Study of the rheological behaviour of monomodal quartz particle beds under stress. A model for the shear yield functions of powders

A model is proposed that describes the yield locus of powders. The model is based on the adhesive contact of elastic spheres theory developed by Johnson, Kendall, and Roberts (JKR model). The proposed model improves the results by applying the Warren Spring Laboratory (WSL) equation and the Coulomb model. Only two parameters are needed in the present model to describe the yield locus of a powder: powder cohesion (C) and the coefficient of friction (μ) of the powder bed. The study verifies the validity of the new equation to describe the yield locus of different powders.The proposed model was used to calculate the cohesion and coefficient of friction of monomodal quartz particle beds with different average particle sizes. The study examines the influence of average particle size (d_s) and bed compactness (?) on these parameters. Cohesion was observed to increase by the sixth power of compactness and inverse of the particle Sauter diameter. The bed coefficient of friction increased asymptotically with bed compactness, this relationship depending on particle size.     

硅粉氮化输送床内气固反应过程数值模拟

以单个硅颗粒氮化反应缩核模型为基础,本文建立了硅颗粒在输送床内反应、辐射与对流传热耦合的数学模型,并借助CFD软件FLUENT对输送床内能质传输过程进行了数值模拟,分析了输送床壁面温度、氮气流量、预热温度、硅粉粒径等因素对输送床内温度场和硅粉氮化率的影响。在数值计算域内将单个颗粒反应过程转化为颗粒群整体反应过程,实时监测颗粒粒径及未反应硅颗粒粒径,为数值模拟颗粒流反应提供一种新思路。当壁面温度高于1723K时,输送床内会出现一高温区加速硅粉氮化反应;反应温度越高、颗粒粒径越小,氮化过程越剧烈,硅粉到达完全氮化所需时间越短。模型表明为使粒径为2.5μm的硅粉达到完全氮化且输送床内最高温度不超过氮化硅的分解温度2173K,应控制输送床壁面温度在1773K,氮化时间在170s以上,预热温度在1273K,粉气质量比为0.2,稀释剂比例为0.5~1。     

Modeling of continuous self‐classifying spiral jet mills part 1: Model structure and validation using mill experiments

The objective of this work is to develop a milling model for a continuous self‐classifying spiral air jet mill. Its foundation is a population balance model with selection and breakage distribution functions that have been related to a minimal number of mill‐dependent and powder‐dependent parameters. Initially, experimentation is required to determine the mill‐dependent parameters for a specific mill, by milling a “base” powder at multiple operating conditions. Powder‐dependent parameters can be determined from either mill experiments or from material characterization measurements that require small amounts of powder (presented in Part 2). Ultimately, the milling model presented successfully predicts the product particle size using as inputs the feed particle‐size distribution and mill operating conditions. Three crystalline powders, sodium bicarbonate, lactose monohydrate, and sucrose, have been used to test the proposed milling model. © 2014 American Institute of Chemical Engineers AIChE J 60: 4086–4095, 2014     

有机硅单体合成的气固流化床的三维数值模拟

为了探索有机硅单体合成气固流化床内硅粉颗粒的流化特性,作者利用计算流体力学CFD软件,采用双欧拉气固两相流模型及SIMPLE算法,模拟了三维的气固流化床内硅粉颗粒的流化特性;分析了气泡生成、长大和破裂的过程,及不同床层高度的固体颗粒运动速度矢量图,不同床层高度处横截面颗粒体积分数变化。结果表明:三维模拟能直观的表现颗粒在流化床中的流化状态,为工业生产及应用提供了有效的依据。     

基于MP-PIC方法的冶金硅氢氯化流化床反应器模拟

冶金硅氢氯化流化床是目前多晶硅生产工艺中实现四氯化硅（silicon tetrachloride, STC）循环利用的关键反应器。基于MP-PIC方法建立了冶金硅氢氯化流化床反应器模型。模型中反应源项采用总包反应动力学进行计算。在网格以及计算颗粒包含的真实颗粒数量无关性检验的基础上,对反应器内三氯氢硅（trichlorosilane, TCS）初始浓度进行了无关性检验。模型验证结果表明,模拟结果与实验值符合良好。反应器特征分析结果表明：TCS产率受化学平衡与气固两相流动因素共同控制;反应器操作压力对TCS产率的影响较小;提高进口H₂/STC摩尔比,可以提高TCS产率但会降低H₂的转化率。     

基于MP-PIC方法的冶金硅氢氯化流化床反应器模拟

冶金硅氢氯化流化床是目前多晶硅生产工艺中实现四氯化硅（silicon tetrachloride, STC）循环利用的关键反应器。基于MP-PIC方法建立了冶金硅氢氯化流化床反应器模型。模型中反应源项采用总包反应动力学进行计算。在网格以及计算颗粒包含的真实颗粒数量无关性检验的基础上,对反应器内三氯氢硅（trichlorosilane, TCS）初始浓度进行了无关性检验。模型验证结果表明,模拟结果与实验值符合良好。反应器特征分析结果表明：TCS产率受化学平衡与气固两相流动因素共同控制;反应器操作压力对TCS产率的影响较小;提高进口H₂/STC摩尔比,可以提高TCS产率但会降低H₂的转化率。     

