Iranian Journal of Chemical Engineering (IJChE)

Abstract "> Experimental measurements of overall gas holdup (εg), residence time distribution (RTD) and liquid mixing time (tm) have been carried out in an air-water system in a 17 lit bubble column with an 11 cm diameter, over a wide range of superficial gas velocity (0.14-1.46 ms-1) and height to diameter ratio (1.36-8.84). The bed expansion method was used to obtain holdup values and the change of slope related to εg versus superficial gas velocity (Usg) indicated the transition point from homogeneous to heterogeneous regime at Usg= 0.7-0.9 ms-1. Besides, the experiments illustrated that H/D ratio had no effect on holdup values. The axial dispersion model was used with semi-closed boundary conditions for prediction of RTD, and hence, the axial dispersion coefficients (Dax). Moreover, fitting results of the model and experimental RTD curves achieved from tracer injection method attained the model parameter, Dax , in three H/D ratios of 4.73, 6.36 and 8.84. Results showed that an elevation in H/D ratio, caused a rise in D ax. On the other hand, the mixing time data declined with an increase in Usg; however enhancement of H/D ratio caused an increase in tm.

Abstract In this paper, the experimental data of the onion drying process by a batch cabinet dryer is investigated. Obtained experimental data including moisture content, shrinkage, and rehydration via random factorial scheme are analyzed. Comparison of data average is carried out with the help of the multi amplitude test of Duncan. Statistical analysis of experimental data shows that time, temperature, and their combined effect have a reasonable impact on the moisture content and rehydration value of dried samples. However, a combined effect of time and temperature on the shrinkage value is not meaningful (P>0.05). The results also show that increasing time and temperature leads to a decrease in the moisture content of the samples, but it increases the value of rehydration and the shrinkage of samples.

Abstract In this study, we examined a scale-up to production of nickel perovskite catalyst, used in the conversion of natural gas to synthesis gas, using the sol-gel method in the laboratory and a bench-scale reactor. The required volume of solvent and catalyst activity in the methane-reforming reaction was determined from the optimum catalyst production conditions at the laboratory scale. This information was then used to design the bench-scale unit. We used heat-transfer models in a non-continuous bench reactor and scale-up fundamentals to achieve the same physical and chemical properties of the catalyst as that in the laboratory sample. A correlation coefficient corresponding to the experiment conditions, including the stirrer geometry, is presented based on the heat transfer equations in stirred tanks. This correlation can be used to estimate the heattransfer coefficient at larger scales, such as in a pilot reactor.

Abstract Electro-Hydrodynamics (EHD) spray is an important process in many engineering operations such as ink-jet printing, spray drying and atomization (liquid jet in air), dispersion and emulsification (liquid jet in liquid). In this work the method of electrospray was employed to form droplets from a high viscous sodium-alginate solution using constant DC voltage. The droplets were cured in calcium chloride solution to produce solid beads. The main objective was to extend the knowledge of EHD spray to high viscous and non-Newtonian liquids (1000-5000 mPa.s). However, the effects of electric field strength, nozzle diameter, flow rate and concentration of liquid were specifically investigated on the size of the beads. Among the parameters studied, voltage had a pronounced effect on the size of the beads as compared to the flow rate, the nozzle diameter and the concentration of alginate liquid. The size of beads was reduced to a minimum value by increasing the voltage in the range of 0-10 kV. However, within the middle part of this range, an unstable transition occurred from the dripping mode to the jet mode where the size distribution was wide. Increasing the height of fall of the droplets improved the sphericity of the beads, because of the increase of flight time for the droplets to relax before gelation.

Abstract In this work, the effects of air stream direction in the channels of indirect evaporative cooler (IEC) on system performance have been investigated. In addition, the dependence of system performance on outdoor air temperature and relative humidity has been studied to determine the allowable conditions for proper operation of the system, with respect to thermal comfort criteria. For this; the different types of IECs were investigated using the CFD technique. Several codes were defined in MATLAB for modeling the parallel flow, counter flow and cross flow layout. The CFD program was validated against theoretical data from the literature and good agreement between the prediction and measurement was achieved. The calculated results show that when the air relative humidity is lower than 70%, the system can prepare a good indoor condition even at 50oC, and a higher performance is achieved by using the IEC with counter current configuration. The results showed that IECs can be successfully used in hot and humid climates to fulfill the indoor thermal comfort conditions.

