Concentration fluctuation and cooperative chain mobility of hyperbranched poly(ε-caprolactone)s investigated by photon correlation spectroscopy

Concentration fluctuation and cooperative chain mobility of the hyperbranched poly(ε-caprolactone)s (HPCLs) as well as that of the linear poly(ε-caprolactone) (LPCL) in their concentrated solutions in tetrahydrofuran (C_polymer=1.0 g/mL) were systematically characterized using photon correlation spectroscopy (PCS). The results were interpreted in conjunction with the architectural characteristics of the HPCLs that are the different lengths of the linear segments and the different branching structures. The HPCLs were synthesized to incorporate the different lengths of the linear segments consisting of 5, 10, and 20 ε-caprolactone units on their backbone chains, and thereby referred to as HPCL-5, -10, and -20, respectively. The relative degrees of branching (DB) of the HPCLs, which is obtained by branching ratio values, were found to be in the order of HPCL-5>-10>-20. From the narrow distribution of relaxation times and q²-dependence of relaxation rates, it was verified that the relaxation processes concerned in this study were due to the concentration fluctuations caused by cooperative chain diffusion. The correlation times, τ_c's, and the corresponding apparent activation energies, E_a's, which provide a direct evaluation for the molecular mobility, were determined by non-linear curve fittings of measured correlation functions with Kohlrausch-Williams-Watts (KWW) equation and by the Arrhenius fits of temperature dependences of τ_c's, respectively. As a result, cooperative chain mobility of the HPCLs in the concentrated solution state was found to be higher than that of the LPCL, and was enhanced with the shorter linear segments and with the higher relative DBs of the HPCLs.     

颗粒团聚对稀相气固流动脉动关联项的影响

研究了颗粒团聚对描述稀相气固两相流的气固相宏观控制方程中待封闭气固脉动关联项的影响规律并建立关键待封闭项代数模型。根据气相速度脉动与固相浓度关联项(漂移速度)控制方程,将漂移速度表达为固相非均匀程度和气固平均滑移速度的代数模型。分别采用两种基于颗粒动理学的欧拉?欧拉框架介尺度方法模拟三维周期条件且固相平均浓度为1%的稀相气固两相流动,第1种方法假设固相速度分布函数f为各项同性的双流体方法(TFM);第2种方法假设f服从各向异性高斯分布的积分矩法(AG)。网格分辨率为颗粒直径dp的1.75倍,气固间动量交换采用Stokes曳力模型,并与文献中采用相同参数设置的欧拉?拉格朗日(E?L)方法模拟结果进行对比。结果表明,AG方法的准确度优于TFM方法,气固平均滑移速度、气固脉动能等更接近E?L方法模拟结果。颗粒聚团的积分尺度小于气相脉动速度的积分尺度,两者均呈各向异性,竖直分量高于水平分量。模拟得到了气固速度脉动关联系数和漂移速度系数。     

Inhibition of catalytic oxidation of carbon/carbon composites by phosphorus

The inhibition effectiveness of thermally deposited phosphorus (P) compounds on the carbon oxidation catalyzed by potassium or calcium acetate has been investigated. The P deposit was formed by impregnating carbon/carbon composite samples with methanol solution of methyl-phosphoric acid or phosphorus oxychloride and heating at ca. 600 °C. An amorphous layer formed by a relatively large amount of P deposit functioned as a barrier for the access of the catalyst to the carbon surface even though it had almost no barrier effect for O₂ access. The catalytic effect of calcium was almost completely suppressed by such deposit, but the effect of potassium was only partially suppressed due to the superior wetting ability and mobility of potassium species. Small amounts of P deposit showed similar inhibition effects on non-catalyzed oxidation, while their effects on catalytic oxidation were not as good. Characterization of P-deposited carbon samples by XPS, XRD, SEM and TPD, as well as ab initio MO calculations, suggested that the inhibition effect mainly resulted from the formation of oxygen-containing P groups which may include metaphosphates, C-O-PO₃ groups and C-PO₃ groups. Those groups are suggested to act as a physical barrier against carbon/catalyst interfacial contact as well as to block the active carbon sites. The presence of bridge oxygen bonded to a carbon site and a P group appears to be a critical factor for maintaining the inhibition effect. Indeed, the loss of such oxygen or connecting bond seems to result in loss of inhibition.     

