Volume diffusion in purification by sublimation

The amount sublimated of large organic molecules varies with the square of the diameter of the tube in which sublimation occurs. This implies that the velocity profile in the tube is nearly flat, consistent with volume diffusion, but not with laminar flow, Knudsen diffusion, or slip flow. However, molecular simulation calculations show that under the conditions used, the velocity profile is near parabolic when there is no deposition on the tube wall, inconsistent with volume diffusion, but in agreement with laminar flow. These calculations also show that deposition on the wall does result in velocity which is almost constant with radial position. The result is a laminar flow profile which is nearly flat, and hence a total flux proportional to the square of the tube diameter, which is observed both in these simulations and in physical experiments. © 2017 American Institute of Chemical Engineers AIChE J, 63: 1757–1764, 2017     

咖啡因提取实验减压升华装置的设计

茶叶中提取咖啡因是有机化学实验中的常规实验,是天然产物提取的典型实验。针对实验存在的其存在收率较低、升华耗时长、升华温度高、产物不好收集等缺点,通过减压升华的方法进行改进,新的升华装置具有装置简单、操作方便、实验安全性得到提高、实验时间大幅缩短、升华过程实现可视化的优点。     

升华提纯法制备三氯叔丁醇高纯试剂的研究

采用升华提纯法制备半水合三氯叔丁醇高纯试剂,探索了升华温度和升华时间对工艺的影响。利用气相色谱仪(GC)和电感耦合等离子体质谱(ICP-MS)测试升华前后半水合三氯叔丁醇的纯度和金属离子含量的变化。参照美国西格玛试剂指标对样品进行检测分析,结果表明半水合三氯叔丁醇在100℃下升华14 h,收率达98.2%,不出现分解,所得试剂纯度高达99.96%,并具有较低的金属离子含量。     

Understanding rate‐limiting processes for the sublimation of small molecule organic semiconductors

Organic small molecules continue to gain attention for application in light‐emitting devices in displays and solid‐state lighting. The purification of these materials by sublimation represents a critical obstacle for their high throughput processing. In this work, we find that the purification of the archetypical hole‐transporting material N,N′‐bis(naphthalen‐1‐yl)‐N,N′‐bis(phenyl)‐benzidine (α‐NPD) is controlled by a combination of viscous flow, Knudsen diffusion, and physical vapor deposition. In contrast with other commercially practiced sublimations, steps like diffusion within the solid feed, desorption from the feed particle surface, and mass transfer within the bed of feed particles, do not significantly affect the sublimation rate. This work provides guidelines for the large‐scale purification of organic semiconductor materials, and possibly for a broader range of high value small molecule specialty materials. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1347–1354, 2014     

Direct measurement of additive migration from low‐density polyethylene as a function of space and time

A method, based on microtoming and GC analysis, for studying migration of additives inside polymers as a function of space and time was validated for Irganox 1076 migrating from low‐density polyethylene to ethanol. The consistency of the mass balance of the total amount of Irganox 1076 in the polymer and the solvent after different incubation times was acceptable considering errors introduced by the analytical procedure. A solution of Fick diffusion equations, fitted to concentration profiles inside the polymer at different incubation times, was found to describe well the transport process as a function of both position and time with a diffusion coefficient of 1.1 × 10^?13 m² s^?1. This value corresponded to the diffusion coefficient obtained using conventional measurements of an Irganox 1076 concentration in ethanol as a function of time. Compared to a stationary solvent, no significant effect was observed on the diffusion coefficient by gently shaking the ethanol. Diffusion coefficients measured at different temperatures using the validated method followed an Arrhenius type of relationship with an activation energy of 113 kJ mol^?1. Conclusively, the method was found to be well suitable for studying additive migration in polymers as a function of both space and time. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3185–3190, 2002     

升华在化工过程中应用

综述了升华技术在化工分离与精制中的原理及特点，介绍了升华在化工分离与精制过程中的应用研究进以及工业开发应用前景。     

升华焓的密度泛函理论研究

采用Politzer经验公式,运用密度泛函理论研究了升华焓的预估方法。基于在B3LYP/6–31G**水平上获得56种化合物稳定构型的基础上,运用Multiwfn软件对分子表面进行了定量分析。对比研究了采用Politzer系数和Rice系数对升华焓预估值的影响,结果表明,Politzer系数和Rice系数所造成的最大误差分别为46.1 kJ/mol和38.9 kJ/mol。为了缩小误差,根据34种化合物升华焓的实验值拟合出了一组新的系数(0.000 409(α),2.005 0(β),–2.82(γ)),并用来计算了22种含能化合物的升华焓。计算结果表明,该组新系数使计算值的最大误差为32.6 kJ/mol,标准偏差为13.8 kJ/mol。     

Effects of the cathode gas diffusion layer characteristics on the performance of polymer electrolyte fuel cells

Polymer electrolyte fuel cell (PEFC) electrodes were prepared by applying different porous gas diffusion half-layers (GDHLs) onto each face of a carbon cloth support, followed by the deposition of a catalyst layer onto one of these half-layers. The performance of PEFCs in H₂/air operation using cathodes with GDHLs presenting different characteristics were compared. The best result was obtained using cathodes with GDHLs having polytetrafluorethylene (PTFE) contents of 30 wt % in the gas side and 15 wt % in the catalyst side. This behaviour was explained in terms of a better water management within the cell.     

