Nashrieh Shimi va Mohandesi Shimi Iran
Year:2007 | Volume:26 | Issue:3|Papers Count:15

For large and complex reacting systems, computational efficiency becomes a critical issue in process simulation, optimization and model-based control. Mechanism simplification is often a necessity to improve computational speed. We present a novel approach to simplification of reaction networks that formulates the model reduction problem as an optimization problem and solves it using genetic algorithm (GA).The aim of simplification kinetics modeling is to derive the simplest reaction system, which retains the essential features of the full system. Numerical results for H2/O2 combustion reaction mechanism illustrate the potential and proficiency of this approach.

The analytical theory of one dimensional, dispersion free miscible displacement of oil by injection gas with nc component has shown that: the MMP is the lowest pressure at which any one of the initial oil, injection gas or crossover key tie lines becomes critical, which means that its length approaches to zero. In this paper, we propose a method for a solving multi component system based on analytical calculation of ternary systems, which simplifies and converts the multi component system into a pseudo ternary system and estimates the minimum miscibility pressure without solving complex and time consuming equations of crossover tie lines.

The major hydraulic phase in all the calcium aluminate cements including ciment fondue is CA (CaAl2O4). Once hydrated, it starts to form the hexagonal crystals of CAH10 and C2AH8 that depending on the time and temperature of hydration convert to the cubic crystals of C3AH6 and AH3. The nature, sequence, crystallinity and microstructure of hydrated phases of commercial refractory calcium aluminate cement were analyzed using differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the formation of different hydrated phases was temperature dependent. At 28 oC, the hydration products at the beginning of hydration were a gel phase, C2AH8 and gibbsite, while CAH10 also formed after four days of hydration, both C2AH8 and CAH10 were converted to C3AH6 at latter ages. The rate of conversion of C2AH8 is dependent on the temperature and time of hydration at temperatures higher than 28 oC and accelerates with increasing temperature, but still takes several weeks for completion at 36 oC.

As we know sol-gel is one of the most important techniques for thin film preparation. In this paper, high transmission silica thin films have been prepared by dip-coating process from a new silicon-alkoxide solution. The prepared sol was stable for 45 days which is very important to characterize the coating process. The optical properties as a function of aging time, withdrawal rate, and heat treatment parameters (temperature and time) have been studied.

Solubility of high molecular weight n-paraffins in supercritical carbon dioxide has been a matter of interest to many researchers. However, not sufficient solubility experimental data are available although the methods by which the experimental data are obtained have many varieties. Utilizing cubic equations of state is an effective method for solubility prediction of  n-paraffin's in supercritical fluids. In this work, five cubic equations of state (EOS) are employed to predict the solubility of six high molecular weight n-paraffins: n-tetracosane, n-pentacosane, n-hexacosane, n-heptacosane, n-octacosane and n-nonacosane, in supercritical carbon dioxide. The EOSs used are van der Waals, Redlich-Kwong and MohsenNia-Modarress-Mansoori (MMM) as two-parameter EOSs and Soave and Peng-Robinson as three-parameter EOSs. The results show that the two-parameter MMM EOS is more accurate in solubility prediction than the other EOSs.

Several investigations have been devoted towards understanding the coating process in fluidized beds. Most of these studies focused on spraying liquid droplets over the bed. However, due to the complexity of these systems, more investigations are still required to quantify the effect of operating conditions on the coating criteria. The present work aims at fundamentally understanding and precisely determining the effect of different operating conditions on coating quality in a jetting fluidized bed where the coating agent is sprayed as liquid droplets into the bed. A 3-D fluidized bed with a spraying nozzle situated in the middle of a perforated distributor plate was used for the experimental study of the effect of the operating condition on the coating quality. Results from the experimental part showed that at maximum jet gas flow rate which the jet didn’t turn into a spout and turbulent stage of fluidization the coating quality improves, but there is a limitation for the binder flow rate (NAR=4.0 E-5) which is recognized in this study and could be use for any fluidized bed coater in the same condition. Based on the experimental results, an empirical function was derived to predict the coating efficiency in different operating conditions and this function also could be used in the mathematical model.

