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Thermodynamic assessment of membrane-assisted chemical looping reforming of glycerol
Chemical Engineering & Processing: Process Intensification 142 (2019) 107564
Contents lists available at ScienceDirect
Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Thermodynamic assessment of membrane-assisted chemical looping reforming of glycerol
T
⁎
Shuai Wang , Bowen Li, Yuxiang Tang, Yurong He School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Chemical looping reforming Hydrogen separation Membrane Glycerol
Chemical looping reforming (CLR) as an extension of chemical looping combustion uses less oxygen carriers to achieve hydrogen production via partial oxidation of fuel. Membrane hydrogen separation can promote hydrogen production and makes it possible to low-temperature reforming. In this work, the CLR performance of glycerol with membrane hydrogen separation is investigated by means of thermodynamic analysis. On the basis of the Gibbs free energy minimization method, the sensitivity of the CLR performance with membrane separation to operating parameters is evaluated. The results show that membrane separation can promote hydrogen production at the cost of more heat requirement. The reaction of oxygen carriers cannot achieve auto-thermal conditions of the system. The integration of sorption-enhanced and membrane separation in the CLR of glycerol is further examined.
1. Introduction Chemical looping reforming (CLR) is a novel reforming technology integrated with chemical looping system, which uses oxygen carriers to achieve fuel oxidation and heat transportation between the reformer (contact with the fuel) and the regenerator (oxidation of reduced oxygen carriers). The schematic diagram of a CLR system is displayed in Fig. 1. In contrast to traditional reforming technologies, the CLR system can achieve auto-thermal condition without external combustion. By avoiding the contact of fuel gas and air, the N2-free reforming products can be generated [1]. In additional, high purity hydrogen can be obtained without separation processes [2]. In recent years, the CLR system has attracted more and more concerns [3–5]. Feng et al. [6] studied the reaction mechanics of methane over oxygen carriers during the CLR process using the density functional theory. It was found that reaction rates significantly depended on oxygen anion diffusion in crystal structure of carriers. Adiya et al. [7] investigated the hydrogen production process from shale gas in a sorption-enhanced CLR system using CaO as sorbent. In contrast to conventional catalytic steam reforming, the system with oxygen carriers could greatly reduce the energy requirement for hydrogen production. Diglio et al. [8] carried out a simulation of the CLR process in a dual fluidized bed based on the hydrodynamic model coupled with 1D steady-state heat and mass balances. The results demonstrated that the reforming performance and process feasibility were determined by the
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temperature and the transported oxygen amount between reactors. García-Labiano et al. [9] experimentally studied the hydrogen production from bioethanol in a CLR system and examined the effect of operating conditions. It was concluded that the ethanol had a high conversion and carbon formation was also restricted. Meanwhile, the auto-thermal condition could be achieved. Glycerol from the biodiesel production is a promising renewable resource, which has been used as fuel for hydrogen production [10–12]. Ni et al. [13] used the potassium promoted Li2ZrO3 as sorbents to conduct an experimental study of sorption-enhanced chemical looping reforming of glycerol. It was found that the coke deposition reduced the catalytic effect and the oxygen transfer effect, resulting in the decrease of fuel conversion. Tippawan et al. [14] conducted a thermodynamic analysis of the sorption-enhanced CLR of glycerol and analyzed the impact of sorbent on the reforming performance. It was found that the hydrogen purity and yield could be promoted via the sorption-enhanced way. Additionally, it was pointed out that carbon formation was hindered by the control of calcium oxide-to-glycerol ratio and steam-toglycerol ratio. Jiang et al. [15] experimentally investigated physicochemical properties of oxygen carriers in a CLR process of glycerol. The results revealed that the phyllosilicate nanotube confinement effect could promote the reactivity of oxygen carrier and hydrogen production. Hydrogen separation can promote the hydrogen production of reforming reactions via shifting chemical equilibrium. Membrane reactors
Corresponding author. E-mail address: [email protected] (S. Wang).
https://doi.org/10.1016/j.cep.2019.107564 Received 4 December 2018; Received in revised form 8 June 2019; Accepted 19 June 2019 Available online 21 June 2019 0255-2701/ © 2019 Elsevier B.V. All rights reserved.
Chemical Engineering & Processing: Process Intensification 142 (2019) 107564
S. Wang, et al.
Nomenclature
y
aik Ak Cep
Greekletters
Ea FH2 ΔGf0 ni P R T
number of kth element atoms in the species i total mass of kth element in the feed [kg] membrane permeation capacity (membrane surface area/ thickness) [m] activation energy for permeation [J mol−1] hydrogen permeation rate [mol h−1] standard Gibbs function of formation mole number of species i pressure [Pa] universal gas constant [J mol−1K−1] temperature [K]
λ φ η
gas phase mole fraction
Lagrange multipliers fugacity coefficient permeation effectiveness factor
Subscripts p r
provide the possibility for potential gas separation [16–18]. Ji et al. [19] investigated the enhancement of methane steam reforming through the sorption-enhanced membrane reactor. It was observed that the sorption-enhanced membrane reactor could reduce the CO fraction and minimized the hydrogen permeation decay. Spallina et al. [20] evaluated the performance of ethanol auto-thermal reforming by means of experimental and numerical methods. It was pointed out that the sealing of the membrane was critical. Kim et al. [21] carried out an experimental study of methane steam reforming in a membrane reactor and analyzed the impact of membrane pressure difference on hydrogen recovery and fuel conversion. The results demonstrated that the hydrogen selectivity and the permeance were decreased in a long-term operation owing to catalytic particle adhesion. Although a membrane reactor has been applied to the reforming process, few reports about the CLR process in a membrane reactor could be available. In this work, the CLR performance of glycerol in a membrane reactor with hydrogen separation is investigated via thermodynamic analysis. On the basis of Gibbs free energy minimization method, the enhanced reforming performance via membrane hydrogen separation is studied and the effect of operating parameters is evaluated. Meanwhile, the auto-thermal condition of the CLR system with membrane separation is also examined.
