MD Simulations of Diffusivities in Methanol‐n‐hexane Mixtures Near the Liquid‐liquid Phase Splitting Region

Molecular Dynamics (MD) simulations have been carried out to determine the self‐diffusivities in binary mixtures of methanol and n‐hexane with varying compositions at four different temperatures of 303.15 K, 307.7 K, 310.7 K, and 313.15 K. The Darken relation was used to determine both the Fick diffusivity and the Maxwell‐Stefan diffusivity. The values of the Fick diffusivity obtained from the simulations are in very good agreement with published experimental data of Clark and Rowley [17]. These diffusivities approach zero near composition regions where liquid‐liquid phase splitting occurs. On the other hand, the Maxwell‐Stefan diffusivity is well‐behaved and appears to be practically insensitive to the complex thermodynamics.     

"人工神经网络"方法用于超临界流体萃取模拟

在15－30MPa和303－323K条件下，用超临界CO2流体萃取沙棘籽油，结果表明，最高沙棘油收率（30MPa,308K)可达到90％以上，对过程进行动力学模拟，建立了超临界萃取过程的人工神经网络（ANN）模型，以MATLAB软件为平台，编制了SFE－ANN模拟程序系统，采用3层BP网络结构，以压力，温度、萃取时间为输入，以萃取出油量为输出对网络进行训练，由此得到的网络可以对萃取速率和单位时间床高方向的萃取出油量进行准确的模拟和预测，与实验结果比较证明，训练样本集误差为0．2％，测试样本集误差为0．5％，模拟误差小于6％。     

甲烷在单壁碳纳米管中扩散的分子动力学模拟

利用分子动力学模拟方法模拟了甲烷在不同直径的单壁碳纳米管（SWCNTs）中的扩散。模拟结果发现：当ε=325 K时,甲烷在直径为0.8 nm的SWCNT中的扩散呈现出单队列扩散;当ε从65 K增加为325 K时,甲烷在直径为1 nm和1.25 nm的SWCNTs中的扩散行为从正常扩散变为超扩散;在直径大于1.6 nm的SWCNTs中,ε值的变化并不影响甲烷在SWCNTs中的扩散行为。     

用改进的形状因子法推算液体混合物的粘度

本文对Ｅｌｙ的形状因子法引进编心因子修正，同时采用新的改进Ｒａｃｋｅｔｔ方程混合规则推算液体混合物密度。改进后的形状因子法在精度上有了显著的提高并能适用于极性系统。不同温度下２６个二元系５１３个粘度数据及３个三元系３６个粘度数据的预测结果，其绝对平均百分偏差（ＡＤＤ）分别为３．１１％和３．１７％。     

乙醇－丙酮体系液体密度的测定和预测

使用ＤＭＡ５５型精密密度计和ＤＭＡ５１２型高压外测量池，测定了乙醇－丙酮体系在温度范围为２８８．１５至３１８．１５Ｋ和压力从常压至１９．５ＭＰａ下的液体密度，并根据密度数据计算了体系的过量体积。应用三重随机局部组成密度模型，预测了该体系在不同压力下的密度和过量体积，取得令人满意的结果。     

Kinetic Monte Carlo Simulations of the Loading Dependence of Diffusion in Zeolites

Kinetic Monte Carlo (KMC) simulations are used to model diffusion of molecules in zeolites. A variety of loading dependences of the Maxwell‐Stefan diffusivity, ?_i, can be realized by allowing the jump frequencies, ν, of molecules to be influenced by the presence of neighboring molecules. Neighboring molecules are assumed to reduce or increase the activation energy for diffusion by δE and the jump frequencies ν are altered by a factor f = exp(δE/RT) for each neighboring molecule. By appropriate choice of ν and f the loading dependence of ?_i can be made to match those obtained from either Molecular Dynamics (MD) simulations or experiment. The major advantage of the KMC simulation strategy is that considerably less computer power is required than for the corresponding MD simulations. Furthermore, KMC simulations allow mixture diffusion to be probed without additional parameter tuning. The KMC simulations also validate the applicability of the quasi‐chemical theory of Reed and Ehrlich (Surf. Sci. 1981 , 105, 603–628) for describing the loading dependence of ?_i.     

