Transreac: a model for the calculation of combined chemical reactions and transport processes and its extension to a probabilistic model

Transreac is a computer model for the simulation of the chemical attack to which mineral building materials are exposed. For reliability analysis, Transreac is used in conjunction with the Monte Carlo simulation method. The probabilistic model was evaluated for cement mortar corroded as a result of sulfate attack. The experimentally derived corrosion effects and their scatter are in good agreement with simulation results. In addition, the probability of failure was predicted and a sensitivity analysis was performed.     

Laminar single phase flow distribution in a multi-tube mini-channel heat exchanger using fractal distribution

Mini-channel technology is a promising solution in a world where there is a growing demand for product miniaturization. In this paper a new geometry for a distributor of a multi-tube mini-channel shell and tube exchanger is proposed which should distribute the flow equally over the tubes. The tubes have an internal diameter of 0.5 mm while the shell has a hydraulic diameter of 1.8 mm. The flow in the heat exchanger is pure counter current flow; no baffles have been used in the shell. Experimental data for laminar single phase water in both the tube and shell side are reported. A mathematical model is developed which can predict the performance of the multi-tube mini-channel that uses the newly proposed distributor. This model considers both heat transfer and pressure drop and makes use of prediction methods proposed in the literature for similar geometries. It has been experimentally validated that the newly proposed distributor gives a good distribution for laminar single phase flows. The mathematical model can predict the experimental performance of the multi-tube mini-channel exchanger within 10% accuracy.     

Monte Carlo simulation of molecular weight distribution and copolymer composition in transamidation reaction of polyamides

Melt mixing of polyamides results in exchange reaction and generation of copolymers. In this work, Monte Carlo method is used to simulate the time evolution of molecular weight distribution (MWD) and copolymer composition during the exchange reaction process between polyamides with AA and BC structure. The influences of initial composition and molecular weight have been investigated. Decrease in the difference between the average molecular weight of two kinds of polyamides results in faster approach of the MWD toward Flory's distribution and higher probability of producing copolymers. When the ratio between the numbers of initial molecules of two homopolymers is increased, the number of generated copolymers is reduced and the wider MWD is obtained. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Three-parameter (3P) weibull distribution for characterization of strength of ceramics showing R-Curve behavior

Strength distribution of advanced ceramics is commonly characterized by two-Parameter (2P) Weibull distribution. However, deviation of strength distribution from 2P-Weibull distribution may occur in ceramics due to various mechanisms. R-curve behavior is one of these mechanisms where increase of fracture resistance with the extension of crack occurs. In such cases, 2P-Weibull distribution may not be the best fitting distribution function based on the goodness-of-fitness tests. This article examines the effectiveness of three-parameter (3P) Weibull distribution function for fitting the strength variation due to R-curve effect by using experimental and virtual strength data. The effect of Weibull parameters, degree of increase in crack resistance and number of samples on effectiveness of fitting via 3P-Weibull distribution is investigated. It is reported that 3P-Weibull distribution function fits the strength distribution better than 2P-Weibull distribution function for materials showing R-curve behavior when the crack resistance curve is steep and Weibull modulus is high. Furthermore, it is shown that at least 100 samples should be used for a reliable estimate when the material exhibits R-curve behavior.     

A simulation of oscillatory behavior in the NO+H2 reaction on Pt(100): effect of diffusion and blocking sites

The catalytic reduction of nitric oxide by H₂ over the Pt(100) surface is studied as a function of partial pressures of the reactants. The mathematical mean field model, originally proposed by Makeev and Nieuwenhuys (J. Chem. Phys. 108 (1998a) 3740) shows kinetic oscillations of the reaction products and near the critical point the dependence of the amplitude of oscillations on the partial pressures ratio p is shown to be very close to that predicted for the Hopf supercritical bifurcation. The model has been extended to include the effect of blocking sites (poisoning) on the catalyst surface. This effect changes the period of the oscillations. Diffusion of the adspecies NO has also been studied and we show in what way it modifies the nonlinear behavior of the kinetic oscillations and the transition to chaos through period-doubling bifurcations. The influence of defects on global coupling as they are increased continuously is also studied. Finally, we use Monte Carlo simulations to show the species distributions and growth as they oscillate in time.     

丙烯腈共聚物序列分布的Monte Carlo模拟

通过Monte Carlo方法,对不同共聚单体的聚丙烯腈组成进行了模拟。通过比较．与文献中提供的实验结果基本一致。同时模拟了衣康酸(1A)、丙烯酸(AA)、甲基丙烯酸(MAA)分别与丙烯腈(AN)在不同配比下共聚物的序列分布,结果表明：在Monte Carlo模型中,可以通过控制单体的配比来控制序列分布,而在实际的合成中,要获得均匀的序列结构,就可以通过降低第二单体投料比法或滴加单体法来实现。     

New aspects of unsaturated polyester resin synthesis. Part 2. Reactant sequence distribution and its effect on cure kinetics