permeate side retentate side
sorption reaction is expressed as below [23]:
CaO(s) + CO2 (g) → CaCO3 (s)
ΔH0r = −179. 2KJ/mol
(1)
In order to conduct a thermodynamic analysis, the Gibbs free energy minimization method is employed and expressed as follows: N
∑ ni (ΔGfi0 + RT ln i=1
ˆ
yi φi P + P0
∑ λk aik ) = 0 k
∑ ni aik = Ak fork = 1, 2, 3, ......w i
(2)
(3)
where the Soavee-Redliche-Kwong equation is adopted to calculate thermodynamic properties [24]. For the membrane-assisted CLR process, a one-dimensional plug flow is assumed where the axial variation of gas compositions is only considered with uniform temperature distribution. The Sieverts’ law is applied to describe the hydrogen permeation through membranes [25,26]:
FH 2 = ηCep kexp (−
Ea 0.5 )(Pr,0.5 H 2 − P p, H 2 ) RT
(4)
where Pr,H2 and Pp,H2 denote hydrogen partial pressures of retentate side and permeate side. Cep represents membrane permeation capacity, depending on the sub-separator number. η is permeation effectiveness factor. k and Ea represent pre-exponential factor and activation energy, which are 1.08 × 10−10 mol/(s m Pa0.5) and 918 0J/mol in this work [26]. In order to model a membrane reactor, some successive sub-reformers and membrane separators are used following the method of Ye et al. [26]. The hydrogen permeation rate is user-defined and implemented on the platform of ASPEN PLUS. The greater the number of sub-separators is, the closer to the real system the predictions are. By conducting a sensitivity analysis of hydrogen production rate to subseparator number, it can be found that the variation of prediction results becomes unclear after the number of sub-separators m reaches 50. Hence, the value of m adopts 50, which is consistent with the prediction of Ye et al. [26].
2. Method The membrane-assisted CLR process with glycerol as fuel is chosen as the objective of this work, involving a series of reactions including glycerol steam reforming, reduction of oxygen carriers with fuel and some side reactions, which are mainly summarized in Table 1 [13,22]. For the CaO-based sorption-enhanced reforming process, the CO2
3. Results and discussion 3.1. Effect of membrane hydrogen separation on gas composition The influence of membrane hydrogen separation on gas compositions in the CLR reactor is evaluated and shown in Fig. 2. It can be observed that membrane separation apparently improves hydrogen production. The methane and water concentrations are greatly reduced, which can be explained that membrane hydrogen separation enhances reforming reactions to move towards the direction in favor of hydrogen yield, resulting in more methane and steam consumed and carbon dioxide produced. As the reactant of methane reforming reactions and the
Fig. 1. Schematic diagram of a CLR system (NiO as oxygen carrier and glycerol as fuel). 2
Chemical Engineering & Processing: Process Intensification 142 (2019) 107564
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Table 1 Main reactions considered during chemical looping reforming of glycerol [13,22]. Reaction description
Reaction equation
Glycerol reforming reaction
C3 H8O3 +3H2 O↔ 3CO2 +7H2 ; ΔH298K = 127.67kJ mol-1 (R1) C3 H8O3 → 3CO+4H2 ; ΔH298K = 215.18kJ mol-1 (R2)
Water-gas shift reaction Reduction reaction of oxygen carriers
CO+H2 O↔ H2 +CO2 ΔH298K = -41.17kJ mol-1 (R3) C3 H8O3 + H2 O+NiO ↔ CO + 2CO2 +5H2 + Ni ΔH298K = 148.74kJ ⋅mol−1 (R4) NiO+CH 4 → Ni+CO+2H2; ΔH298K = 203.75kJ mol-1 (R5) NiO+CO→Ni+CO2 ; ΔH298K = -43.3 kJ mol-1 (R6)
NiO+H2 → Ni+H2 O; ΔH298K = -2.1kJ mol-1 (R7) Methanation
CO+3H2 ↔ CH 4 + H2 O; ΔH298K = -206.11kJ mol-1 (R8) CO2 +4H2 ↔ CH 4 +2H2 O; ΔH298K = -164.94kJ mol-1 (R9) 2CO+2H2 ↔ CH 4 +CO2 ΔH298K = -247.3kJ mol-1 (R10)
Carbon formation
CO+H2 ↔ C+H2 O; ΔH298K = -131.26kJ mol-1 (R11) CO2 +2H2 ↔ C+2H2 O; ΔH298K = -355.06kJ mol-1 (R12) CH 4 ↔ C+2H2; ΔH298K = 74.85kJ mol-1 (R13) 2CO↔CO2 +C; ΔH298K = -172.43kJ mol-1 (R14)
An excessively high temperature cannot significantly promote hydrogen production, which even hinders the membrane permeation capacity and consumes more energy. Fig. 4 illustrates the effect of oxygen carrier to glycerol ratio on the profile of methane yield with reactor temperature in a membrane-assisted CLR reactor. We can observe that a rising temperature can bring about a decrease of methane production. On the one hand, methane reforming reactions are promoted by increasing reactor temperature. On the other hand, the enhancement of hydrogen separation degree under high temperature further improves methane reforming reactions. Furthermore, we can find that methane production is increased as the oxygen carrier to glycerol ratio increases. The variation of carbon dioxide production at different oxygen carrier to glycerol ratios as a function of reactor temperature is shown in Fig. 5. It can be observed that the carbon dioxide yield gets higher as the temperature increases as a result of the promotion effect of high temperature on reforming reactions of glycerol and methane. Simultaneously, an increasing temperature improves membrane separation so as to enhance carbon dioxide production. However, the water-gas shift reaction is restrained as a consequence of an increasing temperature. As a result, the growth rate of carbon dioxide production slows down. We can also find that the higher oxygen carrier to glycerol ratio is, the lower carbon dioxide yield will be. This can be attributed to the fact that the methane conversion rate decreases when the amount of oxygen carriers increases. The methane reforming reaction moves toward the
Fig. 2. Comparisons of gas compositions in a CLR reactor with and without membrane hydrogen separation.
product of water-gas shift reaction, carbon monoxide is generated while being consumed so that the variation of carbon monoxide concentration is not evident. Operating parameters influence the membrane-assisted CLR performance. Here, the effects of different parameters including reactor temperature of 500–650 °C, reactor pressure of 1–3.5 MPa, hydrogen partial pressure in the permeate side of 0.05–0.15 MPa and steam to carbon ratio of 1–3.5 are examined. A base condition is specified at 600 °C with the steam molar flow rate of 6 kmol/h and the steam to carbon ratio of 2.0. The molar flow rate of NiO-based oxygen carrier is 1 kmol/h. The pressures in the retentate side and permeate side of membrane are 2 MPa and 0.1 MPa, respectively.
