Navid Mostoufi

Sequential Modeling of Fluidized Bed Paddy Dryer

Navid Bizmark — Mon, 09 Aug 2010 01:08:40 PDT

A sequential method was developed to model a continuous plug flow fluidized bed dryer. The method is based on dividing the dryer into sections in series with ideal mixing for both solid and gas phases in each section. In order to determine the proper number of sections, drying experiments were carried out using paddy at different operating conditions. It was shown that the number of sections can be correlated to Damköhler number, which includes kinetic and hydrodynamic parameters of the process. The model is able to predict the particle moisture profile along the bed as well as the moisture content of dried product. It was shown that the model fits the experimental data satisfactorily with the correlation coefficient of 0.989. Moreover, the model was tested against available data in literature at different scales and operating conditions for which an error of less than 4.5% was observed in predicting the paddy moisture content at the outlet.

Kinetic modeling of propylene homopolymerization in a gas-phase fluidized-bed reactor

Ahmad Shamiri — Mon, 09 Aug 2010 01:04:25 PDT

A comprehensive mechanistic model describing gas-phase propylene polymeriza¬tion is developed. The kinetics of polymerization is based on a multiple active site for Ziegler-Natta catalyst. The model considers the polymerization reaction to take place in both bubble and emulsion phases. The developed model was used to predict polymer production rate, number and weight average molecular weights, polydispersity index (PDI) and melt flow index (MFI). Results showed that by increasing the superficial gas velocity from 0.1 to 0.7 m/s the proportion of the polymer produced in the bubble phase increases from 7.92% to 13.14 % which highlights the importance of considering the existence of catalyst in the bubble phase. Comparing the developed model with published models of the same reactor revealed that the polymer productivity will be higher using the new model at high catalyst feed rate.

Investigating the synergistic effect of D2EHPA and Cyanex 302 on separation of zinc and manganese

Tahereh Hosseini — Mon, 09 Aug 2010 01:01:15 PDT

Synergistic effect of Cyanex 302 on extraction of zinc and manganese with D2EHPA in sulfate media was investigated. Experiments were carried out in the pH range of 1.0–5.0, temperature of 23, 40 and 60 C with sole D2EHPA and Cyanex 302 as extractant and D2EHPA to Cyanex 302 ratios of 1:3, 1:1 and 3:1. The experimental results showed that the co-extraction of zinc and manganese increased with increasing equilibrium pH and temperature. Increasing the D2EHPA to Cyanex 302 ratio in the organic phase, caused a left shifting of extraction isotherm of zinc and right shifting of extraction isotherm of manganese. Thus, a better separation of zinc over manganese was achieved. At low pHs, separation factor is low when pure D2EHPA is used as extractant, however, using Cyanex 302 as synergist, separation factor increases and results in a better separation of zinc from manganese. Stoichiometric coefficient of zinc for single D2EHPA and Cyanex 302 and their mixture was calculated to be close to 6.

Effect of surface contaminants on oxygen transfer in bubble columns

Maryam Asgharpour — Mon, 09 Aug 2010 00:58:27 PDT

Gas hold-up (εg), sauter mean bubble diameters (d32) and overall oxygen transfer coefficient (kLa) were evaluated at four different alkane concentrations (0.05, 0.1, 0.3 and 0.5 vol.%) in water over the range of superficial gas velocity (ug) of 1.18-23.52×10-3 m/s at 25 ºC in a laboratory-scale bubble column bioreactor. Two types of anionic and non-ionic surfactants were also employed in order to investigate the effect of addition of surfactants to organic-aqueous systems on sauter mean bubble diameter, gas hold-up and overall oxygen transfer coefficient. Influence of addition of alkanes on overall oxygen transfer coefficient and gas hold-up, was shown to be dependent on the superficial gas velocity. At superficial gas velocity below 2.5×10-3 m/s, addition of alkane in air-water medium has low influence on overall oxygen transfer coefficient and also gas hold-up, whereas; at higher gas velocities slight addition of alkane increases overall oxygen transfer coefficient and also gas hold-up. Increase in concentration of alkane resulted in increase in overall oxygen transfer coefficient and gas hold-up and roughly decrease in sauter mean bubble diameter, which was attributed to an increase in the coalescence-inhibiting tendency in the presence of surface contaminant molecules. Bubbles tend to become smaller with decreasing surface tension of hydrocarbon, thus, overall oxygen transfer coefficient increases due to increasing of specific gas–liquid interfacial area (a). Empirical correlations were proposed for evaluating gas hold-up as a function of sauter mean bubble diameter, superficial gas velocity and interfacial surface tension as well as evaluating Sherwood number as a function of Schmidt, Reynolds and Bond numbers.