The Dynamics of Lysozyme from Bacteriophage Lambda in Solution Probed by NMR and MD Simulations

¹⁵N NMR relaxation studies, analyses of NMR data to include chemical shifts, residual dipolar couplings (RDC), NOEs and H^N–H^α coupling constants, and molecular dynamics (MD) simulations have been used to characterise the behaviour of lysozyme from bacteriophage lambda (λ lysozyme) in solution. The lower and upper lip regions in λ lysozyme (residues 51–60 and 128–141, respectively) show reduced ¹H–¹⁵N order parameters indicating mobility on a picosecond timescale. In addition, residues in the lower and upper lips also show exchange contributions to T₂ indicative of slower timescale motions. The chemical shift, RDC, coupling constant and NOE data for λ lysozyme indicate that two fluctuating β‐strands (β3 and β4) are populated in the lower lip region while the N terminus of helix α6 (residues 136–139) forms dynamic helical turns in the upper lip region. This behaviour is confirmed by MD simulations that show hydrogen bonds, indicative of the β‐sheet and helical secondary structure in the lip regions, with populations of 40–60 %. Thus in solution λ lysozyme adopts a conformational ensemble that will contain both the open and closed forms observed in the crystal structures of the protein.     

Interference of Soil Contaminants with Laccase Activity During the Transformation of Complex Mixtures of Polycyclic Aromatic Hydrocarbons in Liquid Media

The biotransformation of 16 polycyclic aromatic hydrocarbons (PAHs) in mixture was investigated in reactors in the presence of purified laccases of the fungus Pycnoporus cinnabarinus, ABTS as a redox mediator, 25% acetonitrile, and Tween 20. Several hydrocarbons from a synthetic mixture, such as anthracene and benzo[ a ]pyrene, were converted up to 80% into quinones, whereas others also belonging to three- and five-ring chemicals were less transformed. Chrysene and benzo[ k ]fluoranthene were not oxidized by the laccase mediator system. Moreover, hydrocarbons extracted from an industrial soil were all recalcitrant to enzymatic attack. This lack of reactivity of the laccases toward the hydrocarbons could be due to the presence of interfering compounds coextracted from the soil, such as metals.     

The Use of Simulation Techniques in Developing Kinetic Models for Polymerization

The demands on simulations of polymerization reactions grow continuously, as do their capabilities. Using two examples it will be demonstrated how the use of detailed kinetic models in such simulations can contribute to the development of experimental strategies for determining rate coefficients, assist in mechanistic analysis and enable feasibility studies about operational strategies. Using the high‐temperature butyl acrylate polymerization as an example, it will be illustrated, based on detailed analytical information, how the kinetic scheme of the butyl acrylate polymerization expands to a complex network of elementary reactions. Based on this model an experimental strategy will be developed for the selective determination of the individual rate coefficients. A comparison of the results from these simulations with experimental data shows that a realization of the developed concept appears to be within reach. The second example is based on a kinetic scheme of the, high‐temperature high‐pressure, polymerization of ethene. In addition, using this kinetic scheme, the capability of nitroxyl species in establishing a controlled radical polymerization mechanism in polymerizations that show high polymerization rates and significant quantities of intra‐ and inter‐molecular branching reactions will be tested. This feasibility study will examine the potential effect a nitroxyl species has on the kinetic mechanism with respect to its stability and the structural characteristics of a polymer that results from such a polymerization.     

CFD Simulations of a Bubble Column Operating in the Homogeneous and Heterogeneous Flow Regimes

Bubble columns are operated either in the homogeneous or heterogeneous flow regime. In the homogeneous flow regime, the bubbles are nearly uniform in size and shape. In the heterogeneous flow regime, a distribution of bubble sizes exists. In this paper, a CFD model is developed to describe the hydrodynamics of bubble columns operating in either of the two flow regimes. The heterogeneous flow regime is assumed to consist of two bubble classes: “small” and “large” bubbles. For the air‐water system, appropriate drag relations are suggested for these two bubble classes. Interactions between both bubble populations and the liquid are taken into account in terms of momentum exchange, or drag‐, coefficients, which differ for the “small” and “large” bubbles. Direct interactions between the large and small bubble phases are ignored. The turbulence in the liquid phase is described using the k‐ϵ model. For a 0.1 m diameter column operating with the air‐water system, CFD simulations have been carried out for superficial gas velocities, U, in the range 0.006–0.08 m/s, spanning both regimes. These simulations reveal some of the characteristic features of homogeneous and heterogeneous flow regimes, and of regime transition.     

