Aerosol Synthesis of Lipid Nanoparticles: Relating Crystallinity to Simulated Evaporation Rates

The degree of crystallinity of nanometer size lipid matrices governs drug loading and release rates. Recently, droplet-phase aerosol synthesis was used to prepare lipid nanoparticles of stearic acid and achieve control over their crystallinity using precursor solvents with differing vapor pressures. The present work aims at examining relationships between solvent evaporation rate and extent of evaporative cooling, during drop evaporation, on the crystallinity of the resulting lipid nanoparticles. A stationary drop model was developed to study evaporation of submicron-sized solution drops, of stearic acid in organic solvents, by including mechanisms of solvent vapor pressure depression by the solute, heat and mass transfer between the drop ensemble and suspending gas, Kelvin (curvature) effect, noncontinuum vapor transfer effects, and changes in activity coefficients of solute and solvent with changing concentrations. It was found that increasing estimated evaporation rates correlated with decreasing measured crystallinity. Higher evaporation rates also led to greater evaporative cooling and lower drop temperatures. The rate of change of supersaturation in solution drops under fast evaporation was shown to be an order of magnitude higher than that for slow evaporation. The modeled evaporation rate and drop temperature depend primarily on vapor pressure and enthalpy of vaporization of the precursor solvent. This suggests that selection of precursor solvents, with desired physical properties, can be used to control crystallinity, and related drug release behavior of lipid nanoparticles made through aerosol synthesis routes.

Copyright 2012 American Association for Aerosol Research     

Determining Aerosol Volatility Parameters Using a “Dual Thermodenuder” System: Application to Laboratory-Generated Organic Aerosols

Thermodenuders (TD) are a tool widely used for measuring aerosol volatility in the laboratory and field. Extracting the parameters that dictate organic aerosol volatility from TD data is challenging because gas-particle partitioning rarely reaches equilibrium inside a TD operating under atmospheric conditions, thus a wide variety of parameter sets can explain observed evaporation. Component volatilities (as represented by saturation vapor pressure, C_sat), cannot be directly extracted due to uncertainties in potential limitations to mass transfer (represented by mass accommodation coefficient, α) and components’ enthalpies of evaporation (ΔH_vap). To address these limitations, we have developed a “dual TD” experimental approach in which one line uses a temperature-stepping TD (TS-TD) with a relatively long residence time (RT) and the other operates isothermally at variable residence time (VRT-TD). Data from this approach are used in tandem with an optimizing evaporation kinetics model to extract the values of parameters dictating volatility (C_sat, and associated values of ΔH_vap and α). The system was evaluated using laboratory generated dicarboxylic acid aerosols (adipic acid and succinic acid). Excellent agreement with previously published evaporation data collected with other TD systems was observed. Parameter values reported in the literature for the tested acids vary widely, but our results are generally consistent with those from studies that allow for nonunity values of α. For example, our results suggest that α for these aerosols are of order 0.1, in agreement with results determined by Saleh et al. (2009, 2012). Modeling results suggest that the addition of VRT-TD data provides tighter constraint on feasible ΔH_vap and α values. The dual TD approach presented here does not rely on equilibration in the TD and thus can be directly applied to extract volatility parameters for more complex laboratory and ambient organic aerosol systems.

Copyright 2015 American Association for Aerosol Research     

On the Determination of the Surface Temperature of an Evaporating Liquid

Some methods for calculating the wet-bulb temperature are compared. These methods are based on different forms of equations relating the heat and mass transfer in evaporation of a liquid into a gas flow. It is hypothesized that the deviation from the similarity between heat and mass transfer is described by the first power of the Lewis number, for which the parameters of the gas mixture are calculated on the phase-transition surface. In this case, the heat-transfer coefficients calculated from the enthalpy and temperature gradients of the gas–vapor mixture are equal to each other. The wet-bulb temperature calculated using this approach is in satisfactory agreement with experimental data.     

