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Table of Content
05 October 2022, Volume 73 Issue 10
    Reviews and monographs
    Progress of ethane-selective adsorbents for efficient purification of ethylene
    Bo ZHANG, Xiaofei CHEN, Siyao ZHAO, Xin ZHOU
    2022, 73(10):  4255-4267.  doi:10.11949/0438-1157.20220599
    Abstract ( 612 )   HTML ( 40)   PDF (3131KB) ( 151 )  
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    Ethylene, the essential raw material in petroleum chemistry industry, is usually produced with ethane impurity. It is of industrial significance to separate ethane from ethylene for the production of polymerization grade ethylene. Conventional distillation operated at high pressure and low temperature suffers from intensive energy cost and crucial technical challenges. Adsorptive separation separates ethane from ethylene at mild condition, hence holds tremendous potential in industrial application. This paper reviews the most recent research progress of ethane-selective adsorbents, including metal-organic frameworks (MOFs), hydrogen-bonded organic frameworks (HOFs) and porous carbon. Moreover, the ethane-selective adsorption mechanisms are generalized and clarified in detail. In addition, the feasible design strategy of ethane-selective adsorbents is summarized, and the current challenges and future research direction are discussed.

    Status and prospect on CO2 adsorption and separation by supported ionic liquids
    Jianmeng WU, Shuang ZHENG, Shaojuan ZENG, Xiangping ZHANG, Can YANG, Haifeng DONG
    2022, 73(10):  4268-4284.  doi:10.11949/0438-1157.20220600
    Abstract ( 538 )   HTML ( 37)   PDF (3919KB) ( 280 )  
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    Population growth and accelerated global industrialization have led to a yearly increase in fossil energy demand, resulting in a rapid increase of carbon dioxide (CO2) emission in the atmosphere, which has led to a series of global climate problems, and CO2 reduction in the context of “carbon peak and carbon neutralization” is imperative. Traditional industrial technologies for CO2 separation are limited by high energy consumption, low selectivity and large solvent loss. Ionic liquids (ILs) have showed unique advantages in the field of CO2 capture and separation due to their extremely low volatility, strong gas affinity and tunable structures. However, ILs, especially after functionalization, usually are highly viscous or solid at room temperature, resulting in poor gas-liquid mass transfer limiting their applications on CO2 absorption and separation. Supported ionic liquids combine the advantages of ILs and porous materials, not only can enhance selective separation and effectively avoid high viscosity caused by direct absorption of ILs, but also can expand the application range of ILs, which have broad development prospect. This work comprehensively summarized the status and progress on physically and chemically supported ILs for CO2 adsorption and separation in recent years, and provided an outlook on the development trend in future.

    Advances in crystallization of ultrafine crystals
    Haifeng ZHENG, Shengzhe JIA, Songcheng WANG, Rui HAN, Dandan HAN, Zhenguo GAO, Junbo GONG
    2022, 73(10):  4285-4297.  doi:10.11949/0438-1157.20220468
    Abstract ( 273 )   HTML ( 26)   PDF (1553KB) ( 95 )  
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    Ultrafine crystals are widely used in the medicine and chemical industry due to their smaller size and larger specific surface area. In this paper, the preparation methods of ultrafine crystals, principles and controlled parameters of particle size are reviewed in detail. The applications of ultrafine crystals in inhalation, energetic materials and poorly water insoluble drugs were summarized. Meanwhile, the stability of ultrafine crystals suspensions and particle aggregation were reviewed. Finally, we outlook the future development of ultrafine crystals.

    Recent advances in photothermal catalysis of CO2 reduction
    Shuai YAN, Haiping YANG, Yingquan CHEN, Xianhua WANG, Kuo ZENG, Hanping CHEN
    2022, 73(10):  4298-4310.  doi:10.11949/0438-1157.20220863
    Abstract ( 381 )   HTML ( 18)   PDF (2996KB) ( 147 )  
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    Photothermal catalytic conversion of CO2 is a new type of CO2 utilization technology. The process is green, economical, and requires no additional energy input. It is one of the current hot research fields. This paper introduced recent advances in photothermal catalytic conversion of CO2 and summarized design methods for photothermal catalysts based on photo-to-thermal conversion. In addition, fabrication strategies were also investigated in the perspective of photothermal catalytic mechanism and the synergy between photo- and thermo- effects was also discussed. This work aims to promote the understanding of photothermal synergy for CO2 reduction reaction and provides reference for the design of photothermal catalysts and CO2 valorization.

    Research progress of vitamin A biosynthesis
    Xinhui WANG, Ying WANG, Mingdong YAO, Wenhai XIAO
    2022, 73(10):  4311-4323.  doi:10.11949/0438-1157.20220838
    Abstract ( 178 )   HTML ( 15)   PDF (2096KB) ( 101 )  
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    Vitamin A is an essential vitamin to maintain human metabolism. It plays an important role in many fields such as food, medicine and skin care products, and is one of the three pillar vitamins with broad application prospects. Vitamin A on the market mainly comes from chemical synthesis and natural extraction. In recent years, with the development of green biomanufacturing, the biosynthesis of vitamin A has made great progress. This review firstly summarized the research status of vitamin A biosynthesis, analyzed and summarized the optimization of vitamin A biosynthesis, then summarized the fermentation production of vitamin A, regulation of product components and storage strategies, and finally summarized and prospected the status quo of vitamin A biosynthesis.

    Density functional theory investigation on the NH3 and CO2 absorption by functional ionic liquids
    Xianhui ZHU, Fu WANG, Jiecheng XIA, Jinliang YUAN
    2022, 73(10):  4324-4334.  doi:10.11949/0438-1157.20220603
    Abstract ( 117 )   HTML ( 6)   PDF (3108KB) ( 60 )  
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    Due to their unique structural tunability, ionic liquids (ILs) can be used as additives to effectively suppress NH3 escape and simultaneously promote CO2 uptake in ammonia-based carbon capture. Revealing the absorption mechanism of CO2 and NH3 by ILs plays an important role in reconstructing new functional ILs. In this paper, an analysis of five bifunctional ionic liquids based on structure optimization, frequency calculation and atomic charge has been carried out by using density functional theory (DFT) at B3LYP/6-31'++G (d, p) level, and the data of the optimized structures have been achieved. Then, the interaction among ILs, CO2 and NH3 has been analyzed. The calculated results demonstrated that [HEBim][His] shows the best stability, of which its interaction energy reaches -415.73 kJ·mol-1 after basis function overlap error correction. The best sites for interaction with NH3 and CO2 were found by analyzing the electrostatic potential and charge of the designed ILs: NH3 mainly forms O—H…N hydrogen bond with the hydroxyl groups of ILs cation, and [HEBim][His] has the strongest ability to absorb NH3, with a hydrogen bonding energy of 38.52 kJ·mol-1, which has a strong hydrogen bonding effect; CO2 mainly forms C—N…C hydrogen bonds with the amino group in the anion, and [HEBim][Ala] has the strongest ability of CO2 absorption, the formed hydrogen bonding energy of 10.15 kJ·mol-1, which is a relatively weak hydrogen bonding. When ILs interact with NH3 and CO2 at the same time, their absorption capacity decreased to different degrees, and the comprehensive absorption effect of [HEBim][His] and [HEBim][Ala] was the best.

    Phase equilibria of aqueous quaternary system Li+, K+, NH4+ // Cl- - H2O at 298.2 K
    Siying REN, Xudong YU, Jun LUO, Xia FENG, Zhixing ZHAO, Zhihao YAO
    2022, 73(10):  4335-4344.  doi:10.11949/0438-1157.20220894
    Abstract ( 102 )   HTML ( 5)   PDF (3855KB) ( 55 )  
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    The equilibrium solid phase composition of the ternary system Li+, NH4+ // Cl- - H2O and the quaternary system Li+, K+, NH4+ // Cl- - H2O were determined by isothermal dissolution method at 298.2 K. The composition of the equilibrium solid phase at the invariant point of the ternary system Li+, NH4+ // Cl- - H2O was determined by wet residue method and X-ray diffraction method. It was found that a solid solution (NH4Cl) x (LiCl·H2O)1-x was formed. The composition of the equilibrium solid phase at the invariant point of the quaternary system Li+, K+, NH4+ // Cl--H2O was determined by X-ray diffraction and SEM. It was found that solid solutions (NH4Cl) x (LiCl·H2O)1-x, (NH4Cl) x (KCl)1-x and (KCl) x (NH4Cl)1-x were formed. The stable phase diagram of the ternary system Li+, NH4+ // Cl--H2O consists of two invariant points, three uninvariant curves and three crystallization regions, the area of crystallization region decreases in the order of NH4Cl > (NH4Cl) x (LiCl·H2O)1-x > LiCl·H2O. The stable phase diagram of the quaternary system Li+, K+, NH4+ // Cl--H2O consists of three invariant points, eight uninvariant curves and six crystallization regions, the area of the crystallization regions decreases in the order of (KCl) x (NH4Cl)1-x > NH4Cl > KCl > (NH4Cl) x (KCl)1-x >(NH4Cl) x (LiCl·H2O)1-x > LiCl·H2O. The results show that in the chloride system at 298.2 K, both lithium ammonium and potassium ammonium can form solid solution, and potassium ammonium is easier to form solid solution than lithium ammonium and precipitate in large quantities, which increases the difficulty of separation of lithium potassium ammonium chloride.

