Supercritical CO2 precipitation of poly(l-lactic acid) in a wide range of miscibility

Polymer microparticles are useful for numerous applications such as stationary phases in chromatography, adsorbents and catalyst supports, as well as for drug delivery systems. In recent decades the application of supercritical fluids for microparticle precipitation has been developed to a point where it is an ideal alternative to conventional processes. In this work poly(l-lactic acid) (PLLA), a biodegradable and biocompatible thermoplastic aliphatic polyester, has been processed using supercritical fluids, particularly by rapid expansion of supercritical solutions (RESS) and supercritical antisolvent (SAS) processes over a wide miscibility range. Particle morphology was greatly improved from irregular blocks to spherical microparticles on applying the SAS process. The effects of changes in polymer concentration, liquid flow rate, nozzle diameter, solvent, pressure and temperature have also been evaluated on the particle size of PLLA in the SAS precipitation. A higher concentration of the initial solution led to a decrease in particle size. Dichloromethane was the best of the chlorinated solvents investigated. The nozzle diameter had a negligible effect on particle size and the highest liquid flow rate gave the largest particle size. A larger particle size was also obtained on increasing the operating temperature. In contrast, the particle size decreased on increasing the operating pressure.     

EFFECT OF CROSS FLOW ON THE SPRAY CHARACTERISTICS OF AN INDUSTRIAL FEED NOZZLE

The spray characteristics of a scaled-down version of an industrial feed nozzle are studied in the presence of a cross flow. Aerated liquid nitrogen is injected through the nozzle to generate the spray. The aeration rate is low and held constant, while two different liquid flow rates are used to produce the spray. A nonuniform wind profile is chosen to represent the cross flow condition. The droplet diameter and velocity measurements are acquired using a phase-Doppler particle analyzer. The results of the present study indicate that the spray momentum flux determines the extent of the jet bending. The droplets are accelerated significantly in the initial jet region as a result of flashing. However, further downstream of the nozzle, the vaporization of the droplets is considered to be negligible. The size-velocity correlation changes significantly for the case where the spray is shifted due to the cross flow.     

空气雾化喷嘴的液滴雾化性能实验研究

采用LSA-Ⅲ型激光粒度仪对一种常用的小流量空气雾化喷嘴的液滴雾化性能进行实验研究。实验主要测定了不同气液比、气相压力和液相压力情况下沿喷雾轴向不同位置处的液滴粒径分布。测定结果表明气液比和气相压力对雾化液滴粒径影响均较大,其中气相压力影响最大,气液比其次,液相压力影响最小。在本实验测定条件下,经过喷嘴雾化后在轴向100、200、300、400和500 mm位置处液滴的表面积平均直径（SMD）和体积平均直径（D43）出现波动性变化。通过对实验测得液滴粒径分布数据的分析,可以得到Rosin-Rammler分布函数中的特征参数和n,为定量计算液滴粒径提供理论基础。     

喷嘴结构改进及其液体射流过程颗粒团聚研究

在重质原料液的射流阶段降低反应温度会导致液体呈现不同的黏度,促使颗粒聚集形成不同尺寸的团聚结构,阻碍了原料液的热量传递,减缓了裂化反应的速率,颗粒团聚是流体焦化反应工艺面临的一个重要而又具有挑战性的问题。选用水-沙系统模拟热态沥青-焦炭系统,利用气罩装置改进喷嘴结构,基于电导信号法测量多黏度液体射流过程的电导信号随时间的变化规律,研究不同条件下流化床内颗粒团聚过程。研究结果表明：多孔气罩装置可以为喷嘴射流创造理想的稀相环境,避免了液滴在射流空腔以及交换区域的聚集和压缩;液体射流在床层扩散过程中可以观察到不同的流化阶段,即颗粒润湿阶段、团聚形成阶段、团聚隔离阶段;较高的气液比可以有效地阻止颗粒团聚,相比于较低的流化气速,较高的气速条件允许高黏度糖水溶液参与液体射流。本研究为多黏度液体射流过程颗粒团聚现象的在线监测提供了理论研究基础,确保了流化床内射流液滴与颗粒表面的良好接触。     

Effect of cross flow on the spray characteristics of an industrial feed nozzle

The spray characteristics of a scaled-down version of an industrial feed nozzle are studied in the presence of a cross flow. Aerated liquid nitrogen is injected through the nozzle to generate the spray. The aeration rate is low and held constant, while two different liquid flow rates are used to produce the spray. A nonuniform wind profile is chosen to represent the cross flow condition. The droplet diameter and velocity measurements are acquired using a phase-Doppler particle analyzer. The results of the present study indicate that the spray momentum flux determines the extent of the jet bending. The droplets are accelerated significantly in the initial jet region as a result of flashing. However, further downstream of the nozzle, the vaporization of the droplets is considered to be negligible. The size-velocity correlation changes significantly for the case where the spray is shifted due to the cross flow.     

