Axial gas phase dispersion in a molten salt oxidation reactor

Gas phase axial dispersion characteristics were determined in a molten salt oxidation reactor (air-molten sodium carbonate salt two phase system). The effects of the gas velocity (0.05–0.22 m/s) and molten salt bed temperature (870–970 °C) on the gas phase axial dispersion coefficient were studied. The amount of axial gas-phase dispersion was experimentally evaluated by means of residence time distribution (RTD) experiments using an inert gas tracer (CO). The experimentally determined RTD curves were interpreted by using the axial dispersions model, which proved to be a suitable means of describing the axial mixing in the gas phase. The results indicated that the axial dispersion coefficients exhibited an asymptotic value with increasing gas velocity due to the plug-flow like behavior in the higher gas velocity. Temperature had positive effects on the gas phase dispersion. The effect of the temperature on the dispersion intensity was interpreted in terms of the liquid circulation velocity using the drift-flux model.     

Hydrodynamic characteristics of a horizontal flow ejector in a rectangular chamber

The effects of the volumetric flow rate of primary motive water, water height, and the geometric parameters of the hydrodynamiccharacteristics of the gas suction rate and gas phase holdup were investigated in a rectangular chamber (0.22×0.26×1.2 m-high) with a horizontal flow ejector. Gas suction rate increased with increasing volumetric flow rate of the primary motive water, mixing tube length and diffuser length, but it decreased with increasing water height and nozzle diameter. The gas phase holdup was directly proportional to gas suction rate, indicating its corresponding increase with the volumetric flow rate of the primary motive water. Conversely, it decreased with increasing water height and nozzle diameter. However, the mixing tube length affected the gas phase holdup minimally compared to other operating parameters. Both the gas suction rate and gas phase holdup correlated with the dimensionless equations of operating parameters.     

Thermoelectric hydrogen sensor using Li x Ni1-x O synthesized by molten salt method

Li-doped NiO was synthesized by molten salt method. LiN_O3-LiOH flux was used as a source for Li doping. NiCl₂ was added to the molten Li flux and then processed to make the Li-doped NiO material. Li:Ni ratios were maintained from 5: 1 to 30: 1 during the synthetic procedure and the chemical compositions after characterization were found from Li_0.08Ni_0.92O to Li_0.16Ni_0.84O. Li doping did not change the basic cubic structural characteristics of NiO as evidenced by XRD studies; however, the lattice parameter decreased from 0.41769 nm in pure NiO to 0.41271 nm in Li_0.16Ni_0.84O. Hydrogen gas sensors were fabricated by using these materials as thick films on alumina substrates. The half surface of each sensor was coated with the Pt catalyst. The sensor, when exposed to the hydrogen gas blended in air, heated up the catalytic surface leaving the rest half surface (without catalyst) cold. The thermoelectric voltage thus built up along the hot and cold surface of the Li-doped NiO made the basis for detecting hydrogen gas. The linearity of the voltage signal vs H₂ concentration was checked up to 4% of H₂ in air (as higher concentrations above 4.65% are explosive in air) using Li_0.10Ni_0.90O as the sensor material. The response time T₉₀ and the recovery time RT90 were less than 25 sec. H₂ concentration from 0.5% to 4% showed a good linearity against voltage. There was minimum interference of other gases and hence H₂ gas can easily be detected.     

Scale-up and optimization of a two-stage molten salt oxidation reactor system for the treatment of cation exchange resins

A prototype two-stage MSO (molten salt oxidation) reactor system with a capacity of 10 kg/h was developed based on the test results of lab-scale and bench-scale MSO systems. This study first discusses the features of the prototype MSO reactor system. The second part of the study attempts to identify the proper conditions of the prototype two-stage MSO reactor system, where each reactor performs different functions. The volatility of radioactive elements doped in the spent resins was first investigated to establish the proper operating conditions of the primary MSO reactor. A parametric model study of the secondary MSO reactor for the oxidation of hydrocarbons from the primary reactor and an experimental validation were then performed to establish the optimum conditions for the two-stage MSO reactor system. The retention of cesium was greatly influenced by the pyrolysis temperature. The highest pyrolysis temperature with cesium retention of ≥99.9% was 790 °C and this was established as the optimum primary reactor temperature. The optimum conditions of secondary MSO reactor for the substantial oxidation of hydrocarbons generated from the primary MSO reactor were determined to be λ (the ratio of actual air feed rate per stoichiometric air rate) of 2, and a temperature of 800 °C.     