Swelling and diffusion characteristics of polar and nonpolar polymers in asphalt

The swelling and diffusion characteristics of a polar polymer [ethylene vinyl acetate (EVA)] and a nonpolar polymer [low-density polyethylene (LDPE)] were studied with swelling experiments of the polymers in asphalt at different temperatures. The study showed that the diffusion mechanisms for LDPE and EVA were different and temperature-dependent. In the case of LDPE, the observed diffusion was anomalous at both swelling temperatures (70 and 90°C). LDPE at 90°C showed sigmoidal solvent-uptake behavior during the initial period of swelling and a sorption overshoot in a later period. EVA showed Fickian transport at 60°C and anomalous diffusion at a higher swelling temperature (70°C) with sigmoidal uptake behavior. An analysis of the diffusion coefficients and the Fourier transform infrared results showed that the diffusing molecules were different in the case of LDPE and EVA, and there were possible polymer–asphalt interactions. Differential scanning calorimetry and swelling studies showed that penetrant-induced crystallization affected the diffusion process in the case of LDPE. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Determination of oxygen diffusion coefficient of poly(methacrylonitrile) II and the calculation of diffusion activation energy

Poly(methacrylonitrile) (PMAN) samples in the shape of a cylinder used in this study were obtained from methacrylonitrile by bulk polymerization. The electron spin resonance (ESR) method has been used to calculate oxygen diffusion coefficient (D) into PMAN samples exposed to high‐energy radiation at different doses in vacuum by observing the ESR radical signal change. In order to calculate the dose‐independent diffusion coefficient (D₀), ln D values were plotted against dose values. The low dose region of this curve was extrapolated to a zero‐dose value, and D₀ was calculated as D₀ = 3.1 × 10⁻⁹ cm²/s. Although D₀ values were in very good agreement with the one obtained for the thin‐film sample of PMAN, the dose dependence of the oxygen diffusion into the cylindrical PMAN samples was observed as being converse of the thin‐film of PMAN,¹ as expected, because of the big difference of the surface/mass values between relatively big cylindrical PMAN samples and thin‐film of PMAN samples. The activation energy (E_a) values of the oxygen diffusion into PMAN were calculated as E_a1 = 27.9 kJ/mol for the 20–60°C temperature range E_a2 = 74.2 kJ/mol for the temperatures above 60°C of the 36 kGy gamma‐irradiated samples. The temperature value of the break point of activation energy was near to the T_g of PMAN.² © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1108–1118, 1999     

Steady microfluidic measurements of mutual diffusion coefficients of liquid binary mixtures

We present a microfluidic method leading to accurate measurements of the mutual diffusion coefficient of a liquid binary mixture over the whole solute concentration range in a single experiment. This method fully exploits solvent pervaporation through a poly(dimethylsiloxane) (PDMS) membrane to obtain a steady concentration gradient within a microfluidic channel. Our method is applicable for solutes which cannot permeate through PDMS, and requires the activity and the density over the full concentration range as input parameters. We demonstrate the accuracy of our methodology by measuring the mutual diffusion coefficient of the water (1) + glycerol (2) mixture, from measurements of the concentration gradient using Raman confocal spectroscopy and the pervaporation‐induced flow using particle tracking velocimetry. © 2017 American Institute of Chemical Engineers AIChE J, 63: 358–366, 2018     

用升华与蒸发联合法干燥食品的物理性质

Drying is an important unit operation in processing of biological resources. The drying process may influence the product properties and quality, which may shrink, break or undergo rheological, physical and biochemical changes. The important parameters responsible for such changes are drying conditions, type of drying technology and residence drying time. Thermal conductivity, thermal-mass diffusivity, enthalpy, porosity and density are the main material property and heat-mass transfer parameters, which are essential for understanding the changes in product quality and for designing and dimensioning the drying processes. In this paper physical properties of food products undergoing a combined sublimation and evaporation were studied. Pieces of vegetables and potatoes were dried in a heat pump fluidized bed dryer at combined modes with temperatures below the freezing point in the beginning and a final drying step at temperatures above the freezing point. Samples of products were tested at different moisture contents with respect to physical properties. Physical properties of leek and potato samples were measured and mass diffusivities were determined from drying kinetic data. Based on bulk density and rehydration measurements it was clearly observed that drying temperature and modes influenced the final product physical properties. The potato cuberun dried with initial atmospheric freeze-drying step had rehydration ability 430% above a run dried only above the freezing point. The average effective mass diffusivity for 5 mm slabs of leek was 0.5×10^-11m^2·s^-1 for the sublimation stage and 2.2×10^-11m^2·s^-1 for the evaooration stage.     

