Copyright (c) 2008 All rights reserved. http://works.bepress.com/navid_mostoufi Recent documents in Navid Mostoufi en-us Wed, 17 Dec 2008 04:29:20 PST 3600 http://works.bepress.com/navid_mostoufi/35 http://works.bepress.com/navid_mostoufi/35 Tue, 16 Dec 2008 01:49:08 PST Nonlinear time series analysis techniques were applied to predict pressure fluctuation data in fluidized beds in two different hydrodynamic states. The method of delays was used to reconstruct the state space attractor to carry out analysis in the reconstructed state space. The state space reconstruction parameters, i.e., time delay and embedding dimension, were determined and the results shown that their values were different for various types of methods introduced in the literature. Chaotic behavior and predictability of fluidized system were determined by introducing two nonlinear dynamic invariants, correlation dimension and entropy, in different ways. The traditional linear autoregression method and state space based prediction methods (SSBPMs), i.e., nearest neighbors and locally linear, and global linear methods, were applied to predict the pressure fluctuation signals. The quality of prediction was assessed by comparison of the predicted data with its original benchmark. In addition, the dynamic invariants of measured and predicted attractor of the pressure signals were compared. The results showed that SSBPMs are preferred to the traditional linear methods. Finally, a continuous uncertainty band of pressure signals of single and multiple bubble regimes for the prediction methods was presented. R. Zarghami Fluidization http://works.bepress.com/navid_mostoufi/34 http://works.bepress.com/navid_mostoufi/34 Tue, 16 Dec 2008 01:46:23 PST A practical population balance model was used to evaluate the bubble size distribution in a bubble column. In addition, the bubble size distribution in the bubble column was measured at different gas velocities by photography and analysis of the pictures. Four types of liquid, i.e., water and three petroleum-based liquids, were used in the experiments. The gas phase was air. It was found that the existing models in the literature are not able to satisfactorily predict the experimentally measured bubble size distribution. The model can be corrected by applying a correction factor to the energy dissipation rate. The corrected model fits the experimental bubble size distribution considerably better than the existing models. The variation of this correction factor is reported for different systems at different gas velocities. S. Homayouni Process Modeling http://works.bepress.com/navid_mostoufi/33 http://works.bepress.com/navid_mostoufi/33 Tue, 16 Dec 2008 01:42:47 PST Experiments have been carried out in a fluidized bed in order to verify the influence of the axial position, particle diameter and the superficial gas velocity on the heat transfer coefficient from a small horizontal tube (Dt = 8 mm) immersed in the fluidized bed. The solid particles used were 280, 490 and 750 lm diameter sand particles, fluidized by air. The experimental results showed that the heat transfer coefficient is increased with increasing the gas velocity, up to a maximum, and then decreases with a slight slope. The heat transfer coefficient was found to decrease by increasing the particle size. The probe position had less influence on the heat transfer coefficients. In order to predict the heat transfer coefficient from the fluidized bed to a horizontally immersed tube, a cluster based model has been proposed. The model predictions were compared with the experimental data of this work as well as those from the literature in a wide range of operating conditions. A close agreement was found between the model predictions and the experimental findings. A. A. Hamidi Fluidization http://works.bepress.com/navid_mostoufi/32 http://works.bepress.com/navid_mostoufi/32 Tue, 16 Dec 2008 01:37:11 PST Cluster formation in the risers of circulating fluidized beds has been verified by experimental works of different investigators. In this work, a model has been developed based on the force and mass balances over the riser and core cross section in order to characterize the core-annulus structure of the fully developed zone by the cluster-based approach. The clusters are considered to be spherical with constant diameter. Radial profiles of cluster velocity, solid mass flux, and solids holdup have been obtained by this model as the intermediate variable. The comparison is made between the results of the model and the experimental data and modeling reported in the literature. The model is able to explain the experimental data satisfactorily, and the core radius which is the key parameter in the core-annulus flow structure has been estimated. The results of this model could be used to understand the flow structure in the riser of a circulating fluidized bed largely adapted for industrial applications. Navid Mostoufi Fluidization http://works.bepress.com/navid_mostoufi/31 http://works.bepress.com/navid_mostoufi/31 Tue, 16 Dec 2008 01:34:29 PST A population balance model is developed to investigate the particle size distribution (PSD) developments in a gas-phase fluidized bed ethylene polymerization reactor. The model considers the combined effects of particle growth and elutriation for size-distributed prepolymer feed. In the proposed model, the bed is divided into several perfectly mixed serial sections. The population balance differential equations written for each section were simultaneously solved to determine the density function of the size distrubution of the polymer particles in each section. The model is able to predict the profiles of the PSD along the reactor height. It was shown that the mean size of the particles is larger at the bottom of the bed and becomes smaller when moving toward the top of the reactor. The size distribution of the polymer particles in the product becomes sharper and their mean size decreases by increasing the superficial gas velocity. Also, the mean size of the particles in the product increases by increasing the temperature of the reactor. Omid Ashrafi Fluidization http://works.bepress.com/navid_mostoufi/30 http://works.bepress.com/navid_mostoufi/30 Tue, 16 Dec 2008 01:31:29 PST In this study, the influence