Ze Sun

Molecular dynamics simulations of the local structures and transport coefficients of molten alkali chlorides.

Systematic results from molecular dynamics simulations of molten alkali chlorides (ACl) serials are presented in detail in this paper. The effects of temperature and cationic size on the structures and transport properties of molten salts have been investigated and analyzed. The local structures of molten ACl have been studied via the analysis of radial distribution functions and angular distribution functions. The coordination number of ACl decreases when ACl melts from solid and increases as cationic radius increases. Molten LiCl takes a distorted tetrahedral complex as the microconfiguration, while other melts have the tendency to keep the original local structure of the corresponding crystal. Temperature has no significant effect on the local structures of molten ACls. The results also show that the Tosi-Fumi potential predicts positive temperature dependences for self-diffusion coefficients and ionic conductivity, and negative temperature dependences for both viscosity and thermal conductivity of molten ACls. Ionic diffusivity decreases as cationic radius increases from LiCl to CsCl. The simulation results are in agreement with the experimental data available in the literature.

Density functional theory study on the thermodynamics and mechanism of carbon dioxide capture by CaO and CaO regeneration

Reducing CO2 emission is one of the most important events to solve the global climate problem. The carbonation reaction of CaO and the reverse reaction are potential methods for CO2 capture and concentration from dilute flue gases at high temperature. In this paper, the thermodynamics and mechanisms of CO2 capture by CaO and CaO regeneration from CaCO3 were studied and identified in the framework of density functional theory (DFT). In the calculation, the exchange-correlation term was approximated by Perdew–Wang (PW91), a function within the generalized gradient approximation (GGA) family. The reaction energies of carbonation reaction and calcination reaction were calculated to be −147.64 kJ mol−1 and 180.60 kJ mol−1, respectively. To study the reaction between CO2 and CaO, the transition states of carbonation and calcination were also analyzed. The results showed that the carbonation of CaO was rather fast, and the activation energy of carbonation reaction was 0 kJ mol−1, which indicated that the reaction process was not the rate-determining step during the process of CO2 capture. The regeneration of CaO by CaCO3 calcination occurred at higher temperature, with the activation energy of 166.85 kJ mol−1, and the rate of calcination was controlled by the chemical reaction.

Preparation and crystallization kinetics of micron-sized Mg(OH) 2 in a mixed suspension mixed product removal crystallizer

Magnesium hydroxide is an important chemical, and is usually obtained from seawater or brine via precipitation process. The particle size distribution of magnesium hydroxide has great effects on the subsequent filtration and drying processes. In this paper, micron-sized magnesium hydroxide with high purity, large particle size and low water content in filter cake was synthesized via simple wet precipitation in a mixed suspension mixed product removal (MSMPR) crystallizer. The effects of reactant concentration, residence time and impurities on the properties of magnesium hydroxide were investigated by X-Ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Malvern laser particle size analyzer. The results show that NaOH concentration and residence time have great effects on the water content and particle size of Mg(OH)2. The spherical Mg(OH)2 with uniform diameter of about 30 μm was obtained with purity higher than 99% and water content less than 31%. Furthermore, the crystallization kinetics based on the population balance theory was studied to provide the theoretical data for industrial enlargement, and the simulation coefficients (R2) based on ASL model and C-R model are 0.9962 and 0.9972, respectively, indicating that the crystal growth rate of magnesium hydroxide can be well simulated by the sizedependent growth models.

Thermal decomposition mechanism of ammonium sulfate catalyzed by ferric oxide

The decomposition mechanism of ammonium sulfate catalyzed by ferric oxide was investigated in this paper. The decomposition kinetics parameters were determined via a global optimization of the Kissinger iterative method using the non-isothermal thermogravimetric analysis data. The products and intermediates were synchronously characterized by X-ray diffraction and mass spectrometry. The obtained results indicate that the decomposition process of ammonium sulfate catalyzed by ferric oxide can be divided into four stages of which the activation energies are 123.64, 126.58, 178.77 and 216.99 kJ·mol−1 respectively. The decomposition mechanisms at the first and the fourth stage both belong to Mample power theorem, the second stage belongs to Avrami-Erofeev equation and the third belongs to contracting sphere (volume) equation. The corresponding pre-exponential factors (A) are calculated simultaneously.

Designing and optimizing a stirring system for a cold model of a lithium electrolysis cell based on CFD simulations and optical experiments

In the electrolysis lithium industry, liquid lithium metal and chloride gas need to be separated quickly because of the recombination of lithium and chloride. A new stirring system can help to separate liquid metal and chloride in lithium electrolysis cells. The stirring system was tried in a cold model to get the right parameters. Computational Fluid Dynamics (CFD) and Particle Image Velocimetry (PIV) were both employed to design and optimize the device parameters which included impeller type, diameter, position and rotational speed. PIV tests and CFD model validation were conducted in a cylindrical stirred tank. Different turbulence models were applied and the standard k–e model was considered as the most suitable one. The results show that: the propeller agitator properties of a low blade number and low installation position were advantageous to the lithium collection. The impeller diameter and rotational speed have positive effects on the expected flow field. The simulation results were applied in cold model experiments, which showed that the simulations are correct and can be used in real separator design.