Physical Properties in Drying of Food Products with Combined Sublimation and Evaporation

Drying is an important unit operation in processing of biological resources. The drying process may influence the product properties and quality, which may shrink, break or undergo rheological, physical and biochemical changes. The important parameters responsible for such changes are drying conditions, type of drying technology and residence drying time. Thermal conductivity, thermal-mass diffusivity, enthalpy, porosity and density are the main material property and heat-mass transfer parameters, which are essential for understanding the changes in product quality and for designing and dimensioning the drying processes. In this paper physical properties of food products undergoing a combined sublimation and evaporation were studied. Pieces of vegetables and potatoes were dried in a heat pump fluidized bed dryer at combined modes with temperatures below the freezing point in the beginning and a final drying step at temperatures above the freezing point. Samples of products were tested at different moistu     

Coupling between mass diffusion and film temperature evolution in gravimetric experiments

In gravimetric experiments, the swelling and the drying of polymer films is used to investigate the thermodynamic properties and the mutual diffusion coefficient of polymer/solvent systems. Usually thermodynamic equilibrium at the interface between the film and the solvent vapor and thermal equilibrium between the film and the surroundings are assumed. In this paper we show that the second assumption may fail. Indeed, during a swelling or drying experiment, the temperature of the film surface changes due to the latent heat of vaporization, which induces a variation of the activity. When the corresponding variation of the solvent content is of the same order than the variation due to the sorption experiment and when the thermal time constant is significant compared to the characteristic mass diffusion time, this thermal effect must be taken into account when analyzing sorption data. We evaluate the consequence of this thermal effect on gravimetric experiments and develop a complete model to take this phenomenon into account when analyzing sorption data. As an example, the mutual diffusion coefficient for the system PIB (polyisobutylene)/toluene is estimated for various solvent concentrations at 25 °C.     

Calculations of diffusion coefficients of cyclic poly(dimethylsiloxanes)

The Kirkwood equation and a Monte-Carlo technique which employs Metropolis sampling have been used to calculate diffusion coefficients (D) for low molar mass cyclic poly(dimethylsiloxanes). Correlations between calculated and experimental values of D show that the effective segment size is larger for cyclic than for linear PDMS. Thus, free-draining flow is reduced for the cyclic species consistent with its greater segment density. The cyclic molecules also show enhanced diffusion coefficients at very low molar masses which may be associated with the rigid toroidal shapes of these molecules.     

Softwood Distillate as a Bio-Based Additive in Wood-Plastic Composites

The possibility of using softwood distillate as a bio-based additive or filler in wood-plastic composites (WPCs) was studied by adding various amounts (1–20 wt%) of distillate to a commercial WPC consisting of thermally treated sawdust in a polypropylene (PP) matrix. Softwood distillate was obtained as a secondary product from industrial ThermoWood® processing and it was further processed in the laboratory. The addition of softwood distillate significantly enhanced the mechanical properties of WPC when the distillate content was 2 wt%; tensile strength increased by 5%, tensile modulus by 3%, flexural strength by 3%, and modulus of elasticity by almost 2% compared with the unmodified WPC. In addition, a considerable decrease (over 16%) in water absorption was observed on distillate addition. Proton-transfer-reaction time-of-flight mass-spectrometry (PTR-TOF-MS) analyses revealed that the addition of softwood distillate increased release rates of volatile organic compounds (VOCs) in general, and that the odor of acetaldehyde and guaiacol is detectable in several WPCs. Overall, softwood distillate had positive effects on this particular WPC.     

Surface diffusion of strongly adsorbing vapors in activated carbon by a differential permeation method

Conventional methods to determine surface diffusion of adsorbed molecules are proven to be inadequate for strongly adsorbing vapors on activated carbon. Knudsen diffusion permeability (B_k) for strongly adsorbing vapors cannot be directly estimated from that of inert gases such as helium. In this paper three models are considered to elucidate the mechanism of surface diffusion in activated carbon. The transport mechanism in all three models is a combination of Knudsen diffusion, viscous flow and surface diffusion. The collision reflection factor f (which is the fraction of molecules undergoing collision to the solid surface over reflection from the surface) of the Knudsen diffusivity is assumed to be a function of loading. It was found to be 1.79 in the limit of zero loading, and decreases as loading increases. The surface diffusion permeability increases sharply at very low pressures and then starts to decrease after it has reached a maximum (B_μm) at a threshold pressure. The initial rapid increase in the total permeability is mainly attributed to surface diffusion. Interestingly the B_μms for all adsorbates appear at the same volumetric adsorbed phase concentration, suggesting that the volume of adsorbed molecules may play an important role in the “surface” diffusion mechanism in activated carbon.