Volatility and Yield of Glycolaldehyde SOA Formed through Aqueous Photochemistry and Droplet Evaporation

Aqueous hydroxyl radical (～10^?12 M) oxidation of glycolaldehyde (1 mM), followed by droplet evaporation, forms secondary organic aerosol (SOA) that exhibits an effective liquid vapor pressure and enthalpy of vaporization of ～10^?7 atm and ～70 kJ/mol, respectively, similar to the mix of organic acids identified in reaction samples. Salts of these acids have vapor pressures about three orders of magnitude lower (e.g., ammonium succinate ～10^?11 atm), suggesting that the gas–particle partitioning behavior of glycolaldehyde SOA depends strongly on whether products are present in the atmosphere as acids or salts. Several reaction samples were used to simulate cloud droplet evaporation using a vibrating orifice aerosol generator. Samples were also analyzed by ion chromatography (IC), electrospray ionization mass spectrometry (ESI-MS), IC-ESI-MS, and for total carbon. Glycolaldehyde SOA mass yields were 50–120%, somewhat higher than yields reported previously (40–60%). Possible reasons are discussed: (1) formation of oligomers from droplet evaporation, (2) inclusion of unquantified products formed by aqueous photooxidation, (3) differences in gas–particle partitioning, and (4) water retention in dried particles. These and similar results help to explain the enrichment of organic acids in particulate organic matter above clouds compared with those found below clouds, as observed previously in aircraft campaigns.

Copyright 2012 American Association for Aerosol Research     

用微扰理论建立水的分子热力学模型

提出1种水的分子热力学模型,从微扰理论出发,建立了自由能及其它热力学函数的关系式.水分子间作用包括硬球、色散、静电及诱导几个部分.通过同时关联0～300°C下饱和水蒸气压及液体密度数据获得分子参数,还预测了水的蒸发焓及饱和水蒸气的比容,比较了文献中处理水的几种理论方法.结果表明,本模型简单,且较接近实际.     

降膜分子蒸馏过程模型研究

通过对降膜分子蒸馏器蒸发表面上液膜流动和传质传热的分析,建立了液膜的传质和传热方程;并在单组分非线性BGK方程的基础上,建立了描述多组分稀薄气体流动的非线性BGK模型方程。通过适当的边界条件将液膜方程和气体方程耦合在一起,得到了降膜分子蒸馏过程的传质与传热数学模型。该模型揭示了液膜温度和浓度在径向和轴向上的变化规律以及气相空间气体的密度、温度和速度的变化规律,适用于恒壁温和绝热壁等不同操作情况。     

煤液化油窄馏分饱和蒸气压和蒸发焓的测定及计算

用三次膨胀法对煤直接液化油窄馏分的饱和蒸气压进行了测定。实验结果显示,在同一测试温度下,饱和蒸气压随窄馏分蒸馏温度的升高而降低,相同蒸馏温度下,随测试温度的增大而增大;煤液化轻油馏分与纯物质饱和蒸气压一样,其对数值与温度的倒数呈现出良好的线性关系。同时还对测量数据进行线性回归,并使用克劳修斯-克拉佩龙方程进行了蒸发焓的计算,结果显示,随蒸馏温度的提高,窄馏分的蒸发焓数据呈上升的趋势。     

HD低温蒸发系统中填料蒸发器传质传热的分析

通过研究系统中填料蒸发器的蒸发传质传热过程以及两相流动特性，采用计算流体力学（computational fluid dynamics，CFD）中离散相与连续相耦合的方法来模拟规整填料内部通道的蒸发传质传热过程，实现了填料蒸发器中两相传质传热的过程以及液滴流动的可视化，为研究气液两相在规整填料内的流动提供了一种模拟方法。通过与实验结果的比较，最终选用RNG k-ε湍流模型来分析规整填料内部气液两相传质传热以及流动情况。数值模拟研究了规整填料板间距对填料内部气液两相传质传热以及液滴运动影响，发现随着板间距的增大，填料内部压力降逐渐降低，出口空气中水蒸气的含量不断减小，液滴蒸发速率降低，液滴进出口质量差减小，气相出口温度逐渐降低，蒸发传质传热效率降低。随着气速的增大，出口空气中水蒸气的含量不断减小，液滴蒸发速率增加，气相出口温度降低，气液两相传质传热效率降低。     

