forma tion (1.4 MPa at 274 K), and the CO2 concentration of this component in flue gases was very low (about 10–15%). The total energy consumption for their separation process was 158.4 MW, which was equivalent to 0.853 kW h/kg-CO2 (Arca et al., 2011). calculated the energy cost of the process of biogas (60.00 mol % CH4/CO2) upgrading to bio-methane (70.00 mol % CH4/CO2) through hydrate formation. Two different process approaches were proposed and analyzed, the first process was CO2 hydrate process (at 275.15 K and 3.0 MPa), where CO2 hydrate was formed selectively from the biogas. And another process was biogas hydrate process (at 275.15 K and 2.5 MPa), where biogas hydrates was first formed, than selectively dissociated in order to release CO2. The results showed that biogas could be upgraded to bio-methane with two different process approaches with at an energy cost of only 0.174–0.180 kWh/kg Biogas. Castellani et al. (2014) investigated the application of gas hydrate technology to capture of CO2 and H2S from biogas. Theoretical energy evaluations showed that hydrate-based pro cess was a feasible technology, competitive with respect to other con ventional separation methods. Castellani et al. (2017, 2018) also investigated an innovative biogas upgrading process, based on gas hy drate technology, and combining it with power-to-gas technology to produce additional synthetic methane. When the electric energy required by the process was provided by photovoltaic panels, the total energy efficiency was about 0.82, higher than the worldwide average energy efficiency for fossil methane, which was 0.75. The energy cost of HBCC from flue gas was significantly higher than that from biogas, because high operating pressure was needed for HBCC from flue gas. However, excessive energy consumption was one of the main barriers for HBCC promotion, and the scale up of biogas upgrading by hydrate
formation has been poorly investigated and different interpretations can be found in literature. In this study, hydrate phase equilibrium data for the CO2/CH4 þ TBAB system were measured to calculate the dissociation enthalpies of hydrate formation. The experiments of the HBCC from biogas at 3.0 MPa, 278 K with 5 wt% TBAB were investigated. In addition, the process simulation by Aspen Plus was established based on the experimental results and the energy consumption of this process were calculated. The process simulation provides a theoretical energy consumption param eter for the industrialization of hydrate-based gas separation. 2. Experimental section 2.1. Materials The mixture gas CH4 (67.00 mol%)/CO2 and CH4 (89.50 mol%)/CO2 with an uncertainty of �0.001 were supplied by Zhao Qing Gao Neng Da Gas Co. Ltd, China. TBAB with a purity of 99% (Guang zhou Jin ke chemical Co. Ltd, China) was weighed by an electronic balance with a uncertainty in the mass fraction is �0.01, the mass fractions of TBAB solution in this work were all 5 wt%, all of the solutions were prepared at the room temperature and atmospheric pressure. Distilled water was used in all experiments. 2.2. Apparatus The experimental apparatus used for the measurements of hydrate phase equilibrium conditions were clearly described in our previous literature (Li et al., 2010). A schematic diagram of the reactor employed for gas separation was shown in Fig. 1. It mainly included a stainless-steel reactor (7.5 cm in diameter, effective volume 1000 cm3), which equipped with a magnetic stirrer (Haian Scientific Research Apparatus Co, China). The reactor was designed to be operated under 20.0 MPa. Two quartz windows (4.5 cm in diameter) were mounted on both sides of the reactor. The temperature of the reactor was controlled by circulating the coolant from a thermostat (Huber CC1-K20B) with a stability of �0.01 K inside the jacket around the reactor. The tempera ture of the reactor was monitored using two Pt100 resistance ther mometers (Westzh WZ-PT100) within 0.01 K in accuracy, which are placed in the middle and bottom of the reactor, respectively. A pressure transducer (Westzh CyB-20S) within 0.01 MPa in accuracy measured the pressure inside the reactor. The aqueous solution of TBAB at a desired composition was introduced into the reactor by a high pressure metering pump (Alipu JX/12.5). The pressures and temperatures of the reactor were recorded by data logger (Agilent 34970A). During the experiment, 2
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Table 1 Dissociation enthalpies of CH4/CO2 hydrates in TBAB solutions. T/K
p/MPa
Z
Hdiss/kJ⋅mol
276.00 280.00 283.20 284.80 286.60
2.85 4.10 5.82 7.10 9.22
0.8984 0.8599 0.8082 0.7715 0.7169
64.60 61.83 58.12 55.48 51.55
1
CH4 fraction in gas phase (mol %)
CO2 separation factor
3.61 8.12 21.65 35.93
92.76 90.07 83.19 80.50
21.2 20.5 5.6 5.0
the compositions of the gas phase were determined by gas chromatog raphy (KeChuang GC9800TCD). 2.3. Procedure
d ln p � �¼ d T1
(3)
ΔHdiss zR
3.2. Two stages of hydrate reaction
The equilibrium hydrate formation conditions for the CO2/CH4 þ TBAB system were measured by an isochoric pressure search method (Fan et al., 2013). After introducing TBAB solution into the evacuated and vacuumed reactor, the reactor was cooled to 278.0 K. When the temperature in the reactor was kept constant, the reactor was vacuumed and purged with CH4/CO2 mixture gas 5 times to ensure the absence of air, and then mixture gas of CH4/CO2 was charged into reactor to 3.0 MPa. Then the stirring was started and set as time zero. During the experiments, the temperature and pressure were recorded, and the gas samples were transferred and analyzed with GC. After the completion of hydrate formation (the system pressure was kept constant), the stirrer was stopped, the vent valve was opened, and the remaining gas was purged. Then, the reactor was warmed to room temperature to make the hydrate dissociate completely. The dissociated gas composition was also determined with GC.
In stage one of hydrate reaction, CO2 capture from CO2 (33.00 mol %)/CH4 mixture gas with 5 wt% TBAB solutions by hydrate formation was measured at 3.0 MPa and 278.0 K. The reaction temperature, pres sure and TBAB concentration were chose according to our previous work (Li et al., 2015). Rv was ranged from 3.61 to 35.93. Table 2 showed that the higher CH4 concentration in residual gas phase and CO2 separation factor were obtained at the operation Rv as 3.61 and 8.12, respectively. As expected, a negative correlation between CH4 concentration in residual gas phase and Rv was obtained. The reason might be that more solution provided more hydrate cavities, under the same gas flow, CO2 enrichment in the hydrate phases easier. CO2 sep aration factor was defined to quantify the volume of separation effi ciency ratio for different Rv, the maximized CO2 separation factor was 25.5. The optimal Rv was existed for maximum separation factor. According to results of stage one of hydrate reaction selection pa rameters, a stage two hydrate separation process was carried out to obtained higher purity CH4/CO2 mixture gas from CO2 (10.50 mol %)/CH4 mixture gas at 3.0 MPa and 278.0 K with 0.05 wt% TBAB. The result showed that CH4 concentration in residual gas phase was enriched from 89.50 mol % to 97.00 mol % at Rv of 7.9. The results of separation experiment showed that the gas mixture was technically feasible to achieve the standards of commercial natural gas by two stages of hy dration process. The CO2 separation factor can reach about 24.9 at Rv of 7.9 with high separation efficiency.
2.4. Gas liquid volume ratio and separation factor Rv is calculated in Eq. (1): RV ¼
the
where p is pressure, T is temperature, z is compressibility factor for gas and the compressibility factor is determined by Peng-Robinson (P-R) equation (Peng et al., 1970), R is universal gas constant (R ¼ 8.314 J/mol⋅K), and ΔHdiss is the dissociation enthalpy of gas hydrates. The phase equilibrium and dissociation enthalpies of CH4/CO2 hy drates in TBAB solutions determined by using Clausius-Clapeyron equation was shown in Table 1. The results showed that the dissocia tion enthalpy of hydrate decrease with the increase of the reaction pressure. Through linear match, the hydrates dissociation enthalpy for this simulation technology system was 64.39 kJ mol 1 at the condition of 278.0 K and 3.0 MPa, the compressibility factor was 0.8957. Therefore, the formation enthalpy of hydrates was 64.39 kJ mol 1 at 278.0 K and 3.0 MPa. The dissociation enthalpy of gas hydrates was measured for calculating the hydrate formation enthalpy.