Leaching of vanadium from LD converter slag using sulfuric acid

Mohsen Aarabi — Mon, 09 Aug 2010 00:56:30 PDT

In order to extract vanadium from LD (Linz-Donawitz) converter slag of steelmaking plant and, an alkaline roasting-acid leaching study was carried out and effect of different parameters on the kinetics of vanadium dissolution was determined. The leaching residue was characterized by XRD, XRF and SEM/EDX analyzer. The maximum vanadium recovery of ca. 95% was achieved at the optimum leaching condition of 70 ºC, S/L: 1/15, acid concentration: 3M and leaching time: 150 minutes. It was shown particle size has a significant effect on the dissolution of vanadium and maximum extraction was achieved at the finest size of below 0.850 mm. Dissolution of vanadium in sulfuric acid showed that there are two stages in the kinetics of leaching. In the first 15 minutes, a sharp increase in the amount of vanadium extracted was observed and at longer times, leaching became slower. Shrinking core model (SCM) was used to describe the kinetics of the slag acid leaching. SCM equations were modified to represent the long time leaching process in which the initial recovery of vanadium (at the beginning of long term period) is not zero. It was found that the kinetics of leaching at low temperature is controlled by chemical reaction for both short and long leaching periods. Rate of leaching is controlled by solid product diffusion regime at high temperature.

Prediction of the maximum heat transfer coefficient between a horizontal tube and gas–solid fluidized beds

Nima Masoumifard — Mon, 09 Aug 2010 00:53:57 PDT

Average heat transfer coefficient between a horizontal tube and fluidized bed reaches a maximum at an optimum superficial gas velocity. Prediction of the optimum superficial gas velocity and maximum heat transfer coefficient in gas-solid fluidized beds was obtained based on a cluster based mechanistic model which properly takes into account the thermal and fluid dynamic properties of the solids and the fluidizing gas. Experiments were carried out in a 15 cm diameter fluidized bed using 280, 490 and 750 μm sand particles fluidized by air with a 8 mm diameter horizontal tube immersed in the bed. The predicted values were compared with the experimental data of this work as well as those from the literature in a wide range of operating conditions with a good agreement. The model presented in this work for prediction of maximum heat transfer coefficient and optimum superficial gas velocity is considerably more reliable than existing correlations.

Non-intrusive characterization of fluidized bed hydrodynamics using vibration signature analysis

Mojgan Abbasi — Mon, 09 Aug 2010 00:50:28 PDT

There are many techniques to characterize the hydrodynamics of fluidized beds but new techniques still are needed for more reliable measurement. Bed vibrations were measured by an accelerometer in a gas-solid fluidized bed in order to characterize the hydrodynamics of the fluidized bed in a non-intrusive manner. Measurements were carried out at different superficial gas velocities and particle sizes. Pressure fluctuations were measured simultaneously. Vibration signals were processed using statistical analysis. For the sake of the evaluation, the vibration technique was used to calculate minimum fluidization velocity. It was shown that minimum fluidization velocity can be determined from the variation of standard deviation, skewness and kurtosis of vibration signals against superficial gas velocity of the bed. Kurtosis was proved to be a new method of analyzing vibration signals. Results indicate that analyzing the vibration signals can be an effective non-intrusive technique to characterize the hydrodynamics of fluidized beds.