Spatial reaction engineering approach as an alternative for nonequilibrium multiphase mass‐transfer model for drying of food and biological materials

Drying is a complex process which involves simultaneous heat and mass transfer. Complicated structure and heterogeneity of food and biological materials add to the complexity of drying. Drying models are important for improving dryer design and for evaluating dryer performance. The lumped reaction engineering approach (L‐REA) has been shown to be an accurate and robust alternative for cost‐effective simulations of challenging drying systems. However, more insightful physics has to be shown spatially. In this study, the REA is coupled with the standard mechanistic drying models to yield the spatial‐REA (S‐REA) as nonequilibrium multiphase mass‐transfer model. The S‐REA consists of a system of equations of conservation with the REA representing the local evaporation and wetting rate. Results of the modeling using the S‐REA match well with the experimental data reported previously. This is the first comprehensive REA approach to model the profiles of water vapor concentration during drying of food and biological materials. This study indicates that the S‐REA can be an accurate nonequilibrium multiphase mass‐transfer model with appropriate account of the local evaporation rate. The overall REA concept is expected to contribute substantially for better and cost‐effective representation of transport phenomena of drying process. © 2012 American Institute of Chemical Engineers AIChE J, 59: 55–67, 2013     

CONVECTWE DRYING MODEL OF SOUTHERN PINE

A transient one dimensional first principles model is developed for the drying of a porous material (wood is used as an example) that includes both heat and mass transfer. Heat transfer by conduction and convection, mass transfer by binary gas diffusion, pressure-driven bulk flow in the gas and liquid, and diffusion of bound water are included in the analysis. The diffusive mass transfer terms are modeled using a Fickian approach, while the bulk flow is modeled assuming Darcian flow. Depending on the state (pendular or funicular) of the moisture in the wood, appropriate terms are considered in the development of the governing mass equations. The results provide distributions within the material of each moisture phase (vapor, liquid, and bound), temperature, and total pressure. Information regarding the drying rate and evaporation rate is also presented. Average distributions are obtained as a function of time, and compared with experimental data from the literature. It is observed that the total pressure within the material can be considerably above one atmosphere during the drying process.     

机械蒸汽再压缩蒸发系统的性能分析

机械蒸汽再压缩（MVR）蒸发系统是一种新型高效节能蒸发技术。它有多个单元设备组成,每个操作节点的控制都对系统运行的稳定性和节能效率至关重要,其中包括进料温度、蒸发压强、蒸汽压缩比、冷凝液温度等。若操作条件不当,不仅会大大降低蒸发效率而且会对设备和管路造成损害。本文建立了一套充分利用能源的MVR蒸发工艺流程,并通过理论分析对每个操作节点进行了质量和能量衡算,同时利用Aspen Plus模拟软件建立了系统的流程模拟图。通过对操作单元的变量控制,研究了循环蒸汽量、补充水的量与进料温度、冷凝液温度、蒸汽压缩比以及蒸发压强等之间的变化关系。由数据分析可得：原料在饱和液体时进料最佳,冷凝液的温度应保持与蒸发温度的有效温差在5～8 ℃时较好,压缩机的蒸汽压缩比控制在1.8～2.2较为合理。同时可利用冷凝液和浓缩液的余热对原料预热,补充水也可从冷凝液中直接取用。     

Some Considerations in Modeling of Moisture Transport in Heating of Hygroscopic Materials