3.2. Effect of reactor temperature Fig. 3 demonstrates the relationship of hydrogen production and reactor temperature at different oxygen carrier to glycerol ratios in a membrane-assisted CLR reactor. It can be seen that the hydrogen production roughly increases with a descending growth rate as the temperature increases. Both reforming reactions and water-gas shift reaction are endothermic. Meanwhile, it can be seen that the hydrogen production is reduced with the increase of oxygen carrier to glycerol ratio. This can be explained that part of oxygen carriers react with hydrogen when the oxygen carrier to glycerol ratio is improved. This implies that a suitable temperature rise at a lower oxygen carrier to glycerol ratio is beneficial to the enhancement of hydrogen production.
Fig. 3. Relationship between hydrogen production and reactor temperature at different oxygen carrier to glycerol ratios in a membrane-assisted CLR reactor. 3
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pressure results in the decrease of methane production. In spite of the improvement of pressure has a negative impact on methane reforming reactions, the enhancement of membrane separation resulting from high pressure promotes the methane reforming reactions. According to the variation of methane production, the influence of the latter reason is more remarkable. This also inspires us that improving the pressure is a feasible way to reduce methane production and hydrogen yield. Fig. 9 demonstrates the relationship of overall reaction heat and reactor pressure at different oxygen carrier to glycerol ratios. The overall reaction heat is enhanced with the increase of pressure. This means that more heat is required under high pressure. From the above analysis of Figs. 7 and 8, we can realize that reforming reactions are promoted owing to the enhancement of hydrogen separation under high pressure so that more heat is required. In addition, we can find that although the increase of oxygen carrier to glycerol ratios can reduce the heat requirement, the auto-thermal condition is still difficult to achieve.
Fig. 4. Variation of methane production with reactor temperature at different oxygen carrier to glycerol ratios in a membrane-assisted CLR reactor.
3.4. Effect of steam to glycerol ratio The impact of steam to carbon ratio on hydrogen production is evaluated in Fig. 10. It can be clearly seen that the profile of hydrogen production with steam to carbon ratio is not monotonous. As the steam to carbon ratio gets higher, methane and glycerol reforming reactions as well as water-gas shift reaction are enhanced owing to a higher steam fraction so that hydrogen production is promoted. However, an excessively high amount of steam results in the reduction of hydrogen yield, decreasing the reactor partial pressure of hydrogen so as to hinder membrane separation. At the NiO/G ratio of 1.0, the hydrogen yield is decreased from 5.31 to 5.19 when the S/C ratio is changed from 1.5 to 3.5, which implies that the descending degree is not significant for a slight variation of S/C ratio. Hence, we can realize that a suitable selectivity of steam to glycerol ratio is essential for reforming reactions and membrane separation enhancement effect. Fig. 11 shows the overall reaction heat variation with steam to carbon ratio. It can be recognized that a rising steam to carbon ratio results in the improvement of overall reaction heat. An increase of steam to carbon ratio promotes endothermic reforming reactions and improves reaction heat, which is similar to the effect of pressure on reaction heat. Meanwhile, we can realize that the reaction heat has a sharp change for the reforming process without oxygen carriers when the steam to carbon ratio is below 1.5. In order to better understand this phenomenon, the variation of carbon formation with steam to carbon ratio is displayed in Fig. 12. We can find that a great deal of carbon is formed as the steam to carbon ratio decreases and the released heat from carbon formation greatly reduces the overall reaction heat.
Fig. 5. Variation of carbon dioxide production with reactor temperature at different oxygen carrier to glycerol ratios in a membrane-assisted CLR reactor.
direction in favor of methane production, resulting in the reduction of carbon dioxide production. Fig. 6 displays the variation of overall reaction heat at different oxygen carrier to glycerol ratios with the temperature in a membraneassisted CLR reactor. It can be observed that the overall reaction heat is increased. Because the overall reaction is endothermic, a rising temperature enhances the reaction degree so as to increase the heat requirement. The growth rate of hydrogen production gets slower as the temperature further increases. Besides, we can observe that the increase of oxygen carrier to glycerol ratio reduces the reaction heat. This can be explained that the exothermic reaction between hydrogen and oxygen carriers is enhanced so as to reduce the overall reaction heat when the amount of oxygen carriers increases. 3.3. Effect of reactor pressure The variation of hydrogen production with reactor pressure at different oxygen carrier to glycerol ratios is displayed in Fig. 7. It can be seen that the hydrogen production increases with a descending growth rate as the pressure increases. Though the increase of pressure restrains the reforming reaction of methane and glycerol, a high pressure in the reactor will expand the differential pressure between both sides of membranes so that hydrogen separation is promoted. This implies that the pressure effect on hydrogen separation plays a significant role in hydrogen production. Fig. 8 shows the effect of reactor pressure on methane production at different oxygen carrier to glycerol ratios. We can observe that a rising
Fig. 6. Variation of overall reaction heat with reactor temperature at different oxygen carrier to glycerol ratios in a membrane-assisted CLR reactor. 4
Chemical Engineering & Processing: Process Intensification 142 (2019) 107564
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Fig. 7. Effect of reactor pressure on hydrogen yield in a membrane-assisted CLR reactor.
Fig. 10. Effect of steam to glycerol ratio on hydrogen yield in a membraneassisted CLR reactor.
Fig. 8. Effect of reactor pressure on methane yield in a membrane-assisted CLR reactor.
Fig. 11. Effect of steam to glycerol ratio on overall reaction heat in a membrane-assisted CLR reactor.