Hydrodynamic characteristics of gas-solid fluidization at high temperature

Shabnam Sanaei — Mon, 09 Aug 2010 00:48:00 PDT

Effect of temperature on the hydrodynamics of bubbling gas-solid fluidized beds was investigated in this work. Experiments were carried out at different temperatures ranged of 25-600 °C and different superficial gas velocities in the range of 0.17-0.78 m/s with sand particles. The time-position trajectory of particles was obtained by the radioactive particle tracking technique at elevated temperature. These data were used for determination of some hydrodynamic parameters (mean velocity of upward and downward-moving particles, jump frequency, cycle frequency and axial/radial diffusivities) which are representative to solids mixing through the bed. It was shown that solids mixing and diffusivity of particles increases by increasing temperature up to around 300 °C. However, these parameters decrease by further increasing the temperature to higher than 300 °C. This could be attributed to the properties of bubble and emulsion phases. Results of this study indicated that the bubbles grow up to a maximum diameter by increasing the temperature up to 300 °C, after which the bubbles become smaller. The results showed that due to the wall effect, there is no significant change in the mean velocity of downward-moving clusters. In order to explain these trends, surface tension of emulsion between the rising bubble and the emulsion phase was introduced and evaluated in the bubbling fluidized bed. The results showed that surface tension between bubble and emulsion is increased by increasing temperature up to 300 °C, however, after that it acts in oppositely.

Flow characteristics in an airlift membrane bioreactor

Amirhossein Khalili — Mon, 09 Aug 2010 00:43:51 PDT

Eulerian, two-fluid CFD simulations were carried out to investigate the hydrodynamic features of changing the level of liquid that are responsible for flux enhancement in the airlift submerged flat-sheet membrane system. The proposed model is in good agreement with the experimental results in terms of gas hold-up. It was shown that by decreasing the liquid level, the intensity of mixing and distribution of liquid phase velocity in the riser was improved while the shear stress on the membrane surface was reduced. With the lower liquid level, the shear stress distribution restricted only to certain parts of the membrane, whereas at higher level of liquid the shear stress spreads over a large part of the membrane. Wall shear stress is probably the major factor responsible for reduction of fouling in membrane systems. Greater shear stress of the liquid will minimize the extent to which particles will settle on the membrane, thus, fouling will be reduced and flux will be enhanced. This study also shows that imaging technique provides valuable information for investigation of the hydrodynamic properties of MBRs. By analyzing specific properties of the bubbles like the bubble diameter and rising velocity in the recorded images, it is possible to make an estimation of: gas hold-up, bubble size distribution, and etc. Based on the results obtained in this study, it is thought that the performance of the airlift MBR can still be improved.

Modeling and optimization of the sugar extraction process

Rahmat Sotudeh-Gharebagh — Mon, 09 Aug 2010 00:39:59 PDT

The dynamic modeling of sugar extraction process from beet was carried out and the resulting first order partial differential equations written for three main constituents of the system (Sugar, Non-sugars and Water), were solved numerically by an implicit finite difference algorithm. The model is able to predict the axial concentration profiles and the process variables such as Brix, Purity and the percentage of sugar and non-sugar components extracted during the process at the exit streams. The results of the model were compared with the experimental data obtained from an industrial plant and a close agreement was observed. In an industrial extraction plant, it is always desired to maintain a constant and uniform flow of materials entering the tower to ensure that the properties of the effluent streams would not be changed. To achieve this, a Proportional Integral (PI) Controller is installed in the process to control the liquid level in the tower.

