MOLECULAR DIFFUSION OF VOLATILE-LIQUID VAPORS INTO AIR

Diffusion coefficients of vapors diffused into stagnant air were determined at room temperature and atmospheric pressure. Experimental data on diffusion coefficients were obtained from the steady-state open-tube evaporation method modified for this study. No fresh air is passed over the top end of a diffusion tube, and amounts of volatile liquids evaporated in the diffusion tube are measured with a balance rather than a change of the liquid level in a diffusion tube. Experimental molecular diffusion coefficients are obtained by applying experimental data of mass losses of volatile liquidsversusevaporation durations to a diffusion equation developed suitable for the novel open-tube evaporation method. Three main experimental errors of the novel open-tube evaporation method are described in detail. Predicted diffusion coefficients are calculated with the Wilke-Lee method and compared with experimental values obtained from this diffusion study.     

Molecular diffusion of volatile-liquid vapors into air

Diffusion coefficients of vapors diffused into stagnant air were determined at room temperature and atmospheric pressure. Experimental data on diffusion coefficients were obtained from the steady-state open-tube evaporation method modified for this study. No fresh air is passed over the top end of a diffusion tube, and amounts of volatile liquids evaporated in the diffusion tube are measured with a balance rather than a change of the liquid level in a diffusion tube. Experimental molecular diffusion coefficients are obtained by applying experimental data of mass losses of volatile liquidsversusevaporation durations to a diffusion equation developed suitable for the novel open-tube evaporation method. Three main experimental errors of the novel open-tube evaporation method are described in detail. Predicted diffusion coefficients are calculated with the Wilke-Lee method and compared with experimental values obtained from this diffusion study.     

Molecular simulations of gas transport in nitrile rubber and styrene butadiene rubber

Nitrile rubber (NBR, 39:61 wt% of acrylonitrile:butadiene) and styrene butadiene rubber (SBR, 50:50 wt% of styrene:butadiene) matrices have been equilibrated by molecular dynamics (MD) simulations. Transition-state approach is used to calculate the diffusion and solubility coefficients of small penetrants in these matrices, indicating quite low values in NBR and reasonable agreement with experimental results. MD simulations have been performed to analyze water diffusion in these matrices. Aggregation of water molecules is observed in the hydrophobic matrix SBR. MD simulations with fictitious nonpolar water molecules inhibit aggregation and lead to enhanced diffusion in SBR. In NBR there is a slight increase in diffusion for fictitious water molecules. The lower diffusion constants in NBR result from slower local relaxation of the matrix due to tighter intermolecular packing and higher cohesive energy density. The free volume distribution that affects solubility coefficients is not a major determining factor for the diffusion coefficients in these matrices.     

Study on the gas permeabilities in styrene-butadiene rubber by molecular dynamics simulation

In this research, molecular dynamics (MD) simulations were used to study the transport properties of small gas molecules in the butadiene-styrene copolymer (SBR). The condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field was applied. The diffusion coefficients were obtained from MD (NVT ensemble) and the relationship between gas permeability; the chemical structure and free volume of butadiene-styrene copolymer were investigated. The results indicated that the diffusion coefficient of oxygen declined with increasing styrene content. The fraction of free volume (FFV) in butadiene-styrene copolymer was calculated. It was concluded that diffusion coefficient increased as the FFV increases, which is in accordance with the analysis of the small molecular hop through the free volume in polymer matrix. Subsequently, the glass transition temperatures of these copolymers were calculated by MD. The result showed that the glass transition temperature increased with increasing styrene content in polymer.     

Study on the gas permeabilities in styrene-butadiene rubber by molecular dynamics simulation

In this research, molecular dynamics (MD)simulations were used to study the transport properties of small gas molecules in the butadiene-styrene copolymer(SBR). The condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field was applied. The diffusion coefficients were obtained from MD (NVT ensemble) and the relationship between gas permeability; the chemical structure and free volume of butadiene-styrene copolymer were investigated. The results indicated that the diffusion coefficient of oxygen declined with increasing styrene content. The fraction of free volume (FFV) in butadiene-styrene copolymer was calculated. It was concluded that diffusion coefficient increased as the FFV increases, which is in accordance with the analysis of the small molecular hop through the free volume in polymer matrix. Subsequently, the glass transition temperatures of these copolymers were calculated by MD. The result showed that the glass transition temperature increased with increasing styrene content in polymer.     