The present study is concerned with measuring and simulating soot formation and combustion in turbulent liquid fuel spray flames. Soot concentrations inside the combustor are measured by filter paper technique. The simulation is based on the solution of the fully-coupled conservation equations for turbulent flow, chemical species kinetic modeling, fuel droplet evaporation and combustion and soot formation/oxidation. The soot formation is modeled by using the soot particle number density and the mass density based on acetylene concentrations. Two oxidation models simulate the rate of soot combustion: the O2-oxidation model, which assumes soot combustion is caused by oxygen molecules, and the O2-OH oxidation model, which assumes soot combustion occurs by both hydroxide radicals and oxygen molecules. The experimental and numerical investigations are conducted for different fuel spray cone angles. The comparison of calculated results against experimental measurements shows good agreement. Both the numerical and experimental results show that the peak value of soot and its location in the furnace depend on fuel spray cone angle. An increase in spray angle enhances the evaporating rate and increases peak temperature near the nozzle. The results also show that the OH radical has major influence on soot combustion especially while O2 oxidation is minimal.

Cu-Mo and Cu-Ti contact structures were fabricated on multi-crystalline silicon substrates to provide a low resistance ohmic contact. Deposition steps are done in an excellent vacuum chamber by means of electron beam evaporation and samples are then annealed for the realization of an efficient alloy layer. The effects of process parameters such as film thickness, annealing duration and temperature on the contact quality have been investigated and optimized for achieving the best special contact resistivity. The specific contact resistance obtained for Cu-Mo and Cu-Ti structures were 8.58x10-6 W-cm2 and 9.72x10-6 W-cm2, respectively. Finally, between the two proposed structures a comparison has been made which is resulted in the selection of Cu-Mo contact as the better structure due to its less resistance and better adhesion to the substrate.

A mathematical model of sorption of Zirconium and Hafnium cations mixture and elution of these ions from a cation exchange resin bed is developed. Sulfuric acid is selected as the eluent for selective separation of Zr and Hf in a packed bed of Dowex cation exchange resin. The column model is a dispersed plug flow model, and assumed to consist of randomly packed synthetic and spherical resin beads with constant diameter. The beads are assumed to be porous containing not only active sites on their solid structure but also solution-filled pores. The model incorporates mass transfer resistances including inter and intra-particle effects within the solid and fluid phases. Intra-particle diffusion is treated by employing pore diffusion model. The adsorption phenomena of the cations on the active sites is expressed by the equilibrium relationship characterized by a form of the extended Langmuir equation. The batch experiments are carried out in order to determine Langmuir parameters at equilibrium, and pore diffusivity at transitional conditions. These parameters are used in the dynamic model of adsorption and desorption of the mentioned cations. Unlike a number of earlier models the present mathematical model does not have assumptions such as the linearity of the sorption isotherms, uniform concentration profile in the resin particles and ideal plug liquid flow along the bed. It is found that the present model predicts the breakthrough and chromatographic curves with a fair degree of accuracy. It is concluded that the eluent  acid concentration and  flow rate of the liquid along the bed are the most two important variables which influence the efficiency of separation.

By proposing a predictive method with no adjustable parameter and by using infinite dilution activity coefficients of components in binary mixtures obtained from UNIFAC model, the binary interaction parameters (K12) in van der Waals mixing rule (vdWMR) and Orbey-Sandler mixing rule (OSMR) have been evaluated. The predicted binary interaction parameters are used in Peng-Robinson-Stryjek-Vera equation of state (PRSV-EoS) to obtain the vapor-liquid equilibrium (VLE) for six strongly polar and asymmetric binary mixtures. The binary interaction parameters evaluated by correlation of VLE experimental data and by PRSV EoS are also used in VLE calculations of the same binary mixtures. The average absolute deviations (AADs%) of the calculated results by both predictive and correlative methods are reported. The results indicate that although the correlative method has a more flexibility to fit the VLE experimental data, the AAD% of the predictive method is comparable and in some cases even better than that of correlative method.

Ferula gumosa Boiss is one of the natural plants of Iran whose exudates (Barijeh) can be used in food industry. In the present study the properties of the gum extracted from tubers of Ferula gumosa were examined. For this purpose, gum was extracted from crude exudates by alcoholic extraction with 90% (v/v) ethanol. The purity of gum was relatively high, and it was composed mostly of polysaccharide. Soybean oil emulsions (10% w/w) were prepared using gum with concentration from 0.2 to 1.0 % (w/w). Surface tension, interfacial tension and creaming measurements were performed for all emulsions. Particle size measurements, light microscopy observations and theological studies were performed for 1.0 % gum emulsion. The purified gum was found to reduce surface tension and interfacial tension of emulsions. Although particle size distribution measurements showed a slight change in particle size pattern after two month, mean diameter of particles remained constant. The creaming tests confirmed a comparatively good overall stability over a period of two months. The rheological measurements showed the shear thinning nature of emulsions prepared with the Barijeh gum. The emulsion stability was attributed to the surface adsorption mechanism.