Fig. 12. Effect of steam to glycerol ratio on overall reaction heat and carbon formation in a membrane-assisted CLR reactor without NiO. Fig. 9. Effect of reactor pressure on overall reaction heat in a membrane-assisted CLR reactor.
depends on the differential pressure between both sides of membranes. When the hydrogen partial pressure of reaction side doesn’t change, the differential pressure is decreased as the hydrogen partial pressure of permeate side increases. Thus the membrane hydrogen separation process is restrained and the hydrogen production is hindered. The variation of methane production with hydrogen partial pressure in the permeate side is plotted in Fig. 14. We can observe that the enhancement of hydrogen partial pressure in the permeate side leads to
3.5. Effect of permeate side hydrogen partial pressure The relationship between hydrogen production and hydrogen partial pressure in the permeate side is displayed in Fig. 13. From the figure, we can find that the hydrogen production is decreased when the partial pressure of permeate side increases. The permeance of hydrogen 5
Chemical Engineering & Processing: Process Intensification 142 (2019) 107564
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production and sorbent to glycerol ratio at different temperatures. The methane production variation shows a descending trend when the sorbent to glycerol ratio increases. The exothermic effect of absorption reaction promotes reforming conversion of methane. Meanwhile, we can find that nearly full methane conversion can be achieved under high temperature. This implies that the combined effect of membrane separation and CO2 sorption is more important under low temperature. Fig. 17 demonstrates the relationship of reaction heat and sorbent to glycerol ratio at different temperatures. We can find that the overall reaction heat is positive when the sorbent to glycerol ratio is low. As the sorbent to glycerol ratio increases, the reaction heat trends to a negative value. More heat is released from CO2 sorption reaction when the sorbent to glycerol ratio increases, providing the required energy of reforming reactions and membrane separation so that the auto-thermal condition can be reached. Furthermore, we can recognize that a higher sorbent to glycerol ratio is required for the auto-thermal condition of the system.
Fig. 13. Relationship between hydrogen production and permeate side hydrogen partial pressure in a membrane-assisted CLR reactor.
4. Conclusion On the basis of the Gibbs free energy minimization method, a thermodynamic evaluation of the membrane-assisted CLR process with glycerol as fuel is conducted. The results demonstrate that membrane separation greatly improves the hydrogen yield in a CLR process of glycerol. The sensitivity analysis to operating parameters including reactor temperature, reactor pressure, hydrogen partial pressure in the permeate side and steam to carbon ratio is carried out. It is found that the increase of reactor temperature and pressure is beneficial to hydrogen production and methane restriction. The effect of steam to glycerol ratios on hydrogen yield is not monotonous where a peak value exists. A high hydrogen partial pressure in the permeate side will hinder the hydrogen yield owing to the reduction of pressure discrepancy between both sides of membranes. It is also found that membrane separation can enhance hydrogen yield at the cost of more energy consumption. In addition, the integration of sorption-enhanced and membrane separation in a CLR system is evaluated. The results demonstrate that the sorption-enhanced reforming can meet the heat requirement of the membrane-assisted reforming process so as to achieve the auto-thermal condition of the system.
Fig. 14. Variation of methane production with permeate side hydrogen partial pressure in a membrane-assisted CLR reactor.
the increase of the methane production. The variation of methane production can be interpreted by the change of hydrogen production. The hydrogen separation is hindered when the hydrogen partial pressure in the permeate side increases, which will suppress methane reforming reactions and reduce methane conversion.
Acknowledgments This research is conducted with financial support from the National Natural Science Foundation of China (51606053), China Postdoctoral Science Foundation funded project (2016T90285) and Chinese
3.6. Effect of sorbent to glycerol ratio The sorption-enhanced reforming is another efficient way for improving hydrogen production, which usually operated under high temperature. However, operating temperature of membrane reactors is relatively low owing to the restriction of membrane material. Here, the combined effect of membrane hydrogen separation and CO2 sorption on reforming process is further investigated. The effect of sorbent to glycerol ratio on hydrogen production at different temperatures is displayed in Fig. 15. It can be found that the increase of sorbent to glycerol ratio will enhance hydrogen production. The absorption reaction consumes carbon dioxide while releasing heat. The exothermic effect can further enhance the overall reaction so as to increase hydrogen production. Meanwhile, we can observe that the growth rate of hydrogen production is reduced as the temperature increases at a high sorbent to glycerol ratio. Compared to low temperature condition, the reforming reactions of glycerol and methane are more likely to be complete under high temperature. Therefore, the enhancement of high temperature on sorption-enhanced reforming reactions in a membrane-assisted CLR reactor is less significant than that of low temperature. From Fig. 16, we can see that the relationship between methane
Fig. 15. Variation of hydrogen production with sorbent to glycerol ratio in a membrane-assisted CLR reactor. 6
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[4] M. Osman, A. Zaabout, et al., Internally circulating fluidized-bed reactor for syngas production using chemical looping reforming, Chem. Eng. J. (2018), https://doi. org/10.1016/j.cej.2018.10.013. [5] S.M. Nazira, J.F. Morgado, et al., Techno-economic assessment of chemical looping reforming of natural gas for hydrogen production and power generation with integrated CO2 capture, Int. J. Greenh. Gas Control 78 (2018) 7–20. [6] Y. Feng, X. Guo, Study of reaction mechanism of methane conversion over Ni-based oxygen carrier in chemical looping reforming, Fuel 210 (2017) 866–872. [7] Z.I.S.G. Adiya, V. Dupont, et al., Chemical equilibrium analysis of hydrogen production from shale gas using sorption enhanced chemical looping steam reforming, Fuel Process Technol. 159 (2017) 128–144. [8] G. Diglio, P. Bareschino, et al., Numerical simulation of hydrogen production by chemical looping reforming in a dual fluidized bed reactor, Powder Technol. 316 (2017) 614–627. [9] F. García-Labiano, E. García-Díez, et al., Syngas/H2 production from bioethanol in a continuous chemical-looping reforming prototype, Fuel Process Technol. 137 (2015) 24–30. [10] C.V. Rodrigues, K.O. Santana, et al., Crude glycerol by transesterification process from used cooking oils: characterization and potentialities on hydrogen bioproduction, Int. J. Hydrogen Energy 41 (2016) 14641–14651. [11] Y. Liu, A. Lawal, Kinetic study of autothermal reforming of glycerol in a dual layer monolith catalyst, Chem. Eng. Process.: Process Intensif. 