Modeling and experimental evaluation of an aerosol generator for very high number currents based on a free turbulent jet

In this paper we report on theoretical and experimental work on aerosol formation in a free turbulent jet. A hot DEHS vapor issues through a circular nozzle into slowly moving cold air. Vapor concentration and temperatures are such that particles are formed via homogeneous nucleation close to the nozzle upon mixing with the surrounding air. The vapor is completely quenched in the nucleation regime so that further particle growth is controlled by coagulation. A simple growth dynamics model is presented and the theory is used to design a generation system that produces liquid aerosols at a very high number current [up to 10¹² particles (s)]. The aerosol properties can be controlled by two easily adjustable parameters. The aerosol properties are related to these parameters by simple scaling laws. The results of measurements of the number current and the average particle size support these scaling laws.     

高塔复合肥熔体造粒工艺粒度成因机理及影响造粒的各种因素

我国现有高塔熔体工艺生产尿基、硝基复合肥,成品粒度波动较大,常有大块肥、扁平粒、细粉的产生,生产过程较难控制。分析高塔熔体造粒工艺成粒机理,原料物性、配方、操作条件对造粒的影响;介绍根据成品粒度的状况及观察喷头造粒情况如何进行生产控制的经验,以生产内外在质优的复合肥。     

A High-Temperature Reducing Jet Reactor for Flame-Based Metal Nanoparticle Production

We present a new flame-based aerosol reactor configuration that combines thermal decomposition and hydrogen reduction to produce metal nanoparticles. This approach uses a fuel-rich hydrogen flame as a source of low-cost energy to initiate particle synthesis, but separates the flame chemistry from the particle formation chemistry. Hot combustion products pass through a nozzle to produce a high-temperature reducing jet. A liquid precursor solution is rapidly atomized, evaporated, and decomposed by the expanding jet, initiating particle formation. In particular, here we have produced carbon-coated copper nanoparticles from an aqueous copper formate precursor solution and characterized them by aerosol mobility distribution measurements, electron microscopy, and x-ray diffraction. Copper serves here as a prototype for non-oxide materials that are generally difficult to produce in flame-based reactors. This work demonstrates that such materials can be produced in substantial quantities with particle diameters below 50 nm in this new process.     

Prediction of the Size of Aluminum-Oxide Particles in Exhaust Plumes of Solid Rocket Motors

The processes of coagulation and aerodynamic fragmentation of liquid particles of aluminum oxide in an accelerating gas flow in the Laval nozzle are analyzed. A formula obtained by an approximate analytical solution of equations of a two-phase flow is proposed to calculate the characteristic particle diameter at the nozzle exit. The limiting particle diameter in the nozzle throat calculated theoretically is close to the mean-mass diameter obtained by numerical simulation of polydisperse two-phase flows with particle coagulation and fragmentation. The formula proposed is in agreement with Hermsen's correlation dependences and is confirmed by numerous published data on measurement of the mean-mass diameter of aluminum-oxide particles in exhaust plumes of small-, medium-, and large-scale solid rocket motors. The formula contains physical parameters whose values are readily calculated and prescribed. The formula is tested by all the parameters that enter into it. Based on a comparison of theoretical calculations and numerous experimental data, the formula is recommended for prediction of the size of aluminum-oxide particles in exhaust plumes of various types of solid rocket motors. Key words: rocket motor, two-phase flow, aluminum oxide, fragmentation of drops.     

Bubble formation in cocurrently upward flowing liquid

The effects of liquid velocity, nozzle diameter, gas chamber volume and gas flow rate on volumes, shapes and growth curves of bubbles formed at a nozzle submerged in a cocurrently upward flowing liquid in a bubble column were experimentally investigated. The bubble volume decreases with increasing liquid flow velocity. The effect of liquid flow velocity on the volume of bubble increases with an increase in the gas flow rate. To simulate bubble formation at a nozzle submerged in cocurrently upward flowing liquid, a revised non-spherical bubble formation model was proposed. Bubble volumes, bubble growth curves and shapes experimentally obtained in this study, as well as in previous experimental studies, are well predicted by the present model.     