Effect of inlet configuration on hydrocyclone performance

Abstract Inlet configuration is important parameter of hydrocyclones, which has great impact on the classification performance. The effects of inlet configuration on the precise classification were studied by computational fluid dynamics under various combinations of inlet diameter and inlet velocity. The results showed that a high sharpness of classification was achieved with specific inlet diameter and inlet velocity. The separation efficiency of the coarse particles by underflow significantly decreased when inlet had an oversize diameter owing to a stronger short-circuit flow. It is resulted from the chaotic flow and the stronger pressure gradient around the vortex finder. Meanwhile, a low separation efficiency of the fine particles by overflow was achieved when inlet velocity was high, which indicated a low sharpness caused by the overlarge centrifugal force.

Design and Optimization of the Preparation of Calcium Carbonate from Calcium Sulfate and Ammonium Bicarbonate

Abstract Simulation on single factor effect was used for the design and optimization of the preparation of calcium carbonate from calcium sulfate (DH) and ammonium bicarbonate. This study shows that simulation on single factor effect is effective because the experimental results are close to predicted results. Furthermore, response surface method based on a central composite design was used to determine the range of parameters to achieve a highly efficient conversion of DH. The results indicate that the significant parameters that affected the conversion of DH were ratio of carbon to sulfur, temperature, concentration of ammonium bicarbonate, and stirring speed. The strength order of factors is as follows: ratio of carbon to sulfur > concentration of ammonium bicarbonate > stirring speed > temperature. A quadratic polynomial equation was established using multiple regression analysis. The optimum parameters were determined as follows: 2.10 for ratio of carbon to sulfur, 320.35 K for temperature, 337.31 rpm for the stirring speed, and 1.75 mol · L−1 for bicarbonate concentration. The corresponding conversion rate of the experimental result was 99.7%, which was highly consistent with the predicted value of 99.9%. Based on model and the optimum parameters, products of vaterite, with ammonium sulfate crystal of grade A, can be obtained. Equipotential lines of conversional rate and desired process conditions were provided as well.

Hydrodynamic characteristics of the two-phase flow field at gas-evolving electrodes: numerical and experimental studies

Gas-evolving vertical electrode system is a typical electrochemical industrial reactor. Gas bubbles are released from the surfaces of the anode and affect the electrolyte flow pattern and even the cell performance. In the current work, the hydrodynamics induced by the air bubbles in a cold model was experimentally and numerically investigated. Particle image velocimetry and volumetric three-component velocimetry techniques were applied to experimentally visualize the hydrodynamics characteristics and flow fields in a two-dimensional (2D) plane and a three-dimensional (3D) space, respectively. Measurements were performed at different gas rates. Furthermore, the corresponding mathematical model was developed under identical conditions for the qualitative and quantitative analyses. The experimental measurements were compared with the numerical results based on the mathematical model. The study of the time-averaged flow field, three velocity components, instantaneous velocity and turbulent intensity indicate that the numerical model qualitatively reproduces liquid motion. The 3D model predictions capture the flow behaviour more accurately than the 2D model in this study.

Coupled electro-thermal field in a high current electrolysis cell or liquid metal batteries

Coupled electro-thermal field exists widely in chemical batteries and electrolysis industry. In this study, a three-dimensional numerical model, which is based on the finite-element software ANSYS, has been built to simulate the electro-thermal field in a magnesium electrolysis cell. The adjustment of the relative position of the anode and cathode can change the energy consumption of the magnesium electrolysis process significantly. Besides, the current intensity has a nonlinear effect on heat balance, and the effects of heat transfer coefficients, electrolysis and air temperature on the heat balance have been released to maintain the thermal stability in a magnesium electrolysis cell. The relationship between structure as well as process parameters and electro-thermal field has been obtained and the simulation results can provide experience for the scale-up design in liquid metal batteries.

Effects of operational and structural parameters on cell voltage of industrial magnesium electrolysis cells

Electric field is the energy foundation of the electrolysis process and the source of the multiphysical fields in a magnesium electrolysis cell. In this study, a three-dimensional numerical model was developed and used to calculate electric field at the steady state through the finite element analysis. Based on the simulation of the electric field, the operational and structural parameters, such as the current intensity, anode thickness, cathode thickness, and anode-cathode distance (ACD), were investigated to obtain the minimum cell voltage. The optimization is to obtain the minimum resistance voltage which has a significant effect on the energy consumption in the magnesium electrolysis process. The results indicate that the effect of the current intensity on the voltage could be ignored and the effect of the ACD is obvious. Moreover, there is a linear decrease between the voltage and the thicknesses of the anode and cathode; and the anodecathode working height also has a significant effect on the voltage.