Table 2 The CH4 concentration in residual gas phase and CO2 separation factor by one stage of hydrate reaction with different Rv. Gas-liquid volume ratio (Rv)
calculated dissociation enthalpy of hydrates by using Clausius-Clapeyron equation, which was shown in Eq. (3):
22400n0 VL
(1)
where n0 is the number of moles of feed mixed gas, VL is the volume of the liquid phase. The separation factor of CO2 is calculated in Eq. (2): . nHCO2 nHCH4 S ¼ gas � gas (2) nCO2 nCH4
4. Process simulation The technology of hydrate-based purification of biogas focused on energy consumption of biogas handing capacity and the purity of products, methane recovery and reaction time of the process. This simulation process was simulated by Aspen Plus 7.2, and scale up with the lab results, complied with the law of the raw material conservation and energy conservation, the mechanical energy consumption of the whole process was calculated. It was designed based on the experimental results to achieve the lowest energy consumption, choosing 3.0 MPa and 278.0 K at the Rv of 8.12 as simulation parameters for first stage of hy drate formation, 3.0 MPa and 278.0 K at the Rv of 7.9 as simulation parameters for second stage of hydrate formation. The concentration of CH4 was 90.07 mol % after stage one of hydrate reaction, after stage two of hydrate reaction, the CO2 concentration was lower than 3 mol%. The
gas
where nCO2 and nH CO2 are the number of moles of CO2 in the gas phase and gas
hydrate phase at the end of the experiments, respectively. nCH4 and nH CH4 are the number of moles of CH4 in the gas phase and hydrate phase at the end of the experiments, respectively. 3. Experimental results and discussion 3.1. Dissociation enthalpies of CH4/CO2 hydrates in TBAB solution
Hydrate phase equilibrium data for the CO2/CH4 þ TBAB system were measured at temperatures from 276 to 288.6 K and pressures from 2.85 to 9.22 MPa with the 0.05 wt% TBAB. Sloan and Fleyfel (1992) 3
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Fig. 2. The schematic diagram of process simulation for two stages of hydrate-based CO2 capture form biogas.
Fig. 3. The simulation process of hydrate-based CO2 capture from biogas.
schematic diagram of this process simulation. was shown in Fig. 2. The feasibility of the HBCC technology from biogas was certificated through analyzing the energy consumption, energy consumption rate and methane recovery. Energy consumption of the process was reduced by adjusting the process parameters, adding energy recovery unit and energy saving operation conditions for separation process. The process of hydrate-based CO2 capture from biogas was sho wn in Fig. 3. The main route for the decarbonization process: the raw gas was pressurized by a two stages of compression with air cooling device, then delivered to the heat exchanger BC3. The raw gas was cooled by refrigerant 1,1,1,2-tetrafluoroethane (R134a), then injected into the first stage hydrate reactor HFORM1. The residual gas of first stage of hydrate formation was pressurized by the compressor GCOMP, and heat exchanged with refrigerant R134a to the certain subcooling degree then entered into the second stage hydrate reactor. The hydrate slurry of first stage hydration was introduced into a hydrate dissociation reactor, and heated by heating system. The water was pumped by B2 to heat
exchanger RC3. The decomposition solution was returned to the hydrate reactor for the next hydrate reaction. Throughout the process, the hy dration process was cooled by the cooling system, and the hydrate dissociation process was heated by the heating system. A heat transfer system transported heat from hydrate reactor to decomposition reactor and excess heat through a heat exchanger RC1, which was cooled down by air. 4.1. Process unit 4.1.1. Gas compression cooling unit Clathrate hydrate was formed at low temperature and high pressure conditions. The export pressure of common biogas was 8–10 kPa, and gaged pressure was 0.11 MPa. The raw biogas needed be pressurized to 3.0 MPa. Fig. 4 showed a two stages of gas compression cooling unit, setting the compression ratio as 5.3. After each stage, the temperature of pressurized feed gas reached about 373.15 K, then cooled by air cooler 4
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Fig. 4. Two stages of gas compression cooling unit for biogas.
Fig. 5. Two stages of hydrate reaction unit.
(AC1, AC2). The pressurized gas was cooled to 303.15 K with air, then cooled to the hydrate formation temperature by cooling system. 4.1.2. Hydrate reaction unit According to the experimental results of stage one hydrate reaction, the methane concentration of residual gas phase reached more than 89.5 mol %. In order to get the product gas that included less than 3.0 mol % carbon dioxide, a two stages of hydrate reaction process was designed. Fig. 5 was the two tage hydrate reaction unit. The cooled and pres surized feed gas were injected into the hydrate formation reactor (HFORM1) and formed hydrate with the TBAB solution. Hydrate for mation was accelerated by mechanical agitation. After stage one hydrate process, the residual gas in gas phase was pressurized by gas compres sion pump (GCOMP) and cooled by heat exchanger (BC4) then sent into another hydrate formation reactor (HFORM2), at the end of the second stage of hydrate formation product gas was obtained. Since Aspen Plus software without hydrate formation module, thus the user model was used for hydrate-based separation process simulation.
Fig. 6. One of hydrate dissociation unit.
4.1.3. Hydrate dissociation unit Fig. 6 showed the hydrate dissociation unit, which hydrate slurry was heated and gases were released from hydrate. At the end of the 5
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Fig. 7. The circulation unit of heat source for hydrate reaction and dissociation. Table 3 The process simulation main parameters of feedstocks and experimental conditions. Exp.no
Total gas flow (mol⋅h 1)
CH4 (mol⋅h
Feed gas1 G1a H1b Feed gas2 G2c H2d
1160 728 432 728 593.32 134.68
777.2 651.56 125.64 651.56 574.52 76.04
a b c d
1
)
CO2 (mol⋅h
1
TBAB solution (mol⋅h 1)
)
382.8 76.44 306.36 76.44 17.8 58.64
1.778 � 105 1.147 � 10
5
Pressure (MPa)
Recovery rate (%)
3.0 2.1
83.83
3.0 2.1
88.18
Gas phase of first stage of hydrate formation. Hydrate phase of first stage of hydrate formation. Gas phase of second stage of hydrate formation. Hydrate phase of second stage of hydrate formation.
Table 4 Energy consumption of major equipment for the simulation progress. type of facility
code
raw material
Fa (kg⋅h
compressor
BCOMP1 BCOMP2 GCOMP RCOMP SP1 SP2 B2 FAN1 FAN2 FAN3
biogas biogas RSb R134a TBABc TBABc water air air air
29.32 29.32 13.82 71.50 800.60 516.05 135.80 1013.30 1013.30 578.40
fluid pump Fan stirrer sum a b c
1
)
Ti (K)
Pi (atm)
To (K)
Po (atm)
Energy cost (W)
303.15 303.15 278.00 268.15 278.00 278.00 283.15 298.15 298.15 298.15
1.09 6.00 20.73 2.20 20.73 20.73 1.00 1.00 1.00 1.00
476.06 477.48 312.93 319.89 278.49 278.49 283.20 298.23 298.41 299.30
5.70 30.00 30.00 8.14 3.00 3.00 2.00 1.01 1.01 1.01
2245.3 2222.3 235.3 843.1 697.7 452.0 12.8 41.7 41.7 190.2 4468.8 11450.9
The amount of flow. Residual gas in gas phase of HFORM 1. The amount of TBAB solutions.
hydrate dissociation, residual solution was pumped back to the hydrate reactor. Based on the law of heat conservation, the enthalpy of hydrate formation and dissociation were generally equal.
W3 were hydrate decomposition heat cycle. High temperature R134a heat changed with water in the heat exchanger RC2, and hot water recycled by fluid pump B2. This heat source circulation unit neither need add additional heat nor cold energy.