Mathematical Model for Batch Drying in an Inert Medium Fluidized Bed

A modified three-phase model is proposed for batch drying of fine powders in an inert medium fluidized bed. The overall heat and mass transfer coefficients between the interstitial gas and solid phases have been determined by the proposed surface-stripping model in which the Biot number is a governing parameter. The effects of gas velocity, inlet gas temperature and mass ratio of starch to inert particles on the drying characteristics of starch in a 0.083 m ID × 0.80 m high medium fluidized bed have been determined. Based on the proposed model, the internal resistance of mass transfer at the powder is equal to the external resistance. The model predicts well the bed temperature, humidity of outlet gas, moisture content of solid particles, heat and mass transfer in an inert medium fluidized bed.     

Modeling of cavern formation in yield stress fluids in stirred tanks

Prediction of cavern formation in yield stress fluids in stirred tanks is of great importance for optimization. A new torus model is developed and then validated by experimental data and computational fluid dynamics simulation. Unlike existing mathematical models, the new torus model assumes that the circular center of the torus should not be outside the impeller swept region as the Reynolds number (Re) increases. Hence the cavern boundary is shaped like an apple torus rather than a horn torus. The new model also considers the cavern‐vessel interactions. At relatively high Re, the new model predicts cavern shape and size better than other models. It correctly captures the cavern outline at various Re, which verified the assumption about torus center. The new model is then used to identify the influence of rheological parameters on cavern formation, and further extended to the cavern prediction of the dual‐impeller system. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3057–3070, 2014     

Analysis of solar‐driven gasification of biochar trickling through an interconnected porous structure

The efficient transfer of high‐temperature solar heat to the reaction site is crucial for the yield and selectivity of the solar‐driven gasification of biomass. The performance of a gas‐solid trickle‐bed reactor constructed from a high thermal conductivity porous ceramic packing has been investigated. Beech char particles were used as the model feedstock. A two‐dimensional finite‐volume model coupling chemical reaction with conduction, convection, and radiation of heat within the packing was developed and tested against measured temperatures and gasification rates. The sensitivity of the gasification rate and reactor temperatures to variations of the packing's pore diameter, porosity, thermal conductivity, and particle loading was numerically studied. A numerical comparison with a moving bed projected a more uniform temperature distribution and higher gasification rates due to the increased heat transfer via combined radiation and conduction through the trickle bed. © 2014 American Institute of Chemical Engineers AIChE J, 61: 867–879, 2015     

助催化剂对微型固定床反应器合成二甲基二氯硅烷的影响

文章利用微型固定床反应器,研究了直接法合成二甲基二氯硅烷的反应过程中助催化剂(Zn粉、三苯基磷和醋酸锌)对二甲基二氯硅烷收率和选择性的影响。研究发现:硅铜触体在加入助催化剂后,诱导期缩短,硅转化率有不同程度的提升,二甲基二氯硅烷的收率也有很大提高,但是选择性却变化不一;同时表明使用微型固定床反应器筛选有机硅单体制备的催化剂,具有量小、便捷、高效的特点。     

流化床制备三氯氢硅工艺参数的优化

流化床制备三氯氢硅是多晶硅生产中比较常见的工艺方法。通过理论计算,选择合适粒径的硅粉,结合实际生产,总结出最优工艺参数,保证三氯氢硅合成系统稳定运行。     

流化床生产多晶硅的研究

在流态化CVD法生产多晶硅的过程中发生了无数的均相反应和异相反应,均相反应十分的复杂,生成的无定形硅粉中含有氢键,是硅粉发生爆炸的关键因素。主要介绍了均相反应和异相反应的反应机理和主要影响因素(反应温度、入口硅烷浓度、进料气速、颗粒的平均直径、反应压力、床层高度等)对多晶硅和硅粉的影响规律。     

磷肥副产硅胶生产碳化硅的实验研究

磷肥副产硅胶产量可观,其主要成分是二氧化硅,是加工硅系列产品很好的原料.碳化硅属第三代半导体材料,经济价值高,应用前景好.概述了以二氧化硅为硅源制备碳化硅粉体的主要方法,并介绍利用磷肥副产硅胶碳热还原法生产碳化硅的实验研究.     

