PARTICLE REMOVAL MECHANISMS IN CRYOGENIC SURFACE CLEANING

Particle removal mechanisms in cryogenic surface cleaning are examined. The effect of impacting solid carbon dioxide particles and the hydrodynamic forces and torques are included in the model. Sliding detachment models and rolling detachment models in conjunction with the theory of critical moment are used. The critical conditions for removal of particles of different sizes are evaluated. For various carbon dioxide pellet diameters, incoming flow angles, nozzle, and cleaning surface separation distances, the critical shear velocities for particle detachment are evaluated.     

MEASURED ATMOSPHERIC NEW PARTICLE FORMATION RATES: IMPLICATIONS FOR NUCLEATION MECHANISMS

Measured production rates of tropospheric ultrafine particles (~3nm diameter) are reported for the first time and are shown to be orders of magnitude greater than nucleation rates predicted by the binary theory of homogeneous nucleation for sulfuric acid and water. Furthermore, the functional dependence of observed particle formation rates on sulfuric acid vapor concentrations is much weaker than predicted by binary theory. We present arguments to show that these discrepancies might be due to the participation of a species such as ammonia which could stabilize subcritical clusters, thereby enhancing nucleation rates. The data suggest that atmospheric nucleation may occur by a collision-limited process, rather than by a condensation/evaporation controlled process as is assumed in the classical theory.     

燃煤过程中颗粒物的形成机理研究进展

介绍了煤粉燃烧过程中颗粒物的形成机理，包括亚微米飞灰和残灰颗粒的主要形成途径．亚微米颗粒主要来自无机物的气化-凝结过程，在高温条件下无机矿物首先以氧化物、次氧化物或原子的形式气化，当温度降低时，无机蒸气通过均相成核、异相冷凝、凝并、团聚等过程形成细微颗粒．残灰由残留在焦炭颗粒中的矿物转化而成，焦炭破碎和表面灰的聚合是决定残灰最终粒径分布的主要过程，除此之外，对于含外来矿物较多的煤种，矿物破碎对残灰颗粒的形成也有十分重要的影响．最后对燃煤过程中颗粒物的形成机理研究提出了建议．     

悬浮体中团聚体的产生机理及其对材料结构与性能的影响

研究了陶瓷胶态成型过程悬浮体中产生团聚体的微观机理,根据液体介质中陶瓷粉体颗粒之间的作用势,建立了团聚体产生的微观结构模型.利用环境扫描电子显微镜对悬浮体中粉体团聚的显微结构进行"准"直接观察,发现悬浮体中存在2种团聚体形态:Ⅰ型团聚体(硬团聚)和Ⅱ型团聚体(软团聚),揭示了团聚体的产生是由于浆料的固相含量偏离了稳定固相含量.研究发现:悬浮体中的团聚体会遗传到陶瓷体中,并对陶瓷构件的力学性能有显著影响.探讨了团聚体对烧结体的结构及力学性能影响的原因.当悬浮体中固相含量低于稳定固相体积分数ψ0时,悬浮体中会形成松散团聚即软团聚;当固相含量超过ψ0时,悬浮体中会形成紧密团聚即硬团聚:当固相含量等于ψ0时,悬浮体具有均匀分散的稳定结构.由于悬浮体固相含量偏离稳定周相含量ψ0而在悬浮体中产生的团聚体,会由于原位周化而遗传到坯体之中,影响烧结体结构的均匀性.     

电子束辐射GeS2-Ga2S3-CdS硫系玻璃的二次谐波发生效应

用熔融一急冷法制备了在可见光区有较高透过率的GeS2-Ga2S3-CdS硫系玻璃,在电子束辐射的玻璃样品中观察到明显二次谐波发生效应,得到了对称性良好的Maker条纹.研究结果表明:当激光入射角在±(40°～50°)左右时,二阶非线性光学效应的相对强度出现最大值.通过表面化学蚀刻法进行了极化机理分析,发现在电子束辐射下,玻璃表面很薄的一层区域内形成局域直流强电场,破坏了玻璃的宏观各向同性,极化区域厚度大于理论计算结果.     

FREQUENCY DEPENDENCE IN DIELECTROPHORESIS: TRAJECTORIES AND PARTICLE BUILDUP

A study of the effect of frequency of a sinusoidal field on dielectrophoretic capture is presented, using numerical analysis to compute particle trajectories. It is shown that at certain critical frequencies the trapping efficiency drops sharply due to weakening of the dielectrophoretic pull. Experiments in which the capture performance was visually recorded by photographing the particle buildup on a single rod at increasing frequencies are reported. Results are discussed in the light of the numerical analysis.     