4.1.4. Heat source circulation unit As shown in Fig. 7, R1-R6 were R134a refrigerant circulation path, and R134a was throttled by VALVE, then entered BC4 evaporation heat, cooling the residual gas from first stage of hydrate formation, the raw gas cooled in BC3. Refrigerant heat exchanged in BC4 and BC3 by endothermic through phase transition from liquid to gaseous state. In order to achieve the hydration temperature, the pressure of R134a was no more than 349.96 kPa after the throttle. Generated low pressure steam was pressurized by the compressor (RCOMP), then injected into the heat exchanger RC2, which heated the dissociation reaction circu lating water. Finally, highpressure refrigerant vapor cooled to liquid through a heat exchanger RC1 with air, transported into next cycle. W1-
4.1.5. Mechanical stirring unit Experimental process requires mechanical agitation to strengthen the heat and mass transfer of hydrate formation process. Stirring power N calculated in Eq. (4): N ¼ φρn3 d5
(4)
where ρ is the solution density, n is the rotational speed, d is the diameter of blade turbine. The power factor φ affected by stirrer configuration and Reynolds number (Re), calculation with Rushton φ -Re curve in Eq. (5):
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Re ¼
nρd2
μ
(5)
where μ is the solution viscosity, n is the rotational speed, ρ is the so lution density, d is the diameter of blade turbine. In geometrically similar system, in order to obtain the same effect in the hydrate reaction, a blade tip speed uT ¼ πnd is needed to be un changed, that is calculated in Eq. (6): n1 d1 ¼ n2 d2
(6)
where n1 and d1 are rotational speed and the diameter of blade turbine in laboratory stirrer, respectively. n2 and d2 are rotational speed and the diameter of blade turbine in the simulation process, respectively. In this simulation process, the amount of feed was enlarged 1000 times, equal to the reactor geometry enlarged 10 times. According to the principle of geometric similarity, the size of bladed paddle proportional amplification was calculated in Eq. (7): N2 n32 d52 d22 ¼ ¼ ¼ 100 N1 n31 d52 d21
(7)
The stirrer speed was 600 r/min, and the stirring power was 27.5 W. Therefore, the stirring power of stage one hydrate reaction was calcu lated as 2750 W. The amount of solution for stage two hydration was 62.5% of the stage one hydration. In this case, the stirring power of stage two of hydrate reaction was calculated as 1718.8 W. The total stirrer energy consumption of the simulation process was 4468.8 W. 4.2. Simulation results and discussions In this work, a two stages hydrate reaction separation process was designed to calculate the main mechanical energy costs. Hydrate-based separation simulation process for CH4 (67.00 mol%)/CO2 mixture gas with 5 wt% TBAB solutions, the initial pressure and temperature of raw gas was 0.11 MPa and 303.15 K, respectively. The hydrate formation of stage one and stage two were carried out at 3.0 MPa and 278 K, and the mole flow was 1160 mol/h and 728 mol/h, respectively. There were some rules defined for the process simulation. A certain temperature difference was required between logistics, assumed minimum temper ature difference between the heat exchanger of the logistics process with 5.0 K. The pressure and temperature of air were 0.1 MPa and 298.15 K, respectively. The amount of solution back to hydrate formation reactor was equal to 25% of the total solution. Assuming that during the hydrate formation and decomposition, heat transfer process with the cooling and heating system, there was no heat exchanged with the environment. The mass flow of raw biogas for the simulation process were 29.32 kg h 1. The amount of product were 10.57 kg h 1, and the CH4 concentration was 97.0 mol %. Table 3 listed the main parameters and experimental conditions for the simulation process. The hydrate reaction is exothermic process, and reaction heat can be calculated in Eq. (8): Q¼
MΔHdiss
(8)
where Q is enthalpy of hydrate formation process, and M is the moles of hydrates. ΔHdiss is the dissociation enthalpy of gas hydrates and has measured at experimental section. The decomposition thermal of this simulation process was 36.49 kJ/ h calculated by Eq. (8). Therefore, the cooling for hydrate formation was 0.01 kW, and easily supplied from the cooling system (Arca et al., 2011). The heat source circulation unit provided the heating and cooling power for the whole process. In this case, the final cost for the whole process would be only electric power. As seen in Table 3, the CH4 recovery of this separation process was 73.9%, and the CH4 recovery of second stage of hydrate formation slightly higher than those of first stage of hydrate formation. The final pressure was reduced 0.9 MPa due to the formation of hydrate crystals in the reactor.
Fig. 8. Energy cost distributions for the simulation progress: (a), total energy cost; (b), energy cost of compressor; (c), energy cost of fluid pump.
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Fig. 9. Energy recovery apparatus for the exhaust gas. Table 5 Energy consumption for different separation technologies. Energy costs (KWh/kg biogas)
PSA
WS
CA
MS
CU
Hy
Hy’
0.209 ~0.235
0.174 ~0.260
0.582
0.174 ~0.200
0.284 ~0.639
0.357 ~0.390
0.204 ~0.223
energy consumption of fluid pump was lower Rv to achieve the required hydration rate. 4.3. Energy conservation 4.3.1. Energy consumption ratio Hydrate-based gas mixture separation was a non-spontaneous pro cess. Energy consumption must be needed during the separation process according to the laws of thermodynamics. Assumed that the separation process was reversible at constant temperature, the energy consumption at these conditions was minimized. Minimum separation energy of bi nary mixed gas was calculated in Eq. (9): � � x ln x þ ð1 xÞlnð1 xÞ y ln y þ ð1 yÞlnð1 yÞ (9) Wmin;T ¼ RT x y where R is the ideal gas constant and value of 8.314 J/(mol⋅K). T is the temperature of experiment procedure; x and y represent the mole con centration of the feed mixed gas and separated mixed gas. Where is the ideal gas constant and value of 8.314 J/(mol�K). is the temperature of experiment procedure; and represent the mole concen tration of the feed mixed gas and separated mixed gas. The concept of energy consumption ratio (ECR) was introduced to compare the effect of different separation processes. ECR was defined as the ratio of the actual energy consumption to minimum energy con sumption for the ideal reversible process. In order to lower the energy consumption per unit, we should strive to reduce the value of ECR by experiment optimization. Based on the experimental and process simulation, x and y were 67.00 mol% and 97.00 mol%, respectively. The temperature was 278 K, and obtained minimum separation power is 1866.66 J/mol. For this separation process, the minimum separation power was 601.48 W.
Fig. 10. Energy consumption comparisons for different separation technolo gies: low value of energy consumption range; high value of energy con sumption range.
Table 4 showed the energy consumptions of major equipment for the simulation progress. The total energy consumption was 11.45 kW. In terms of methane production, the energy cost was 1.17 kWh/kg CH4, while the CO2 remove energy cost was about 1.29 kWh/kg CO2. Ac cording to the handing capacity of the feed gas, the energy cost was 0.390 kWh/kg biogas. Fig. 8a showed the total energy consumption distributions of the simulation progress. The energy cost of compressor occupied 48.4% of the total energy cost which was the largest ratio. Moreover, the energy cost of stirrer, fluid pump and fan occupied 39.0%, 10.2% and 2.4%, respectively. As seen in Fig. 8b, the energy cost of compression pump BCOMP1 and BCOMP2 were the main energy cost sections of total en ergy cost of compressor and approximately 4467.6 W. Fig. 8c showed the energy cost of fluid pump. As shown in Fig. 8c, hydrate slurry de livery pump SP1 and SP2 occupied 98.9% of total energy cost of fluid pump, the energy cost of hydrate heat circulating pump B2 was only 12.8 W. The energy consumption of SP1 and SP2 was related to the amount of hydrate formation solution. In this case, the key to reduce the
4.3.2. Energy recovery of exhaust gas The pressure and temperature for the exhaust gas of second stage of hydrate formation were 2.1 MPa and 278 K. Fig. 9 showed a process for the recovery of the pressure energy of exhaust gas, the expansion power obtained by exhaust gas through expander which was used to increase the pressure of feed gas. The temperature of expanded exhaust gas was reduced and exchanging heat with preliminary pressurized feed gas. By this way, the energy of exhaust gas was used to preliminary pressurized 8
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and cooling the feed gas. Through the energy recovery unit, the pressure and temperature of the exhaust gas were 0.8 MPa and 298 K. The pres sure and temperature of the feed gas reduced approximately 200 KPa and 7 K, respectively. Saving energy consumption 565.6 W, approxi mately 4.9% of total energy consumptions, Total energy consumptions dropped to 10885.3 W, and energy consumption ratio reduced from 19.03 to 18.09.