95 (2015) 276–283. [12] B. Jiang, B. Dou, et al., Hydrogen production from chemical looping steam reforming of glycerol by Ni-based oxygen carrier in a fixed-bed reactor, Chem. Eng. J. 280 (2015) 459–467. [13] Y. Ni, C. Wang, et al., High purity hydrogen production from sorption enhanced chemical looping glycerol reforming: application of NiO-based oxygen transfer materials and potassium promoted Li2ZrO3 as CO2 sorbent, Appl. Therm. Eng. 124 (2017) 454–465. [14] P. Tippawan, T. Thammasit, et al., Using glycerol for hydrogen production via sorption-enhanced chemical looping reforming: thermodynamic analysis, Energy Convers. Manage. 124 (2016) 325–332. [15] B. Jiang, L. Li, et al., Hydrogen generation from chemical looping reforming of glycerol by Cedoped nickel phyllosilicate nanotube oxygen carriers, Fuel 222 (2018) 185–192. [16] M.R. Rahimpour, F. Samimi, et al., Palladium membranes applications in reaction systems for hydrogen separation and purification: a review, Chem. Eng. Process.: Process Intensif. 121 (2017) 24–49. [17] S. Foresti, G. Di Marcoberardino, et al., A comprehensive model of a fluidized bed membrane reactor for small-scale hydrogen production, Chem. Eng. Process.: Process Intensif. 127 (2018) 136–144. [18] B. Anzelmo, J. Wilcox, et al., Hydrogen production via natural gas steam reforming in a Pd-Au membrane reactor. Comparison between methane and natural gas steam reforming reactions, J. Membr. Sci. 56 (2018) 113–120. [19] G. Ji, M. Zhao, et al., Computational fluid dynamic simulation of a sorption-enhanced palladium membrane reactor for enhancing hydrogen production from methane steam reforming, Energy 147 (2018) 884–895. [20] V. Spallina, G. Matturro, et al., Direct route from ethanol to pure hydrogen through autothermal reforming in a membrane reactor: experimental demonstration, reactor modelling and design, Energy 143 (2018) 666–681. [21] C. Kim, J. Han, et al., Methane steam reforming using a membrane reactor equipped with a Pd-based composite membrane for effective hydrogen production, Int. J. Hydrogen Energy 43 (2018) 5863–5872. [22] S. Wang, X. Song, et al., Thermodynamic evaluation of glycerol autothermal reforming in membrane reactors, Int. J. Hydrogen Energy 41 (2016) 17864–17870. [23] S. Wang, X. Yang, et al., Evaluation of sorption-enhanced reforming of biodiesel byproduct in fluidized beds by means of CFD approach, Fuel 214 (2018) 115–122. [24] Y. Li, Y. Wang, et al., Thermodynamic analysis of autothermal steam and CO2 reforming of methane, Int. J. Hydrogen Energy 33 (2008) 2507–2514. [25] A. Sieverts, G. Zapf, Solubility of H and d in solid Pd (I), Z. Phys. Chem. 174 (1935) 359–364. [26] G. Ye, D. Xie, et al., Modeling of fluidized bed membrane reactors for hydrogen production from steam methane reforming with Aspen Plus, Int. J. Hydrogen Energy 34 (2009) 4755–4762.
Fig. 16. Variation of methane production with sorbent to glycerol ratio in a membrane-assisted CLR reactor.
Fig. 17. Variation of overall reaction heat with sorbent to glycerol ratio in a membrane-assisted CLR. Reactor.
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7
Contents lists available at ScienceDirect
Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Thermodynamic assessment of membrane-assisted chemical looping reforming of glycerol
T
⁎
Shuai Wang , Bowen Li, Yuxiang Tang, Yurong He School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Chemical looping reforming Hydrogen separation Membrane Glycerol
Chemical looping reforming (CLR) as an extension of chemical looping combustion uses less oxygen carriers to achieve hydrogen production via partial oxidation of fuel. Membrane hydrogen separation can promote hydrogen production and makes it possible to low-temperature reforming. In this work, the CLR performance of glycerol with membrane hydrogen separation is investigated by means of thermodynamic analysis. On the basis of the Gibbs free energy minimization method, the sensitivity of the CLR performance with membrane separation to operating parameters is evaluated. The results show that membrane separation can promote hydrogen production at the cost of more heat requirement. The reaction of oxygen carriers cannot achieve auto-thermal conditions of the system. The integration of sorption-enhanced and membrane separation in the CLR of glycerol is further examined.
1. Introduction Chemical looping reforming (CLR) is a novel reforming technology integrated with chemical looping system, which uses oxygen carriers to achieve fuel oxidation and heat transportation between the reformer (contact with the fuel) and the regenerator (oxidation of reduced oxygen carriers). The schematic diagram of a CLR system is displayed in Fig. 1. In contrast to traditional reforming technologies, the CLR system can achieve auto-thermal condition without external combustion. By avoiding the contact of fuel gas and air, the N2-free reforming products can be generated [1]. In additional, high purity hydrogen can be obtained without separation processes [2]. In recent years, the CLR system has attracted more and more concerns [3–5]. Feng et al. [6] studied the reaction mechanics of methane over oxygen carriers during the CLR process using the density functional theory. It was found that reaction rates significantly depended on oxygen anion diffusion in crystal structure of carriers. Adiya et al. [7] investigated the hydrogen production process from shale gas in a sorption-enhanced CLR system using CaO as sorbent. In contrast to conventional catalytic steam reforming, the system with oxygen carriers could greatly reduce the energy requirement for hydrogen production. Diglio et al. [8] carried out a simulation of the CLR process in a dual fluidized bed based on the hydrodynamic model coupled with 1D steady-state heat and mass balances. The results demonstrated that the reforming performance and process feasibility were determined by the
⁎
temperature and the transported oxygen amount between reactors. García-Labiano et al. [9] experimentally studied the hydrogen production from bioethanol in a CLR system and examined the effect of operating conditions. It was concluded that the ethanol had a high conversion and carbon formation was also restricted. Meanwhile, the auto-thermal condition could be achieved. Glycerol from the biodiesel production is a promising renewable resource, which has been used as fuel for hydrogen production [10–12]. Ni et al. [13] used the potassium promoted Li2ZrO3 as sorbents to conduct an experimental study of sorption-enhanced chemical looping reforming of glycerol. It was found that the coke deposition reduced the catalytic effect and the oxygen transfer effect, resulting in the decrease of fuel conversion. Tippawan et al. [14] conducted a thermodynamic analysis of the sorption-enhanced CLR of glycerol and analyzed the impact of sorbent on the reforming performance. It was found that the hydrogen purity and yield could be promoted via the sorption-enhanced way. Additionally, it was pointed out that carbon formation was hindered by the control of calcium oxide-to-glycerol ratio and steam-toglycerol ratio. Jiang et al. [15] experimentally investigated physicochemical properties of oxygen carriers in a CLR process of glycerol. The results revealed that the phyllosilicate nanotube confinement effect could promote the reactivity of oxygen carrier and hydrogen production. Hydrogen separation can promote the hydrogen production of reforming reactions via shifting chemical equilibrium. Membrane reactors
Corresponding author. E-mail address: [email protected] (S. Wang).
https://doi.org/10.1016/j.cep.2019.107564 Received 4 December 2018; Received in revised form 8 June 2019; Accepted 19 June 2019 Available online 21 June 2019 0255-2701/ © 2019 Elsevier B.V. All rights reserved.