COMPETING DRYING AND REACTION MECHANISMS IN THE FORMATION OF SOL-TO-GEL FILMS, FIBERS, AND SPHERES

Drying of films, fibers, and spheres undergoing sol-to-gel transformation is greatly affected by the strong dependence of the diffusion coefficients on composition, molecular weight, and temperature. This dependence is probed by solving the equations of mass and heat transfer by diffusion or conduction and associated convection, along with chemical reaction. A one-dimensional analysis is used that encompasses the planar, cylindrical, and spherical configurations. The solutions are obtained by Galerkin's method with finite element basis functions and entail large-scale computation.

Skinning and solidification phenomena in silica sol-to-gel systems are greatly affected not only by falling diffusion coefficients, but also by the competition between drying and reaction. The kinetics of silica reactions are modeled with the statistical scheme of Kay and Assink (1988). Gelation is predicted by the recursive technique of Bailey et al. (1990). Results show that at intermediate reaction rates, high molecular weight silica polymers are formed first near the evaporating free surface and gelation proceeds from that surface inwards, but at high or low reaction rates, gelation occurs before or after drying, respectively. These results help predict conditions in which uniform films can be cast, uniform fibers can be drawn, and uniform microspheres can be formed by spray drying.     

COMPETING DRYING AND REACTION MECHANISMS IN THE FORMATION OF SOL-TO-GEL FILMS,FIBERS, AND SPHERES

ABSTRACT

Drying of films, fibers, and spheres undergoing sol-to-gel transformation is greatly affected by the strong dependence of the diffusion coefficients on composition, molecular weight, and temperature. This dependence is probed by solving the equations of mass and heat transfer by diffusion or conduction and associated convection, along with chemical reaction. A one-dimensional analysis is used that encompasses the planar, cylindrical, and spherical configurations. The solutions are obtained by Galerkin's method with finite element basis functions and entail large-scale computation.

Skinning and solidification phenomena in silica sol-to-gel systems are greatly affected not only by falling diffusion coefficients, but also by the competition between drying and reaction. The kinetics of silica reactions are modeled with the statistical scheme of Kay and Assink (1988). Gelation is predicted by the recursive technique of Bailey et al. (1990). Results show that at intermediate reaction rates, high molecular weight silica polymers are formed first near the evaporating free surface and gelation proceeds from that surface inwards, but at high or low reaction rates, gelation occurs before or after drying, respectively. These results help predict conditions in which uniform films can be cast, uniform fibers can be drawn, and uniform microspheres can be formed by spray drying.     

Instability in Unsteady-State Evaporation of Binary Solutions into an Inert Gas

The critical time of transition to the convective mode of the unsteady-state evaporation of a horizontal layer of a binary solution into a stagnant gas is theoretically and experimentally determined. It is shown that the slow diffusion mode of evaporation can be destabilized and replaced by the convective mode if the molecular weight of one of the liquid components is higher than the molecular weight of the gas and the molecular weight of the other liquid components is lower than the molecular weight of the gas. The neutral stability curve of the diffusion mode is determined using the Navier-Stokes equation in the Boussinesq approximation, the continuity equation, and the convective diffusion equations of the evaporating components. The transition between the modes is faster than the molecular diffusion; therefore, the depth of penetration of the vapor of the light component into the gas in the diffusion mode can be taken as the linear scale of the phenomenon considered. By the Galerkin method, the neutral stability curve of the diffusion mode of evaporation is determined and the critical time of bifurcation of the mode is found as a function of the composition of an aqueous butanol solution. The theoretical and experimental data are in good agreement.     

Preparation of isotactic polypropylene/polystyrene blends by diffusion and subsequent polymerization of styrene in isotactic polypropylene pellets

Isotactic polypropylene (iPP) pellets were used to prepare isotactic polypropylene/polystyrene (iPP/PS) blends by diffusion and subsequent polymerization of styrene in water medium, with initiator benzoyl peroxide (BPO) added after diffusion of styrene. Two methods, differencing in whether the excess monomer was removed after diffusion, were used and parameters influencing PS contents were investigated. Diffusion kinetic study showed that the diffusion coefficient at 90 °C is 2.8 times that at 80 °C due to the α relaxation of iPP segments. Investigation on the distribution state of styrene in the mixture before diffusion revealed that most styrene adsorbed on the surface of iPP pellets, and thus the diffusion behavior of styrene into iPP pellets in water medium is similar to that in bulk styrene. Phase morphology of a typical iPP/PS blend showed an average particle size of about 90 nm in the inner part of the iPP pellets. The diametrical distributions of PS showed that styrene can diffuse up to the center of the pellets at 90 °C. It is important to note that the depth of styrene is limited when the initiator BPO is added along with styrene, providing a good explanation for the previously reported ineffective solid-state modification of melt-extruded iPP pellets.     