References Andriani, D., Wresta, A., Atmaja, T.D., Saepudin, A., 2014. A review on optimization production and upgrading biogas through CO2 removal using various techniques. Appl. Biochem. Biotechnol. 172, 1909–1928. Arca, S., Poletti, L., Poletti, R., D’Alessandro, E., 2011. Upgrading of biogas technology through the application of gas hydrates. ICGH 2011 (July), 17–21. BC Innovation Council, 2008. Feasibility Study-Biogas Upgrading and Grid Injection in the Fraser Valley. British Columbia. Beil, M., Uwe, H., 2010. Technical Success of the Applied Biogas Upgrading Methods. Publisher: biogasmax.eu. Brown, E., Khan, M.N., Salmin, D., Wells, J., Koh, C.A., 2016. Cyclopentane hydrate cohesion measurements and phase equilibrium predictions. J. Nat. Gas Sci. Eng. 36, 1435–1440. Castellani, B., Rossi, F., Filipponi, M., Nicolini, A., 2014. Hydrate-based removal of carbon dioxide and hydrogen sulphide from biogas mixtures: experimental investigation and energy evaluations. Biomass Bioenergy 70, 330–338. Castellani, B., Morini, E., Bonamente, E., Rossi, F., 2017. Experimental investigation and energy considerations on hydrate-based biogas upgrading with CO2 valorization. Biomass Bioenergy 105, 364–372. Castellani, B., Rinaldi, S., Bonamente, E., Nicolini, A., Rossi, F., Cotana, F., 2018. Carbon and energy footprint of the hydrate-based biogas upgrading process integrated with CO2 valorization. Sci. Total Environ. 615, 404–411. Dashti, H., Yew, L.Z., Lou, X., 2015. Recent advances in gas hydrate-based CO2 capture. J. Nat. Gas Sci. Eng. 23, 195–207. Dyadin, Y.A., Udachin, K.A., 1984. Clathrate formation in water-peralkylonium salts systems. J. Inclusion Phenom. 2, 61–72. Fan, S.S., Li, Q., Nie, J.H., Lang, X.M., Wen, Y.G., Wang, Y.H., 2013. Semiclathrate hydrate phase equilibrium for CO2/CH4 gas mixtures in the presence of tetrabutylammonium halide (bromide, chloride, or fluoride). J. Chem. Eng. Data 58, 3137–3141. Grande, C.A., Blom, R., 2014. Cryogenic adsorption of methane and carbon dioxide on zeolites 4A and 13X. Energy Fuels 28, 6688–6693. Kadam, R., Panwar, N.L., 2017. Recent advancement in biogas enrichment and its applications. Renew. Sustain. Energy Rev. 73, 892–903. Kapoor, R., Subbarao, P.M.V., Vijay, V.K., Shah, G., Sahota, S., Singh, D., Vermaa, M., 2017. Factors affecting methane loss from a water scrubbing based biogas upgrading system. Appl. Energy 208 (15), 1379–1388. Khan, I.U., Othman, M.H.D., Hashim, H., Matsuura, T., Azelee, I.W., 2017. Biogas as a renewable energy fuel-A review of biogas upgrading, utilisation and storage. Energy Convers. Manag. 150, 277–294. Kumar, R., Linga, P., Moudrakovski, I., Ripmeester, J.A., Englezos, P., 2008. Structure and kinetics of gas hydrates from methane/ethane/propane mixtures relevant to the design of natural gas hydrate storage and transport facilities. AIChE J. 54, 2132–2144. Li, L.L., Fan, S.S., Chen, Q.X., Yang, G., Zhao, J.Z., Wei, N., Wen, Y.G., 2019. Experimental and modeling phase equilibria of gas hydrate systems for postcombustion CO2 capture. J. Taiwan Inst. Chem. Eng. 96, 35–44. Li, Q., Fan, S.S., Wang, Y.H., Lang, X.M., Chen, J., 2015. CO2 removal from biogas based on hydrate formation with tetra-n-butylammonium bromide solution in the presence of 1-butyl-3-methylimidazolium tetrafluoroborate. Energy Fuels 29 (5), 3143–3148. Li, S.F., Fan, S.S., Wang, J.Q., Lang, X.M., Wang, Y.H., 2010. Semiclathrate hydrate phase equilibria for CO2 in the presence of tetra-N-butyl. J. Chem. Eng. Data 55 (9), 3212–3215. Li, X.S., Xu, C.G., Chen, Z.Y., Wu, H.J., 2011. Hydrate-based pre-combustion carbon dioxide capture process in the system with tetra-N-butyl ammonium bromide solution in the presence of cyclopentane. Energy 36, 1394–1403. Linga, P., Kumar, R., Englezos, P., 2007. Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures. Chem. Eng. Sci. 62, 4268–4276. Liu, M., Jiang, C.Y., Liu, Q.F., Chen, J., 2019. Separation of C3H8 and C3H6 from butyl alcohol-octyl alcohol vent gas mixture via hydrate formation in the presence of SDS and THF in tap-water system. J. Chem. Eng. Data 64 (3), 1244–1249. Ma, C.Y., Liu, C., Lu, X.H., Jia, X.Y., 2018. Techno-economic analysis and performance comparison of aqueous deep eutectic solvent and other physical absorbents for biogas upgrading. Appl. Energy 225 (1), 437–447. Mech, D., Gupta, P., Sangwai, J.S., 2016. Kinetics of methane hydrate formation in an aqueous solution of thermodynamic promoters (THF and TBAB) with and without kinetic promoter (SDS). J. Nat. Gas Sci. Eng. 35, 1519–1534. Oyama, H., Shimada, W., Ebinuma, T., Kamata, Y., Takeya, S., Uchida, T., Nagao, J., Narita, H., 2005. Phase diagram, latent heat, and specific heat of TBAB semiclathrate hydrate crystals. Fluid Phase Equilib. 234, 131–135. Peng, D.Y., Robinson, D.B., 1970. A new two-constant equation of state. Ind. Eng. Chem. Fundam. 59–64. Sloan, E.D., Fleyfel, F., 1992. Hydrate dissociation enthalpy and guest size. Fluid Phase Equilib. 76, 123–140. Tajima, H., Yamasaki, A., Kiyono, F., 2004. Energy consumption estimation for greenhouse gas separation processes by clathrate hydrate formation. Energy 29, 1713–1729. Wang, Y.H., Lang, X.M., Fan, S.S., 2013. Hydrate capture CO2 from shifted synthesis gas, flue gas and sour natural gas or biogas. J. Energy Chem. 22, 39–47. Xu, C.G., Li, X.S., 2014. Research progress of hydrate-based CO2 separation and capture from gas mixtures. RSC Adv. 4, 18301–18316. Yan, Y., Zhang, Z., Zhang, L., Chen, Y., Tang, Q., 2014. Dynamic modeling of biogas upgrading in hollow fiber Membrane contactors. Energy Fuels 28, 5745–5755. Yin, J., Qin, C., An, H., Liu, W., Feng, B., 2012. High-temperature pressure swing adsorption process for CO2 separation. Energy Fuels 26, 169–175.
4.3.3. Multi stage pressurized The pressure ratio of feed biogas to raw biogas was 28. This process simulation used two stages of compression. In this case, single stage pressure ratio was about 5.3. Compressors were the main energy con sumption apparatus. The energy cost of compressors for two stages, three stages and four stages pressurized were calculated, and the results were 4467.6 W, 4186.0 W and 4048.7 W, respectively. These results showed that four stages of pressurized operation can reduce energy consumption to 418.9 W, approximately 3.6% of the total energy con sumption. Total energy consumption was reduced from 11450.9 W to 10466.4 W, and energy consumption ratio was reduced to 17.40. However, the pressure of the outlet gas was still about 0.8 MPa. Table 5 and Fig. 10 showed the energy costs of hydrate-based sep aration technologies, which were compared with other existing tech nologies (Beil et al., 2010; BC Innovation Council, 2008). Hydrate (Hy) based separation technology was advantageous in energy consumption compared to Chemical Absorption (CA) and Cryogenic Upgrading (CU). Comparing with PSA, WS and MS, the energy consumption of hydrate separation was still relatively high. The main energy cost was the stir ring device. With the implement of static hydration enhancement technology, the energy consumption of hydrate separation (Hy’) can be reduced to 0.204–0.223 kWh/kg biogas, similar to the energy cost of PSA. 5. Conclusions The gas separation experiments for CO2 separation from CH4 (67.00 mol%)/CO2 through forming hydrate at 3.0 MPa and 278.0 K in the present of 5.0 wt% TBAB were studied. The experimental results showed that higher purity product gas was obtained at lower Rv. Methane content increased from 67.00 mol% to 92.76 mol% with Rv as 3.61. However, too large or too small Rv would decrease the separation factor. Therefore, there existed an optimal Rv value for gas separation. The product gas content of 97 mol% CH4 was obtained by two stages of hydration at 3.0 MPa and 278.0 K with 5.0 wt% TBAB. The process simulation was established with Aspen Plus based on the experiments under the same conditions and enlarged 1000 times, which analyzed the energy consumption for major mechanical energy consuming equipment. The simulation results showed that the energy cost was only 0.357–0.390 kWh/kg Biogas with two stages of hydration process, lower than the traditional chemical absorption method, resulting in a cost effective process. The application of static hydration enhancement technology could reduce the energy consumption to 0.204–0.223 kWh/kg biogas. Acknowledgement This work was supported by the National Natural Science Foundation of China (21736005) and CAS Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, (No. Y807km1001). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jngse.2019.103008.