The distribution of unsaturations in the prepolymer of a typical unsaturated polyester (UP) resin (maleic anhydride, phthalic anhydride and 1,2‐propylene glycol) has been shown to influence the kinetics of the cure process with styrene monomer. Segments containing double bonds in close proximity appear to lower the reactivity of the resin due to steric hindrance, as indicated by the fact that the rate of cure and the final degree of cure, measured by differential scanning calorimetry (DSC), increase as the average sequence length (SL) of maleic units decreases. This implies that the reactivity of UP resins may be improved by synthesis of prepolymers with certain reactant sequence‐length distributions. The copolymer formed by the melt condensation process of maleic anhydride, phthalic anhydride and 1,2‐propylene glycol in the absence of a transesterification catalyst has a non‐random structure with a tendency towards blockiness. This was established using ¹H NMR analysis in tandem with deterministic and Monte Carlo modelling techniques. Copyright © 2003 Society of Chemical Industry     

Pore size distribution and porosity influence on Sorptivity of ceramic tiles: From experimental data to fractal modelling

The Sorptivity is a coefficient very important to characterize porous materials. It is associated to principal properties such as mechanical durability, thermal and electrical conductivity, etc. In this work, the Sorptivity coefficient of several systems of porous ceramics has been measured following the experimental procedure. In different situations, this very simple test could be not performed; in cultural heritage or during an optimised industrial process. Major reasons for this inability include that it would demand great quantitates of materials impossible to withdraw from the protected building, as well as the experimental test can last for several days, which reduces the possibility to correct/improve the industrial production process. In this regards, being very useful to have analytical formulas in order to calculate Sorptivity coefficient, an Intermingled Fractal Units model has been proposed. Starting from its capability to reproduce entirely the pore size distributions of porous materials, IFU is used to simulate water absorption process and to estimate the Sorptivity coefficient. The obtained results are in good agreement with experimental data and others two models predictions. This fact allows considering IFU model as a future tool for design materials and to predict their service life.     

Wall friction in the compaction of pharmaceutical powders: measurement and effect on the density distribution

In this paper, the axial density profile of tablets of microcrystalline cellulose (MCC) powder compacted in nonlubricated die is investigated by finite element modelling (FEM). The Drucker-Prager/Cap model was adopted for the compaction behavior of powder. The material parameters of the model, including the die wall friction coefficient, were estimated from experimental data of die compaction where the initial density of powder is taken uniform. Changes of Young's modulus with density was measured with a four-point beam bending test. The results of the simulation of the compression and the decompression steps were used to calculate the axial density distribution. Comparison with the measured data presented in [A. Michrafy, M.S. Kadiri, J.A.D. Dodds, Wall friction and its effects on the density distri-bution in the compaction of pharmaceutical excipients, Chem. Eng. Research and Design, Vol. 81, Part A, September (2003)] is discussed.     

MoO3掺杂对二氧化硅吸附Cu(Ⅱ)影响的Monte Carlo模拟

利用Monte Carlo方法,分别模拟二氧化硅和钼掺杂二氧化硅对Cu(Ⅱ)的吸附行为。结果表明,Cu(Ⅱ)被吸附于该纳米材料的表面和原子间隙之中;模拟发现加入少量氧化钼于二氧化硅中不会显著改变对Cu(Ⅱ)的吸附能力。同时采用广义梯度近似(GGA)的密度泛函理论(DFT)对其过程进行验证,并通过微量热技术佐证了二氧化硅及钼掺杂二氧化硅对水中Cu(Ⅱ)的吸附行为,实验发现,吸附过程的ΔH<0,ΔS<0,范德华力为吸附驱动力,分子模拟与实验结果相吻合。     

3-D Monte Carlo simulation of particle deposition on a permeable surface

Monte Carlo simulations are performed to investigate deposition phenomena of sticky particles that undergo diffusive, convective, and gravitational influences on a highly permeable surface. Deposited structures are studied in 3-D space using a Pelcet number as the key parameter. Fractal dimensions of the particle trajectory and deposit structure indicate that a transition from diffusion-limited to convection (and gravitation)-limited transport occurs at the Peclet number of Pe = 10^− 0.84 = 0.146. Analysis of the lateral density shows three distinct regions of deposit structures (defined in this study): founding, grown, and progressing regions. A penetration depth is defined to explain the relationship between particle transport and the evolution of deposit structures.     