Concentration control of volatile organic compounds by ionic liquid absorption and desorption

Volatile organic compounds (VOCs) are difficult to be eliminated safely and effectively because of their large concentration fluctuations. Thus, maintaining a stable concentration of VOCs is a significant study. In this research, H₂O, Tween-80,[Emim]BF₄,[Emim]PF₆, and[Hnmp]HSO₄ were applied to absorb and desorb simulated VOCs. The ionic liquid[Emim]BF₄ demonstrated the best performance and was thus selected for further experiments. As the ionic liquid acted as a buffer, the toluene concentration with a fluctuation of 2000-20000 mg·m^-3 was stabilized at 6000-12000 mg·m^-3. Heating distillation (90℃) was highly efficient to recover[Emim]BF₄ from toluene. The regenerated[Emim]BF₄ could retain its initial absorption capacity even after multiple cycles. Moreover,[Emim]BF₄ had the same buffer function on various aromatic hydrocarbons.     

氯气管道积水引起压力波动的规律及处理

分析氯气管道积水引起压力波动的原因,根据压力波动规律总结积水部位的判断方法,并结合实例,证明该方法简洁实用.     

Modeling of a metal monolith catalytic reactor for methane steam reforming-combustion coupling

A novel metal monolith reactor for coupling methane steam reforming with catalytic combustion is proposed in this work, the metal monolith is used as a co-current heat exchanger and the catalysts are deposited on channel walls of the monolith. The transport and reaction performances of the reactor are numerically studied utilizing heterogeneous model based on the whole reactor. The influence of the operating conditions like feed gas velocity, temperature and composition are predicted to be significant and they must be carefully adjusted in order to avoid hot spots or insufficient methane conversion. To improve reactor performance, several different channel arrangements and catalyst distribution modes in the monolith are designed and simulated. It is demonstrated that reasonable reactor configuration, structure parameters and catalyst distribution can considerably enhance heat transfer and increase the methane conversion, resulting in a compact and intensified unit.     

Extra low friction coefficient caused by the formation of a solid-like layer: A new lubrication mechanism found through molecular simulation of the lubrication of MoS2 nanoslits

Monolayer molybdenum disulfide (MoS₂) is a novel two-dimensional material that exhibits potential application in lubrication technology. In this work, molecular dynamics was used to investigate the lubrication behaviour of different polar fluid molecules (i.e., water, methanol and decane) confined in monolayer MoS₂ nanoslits. The pore width effect (i.e., 1.2, 1.6 and 2.0 nm) was also evaluated. Results revealed that decane molecules exhibited good lubricating performance compared to the other two kinds of molecules. The friction coefficient followed the order of decane < methanol < water, and decreased evidently as the slit width increased, except for decane. Analysis of the spatial distribution and mobility of different confined fluid molecules showed that a solid-like layer was formed near the slit wall. This phenomenon led to the extra low friction coefficient of confined decane molecules.     

Diffusion of single alkane molecule in carbon nanotube studied by molecular dynamics simulation

Full atomistic molecular dynamics simulations have been used to study the diffusion of alkane molecule in single wall carbon nanotube (SWCNT), with different alkane chain lengths and nanotube diameters. In this paper, we calculated the self-diffusion coefficient, mean-square gyration and bond-orientation order parameter of alkane molecule and the average intermolecular interaction energy per segment between SWCNT and alkane. Furthermore, structure of alkane in SWCNT was characterized through the radial distribution function, with results showing that the self-diffusion coefficient is related to the nanotube diameter. The component of mean-square gyration in z-direction scales with alkane chain length in SWCNT(9,9) like N1.07±0.04, which is in good agreement with the prediction from scaling theory for polymers. The obtained results show that nanotube diameter and alkane chain length are important factors affecting the behavior of one-dimensional confined alkanes.     