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Journal of Natural Gas Science and Engineering 72 (2019) 103008 Zhong, D.L., Wang, Y.R., Lu, Y.Y., Wang, W.C., Wang, J.L., 2016. Phase equilibrium and kinetics of gas hydrates formed from CO2/H2 in the presence of tetrahydrofuran and cyclohexane. J. Nat. Gas Sci. Eng. 35, 1566–1572.
Zheng, J.J., Lee, Y.K., Babu, P., Zhang, P., Linga, P., 2016. Impact of fixed bed reactor orientation, liquid saturation, bed volume and temperature on the clathrate hydrate process for pre-combustion carbon capture. J. Nat. Gas Sci. Eng. 35, 1499–1510. Zhong, D.L., Daraboina, N., Englezos, P., 2013. Recovery of CH4 from coal mine model gas mixture (CH4/N2) by hydrate crystallization in the presence of cyclopentane. Fuel 106, 425–430.
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Journal of Natural Gas Science and Engineering journal homepage: http://www.elsevier.com/locate/jngse
Experimental and process simulation of hydrate-based CO2 capture from biogas Qi Li a, b, c, Shuanshi Fan a, *, Qiuxiong Chen b, Guang Yang b, Yunwen Chen b, Luling Li b, Gang Li a a
Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, PR China b Shenzhen Gas Corporation Ltd., Shenzhen, 518040, PR China c CAS Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Guangzhou, 510640, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Biogas Clathrate hydrates Process simulation Energy consumption Carbon dioxide capture
This study experimentally investigated the effects of operation conditions for the CO2 capture from biogas by clathrate hydrate formation. The experiments were tested at 3.0 MPa and 278 K in the present of 5.0 wt % tetran-butylammonium bromide (TBAB). In addition, the dissociation enthalpies of CH4/CO2 hydrates in TBAB so lution was calculated for energy consumption. The hydrate process simulation with Aspen Plus was carried out at the same experimental conditions to simulate the actual hydrate-based CO2 capture (HBCC), and the energy consumption of this process was analyzed. Experimental results show that the CH4 fraction in the residual gas phase increased with the decrease of gas liquid volume ratio (Rv). Rv decreased from 35.93 to 3.61 while the concentration of CH4 in residual gas increased from 80.5 mol % to 92.76 mol %. The CH4 concentration in re sidual gas phase was enriched from 67.00 mol% to 97.00 mol% by two stages of HBCC. Process simulation results showed that the energy cost was 1.17 kWh/kg CH4. According to the handing capacity of the feed gas, the energy cost was 0.390 kWh/kg biogas, which was lower than that by chemical absorption method. After adjusting the process parameters and increasing exhaust gas energy recovery, the energy consumption was reduced to 0.357 kWh/kg biogas. The energy consumption could be decreased to 0.204–0.223 kWh/kg biogas by using static hydration enhancement technology.
1. Introduction Because of its capability for continuous energy production, and very low grade feedstock, such as biomasses and wastes, along with energy crops, biogas becomes a kind of the most important renewable energy source (Khan et al., 2017). Raw biogas is mainly composed of methane (CH4, 40–75% v/v) and carbon dioxide (CO2, 15–60% v/v), along with trace amounts of hydrogen sulfide (H2S), oxygen (O2), carbon monoxide (CO) and nitrogen (N2) generally not greater than 2% v/v (Andriani et al., 2014). Biogas produced from organic materials can be used directly to generate power, but the large volume of CO2 reduce its heating value, and limiting economic feasibility to use. On average, the calorific value of natural gas is 35.8 MJ/m3, while that of biogas is 21.5 MJ/m3 (Kadam et al., 2017). Removing CO2 from the biogas increases the calorific value for the concentration of methane is increased. Pressure Swing
Adsorption (PSA) (Yin et al., 2012), Water Scrubbing (WS) (Kapoor et al., 2017), Chemical Absorption (CA) (Ma et al., 2014), Membrane Separation (MS) (Yan et al., 2014) and Cryogenic Upgrading (CU) sep aration (Grande et al., 2014) are effective techniques applied to capture CO2 and other small amount components from biogas, which have been studied for years. In addition, gas hydrate separation technology was a promising method for CO2 capture (Dashti et al., 2014; Zheng et al., 2016; Li et al., 2019) and attracted numerous attentions in recent years. The basis for the separation was the selective partition of CO2 between the hydrate phase and the gas phase. The HBCC can be of high capacity and simple to operate, for its unique physical properties such as guest substance selectivity, large gas storage capacity and large value of the enthalpy of formation/decomposition (Wang et al., 2014). Moreover, the using of promoters such as cyclopentane (Zhong et al., 2013, 2016; Brown et al., 2016), tetrahydrofuran (Linga et al., 2007; Mech et al., 2016; Liu et al., 2019), propane (Kumar et al., 2008) and TBAB (Li et al.,
* Corresponding author. Room 302, YiFu Engineering Building, School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou, 510640, PR China. E-mail address: [email protected] (S. Fan). https://doi.org/10.1016/j.jngse.2019.103008 Received 20 August 2019; Received in revised form 18 September 2019; Accepted 23 September 2019 Available online 25 September 2019 1875-5100/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. The schematic diagram of the devices for gas separation based on hydrate formation.
2011, 2015; Fan et al., 2013) moderated the conditions of hydrate for mation. Meanwhile, TBAB forms semiclathrate hydrates at ambient conditions of atmosphere pressure and 285.15 K (Dyadin et al., 1984; Oyama et al., 2005), which can trap small gas molecules like CH4 and CO2 in the small cavities (S-cage, 512) at favorable temperatures and pressures. The technology of HBCC from various types of gas mixture (e.g., shifted synthesis gas, flue gas, and the sour natural gas or biogas) have been studied and proved to be promising and economically feasible compared with traditional techniques (Wang et al., 2013; Xu et al., 2014). Recently, researchers put attentions to evaluate the feasibility of HBCC through gas hydrate formation by simulating and analyzing the energy costs. Tajima et al. (2004) analyzed the energy cost for CO2 capturing through the formation of CO2 hydrate for a practical appli cation to a flue gas (CO2/N2), and found the major contributors to the energy consumption can be attributed to the compression process, because high pressure conditions are necessary for CO2 hydrate forma tion (1.4 MPa at 274 K), and the CO2 concentration of this component in flue gases was very low (about 10–15%). The total energy consumption for their separation process was 158.4 MW, which was equivalent to 0.853 kW h/kg-CO2 (Arca et al., 2011). calculated the energy cost of the process of biogas (60.00 mol % CH4/CO2) upgrading to bio-methane (70.00 mol % CH4/CO2) through hydrate formation. Two different process approaches were proposed and analyzed, the first process was CO2 hydrate process (at 275.15 K and 3.0 MPa), where CO2 hydrate was formed selectively from the biogas. And another process was biogas hydrate process (at 275.15 K and 2.5 MPa), where biogas hydrates was first formed, than selectively dissociated in order to release CO2. The results showed that biogas could be upgraded to bio-methane with two different process approaches with at an energy cost of only 0.174–0.180 kWh/kg Biogas. Castellani et al. (2014) investigated the application of gas hydrate technology to capture of CO2 and H2S from biogas. Theoretical energy evaluations showed that hydrate-based pro cess was a feasible technology, competitive with respect to other con ventional separation methods. Castellani et al. (2017, 2018) also investigated an innovative biogas upgrading process, based on gas hy drate technology, and combining it with power-to-gas technology to produce additional synthetic methane. When the electric energy required by the process was provided by photovoltaic panels, the total energy efficiency was about 0.82, higher than the worldwide average energy efficiency for fossil methane, which was 0.75. The energy cost of HBCC from flue gas was significantly higher than that from biogas, because high operating pressure was needed for HBCC from flue gas. However, excessive energy consumption was one of the main barriers for HBCC promotion, and the scale up of biogas upgrading by hydrate