Synthesis of Ti3SiC2 MAX phase powder by a molten salt shielded synthesis (MS3) method in air

Titanium silicon carbide (Ti₃SiC₂) powder was synthesized by molten salt shielded synthesis route of elemental reactants. Potassium bromide (KBr) was used for gas-tight encapsulation of the consolidated reaction mixture for further high temperature processing. The synthesis of Ti₃SiC₂ powder was carried out in air, the salt cladding and molten salt pool provided for the protection of the material against oxidation both at low and high temperature. The process yielded free flowing Ti₃SiC₂ powders without the need of a milling step. Al addition to the reaction mixture resulted in a high purity (96 wt. %) of Ti₃SiC₂ at a synthesis temperature of 1250 °C. The synthesized micro-metric Ti₃SiC₂ can be milled to nano-metric powders.     

Heat transfer characteristics of molten plastics in a vertical falling film reactor

The heat transfer efficiency during the pyrolysis process is a key factor to be considered in the design of pyrolysis reactors. In this study, the average apparent heat transfer characteristics of molten plastic pyrolysis in a vertical falling film reactor were explored by experiments and numerical simulation and the apparent heat transfer coefficients were determined. In addition, the temperature distribution and the thickness of the liquid film in the reactor were predicted and the influences of pyrolysis temperatures on the average apparent heat transfer coefficients were discussed.     

A novel concept reactor design for preventing salt deposition in supercritical water

Reactor plugging and corrosion are the key problems which hinder commercial applications of supercritical water oxidation and gasification, and can be efficiently overcome by preventing salt deposition on internal surface of reactor. In this work the problems caused by salt deposition and the correspondingly main solutions are further reviewed objectively. A novel reactor is designed and manufactured with a feed rate of about 100 L/h for sewage sludge treatment. The reactor combines the characteristics of Modar reactor and transpiring wall reactor for the first time, which is expected to prevent reactor plugging and corrosion as well as to decrease catalyst deactivation rate. The reactor is the core equipment of the first pilot-scale plant for supercritical water oxidation in China. Further optimizations of reactor configuration and operational parameters need plenty of experiments and/or a long-time test with sewage sludge in the subsequent work.     

气体穿过筛板上固液两相流层时的阻力特性

对气体穿过筛板上固液两相流层的流动特性进行了研究,关联出了气体穿过筛板上固液两相流层时泡沫工况下的筛板压降公式,为工程应用提供实验基础。     

Template formation of magnesium aluminate (MgAl2O4) spinel microplatelets in molten salt

Magnesium aluminate, MgAl₂O₄ (MA), microplatelets were synthesized using a molten salt technique. -Alumina platelets partially decomposed from aluminium sulphate were reacted with either commercial magnesium oxide or magnesium nitrate in the molar ratio 1:1 to synthesize spinel platelets. Molten salts such as chloride, MCl (M = Li, Na, and K) and potassium sulphate were used for MA synthesis and the salt to oxide ratio was kept at 3:1 for all compositions. Reactants and molten salt mixes were fired in an alumina crucible for 3 h at from 800 to 1150 °C. XRD revealed complete MA without formation of any secondary phase for powders fired for 3 h at 1100 °C. Electron microscopy revealed the MA platelet morphology and size was the same as the -alumina platelets indicating a ‘template process’ during molten salt synthesis.     

A study on the start-up and performance of a kW-class molten carbonate fuel cell (MCFC) stack

A kW-class internal-manifolded molten carbonate fuel cell (MCFC) stack (52 cells) was assembled with inorganic adhesive under a suitable stacking pressure. The organic compounds in the matrices were burnt out under the conditions of slow and uniform elevation of temperature and big flow of oxygen gas in the stack. The stacking pressure dropped with elevating temperature. The output power of the stack at 150 mA cm⁻² was 1025.5 W when the reactant gas pressure and utilization were 0.5 MPa and 20%, respectively. The thermal-electrical efficiency of the stack was enhanced by increasing the pressure of the reactant. However, it was contrarily decreased when current density was increased.     