Integrated FCC riser—regenerator dynamics studied in a fluid catalytic cracking pilot plant

In this paper a dynamic simulator of the fluid catalytic cracking (FCC) pilot plant, operating in the Chemical Process Engineering Research Institute (CPERI, Thessaloniki, Greece), is presented. The operation of the pilot plant permits the execution of case studies for monitoring of the dynamic responses of the unit, by imposing substantial step changes in a number of the manipulated variables. The comparison between the dynamic behavior of the unit and that predicted by the simulator arise useful conclusions on both the similarities of the pilot plant to commercial units, along with the ability of the simulator to depict the main dynamic characteristics of the integrated system. The simulator predicts the feed conversion, coke yield and heat of catalytic reactions in the FCC riser on the basis of semi-empirical models developed in CPERI and simulates the regenerator according to the two-phase theory of fluidization, with a dilute phase model taking account of postcombustion reactions. The riser and regenerator temperature, the stripper and regenerator pressure drop and the composition of the regenerator flue gas are measured on line and are used for verification of the ability of the simulator to predict the dynamic transients between steady states in both open- and closed-loop unit operation. All the available process variables such as the reaction conversion, the coke yield, the carbon on regenerated catalyst and the catalyst circulation rate are used for the validation of the steady-state performance of the simulator. The comparison between the dynamic responses of the model and those of the pilot plant to step changes in the feed rate and preheat temperature reveals the ability of the simulator to accurately depict the complex pilot process dynamics in both open- and closed-loop operation. The dynamic simulator can serve as the basis for the development of a model-based control structure for the pilot plant, alongside its use as a tool for off-line process optimization studies.     

Study on the active centers of methane coupling catalysts by means of fluorescence emission of Eu

Eu³⁺ ion was adopted as a probe to detect the probability of entrance of alkali elements into the crystal lattice of MgO, CaO and La₂O₃ by means of its characteristic emission. Based on the experimental data it is concluded that Li⁺ and Na⁺ ions can substitute Mg²⁺ and Ca²⁺ ions and only a small amount of K⁺ ion can enter into the lattice of CaO. Whilst Li⁺ ion can not enter into the lattice of lanthana. The conclusion of this investigation is in good agreement with that obtained by Lunsford by ESR studies.     

Morphological features and mechanical behavior of one- and two-phase polymeric materials simulated by molecular dynamics

Single phase amorphous polymeric materials and two-phase polymer liquid crystals (PLCs) have been created on the computer and their behavior simulated using molecular dynamics. An external force was applied on the material and its response computed along time. The influence of several parameters was investigated, such as the concentration of the rigid LC second phase and the existence of regions of different orientation across the thickness of the material.A simplified 3-region model, such as that used to model the skin-core structure resulting from injection molding, was used. The influence of the relative size of each region with different properties was determined. Thicker skin regions increase the rigidity of the material, due to their higher orientation in the direction of force application. The concentration of the reinforcing LC second phase has a similar effect, also resulting in a more brittle behavior. The simulations have provided a better understanding of these phenomena.A method for calculating the true stress during simulation of computer-generated materials (CGMs) is proposed. The true stress behavior was found to differ qualitatively from the engineering stress when the structure of the material allows for considerable changes in cross-sectional area at large-scale deformation.     