formation has been poorly investigated and different interpretations can be found in literature. In this study, hydrate phase equilibrium data for the CO2/CH4 þ TBAB system were measured to calculate the dissociation enthalpies of hydrate formation. The experiments of the HBCC from biogas at 3.0 MPa, 278 K with 5 wt% TBAB were investigated. In addition, the process simulation by Aspen Plus was established based on the experimental results and the energy consumption of this process were calculated. The process simulation provides a theoretical energy consumption param eter for the industrialization of hydrate-based gas separation. 2. Experimental section 2.1. Materials The mixture gas CH4 (67.00 mol%)/CO2 and CH4 (89.50 mol%)/CO2 with an uncertainty of �0.001 were supplied by Zhao Qing Gao Neng Da Gas Co. Ltd, China. TBAB with a purity of 99% (Guang zhou Jin ke chemical Co. Ltd, China) was weighed by an electronic balance with a uncertainty in the mass fraction is �0.01, the mass fractions of TBAB solution in this work were all 5 wt%, all of the solutions were prepared at the room temperature and atmospheric pressure. Distilled water was used in all experiments. 2.2. Apparatus The experimental apparatus used for the measurements of hydrate phase equilibrium conditions were clearly described in our previous literature (Li et al., 2010). A schematic diagram of the reactor employed for gas separation was shown in Fig. 1. It mainly included a stainless-steel reactor (7.5 cm in diameter, effective volume 1000 cm3), which equipped with a magnetic stirrer (Haian Scientific Research Apparatus Co, China). The reactor was designed to be operated under 20.0 MPa. Two quartz windows (4.5 cm in diameter) were mounted on both sides of the reactor. The temperature of the reactor was controlled by circulating the coolant from a thermostat (Huber CC1-K20B) with a stability of �0.01 K inside the jacket around the reactor. The tempera ture of the reactor was monitored using two Pt100 resistance ther mometers (Westzh WZ-PT100) within 0.01 K in accuracy, which are placed in the middle and bottom of the reactor, respectively. A pressure transducer (Westzh CyB-20S) within 0.01 MPa in accuracy measured the pressure inside the reactor. The aqueous solution of TBAB at a desired composition was introduced into the reactor by a high pressure metering pump (Alipu JX/12.5). The pressures and temperatures of the reactor were recorded by data logger (Agilent 34970A). During the experiment, 2
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Table 1 Dissociation enthalpies of CH4/CO2 hydrates in TBAB solutions. T/K
p/MPa
Z
Hdiss/kJ⋅mol
276.00 280.00 283.20 284.80 286.60
2.85 4.10 5.82 7.10 9.22
0.8984 0.8599 0.8082 0.7715 0.7169
64.60 61.83 58.12 55.48 51.55
1
CH4 fraction in gas phase (mol %)
CO2 separation factor
3.61 8.12 21.65 35.93
92.76 90.07 83.19 80.50
21.2 20.5 5.6 5.0
the compositions of the gas phase were determined by gas chromatog raphy (KeChuang GC9800TCD). 2.3. Procedure
d ln p � �¼ d T1
(3)
ΔHdiss zR
3.2. Two stages of hydrate reaction
The equilibrium hydrate formation conditions for the CO2/CH4 þ TBAB system were measured by an isochoric pressure search method (Fan et al., 2013). After introducing TBAB solution into the evacuated and vacuumed reactor, the reactor was cooled to 278.0 K. When the temperature in the reactor was kept constant, the reactor was vacuumed and purged with CH4/CO2 mixture gas 5 times to ensure the absence of air, and then mixture gas of CH4/CO2 was charged into reactor to 3.0 MPa. Then the stirring was started and set as time zero. During the experiments, the temperature and pressure were recorded, and the gas samples were transferred and analyzed with GC. After the completion of hydrate formation (the system pressure was kept constant), the stirrer was stopped, the vent valve was opened, and the remaining gas was purged. Then, the reactor was warmed to room temperature to make the hydrate dissociate completely. The dissociated gas composition was also determined with GC.
In stage one of hydrate reaction, CO2 capture from CO2 (33.00 mol %)/CH4 mixture gas with 5 wt% TBAB solutions by hydrate formation was measured at 3.0 MPa and 278.0 K. The reaction temperature, pres sure and TBAB concentration were chose according to our previous work (Li et al., 2015). Rv was ranged from 3.61 to 35.93. Table 2 showed that the higher CH4 concentration in residual gas phase and CO2 separation factor were obtained at the operation Rv as 3.61 and 8.12, respectively. As expected, a negative correlation between CH4 concentration in residual gas phase and Rv was obtained. The reason might be that more solution provided more hydrate cavities, under the same gas flow, CO2 enrichment in the hydrate phases easier. CO2 sep aration factor was defined to quantify the volume of separation effi ciency ratio for different Rv, the maximized CO2 separation factor was 25.5. The optimal Rv was existed for maximum separation factor. According to results of stage one of hydrate reaction selection pa rameters, a stage two hydrate separation process was carried out to obtained higher purity CH4/CO2 mixture gas from CO2 (10.50 mol %)/CH4 mixture gas at 3.0 MPa and 278.0 K with 0.05 wt% TBAB. The result showed that CH4 concentration in residual gas phase was enriched from 89.50 mol % to 97.00 mol % at Rv of 7.9. The results of separation experiment showed that the gas mixture was technically feasible to achieve the standards of commercial natural gas by two stages of hy dration process. The CO2 separation factor can reach about 24.9 at Rv of 7.9 with high separation efficiency.
2.4. Gas liquid volume ratio and separation factor Rv is calculated in Eq. (1): RV ¼
the
where p is pressure, T is temperature, z is compressibility factor for gas and the compressibility factor is determined by Peng-Robinson (P-R) equation (Peng et al., 1970), R is universal gas constant (R ¼ 8.314 J/mol⋅K), and ΔHdiss is the dissociation enthalpy of gas hydrates. The phase equilibrium and dissociation enthalpies of CH4/CO2 hy drates in TBAB solutions determined by using Clausius-Clapeyron equation was shown in Table 1. The results showed that the dissocia tion enthalpy of hydrate decrease with the increase of the reaction pressure. Through linear match, the hydrates dissociation enthalpy for this simulation technology system was 64.39 kJ mol 1 at the condition of 278.0 K and 3.0 MPa, the compressibility factor was 0.8957. Therefore, the formation enthalpy of hydrates was 64.39 kJ mol 1 at 278.0 K and 3.0 MPa. The dissociation enthalpy of gas hydrates was measured for calculating the hydrate formation enthalpy.
Table 2 The CH4 concentration in residual gas phase and CO2 separation factor by one stage of hydrate reaction with different Rv. Gas-liquid volume ratio (Rv)
calculated dissociation enthalpy of hydrates by using Clausius-Clapeyron equation, which was shown in Eq. (3):
22400n0 VL
(1)
where n0 is the number of moles of feed mixed gas, VL is the volume of the liquid phase. The separation factor of CO2 is calculated in Eq. (2): . nHCO2 nHCH4 S ¼ gas � gas (2) nCO2 nCH4
4. Process simulation The technology of hydrate-based purification of biogas focused on energy consumption of biogas handing capacity and the purity of products, methane recovery and reaction time of the process. This simulation process was simulated by Aspen Plus 7.2, and scale up with the lab results, complied with the law of the raw material conservation and energy conservation, the mechanical energy consumption of the whole process was calculated. It was designed based on the experimental results to achieve the lowest energy consumption, choosing 3.0 MPa and 278.0 K at the Rv of 8.12 as simulation parameters for first stage of hy drate formation, 3.0 MPa and 278.0 K at the Rv of 7.9 as simulation parameters for second stage of hydrate formation. The concentration of CH4 was 90.07 mol % after stage one of hydrate reaction, after stage two of hydrate reaction, the CO2 concentration was lower than 3 mol%. The
gas
where nCO2 and nH CO2 are the number of moles of CO2 in the gas phase and gas
hydrate phase at the end of the experiments, respectively. nCH4 and nH CH4 are the number of moles of CH4 in the gas phase and hydrate phase at the end of the experiments, respectively. 3. Experimental results and discussion 3.1. Dissociation enthalpies of CH4/CO2 hydrates in TBAB solution
Hydrate phase equilibrium data for the CO2/CH4 þ TBAB system were measured at temperatures from 276 to 288.6 K and pressures from 2.85 to 9.22 MPa with the 0.05 wt% TBAB. Sloan and Fleyfel (1992) 3
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Journal of Natural Gas Science and Engineering 72 (2019) 103008
Fig. 2. The schematic diagram of process simulation for two stages of hydrate-based CO2 capture form biogas.