The Use of Spectrofluorimetry for Monitoring the Bioaccumulation and the Biotransformation of Polycyclic Aromatic Hydrocarbons in Daphnia magna

A rapid and direct spectrofluorimetric method was tested in order to monitor the bioaccumulation of PAHs in Daphnia magna in a control media. After exposure to water containing benzo[ a ]pyrene or fluoranthene, daphnids were put in solvent, sonicated, and filtered. The fluorescence spectrum observed in the filtrate was recorded. Results were compared to HPLC measurements of the same pools of organisms. In fluoranthene experiments, the fluorescence peak of the daphnid extract spectrum was linearly related to the PAH content as measured with HPLC. In benzo[ a ]pyrene experiments, other fluorescent compounds progressively appeared in the sample. They were assumed to be metabolites. A linear regression involving fluorescence intensities at two different wavelengths was necessary for a satisfactory correlation with HPLC measurements. A water extraction was performed to isolate metabolites. None or very few fluoranthene metabolites were isolated in the aqueous phase, whereas increasing benzo[ a ]pyrene metabolites were observed while the exposure time increased.     

搅拌釜内流场实验研究与数值模拟的进展

概述了搅拌釜内流场实验研究与数值模拟的进展,介绍了速度场、混合时间、循环流量和温度场的各种测量方法,同时就搅拌釜内流场数值模拟中涉及的各种湍流模型以及旋转桨叶的处理方法进行了对比分析。最后指出传热特性、多相流以及多参数耦合数学模型是搅拌釜研究的重点。     

Simulations of chemical absorption in pilot-scale and industrial-scale packed columns by computational mass transfer

A complex computational mass transfer model (CMT) is proposed for modeling the chemical absorption process with heat effect in packed columns. The feature of the proposed model is able to predict the concentration and temperature as well as the velocity distributions at once along the column without assuming the turbulent Schmidt number, or using the experimentally measured turbulent mass transfer diffusivity. The present model consists of the differential mass transfer equation with its auxiliary closing equations and the accompanied formulations of computational fluid dynamics (CFD) and computational heat transfer (CHT). In the mathematical expression for the accompanied CFD and CHT, the conventional methods of k-ε and are used for closing the momentum and heat transfer equations. While for the mass transfer equation, the recently developed concentration variance and its dissipation rate ε_c equations (Liu, 2003) are adopted for its closure. To test the validity of the present model, simulations were made for a pilot-scale randomly packed chemical absorption column of 0.1 m ID and 7 m high, packed with 1/2^″ ceramic Berl saddles for CO₂ removal from gas mixture by aqueous monoethanolamine (MEA) solutions (Tontiwachwuthikul et al., 1992 ) and an industrial-scale randomly packed chemical absorption column of 1.9 m ID and 26.6 m high, packed with 2^″ stainless steel Pall rings for CO₂ removal from natural gas by aqueous MEA solutions (Pintola et al., 1993). The simulated results were compared with the published experimental data and satisfactory agreement was found between them in both concentration and temperature distributions. Furthermore, the result of computation also reveals that the turbulent mass transfer diffusivity D_tvaries along axial and radial directions. Thus the common viewpoint of assuming constant D_t throughout the whole column is questionable, even for the small size packed column. Finally, the analogy between mass transfer and heat transfer in chemical absorption is demonstrated by the similarity of their diffusivity profiles.     

计算机数值仿真高粘变物性流体管内传热分析

高粘变物性流体传热过程在生产中广泛出现 ,其对产品质量、能耗等有重要影响 ,深入探讨高粘变物性流体流动传热规律有重要意义。文中采用SIMPLER方法 ,以苯乙烯 -聚苯乙烯物系为例 ,对高粘假塑性流体管内传热进行了仿真计算。结果表明 ,压力降受物性影响较大 ;采用较小管径是减小径向温差的有效措施     

Application of SPME in the Analysis of Polynuclear Aromatic Hydrocarbons in Water by HPLC with DAD Detection

Determination of polynuclear aromatic hydrocarbons is very important in many laboratories of water quality control. The new Council Directive 98/78/EC states that water intended for human consumption shall be monitored on these parameters and meet the parametric values set out in Annex I (less than 0,10 μg/L for the sum of the concentrations of the specified compounds). The usual methodology of such analysis is based on liquid-liquid extraction followed by evaporation/concentration and clean-up steps. The coupling of Solid Phase Microextraction (SPME), a recent sample preparation technique to HPLC and fluorescence or diode-array detection (DAD) is extremely favourable for these compounds, with the aim to reduce the solvent usage and to simplify the procedure. This technique has been applied to the analysis of the following PAHs: fluoranthene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, benzo[ghi]perylene and indeno[1,2,3-cd]pyrene in model aqueous solutions. In this study we describe the optimisation of SPME-HPLC in the analysis of these compounds. The results obtained suggest that this method can be an alternative to the other sample preparation methods already used in this field.     