Catalyst design for methane oxidative coupling by using artificial neural network and hybrid genetic algorithm

A new method for catalyst design was discussed based on artificial neural network, which was developed to simulate the relations between catalyst components and catalytic performance in the previous research. For enhancing efficiency of catalyst design, a new hybrid GA tested by TSP was generated for global optimization to design the ‘optimal’ catalyst. A multi-turn design strategy was described. Based on the previous research, the design method was applied for designing multi-component catalyst for methane oxidative coupling, some better catalysts, in which C₂ hydrocarbon yields were greater than 25% were designed. When reacting on the best catalyst, GHSV was , CH₄:O₂ was 3, reaction temperature was , methane conversion and C₂ hydrocarbon selectivity were 37.79% and 73.50%, respectively (C₂ hydrocarbon yield was 27.78%), which was higher than that of previous reported catalysts on no diluted gas condition, and showed a better prospect for industrialization of methane oxidative coupling. The research also showed that the new catalyst design method is highly efficient and universal.     

Composition-structure-properties relationship of lithium-calcium borosilicate glasses studied by molecular dynamics simulation

The composition-structure-properties relationship of the lithium-calcium borosilicate (LCBS) glasses, which have a composition of 0.4[(1-x)Li₂O-xCaO]-0.6[(1-y)B₂O₃-ySiO₂] with x in the range of 0–1 and y in the range of 0.33–0.83, is investigated by the molecular dynamics (MD) simulation with the Buckingham potential. The structure of the silicon-oxygen tetrahedron is relatively independent of the glass compositions; however, the structure of the boron-oxygen polyhedron and the local environment around the modifier cations change significantly with increasing [SiO₂]/[B₂O₃] ratio (K) and CaO content. The relationships between glass composition and simulated linear thermal expansion coefficient (α_L), glass transition temperature (T_g), self-diffusivity (D), activation energy of electrical conductivity (Ea^σ) and fragility (m) are strongly affected by the change of glass network structure, and consistent with those of experimental results.     

Motional coherency in chain dynamics of polybutadiene studied by molecular dynamics simulations

Molecular dynamics simulations of 1,4-polybutadiene in bulk amorphous state were performed. Results were compared with the recent neutron spin-echo measurements. To investigate motional coherency the relaxation rates for the collective and self-motions, the collective and self-relaxation rates, were evaluated for the short and long time regimes of the normalized intermediate scattering functions. The scattering vector dependence of the collective relaxation rates estimated for both fast and slow processes indicated a minimum at scattering vector q = 1.5 Å⁻¹, corresponding to the position of a peak in the static structure factor. The self-relaxation rates increased monotonously with q. A phenomenon known as de Gennes narrowing was reproduced well in the simulation and found to be originated from the inter-molecular correlation. The collective relaxation rate evaluated for fast process appeared to modulate around a peak of q = 2.9 Å⁻¹, corresponding to the intra-molecular correlation.     

Material properties of the cross-linked epoxy resin compound predicted by molecular dynamics simulation

Molecular dynamics (MD) simulations were conducted to estimate the material properties of the cross-linked epoxy resin compound. A periodic amorphous structure of the cross-linked epoxy resin compound was constructed and it was simulated by continuous accumulation of structure configurations at various temperatures. Based on the simulation results, glass transition temperature (T_g), linear thermal expansion coefficients and Young's modulus of the cross-linked epoxy resin compound were predicted. The predicted values of these material properties are in good agreement with the experimental values in the literature.     

刚-柔组合搅拌桨强化流体混沌混合

合理设计搅拌反应器的桨叶,强化流体流动与混合行为,是实现流体高效、节能混合的重要手段。柔性体与刚性体组合,可设计出具有多体运动行为的刚-柔组合搅拌桨。结合PIV流场观测和CFD模拟,对比分析了刚性桨和刚-柔组合桨对流场结构及流体混沌混合行为的影响。结果表明,与刚性搅拌桨相比,刚-柔组合桨的柔性端强化能量传递,流体流速衰减速率降低25%,有利于搅拌桨输入能量在流场结构内的有效分配。传统刚性六凹叶和六直叶涡轮桨搅拌反应器内流体形成的流线结构具有明显的周期吸引子,其时均流场的分形维数分别为1.9046和1.9138。刚-柔组合六直叶涡轮桨搅拌反应器内流体流线呈明显的准周期性吸引子性质,其流场分形维数为1.9337,而刚-柔组合六凹叶涡轮桨搅拌反应器内流体流线具有典型的混沌吸引子性质,其流场分形维数为1.9545。刚-柔组合搅拌桨可改变流体流线的吸引子来调控流场的多尺度结构,强化流体混沌混合,实现高效节能操作。     