of hydrodynamic models on dynamics of the fluidized bed polyethylene production process has been investigated via modeling. The overall fluidized bed model consists of two integrated sub-models. The hydrodynamic sub-models, which are the simple and dynamic two-phase, well mixed and tanks in series models, were used in the study along with the reaction submodel from the literature. The hydrodynamic sub-models differ from each other in terms of flow structure and the presence of solids in the disperse phase. The conventional PID controllers were used to control the reactor conditions for these different hydrodynamic conditions. The results of this study showed that a choice of a hydrodynamic model for the fluidized bed polyethylene reactor has a minor effect on the dynamic response and the grade transition of the polyethylene production process and a simple hydrodynamic sub-model can be used for the proper dynamic modeling and control of the process. A. Sarvaramini Fluidization http://works.bepress.com/navid_mostoufi/29 http://works.bepress.com/navid_mostoufi/29 Tue, 16 Dec 2008 01:28:59 PST A two-phase flow model is adapted in order to predict the performance of a fluidized bed reformer using the sequential modular simulator. Since there are physical and chemical phenomena interacting in the reformer, two sub-models appear to be necessary to describe the overall model. These are the hydrodynamic and reaction sub-models. The hydrodynamic sub-model is based on the dynamic two-phase model and the reaction sub-model is derived from the literature. In the overall model, the bed is divided into several sections. At each section, the flow of the gas is considered as plug flow through the bubble phase and to be perfectly mixed through the emulsion phase. Two sets of experimental data from the literature at different hydrodynamic regimes were used in order to validate the proposed model. A close agreement was observed between the model predictions and the experimental data. The model proposed in this work may be used as a framework for the development of sophisticated models for non-ideal reactors inside process simulators. R. Habibi Fluidization http://works.bepress.com/navid_mostoufi/28 http://works.bepress.com/navid_mostoufi/28 Tue, 16 Dec 2008 01:25:13 PST A hybrid optimization technique, GA-SQP, is developed in which the genetic algorithm (GA) which is a stochastic method is combined with the sequential quadratic programming (SQP) method which is a deterministic method. This method was used to determine the kinetic parameters of the set of highly nonlinear hydrogenation reactions. Catalyst deactivation was also taken into account. The ability of GA and SQP in solving this type of problem was investigated. It was shown that although the SQP is fast, it is not able to solve this problem properly and is very sensitive to the choice of initial point. The GA was able to solve the problem after a large number of generations. It was shown that the new GA-SQP hybrid method is able to determine the final solution considerably faster than the GA while it is not sensitive to the initial point. Behrang Mansoornejad Process Modeling http://works.bepress.com/navid_mostoufi/27 http://works.bepress.com/navid_mostoufi/27 Tue, 16 Dec 2008 01:22:18 PST Dynamic simulation and optimization of a hydrogenation reactor system were developed and investigated in the present work. The process mainly consists of three adiabatic fixed bed hydrogenation reactors in series in addition to one heater before the first reactor and three coolers after each reactor for interstage cooling. The feed flow rate to the unit, the feed temperature or the carbon monoxide content of the feed may change and cause variations of the outlet temperature of reactors. Therefore, it is essential to control the inlet temperature of each reactor. There is a temperature controller before each reactor and also one after the third reactor. The tuning parameters of the controllers were optimized in three different cases, taking into account the process constraints. In each case, a special disturbance was forced to the process. Because of catalyst deactivation, the inlet temperature of reactors must be increased during the running period in order to have the desired conversion. Therefore, the optimal inlet temperature profile was determined based on SOR, MOR and EOR data. Optimization of the process was done by the use of a hybrid GASQP method. The new hybrid method was developed to overcome the difficulties of both methods. The genetic algorithm (GA) which is a stochastic method, is relatively slow, but is not sensitive to the initial point. In contrast, sequential quadratic programming (SQP) method is a deterministic method which is fast, but very sensitive to the initial point and gets trapped in local optima. In the newly developed hybrid method, the SQP method speeds the solving procedure, while the GA enables the algorithm to escape from local optima. An industrial acetylene hydrogenation system was used to provide the necessary data to adjust kinetics and to validate the approach. Behran Mnsoornejad Process Modeling http://works.bepress.com/navid_mostoufi/26 http://works.bepress.com/navid_mostoufi/26 Tue, 16 Dec 2008 01:18:24 PST A simulation model is developed using ASPEN PLUS to predict the performance of a fluidized bed membrane reformer. Because there are physical and chemical phenomena interacting in the fluidized bed membrane reformer, two submodels seem necessary in the model. These submodels are the hydrodynamic and reaction submodels. The hydrodynamic submodel is based on the dynamic two-phase model, and the reaction submodel is derived from the literature. The reformer is divided into two regions: a dense bed and freeboard. The dense bed is divided into several sections. At each section, the flow of the gas is considered as the plug flow through the membrane and bubble phases and perfectly mixed through the emulsion phase. The sets of the experimental data were used from the literature to validate the model. Close agreement was observed between the model predictions and experimental data. This model can be used for the simulation of nonideal fluidized bed membrane reactors inside the ASPEN PLUS process simulator. A. Sarvar-Amini Fluidization