Hamid Arastoopour

Simulation of particles and gas flow behavior in the riser section of a circulating fluidized bed using the kinetic theory approach for the particulate phase

Abstract Gas/particle flow behavior in the riser section of a circulating fluidized bed (CFB) was simulated using a computational fluid dynamics (CFD) package by Fluent. Fluid catalytic cracking (FCC) particles and air were used as the solid and gas phases, respectively. A two-dimensional, transient and isothermal flow was simulated for the continuous phase (air) and the dispersed phase (solid particles). Conservation equations of mass and momentum for each phase were solved using the finite volume numerical technique. This approach treats each phase separately, and the link between the gas and particle phases is through drag, turbulence, or energy dissipation due to particle fluctuation. Gas and particle flow profiles were obtained for velocity, volume fraction, pressure, and turbulence parameters for each phase. The computational values agreed reasonably well with the available experimental results. Our computational results showed that the inlet and outlet design have significant effects on the overall gas and solid flow patterns and cluster formations in the riser. However, the effect of the initial condition tended to disappear after some time. The main frequencies of oscillations of the system were obtained in different regions of the riser. These frequencies are important in comparing the computational results with the available time-averaged experimental data.

Numerical simulation and experimental analysis of gas/solid flow systems: 1999 Fluor-Daniel Plenary lecture

Gas/solid flow systems, including fluidized beds, are an essential part of many chemical processes. The optimum design and scale-up of these systems requires a through understanding of gas/solid flow patterns. Achievement of this understanding involves the development of experimental flow measurement techniques, and experimentally verified multiphase flow equations and numerical simulation tools. This paper provides a brief review of current techniques in solid flow measurement and recent contributions of the multiphase flow approach to gas/solid flow systems and fluidization. It features a review of the Eulerian approach, as well as different governing and constitutive equations, including the kinetic theory approach for cohesive and non-cohesive particles. The effects of inlet, outlet and boundary conditions are also discussed. Two- and three-dimensional transient numerical simulations of gas/solid flow patterns in circulating and bubbling fluidized beds are presented, along with a comparison of the predicted flow parameters with large-scale experimental data. In addition, recent improvements in the computational code to simulate a real process with complex geometries, developed in partnership with Fluent and AEA Technology, are highlighted. The paper is concluded with a discussion of the limitations and opportunities for research and development in this area.

Hydrodynamic analysis of dilute gas—solids flow in a vertical pipe

Abstract Dilute gas-solids systems are widely used as a proper reactor in many chemical, gas, food processing and waste treatment industries. The optimal utilization of this technology requires a good understanding of gas and particle flow patterns and the capability to predict such behavior. To respond to this need, the Two-Dimensional Steady State Two-Phase Hydrodynamic Model is developed to describe gas—solids flow behavior in dilute gas—solids flow systems. The model considers particles of same size and their boundary layers as a separate continuum phase. Furthermore, viscous dissipation forces and phase interaction forces are incorporated in the model. Numerical solution was obtained using the Method of Lines which is incorporated in FORSIM computer code. The predicted system variables agreed with the observed behavior of the experiments reported in the literature. The predicted average of the pressure drop profile using this model agreed well with the available experimental data in the literature.

Particle—particle interaction force in a dilute gas—solid system

Abstract An experiment to measure pressure drop and solids velocities in a dilute pneumatic conveying pipe has been designed and constructed. A model based on momentum balance over particles has been developed to translate pressure drop and solids velocities data into an expression for particle—particle interaction. A correlation for particle—particle interaction with the relative velocity of 7 to 16 m/s has been developed. This correlation describes our experimental data within the 20% deviation.

Extension of kinetic theory to cohesive particle flow

Abstract Particle flow is significantly affected by the agglomeration of particles due to the inter-particle force when fine particles, such as Geldarts Group C particles, are used. In order to quantify cohesive particle flow behavior, governing equations based on the kinetic theory for cohesive particle flow were developed. In the derivation of the governing equations for cohesive particle flow, we defined a new distribution function of the instantaneous velocity of the particle relative to the average velocity based on the volume fraction of the particulate phase that is conserved upon agglomeration. Furthermore, to account for the effect of diameter growth on agglomeration, we considered the change in number of particles due to agglomeration in the derivation of a new conservation of the number of particles equation. Based on our distribution function, governing equations were derived; namely, mass and momentum plus energy and conservation of number of particles equations for cohesive particle flow. This set of equations is capable of describing cohesive particle flow behavior as well as particle diameter variation due to agglomeration. Finally, to show the validity of our model, we analyzed the homogeneous simple shear flow of particles under the agglomeration condition. The predicted flow properties, such as shear viscosity, normal viscosity, and particle growth, agreed well with the expected trends.