Molecular dynamics study of a macromolecular model of hydrous sodium aluminosilicate geopolymer with charge balancing by extra-structural aluminum for prediction of its properties

Geopolymer is a material with unique properties and has various uses. This substance is mainly amorphous, and its qualitative characteristics are related to its binder phase that is called the hydrous sodium aluminosilicate geopolymer. The molecular structural model of this geopolymer includes ${\text{Q}}^4\left(4\text{Al}\right),{\text{Q}}^4\left(3\text{Al}\right),{\text{Q}}^4\left(2\text{Al}\right)$, and ${\text{Q}}^4\left(1\text{Al}\right)\text{Si}$ units, which have been balanced in terms of electric charge by extra-framework $\text{Al}$ and ${\text{Na}}^{+}$ ions. In this study, we calculated the density, Young's modulus, and RDF curve of the geopolymer from the molecular dynamics simulation. The results of the simulation were in good agreement with the results of the laboratory obtained from several studies.     

二元混合物环丁酚和对二甲苯、乙苯在温度范围为303.15—353.15K下的密度、黏度及其相关性质

Densities and viscosities of the binary systems of sulfolane ＋ ethylbenzene, sulfolane ＋ p-xylene have been experimentally determined in temperature interval 303.15—353.15 K and at atmospheric pressure for the whole composition range. The excess molar volumes and viscosity deviations were computed. The computed quantities have been fitted to Redlich-Kister equation. Excess molar volumes and viscosity deviation show a systematic change with increasing temperature. Two mixtures exhibit negative excess volumes with a minimum which occurs approximately at χ = 0.5. The effect of the size, shape and interaction of components on excess molar volumes and viscosity deviations is discussed.     

Thermodynamic Modeling of the Use of Alkane Mixtures as Working Fluids in Absorption Cycles

The main objective is to show the effect of the choice of the most adequate combination of activity‐fugacity coefficient models on the vapor‐liquid phase equilibrium (VLE) data quality and on the calculation of the enthalpies of the different vapor and liquid streams in a refrigeration absorption machine. Six alkane binary mixtures, i.e. propane or n‐butane as refrigerants and n‐heptane, n‐octane, or n‐decane as absorbents, were considered as working fluids for their proven positive characteristics. Two activity coefficient models, UNIFAC and NRTL, and two equations of state, virial and Peng‐Robinson (PR), were tested. The obtained results showed that the UNIFAC‐PR combination reproduced the VLE data reasonably well; hence, it should be recommended for this type of refrigeration systems operating with alkanes.     

Use of numerical modeling in design for co-firing biomass in wall-fired burners

Co-firing biomass with coal or gas in the existing units has gained increasing interest in the recent past to increase the production of environmentally friendly, renewable green power. This paper presents design considerations for co-firing biomass with natural gas in wall-fired burners by use of numerical modeling. The models currently used to predict solid fuel combustion rely on a spherical particle shape assumption, which may deviate a lot from reality for big biomass particles. A sphere gives a minimum in terms of the surface-area-to-volume ratio, which impacts significantly both motion and reaction of a particle. To better understand the biomass combustion and thus improve the design for co-firing biomass in wall-fired burners, non-sphericity of biomass particles is considered. To ease comparison, two cases are numerically studied in a long gas/biomass co-fired burner model. (1) The biomass particles are assumed as solid or hollow cylinders in shape, depending on the particle group. To model accurately the motion of biomass particles, the forces that could be important are all considered in the particle force balance, which includes a drag for non-spherical particles, an additional lift due to particle non-sphericity, and a “virtual-mass” force due to relatively light biomass particles, as well as gravity and a pressure-gradient force. Since the drag and lift forces are both shape factor- and orientation-dependent, coupled particle rotation equations are resolved to update particle orientation. To better model the reaction of biomass particles, the actual particle surface area available and the average oxygen mass flux at particle surface are considered, both of which are shape factor-dependent. (2) The non-spherical biomass particles are simplified as equal-volume spheres, without any modification to the motion and reaction due to their non-sphericity. The simulation results show a big difference between the two cases and indicate it is very significant to take into account the non-sphericity of biomass particles in order to model biomass combustion more accurately. Methods to improve the design for co-firing biomass in wall-fired burners are finally suggested.