Carbothermic reduction synthesis of Ti(C,N) powder in the presence of molten salt

In the presence of sodium chloride (NaCl), Ti(C, N) powder was successfully obtained by the carbothermal reduction of TiO₂ in lab-scale experiments. The effects of NaCl addition and reaction temperature on the formation of the powder were studied in the temperature range of 1100–1600 °C, the reaction time used in all cases was 3 h. The final powder was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results indicated that addition of NaCl played a facilitating role in the formation process of Ti(C, N). Ti(C, N) was detected at 1100–1200 °C, and the yield of powder was purer at about 1300 °C when 10 wt.% NaCl was added. The as-prepared Ti(C, N) was uniform in shape and the particle size was about 5–8 μm. With increasing temperature, the residual carbon content in the products decreased but the degree of oxidation increased at temperatures above 1300 °C. The possible mechanism involved in the reactions was discussed.     

Ceramic foam as a potential molten salt oxidation catalyst support in the removal of soot from diesel exhaust gas

An exploratory study has been carried out to determine the potential of open-pore ceramic foam as support for molten-salt diesel soot oxidation catalysts. These catalysts are based on eutectic mixtures of Cs₂O, V₂O₅, MoO₃, and Cs₂SO₄. Open-pore ceramic foams are very interesting, since they can act as supports and as soot filters. 50 ppi -alumina foam has been tested as support substance. Attractive methods have been developed for depositing the molten salt on the external surface of the pore walls of the foam. The combustion of ‘loose-contact’ synthetic soot has been analysed with a micro-flow reactor. The oxidation rate is a function of the amount of molten salt deposited on the foam. Stable rates in the range from 5 to 25 μg_soot/g_soot,initial/s at 650 K have been observed. These rates are in the same order of magnitude as measured for the best catalytic fuel additives; and, therefore, catalytic foams are very promising. The initially minor ‘deactivation’ that occurs after the thermal treatment up to 100 h at 725 K probably has a physical explanation. During the treatment, the molten salt redistributes to a configuration that has less catalyst available for accelerating the soot oxidation.     

Molten waste plastic pyrolysis in a vertical falling film reactor and the influence of temperature on the pyrolysis products

Molten plastics are characterised with high viscosity and low thermal conductivity. Applying falling film pyrolysis reactor to deal with waste plastics can not only improve heat transfer efficiency, but also solve the flow problem.In this work, the pyrolysis process of molten polypropylene(PP) in a vertical falling film reactor is experimentally studied, and the influence of heating temperature on pyrolysis products is discussed. It has been found that with the temperature increases from 550 ℃ to 625 ℃, the yield of pyrolysis oil decreases from 74.4 wt%(± 2.2 wt%) to53.5 wt%(±1.3 wt%). The major compositions of the pyrolysis oil are C_9, C_(12) and C_(18), and β-scission reactions are predominant. The content of the light fraction C_6-C_(12) of pyrolysis oil is 69.7 wt%. Compared with other pyrolysis reactors, the yield of oil from vertical falling film pyrolysis reactor is slightly higher than that from tubular reactor,equal to that from rotary kiln reactor, and slightly lower than that in medium fluidised-bed reactor.     

Air oxidation of aqueous cyanides in a countercurrent fixed bed reactor

This study was conducted to discuss the removal of cyanides from water by air oxidation. Experiments were carried out in a countercurrent fixed bed reactor for three different values of temperature, concentration, gas and liquid flow rates. It was operated at pH 12 by using delrin (formaldehyde polymer) as packing material. Effects of some operating parameters on the conversion were studied, and it was observed that the conversion percent increased by increasing temperature and decreasing gas and liquid flow rates. Effect of concentration was not steady. A conversion of 89% was achieved under optimum conditions while it ranges from 44 to 79% at room temperature.     

Neodymium(III) cathodic processes in molten fluorides

The electrochemical behaviour of the Nd(III)/Nd(0) system has been investigated in several molten media and more particularly in LiF-CaF₂. A preliminary study based both on thermodynamic and experimental data showed that it is not possible to observe the Nd(III)/Nd(0) system in LiF-KF and LiF-NaF melts; because the K⁺ and Na⁺ cation reduction waves hide the Nd³⁺ reduction wave. Then, the Nd(III)/Nd(0) system has been investigated at 810 °C using solutions of NdF₃ in fluoride solvents without K⁺ and Na⁺ ions, such as LiF-CaF₂, by cyclic voltammetry, chronopotentiometry and square wave voltammetry. Experimental results show that neodymium trifluoride is reduced in Nd(0) in a one-step process exchanging three electrons (Nd(III) + 3e⁻ → Nd(0)). The electrode process is shown to be diffusion controlled. Nd(III) diffusion coefficient is in the range of 1.1-1.3 × 10⁻⁵ cm² s⁻¹ at 810 °C.     