    Fluid dynamics and transport phenomena
    Influence of wavy-structured superhydrophobic surfaces on coalescence-induced droplet jumping
    Yingjie LI, Qixia LI, Hong WANG, Xun ZHU, Rong CHEN, Qiang LIAO, Yudong DING
    2022, 73(10):  4345-4354.  doi:10.11949/0438-1157.20220680
    Abstract ( 143 )   HTML ( 5)   PDF (3494KB) ( 64 )  
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    Spontaneous coalescence of droplets is widespread in nature and industry. How to efficiently remove coalesced droplets is an important part of strengthening droplet condensation heat transfer and preventing icing. In this paper, the coalescence-induced droplet jumping with different radius ratios on superhydrophobic horizontal wall and superhydrophobic wavy wall is studied by numerical simulation. The horizontal and vertical velocities of the droplets coalescing on the flat wall surface differ by 1—2 orders of magnitude, the horizontal displacement of the droplets is small, so it's difficult to remove them effectively after coalescing. However, on the wavy wall surface, because the liquid bridge impinges on the inclined surface, the droplet is subjected to a large horizontal component force, and the horizontal velocity of the droplet remains at the same order of magnitude as the vertical velocity after coalescence, and the horizontal displacement increases significantly. Moreover, the wave structure has a significant effect. With the increase of the wave aspect ratio, the horizontal displacement of the droplet increases and the bounce height decreases, which can effectively promote the horizontal movement of the droplet. When the aspect ratio is 0.21, the promotion effect is close to the peak value. The findings provide a new reference for the efficient removal of coalesced droplets.

    Particle velocity and concentration distribution in double-exit staged entrained-flow gasifier
    Huaxing ZHU, Jingxiao WANG, Jianliang XU, Zhenghua DAI, Haifeng LIU
    2022, 73(10):  4355-4365.  doi:10.11949/0438-1157.20220608
    Abstract ( 95 )   HTML ( 3)   PDF (1623KB) ( 51 )  
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    Staged utilization of coal is one of the main ways of efficient and low-carbon utilization of coal, a new type of staged entrained-flow gasifier is investigated for production of pyrolysis gas and synthesis gas, the upper part of the gasifier is a coal pyrolysis chamber and the lower part is a coal coke gasification chamber. In order to reveal the characteristics of the gas-solid flow in the gasifier, PV6M particle velocimeter was used to measure the velocity and concentration distribution of solid particles in the gasifier, and the gas-solid flow field in the gasifier was simulated with CFD software. The results indicated that the particle velocity was higher in the jet development region and the refraction flow development region after jet collision. The particle velocity in the side area was lower and the phenomenon of backflow appeared. Under the effect of inertia and airflow traction, most of the particles in the pyrolysis chamber flowed into the gasification chamber. The radial particle concentration centering in the upper part of the pyrolysis chamber was higher than the side wall. The radial particle concentration centering in the lower part of the gasification chamber was higher in the side area. The pyrolysis chamber to gasification chamber inlet air ratio, nozzle angle and particle diameter had an important influence on the particle outflow distribution at the gasifier outlet. The deflection angle of the pyrolysis nozzle and the Stokes number of the particles increased, and the proportion of the particles flowing out from the outlet of the pyrolysis chamber decreased.

    Comparative study on the performance of cascaded latent heat storage system enhanced by fins and porous media
    Yongliang SHEN, Pengwei ZHANG, Shuli LIU
    2022, 73(10):  4366-4376.  doi:10.11949/0438-1157.20220670
    Abstract ( 83 )   HTML ( 2)   PDF (3361KB) ( 39 )  
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    Aiming at the problem of low heat transfer rate caused by the poor thermal conductivity of the phase change material, 3-D numerical simulation is used to study the enhancement effect of fins and porous media on the discharging performance of the cascaded latent heat storage system. Gradient porosity is proposed to further improve the discharging performance of the system, and different enhancement methods are analyzed and compared from the two aspects of the discharging rate and efficiency. The results show that adding fins has a more significant effect on heat transfer enhancement in the sensible heat discharging process, while adding porous media is more significant in the latent heat discharging process. Adding only porous media during the whole discharging process shows better thermal performance than adding only fins. When adding fins and porous media at the same time, the discharging performance of the system is the best, and the complete solidification time of PCMs is reduced by 40%. There is no significant difference in the discharging efficiency of the system under the three porosity gradient conditions, but in the case of negative gradient porosity, the discharging rate is higher and more uniform. Compared with the case of positive gradient porosity, negative gradient porosity has better thermal performance.

    Investigation of bubble behaviors in gas-liquid two-phase flow in helically coiled tube based on wire mesh sensor
    Juntao DAI, Li LIU, Shuai LIU, Hanyang GU, Ke WANG
    2022, 73(10):  4377-4388.  doi:10.11949/0438-1157.20220610
    Abstract ( 117 )   HTML ( 5)   PDF (4656KB) ( 51 )  
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    The heat transfer surface of the nuclear reactor steam generator consists of helical tube bundles. As the spatial spiral structure of the tubes, the slip velocity between phases increases due to the combined effect of gravity, centrifugal force and buoyancy. Accordingly, the distribution of the bubbles in bubbly flow and plug flow shows an asymmetric profile, leading to a significant effect on the heat transfer performance and even the occurrence of departure from nucleate boiling (DNB). In this paper, we employ the developed conductive wire mesh sensor and data post-processing algorithm to investigate the flow field and bubble behaviors in both bubbly and plug flow in a helically coiled tube. Reconstruction of the time and space distribution of the flow field based on the proposed algorithm provides an in-depth knowledge of the characteristics of the bubbles. Based on the present study, the geometric structure of the helically coiled tube can be optimized to avoid heat transfer deterioration, which provides basic experimental data and optimization for the design of the helically coiled tube evaporator. The results indicate that both the increase of the superficial gas velocity and superficial liquid velocity can promote the bubble coalescence. The increase of the gas velocity increases the instability of the gas-liquid interface, and the higher liquid velocity splits the gas plug into several small bubbles in the plug flow.

    Study on flow pattern and transition mechanism of gas-liquid two-phase flow in swirl field under pulsating flow
    Rui YANG, Baojin ZHU, Chao LYU, Lei ZHANG, Yingsong XIAO
    2022, 73(10):  4389-4398.  doi:10.11949/0438-1157.20220769
    Abstract ( 77 )   HTML ( 1)   PDF (2273KB) ( 33 )  
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    Using high-speed camera technology, the dynamic behavior of bubbles and gas-liquid two-phase flow pattern in the cyclone separator were studied under the condition of flow pulsation. It was found that the flow fluctuated in the range of 3.62—4.18 m3/h during the full pulsation cycle. In the flow-enhancing section, the gas core migrates toward the overflow as a whole. And the gas core migrates toward the bottom cone as a whole in the flow-decreasing section, and the size and shape of the gas core changes periodically. Through the analysis of special frames in the pulsation period, it is concluded that the gas-liquid two-phase flow patterns in the swirl field under the condition of flow pulsation mainly include: bubble flow, plug flow, slug flow, silk flow and wavy flow. According to the experiment, the converted velocity of gas-liquid two-phase is obtained. And the limit diagram of the flow pattern conversion of gas-liquid two-phase flow is determined under pulsating conditions. The coalescence and fragmentation behavior between bubbles is the main reason for generating the gas-liquid two-phase flow pattern. Ultimately, an evaluation model was constructed to characterize the relationship between cross-section gas content and separation efficiency.

    Heat transfer enhancement mechanism of phase change heat storage system based on two-dimensional gradient dendritic fins
    Xinyu ZHANG, Xiaohong YANG, Yannan ZHANG, Jiakun XU, Xiao GUO, Rui TIAN
    2022, 73(10):  4399-4409.  doi:10.11949/0438-1157.20220669
    Abstract ( 109 )   HTML ( 3)   PDF (3327KB) ( 70 )  
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    A double-dish photothermal-photoelectric heat storage and power generation system was designed. The heat transfer characteristics of phase change heat storage system were studied. Then the phase change heat storage models of six longitudinal fins, snowflake fins and gradient dendritic fins were established. The Fluent software was used to simulate the heat storage and release process of paraffin. Finally, the heat transfer mechanism of paraffin melting and solidification was analyzed through the change of unsteady heat transfer temperature field and velocity field. The results show that the paraffin melting process is accompanied by the synergistic effect of heat conduction and natural convection, and the convective heat transfer in the solidification process is weak and mainly based on heat conduction. From the perspective of field synergy, the gradient dendritic fins are used to make the spatial temperature distribution more uniform, which can improve the synergy degree of fluid velocity field and temperature field. The melting temperature of paraffin is 315, 340 and 360 K respectively, and the complete melting time is 224, 374 and 703 s respectively. Complete solidification time is 3439, 1089, 842 s. It can be seen that with the increase of melting temperature, the complete melting time increases and the complete solidification time shortens. Therefore, the melting temperature, initial and final temperature of heat storage and release and heat storage capacity should be considered when choosing phase change materials.