Evaporation and pervaporation of a binary mixture from an inert carrier liquid

Bench-Scale experiments were carried out with the ternary mixture 2-propanol/water/glycerol. In a cylindrical vessel, the stirred and thermostated liquid was exposed to a preheated and either dry or humidified air stream. The mixture was evaporated either from a free surface or from the surface of a porous plate. On evaporating the mixture from a porous plate into a dry air stream, the less volatile component water escapes preferentially; in the case of evaporation from a free surface, water is retained in the mixture. In the former case, selectivity is liquid diffusion controlled, in the latter, the gas-side mass transfer and the thermodynamic equilibrium are the controlling mechanisms. On reducing the gas flow rate, the more volatile alcohol evaporates preferentially in both cases; the selectivity is controlled only by thermodynamic equilibrium. Humidification of the air stream with water vapour causes preferential evaporation of the alcohol at an increased rate. Furthermore, instabilities in the liquid boundary layer due to density gradients and surface tension effects may affect the selectivity of evaporation. Experimental results show that the selectivity of evaporation can be manipulated by choosing appropriate evaporation conditions. The theory, which applies the generalized Stefan-Maxwell equations to diffusion in the liquid, can describe these effects.     

Crosslink density and diffusion mechanisms in blend vulcanizates loaded with carbon black and paraffin wax

Results on blends containing different ingredients have revived the interest on the interaction parameter and crosslink density of structures that contain soft fillers. Composite blend of natural rubber and styrene butadiene rubber loaded with the percolation concentration of high abrasion furnace carbon black and different concentrations with paraffin wax were prepared. The applicability of the blends for liquid diffusion has been examined through the changes in the interaction parameter and the crosslink density at different temperatures. The transport mechanism through the vulcanized blends is governed by Fickian diffusion law at room temperatures and it becomes non‐Fickian at higher temperatures. The diffusion coefficients were calculated and found to have an activated behavior with temperature from which the activation energy was calculated. The change in entropy, enthalpy, and Gibbs free energy are also studied at different temperatures. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Fat-simulating and accelerating solvents for polyolefins and MWD of solvent extracts of polyethylenes

Migration kinetics of straight-chain oligomers and antioxidants from several polyolefins at different temperatures into various solvents have been studied by radioactive tracer techniques. Anhydrous ethanol appears to be a well suited food-oil or liquid-fat simulant for extracting different types of migrants from polyolefins. Pure and mixed triglycerides are also good oil or fat simulants, but the triglycerides offer no simpler analytical procedures than the use of oil or fat themselves. n-Octanol may also be considered as a reasonable oil or fat simulant; however, its action depends somewhat on the choice of migrants. The accelerating action of n-heptane over that of oil or simulants is quantitatively demonstrated. The accelerating effects are greater for migration systems with lower diffusion coefficients. The diffusion coefficients for migration into n-heptane are about 20 times greater than the diffusion coefficients into ethanol or oil for otherwise identical migration systems yielding diffusion coefficients of about 10^?7 cm²s^?1 into oil or ethanol. For systems yielding diffusion coefficients into oil or ethanol of about 10^?12 cm²s^?1, the corresponding diffusion coefficients into n-heptane are about 1000 times greater. The molecular weight distributions (MWDs) of the n-heptane and ethanol extracts of polyolefins have been analysed. n-Heptane can not only accelerate the migration of the individual migrant but also remove oligomer species that are slightly soluble or present at low levels in the oil or simulant extracts.     

Permeation of N-butane,propane and ethane through ethylcellulose

The permeation of n-butane, propane, and ethane in ethylcellulose has been measured over a pressure range from 25 to 200 mm Hg and over the temperature range from 30 to 70°C. The permeation and diffusional time lag of each of the three gases in ethylcellulose is pressure dependent. Transport of the gases through ethylcellulose can be described by the partial immobilization model. It was found that, in general, the Langmuir-mode species diffusion coefficients are lower than the Henry's law species diffusion coefficients. The logarithm of diffusion coefficients at zero penetrant concentration varies linearly with the square of the molecular diameter of n-butane, propane, and ethane permeating through ethylcellulose. This relationship suggests that the diffusion process depends upon the availability of sufficient cross-sectional area for the penetrant to diffuse. An Arrhenius temperature dependence was observed for permeation coefficients and diffusion coefficients for n-butane, propane, and ethane in ethylcellulose. The activation energy of diffusion at zero penetrant concentration is directly proportional to the square of the gas molecular diameter and the entropy of activation. This observation is consistent with the view that the activation energy of diffusion is associated with the energy required to produce a space of sufficient cross-section for the diffusion molecule to pass.     