Fig. 3. The simulation process of hydrate-based CO2 capture from biogas.
schematic diagram of this process simulation. was shown in Fig. 2. The feasibility of the HBCC technology from biogas was certificated through analyzing the energy consumption, energy consumption rate and methane recovery. Energy consumption of the process was reduced by adjusting the process parameters, adding energy recovery unit and energy saving operation conditions for separation process. The process of hydrate-based CO2 capture from biogas was sho wn in Fig. 3. The main route for the decarbonization process: the raw gas was pressurized by a two stages of compression with air cooling device, then delivered to the heat exchanger BC3. The raw gas was cooled by refrigerant 1,1,1,2-tetrafluoroethane (R134a), then injected into the first stage hydrate reactor HFORM1. The residual gas of first stage of hydrate formation was pressurized by the compressor GCOMP, and heat exchanged with refrigerant R134a to the certain subcooling degree then entered into the second stage hydrate reactor. The hydrate slurry of first stage hydration was introduced into a hydrate dissociation reactor, and heated by heating system. The water was pumped by B2 to heat
exchanger RC3. The decomposition solution was returned to the hydrate reactor for the next hydrate reaction. Throughout the process, the hy dration process was cooled by the cooling system, and the hydrate dissociation process was heated by the heating system. A heat transfer system transported heat from hydrate reactor to decomposition reactor and excess heat through a heat exchanger RC1, which was cooled down by air. 4.1. Process unit 4.1.1. Gas compression cooling unit Clathrate hydrate was formed at low temperature and high pressure conditions. The export pressure of common biogas was 8–10 kPa, and gaged pressure was 0.11 MPa. The raw biogas needed be pressurized to 3.0 MPa. Fig. 4 showed a two stages of gas compression cooling unit, setting the compression ratio as 5.3. After each stage, the temperature of pressurized feed gas reached about 373.15 K, then cooled by air cooler 4
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Journal of Natural Gas Science and Engineering 72 (2019) 103008
Fig. 4. Two stages of gas compression cooling unit for biogas.
Fig. 5. Two stages of hydrate reaction unit.
(AC1, AC2). The pressurized gas was cooled to 303.15 K with air, then cooled to the hydrate formation temperature by cooling system. 4.1.2. Hydrate reaction unit According to the experimental results of stage one hydrate reaction, the methane concentration of residual gas phase reached more than 89.5 mol %. In order to get the product gas that included less than 3.0 mol % carbon dioxide, a two stages of hydrate reaction process was designed. Fig. 5 was the two tage hydrate reaction unit. The cooled and pres surized feed gas were injected into the hydrate formation reactor (HFORM1) and formed hydrate with the TBAB solution. Hydrate for mation was accelerated by mechanical agitation. After stage one hydrate process, the residual gas in gas phase was pressurized by gas compres sion pump (GCOMP) and cooled by heat exchanger (BC4) then sent into another hydrate formation reactor (HFORM2), at the end of the second stage of hydrate formation product gas was obtained. Since Aspen Plus software without hydrate formation module, thus the user model was used for hydrate-based separation process simulation.
Fig. 6. One of hydrate dissociation unit.
4.1.3. Hydrate dissociation unit Fig. 6 showed the hydrate dissociation unit, which hydrate slurry was heated and gases were released from hydrate. At the end of the 5
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Fig. 7. The circulation unit of heat source for hydrate reaction and dissociation. Table 3 The process simulation main parameters of feedstocks and experimental conditions. Exp.no
Total gas flow (mol⋅h 1)
CH4 (mol⋅h
Feed gas1 G1a H1b Feed gas2 G2c H2d
1160 728 432 728 593.32 134.68
777.2 651.56 125.64 651.56 574.52 76.04
a b c d
1
)
CO2 (mol⋅h
1
TBAB solution (mol⋅h 1)
)
382.8 76.44 306.36 76.44 17.8 58.64
1.778 � 105 1.147 � 10
5
Pressure (MPa)
Recovery rate (%)
3.0 2.1
83.83
3.0 2.1
88.18
Gas phase of first stage of hydrate formation. Hydrate phase of first stage of hydrate formation. Gas phase of second stage of hydrate formation. Hydrate phase of second stage of hydrate formation.
Table 4 Energy consumption of major equipment for the simulation progress. type of facility
code
raw material
Fa (kg⋅h
compressor
BCOMP1 BCOMP2 GCOMP RCOMP SP1 SP2 B2 FAN1 FAN2 FAN3
biogas biogas RSb R134a TBABc TBABc water air air air
29.32 29.32 13.82 71.50 800.60 516.05 135.80 1013.30 1013.30 578.40
fluid pump Fan stirrer sum a b c
1
)
Ti (K)
Pi (atm)
To (K)
Po (atm)
Energy cost (W)
303.15 303.15 278.00 268.15 278.00 278.00 283.15 298.15 298.15 298.15
1.09 6.00 20.73 2.20 20.73 20.73 1.00 1.00 1.00 1.00
476.06 477.48 312.93 319.89 278.49 278.49 283.20 298.23 298.41 299.30
5.70 30.00 30.00 8.14 3.00 3.00 2.00 1.01 1.01 1.01
2245.3 2222.3 235.3 843.1 697.7 452.0 12.8 41.7 41.7 190.2 4468.8 11450.9
The amount of flow. Residual gas in gas phase of HFORM 1. The amount of TBAB solutions.
hydrate dissociation, residual solution was pumped back to the hydrate reactor. Based on the law of heat conservation, the enthalpy of hydrate formation and dissociation were generally equal.
W3 were hydrate decomposition heat cycle. High temperature R134a heat changed with water in the heat exchanger RC2, and hot water recycled by fluid pump B2. This heat source circulation unit neither need add additional heat nor cold energy.
4.1.4. Heat source circulation unit As shown in Fig. 7, R1-R6 were R134a refrigerant circulation path, and R134a was throttled by VALVE, then entered BC4 evaporation heat, cooling the residual gas from first stage of hydrate formation, the raw gas cooled in BC3. Refrigerant heat exchanged in BC4 and BC3 by endothermic through phase transition from liquid to gaseous state. In order to achieve the hydration temperature, the pressure of R134a was no more than 349.96 kPa after the throttle. Generated low pressure steam was pressurized by the compressor (RCOMP), then injected into the heat exchanger RC2, which heated the dissociation reaction circu lating water. Finally, highpressure refrigerant vapor cooled to liquid through a heat exchanger RC1 with air, transported into next cycle. W1-
4.1.5. Mechanical stirring unit Experimental process requires mechanical agitation to strengthen the heat and mass transfer of hydrate formation process. Stirring power N calculated in Eq. (4): N ¼ φρn3 d5
(4)
where ρ is the solution density, n is the rotational speed, d is the diameter of blade turbine. The power factor φ affected by stirrer configuration and Reynolds number (Re), calculation with Rushton φ -Re curve in Eq. (5):
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Journal of Natural Gas Science and Engineering 72 (2019) 103008
Re ¼
nρd2
μ
(5)
where μ is the solution viscosity, n is the rotational speed, ρ is the so lution density, d is the diameter of blade turbine. In geometrically similar system, in order to obtain the same effect in the hydrate reaction, a blade tip speed uT ¼ πnd is needed to be un changed, that is calculated in Eq. (6): n1 d1 ¼ n2 d2
(6)
where n1 and d1 are rotational speed and the diameter of blade turbine in laboratory stirrer, respectively. n2 and d2 are rotational speed and the diameter of blade turbine in the simulation process, respectively. In this simulation process, the amount of feed was enlarged 1000 times, equal to the reactor geometry enlarged 10 times. According to the principle of geometric similarity, the size of bladed paddle proportional amplification was calculated in Eq. (7): N2 n32 d52 d22 ¼ ¼ ¼ 100 N1 n31 d52 d21
(7)
The stirrer speed was 600 r/min, and the stirring power was 27.5 W. Therefore, the stirring power of stage one hydrate reaction was calcu lated as 2750 W. The amount of solution for stage two hydration was 62.5% of the stage one hydration. In this case, the stirring power of stage two of hydrate reaction was calculated as 1718.8 W. The total stirrer energy consumption of the simulation process was 4468.8 W. 4.2. Simulation results and discussions In this work, a two stages hydrate reaction separation process was designed to calculate the main mechanical energy costs. Hydrate-based separation simulation process for CH4 (67.00 mol%)/CO2 mixture gas with 5 wt% TBAB solutions, the initial pressure and temperature of raw gas was 0.11 MPa and 303.15 K, respectively. The hydrate formation of stage one and stage two were carried out at 3.0 MPa and 278 K, and the mole flow was 1160 mol/h and 728 mol/h, respectively. There were some rules defined for the process simulation. A certain temperature difference was required between logistics, assumed minimum temper ature difference between the heat exchanger of the logistics process with 5.0 K. The pressure and temperature of air were 0.1 MPa and 298.15 K, respectively. The amount of solution back to hydrate formation reactor was equal to 25% of the total solution. Assuming that during the hydrate formation and decomposition, heat transfer process with the cooling and heating system, there was no heat exchanged with the environment. The mass flow of raw biogas for the simulation process were 29.32 kg h 1. The amount of product were 10.57 kg h 1, and the CH4 concentration was 97.0 mol %. Table 3 listed the main parameters and experimental conditions for the simulation process. The hydrate reaction is exothermic process, and reaction heat can be calculated in Eq. (8): Q¼
MΔHdiss
(8)
where Q is enthalpy of hydrate formation process, and M is the moles of hydrates. ΔHdiss is the dissociation enthalpy of gas hydrates and has measured at experimental section. The decomposition thermal of this simulation process was 36.49 kJ/ h calculated by Eq. (8). Therefore, the cooling for hydrate formation was 0.01 kW, and easily supplied from the cooling system (Arca et al., 2011). The heat source circulation unit provided the heating and cooling power for the whole process. In this case, the final cost for the whole process would be only electric power. As seen in Table 3, the CH4 recovery of this separation process was 73.9%, and the CH4 recovery of second stage of hydrate formation slightly higher than those of first stage of hydrate formation. The final pressure was reduced 0.9 MPa due to the formation of hydrate crystals in the reactor.