Pulverization of rubber granulates using the solid state shear extrusion process. Part II. Powder characterization

Abstract Rubber powder obtained from the solid state shear extrusion (SSSE) process and the unprocessed rubber granulates were analyzed using physical, thermal, and chemical characterization methods. A portion of the granulates and two different size fractions of the powder produced by several passes of the rubber through the extruder were sampled by sieves and then characterized. Particle size distribution of the samples was determined using a laser diffraction technique. The shape of the particles was observed with an optical microscope, and the details of the particle surfaces were visualized using a scanning electron microscope. The total surface area was obtained using a BET method. Thermal analysis techniques were used to determine the composition, thermal, and thermo-oxidative degradation characteristics. The cross-link density and gel fraction of the rubber were determined using swelling and Soxhlet extraction methods, respectively. The microscopy study revealed that the particles generally had irregular shapes with rough surfaces, whereas the granulates had angular shapes with smooth surfaces. The larger particles produced by the SSSE process were mainly agglomerates of smaller particles. The total and external surface areas of the particles produced by the SSSE process were significantly greater than those of a cryogenically ground rubber of a similar size range. The extent of thermo-oxidation depended on the external and total surface areas of the samples. However, characteristic temperatures and kinetic parameters of the thermal degradation in the nitrogen environment were not affected by the size or surface area. The composition of the granulates was the same as that of the particles. However, the cross-link density and gel fraction of the rubber particles were smaller than those of the granulates suggesting the cleavage of chemical bonds due to the high mechanical stresses and possible oxidation during the SSSE process. The particle agglomeration during the reprocessing was instrumental in the alteration of the rubber properties.

Analysis of IGT pneumatic conveying data and fast fluidization using a thermohydrodynamic model

Abstract Pressure drop and choking were calculated for dense-phase vertical pneumatic conveying of solids using a model employing two continuity and two momentum equations. The relative equation of motion studied has been previously derived using the methods of non-equilibrium thermodynamics. The calculated pressure drops and choking velocities were found to be in a good agreement with IGT high-pressure lift-line experiments. To compare calculations with data, it was necessary to assume an effective particle size and an inlet void fraction which were not measured. Uncertainties in drag correlations for various flow regimes do not at present permit an equally good mathematical description of fast fluidization.

Pulverization of rubber granulates using the solid-state shear extrusion (SSSE) process:: Part I. Process concepts and characteristics

A single screw extruder was used to pulverize rubber granulates at high shear and compression without using cryogenic fluid for cooling. This process, solid-state shear extrusion (SSSE), is based on the large compressive shear deformation of rubber granulates, which results in the storage of a large amount of strain energy and the formation of cracks. When the stored energy reaches a critical level, the granulate cannot sustain itself. As a result, the stored elastic energy is converted into surface energy through the formation of new surfaces and, in turn, pulverization occurs. The stored elastic energy is dependent on the viscoelastic response of rubber granulates to the processing condition. The independent variables of the process were identified as the degree of compression of the rubber, number of extruder passes, barrel wall temperatures, rotation rate of the extruder screw, and feed rate of the granulates. The effects of these variables on the dependent variables, such as material and screw temperatures, particle size distribution (PSD), torque, and mechanical power consumption at steady state, were systematically studied. Fine rubber particles were obtained when the granulates were compressed sufficiently, and loss of strain energy due to viscoelastic stress relaxation was minimized by significant cooling in the pulverization zone. Agglomeration of rubber particles was found to be competing with the pulverization process.

Fluidization behavior of particles under agglomerating conditions

Abstract To obtain fundamental information on the fluidization behavior of sticky particles, a visually observable fluidized bed, which can be operated under conditions in which agglomeration of the bed materials occurs, was designed and constructed. Polyethylene particles and silica sand coated with a thin layer of low melting point polymer were chosen as bed materials. Two types of fluidizing gas distributors were used: a porous plate and a porous plate with an independently fed jet at the center. Our fluidized bed was operated in continuous-feed mode and in batch mode. The weight percentage of the aggomerates was measured at different residence times, jet air velocities, and jet and auxiliary air temperatures, using different particle sizes, coating thicknesses, and concentrations of coated particles in the fluidized bed. In the case of polyethylene particles as bed material in the batch system, the amount of agglomerates generated increased linearly with residence time and increased exponentially with either auxiliary air or jet air temperatures. Within the operating conditions of our study, the amount of agglomerates generated increased with jet nozzle size at the constant jet air flow rate. Under conditions of high temperature, a low jet velocity resulted in excessive agglomeration, which led to catastrophic sintering and defluidization. In the case of coated particles as bed materials in our batch fluidized bed, the amount of agglomerates generated increased with residence time and then leveled off. It increased sharply with jet air temperature, coating thickness, concentration of coated particles and decreasing particle sizes in the range of our experimental conditions.

A New Recycling Technology: Compression Molding of Pulverized Rubber Waste in the Absence of Virgin Rubber

Recycling of rubber waste poses a challenging environmental, economical, and social problem. In the present study, we propose a new two-stage recycling process to reuse a rubber waste. First, the granulates of the waste were pulverized into small particles using a single screw extruder in the Solid State Shear Extrusion (SSSE) process. Then, the produced powder was compression molded in the absence of virgin rubber. The slabs prepared at various molding conditions were subjected to mechanical, chemical, and microscopic tests. It is found that the slabs have high extensibility with low-medium tensile strength. Compressive creep of the powder, self-adhesion of rubber molecules, and interchange reactions of polysulfidic crosslinks are proposed as the basis of particle bonding.