    Catalysis, kinetics and reactors
    Influence of oxygen content in industrial kiln flue gas on NH3-SCR denitration reaction of vanadium-titanium catalysts
    Yuxian XIE, Tao LIU, Sheng SU, Lijun LIU, Yuxiu ZHONG, Zhiwei MA, Kai XU, Yi WANG, Song HU, Jun XIANG
    2022, 73(10):  4410-4418.  doi:10.11949/0438-1157.20221034
    Abstract ( 99 )   HTML ( 5)   PDF (1281KB) ( 44 )  
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    Currently, industrial kilns mainly use vanadium-titanium-based SCR catalysts to control nitrogen oxides (NO x ) in flue gas. However, the flue gas generated from industrial kilns contains high oxygen content, and the mechanism in the effect of high O2 content on the SCR reaction process over vanadium-titanium catalyst is still unclear. Therefore, the denitration activity of vanadium-titanium catalysts under different O2 contents was experimentally studied. The effects of different O2 contents on the physical and chemical structure of vanadium-titanium catalysts were systematically analyzed by various characterization methods. At the same time, combined with the in-situ infrared technology, the mechanism of SCR denitration reaction of vanadium-titanium catalysts with different O2 contents was further revealed. The results show that high O2 content can improve the low-temperature denitration efficiency of vanadium-titanium catalysts to a certain extent at the reaction temperature of 150—400℃. The increase of the vanadium loading can make the denitration activity temperature window expanding and moving to the low temperature region. After the catalyst undergoes NH3-SCR denitration reaction under different O2 content conditions, its physical structure almost unchanged. Higher O2 content during the reaction can accelerate the SCR denitration reaction by promoting the acid cycle and redox cycle on the catalyst surface. The active intermediates NH3 (L) and NH4+(B) are consumed faster in the SCR reaction process at higher O2 content, and high O2 content can promote the formation of nitrate species and activate the bridge nitrates, thereby enhancing SCR denitration activity of the vanadium-titanium catalyst.

    Fluidized bed reactor simulation for hydrochlorination of metallurgical silicon based on MP-PIC method
    Shiming WU, Haoning CHEN, Yuan ZONG, Zhimei XU, Ling ZHAO
    2022, 73(10):  4419-4428.  doi:10.11949/0438-1157.20220701
    Abstract ( 80 )   HTML ( 6)   PDF (1560KB) ( 40 )  
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    Metallurgical silicon hydrochlorination fluidized bed is the key reactor to realize the recycling of silicon tetrachloride (STC) in the current polysilicon production process. Based on the MP-PIC method, a metallurgical hydrochlorination fluidized bed reactor model was established. In the model, overall kinetics was used to calculate the reaction source term. On the base of independence test of grid and the number of particles per parcel, the independence test of the initial concentration of trichlorosilane (TCS) was carried out. The model validation results showed that the simulation results were in good agreement with the experimental results. The reactor analysis showed that the yield of TCS is controlled by chemical equilibrium and gas-solid two-phase flow. From the operation condition analysis, the operation pressure of the reactor has little effect on the yield of TCS. Increasing the molar ratio of imported H2/STC can effectively improve the yield of TCS, but reduce the conversion of H2.

    Promotion effects of NbO x doping to Pt/TiO2 on catalytic combustion of vinyl chloride
    Yiyin GAO, Rui FU, Li WANG, Yun GUO
    2022, 73(10):  4429-4437.  doi:10.11949/0438-1157.20220649
    Abstract ( 76 )   HTML ( 5)   PDF (1950KB) ( 29 )  
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    The role of NbO x in catalytic combustion of vinyl chloride (VC) was studied by preparing Pt/Nb x /TiO2. XRD, XPS, H2-TPR, NH3-TPD and Py-FT-IR were used to characterize the effects of NbO x on the catalyst structure, redox and acidity. The activity results presented that the presence of NbO x significantly promoted reaction activity of Pt/TiO2, and the T90 was 246℃ over Pt/Nb0.09/TiO2 with the molar ratio of Nb/Ti at 0.09 for vinyl chloride catalytic combustion. Compared with unmodified Pt/TiO2 catalyst, the value of T90 shifted to lower temperature by 69℃. The existence of NbO x also affected the total concentration and distribution of chlorinated by-products during VC catalytic combustion. The characterization results illustrated that the introduction of NbO x further increased the interaction between Pt and support (TiO2) and the concentration of surface active oxygen species on the catalyst surface which promoted the redox ability of the catalyst. The total acid amount decreased with the increase in NbO x content, especially the Lewis acid amount. Hence, the total acid amount and acid distribution was not the key factor to affect the activity. The redox property at low temperature played an important role in the increase of activity.

    Effect of aging process on copper manganese composite catalyst
    Kailun FANG, Shuaishuai CHEN, Jiawei FU, Xin JIANG
    2022, 73(10):  4438-4447.  doi:10.11949/0438-1157.20220798
    Abstract ( 114 )   HTML ( 8)   PDF (1919KB) ( 39 )  
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    Aging process is a significant process in the preparation of Cu-Mn composite oxide catalyst by co-precipitation method, during which the rapid structural change process of precipitates at the very initial stage of formation remain to be explored. In this paper, Cu-Mn catalyst was prepared in a microreactor and aged in an extended section to study the effect of very short aging time on the structure of Cu-Mn precipitate and catalyst. High resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to analyze the species and structural properties of precursors and catalysts obtained at different aging time. The results show that MnCO3 in the precipitate has rapidly completed the transformation from amorphous to crystalline within a few minutes of aging, while the process of Cu2+ entering the crystalline MnCO3 structure takes tens of minutes. The formation of crystalline MnCO3 separates Cu and Mn, while the formation of Cu-Mn composite carbonate gradually improves the dispersion of Cu and Mn. This leads to a regular change in the structural parameters of the catalyst, which makes the catalyst performance first rapidly deteriorate and then slowly improve with aging time.

    Employing PdCo/SiO2 catalyst in high activity dehydrogenation reaction of heterocyclic H2 storage carrier
    Xiang GONG, Linsen LI, Zhao JIANG
    2022, 73(10):  4448-4460.  doi:10.11949/0438-1157.20220857
    Abstract ( 102 )   HTML ( 2)   PDF (4336KB) ( 78 )  
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    N-Ethylcarbazole/dodecahydro-N-ethylcarbazole (NECZ/12H-NECZ) system was regarded as a promising hydrogen storage solution in the liquid organic hydrogen carriers (LOHC) filed. However, the design and development of high activity and selectivity dehydrogenation catalysts still chocked off the industrial application of NECZ/12H-NECZ system. Aiming to improve H2 release efficiency of 12H-NECZ, bimetal catalyst Pd1Co3/SiO2 (Pd content:1.25%(mass)) with high dehydrogenation activity and high dehydrogenation selectivity was synthesized in this work. The structure properties were characterized by XPS, XRD and HRTEM. And its catalytic performance was evaluated in the dehydrogenation reaction of 12H-NECZ. Compared with commercial 5.0%(mass) Pd/SiO2, it was indicated that the formation of PdCo alloy by the introducing of metal Co facilitated the dehydrogenation efficiency of 12H-NECZ and selectivity of NECZ. Both the reaction kinetics results and DFT calculation analyses showed the obvious decreasing of energy barriers of three elementary reactions by employing bimetal catalyst. Especially in the 2ndelementary reaction (8H-NECZ to 4H-NECZ), its efficiency was boosted a lot. The research content in this paper provides new ideas for revealing the mechanism of 12H-NECZ dehydrogenation reaction and designing and developing efficient catalysts for dehydrogenation reaction.

    Poisoning mechanism and antioxidant performance of TiAl-based carbonyl sulfur hydrolysis catalyst
    Jun HUANG, Yiliang LIU, Peng WU, Kai SHEN, Yaping ZHANG
    2022, 73(10):  4461-4471.  doi:10.11949/0438-1157.20220883
    Abstract ( 72 )   HTML ( 0)   PDF (4530KB) ( 34 )  
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    Desulfurization of blast furnace gas is the key to achieving ultra-low emissions in the steel industry with multiple processes and the entire process. In this paper, Ti0.5Al and K0.2Ti0.5Al catalysts were prepared by co-precipitation method, and the catalytic performance of COS hydrolysis under oxygen-containing atmosphere was investigated. Besides, the effect law of oxygen volume fraction on COS conversion and H2S selectivity was analyzed. A series of characterization methods were used to systematically investigate the physicochemical properties of the catalysts before and after deactivation, and the effect of O2 on the mechanism of COS hydrolysis reaction was investigated by in situ DRIFTS simulation. The activity test results showed that the initial COS conversion of Ti0.5Al catalyst was close to 90% while the efficiency gradually decreased to below 60% as the reaction time increased. As a comparison, the COS conversion of the K0.2Ti0.5Al catalyst could still be maintained at 93.44% after 22 h of continuous reaction under 0.5% (vol) O2 atmosphere. The characterization results showed that the specific surface area is greatly reduced after deactivation, and the surface basicity is significantly weakened. In addition, sulfation of the active center Al atoms was the main cause of catalyst deactivation, while sulfate deposition was a secondary cause. The in situ DRIFTS results showed that the introduction of K significantly attenuated the adsorption of O2 on the catalyst surface and blocked the oxidation of intermediate transition species, which were the key for K to improve the antioxidant performance of the catalyst.