Fig. 8. Energy cost distributions for the simulation progress: (a), total energy cost; (b), energy cost of compressor; (c), energy cost of fluid pump.
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Fig. 9. Energy recovery apparatus for the exhaust gas. Table 5 Energy consumption for different separation technologies. Energy costs (KWh/kg biogas)
PSA
WS
CA
MS
CU
Hy
Hy’
0.209 ~0.235
0.174 ~0.260
0.582
0.174 ~0.200
0.284 ~0.639
0.357 ~0.390
0.204 ~0.223
energy consumption of fluid pump was lower Rv to achieve the required hydration rate. 4.3. Energy conservation 4.3.1. Energy consumption ratio Hydrate-based gas mixture separation was a non-spontaneous pro cess. Energy consumption must be needed during the separation process according to the laws of thermodynamics. Assumed that the separation process was reversible at constant temperature, the energy consumption at these conditions was minimized. Minimum separation energy of bi nary mixed gas was calculated in Eq. (9): � � x ln x þ ð1 xÞlnð1 xÞ y ln y þ ð1 yÞlnð1 yÞ (9) Wmin;T ¼ RT x y where R is the ideal gas constant and value of 8.314 J/(mol⋅K). T is the temperature of experiment procedure; x and y represent the mole con centration of the feed mixed gas and separated mixed gas. Where is the ideal gas constant and value of 8.314 J/(mol�K). is the temperature of experiment procedure; and represent the mole concen tration of the feed mixed gas and separated mixed gas. The concept of energy consumption ratio (ECR) was introduced to compare the effect of different separation processes. ECR was defined as the ratio of the actual energy consumption to minimum energy con sumption for the ideal reversible process. In order to lower the energy consumption per unit, we should strive to reduce the value of ECR by experiment optimization. Based on the experimental and process simulation, x and y were 67.00 mol% and 97.00 mol%, respectively. The temperature was 278 K, and obtained minimum separation power is 1866.66 J/mol. For this separation process, the minimum separation power was 601.48 W.
Fig. 10. Energy consumption comparisons for different separation technolo gies: low value of energy consumption range; high value of energy con sumption range.
Table 4 showed the energy consumptions of major equipment for the simulation progress. The total energy consumption was 11.45 kW. In terms of methane production, the energy cost was 1.17 kWh/kg CH4, while the CO2 remove energy cost was about 1.29 kWh/kg CO2. Ac cording to the handing capacity of the feed gas, the energy cost was 0.390 kWh/kg biogas. Fig. 8a showed the total energy consumption distributions of the simulation progress. The energy cost of compressor occupied 48.4% of the total energy cost which was the largest ratio. Moreover, the energy cost of stirrer, fluid pump and fan occupied 39.0%, 10.2% and 2.4%, respectively. As seen in Fig. 8b, the energy cost of compression pump BCOMP1 and BCOMP2 were the main energy cost sections of total en ergy cost of compressor and approximately 4467.6 W. Fig. 8c showed the energy cost of fluid pump. As shown in Fig. 8c, hydrate slurry de livery pump SP1 and SP2 occupied 98.9% of total energy cost of fluid pump, the energy cost of hydrate heat circulating pump B2 was only 12.8 W. The energy consumption of SP1 and SP2 was related to the amount of hydrate formation solution. In this case, the key to reduce the
4.3.2. Energy recovery of exhaust gas The pressure and temperature for the exhaust gas of second stage of hydrate formation were 2.1 MPa and 278 K. Fig. 9 showed a process for the recovery of the pressure energy of exhaust gas, the expansion power obtained by exhaust gas through expander which was used to increase the pressure of feed gas. The temperature of expanded exhaust gas was reduced and exchanging heat with preliminary pressurized feed gas. By this way, the energy of exhaust gas was used to preliminary pressurized 8
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and cooling the feed gas. Through the energy recovery unit, the pressure and temperature of the exhaust gas were 0.8 MPa and 298 K. The pres sure and temperature of the feed gas reduced approximately 200 KPa and 7 K, respectively. Saving energy consumption 565.6 W, approxi mately 4.9% of total energy consumptions, Total energy consumptions dropped to 10885.3 W, and energy consumption ratio reduced from 19.03 to 18.09.
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4.3.3. Multi stage pressurized The pressure ratio of feed biogas to raw biogas was 28. This process simulation used two stages of compression. In this case, single stage pressure ratio was about 5.3. Compressors were the main energy con sumption apparatus. The energy cost of compressors for two stages, three stages and four stages pressurized were calculated, and the results were 4467.6 W, 4186.0 W and 4048.7 W, respectively. These results showed that four stages of pressurized operation can reduce energy consumption to 418.9 W, approximately 3.6% of the total energy con sumption. Total energy consumption was reduced from 11450.9 W to 10466.4 W, and energy consumption ratio was reduced to 17.40. However, the pressure of the outlet gas was still about 0.8 MPa. Table 5 and Fig. 10 showed the energy costs of hydrate-based sep aration technologies, which were compared with other existing tech nologies (Beil et al., 2010; BC Innovation Council, 2008). Hydrate (Hy) based separation technology was advantageous in energy consumption compared to Chemical Absorption (CA) and Cryogenic Upgrading (CU). Comparing with PSA, WS and MS, the energy consumption of hydrate separation was still relatively high. The main energy cost was the stir ring device. With the implement of static hydration enhancement technology, the energy consumption of hydrate separation (Hy’) can be reduced to 0.204–0.223 kWh/kg biogas, similar to the energy cost of PSA. 5. Conclusions The gas separation experiments for CO2 separation from CH4 (67.00 mol%)/CO2 through forming hydrate at 3.0 MPa and 278.0 K in the present of 5.0 wt% TBAB were studied. The experimental results showed that higher purity product gas was obtained at lower Rv. Methane content increased from 67.00 mol% to 92.76 mol% with Rv as 3.61. However, too large or too small Rv would decrease the separation factor. Therefore, there existed an optimal Rv value for gas separation. The product gas content of 97 mol% CH4 was obtained by two stages of hydration at 3.0 MPa and 278.0 K with 5.0 wt% TBAB. The process simulation was established with Aspen Plus based on the experiments under the same conditions and enlarged 1000 times, which analyzed the energy consumption for major mechanical energy consuming equipment. The simulation results showed that the energy cost was only 0.357–0.390 kWh/kg Biogas with two stages of hydration process, lower than the traditional chemical absorption method, resulting in a cost effective process. The application of static hydration enhancement technology could reduce the energy consumption to 0.204–0.223 kWh/kg biogas. Acknowledgement This work was supported by the National Natural Science Foundation of China (21736005) and CAS Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, (No. Y807km1001). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jngse.2019.103008.
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Zheng, J.J., Lee, Y.K., Babu, P., Zhang, P., Linga, P., 2016. Impact of fixed bed reactor orientation, liquid saturation, bed volume and temperature on the clathrate hydrate process for pre-combustion carbon capture. J. Nat. Gas Sci. Eng. 35, 1499–1510. Zhong, D.L., Daraboina, N., Englezos, P., 2013. Recovery of CH4 from coal mine model gas mixture (CH4/N2) by hydrate crystallization in the presence of cyclopentane. Fuel 106, 425–430.
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