Chemical Engineering & Processing: Process Intensification 142 (2019) 107564
S. Wang, et al.
Nomenclature
y
aik Ak Cep
Greekletters
Ea FH2 ΔGf0 ni P R T
number of kth element atoms in the species i total mass of kth element in the feed [kg] membrane permeation capacity (membrane surface area/ thickness) [m] activation energy for permeation [J mol−1] hydrogen permeation rate [mol h−1] standard Gibbs function of formation mole number of species i pressure [Pa] universal gas constant [J mol−1K−1] temperature [K]
λ φ η
gas phase mole fraction
Lagrange multipliers fugacity coefficient permeation effectiveness factor
Subscripts p r
provide the possibility for potential gas separation [16–18]. Ji et al. [19] investigated the enhancement of methane steam reforming through the sorption-enhanced membrane reactor. It was observed that the sorption-enhanced membrane reactor could reduce the CO fraction and minimized the hydrogen permeation decay. Spallina et al. [20] evaluated the performance of ethanol auto-thermal reforming by means of experimental and numerical methods. It was pointed out that the sealing of the membrane was critical. Kim et al. [21] carried out an experimental study of methane steam reforming in a membrane reactor and analyzed the impact of membrane pressure difference on hydrogen recovery and fuel conversion. The results demonstrated that the hydrogen selectivity and the permeance were decreased in a long-term operation owing to catalytic particle adhesion. Although a membrane reactor has been applied to the reforming process, few reports about the CLR process in a membrane reactor could be available. In this work, the CLR performance of glycerol in a membrane reactor with hydrogen separation is investigated via thermodynamic analysis. On the basis of Gibbs free energy minimization method, the enhanced reforming performance via membrane hydrogen separation is studied and the effect of operating parameters is evaluated. Meanwhile, the auto-thermal condition of the CLR system with membrane separation is also examined.
permeate side retentate side
sorption reaction is expressed as below [23]:
CaO(s) + CO2 (g) → CaCO3 (s)
ΔH0r = −179. 2KJ/mol
(1)
In order to conduct a thermodynamic analysis, the Gibbs free energy minimization method is employed and expressed as follows: N
∑ ni (ΔGfi0 + RT ln i=1
ˆ
yi φi P + P0
∑ λk aik ) = 0 k
∑ ni aik = Ak fork = 1, 2, 3, ......w i
(2)
(3)
where the Soavee-Redliche-Kwong equation is adopted to calculate thermodynamic properties [24]. For the membrane-assisted CLR process, a one-dimensional plug flow is assumed where the axial variation of gas compositions is only considered with uniform temperature distribution. The Sieverts’ law is applied to describe the hydrogen permeation through membranes [25,26]:
FH 2 = ηCep kexp (−
Ea 0.5 )(Pr,0.5 H 2 − P p, H 2 ) RT
(4)
where Pr,H2 and Pp,H2 denote hydrogen partial pressures of retentate side and permeate side. Cep represents membrane permeation capacity, depending on the sub-separator number. η is permeation effectiveness factor. k and Ea represent pre-exponential factor and activation energy, which are 1.08 × 10−10 mol/(s m Pa0.5) and 918 0J/mol in this work [26]. In order to model a membrane reactor, some successive sub-reformers and membrane separators are used following the method of Ye et al. [26]. The hydrogen permeation rate is user-defined and implemented on the platform of ASPEN PLUS. The greater the number of sub-separators is, the closer to the real system the predictions are. By conducting a sensitivity analysis of hydrogen production rate to subseparator number, it can be found that the variation of prediction results becomes unclear after the number of sub-separators m reaches 50. Hence, the value of m adopts 50, which is consistent with the prediction of Ye et al. [26].
2. Method The membrane-assisted CLR process with glycerol as fuel is chosen as the objective of this work, involving a series of reactions including glycerol steam reforming, reduction of oxygen carriers with fuel and some side reactions, which are mainly summarized in Table 1 [13,22]. For the CaO-based sorption-enhanced reforming process, the CO2
3. Results and discussion 3.1. Effect of membrane hydrogen separation on gas composition The influence of membrane hydrogen separation on gas compositions in the CLR reactor is evaluated and shown in Fig. 2. It can be observed that membrane separation apparently improves hydrogen production. The methane and water concentrations are greatly reduced, which can be explained that membrane hydrogen separation enhances reforming reactions to move towards the direction in favor of hydrogen yield, resulting in more methane and steam consumed and carbon dioxide produced. As the reactant of methane reforming reactions and the
Fig. 1. Schematic diagram of a CLR system (NiO as oxygen carrier and glycerol as fuel). 2
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Table 1 Main reactions considered during chemical looping reforming of glycerol [13,22]. Reaction description
Reaction equation
Glycerol reforming reaction
C3 H8O3 +3H2 O↔ 3CO2 +7H2 ; ΔH298K = 127.67kJ mol-1 (R1) C3 H8O3 → 3CO+4H2 ; ΔH298K = 215.18kJ mol-1 (R2)
Water-gas shift reaction Reduction reaction of oxygen carriers
CO+H2 O↔ H2 +CO2 ΔH298K = -41.17kJ mol-1 (R3) C3 H8O3 + H2 O+NiO ↔ CO + 2CO2 +5H2 + Ni ΔH298K = 148.74kJ ⋅mol−1 (R4) NiO+CH 4 → Ni+CO+2H2; ΔH298K = 203.75kJ mol-1 (R5) NiO+CO→Ni+CO2 ; ΔH298K = -43.3 kJ mol-1 (R6)
NiO+H2 → Ni+H2 O; ΔH298K = -2.1kJ mol-1 (R7) Methanation
CO+3H2 ↔ CH 4 + H2 O; ΔH298K = -206.11kJ mol-1 (R8) CO2 +4H2 ↔ CH 4 +2H2 O; ΔH298K = -164.94kJ mol-1 (R9) 2CO+2H2 ↔ CH 4 +CO2 ΔH298K = -247.3kJ mol-1 (R10)
Carbon formation
CO+H2 ↔ C+H2 O; ΔH298K = -131.26kJ mol-1 (R11) CO2 +2H2 ↔ C+2H2 O; ΔH298K = -355.06kJ mol-1 (R12) CH 4 ↔ C+2H2; ΔH298K = 74.85kJ mol-1 (R13) 2CO↔CO2 +C; ΔH298K = -172.43kJ mol-1 (R14)
An excessively high temperature cannot significantly promote hydrogen production, which even hinders the membrane permeation capacity and consumes more energy. Fig. 4 illustrates the effect of oxygen carrier to glycerol ratio on the profile of methane yield with reactor temperature in a membrane-assisted CLR reactor. We can observe that a rising temperature can bring about a decrease of methane production. On the one hand, methane reforming reactions are promoted by increasing reactor temperature. On the other hand, the enhancement of hydrogen separation degree under high temperature further improves methane reforming reactions. Furthermore, we can find that methane production is increased as the oxygen carrier to glycerol ratio increases. The variation of carbon dioxide production at different oxygen carrier to glycerol ratios as a function of reactor temperature is shown in Fig. 5. It can be observed that the carbon dioxide yield gets higher as the temperature increases as a result of the promotion effect of high temperature on reforming reactions of glycerol and methane. Simultaneously, an increasing temperature improves membrane separation so as to enhance carbon dioxide production. However, the water-gas shift reaction is restrained as a consequence of an increasing temperature. As a result, the growth rate of carbon dioxide production slows down. We can also find that the higher oxygen carrier to glycerol ratio is, the lower carbon dioxide yield will be. This can be attributed to the fact that the methane conversion rate decreases when the amount of oxygen carriers increases. The methane reforming reaction moves toward the
Fig. 2. Comparisons of gas compositions in a CLR reactor with and without membrane hydrogen separation.