    Degradation of chlorobenzene by non-thermal plasma with Mn based catalyst
    Xiujuan SHI, Wenjun LIANG, Guobin YIN, Jinzhu WANG
    2022, 73(10):  4472-4483.  doi:10.11949/0438-1157.20220696
    Abstract ( 81 )   HTML ( 7)   PDF (3285KB) ( 33 )  
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    Chlorobenzene (CB), a typical representative of chlorinated volatile organic compounds (CVOCs), was selected as the research object. Mn based catalysts were prepared by impregnation method with manganese nitrate (MN) and manganese acetate (MA) as precursors, respectively. The effects of non-thermal plasma cooperating with Mn based catalysts on the degradation of CB and the inhibition of ozone generation, a by-product of the reaction, were investigated. It is found that increasing the discharge voltage can improve the degradation efficiency of CB for different reaction systems. The introduction of catalyst can greatly improve the degradation performance of CB. Compared with MnO x (MN)/γ-Al2O3, the introduction of MnO x (MA)/γ-Al2O3 has better degradation effect and higher inhibition performance on ozone generation. The catalysts before and after the reaction were characterized by N2 adsorption-desorption, scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). It was found that the discharge had no effect on the pore size and crystal structure of the catalyst. The variation of Cl elements during the degradation of CB were analyzed by Cl selectivity and tail GC-MS. Compared with MnO x (MN)/γ-Al2O3 catalyst, the specific surface area of ​​the MnO x (MA)/γ-Al2O3 catalyst is relatively large, and the active components have higher and more uniform dispersion, resulting in more ozone of the reaction system being decomposed into active oxygen atoms on the catalyst surface, which improves the degradation performance of CB and inhibits the formation of ozone in the reaction system.

    NH4Cl assisted preparation of Ni-N-CNTs catalyst and its performance for electrochemical CO2 reduction
    Shide WU, Feng YI, Dan PING, Yifei ZHANG, Jian HAO, Guoji LIU, Shaoming FANG
    2022, 73(10):  4484-4497.  doi:10.11949/0438-1157.20220507
    Abstract ( 84 )   HTML ( 10)   PDF (3566KB) ( 68 )  
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    The resource utilization of carbon dioxide (CO2) is an important means to achieve “carbon peak, carbon neutrality”. Among various CO2 conversion technologies, electrochemical CO2 reduction is considered to be one of the most promising carbon reduction technologies due to its mild reaction conditions and simple process. The key lies in the development of high-efficiency and high-stability electrocatalysts. Transition metal-nitrogen-carbon (M-N-C) material is an effective catalyst for electrochemical CO2 reduction to CO. However, the inevitable aggregation of active metal atoms and the serious N loss during the pyrolysis preparation usually lead to the reduction of active sites density and then the degradation of catalytic performance. Herein, this paper proposes the construction of Ni-N-carbon nanotubes (Ni-N-CNTs) catalysts for electrochemical CO2 reduction via NH4Cl-assisted pyrolysis and associated acid leaching method. The catalysts are prepared by using dicyandiamide as carbon and nitrogen sources, the nickel acetylacetonate as metal source and the NH4Cl as additional nitrogen source and pore-forming agent. The influence of NH4Cl addition on the structure and catalytic performance of catalysts is investigated in detail. The characterization results show that the addition of NH4Cl is beneficial to the formation of nanotube-like morphology and hierarchical pore structure of the catalyst. At the same time, it is beneficial to increase the content of Ni-Nx (1.6%, mole fraction) and pyridinic-N (1.75%, mole fraction) species in the catalyst. A series of catalytic performance results indicate that the catalyst active site is the atomic Ni-Nx, and the presence of pyridinic-N species also contribute to an enhanced catalytic performance. When the mass ratio of NH4Cl to the total mass of nitrogen and metal sources is 1∶1, the derived Ni-N-CNTs-1 catalyst exhibits the best catalytic performance. The CO Faradaic efficiency is as high as 92% with a large CO partial current density of 8 mA·cm-2 at a low potential of -0.65 V (vs RHE). In addition, this catalyst also shows good operation stability for 12 h without obvious performance decay. The catalyst preparation process is simple and the preparation conditions are controllable. This work will provide a practical and feasible approach to build highly efficient M-N-C catalyst for CO2 electroreduction.

    Mechanism of solvent effect on hydrogenation of lignin-derived phenolic compounds
    Hongyun YOU, Jingjun LIN, Kaiyue HUANG, Riyang SHU, Zhipeng TIAN, Chao WANG, Ying CHEN
    2022, 73(10):  4498-4506.  doi:10.11949/0438-1157.20220826
    Abstract ( 108 )   HTML ( 5)   PDF (1746KB) ( 42 )  
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    In this paper, the effect of solvent effect on hydrogenation of lignin phenolic compounds in Ru/C and Al2O3 catalyst systems was systematically studied. In ethanol solvent, phenol can be completely converted into cyclohexanol at 35℃, which exhibited the best hydrogenation effect. The research showed that the hydrogenation effect of polar solvent was better than nonpolar solvents, because the catalyst enabled to be evenly dispersed in the polar solvent, which strengthened the mass transfer and diffusion of the catalyst and the reactants. This study also established the relationship between Kamlet-Taft expression parameters and the conversion rate of phenol, and the effect of each parameter was analyzed. Besides, this work also elaborated the detailed phenol hydrogenation reaction pathway and mechanism in Ru/C and Al2O3 catalytic system. The catalytic system was also applied to the hydrogenation of other lignin-derived phenolic model compounds, which also achieved a good result. Most of the phenolic compounds were transformed into corresponding cyclic alcohols through hydrogenation with a high stability.

    Separation engineering
    Molecular simulation study on adsorption and separation of H2/He mixtures by M-MOF-74
    Yujie WANG, Shenhui LI, Zhiping ZHAO
    2022, 73(10):  4507-4517.  doi:10.11949/0438-1157.20220531
    Abstract ( 85 )   HTML ( 5)   PDF (6082KB) ( 44 )  
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    In order to explore whether the organometallic framework MOF-74 can be used as an excellent solid adsorbent to separate H2 in H2/He mixture and achieve the purpose of purifying He, the adsorption performance and adsorption mechanism of H2, He and H2/He mixture on M-MOF-74 (M=Mg, Co, Ni, Cu, Zn) were studied by molecular simulation. The results show that the selectivity of pure H2 to pure He on Ni-MOF-74 is 6.58 under the conditions of 1 bar pressure and 25 °C. The amount of H2 adsorbed on Mg-MOF-74 is the largest (0.19 mmol·cm-3) in the same condition, which is 6.46 times of He. When the concentration of the H2/He mixture was changed, it had no great effect on its adsorption separation factor on M-MOF-74, indicating that the concentration change would not affect the ability of the adsorption sites on M-MOF-74 to accommodate H2 and He. The adsorption site and adsorption heat analysis showed that the metal ion unsaturated sites on MOF-74 could significantly enhance its adsorption capacity for H2. The results provide a theoretical basis for judging whether M-MOF-74 has the potential to separate H2/He mixtures and quantitatively analyzing the contribution of metal unsaturated sites of MOFs to the separation of H2/He mixtures.

    Global multi-objective optimization of trimethyl orthoformate-acetic acid extractive distillation
    Xu LIU, Songlin XU, Yanfei WANG
    2022, 73(10):  4518-4526.  doi:10.11949/0438-1157.20220747
    Abstract ( 92 )   HTML ( 6)   PDF (1303KB) ( 39 )  
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    The highest azeotrope exists between the reactant trimethyl orthoformate (TMOF) and the product acetic acid (HAc) in the production of 3- (methoxy methotenyl) - 2 (3H) benzofuranone, which leads to the accumulation of reactants and the loss of raw materials and is not conducive to the forward reaction. For TMOF-HAc system, vapor-liquid equilibrium was calculated and analyzed by Hayden-O'Connell's UNIFAC group contribution method with modified fugability coefficient. After error verification, NRTL-HOC model was used as the basis of simulation distillation. The azeotrope can be separated by empirical extractive distillation. The entrainment performance of HAc with N-methyl acetamide (NMA) and N-methyl pyrrolidone (NMP) was compared, and NMP was selected as the extractant. Conventional extractive distillation (CED), side-stream (liquid) extractive distillation (SED) and extractive dividing-wall column (EDWC) were designed with the objective of molar purity of system recovered components, reboiler thermal load (Q) and annual total cost (TAC). Sensitivity analysis was used to adjust the parameters of the three processes in advance. Based on the multi-objective constraints, the optimal parameter range was divided as the initial data of Box-Behnken response surface method (BBD-RSM) optimization, and then the global optimization of the three processes was further conducted by BBD within the specified range. The most economical scheme was formulated by optimizing the data regression multi-objective equation, and the mole purity was 99.8% HAc and 99.9% TMOF. The results show that EDWC and SED can save more economic input and reboiler heat load than CED under the same separation purity condition. SED can save 10.37% TAC and 6.88% heat load, and EDWC can save 10.65% TAC and 10.53% heat load. The results show that there is a good fitting relationship between the predicted value and the actual value, and the prediction errors of TAC and Q by CED, SED and EDWC are all less than 1%. All three process schemes can provide theoretical basis for actual chemical production.

    Preparation of Pebax/a-MoS2/MIP-202 mixed matrix membranes for CO2 separation
    Zhuo JIN, Yonghong WANG, Xinru ZHANG, Xue BAI, Jinping LI
    2022, 73(10):  4527-4538.  doi:10.11949/0438-1157.20220628
    Abstract ( 71 )   HTML ( 4)   PDF (3254KB) ( 34 )  
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    To obtain high-performance CO2/N2 separation membranes, a dual-functional filler prepared by mechanochemistry reaction of oxygen-etched molybdenum disulfide (a-MoS2) and metal-organic framework material MIP-202 was used as the dispersed phase, and polyether block amide (Pebax-1657) was served as a continuous phase, and Pebax/a-MoS2/MIP-202 mixed matrix membranes (MMMs) were prepared by solution casting. The chemical structure of filler was characterized by FT-IR. The chemical structure, microscopic structure, thermal stability and physical mechanical properties of MMMs were characterized by ATR-FTIR, SEM, TG and mechanical property testing. The effects of water content, dual-functional filler ratio, dual-functional filler content, feed pressure and operating temperature on the gas separation performance of the membranes were studied, and the long-time stability of MMMs under simulated flue gas (CO2/N2 volume ratio 15/85) conditions was investigated. The results showed that the best CO2 permeability of MMMs was 380 Barrer with a CO2/N2 selectivity of 124.7 at 25℃ and 0.1 MPa, surpassing the upper bound proposed by McKeown et al in 2019, when the mass ratio of a-MoS2 to MIP-202 was 5∶5 and the dual-functional filler content was 6%(mass). The separation performance of MMMs was not significantly degraded over 360 h testing, with an average CO2 permeability of 358 Barrer and CO2/N2 selectivity of 120.1. This is mainly due to the synergistic improvement of the gas separation performance of the membrane by a-MoS2 and MIP-202.

