Sharif Digital Repository

A novel method is proposed in order to find the kinetics of dynamic recovery in the form of Avrami relation using hot flow curves. The relation is derived on the basis of variation of dislocation density with strain. The model is applied on an Al-Mg alloy  

Extraction of lipoxygenase-1 was investigated by using the aqueous two-phase system formed by sodium sulfate-poly(ethylene glycol)-buffer. The extraction was performed in a 47-mm inner diameter spray column, operating in a semibatch manner. The effects of dispersed-phase flow rate and phase compositions on fractional dispersed-phase hold up (εD) and volumetric masstransfer coefficient (KDa) were respectively studied. It was found that the hold-up and volumetric mass-transfer coefficient increased with an increase in dispersed-phase velocity and decreased with increasing phase composition. Correlations have been developed by nonlinear regression for estimation of fractional dispersed-phase... 

Hydrocracking is one of the most versatile petroleum refining processes for production of valuable products including gasoline, gas oil, and jet fuel. In this paper, a five-parameter continuous lumping model was used for kinetic modeling of hydrocracking of vacuum gas oil (VGO). The model parameters were estimated from industrial data obtained from a fixed bed reactor operating at an average temperature of 400°C and residence time of 0.3 h. Product distributions were obtained in terms of the weight fraction of various boiling point cuts. The model parameters were estimated using the Nelder-Mead optimization procedure and were correlated with temperature. Comparison of experimental and... 

In this work, a new model based on multilayer kinetic adsorption mechanism has been proposed to account asphaltene adsorption in porous media under dynamic condition and the model was verified using experimental data obtained in this work and also with those reported in the literature. In the proposed model two steps are considered for asphaltene adsorption. The first step is taken as adsorption of asphaltenes on the surface of the porous media and the second step is taken as adsorption of asphaltenes on the asphaltenes already adsorbed on the porous media. The Crank-Nicholson method, central difference in space and trapezoidal rule in time, giving second order convergence in time was... 

A reduction in catalyst's activity with time-on-stream and the formation of side products are two of the problems associated with catalytic propane dehydrogenation (PDH). Previous studies have indicated that the presence of small amounts of oxygenated additives such as water can reduce coke formation and enhance catalyst activity. The aim of the present work was to develop an appropriate kinetic model for PDH over a commercial Pt-Sn/γ-Al 2O3 catalyst in the presence of small amounts of water. Experimental data were obtained from a previous study where catalytic PDH was carried out in a bench scale reactor system at atmospheric pressure in the temperature range of 575-620°C in the presence of... 

Propane dehydrogenation was carried over a commercial Pt-Sn/γ-Al 2O3 catalyst at atmospheric pressure and reaction temperatures of 580, 600, and 620°C and WHSV of 11 h-1 in an experimental tubular quartz reactor. Propane conversions were measured for catalyst time on stream of up to nine days. The amounts of coke deposited on the catalyst were measured after one, three, six, and nine days on stream using a thermogravimetric differential thermal analyzer (TG-DTA) for each reaction temperature. The coke formation kinetics was successfully described by a coke formation model based on a monolayer-multilayer mechanism. In addition, catalyst deactivation was presented by a time-dependant... 

This research aimed at finding the factors with more impact on the bioleaching of a spent hydrocracking catalyst by thermophilic acidophilic archaea called Acidianus brierleyi and determining the optimal condition in order to reach the maximum metals recovery. Initially, four more significant factors affecting the bioleaching efficiency including pH, pulp density, inoculation and elemental sulfur concentration, were screened and identified among ten key factors using Plackett-Burman factorial design. Next, a central composite design was applied to obtain the optimum conditions for achieving the maximum efficiency of the bioleaching process. pH 1.6, pulp density 0.6% (w/v), inoculation 4%... 

In this research, an algorithm based on the Q-state Potts model is presented for modeling the austenite to ferrite transformation. In the algorithm, it is possible to exactly track boundary migration of the phase formed during transformation. In the algorithm, effects of changes in chemical free energy, strain free energy and interfacial energies of austenite-austenite, ferrite-ferrite and austenite-ferrite during transformation are considered. From the algorithm, the kinetics of transformation and mean ferrite grain size for different cooling rates are calculated. It is found that there is a good agreement between the calculated and experimental results  

The current study was carried out to investigate the kinetics of reactions for the pressure oxidative leaching of a molybdenite concentrate in less than one hour. The effects of oxygen pressure, stirring speed, pulp density, acid concentration and temperature on the leaching rate of the molybdenum were studied. It was found that about 85% of molybdenite was oxidized to molybdic oxide precipitate with remaining molybdenum dissolved in the leaching liquor. Analysis of experimental data showed that the reaction is chemically controlled with activation energy of 68.8 kJ/mol. Furthermore SEM images showed no significant boundary diffusion layer and it was noted that the molybdic oxide... 

In this work, the effect of carbon dioxide partial pressure on the calcination kinetics of high purity zinc carbonate hydroxide has been studied. Non-isothermal analysis has been performed on samples at different CO 2 partial pressures by TGA and DTA. It has been found that the calcination behaviour of this material corresponds to the shrinking core model and the reaction mechanism is phase boundary controlled. The calcination reaction of zinc carbonate hydroxide starts at 240 °C. Increasing the carbon dioxide partial pressure can result in an increase in the reaction start temperature of up to 30°C. The activation energy for the reaction is calculated as 180 ± 5 kJ/mol at... 