Hygroscopic materials are those in which the equilibrium pressure of water vapor changes with moisture content and temperature, such as food, soil or wood, etc. Heat and moisture transports are coupled in heating of hygroscopic materials. One of the major links between temperature and moisture changes is water evaporation. There have been different formulations on modeling of evaporation in the past. A typical approach (Model 1 in this article) is to equate the evaporation rate to the rate of local moisture loss. The first part of this paper illustrates that such an approach is physically incorrect based on fundamental conservation relationships. A conservation-based coupled heat and moisture transfer model (Model 2) is presented here based on previous multiphase transport models. It shows that total evaporation rate over the entire material is included in Model 1 while the local evaporation rate is not. The situations when Model 1 may or may not generate large errors are discussed. The second part of this article completes the modeling of evaporation using Model 2. Two types of formulations are given depending on the phase equilibrium of moisture in the hygroscopic materials. When phase equilibrium between water and vapor is assumed for any location at any time, vapor pressure is provided as known variables. In a nonequilibrium approach, evaporation rate needs to be provided. The latter poses numerical difficulties near the material surface, which arises from the possibility that equilibrium state may have a large change near the surface. Further discussions were made on the physical and numerical considerations in using both approaches.     

Investigation of the saturated vapor pressure of selenium and indium sesquiselenide In0.4Se0.6

The temperature dependences of the saturated vapor pressure of selenium (700–1100 K) and indium sesquiselenide In_0.4Se_0.6 (1123–1373 K) are investigated using the gravimetric method. The saturated vapor pressures obtained in this study for selenium and the In_0.4Se_0.6 compound are in satisfactory agreement with the data available in the literature. The enthalpy of vaporization of In_0.4Se_0.6 is estimated.     

盐水液滴降压蒸发析盐过程传热传质特性

针对单个盐水(NaCl溶液)液滴在降压环境下蒸发析盐的传热传质过程建立了数学模型。模型考虑了多孔盐壳在液滴表面的形成过程,降压过程引起的气流运动,液核通过多孔介质的传质扩散,以及液滴表面的蒸发换热和对流换热。将实验数据与计算结果对比,验证了模型的有效性。通过模型计算获得了液滴表面温度及液滴质量随时间的变化。结果表明盐水液滴在降压环境下蒸发析盐过程的温度变化分为4个阶段:温度骤降阶段、温度回升阶段、平衡温度阶段和温度上升阶段。平衡温度阶段,盐壳界面运动较慢,随蒸发进行,液核尺寸逐渐减小,盐壳界面运动速度加快。理论分析了环境压力对盐水液滴蒸发析盐过程的影响,环境压力越低,平衡温度越低,盐分完全析出时间越短。     

CONVECTWE DRYING MODEL OF SOUTHERN PINE

ABSTRACT

A transient one dimensional first principles model is developed for the drying of a porous material (wood is used as an example) that includes both heat and mass transfer. Heat transfer by conduction and convection, mass transfer by binary gas diffusion, pressure-driven bulk flow in the gas and liquid, and diffusion of bound water are included in the analysis. The diffusive mass transfer terms are modeled using a Fickian approach, while the bulk flow is modeled assuming Darcian flow. Depending on the state (pendular or funicular) of the moisture in the wood, appropriate terms are considered in the development of the governing mass equations. The results provide distributions within the material of each moisture phase (vapor, liquid, and bound), temperature, and total pressure. Information regarding the drying rate and evaporation rate is also presented. Average distributions are obtained as a function of time, and compared with experimental data from the literature. It is observed that the total pressure within the material can be considerably above one atmosphere during the drying process.     

Examining the Suitability of the Reaction Engineering Approach (REA) to Modeling Local Evaporation/Condensation Rates of Materials with Various Thicknesses

The reaction engineering approach (REA) is examined here to investigate its suitability as the local evaporation rate to be used in multiphase drying. For this purpose, REA is first implemented to model the convective drying of materials with various thicknesses. The relative activation energy, as the fingerprint of REA, generated from one size of a material is used to model the convective drying of the same material with different thicknesses. Because the results indicate that REA parameters can model the drying of materials with various thicknesses, REA can be scaled down to describe the local evaporation rate (at the microscale as affected by local composition and temperature). The relative activation energy is used to describe the global drying rate in modeling the local evaporation rate. REA is combined with a system of equations of conservation of heat and mass transfer in order to yield the spatial reaction engineering approach (S-REA) as a nonequilibrium multiphase drying model. By using S-REA, the spatial profiles of moisture content, concentration of water vapor, temperature, and local evaporation rate can be generated, which can assist in comprehending the transport phenomena.     