FREQUENCY DEPENDENCE IN DIELECTROPHORESIS: TRAJECTORIES AND PARTICLE BUILDUP

A study of the effect of frequency of a sinusoidal field on dielectrophoretic capture is presented, using numerical analysis to compute particle trajectories. It is shown that at certain critical frequencies the trapping efficiency drops sharply due to weakening of the dielectrophoretic pull. Experiments in which the capture performance was visually recorded by photographing the particle buildup on a single rod at increasing frequencies are reported. Results are discussed in the light of the numerical analysis.     

CENTRIFUGAL FLUIDIZATION: EFFECTS OF PARTICLE DENSITY AND SIZE DISTRIBUTION

Based on the model of the centrifugal fluidized bed as a homogeneous viscous rotating fluid, the equation for conservation of momentum in the tangential direction is solved to obtain expressions for bed pressure drop and the radial variations of tangential velocity within the bed region. Comparisons with available data on bed pressure drop indicate that the fluidized bed rotates as a rigid body. Experimental data are also presented on minimum fluidization velocity and bed pressure drop with different particle densities and size distributions, verifying the validity of the available models for packed bed and fluidized bed pressure drop and minimum fluidization velocity in a centrifugal fluidized system.     

CENTRIFUGAL FLUIDIZATION: EFFECTS OF PARTICLE DENSITY AND SIZE DISTRIBUTION

Based on the model of the centrifugal fluidized bed as a homogeneous viscous rotating fluid, the equation for conservation of momentum in the tangential direction is solved to obtain expressions for bed pressure drop and the radial variations of tangential velocity within the bed region. Comparisons with available data on bed pressure drop indicate that the fluidized bed rotates as a rigid body. Experimental data are also presented on minimum fluidization velocity and bed pressure drop with different particle densities and size distributions, verifying the validity of the available models for packed bed and fluidized bed pressure drop and minimum fluidization velocity in a centrifugal fluidized system.     

SINGLE-STAGE AND MULTISTAGE MASS TRANSFER: SIMPLE RATE-BASED ANALYTICAL SOLUTION

An analytical solution to steady-state mass transfer in a binary system, of general application in mass transfer processes, was found by straightforward mathematical analysis. The analysis resulted in simple closed-form equations that define single-stage and multistage mass transfer, for cross-flow and co-current flow, entirely as expressions of the transfer unit concept. The traditional equilibrium stage and efficiency model is not validated by the analysis.     

SINGLE-STAGE AND MULTISTAGE MASS TRANSFER: SIMPLE RATE-BASED ANALYTICAL SOLUTION

An analytical solution to steady-state mass transfer in a binary system, of general application in mass transfer processes, was found by straightforward mathematical analysis. The analysis resulted in simple closed-form equations that define single-stage and multistage mass transfer, for cross-flow and co-current flow, entirely as expressions of the transfer unit concept. The traditional equilibrium stage and efficiency model is not validated by the analysis.     

UNDERSTANDING REACTIVE HAZARDS USING MOLECULAR SIMULATION: MECHANISMS OF HYDROXYLAMINE DECOMPOSITION

Reactive hazards have been a significant concern for the chemical process industries. Without sufficient control of chemical hazards and risks, reactive incidents have caused catastrophic consequences for humans and the environment worldwide. In order to help understand and evaluate reactive hazards, calorimetric investigations have been employed to characterize reactive chemical behavior. However, with the development of computer resources and computational chemistry, molecular simulation has become a powerful and reliable tool to complement and guide experimental testing. In this article, hydroxylamine (HA) calculations demonstrate the application of molecular simulation for understanding and controlling reactive hazards. Using the quantum mechanical software Gaussian 03, HA bimolecular or water-catalyzed HA isomerization to ammonia oxide in aqueous solution was identified as the most likely decomposition pathways. Based on these molecular simulation results, recommendations are provided for safer handling of HA.     

UNDERSTANDING REACTIVE HAZARDS USING MOLECULAR SIMULATION: MECHANISMS OF HYDROXYLAMINE DECOMPOSITION

Reactive hazards have been a significant concern for the chemical process industries. Without sufficient control of chemical hazards and risks, reactive incidents have caused catastrophic consequences for humans and the environment worldwide. In order to help understand and evaluate reactive hazards, calorimetric investigations have been employed to characterize reactive chemical behavior. However, with the development of computer resources and computational chemistry, molecular simulation has become a powerful and reliable tool to complement and guide experimental testing. In this article, hydroxylamine (HA) calculations demonstrate the application of molecular simulation for understanding and controlling reactive hazards. Using the quantum mechanical software Gaussian 03, HA bimolecular or water-catalyzed HA isomerization to ammonia oxide in aqueous solution was identified as the most likely decomposition pathways. Based on these molecular simulation results, recommendations are provided for safer handling of HA.     