Impact of gravity on the bubble‐to‐pulse transition in packed beds

A model based on two‐phase volume‐averaged equations of motion is proposed to examine the gravity dependence of the bubble‐to‐pulse transition in gas‐liquid cocurrent down‐flow through packed beds. As input, the model uses experimental correlations for the frictional pressure drop under both normal gravity conditions and in the limit of vanishing gravity, as well as correlations for the liquid‐gas interfacial area per unit volume of bed in normal gravity. In accordance with experimental observations, the model shows that, for a given liquid flow, the transition to the pulse regime occurs at lower gas‐flow rates as the gravity level or the Bond number is decreased. Predicted transition boundaries agree reasonably well with observations under both reduced and normal gravity. The model also predicts a decrease in frictional pressure drop and an increase in total liquid holdup with decreasing gravity levels. © 2013 American Institute of Chemical Engineers AIChE J 60: 778–793, 2014     

Drop impact and agglomeration under static powder bed conditions

The influence of impact conditions (reported in terms of Weber and Reynolds numbers) on nucleus formation was studied for single drops striking a static glass bead bed. Results from high speed images showed that the nucleation rate is not influenced by liquid physical properties (density, surface tension, viscosity) for drops that spread significantly (30 < We < 233). Results also showed that nucleus size is determined by how much liquid penetrates into the bed during drop spreading, so does depend on surface tension and liquid density. A corresponding analytical model, derived from first principles, predicts nucleus size to with 1.5% using only liquid physical and powder bed properties, plus the experimentally measured drop spreading behavior. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Production of annatto concentrates in spouted beds

Annatto is composed of a natural reddish-yellow pigment, which is commonly used in the food, cosmetic and pharmaceutical industries. This pigment is easily extracted from Bixa orellana seeds by particle impact and attrition rather than by solvent extraction. Seeds must be dried at a safe temperature to preserve the pigment quality. Experiments show that the annatto powder produced in a non-conventional spouted bed has a high pigment content and is thus suitable for commercial use. A comparative analysis of the bixin extraction in different systems demonstrates that this spouted bed unit can be competitive for commercial applications. Hydrodynamic and drying parameters were studied to improve the powder production rate in this unit.     

Multiscale modeling for surface composition of spray‐dried two‐component powders

Spray drying is a primary process for manufacturing various powder products. One of the most important properties of powders is the ability to get wet. Surface chemical composition critically influences this property. Furthermore, surface composition also influences the efficiency of production as it affects the stickiness of the powder. This work is an attempt to analyze the surface compositions of spray‐dried two‐component powders produced under various conditions using an innovative multiscale modeling approach. A molecular‐level geometrical interpretation is seamlessly coupled with a continuum diffusion model. The predictions are compared with the measurements done on the protein–lactose system using X‐ray photoelectron spectroscopy. Sample calculations for the system have demonstrated that the new approach helps reveal surface formation mechanisms much better than that explained with the monoscale continuum approach. This work provides a good basis for a fruitful area of study toward surface composition‐focused powder quality control that will have a positive impact in industries. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2416–2427, 2014     

Two dimensional numerical computation of a circulating fluidized bed biomass gasifier

A two dimensional model for an atmospheric CFB biomass gasifier has been developed which uses the particle based approach and integrates and simultaneously predicts the hydrodynamic and gasification aspects. Tar conversion is taken into account in the model. The model calculates the axial and radial distribution of syngas mole fraction and temperature both for bottom and upper zones. The proposed model addresses both hydrodynamic parameters and reaction kinetic modeling. Results are compared with and validated against experimental data from a pilot scale air blown CFB gasifier which uses different types of biomass fuels given in the literature. Developed model efficiently simulates the radial and axial profiles of the bed temperature and H₂, CO, CO₂ and CH₄ volumetric fractions and tar concentration versus gasifier temperature. The minimum error of comparisons is about 1% and the maximum error is less than 25%.     

A bubbling fluidized bed solar reactor model of biomass char high temperature steam-only gasification

A two phase biomass char (biochar) steam gasification model based on the systems kinetics is developed in a bubbling fluidized bed with concentrated solar heat as source of energy. The model calculates the dynamic and steady state profiles, as well as the complex parameters of fluidized beds. This robust model is capable of predicting the temperature and concentration profiles of gases in the bubble, emulsion gas and solid phases. The Rosseland equation is used to calculate the radiative transfer within the bed. Due to the nature of the fluidized bed, the small bed thermal conductivity and bigger void between particles, there is a large temperature gradient throughout the bed, indicating that the system is highly non-isothermal. The set-up of a fluidized bed with solar irradiation in the upper side of the reactor is found to be a less efficient gasifying system in comparison with a packed bed, but could be optimized if the source of heat is changed to the bottom of the reactor. The trends and responses of the model are in good agreement with the experimental trends reported in the literature. Hydrogen is the principal product followed by carbon monoxide, the carbon dioxide production is small and the methane production is negligible.     

Binder jetting additive manufacturing of silicon carbide ceramics: Development of bimodal powder feedstocks by modeling and experimental methods

A limitation of binder jetting additive manufacturing is the low density of fabricated parts. Mixing powders with different sizes is a promising approach to increase powder bed packing density and, hence, printed part density. However, in previous studies mixed powder feedstock was prepared by trial and error method. In this research, both modeling and experimental methods were used to prepare the bimodal powder feedstocks. Analytical packing model was introduced for irregular powders. A bimodal powder was prepared by mixing two different-sized silicon carbide powders (i.e. coarse and fine) using ball mill, and their tap densities were measured. Silicon carbide plates were printed using the coarse and bimodal powders by a commercial binder jetting system. Results showed that the modeling method could predict the tap density of bimodal powders with high accuracy. The printed parts from bimodal powder achieved higher green densities than those from the unimodal powder.