Non-intrusive monitoring of bubbles in a gas-solid fluidized bed using vibration signature analysis

Mojgan Abbasi — Mon, 09 Aug 2010 00:36:50 PDT

A reliable method was developed to study bubble behavior by analysis of vibration signals in fluidized beds. The advantage of this method is that the vibration probe is in indirect contact with the process. Accelerometers were used to record vibration signals generated by particle flow through the fluidized bed at various superficial gas velocities and particle sizes. Measurement of vibration signals, sampled at 25 KHz for 30 s, enabled investigation of changes in flow structure related to flow regime transitions. To study bubble behavior under different conditions, different particle sizes were used in the experiments. The measurements were extensively analyzed using wavelet and Fast Fourier transforms. Results indicate that the vibration frequency generated by bubbles is between 1000 and 3000 Hz. The vibration analysis was effectively used to detect minimum fluidization and transition from bubbling to slugging in gas-solid fluidized beds.

Determination of hydrodynamic behavior of gas-solid fluidized beds using statistical analysis of acoustic emissions

Nasim Salehi-Nik — Mon, 09 Aug 2010 00:34:34 PDT

In the processes involving the movement of solid particles, acoustic emissions are caused by particle friction, collision and fluid turbulence. Particle behavior can therefore be monitored and characterized by assessing the acoustic emission signals. Herein, extensive measurements were carried out by microphone at different superficial gas velocities with different particle sizes. Acoustic emission signals were processed using statistical analysis from which the minimum fluidization velocity was determined from the variation of standard deviation, skewness and kurtosis of acoustic emission signals against superficial gas velocity. Initial minimum fluidization velocity, corresponding to onset of fluidization of finer particles in the solids mixture, at which isolated bubbles occur, was also detected by this method. It was shown that the acoustic emission measurement is highly feasible as a practical method for monitoring the hydrodynamics of gas-solid fluidized beds.

Effect of geometry of the plenum chamber on gas distribution in a fluidized bed

Ali Mohammadkhah — Mon, 09 Aug 2010 00:31:52 PDT

Computational fluid dynamic technique was used to simulate a fluidized bed with eight different plenum chambers of different design. Effect of geometry of the plenum chamber on velocity profile on the distributor and pressure drop across the distributor was investigated. The plenums were selected such that the influence of characteristics such as body shape, direction and location of gas entrance, entrance type, existence and shape of internal baffle could be studied. Results indicate that the geometry of the body and existence of the baffle are the most important characteristics affecting the flow uniformity above the distributor. It was shown that the shape of the baffle does not have a remarkable impact on velocity profile. Direction and location of the gas entrance have a significant effect on the distribution quality. Downward injection of gas and placing the gas entrance far enough from the distributor increases the uniformity of flow over the distributor. It was shown that geometry of the plenum chamber affects the total pressure drop across the distributor. A correction factor was introduced to quantify the effect of geometry of the plenum on pressure drop of the perforated plate distributor.

A shrinking particle - shrinking core model for leaching of a zinc ore containing silica

Vida Safari — Mon, 09 Aug 2010 00:30:02 PDT

A new mathematical model was developed for leaching of zinc ores containing silicates such as hemimorphite which produce a gel during leaching with sulfuric acid. This model is based on the shrinking core model in which the particle size and the reacting core shrink simultaneously. It was shown that the actual dissolution time of the ore particles is longer than the time corresponding to the dissolution of chemical zinc oxide itself. It was suggested that because of the existence of silicates in the ore, a gelatinous layer was formed around the reacting core. Since the gel product is soft, it breaks apart when the particles collide and as a result, the particles shrink. However, a thin gelatinous layer always covers the reacting core which increases the mass transfer resistance and increases the leaching time. This model was applied to leaching of a zinc-rich tailing containing hemimorphite and the thickness of the gelatinous layer as well as the diffusion coefficient in this layer was determined.