CHAR GASIFICATION: PARTICLE MODEL COMPARISON IN A STIRRED TANK REACTOR

A simple well mixed model is developed to study the effect of single particle models for the gasification of char in a reactor. Two models, a lumped model and a distributed model, are used to describe the processes in the particle. The model consists of the conservation equations along with the residence time distributions and population balance equations. It is found that neglecting the conversion distribution function in the bed causes considerable differences at high temperatures and large particle sizes. The lumped model is rather insensitive to the mean residence time of the particles and the pressure, compared to the dependence observed in the distributed system. The difference in the design parameters predicted by the models is large and further, the particle size influences these parameters to a greater extent in the distributed system, the lumped system being almost independent of the particle size.     

CHAR GASIFICATION: PARTICLE MODEL COMPARISON IN A STIRRED TANK REACTOR

A simple well mixed model is developed to study the effect of single particle models for the gasification of char in a reactor. Two models, a lumped model and a distributed model, are used to describe the processes in the particle. The model consists of the conservation equations along with the residence time distributions and population balance equations. It is found that neglecting the conversion distribution function in the bed causes considerable differences at high temperatures and large particle sizes. The lumped model is rather insensitive to the mean residence time of the particles and the pressure, compared to the dependence observed in the distributed system. The difference in the design parameters predicted by the models is large and further, the particle size influences these parameters to a greater extent in the distributed system, the lumped system being almost independent of the particle size.     

MOLECULAR MECHANISMS OF CELL ADHESION: NEW PERSPECTIVES FROM SURFACE FORCE MEASUREMENTS

This review highlights recent surface force measurements of the molecular interactions of protein adhesives. These studies identified novel molecular-binding mechanisms that are intimately linked to the protein structures. Central to these findings is the precise force and distance sensitivity achieved with the surface force apparatus. They illustrate that a mechanistic understanding of the nanomechanical properties of biomolecular adhesives requires measurements of both the strength of adhesion and the range of the protein interactions.     

MOLECULAR MECHANISMS OF CELL ADHESION: NEW PERSPECTIVES FROM SURFACE FORCE MEASUREMENTS

This review highlights recent surface force measurements of the molecular interactions of protein adhesives. These studies identified novel molecular-binding mechanisms that are intimately linked to the protein structures. Central to these findings is the precise force and distance sensitivity achieved with the surface force apparatus. They illustrate that a mechanistic understanding of the nanomechanical properties of biomolecular adhesives requires measurements of both the strength of adhesion and the range of the protein interactions.     

LIQUID DISPERSION MECHANISMS IN AGITATED TANKS: PART I. PITCHED BLADE TURBINE

An oil into water dispersion, created by a pitched blade turbine, was observed using high speed, stereoscopic motion pictures. Two different dispersion mechanisms were responsible for the break-up of the oil drops, even though both mechanisms occurred in the vortex system trailing from the impeller blade tips. The first mechanism could be described as ligament stretching, since large oil drops were stretched by fluid shear to form elongated ligaments, which subsequently ruptured into small drops. The second mechanism was turbulent fragmentation, where large oil drops were shattered into large droplet clouds the instant they entered the trailing vortex system. Observations of the oil drops undergoing ligament stretching also indicated that velocities in the trailing vortex system were proportional to impeller tip speed.     

LIQUID DISPERSION MECHANISMS IN AGITATED TANKS: PART I. PITCHED BLADE TURBINE

An oil into water dispersion, created by a pitched blade turbine, was observed using high speed, stereoscopic motion pictures. Two different dispersion mechanisms were responsible for the break-up of the oil drops, even though both mechanisms occurred in the vortex system trailing from the impeller blade tips. The first mechanism could be described as ligament stretching, since large oil drops were stretched by fluid shear to form elongated ligaments, which subsequently ruptured into small drops. The second mechanism was turbulent fragmentation, where large oil drops were shattered into large droplet clouds the instant they entered the trailing vortex system. Observations of the oil drops undergoing ligament stretching also indicated that velocities in the trailing vortex system were proportional to impeller tip speed.     

CHEMICAL MECHANISMS OF CATALYTIC CRACKING OVER SOLID ACIDIC CATALYSTS: ALKANES AND ALKENES

The review discusses chemical mechanisms of most important reactions that take place in the course of catalytic cracking of alkenes and alkanes over solid acidic catalysts. The main subject of the review is the mechanism of the principal cracking step, fission of C─C bonds in aliphatic hydrocarbons. The cracking mechanism of alkenes via carbenium ion intermediates (with complications caused by oligomerization reactions) is well established. In contrast, the C─C bond fission mechanism in alkanes is still a controversial subject. The review compares merits and difficulties of different mechanisms proposed in the literature: several carbenium-ion mechanisms, the carbonium-ion mechanism, the chain-reactions mechanism with participation of both carbonium and carbenium ions, and the oxonium-ion mechanism. Reactions accompanying C─C-bond fission in cracking reactions, hydrogen redistribution, formation of cycloalkanes, light aromatic compounds, and coke are also briefly discussed.