Numerical simulation of fixed bed reactor for oxidative coupling of methane over monolithic catalyst

A three-dimensional geometric modelwas set up for the oxidative coupling of methane (OCM) fixed bed reactor loaded with Na₃PO₄-Mn/SiO₂/cordierite monolithic catalyst, and an improved Stansch kinetic model was established to calculate the OCMreactions using the computational fluid dynamicsmethod and Fluent software. The simulation conditions were completely the same with the experimental conditions that the volume velocity of the reactant is 80 ml·min^-1 under standard state, the CH₄/O₂ ratio is 3 and the temperature and pressure is 800 ℃ and 1 atm, respectively. The contour of the characteristic parameters in the catalyst bed was analyzed, such as the species mass fractions, temperature, the heat flux on side wall surface, pressure, fluid density and velocity. The results showed that the calculated valuesmatchedwell with the experimental values on the conversion of CH₄ and the selectivity of products (C₂H₆, C₂H₄, CO,CO₂ and H₂) in the reactor outlet with an error range of ±4%. The mass fractions of CH₄ and O₂ decreased from 0.600 and 0.400 at the catalyst bed inlet to 0.445 and 0.120 at the outlet, where the mass fractions of C₂H₆, C₂H₄, CO and CO₂ were 0.0245, 0.0460, 0.0537 and 0.116, respectively. Due to the existence of laminar boundary layer, the mass fraction contours of each species bent upwards in the vicinity of the boundary layer. The volume of OCM reaction was changing with the proceeding of reaction, and the total moles of products were greater than reactants. The flow field in the catalyst bed maintained constant temperature and pressure. The fluid density decreased gradually from 2.28 kg·m^-3 at the inlet of the catalyst bed to 2.18 kg·m^-3 at the outlet of the catalyst bed, while the average velocity magnitude increased from 0.108 m·s^-1 to 0.120 m·s^-1.     

Study of Materials Deformation in Nanometric Cutting by Large-scale Molecular Dynamics Simulations

Nanometric cutting involves materials removal and deformation evolution in the surface at nanometer scale. At this length scale, atomistic simulation is a very useful tool to study the cutting process. In this study, large-scale molecular dynamics (MD) simulations with the model size up to 10 millions atoms have been performed to study three-dimensional nanometric cutting of copper. The EAM potential and Morse potential are used, respectively, to compute the interaction between workpiece atoms and the interactions between workpiece atoms and tool atoms. The material behavior, surface and subsurface deformation, dislocation movement, and cutting forces during the cutting processes are studied. We show that the MD simulation model of nanometric cutting has to be large enough to eliminate the boundary effect. Moreover, the cutting speed and the cutting depth have to be considered in determining a suitable model size for the MD simulations. We have observed that the nanometric cutting process is accompanied with complex material deformation, dislocation formation, and movement. We find that as the cutting depth decreases, the tangential cutting force decreases faster than the normal cutting force. The simulation results reveal that as the cutting depth decreases, the specific cutting force increases, i.e., “size effect” exists in nanometric cutting.     

Study on nanometric cutting of germanium by molecular dynamics simulation

Three-dimensional molecular dynamics simulations are conducted to study the nanometric cutting of germanium. The phenomena of extrusion, ploughing, and stagnation region are observed from the material flow. The uncut thickness which is defined as the depth from bottom of the tool to the stagnation region is in proportion to the undeformed chip thickness on the scale of our simulation and is almost independent of the machined crystal plane. The cutting resistance on (111) face is greater than that on (010) face due to anisotropy of germanium. During nanometric cutting, both phase transformation from diamond cubic structure to β-Sn phase and direct amorphization of germanium occur. The machined surface presents amorphous structure.