نشریه شیمی و مهندسی شیمی ایران
سال بیست و نهم شماره 2 (پیاپی 57، تابستان 1389)

In this article closed-loop behavior of non-linear unstable systems using model predictive control has been investigated. Closed-loop stability in presence of input constraint is discussed and related theorems have been stated. Two cases are considered. First it is assumed that all states are available and thereafter stability based on application of observer has been investigated. Since closed-loop performance is affected by the observer rate of convergence, in the second case, a high gain observer has been used and effect of observer gain on the system performance has been demonstrated.

Granulation is one of the drying processes in which a melt is sprayed to small size droplets through a fixed orifice system or a rotating bucket into a cooling medium which is usually air. During falling down through the uprising air, droplets will cool until the first outer layer starts to solidify. The solidified layer grows and the boundary of solid and melt moves towards the center as heat is converted to air. Fully solidified and dry particle are expected to settle on the floor of the tower.A mathematical model is proposed for the urea granulation tower of the Shiraz Petrochemical Complex. In this model the urea granulation process has been simulated by simultaneous solution of continuity, hydrodynamic, mass and heat transfer equations. The particle size distribution produced by the rotating bucket was determined by the urea product distribution obtained on the floor. Particle trajectory temperature and moisture distribution of the particles and the cooling air along the height of the tower were calculated from the mathematical model. The air temperature profile resulted from the model was compared to the plant data. The model predicted data were consistent with the plant data indicating the validity of model

In this paper mass transfer in a 1.5 m height pulsed sieve-plate extraction column have been investigated applying “plug flow”, “axial dispersion” and “stage wise backflow” models and the obtained results with experimental data have been compared. The used chemical systems were water-acetone-toluene and water-acetone-normal butyl acetate and mass transfer direction was from organic phase to aqueous phase. In spite of simplicity, the error of plug model is high. For system of butyl-acetate- acetone-water in lower zones of column, concentration variation of solute in axial direction is very fast, therefore the errors of axial dispersion and stage-wise backflow models are high. The error of all models for prediction of concentration profile of solute in continueous phase is more than dispersed phase

Green tea leaves contain anti-cancer antioxidants which can be used in drug, food, hygienic and cosmetic products. In this article, the extraction mechanism and kinetics of these components are investigated. Therefore, Diffusion coefficients of C, EC, EGC, EGCG and Caffeine of Iranian green tea leaves in water were calculated. A two diffusion coefficient model (slow and fast) was used for mass transfer modeling of the system. According to the results, the intra-particle diffusion step is the rate-controlling step of the extraction process. Furthermore, the optimum extraction conditions (temperature and time) of these components were determined as the following: Caffeine (80 °C, 20 min), C (90 °C, 80 min), EC (80 °C, 20 min), EGC (80 °C, 20 min) and EGCG (90 °C, 20 min).

In this paper the influence of hydrogen during propene polymerization with 4th generation of Ziegler-Natta catalyst was investigated in terms of variations in catalyst activity, isotacticity, melt flow index and melting temperature of resulting powder. SEM images, bulk density and particle size distribution were used to analyze the morphology of polymer and catalyst particles. Catalyst activity and isotacticity were found to significantly increase with the addition of hydrogen, but there was a maximum which after that activity and isotacticity decreased. Melting temperature also showed this trend. Melt flow index of polymers was increased by addition of hydrogen which in turn showed the molecular weight of polymers was decreased; therefore hydrogen plays an important role in chain transfer reaction. Particle size of polymers was obviously increased in the presence of hydrogen. Particle size distribution plots of catalyst and resulting polymers illustrated that replication phenomenon had occurred. SEM images demonstrated that there was core-shell structure in all resulting powders. Increase in the polymerization rate in the present of hydrogen caused to improve morphology of polymers

Industrial Pt-Sn/γ-Al2 O3 catalyst is very active and selective in the dehydrogenation of propane. The aim of this investigation was to study the effect of addition of small amount of water on the performance of industrial Pt-Sn/γ-Al2O3. Propane dehydrogenation was carried out at 575, 600 and 620 and it was formed that the optimum amount of water for these temperatures were 83.5, 125.4, 139.3 ppm, respectively. The addition of water in these optimum amounts resulted in an increase in propane conversion and propylene yield and reduction in coke formation. In the presence of water, however, the selectivity for propylene decreased and COx were produced

In the present study, the kinetic modeling of monosodium phosphate to sodium- hexametaphosphate reaction is considered in a micro-reactor. The corresponding experiments were carried out at 650-730 °C in different resident times and the experimental data i.e. monosodium phosphate conversions were recorded. A power law model equation was chosen as kinetic rate equation and the equation’s parameters were estimated from experimental data using genetic algorithm method. The comparison between the experimental data and model predictions showed that the presented model has a good agreement with experimental data with average absolute relative deviation of 3.39%.