Progressive conversion/shrinking core (PC-SC) models of constant-size cylinders were exploited to interpret the decarburization reactions of MgO-C-Si bricks heated up under blown air. Chemical adsorption/solid (or pore) diffusion mechanisms governed the reaction rate. With 5% silicon, chemical adsorption vanished at 1000 and 1100°C. The oxidation rate lowered then with temperature. This was due apparently to the blocking of the pore-end gorges by the voluminous compounds (like Forstrite). Arrhenius plots of the specific rates yielded the activation energies of the prevailing steps. Without Si antioxidant, three steps were appreciated having activation energies of 51.65 (for chemical... 

In micron-scale, powder consolidation process is driven by diffusion phenomenon, while in nano-scale the higher surface energy of particles leads to some anomalous behaviors within the process. In order to investigate the nano-sintering occurrence, an atomistic approach is employed via molecular dynamics simulations. Within this approach, the effect of particle size and temperature is examined. The study of particle structure emphasizes on a transition on the governing mechanism of process depending on the material energy levels. The results show that in a specific particle size at low temperatures, the main sintering mechanism is the plastic deformation, while at elevated temperatures it... 

In this study, kinetics, equilibrium, and mechanisms of SDBS adsorption onto carbonate rock in presence/absence of alkaline/electrolyte, which is not well discussed in the available literature, is analyzed through batch experiments. Analysis of kinetic data showed that adsorption rate of SDBS onto carbonate is controlled by both boundary layer and intraparticle diffusion, also adsorption kinetics meets pseudo second-order model. The coefficient of kinetic model is a linear function of initial and equilibrium concentrations. The adsorption isotherm experiences four distinct regions, with a rising trend in the first regions until reaching to a maximum after which decreases slightly, as the... 

Tributhyl phosphate (TBP) decomposition in supercritical water oxidation (SCWO) was performed with and without catalyst, at different temperatures ranging from 370 to 480 °C, and different reaction times. Ag2O, CuO, Fe2O3, MgO, and ZnO synthesized by supercritical water were examined in TBP decomposition as the catalyst. TBP structure decomposed under the non-catalytic reaction; nevertheless, the results indicated that the use of catalysts improved the reaction efficiency, which calculated based on total organic carbon (TOC) removal, by far. However, TOC removal using Fe2O3 was 20% higher than the non-catalytic reaction. Having the ability in completion of the redox cycle, iron oxide... 

In this study, kinetics, equilibrium, and mechanisms of SDBS adsorption onto carbonate rock in presence/absence of alkaline/electrolyte, which is not well discussed in the available literature, is analyzed through batch experiments. Analysis of kinetic data showed that adsorption rate of SDBS onto carbonate is controlled by both boundary layer and intraparticle diffusion, also adsorption kinetics meets pseudo second-order model. The coefficient of kinetic model is a linear function of initial and equilibrium concentrations. The adsorption isotherm experiences four distinct regions, with a rising trend in the first regions until reaching to a maximum after which decreases slightly, as the... 

Al-Si3N4 couples were heat-treated at 850-1150°C for 250 hours. The thickness of the interacted area was measured by scanning electron microscopy (SEM) and scanning/transmission electron microscopy (TEM/STEM). The interaction rate increases exponentially with inverse temperature, with an activation energy of 194.23 kJ/mol and diffusion pre-coefficient of 5 × 10−9 m2/s, indicating that the interaction is diffusion-dependent. As the results showed, the interfacial area is comprised of Al alloy channels, Si precipitates, and AlN grains. Al-Si transfer through the solid solution (Si3-xAlxN4-y) at the interface of Al alloy and β-Si3N4 grains controls the kinetic of the interaction. When... 

Objective: To assess adequacy of the National Institute for Occupational Safety and Health (NIOSH) Lifting Equation (NLE) in controlling lumbar spine loads below their recommended action limits during asymmetric load-handling activities using a detailed musculoskeletal model, that is, the AnyBody Modeling System. Background: The NIOSH committee employed simplistic biomechanical models for the calculation of the spine compressive loads with no estimates of the shear loads. It is therefore unknown whether the NLE would adequately control lumbar compression and shear loads below their recommended action limits during asymmetric load-handling activities. Method: Twenty-four static stoop lifting... 

Gaseous reduction of manganese ores has been studied as it is beneficial for both understanding the common ferromanganese production process and the development of new processes. Thermodynamics, kinetics, and the mechanisms of reduction by methane have been reviewed, supported by thermodynamic calculations from the commercial HSC Chemistry and FactSage software. While there were similarities in reduction process by CO and H2, methane was a more efficient reductant and yielded reduction levels beyond MnO, which was due to the large thermodynamic driving force for carbide formation by metastable methane. The practical aspects of gaseous reduction by methane, such as application of a... 

In this paper, the performance of immobilized packed bed glucose isomerase enzyme was mathematically modeled. A modified Michaelis-Menten type relation was used to describe the enzyme kinetics. Mass transfer inside the biocatalyst particle and through the bed column was analyzed simultaneously. Using measured data, physicochemical properties including diffusivity, viscosity and density of sugar solutions were correlated with its concentrations and were used to provide precision in solving the set of model equations. Model equations were solved using the Runge-Kutta and Gauss-Seidel algorithms and finite difference numerical method in MATLAB environment. Model output was used to demonstrate... 

In the present work, the synergetic effect of Ti-based catalysts (TiH2 and TiO2 particles) on hydrogen desorption kinetics of nanostructured magnesium hydride was investigated. Nanostructured 84mol% MgH2-10%mol TiH2-6%mol TiO2 nanocomposite powder was prepared by high-energy ball milling and subjected to thermal analyses. Evaluation of the absorption/desorption properties revealed that the addition of the Ti-based catalysts significantly improved the hydrogen storage performance of MgH2. A decrease in the decomposition temperature (as high as 100°C) was attained after co-milling of MgH2 with the Ti-based catalysts. Meanwhile, solid-state chemical reactions between MgH2 and TiO2 nanoparticles...