连铸二冷喷嘴内部雾化效果的数值模拟分析

连续铸造二次冷却(连铸二冷)中常用的喷嘴为内混式气水雾化喷嘴。其所喷射的二冷水对铸坯的质量有着重要的影响。根据某钢厂所用的内混式喷嘴数据,介绍了喷嘴雾化的原理,对常用工况下喷嘴内部的喷射芯喷出的液柱破碎情况进行了理论计算。采用CFD软件建立了喷嘴的物理模型,对其内部流场进行了数值模拟。根据模拟结果,分析了液柱在不同工况下的形态变化;计算了液柱We数的模拟值并与理论值进行了对比。综合考虑液柱形态及We数,表明增加进气压力能有效提升液柱破碎的程度,工况3(水压0.2 Mpa,气压0.2 Mpa)及工况4(水压0.2 Mpa,气压0.25 Mpa)具有较好的雾化效果。     

Designing a Jet Mill Nozzle of Maximum Efficiency

A method of calculation for an accelerating jet mill nozzle profile which provides a maximum kinetic energy for solid particles at the nozzle outlet, is developed. The gas‐solid flow in the nozzle is described by a well‐known one‐dimensional model. The effect of both solid loading and particle size on the optimal nozzle profile, as well as on the main flow parameters is studied. Distribution of particle velocities in the computed optimal nozzle is compared to that in a conical convergent‐divergent nozzle of the same length. Computations demonstrated that optimized nozzles are profitable for fairly low solid loadings. The kinetic energy of solids at the optimized nozzle outlet is about 50% higher than that in a conical nozzle if the solid/gas mass flow ratio equals unity.     

PARTICLE TRANSMISSION EFFICIENCY THROUGH THE NOZZLE OF THE API AEROSIZERTM

This study has investigated the particle transmission efficiency through the nozzle of the API Aerosizer^TM numerically. Two-dimensional flow field in the nozzle was first simulated. Particle trajectories for both liquid and solid particles were then calculated to obtain the particle transmission efficiency under various conditions. This study shows that particle aerodynamic diameter, particle materials, particle density and laser beam diameter influence the transmission efficiency. The transmission efficiency is found to increase with increasing particle diameter when the particle aerodynamic diameter is less than several micrometers. The efficiency for liquid particles drops significantly when particle aerodynamic diameter increases from several micrometers because of particle impaction loss in the nozzle. For solid particles, the relationship of the efficiency with particle diameter is found to be more complicated. For particles less than several micrometers in aerodynamic diameter, solid particles behave similarly to the liquid particles. However, as particles are greater than several micrometers, the effect of solid particle bounce is to increase the transmission efficiency with increasing aerodynamic diameter until particles become large enough so that plastic deformation occurs in the particles. Then the transmission efficiency will decrease with increasing particle aerodynamic diameter.     

同轴四通道喷嘴气流式雾化特性影响因素研究

提出一种新型同轴四通道喷嘴——由内到外采用气-液-气-液设置,最外环液体可以将合成气和氧气隔离,大幅降低喷嘴出口温度,可望延长气化炉中喷嘴使用寿命。为研究喷嘴雾化效果的影响因素,以水和空气为介质,利用马尔文激光粒度分析仪对同轴四通道喷嘴的气流式雾化液滴索特平均直径进行实验研究。发现对雾化效果影响程度从大到小依次为通道三、通道四、通道二和通道一;增大通道二、四液量分配比可以降低雾化粒径;增大外环液膜厚度会增大雾化粒径;通道一、三气量分配比对雾化颗粒的影响是非单调性的,雾化粒径先增大后减小。基于实验结果进行数值分析,拟合获得了同轴四通道喷嘴雾化液滴粒径关系式。     

Comparison of experimental results and numerical predictions of drop diameter from a single submerged nozzle in a liquid‐liquid system

This paper presents a comparison of experimental results and numerical predictions of drop formation from a single submerged nozzle for a liquid‐liquid system. The theoretical model is a modification of previous models used for a two‐stage drop formation mechanism. The model has been tested against experimental data for kerosene drop formation in distilled water using a range of different nozzle diameters. In addition, our liquid‐liquid model has been compared with both experimental and predicted results from published literature. These comparisons demonstrate that for liquid‐liquid systems, the present predictions of drop diameter versus dispersed phase nozzle velocity are in overall agreement with both the present and previous experimental results. In addition, the present model predictions are more accurate than those of previous models for liquid‐liquid systems.     