Scaling and under deposit corrosion are serious problems of sugar plants especially at the evaporators. The deposits which are formed on the internal surfaces of the evaporator tubes prevent the heat transfer and consequently reduce the evaporation performance. To overcome the scale formation problem, antiscalants are applied. In this research the scale inhibition efficiency of four types of antiscalants (applicable at sugar plants) including: 1. Polyacrylic acid, 2. Combination of 75% polyacrylic acid and 25% aminomethylpropyl sulfonic acid, 3. Poly maleic acid and 4. Sodium salt of poly carboxylic acid, were compared. In order to estimate the inhibition performance of antiscalants, the hardness of juices was measured at the presence of antiscalants and compared with control. At the end, the performance and optimum dosages of different antiscalants were measured and compared. The results showed that sodium salt of poly carboxylic acid has 95% scale inhibition performance in the evaporators of sugar plants.

Study of the problematic aspects of heavy hydrocarbon deposition has been of a great priority for the last several decades due to its unclear sources. Oil companies have spent a huge effort to develop software in order to predict this phenomenon. One of the difficulties in gas injection process is the asphaltene precipitation for which the value should be estimated beforehand. In this paper, the amount of asphaltene deposition is determined using sweet (dry) and sour (heavy) gas injection. Several samples were tested in a slim tube using different pressure levels while the temperature was kept constant at 237˚F. In these experiments, the amount of asphaltene deposited in the porous media was measured and the effects of the asphaltene deposition on the permeability of porous media are investigated. Experimental results showed that sour gas injection causes less asphaltene precipitation compared to sweet gas injection. Moreover, the asphaltene precipitation was decreased when the pressure was higher than the minimum miscibility pressure.

In order to decrease the Kappa number and bleaching environmental impacts, the effect of several ionic surfactants on delignification and bleachability of bagasse soda pulping were investigated. Results indicated that the applying 1% of some surfactant decreased significantly the Kappa number at the same level of pulp yield. Moreover, addition of surfactant reduced alkali consumption, pulping reject and improved brightness and freeness of pulp. Compared to reference pulp, the pulp obtained with surfactant application produced higher brightness at the same level of chlorine dioxide charge during the bleaching of pulp with DoED1 and DoEpD1 sequences. Resulted pulps had a better Mechanical properties regarding to conventional reference pulp.

Extracted bentonite from different mines has various physical and chemical properties and as a result it would be possible to apply them in various requestes. The main mineral of bentonite is montmorillonite and the amount of montmorillonite, associated minerals and the type of bentonite affect on its properties. In this study, the samples were obtained from active mines and their X-ray diffraction pattern was studied, then the chemical analysis and swelling index investigated. Based on the results the samples 1 and 2 obtained from kheir Abad bentonite mine had relatively high montmorillonite content, while sample 2 was sodium bentonite and sample1 was calcium bentonite. The sample 3 taken from Horjand bentonite mine had relatively high montmorillonite content and qualitative more impurities rather than the other samples and was calcium bentonite. Sample 4 from Tang Ghoohan had relatively high montmorillonite and was calcium bentonite. Cation exchange capacity (CEC) of bentonite samples was determined using copper ethylendiamine complex and methylene blue methods. Copper ethylenediamine complex method in all of the samples was more accurate than methylene blue method. The result showed that methylene blue method was not reliable in determination of CEC of calcium bentonite.

In this study efficiency of sugarcane Baggass in removal of cadmium (П) from aqueous media was investigated. Different experimental conditions such as pH, temperature, bagass weight, particle size, exchange time, etc. were studied and optimum conditions were selected. At pH=6 Cd (П) ions have recovery over than 95%. Exchange speed is very good and has preference over classical exchangers. Also, it is able to eliminate heavy metal in ppm level and higher. Adsorption data on baggass follows both the Freundlich and Langmuir models.The data are better fitted by the Langmuir isotherm as compaired to Freundlich model.