Bi(Ⅲ)在NaCl-KC1熔盐体系中的电化学行为

为开发环境友好型铋提取技术,在700℃下采用循环伏安、方波伏安和计时电位等方法研究了NaCl?KCl熔盐体系中Bi(III)在玻碳电极上的电化学行为。在–0.3 V (vs. Ag/AgCl)电位下以玻碳电极为工作电极对NaCl?KCl?BiCl3进行恒电位电解。结果表明,Bi(III)在NaCl?KCl熔盐体系中的还原反应是一步得到3个电子的准可逆反应Bi3++3e?=Bi,起始还原电位为0.05 V (vs. Ag/AgCl),该反应受扩散控制。Berzins-Delahay方程和Sand方程计算的700℃下Bi(III)在熔盐中的扩散系数分别为0.83×10–5和1.0×10–5 cm2/s。阴极产物为致密纯金属Bi,不含杂质。     

Reduction process of uranium(IV) and uranium(III) in molten fluorides

This study focused on the electroreduction process of uranium cations in molten fluorides. It involved cyclic voltammetry, chronopotentiometry with and without current reversal, and square wave voltammetry.The results indicate a two-step reduction process for uranium(IV). The first step U(IV)/U(III) exchanging one electron corresponds to a soluble/soluble system and is limited by U(IV) diffusion with D_U(IV) = 1.25 ± 0.35 × 10⁻⁵ cm² s⁻¹ in LiF-NaF at 720 °C.In order to perform a thorough study of the second step U(III)/U(0) in the reduction process, the melt was chemically reduced in U(III) with U metal as reducing agent. Alternatively to the use of LiF-NaF where U metal is unstable at 720 °C, the chemical reduction of U(IV) in U(III) was performed in a LiF-CaF₂-UF₄ solution containing U metal at 810 °C. It has been confirmed that the reduction of U(III) proceeds in one step exchanging three electrons and by a diffusion controlled process with D_U(III) = 2.2 ± 0.7 × 10⁻⁵ cm² s⁻¹ in LiF-CaF₂ at 810 °C.     

Investigating the flow characteristics of air-lift pumps operating in gas-liquid two-phase flow

Experiments were conducted to investigate the flow characteristics in the riser pipe and the suction pipe of airlift pump at a series of air flow rates and submergence ratios by using a high speed camcorder and a Laser Doppler Velocimetry system(LDV). A modified model was developed to predict the performance of airlift pump operating in gas-liquid two-phase flow. The results show that an unstable flow structure composed by a water falling film,a bubbly mixture, a water ascending film appearing alternately in riser pipe dominates the performance of airlift pump at large air flow rates. The bubbly mixture with a strongest capacity for pumping water first increases to its maximum and then slightly decreases. In suction pipe, the average velocity shows a flat profile and increases with increasing submergence ratio. Moreover, the predicted results of modified model are in good agreement with the experimental data in a margin of ± 12%.     

Effects of relative gas flow direction in the anode and cathode on the performance characteristics of a Molten Carbonate Fuel Cell

In this work, three-dimensional numerical simulation of conservation equations for mass, momentum, energy, species and electrochemical reaction has been carried out to compare the performance characteristics of a Molten Carbonate Fuel Cell (MCFC) with three different flow types, co-flow, counter-flow and cross-flow. Depending on the flow types, distributions of pressure difference, temperature and current density in the electrolyte matrix were examined and the fractions of various losses were scrutinized. The simulation results show that the co-flow type has the lowest pressure difference across the matrix and the distributions of temperature and current density are more uniform than other types. However, it was found that since irreversible losses due to ohmic resistance, anode activation and cathode activation are smallest in the counter-flow type, best performance can be expected by the MCFC with the counter-flow type.     

分离式热管倾斜布置蒸发管流动特性的试验研究

采用加热石英玻璃管模拟分离式热管倾斜布置蒸发段,通过可视化试验和壁温侧量,对蒸发段工质的流型进行了观测和分析,发现在蒸发段前部存在一种脉冲对流的流动状态,分析了其流动特点;着重研究了倾斜角度、充液量和热流密度对流动规律的影响;由试验数据回归整理了相应的合理充液率关系式,与试验数据吻合较好,并与垂直蒸发段的流动特性进行了比较。