A method based on the Genetic Algorithm (GA) was developed to study the phase behavior of multicomponent and multiphase systems. Upon application of the GA to the thermodynamic models which are commonly used to study the VLE, VLLE and LLE phase equilibria, the physically meaningful values for the Binary Interaction Parameters (BIP) of the models were obtained. Using the method proposed in this work the activity coefficients for components at infinite dilution, obtained from the local composition based models, can be accurately predicted comparing to the experimental data available in the literature. In this work, a Global Optimization Procedure (GOP) based on the GA was developed to obtain the binary interaction parameters of the Wilson, NRTL and the UNIQUAC models for a number of systems at various temperatures. The VLE, VLLE and LLE values of the binary interaction parameters for the activity coefficients models were compared with those reported in the literature for the systems studied in this work. The results showed that the values reported for the binary interaction parameters can predict the activity coefficients at infinite dilutions for the components in the VLE, VLLE and LLE systems studied in this work. In order to confirm the accuracy of the results, the values for the activity coefficients at infinite dilution in binary solutions were compared with those reported in the literature. The comparison showed that the models studied using the proposed method can predict the physically meaningful binary interaction parameters among the species present in solutions. In addition to the accuracy, simplicity, generality and short CPU computation time of the proposed method should be mentioned as some of its clearest advantages.

The first crystal structure of [Cu (terpy)2] (PF6)2 , where terpy=2,2':6',2"-terpyridine is reported. Green crystals of [Cu (terpy)2] (PF6)2 were grown by ether diffusion into an acetonitrile solution of the complex. Crystal structure of this complex is tetragonal (space group: p`4 21c) with a=8.8916 (3), c=20.0214 (13) A, and z=2. The structure was refined by using 1828 independent reflections with I>2s(I) to a R factor of 0.0809. The terpy ligands are tridentate and linked to the Cu (II) via three N atoms. The CuN6 entities have compressed octahedral geometry with the shortest Cu-N bonds that form the axial positions. This complex represents z-in distortion at 293K and has a D2d point group. The effective magnetic moment (meff) of [Cu (terpy)2] (PF6)2 was measured to be 1.95BM by Evans method. Cyclic voltammetry experiment of the complex shows that the Cu (II/I) couple is quasi-reversible.

Asphaltenes are the n-pentane or n-heptane's insoluble fractions of crude oil that remain in solution under reservoir temperature and pressure conditions. They are destabilized and start to precipitate when the pressure, temperature and/or composition changes occur during primary production. The precipitated asphaltene particles will then grow in size and may start to deposit onto the production string and/or flow lines, causing operational problems. In this paper, a combination of the modified Peng-Robinson (PR) EOS and regular solution theory is used to estimate the onset of asphaltene precipitation of some oils. The PR EOS was modified using contribution methods for the asphaltenes which are assumed to be polymeric-like compounds consisting of aggregates of monodisperse asphaltene monomers. The modified EOS with the Peneloux correction was used to estimate the molar volumes and solubility parameter of the four solubility classes (Saturates, Aromatics, Resins, Asphaltenes) of bitumen's. The predicted parameter of EOS was used to determine the onset of asphaltene precipitation from bitumen upon the addition of heptane and predictions were compared with measured onsets. Liquid-Liquid equilibrium was assumed between an oil phase and an asphaltene-rich phase. The asphaltene were divided into several pseudo components based on the Schultz-Zimm distribution function. Application of this model will help operators to better forecast the onset of asphaltene precipitation as it relates to flow rate history of the well and the pace of plugging induced by asphaltenes, and thus better plan remedial measures. The agreement between the predicted and measured onsets is very good.

The ability of natural clinoptilolite and its modified forms were studied for removal of PAHs from n-paraffin. Surfactant modified zeolite (SMZ) was obtained by modifying clinoptilolite with hexadecyltrimethylammonium ion. Homoionic forms of zeolite were obtained by ion exchange process using inorganic salts of desirable cat ions (e.g. Ag+, Fe3+…). Natural clinoptilolite and its cation exchanged forms show small affinity toward PAHs, due to their hydrophil nature, whereas SMZ removed more than 50 percent of the initial PAHs contents of n-paraffin. The structure of zeolite did not change upon modification. Regeneration of zeolite was performed efficiently with chloroform.