CFD analysis of hydrodynamic, heat transfer and reaction of three phase riser reactor

Catalytic cracking reaction and vaporization of gas oil droplets have significant effects on the gas solid mixture hydrodynamic and heat transfer phenomena in a fluid catalytic cracking (FCC) riser reactor. A three-dimensional computational fluid dynamic (CFD) model of the reactor has been developed considering three phase hydrodynamics, cracking reactions, heat and mass transfer as well as evaporation of the feed droplets into a gas solid flow. A hybrid Eulerian-Lagrangian method was applied to numerically simulate the vaporization of gas oil droplets and catalytic reactions in the gas-solid fluidized bed. The distributions of volume fraction of each phase, gas and catalyst velocities, gas and particle temperatures as well as gas oil vapor species were computed assuming six lump kinetic reactions in the gas phase. The developed model is capable of predicting coke formation and its effect on catalyst activity reduction. In this research, the catalyst deactivation coefficient was modeled as a function of catalyst particle residence time, in order to investigate the effects of catalyst deactivation on gas oil and gasoline concentrations along the reactor length. The simulation results showed that droplet vaporization and catalytic cracking reactions drastically impact riser hydrodynamics and heat transfer.     

Investigation of the saturated vapor pressure of the Cu2Se and CuInSe2 compounds

The saturated vapor pressure of the Cu₂Se and CuInSe₂ compounds is investigated using the gravimetric method in the temperature ranges 1282–1450 and 1173–1423 K, respectively. It is demonstrated that, in the case of congruent vaporization, the temperature dependences of the saturated vapor pressure of the Cu₂Se and CuInSe₂ compounds in the temperature ranges under investigation are adequately described by the equation logP = ?A/T + B. The enthalpy and entropy of vaporization are estimated for both compounds.     

Modeling of Two‐Phase Flashing Flow of Multicomponent Mixtures in Large Diameter Pipes

A two‐phase flashing flow model is developed to predict the distributions of pressure, temperature, velocity and evaporation rate in a transfer line, which is a typical example of a two‐phase flow pipe in the petrochemical industry. The model is proposed based on the pressure drop model and the multi‐stage flash model. The results indicate that pressure drop, temperature drop, and change of evaporation rate mainly occur in the transition section and the junction site of the transfer line. The predictions of the model have been tested with reliable field data and the good agreement obtained may lead to a better understanding of the two‐phase flashing flow phenomenon, as well as demonstrating the feasibility of applying the model into the design and optimization of pipelines.     

Distortion of Size Distributions by Condensation and Evaporation in Aerosol Instruments

Aerosols that contain volatile species or condensable vapors may be altered by changes in temperature, pressure, and vapor concentration. When such changes occur within aerosol sampling instruments, the measured size distribution can be distorted significantly. The distortion of particle size distributions in a number of commonly used aerosol instruments, including cascade impactors, both conventional and low pressure instruments, and optical particle counters, is explored both theoretically and experimentally in this paper. Ammonium sulfate aerosols in humid atmospheres have been used to test the instruments. In a low pressure impactor in which the pressure is intentionally reduced to facilitate the collection of small particles, a water containing particle may shrink due to evaporation as the pressure is reduced. However, if the sample flow is also accelerated to high velocities, aerodynamic cooling can lead to condensation of water vapor and particle growth. Either of these competing effects may lead to erroneous estimates of the particle size distribution. Optical particle counters generally use a recirculated sheath airflow. Pumps and electrical dissipation heat this air, leading to a temperature increase that shifts the vapor equilibrium, causing a decrease in particle size due to evaporation. Modifications have been made to avoid this distortion in measured size distributions.