Monte Carlo simulation of sequence distributions of acrylonitrile copolymers

A Monte Carlo simulation examining the effect of monomer ratios on the composition and sequence distribution of acrylonitrile(AN) copolymers with N‐vinyl pyrrolidone (NVP), itaconic acid (IA), and acrylic acid (AA) as comonomers has been developed. The Kelen–Tudos method was used to estimate monomer reactivity ratios. The results of the simulation are consistent with the academic conclusion and are as foreseen by the experimental data. The average number of NVP identical monomers in a sequence length of AN/NVP copolymer chain increases continuously and the average number of AN identical monomers in a sequence length shows a prominent decrease with an increase of NVP concentration in the feed. Changes in the monomer average number of AN/IA and AN/AA copolymers in a sequence length were the same as those of AN/NVP copolymer with an increase of comonomer concentration in the feed. The optimum weight ratio of AN with comonomers for manufacturing carbon fibers is 98/2. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 483–488, 2005     

Effect of interchain interactions on the translocation of polymer chains through small holes

The translocation of polymer chains through a small hole was simulated with the dynamic Monte Carlo method. The dependence of the relaxation time (τ₁) and escaping time (τ₂) on the chain concentration (C) was studied. The interchain interaction played an important role in the translocation process. Different behaviors were discovered at low and high C regions. τ₁ presented a power law behavior with C at low and high C values with exponents of ?1 and ?3, respectively. τ₂ was roughly independent of C at low C values but decreased with power law exponent ?1.3 at high C values. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1200–1205, 2007     

Combination of a Monte Carlo approach with the contact time distribution concept for the steady-state modeling of an isothermal heterogeneous coordinated anionic ring opening polymerization reactor

In this paper, the possibility of combining a molecular level Monte Carlo simulation with a chemical engineering model of an isothermal fixed-bed reactor is demonstrated. This approach is applied to the isothermal heterogeneous coordinated anionic ring opening polymerization of ε-caprolactone. The contact time distribution (CTD) concept as defined by Orcutt et al. (Chem. Eng. Prog. Symp. Ser. 38 (58) (1962) 1), Nauman and Collinge (Chem. Eng. Sci. 23 (1968a) 1309) and Shinnar et al. (Chem. Eng. Sci. 27 (1972) 1627) is used. The contact time is the time spent by a reactant within the pores of the pellets constituting the fixed-bed. The classical monodisperse model represents the hydrodynamic and mass transfer in the reactor. The CTD is calculated from this model according to Shinnar et al. theory. From that point, the reactor outlet monomer conversion can be calculated analytically, the ε-caprolactone consumption being a first-order process. Furthermore, the molecular size distribution (MSD) of the oligomers at the reactor outlet is computed on the basis of Monte Carlo simulations. Comparisons with experimental data are given.     

Monte Carlo simulation of polyampholyte-nanoparticle complexation

The complexation between a polyampholyte and a charged particle was investigated via Monte Carlo simulations on an off-lattice. The simulations revealed that there are three regions for the conformation of the complex formed between a positively charged particle and a polyampholyte chain. When the charge density and the size of the particle were small, the chain adsorbed on the particle surface maintained to a large extent its configuration from the bulk (spherical, dumb-bell, necklace or rod-like). By increasing the charge density and the particle size, the polyampholyte chain has collapsed on the surface. Further increases of the charge density and size of the particle caused a segregation of the beads of the sub-chains with mostly positive charges from those with mostly negative charges, those richer in positive charges being repelled by the particle. The simulation results are compared with some analytical results.     

The effects of different cement dosages, slumps, and pumice aggregate ratios on the thermal conductivity and density of concrete

Thermal conductivity coefficients of concretes made up of mixtures of pumice aggregate (PA) and normal aggregate were measured. To determine the effect of PA ratio, different cement dosage, and slumps on the thermal conductivity of concrete, 25%, 50%, 75%, and 100% pumice ratios were used in place of normal aggregate by volume, 200-, 250-, 350-, 400-, and 500-kg/m³ cement dosages, and 3±1-, 5±1-, and 7±1-cm slumps were used in this study. The analysis of the test results leads to the conclusion that PA decreased the density and thermal conductivity of concretes up to 40% and 46%, respectively. Increasing the cement dosage in the mixtures caused both density and thermal conductivity of the concrete to increase. The effect of slump on the density and thermal conductivity fluctuated.     

Complex impedance spectra of porous electrode with fractal structure

Transmission line model (TLM) is often used for the analysis of porous electrode since the current distribution in the pore can be calculated. In the present paper, the TLM is applied to the impedance analysis of complicated porous electrode with fractal structure. Activated carbon is used as electrode material for electric double layer capacitor and its pore has branch structure. Considering the fractal structure of the activated carbon, we proposed new equivalent circuit with TLM for three sizes of pores: macropore, mesopore and micropore. In this model, we defined the ratio ξ of electric double layer parts in whole inner wall of the pore because the inner wall is separated to electrolyte/electrode interface and branch base parts. Calculated electrochemical impedance of blocking electrode shows the lumped constant region in low frequency range and the distributed constant region in low frequency range. General TLM yields the straight line with slope of 45° in the distributed constant region on the Nyquist plots. In the present analysis, it was found that the slope was 22.5° at ξ = 0 in the case of double-pores (macropore and mesopore) and 11.25° at ξ = 0 in the case of triple-pores (macropore, mesopore and micropore).     

二维磁畴结构形成的蒙特卡洛方法研究

采用含长程相互作用以及各向异性能的哈密顿,应用蒙特卡罗方法对二维结构系统在不同参数下的磁结构进行了计算和模拟,发现系统的微观磁结构与长程交换耦合系数密切相关,在不同作用下系统呈现了不同的磁畴结构。