局部组成型的互扩散系数模型

根据局部组成概念建立了混合物中互扩散系数与自扩系数的关系,并以分子集团迁移的概念建立了混合物中组分的自扩散系数的预测方法,由此预测二元系的自扩散系数和互扩散系数与实验结果符合良好,比经典的Ｈａｒｔｌｅｙ模型有了很大地改进。     

全氟烷基表面活性剂吸附特性研究

针对全氟烷基季铵碘化物(Le-134)、全氟烷基磷酸酯(Le-107)和全氟烷基聚醚(Le-180)三种表面活性剂水溶液的平衡态表面张力和吸附动力特性进行了研究。临界胶束浓度的大小关系为Le-180 (15×10^-6) -6) -6);饱和吸附量Г_max大小关系为 Le-107 相似文献   

尼龙6固相聚合研究

对不同聚合方式、不同尼龙 6预聚体切片以及固相缩聚过程中缩聚副产物小分子在聚合物固体内或表面的扩散 ,在不同聚合温度和聚合时间进行了研究。结果表明 ,在本实验条件下 ,副产物小分子水在尼龙 6固体聚合物中的扩散速度是尼龙 6固相缩聚反应速度的控制步骤 ,而与气体副产物水分子在尼龙 6聚合物固体表面的扩散速度几乎无关。在相同条件下固定床固相聚合时能获得较高的数均相对分子质量尼龙 6产品。数均相对分子质量随聚合温度升高、聚合时间增加、预聚体切片粒子直径减小而增加     

氨基酸扩散系数的分子动力学模拟

采用分子动力学模拟的方法模拟了298. 15 K正则系综下甘氨酸、丙氨酸等12种氨基酸在水中的扩散过程，扩散系数的计算采用微分-区限变分法。计算结果表明：相同浓度下按不同分子数样本计算得到的扩散系数有较大差别，且分子数越多则模拟结果越接近于实验值。5种氨基酸在水中的扩散系数与文献值相对比，误差小于7%。还用同样的方法模拟了氧气在水中的扩散过程，模拟结果与实验结果吻合也较好。实验表明采用分子模拟手段可以获得具有工业应用价值的扩散系数，从而有助于计算传质学的发展。     

Diffusion of Saccharides and Sugar Alcohol Sorbitol in Chitosan Membranes and Beads

Diffusion coefficients of different sugars and the sugar alcohol sorbitol have been determined by two different experimental setups: mass transfer through a flat membrane in a diffusion cell and effusion experiments with beads in a batch stirred vessel. Diffusion experiments show highly consistent data with low uncertainties and high regression coefficients, while effusion experiments suffer from larger experimental uncertainties. For the diffusion through a chitosan membrane at 25 °C, typically one‐third of the free molecular diffusion coefficient is found, whereas at 40 °C only one‐quarter of it is reached. The influence of molecular size, structure, and temperature on the transport properties is discussed in detail.     

超疏水表面太阳能加热金-水纳米流体液滴蒸发特性

实验研究了超疏水表面上太阳能加热金纳米流体液滴蒸发特性。用高速摄像机和红外摄像机同步触发记录了2 μl不同浓度金纳米流体液滴在超疏水表面的蒸发过程。通过一系列实验，观察对比不同浓度金纳米流体液滴蒸发过程中体积、接触角、接触直径、液滴表面温度以及蒸发速率等动态特性。结合水蒸气扩散模型以及红外温度图分析液滴在超疏水表面上的蒸发过程中蒸发通量变化以及表面温度变化等特性。发现不同浓度纳米流体液滴蒸发速率基本一致；超疏水表面上液滴蒸发以常接触角模式为主，后期呈现混合模式蒸发；液滴蒸发过程中，液滴上半部分蒸发通量大，致使液滴表面温度较低。     

Diffusion distillation - A new separation process for azeotropic mixtures Part 1: Selectivity and transfer efficiency

A new separation process for azeotropic mixtures3̄diffusion distillation—is proposed. A liquid mixture is evaporated below the boiling temperature, diffuses through an inert gas gap and is recondensed. Hence the separation effect is not only based on the relative volatility of the components concerned but also on their diffusivity in the inert gas. In a wetted-wall column consisting of two concentric tubes, a significant separation effect can be achieved. Several experiments were carried out with a binary isopropanol-water mixture and a ternary isopropanol-water-methanol mixture at different evaporation and condensation temperatures, with different inert gases and annular widths in the wetted-wall column. The experimental results were well described by the vapour-liquid equilibrium and the Stefan-Maxwell equations, that is, by steady-state molecular diffusion.