液固外循环流化床换热器主压降的实验研究

以喷嘴为颗粒循环装置的液固外循环流化床换热器为研究对象,考察颗粒直径、口径比、喷嘴安装位置、颗粒初始加入量及流体黏度对主压降的影响,得出了稳定操作情况下,液固外循环流化床换热器主压降与上述因素之间的经验关联式,计算值与实验值吻合较好,为外循环流化床换热器的设计计算提供依据。     

超临界CO2辅助膨胀对硝苯地平颗粒纳微化的影响

以超临界CO2为一相,硝本地平-二氯甲烷溶液为另一相,通过同轴喷嘴节流膨胀雾化制备硝本地平纳微颗粒,研究了制备过程硝本地平-二氯甲烷溶液质量分数和进料流率、CO2温度和压力、喷嘴孔径对硝苯地平纳微化颗粒尺寸大小、粒度分布和形貌的影响.根据SEM照片进行颗粒统计结果显示,用超临界CO2辅助膨胀雾化硝苯地平-二氯甲烷溶液,...     

Experimental investigation and model improvement on the atomization performance of single-hole Y-jet nozzle with high liquid flow rate

Y-jet nozzle, as an efficient multi-hole internal-mixing twin-fluid atomizer, has been widely used for liquid fuel spray in many industrial processes. However, single-hole Y-jet nozzle with high liquid flow rate is indispensable in some confined situations due to a small spray cone angle. In this paper, the atomization performance of single-hole Y-jet nozzles with high liquid mass flow rates ranging from 400 to 1500 kg/h for practical semidry flue gas desulfurization processes was investigated by the laser particle size analyzer, and the effects of spray water pressure, atomizing air pressure and air to liquid mass flow ratio on the liquid mass flow rate and the droplet size distribution were analyzed. Moreover, the secondary atomization model was modified on the basis of previous random atomization model of Y-jet nozzle. The predicted results agreed well with the experimental ones, and the improved atomization model of Y-jet nozzle was well validated to design the nozzle geometry and to predict the droplet size distributions for single-hole Y-jet nozzle with high liquid mass flow rate.     

A study on flow through hydroentangling nozzles and their degradation

Hydroentangling is a technique for mechanically bonding loose filaments or fibers arranged in a web. The efficiency with which the web is entangled depends on the peculiar properties of laminar high-speed waterjets used. The characteristics of such waterjets strongly depend on the operating pressure and the nozzle inlet sharpness which influence the dynamics of fluid flow. In this study, we report on experiments and CFD simulations aimed at improving our knowledge of such two-phase flows. In particular, we simulate the formation and growth of the cavitation cloud inside a sharp-edge hydroentangling nozzle at pressures ranging from 10 to . Our experimental results run at the same pressures, confirm that nozzle cavitation will cause “hydraulic flip”. Once hydraulic flip occurs, atomizing waterjets will turn into constricted laminar waterjets with long intact lengths—a necessary condition for hydroentangling. It has been observed that the nozzle inlet deteriorates under high pressures. Our CFD simulations show a striking similarity between the contours of shear stress at the nozzle inlet and the nozzle wear pattern. These findings together with the SEM elemental analysis at the nozzle inlet reveal the potential for metal oxidation around the inlet, implicating stress-induced corrosion as a major contributor to the nozzle wear. Cavitation might also be one of the mechanisms responsible for the above-mentioned wear at the inlet edge. Additionally, our water-borne solid particle tracking, confirms SEM experimental results that particle deposition can potentially play a considerable role in the deterioration of the nozzle inlet shape.     

Jet breakup and droplet formation in near-critical regime of carbon dioxide-dichloromethane system

The jet breakup and droplet formation mechanism of a liquid in the near-critical conditions of a solvent-antisolvent system is examined with high-speed visualization experiments and simulated using a front tracking/finite volume method. The size of droplets formed under varying system pressure at various jet breakup regimes is measured with a Global Sizing Velocimetry, using the shadow sizing method. A stainless steel nozzle with 0.25 mm I.D and 1.6 mm O.D was used in this study. Experiments were performed at fixed temperature of 35 °C and system pressure in the range from 61 to 76 bar in the near-critical regime of the DCM-CO₂. At the near mixture critical regime for DCM-CO₂ mixture, the miscibility between the two fluid phases increases and the interfacial tension diminishes. This phase behavior has important applications in particle formation using gas antisolvent (GAS) and supercritical antisolvent (SAS) processes. The jet breakup and droplet formation in the near-critical regime is strongly dependent on the changes in interface tension and velocity of the liquid phase. An understanding of the droplet formation and jet breakup behavior of DCM-CO₂ in this regime is useful in experimental design for particle fabrication using SAS method.