Abstract Common methods to design heat exchanger networks (HENs) by pinch technology usually need an evolutionary step to reduce the number of heat transfer units. This step is called loop breaking and is based on the removal of exchangers that impose minimum increase on utility consumption. Loops identification and breaking is a tedious task and becomes more complicated in large networks. This paper presents a rapid nonlinear programming (NLP) formulation for the evolution of HENs in which loop identification is not required. The objective of the NLP is the minimization of HENs annual cost, which is not considered in current methods. In this method a search is done to find the best units elimination of which improves HENs annual cost. The search continues until the minimum number of units (MNU) is achieved and the exchangers that must be removed from the network are specified. The method was applied to some networks reported in the literature and better results were obtained. Also, the convergence of the presented method is very fast and it can be applied to different networks designed by pinch technology.

Abstract Synthesis of heat exchanger networks (HENs) is inherently a mixed integer and nonlinear programming (MINLP) problem. Solving such problems leads to difficulties in the optimization of continuous and binary variables. This paper presents a new efficient and robust method in which structural parameters are optimized by genetic algorithm (G.A.) and continuous variables are handled due to a modified objective function for maximum energy recovery (MER). Node representation is used for addressing the exchangers and networks are considered as a sequence of genes. Each gene consists of nodes for generating different structures within a network. Results show that this method may find new or near optimal solutions with a less than 2% increase in Hen annual costs.

Abstract "> In this research an extensive literature search into the mechanism of cavitation dynamics and its consequences was conducted. A cavitation venturie rig was designed and constructed, and the cavitation number was checked mathematically. A new camera technique was developed to capture the cavitation cloud. Carbon steel (AISI 1020), stainless steel (AISI 304 and 316), ferritic alloy steel (B1274), brass (C27000), phosphor bronze (C51000) and aluminum bronze (C60800) test pieces were prepared and tested in the venturie tube. Aluminum bronze was the most and carbon steel the least resistant of these alloys, the measured mechanical properties could not systematically be correlated with cavitation resistance. The microscopic examinations showed that cavitation induced plastic deformation had caused the misalignment of polishing lines. A field study into the susceptibility of an Iranian sugar cane company’s centrifugal pumps showed that deviation from the manuals hydraulic settings caused cavitation and that the gray cast iron had a poor resistance. Besides the venturie tube, other configurations were considered, of which an oil industry choke valve was constructed and examined for cavitation.

Abstract > The motion and shape of a flattened drop and bubble through another continuous liquid phase (conveying phase) are investigated experimentally, using a narrow gap HeleShaw cell. Seven different liquid-liquid systems were tested. In all cases the continuous phase was the more viscous wetting phase. A number of observations on the shape and motion of the elongated flattened drops are noted and discussed. In the capillaritydominated (Ca<1.69×10-6) region, the irregular shape of the discontinuous phase drops changed with time and position, and the drop velocity was much lower than that of the conveying phase. Three different shapes of stabilized elongated drops were observed at higher Capillary numbers. The conditions that lead to the appearance of stabilized elongated drops are discussed. The velocities of these stabilized elongated flattened drops were 2 to 4.7 times higher than that of the conveying liquid. A correlation to predict the elongated flattened drop shape as a function of the dimensionless parameters governing the system is developed based on the experimental results.

Abstract An experimental study was carried out to obtain the mean convective heat transfer coefficient of aqueous carboximethyl cellulose (CMC) solutions in double-pipe heat exchangers. Co-current and counter- flows were considered in the investigation. From the experimental data a Nusselt number was calculated for a wide range of Graetz number, Reynolds number, and CMC concentrations. Increasing the Graetz and Reynolds numbers promote the heat transfer and variation of CMC concentration has a noticeable effect on the Nusselt number.

Abstract The present study attempted deriving and solving the falling- cylinder governing equations for power law model non-Newtonian fluids. Based on this theoretical study, a novel falling- cylinder rheometer (FCR) was designed and constructed to measure the Theological properties of non-Newtonian fluids. Different falling cylinders with predesigned densities were used to determine the apparent viscosity of polyvinyl alcohol (PVA) solutions in water with various concentrations. The results indicate that all PVA solutions obey the power law model with the power law index as well as the consistency index changing linearly with concentration. Increasing concentration of the solution decreases power law index, while enhances consistency index and apparent viscosity.