    Preparation of 2D lamellar CD-MOF membranes for accurate separation of mixed solvents
    Houhu ZHANG, Xiaoli WU, Chongchong CHEN, Jingjing CHEN, Jingtao WANG
    2022, 73(10):  4539-4550.  doi:10.11949/0438-1157.20220543
    Abstract ( 79 )   HTML ( 6)   PDF (4989KB) ( 44 )  
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    With the rapid development of membrane technology in the field of separation, membrane technology has received extensive attention in the application of replacing traditional high-energy distillation in mixed solvent separation. However, preparing separation membranes with uniform subnanometer pore remains a tough challenge. In this paper, isotropic cyclodextrin metal-organic framework (CD-MOF) cubic particles were induced by benzoic acid to generate splintered structure, and then the two-dimensional CD-MOF nanosheets were prepared by liquid phase ultrasonic stripping method, which were further used as building units to assemble lamellar MOF membrane. Importantly, CD-MOF nanosheets contain abundant, connected and homogeneous intrinsic subnanometer pores (0.78 nm), which can recognize the tiny size differences between molecules to achieve accurate separation of mixed solvents. For instance, the separation factor of lamellar CD-MOF membrane for the mixed solvent of 1,3,5-triisopropylbenzene and diisopropylbenzene dissolved in benzene (molar ratio of 1∶3) is up to 7.4. In addition, the rejection of methyl orange dye (1.0 nm) dissolved in methanol reaches 99.6%, and the methanol permeance reaches 84.3 L·m-2·h-1·bar-1.

    Process system engineering
    Full-cycle optimization of acetylene conversion distribution for acetylene hydrogenation beds-in-series reactor
    Zheng WANG, Feng XU, Xionglin LUO
    2022, 73(10):  4551-4564.  doi:10.11949/0438-1157.20220753
    Abstract ( 87 )   HTML ( 6)   PDF (3180KB) ( 31 )  
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    During the commercial operation of acetylene hydrogenation three-beds-in-series reactor, most of acetylene hydrogenation reaction is in the first bed, the large amount of heat released by the hydrogenation reaction makes the temperature in the bed higher than the optimal reaction temperature range,which leads to the decrease of ethylene selectivity and the decrease of ethylene yield. This problem was not taken into account in the whole cycle operation optimization. Therefore, this paper first considers the effect of temperature on the accumulation of green oil, and corrects the kinetic equation of catalyst deactivation. Secondly, in order to ensure that the temperature in each bed of the reactor is within the most suitable temperature range for reaction, two schemes of acetylene conversion rate distribution in each bed of the reactor are given from the perspectives of chemical reaction engineering theory and safety in actual production process. Finally, the constraint of acetylene conversion rate is added to the conventional full-cycle operation optimization model, and the full-cycle operation optimization model of acetylene conversion rate distribution is established, and two schemes of acetylene conversion rate distribution are optimized. The optimization results show that the ethylene yield optimized by the two acetylene conversion distribution schemes is much higher than that optimized by the conventional operation, and the ethylene yield is the highest when the acetylene conversion scheme is 33∶33∶33. Considering the safety in the actual production process, the acetylene conversion distribution scheme is 43∶47∶10, which has a better result.

    Process research of methanol production by CO2 coupled green hydrogen with different CO2 capture technologies
    Dong JI, Jian WANG, Ke WANG, Jingwei LI, Wenliang MENG, Yong YANG, Guixian LI, Dongliang WANG, Huairong ZHOU
    2022, 73(10):  4565-4575.  doi:10.11949/0438-1157.20220778
    Abstract ( 104 )   HTML ( 5)   PDF (1678KB) ( 57 )  
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    The burning of large amounts of fossil fuels has led to a rise in greenhouse gas emissions and global warming. The world has proposed a global agreement to reduce CO2 emissions, and China has proposed the goal of “carbon peaking and carbon neutrality”. CO2 capture and conversion to liquid fuels and chemicals are one of the effective carbon reduction measures under the carbon peaking and carbon neutrality targets, which not only can realize the resource utilization of CO2, but also alleviate the national energy security problem. In this paper, we analyze the CO2 to methanol process coupling green hydrogen, which is based on four different CO2 capture technologies, taking CO2 capture from flue gas of coal-fired power generation and CO2 to methanol as the research objects. Rigorous steady-state modeling and simulation of the CO2-to-methanol process with four different CO2 capture technologies are carried out, and compare the technical-economic performance of the CO2-to-methanol process under different cases of CO2 capture technologies. The results show that the energy consumption per unit of methanol is 7.81, 5.48, 5.91 and 4.66 GJ/t CH3OH for the MEA case, PCS case, DMC case and GMS case, respectively. And the GMS case has the lowest energy consumption, followed by the PCS case, but with the development of more efficient phase change solvents, the energy consumption will be reduced to 2.29—2.58 GJ/ t CH3OH for the PCS case. The total production costs of the four cases are 4314, 4204, 4279 and 4367 CNY/ t CH3OH, respectively, and the PCS case has the lowest cost and is more economically competitive. The comprehensive analysis shows that the PCS case has the best performance and can be used as the best carbon capture technology for coal-fired power plants, providing directions for efficient CO2 synthesis of fuel chemicals to alleviate fossil fuel shortage and environmental pollution problems.

    Surface and interface engineering
    Coupling lubrication model of porous mechanical seal and seal performance analysis
    Xiangkai MENG, Lingchao MENG, Yi MA, Jinbo JIANG, Xudong PENG
    2022, 73(10):  4576-4584.  doi:10.11949/0438-1157.20220845
    Abstract ( 66 )   HTML ( 2)   PDF (2593KB) ( 33 )  
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    Considering the mass transfer coupling relationship between the seepage of the sealing fluid in the porous material and the liquid film, the hydrostatic lubrication model of the porous mechanical face seal was developed. The finite element method was applied to solve the lubrication equation of liquid film and the seepage governing equation of fluid in the porous material. The effects of parameters such as film thickness, permeability, and geometrical parameters of porous ring on the sealing performance were investigated. The mechanism of porous mechanical face seals was revealed. The results show that the porous mechanical face seal can form lubrication film in the seal gap due to the hydrostatic effect. Compared with ordinary parallel end face seals, its liquid film bearing capacity and axial stiffness are greater. As the permeability of porous matrix increases, the leakage rate and opening force of the seal increase gradually, while the stiffness of the liquid film gradually decreases. The increase of film thickness will lead to the increase of leakage rate and the decrease of opening force, while the liquid film stiffness first increases and then decreases, and the maximum stiffness under different permeability corresponds to different film thickness. The present results can provide new ideas and theoretical guidance for the engineering design of porous mechanical face seals.

    Effect of irregular V-shaped surface texture on the performance of mechanical face seal
    Wenjing ZHAO, Zhirong TU, Xiangkai MENG, Jinbo JIANG, Xudong PENG
    2022, 73(10):  4585-4593.  doi:10.11949/0438-1157.20220898
    Abstract ( 61 )   HTML ( 4)   PDF (1325KB) ( 39 )  
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    As for the end face of a new irregular V-shaped surface textured mechanical seal, the lubrication model of the liquid film was established, and the Reynolds equation was solved by the finite element method (FEM). The influences of the area ratio (AR), depth ratio, characteristic number and pressure of seal medium on the seal performance such as load-carrying capacity (LCC), leakage rate, friction coefficient and liquid film stiffness of the seal face were investigated, and the classical triangular texture and circular textured face seals were comparatively analyzed. The results showed that the new irregular V-shaped texture on the seal face had the function of collecting and converging liquid film and enhanced the hydrodynamic effect. The AR, depth ratio, characteristic number and pressure of the sealing medium had a great influence on the seal performance of the V-shaped and triangular textures, but had little effect on that of the circular textures. The seal performance of the new V-shaped surface textured mechanical seal was relatively better based on the research geometry and working conditions, and its LCC, friction coefficient and liquid film stiffness were slightly better than those of the circular texture, and far better than that of the circular texture. Although the leakage rate of the V-shaped texture was slightly lower than that of the triangular texture, it presented an increasing trend. When the AR was greater than 10% and the depth ratio was greater than 1.00, the V-shaped texture had more advantages than the triangular and circular texture. And when the depth ratio was about 1.77 and 1.50, the V-shaped texture had the maximum LCC and liquid film stiffness, respectively. Under the different sealing pressures, the liquid film stiffness of the V-shaped texture was twice that of the triangle texture. The findings can provide support for the design and development of textured face seals.