product of water-gas shift reaction, carbon monoxide is generated while being consumed so that the variation of carbon monoxide concentration is not evident. Operating parameters influence the membrane-assisted CLR performance. Here, the effects of different parameters including reactor temperature of 500–650 °C, reactor pressure of 1–3.5 MPa, hydrogen partial pressure in the permeate side of 0.05–0.15 MPa and steam to carbon ratio of 1–3.5 are examined. A base condition is specified at 600 °C with the steam molar flow rate of 6 kmol/h and the steam to carbon ratio of 2.0. The molar flow rate of NiO-based oxygen carrier is 1 kmol/h. The pressures in the retentate side and permeate side of membrane are 2 MPa and 0.1 MPa, respectively.
3.2. Effect of reactor temperature Fig. 3 demonstrates the relationship of hydrogen production and reactor temperature at different oxygen carrier to glycerol ratios in a membrane-assisted CLR reactor. It can be seen that the hydrogen production roughly increases with a descending growth rate as the temperature increases. Both reforming reactions and water-gas shift reaction are endothermic. Meanwhile, it can be seen that the hydrogen production is reduced with the increase of oxygen carrier to glycerol ratio. This can be explained that part of oxygen carriers react with hydrogen when the oxygen carrier to glycerol ratio is improved. This implies that a suitable temperature rise at a lower oxygen carrier to glycerol ratio is beneficial to the enhancement of hydrogen production.
Fig. 3. Relationship between hydrogen production and reactor temperature at different oxygen carrier to glycerol ratios in a membrane-assisted CLR reactor. 3
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pressure results in the decrease of methane production. In spite of the improvement of pressure has a negative impact on methane reforming reactions, the enhancement of membrane separation resulting from high pressure promotes the methane reforming reactions. According to the variation of methane production, the influence of the latter reason is more remarkable. This also inspires us that improving the pressure is a feasible way to reduce methane production and hydrogen yield. Fig. 9 demonstrates the relationship of overall reaction heat and reactor pressure at different oxygen carrier to glycerol ratios. The overall reaction heat is enhanced with the increase of pressure. This means that more heat is required under high pressure. From the above analysis of Figs. 7 and 8, we can realize that reforming reactions are promoted owing to the enhancement of hydrogen separation under high pressure so that more heat is required. In addition, we can find that although the increase of oxygen carrier to glycerol ratios can reduce the heat requirement, the auto-thermal condition is still difficult to achieve.
Fig. 4. Variation of methane production with reactor temperature at different oxygen carrier to glycerol ratios in a membrane-assisted CLR reactor.
3.4. Effect of steam to glycerol ratio The impact of steam to carbon ratio on hydrogen production is evaluated in Fig. 10. It can be clearly seen that the profile of hydrogen production with steam to carbon ratio is not monotonous. As the steam to carbon ratio gets higher, methane and glycerol reforming reactions as well as water-gas shift reaction are enhanced owing to a higher steam fraction so that hydrogen production is promoted. However, an excessively high amount of steam results in the reduction of hydrogen yield, decreasing the reactor partial pressure of hydrogen so as to hinder membrane separation. At the NiO/G ratio of 1.0, the hydrogen yield is decreased from 5.31 to 5.19 when the S/C ratio is changed from 1.5 to 3.5, which implies that the descending degree is not significant for a slight variation of S/C ratio. Hence, we can realize that a suitable selectivity of steam to glycerol ratio is essential for reforming reactions and membrane separation enhancement effect. Fig. 11 shows the overall reaction heat variation with steam to carbon ratio. It can be recognized that a rising steam to carbon ratio results in the improvement of overall reaction heat. An increase of steam to carbon ratio promotes endothermic reforming reactions and improves reaction heat, which is similar to the effect of pressure on reaction heat. Meanwhile, we can realize that the reaction heat has a sharp change for the reforming process without oxygen carriers when the steam to carbon ratio is below 1.5. In order to better understand this phenomenon, the variation of carbon formation with steam to carbon ratio is displayed in Fig. 12. We can find that a great deal of carbon is formed as the steam to carbon ratio decreases and the released heat from carbon formation greatly reduces the overall reaction heat.