Modeling of stagewise feeding in fluidized bed reactor of oxidative coupling of methane

Mehdi Daneshpayeh — Mon, 09 Aug 2010 00:26:45 PDT

Influence of distributed reactant injection on the performance of fluidized bed reactor of oxidative coupling of methane (OCM) was investigated by means of reactor modeling and simulation. A two-phase hydrodynamic model was coupled with a comprehensive kinetic model of OCM over Mn/Na2WO4/SiO2 catalyst to predict the performance of the reactor. In the stage-wise feeding, flow of air is divided into few portions. One portion is introduced to the fluidized bed through the distributor at the bottom and other portions are injected inside the bed. It was shown that at low temperatures C2 yield in the stage-wise feeding configuration is higher than the conventional co-feeding whereas at high temperatures, yield of products in stage-wise feeding is lower than co-feeding. Therefore, optimum operating temperature of the reactor can be decreased from 800 C in co-feeding to 750 C with two secondary air injections, while keeping the same conversion and yield. Moreover, it was shown that oxygen distribution along the bed decreases making the reactor performance to be less sensitive to change in temperature. In general, when applying stage-wise feeding configuration, the reactor can operate at lower temperature, effect of temperature change on the performance of the reactor would be less and removing the heat of reaction becomes more effective when compared to co-feeding configuration. These results in widening the safe range of operating conditions, providing a more stable process with better temperature control and reducing other problems of high temperature processes in the OCM fluidized bed reactor.

Probabilistic approach to particle-wall contact time in fluidized beds

Reza Zarghami — Mon, 09 Aug 2010 00:24:32 PDT

Heat transfer between submerged surface and particles in fluidized bed is affected by particles migration to and from the exchanger surface. A new probabilistic model was developed for particles migration and particle-wall contact time distribution based on a classical gamma function. The existing models suggest a decrease of contact time with increasing the gas velocity. However, it has been experimentally shown that the contact time increases in turbulent regime by increasing the gas velocity. A theoretical probabilistic model was developed to represent such a trend. The model is in good agreement with the experimental data.

Clusters identification and characterization in a gas-solid fluidized bed by the wavelet analysis

Forough Afsahi — Mon, 09 Aug 2010 00:14:28 PDT

The local solid flow structure of the bubbling fluidized bed of sand particles was investigated in order to identify and characterize the clusters. Extensive experiments were carried out using an optical fiber probe, measuring the velocity and the diameter of clusters. Under all operating conditions, ascending and descending clusters co-existed at all measurement locations. The locus of the inversion point at which the directions of cluster motion changed was determined. The velocity of the ascending clusters was a function of both superficial gas velocity and the radial and axial position. With increasing superficial gas velocity, both the velocity and the diameter of ascending clusters decreased near the wall. However, the velocity of descending clusters depended mainly on superficial gas velocity and the largest clusters existed closer to the wall. The results of this study help to explain cluster hydrodynamics in fluidized beds.

Kinetic modeling of oxidative coupling of methane over Mn/Na2WO4/SiO2 catalyst

Mehdi Daneshpayeh — Mon, 09 Aug 2010 00:10:05 PDT

A comprehensive kinetic model for oxidative coupling of methane (OCM) on Mn/Na2WO4/SiO2 catalyst was developed based on a microcatalytic reactor data. The methane conversion and ethylene, ethane, carbon monoxide and carbon dioxide selectivities were obtained in a wide range of operating conditions including 750

Dynamic optimization of the benzene extractive distillation unit

Azadeh Ghaee — Mon, 09 Aug 2010 00:06:25 PDT

A mathematical model has been developed for describing the dynamic operation of the N-formylmorpholine extractive distillation column and the corresponding solvent recovery column in the benzene extraction plant. The NRTL equation was used to calculate the equilibrium and thermodynamic properties of the mixtures. The validity of the model in terms of temperature, pressure and split fraction was examined using actual plant data at steady-state conditions. Comparison between model results and plant data shows good consistency. In order to improve the control of the process and selection of the optimal control strategy, the model was used to find the optimum values of the constants of the controllers with nelder-mead algorithm during unsteady-state operation by minimizing the deviation from steady-state conditions. The outcome of this study could be used by operators and engineers to increase the productivity of the unit.