    Study on the effect of Ni-P-PTFE composite coating on the deposition characteristics of particulate fouling
    Jingtao WANG, Fanfu SONG, Zhiming XU, Yuting JIA
    2022, 73(10):  4594-4602.  doi:10.11949/0438-1157.20220909
    Abstract ( 37 )   HTML ( 0)   PDF (1677KB) ( 15 )  
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    To explore the effect of Ni-P-PTFE composite coating on particle fouling characteristics, the Ni-P-PTFE composite coating was prepared on the surface of carbon steel by electroless plating process. Taking TiO2 nanoparticles as the research object, the particle fouling deposition characteristics of Ni-P-PTFE composite coatings in TiO2 suspensions under different surface energies (PTFE concentrations) were studied through experiments and theoretical analysis. The results show that the Ni-P-PTFE composite coating has better inhibition effect on TiO2 particulate deposition compared with carbon steel. With the increase of PTFE concentration, the surface energy of the composite coating decreases and the fouling deposition tends to decrease. The deposition of TiO2 particulate fouling in the Ni-P-PTFE composite coating is the smallest at the surface energy of 26.8 mJ/m2 (PTFE=12 ml/L). The experimental results are consistent with the optimal surface energy results calculated by the extended DLVO theory, and also provide a guiding basis for the anti-fouling of different types of particles deposited on heat exchange surfaces.

    Biochemical engineering and technology
    Effects of voltage perturbation on the activities of microorganisms and key enzymes in EAD metabolic flux
    Haibo LIU, Nan WANG, Hongzhou LIU, Tiezhu CHEN, Jianchang LI
    2022, 73(10):  4603-4612.  doi:10.11949/0438-1157.20220502
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    In order to explore the effect of voltage perturbation on the activities of microorganisms and key enzymes in the metabolic flux of the electrochemical anaerobic digestion(EAD), a single-chamber EAD reactor was used in the experiment, the electrolysis voltage was used as the perturbation variable, and a metabolic model was constructed by using CNA. Quantitative changes are discussed. The results showed that after the voltage disturbance, the methanogenesis pathway was biased towards hydrogenotrophic methanogenesis, accounting for about 38.5%, and the 0.6 V disturbance showed the best methane flux of 0.5222 g. The main sources of H2 in the EAD system are the cathode and NADH, and the perturbation of the electrolysis voltage will affect the production of H2, which in turn will affect the methane production flux. After 0.6 V perturbation, the relative abundance of Trichloromonas in the biofilm was as high as 40.2%, which was 1.52 times and 1.13 times that of 1.0 V and 1.4 V, respectively. Meanwhile, CoI, PTA, AK and CoF420 all showed the best enzyme activity levels. The activity levels of key enzymes and the relative abundance of Trichloromonas showed good consistency with the methanogenesis flux, and the microbial diversity in the anode biofilm was also an important factor affecting the methanogenesis flux in EAD.

    Energy and environmental engineering
    Dissipative particle dynamics simulation of the stability of heavy oil
    Dong GUAN, Linzhou ZHANG, Suoqi ZHAO, Chunming XU
    2022, 73(10):  4613-4624.  doi:10.11949/0438-1157.20220768
    Abstract ( 92 )   HTML ( 0)   PDF (4580KB) ( 39 )  
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    The stability of heavy oil is related to the safety of its exploitation, storage, transportation, and processing. An accurate method for determining the stability of heavy oil has important guiding significance for the production and processing of heavy oil. However, the empirical method for determining the stability of heavy oil has certain limitations. Establishing the judgment method of heavy oil stability from the mechanism will improve the prediction accuracy of heavy oil stability. In this paper, the aggregation states of molecules in different heavy oil systems are simulated based on dissipative particle dynamics (DPD). The simulation results show that the asphaltene aggregation rate of the heavy oil system is consistent with the judgment results of the colloidal instability index (C.I.I.) and stability judgment diagram, which verifies the accuracy of the simulation. Based on the simulation results, this paper discusses the limitations of C.I.I. and stability judgment diagram and puts forward an improved stability judgment diagram for rapid judgment of heavy oil stability. The determination method of heavy oil stability proposed in this paper is expected to be used in practical industrial processes.

    Study on the effect of gas channel arrangement on the performance of air-cooled fuel cells
    Ming PENG, Qiangfeng XIA, Lixiang JIANG, Ruiyuan ZHANG, Lingyi GUO, Li CHEN, Wenquan TAO
    2022, 73(10):  4625-4637.  doi:10.11949/0438-1157.20220764
    Abstract ( 81 )   HTML ( 1)   PDF (1925KB) ( 45 )  
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    Air-cooled fuel cells have great advantages in unmanned aerial vehicle application, but they still face problems such as low performance and complex water and heat management. In this study, effects of co-flow, counter-flow and cross-flow gas channel arrangements on the performance of air-cooled fuel cell are studied. The coupling mechanism between various physical quantities is also discussed. It is found that gas channel arrangements have a significant influence on the distribution of key physical quantities of air-cooled fuel cells. Affected by the uneven distribution of dissolved water content, the current density distribution in the cathode catalytic layer of cross-flow arrangement presents an isolated point distribution. Due to good thermal management of cross-flow gas channel arrangement, it can significantly improve the water content of membrane electrode, and then improve the cell performance. Under the operating voltage of 0.6 V, the current density of cross-flow gas channel arrangement is 20% higher than that of co-flow gas channel arrangement. In addition, it is also found that the reduction of ambient humidity will significantly reduce the performance of air-cooled fuel cells.

    The variety regulation and mechanism of high silica and alumina coal ash fusion temperature caused by Ca-Fe binary flux under different atmosphere
    Wenju SHI, Jin BAI, Lingxue KONG, Jingpei CAO, Wen LI
    2022, 73(10):  4638-4647.  doi:10.11949/0438-1157.20220844
    Abstract ( 86 )   HTML ( 7)   PDF (2527KB) ( 25 )  
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    The high ash fusion temperature (AFT) of ZGE coal is easy to cause the blockage of the slag discharging of the liquid slag tapping gasifier. It is necessary to add flux to reduce the AFT. Thus, the effects of CaO/Fe2O3 (Ca/Fe) additives on AFT of ZGE coal ash were measured by the AFT analyzer. The synergetic fluxing mechanism of Ca-Fe binary addition in argon atmosphere and mild reducing atmosphere was studied by combination of thermomechanical analysis (TMA), thermogravimetric and differential scanning calorimetry (TG-DSC), XRD and thermodynamic software FactSage. The results showed that the AFT decreases first and then increases with growing Ca/Fe ratio in both weak reduction and argon atmosphere, and the AFT is the lowest when Ca/Fe=1/1, but the AFT in mild reducing atmosphere is lower than in argon atmosphere. The corresponding shrinkage degrees of coal ash with different Ca-Fe additives at deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT) and flow temperature (FT) are quite different, and the melting temperature range under argon atmosphere (the shrinkage rate of the ash column in DT—FT) is significantly higher than that under weak reducing atmosphere. Nevertheless, the shrinkage curves can be divided into three parts no matter in mild reducing atmosphere or in argon atmosphere, and the shrinkage degree in the first and the second stage under mild reducing atmosphere is larger than that in argon atmosphere. The further investigation demonstrated that the first shrinkage is caused by the solid sintering induced by chemical reactions among components, the shrinkage in the second stage was related to the liquid sintering caused by initial slag, and the third shrinkage determined the value of AFT. The mineral transition displayed that the minerals in ash existed as exclusive anorthite or mullite owns high AFT, but the eutectics can be formed between them under appropriate condition to decrease AFT. Moreover, Fe2+ facilitated to form eutectics. The formation temperature of the eutectic in the weak reducing atmosphere is lower than that in the argon atmosphere.

    Effects of atmosphere and chemical composition on fusion characteristics of high-iron coal ash
    Chong HE, Jin BAI, Jing GUO, Lingxue KONG, Hao LU, Huaizhu LI, Yuhong QIN, Wen LI
    2022, 73(10):  4648-4658.  doi:10.11949/0438-1157.20220770
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    Using ash melting point apparatus, XRD and thermodynamic simulation, the influence mechanism of atmosphere and chemical composition on the melting characteristics of high-speed iron coal ash was studied. The results indicate that the fusion temperature of high-iron coal ash decreases with the increasing iron content, or calcium content, or S/A (the mass ratio of SiO2 to Al2O3). Further, the fusion behavior of high calcium or high S/A and high-iron ash under mild reducing atmosphere (MR) follows the “melting-dissolution” mechanism due to the existence of the initial melting stage, while the fusion of low calcium or low S/A and high-iron ash under air belongs to the “softening-melting” mechanism. The iron accelerates the melting of quartz and anorthite under MR, and the calcium promotes the melting of quartz and corundum or transformations of these minerals to the calcium-based aluminosilicates. Under MR, the liquid content increases with the iron content or S/A, and the liquid viscosity decreases with the increasing calcium content or iron content, improving the mass transfer. In contrast, the crystallization of iron-containing solid solutions which were found in the low-calcium or low S/A and high iron ash leads to a higher liquid viscosity or lower liquid content under air, inhibiting the mass transfer of ash melting.