Fig. 5. Variation of carbon dioxide production with reactor temperature at different oxygen carrier to glycerol ratios in a membrane-assisted CLR reactor.
direction in favor of methane production, resulting in the reduction of carbon dioxide production. Fig. 6 displays the variation of overall reaction heat at different oxygen carrier to glycerol ratios with the temperature in a membraneassisted CLR reactor. It can be observed that the overall reaction heat is increased. Because the overall reaction is endothermic, a rising temperature enhances the reaction degree so as to increase the heat requirement. The growth rate of hydrogen production gets slower as the temperature further increases. Besides, we can observe that the increase of oxygen carrier to glycerol ratio reduces the reaction heat. This can be explained that the exothermic reaction between hydrogen and oxygen carriers is enhanced so as to reduce the overall reaction heat when the amount of oxygen carriers increases. 3.3. Effect of reactor pressure The variation of hydrogen production with reactor pressure at different oxygen carrier to glycerol ratios is displayed in Fig. 7. It can be seen that the hydrogen production increases with a descending growth rate as the pressure increases. Though the increase of pressure restrains the reforming reaction of methane and glycerol, a high pressure in the reactor will expand the differential pressure between both sides of membranes so that hydrogen separation is promoted. This implies that the pressure effect on hydrogen separation plays a significant role in hydrogen production. Fig. 8 shows the effect of reactor pressure on methane production at different oxygen carrier to glycerol ratios. We can observe that a rising
Fig. 6. Variation of overall reaction heat with reactor temperature at different oxygen carrier to glycerol ratios in a membrane-assisted CLR reactor. 4
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Fig. 7. Effect of reactor pressure on hydrogen yield in a membrane-assisted CLR reactor.
Fig. 10. Effect of steam to glycerol ratio on hydrogen yield in a membraneassisted CLR reactor.
Fig. 8. Effect of reactor pressure on methane yield in a membrane-assisted CLR reactor.
Fig. 11. Effect of steam to glycerol ratio on overall reaction heat in a membrane-assisted CLR reactor.
Fig. 12. Effect of steam to glycerol ratio on overall reaction heat and carbon formation in a membrane-assisted CLR reactor without NiO. Fig. 9. Effect of reactor pressure on overall reaction heat in a membrane-assisted CLR reactor.
depends on the differential pressure between both sides of membranes. When the hydrogen partial pressure of reaction side doesn’t change, the differential pressure is decreased as the hydrogen partial pressure of permeate side increases. Thus the membrane hydrogen separation process is restrained and the hydrogen production is hindered. The variation of methane production with hydrogen partial pressure in the permeate side is plotted in Fig. 14. We can observe that the enhancement of hydrogen partial pressure in the permeate side leads to
3.5. Effect of permeate side hydrogen partial pressure The relationship between hydrogen production and hydrogen partial pressure in the permeate side is displayed in Fig. 13. From the figure, we can find that the hydrogen production is decreased when the partial pressure of permeate side increases. The permeance of hydrogen 5
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production and sorbent to glycerol ratio at different temperatures. The methane production variation shows a descending trend when the sorbent to glycerol ratio increases. The exothermic effect of absorption reaction promotes reforming conversion of methane. Meanwhile, we can find that nearly full methane conversion can be achieved under high temperature. This implies that the combined effect of membrane separation and CO2 sorption is more important under low temperature. Fig. 17 demonstrates the relationship of reaction heat and sorbent to glycerol ratio at different temperatures. We can find that the overall reaction heat is positive when the sorbent to glycerol ratio is low. As the sorbent to glycerol ratio increases, the reaction heat trends to a negative value. More heat is released from CO2 sorption reaction when the sorbent to glycerol ratio increases, providing the required energy of reforming reactions and membrane separation so that the auto-thermal condition can be reached. Furthermore, we can recognize that a higher sorbent to glycerol ratio is required for the auto-thermal condition of the system.
Fig. 13. Relationship between hydrogen production and permeate side hydrogen partial pressure in a membrane-assisted CLR reactor.
4. Conclusion On the basis of the Gibbs free energy minimization method, a thermodynamic evaluation of the membrane-assisted CLR process with glycerol as fuel is conducted. The results demonstrate that membrane separation greatly improves the hydrogen yield in a CLR process of glycerol. The sensitivity analysis to operating parameters including reactor temperature, reactor pressure, hydrogen partial pressure in the permeate side and steam to carbon ratio is carried out. It is found that the increase of reactor temperature and pressure is beneficial to hydrogen production and methane restriction. The effect of steam to glycerol ratios on hydrogen yield is not monotonous where a peak value exists. A high hydrogen partial pressure in the permeate side will hinder the hydrogen yield owing to the reduction of pressure discrepancy between both sides of membranes. It is also found that membrane separation can enhance hydrogen yield at the cost of more energy consumption. In addition, the integration of sorption-enhanced and membrane separation in a CLR system is evaluated. The results demonstrate that the sorption-enhanced reforming can meet the heat requirement of the membrane-assisted reforming process so as to achieve the auto-thermal condition of the system.
Fig. 14. Variation of methane production with permeate side hydrogen partial pressure in a membrane-assisted CLR reactor.
the increase of the methane production. The variation of methane production can be interpreted by the change of hydrogen production. The hydrogen separation is hindered when the hydrogen partial pressure in the permeate side increases, which will suppress methane reforming reactions and reduce methane conversion.
Acknowledgments This research is conducted with financial support from the National Natural Science Foundation of China (51606053), China Postdoctoral Science Foundation funded project (2016T90285) and Chinese
3.6. Effect of sorbent to glycerol ratio The sorption-enhanced reforming is another efficient way for improving hydrogen production, which usually operated under high temperature. However, operating temperature of membrane reactors is relatively low owing to the restriction of membrane material. Here, the combined effect of membrane hydrogen separation and CO2 sorption on reforming process is further investigated. The effect of sorbent to glycerol ratio on hydrogen production at different temperatures is displayed in Fig. 15. It can be found that the increase of sorbent to glycerol ratio will enhance hydrogen production. The absorption reaction consumes carbon dioxide while releasing heat. The exothermic effect can further enhance the overall reaction so as to increase hydrogen production. Meanwhile, we can observe that the growth rate of hydrogen production is reduced as the temperature increases at a high sorbent to glycerol ratio. Compared to low temperature condition, the reforming reactions of glycerol and methane are more likely to be complete under high temperature. Therefore, the enhancement of high temperature on sorption-enhanced reforming reactions in a membrane-assisted CLR reactor is less significant than that of low temperature. From Fig. 16, we can see that the relationship between methane
Fig. 15. Variation of hydrogen production with sorbent to glycerol ratio in a membrane-assisted CLR reactor. 6
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Fig. 16. Variation of methane production with sorbent to glycerol ratio in a membrane-assisted CLR reactor.
Fig. 17. Variation of overall reaction heat with sorbent to glycerol ratio in a membrane-assisted CLR. Reactor.
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