    Kinetics of methane hydrate formation in droplets of low-dose superabsorbent resin solution
    Huihui HU, Liang YANG, Daoping LIU, Ke ZHANG
    2022, 73(10):  4659-4667.  doi:10.11949/0438-1157.20220722
    Abstract ( 53 )   HTML ( 0)   PDF (2115KB) ( 25 )  
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    Rapid formation of hydrate with high gas storage capacity is vital for the application of gas hydrates technology. In this paper, water, hydrophobic fumed nano-silica and low-dose [0.1%—1.0%(mass)] superabsorbent resin were mixed and dispersed at a high speed in a mixer to prepare a superabsorbent resin-modified dry water. The modified dry water is essentially a free-flowing swarm of scattered microdroplets held together by polymers with high molecular weight. The scattered microdroplets were employed for methane hydration and storage at 8.0 MPa and 274.2 K, and the kinetics of methane hydrate formation was investigated. The results indicate that the loose polymer droplets significantly increase the specific surface area of the liquid-phase continuous water, hence providing many pathways for gas diffusion to the surface of the droplet. Methane hydration is rapidly created in this droplet. The gas storage rate may reach 5.15—8.78 cm3·g-1·min-1, and the gas storage capacity is as high as 158.0—175.0 cm3·g-1. The microdroplets with a mass fraction of 0.3% showed the fastest storage rate and the highest gas storage capacity, and the gas storage capacity of the first six times in the process of circulating hydration gas storage exceeded 120 cm3·g-1. The findings have significant implications for the large-scale use of hydrate storage and transportation natural gas technology.

    Construction and verification of BGL coal gasification kinetic model
    Lihe ZHANG, Fan ZHANG, Changlun LI, Deping XU, Zhengang XU, Yonggang WANG
    2022, 73(10):  4668-4678.  doi:10.11949/0438-1157.20220815
    Abstract ( 86 )   HTML ( 2)   PDF (1425KB) ( 32 )  
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    At the moment of energy saving and emission reduction, more and more attention is paid to the equipment development and performance optimization of the BGL coal gasification technology. The analysis and prediction of BGL coal gasification performance still mainly use the equilibrium model and the overall reaction model, but the prediction accuracy is still insufficient. Aiming at the BGL coal gasification process, the research improves the reduced core model, subdivides the carbon core into the interface reaction zone and the inner reaction zone, and improves the adaptability of the model. The final yield and composition of coal pyrolysis gas are predicted by combining device calibration data and pyrolysis data, which simplifies the iterative optimization process. The general pyrolysis kinetic model is used to predict the height of coal seam, and the calculated value is 1.22 m, which is more in line with the actual situation. The calculation results of BGL one-dimensional coal gasification model constructed in this work show that the calculated value of crude gas composition is very close to the calibration check value. The gas phase temperature near the burner is 2012℃, and the particle phase temperature is 1978℃, which is consistent with the empirical estimation value. The crude gas outlet temperature is 549℃, and the measured crude gas outlet temperature is about 538℃.

    The treatment of anionic surfactant wastewater using electrode ultrafiltration membrane bioreactor
    Lanhe ZHANG, Lu WANG, Zimeng LI, Hong TANG, Jingbo GUO, Yanping JIA, Mingshuang ZHANG
    2022, 73(10):  4679-4691.  doi:10.11949/0438-1157.20220862
    Abstract ( 98 )   HTML ( 7)   PDF (6591KB) ( 34 )  
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    A large amount of anionic surfactant wastewater is produced in the process of cosmetics production. It has the characteristics of high organic concentration and easy to produce foam, which influence the treatment efficiency and stable operation of ordinary ultrafiltration membrane bioreactor (UMBR). In this study, a self-made electrode ultrafiltration membrane bioreactor (EMBR) was used to treat anionic surfactant wastewater. The effects of current intensity on the treatment efficiency of the pollutants and activated sludge properties were investigated, and the mechanism of membrane pollution was studied. The results showed that compared with the UMBR, under the action of electric field energy, the organic matter content in the filter cake layer of EMBR is lower, the transmembrane pressure difference (TMP) is reduced by about 50%, and the membrane fouling is less. When the current intensity was 10 mA, the removal rate of chemical oxygen demand (COD) and microbial activity were the highest, and were 97.92% and 41.6 mg/(g TSS·h), respectively. The organic content in the cake layer was the lowest and protein (PN), polysaccharide (PS) and humic acids (HA) were 5.6 mg/L, 8.02 mg/L and 0.85 mg/L, respectively. A relative low current intensity can promote the improvement of microbial activity and pollutant removal rate, and effectively control membrane pollution.

    Study on purification of toluene waste gas by ultrasonic atomization/surfactants-enhanced absorption coupled with biological scrubbing
    Xiaosong HOU, Chenxing LIU, Ailing REN, Bin GUO, Yuanming GUO
    2022, 73(10):  4692-4706.  doi:10.11949/0438-1157.20220590
    Abstract ( 46 )   HTML ( 1)   PDF (4299KB) ( 16 )  
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    To improve the efficiency of the biological method for the purification of hydrophobic VOCs, an ultrasonic atomization/surfactants-biological washing reactor (USBWR), featuring micron-sized droplets combined with surfactants was constructed in this study. The removal capacity and recovery performance of USBWR for toluene exhaust were investigated, the optimal process conditions and droplet size distribution of USBWR were discussed, the microbial community structure of the system was analyzed, and the difference in purification performance between USBWR and traditional biological washing reactor (TBWR) was compared. The results show that USBWR has higher toluene removal capacity and removal load than TBWR system, which is more suitable for non-continuous working conditions of enterprises. Under the conditions of inlet gas concentration of 2000 mg·m-3 and atomization volume of 450 ml·h-1, the best process conditions of USBWR were optimized by response surface method with washing solution pH of 7.07, residence time of 54.60 s and liquid-gas ratio of 0.23, and the removal rate of USBWR reached 97.26%. When the compound surfactant solution (50 mg·L-1 saponin + 500 mg·L-1 sodium citrate + 200 mg·L-1 citric acid + 50 mg·L-1 sodium chloride) obtained from the pre-screening laboratory was applied to the ultrasonic atomizer, the droplet size was less than 15 μm, the median diameter is (6.911±0.326) μm, the specific surface area is (359.60±50.02) m2·kg-1, with small and uniform droplets, which is conducive to making the gas-liquid contact more adequate. The main microbial phyla in the USBWR system are Proteobacteria, Bacteroidota and Chloroflexi. Compared with the TBWR system, the USBWR system promotes the growth of the dominant bacteria Proteobacteria. The enrichment growth is more conducive to the degradation of toluene waste gas.

    Material science and engineering, nanotechnology
    Preparation and properties research of poly(m-phenylene isophthalamide) flat-sheet membrane
    Xi ZHENG, Tao WANG, Yongsheng REN, Zhenzhen ZHAO, Xueqi WANG, Zhiping ZHAO
    2022, 73(10):  4707-4721.  doi:10.11949/0438-1157.20220715
    Abstract ( 87 )   HTML ( 7)   PDF (7114KB) ( 42 )  
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    PMIA flat-sheet membranes were prepared by non-solvent induced phase inversion method. Specifically, poly(m-phenylene isophthalamide) (PMIA) as polymer materials, lithium chloride (LiCl), polyethylene glycol (PEG-400) and polyvinyl pyrrolidone (PVP) as additives were blended to form casting solution. The effects of polymer concentration, additive type and content on the structures and properties of PMIA membranes were systematically investigated. The results exhibited that with the increase of polymer concentration and LiCl content, the viscosity of the casting solution was enhanced, resulting in the decrease of pore size and pure water flux. While the polymer chains were stretched with the increasing content of PEG, which made pore size and water flux of membranes improved. Then, with increasing PVP additive, the pure water flux of the membrane was increased firstly and then decreased, and the hydrophilicity of the membrane became worse. When the mass fraction of PMIA was 9%, LiCl was 2.8% and PVP was 1.2%, the pure water flux of the membrane was as high as 1421.55 L·m-2·h-1·bar-1 with 80% rejection of bovine serum albumin (BSA) maintained. The high permeability provided a new idea for the preparation of high-performance membrane materials.

    Two-phase model for continuous final stage melt copolycondensation of PET/PEG copolymer in steady-state analysis
    Yaman ZHANG, Yulin XING, Jie JIANG, Ling ZHAO, Zhenhao XI
    2022, 73(10):  4722-4733.  doi:10.11949/0438-1157.20220621
    Abstract ( 92 )   HTML ( 1)   PDF (2409KB) ( 35 )  
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    For the high-performance copolyester PET/PEG polycondensation process, a two-phase steady-state model of continuous melt polymerization in a disk reactor was established. The effects of reaction temperature, pressure, residence time and mass transfer parameters on the vapor phase composition, the number average molecular weight, by-product concentration and carboxyl end group concentration were analyzed. The results showed that the volatile components were mainly produced in the first half of the reactor. After z > 0.4, the total amount of volatile had been very small. The proportion of ethylene glycol to the vapor phase was very high, about 90%, while the content of diethylene glycol was very low, only about 0.5%. The molecular weight of copolymer increased with the increase of reactor temperature, vacuum degree, residence time and mass transfer coefficient, when the mass transfer coefficient was greater than 0.1 s-1, the molecular weight of the copolyester at the reactor outlet almost did not change. At this time, it was no longer controlled by mass transfer, and the molecular weight of the final product could reach about 26000.

    Crosslinking structure and mechanical properties of thermoplastic phenolic resin modified with siloxane prepolymer
    Li XU, Qianqiu WU, Zixuan LEI, Jiaxuan LI, Yuhong LIU
    2022, 73(10):  4734-4744.  doi:10.11949/0438-1157.20220579
    Abstract ( 68 )   HTML ( 1)   PDF (3066KB) ( 28 )  
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    In this study, the crosslinking networks with different topological structures were constructed by co-curing and curing physical state control of epoxy siloxane (ES) and phenolic resins with different degree of prepolymer. The crosslinking networks with different degree of prepolymer siloxane prepolymer (PES) modified thermoplastic phenolic resins (NR-PES) were investigated and the construction methods for their strengthening and toughening were discussed. On this basis, the regulation to the crosslinking microstrucutre and the strengthening and toughening strategy of NR-PES are systematically discussed. Firstly, based on the analysis of curing reaction and physical state of NR-PES by DSC and rheology, a series of NR-PES with different crosslinking microstructures were obtained. Next, DMA, TGA and mechanical tests were used to study the influence of PES with different prepolymerization degrees on the crosslinking network and properties of NR-PES. As PES polymerization degree is low, the crosslinking density of NR-PES is low, resulting in inferior thermal stability and bending strength. With the increase of PES polymerization degree, the crosslinking density of NR-PES keeps increasing, and the thermal stability and bending strength also increase except that KIC keeps decreasing. Especially, as the polymerization degree of PES is 30%, 2-NR-PES showed excellent thermal stability, bending strength and fracture toughness with value of 53.43% (C800℃), 20.51 MPa and 0.389 MPa·m1/2, respectively. However, when the polymerization of PES is further increase, the thermal stability, bending strength and fracture toughness of NR-PES will be significantly reduced.

    Preparation of coal-based porous carbon nanosheets by molten salt strategy as anodes for sodium-ion batteries
    Boyang REN, Xiaogang CHE, Siyu LIU, Man WANG, Xinghua HAN, Ting DONG, Juan YANG
    2022, 73(10):  4745-4753.  doi:10.11949/0438-1157.20220473
    Abstract ( 111 )   HTML ( 8)   PDF (2978KB) ( 66 )  
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    The development of low-cost anodes with high charge-storage performance for sodium-ion batteries is the key to its commercialization. Herein, two-dimensional porous carbon nanosheets (CTx) were controllably prepared using coal liquefaction residue with rich aromatic frameworks as carbon precursors by taking advantage of KCl/CaCl2 molten salt, and their electrochemical performance as the anodes for sodium-ion batteries was further explored. It was found that the microstructure of coal-based CTx could be optimized by regulating the carbonization temperature, and the as-obtained sample at 1000℃ (CT1000) exhibits a high specific surface area and abundant defect structure. Therefore, a high reversible specific capacity of 221.4 mAh·g-1 was achieved at a current density of 0.1 A·g-1 as the anodes for sodium-ion batteries, and the specific capacity could be maintained at 124.4 mAh·g-1 when the current density was increased to 10 A·g-1, demonstrating excellent rate performance. Furthermore, the CT1000 electrode delivers good cycling stability with a specific capacity retention of 94.2% after 2000 cycles at a current density of 1 A·g-1.

    Process safety
    Inhibition of hydrogen/air detonation propagation by inert gases
    Mingchuan LEI, Jianliang YU, Xingqing YAN, Xianshu LYU, Yujie HOU, Xiaobing ZHAN
    2022, 73(10):  4754-4761.  doi:10.11949/0438-1157.20220795
    Abstract ( 114 )   HTML ( 2)   PDF (1449KB) ( 31 )  
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    Under normal temperature and pressure conditions, an experimental study on the suppression of hydrogen/air detonation by inert gas was carried out in a stainless steel pipe with an inner diameter of 52 mm. By changing the equivalence ratio (0.6, 0.8, 1.0, 1.2, 1.4) and inert gas species (CO2, N2, Ar), the effects of three inert gases on the detonation flame velocity were discussed. The results indicate that the detonation wave is decoupled and the flame velocity declines obviously as hydrogen/air detonation passes through the interface between combustible gas and inert gas. Flame velocity descending process can be divided into three stages: fast descent stage, slow fluctuating descent stage and flame extinction stage. CO2 has the most obvious inhibition effect, and then followed by Ar and N2. Compared with the specific heat difference, the molecular weight difference of Ar and N2 plays a dominant role in detonation suppression. Compared with the stoichiometric concentration, the attenuation degree of detonation in inert medium is larger in both fuel-lean and fuel-rich conditions. Especially, the attenuation degree of detonation in inert medium is more obvious in fuel-rich condition.

    Experimental and theoretical study on the pyrolysis mechanism of (C2F5)3N
    Tianshui LIANG, Xinke WANG, Dezhi LIU, Wei ZHONG
    2022, 73(10):  4762-4768.  doi:10.11949/0438-1157.20220725
    Abstract ( 102 )   HTML ( 4)   PDF (1255KB) ( 50 )  
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    Fluorine amines are one of the most promising nitrogen-containing compounds as halon substitutes. As a typical fluorine amine, perfluorotriethylamine has good fire-extinguishing effect. In order to reveal the pyrolysis mechanism of (C2F5)3N, its pyrolysis products under different temperature conditions are obtained by using GC-MS, and then the pyrolysis reaction paths are theoretically calculated by using Gaussian. The results show that the initial pyrolysis temperature of (C2F5)3N is about 600℃, and the complete pyrolysis is around 750℃, by keeping the residence time at 10 s. The pyrolysis products are C4F9N, C3F7N, C2F6 and C3F8. The volume fraction of C4F9N is the largest at a low pyrolysis temperature, while the volume fraction of C3F7N is the largest at a high pyrolysis temperature. In the calculation of the pyrolysis reaction path of (C2F5)3N, there is one reaction pathway would generate C3F8 after the breaking of C—C bond in the (C2F5)3N molecule. In another pyrolysis reaction path of (C2F5)3N, C2F6 and the stable product C4F9N would be generated after the breaking of C—N bond. The breaking of C—N bond in (C2F5)3N would produce an unstable product N(C2F5)2, the followed breaking of the C—C bond will generate C3F7N. The main pyrolysis products of (C2F5)3N are C4F9N and C3F7N, the two products have CN double bonds and can more easily interact with the combustion active.

    Comparison of explosion characteristics of methane/graphite powder and methane/pulverized coal
    Bei PEI, Mengjiao XU, Shuangming WEI, Jiaqi GUO, Shiliang LI, Ziwei HU
    2022, 73(10):  4769-4779.  doi:10.11949/0438-1157.20220850
    Abstract ( 90 )   HTML ( 1)   PDF (3064KB) ( 24 )  
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    In this paper, the similarities and differences of explosion characteristics of methane/graphite powder and methane/pulverized coal composite systems are studied by means of 20 L spherical explosion system. The results show that the methane concentration has an important influence on the explosion characteristics of methane/graphite powder two-phase systems and methane/pulverized coal two-phase systems, when the concentration of methane is 6%(vol), the pressure curve of methane/graphite powder system changes from single peak to double peak with the increase of graphite powder particle size, graphite powder with three particle sizes (D50:7, 18,75 μm) reaches the maximum explosion pressure (0.691, 0.657, 0.611 MPa) at 60, 200, and 30 g/m3 respectively. The maximum value of methane/pulverized coal system is 0.724 MPa at 400 g/m3, which is higher than that of methane/graphite powder system. When the methane concentration is close to the equivalence ratio, the explosion pressure peaks of the three particle sizes of graphite powder show a decreasing trend, the smaller the particle size of graphite powder, the smaller the explosion pressure peak of the methane/graphite powder two-phase system, the methane/graphite powder system reaches the maximum value when the mass concentration is 10 g/m3. The explosion pressure of methane/pulverized coal system reaches the maximum value of 0.776 MPa at 60 g/m3, the deflagration index of methane/pulverized coal is higher than that of graphite powder/methane system, because the volatile content of pulverized coal is high, the combustion of pulverized coal is more sufficient, and coke participated in the explosion process. The volatile content of graphite powder is low, and the carbon content is much higher than that of pulverized coal. Only a small part of graphite powder participated in the explosion. The research results will provide guidance for the prevention and control of gas powder two-phase mixture explosion.

    Study on combustion instability of hydrogen methane-doped in vertical circular tubes
    Yi WU, Xiaoping WEN, Sumei ZHANG, Zhidong GUO, Haoxin DENG, Wentao JI
    2022, 73(10):  4780-4790.  doi:10.11949/0438-1157.20220805
    Abstract ( 73 )   HTML ( 2)   PDF (2348KB) ( 47 )  
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    In order to study the instability of hydrogen-doped methane combustion in vertical circular tube, a transparent circular combustion tube (radius r=30 mm, tube length L=600 mm) with upper end opening and lower end closing was built. The flame was ignited at the opening end and propagated to the closing end. Under the condition of chemical equivalence ratio, the experiment was carried out by changing the volume fraction of hydrogen. The results show that when the volume fraction of hydrogen γ50%, a large number of small-size cellular structures appear in the flame propagation process, and gradually evolve into the flame front structure of the approximate plane, but this phenomenon does not appear in the case of γ<50%. The primary unstable oscillation and the causes of the secondary unstable oscillation are analyzed. The rapid change of flame surface area is the main reason for the formation of primary instability oscillation, and the limited amplitude acoustic oscillation produced in the combustion process is the reason for the secondary instability oscillation. The sensitivity analysis of the component flow rate of the reaction process under different operating conditions can be concluded that the chain reaction R1 (H +O2̿O+OH) is the dominant reaction to promote the combustion reaction rate. The maximum pressure in the flame propagation process appears in the secondary instability oscillation stage. Due to the opening sound pressure loss and the opening premixed gas loss, the maximum pressure value decreases with the increase of hydrogen volume fraction.