Search

The influence of cohesive interparticle force (IPF) on gas dynamics in a bubbling gas–solid fluidized bed was numerically investigated while the bed hydrodynamics was described with the help of computational fluid dynamics and discrete element method (CFD-DEM). The model was validated by experimental results in terms of total bed pressure drop profile, probability density distribution of instantaneous local bed voidage signals, and Eulerian solid velocity field. The results showed that the model can satisfactorily predict the hydrodynamics of a bubbling gas–solid fluidized bed that is impacted by the presence of cohesive IPF. The validated CFD-DEM model was adopted to delineate the effects of IPF on the distribution of fluidizing gas between the bubble and emulsion phases and bubble characteristics, such as bubble stability and rise path, which could hardly be explored experimentally. Simulation results revealed that the presence of IPF in the bubbling bed alters the distribution of the fluidizing gas between the bubble and emulsion phases in favor of an increase in the propensity of gas to pass through the bed in the emulsion phase. The results also indicated that the bubble stability increases and the straight rise path of bubbles changes to a tortuous path when enhancing IPF.

A novel pyrolusite fluidized bed (PFB) contactor, which we recently developed for dissolved manganese (Mn(II)) removal through surface adsorption and subsequent oxidation by free chlorine, was modeled in this study. The hydrodynamic behavior of the filter media and water in the fluidized bed was described by the axial dispersion model. The model incorporated the effects of axial mixing in the liquid and solid phases, mass transfer resistance in a laminar fluid boundary layer surrounding a media grain, and a second order oxidation rate expression. The experimental data from lab-scale and field pilot-scale contactors was adopted for the model development and its validation. For the former, the data was employed to estimate the oxidation rate constant, the mass transfer coefficient, and the axial solid phase dispersion coefficient for the model. The model simulations matched the experimental data with less than 20% error across a wide range of Mn(II) and free chlorine concentrations and hydraulic loading rates that might be encountered in a drinking water treatment plant. The sensitivity analysis showed that the time to breakthrough is most sensitive to the adsorption isotherm constants and the oxidation rate constant. These observations indicate that the alluded parameters mainly control the performance of the PFB contactor as well as the process stability. Finally, a sample application of the model to acquire operational inputs was illustrated by analyzing the effect of free chlorine concentration on Mn(II) removal performance and breakthrough time within a PFB contactor.

Despite the increase in applications of renewable energy sources, coal combustion for electricity production remains important. The use of alkali and alkaline earth sorbents to address the emissions or their introduction during cocombustion of biomass can cause defluidization of fluidized bed combustors. This article presents the results obtained from an extensive experimental campaign aimed at exploring the performance of different defluidization inhibition measures and defluidization outcomes of a pilot-scale atmospheric bubbling fluidized bed combustor cofired with ReEngineered Feedstock (ReEF) and coal. ReEF is a solid fuel made from the nonrecyclable fraction of municipal solid waste to which alkaline air emission control reagents are physically bound. The cocombustion of coal and ReEF combines fuel- and sorbent-blending techniques for improved combustion and higher efficiency emission reduction. An earlier study of the group proved these favorable aspects during the cocombustion of a granular bitumi...

This study focuses on the hydrodynamic similarities of gas-solid fluidized beds influenced by the presence of interparticle forces (IPFs) at high temperature and induced by a polymer coating approach at near-ambient conditions. The level of IPFs was controlled through the formation of eutectics on the surface of base particles at high temperature. It was adjusted by the temperature of the inlet gas when adopting particles, coated with a thin layer of polymer having a low glass transition temperature, in a gas-solid fluidized bed in the case of polymer coating approach. Similarities were present between the results of the two parts of the study, i.e., in both cases, the minimum fluidization velocity and the bubble size growth rate with the superficial gas velocity in the bubbling regime increased with IPFs while the frequency of macro-structures and the average in-bed differential pressure drop in the bubbling regime decreased with IPFs. The polymer coating approach is therefore capable of simulating most of the conditions of a high temperature fluidized bed operated under the influence of IPFs at near-ambient conditions.

Identifying the onset of an agglomeration phenomenon at an early stage in processes utilizing gas-solid fluidized beds that operate under the influence of cohesive interparticle forces affords enough time to apply counteractive strategies and avoid a disastrous agglomeration of particles potentially leading to complete bed defluidization. In this paper, we compare the performance of different leading approaches proposed in the open literature for the advanced detection of defluidization. The approaches include the single-signal-monitoring of evolutions of total bed pressure drop, standard deviation of pressure signals, or S-value from the attractor comparison analysis as well as the simultaneous-monitoring of temperature and in-bed differential pressure signals during the process. The results show that the simultaneous-monitoring of temperature and in-bed differential pressure signals provided the best prediction of the onset of agglomeration while it demonstrated the least sensitivity to the changes in gas velocity, operating temperature, and bed inventory.

Iron and manganese removal is a common water treatment goal in groundwater systems. Pilot-scale pyrolucite fixed and fluidized bed contactors were tested in parallel to examine the iron and manganese removal from a natural groundwater which was supplemented with iron and manganese in order to challenge both processes. The results showed that the design of the distributor plate located at the bottom of the pyrolucite fluidized bed (PFB) contactor had crucial impact on the release of particulate manganese from the filter media. Using an optimal distributor plate, the PFB contactor provided effective manganese removal (

This study presents a simple approach for the early detection of agglomeration in a bubbling gas-solid fluidized bed. This monitoring approach is based on the simultaneous measurements of local temperatures and the in-bed differential pressure drop from the well-stabilized section of the bed. Defluidization experiments (800–1000 °C) showed that when a bubbling gas-solid fluidized bed approaches complete defluidization the average in-bed differential pressure drop progressively decreases from a reference value obtained under normal conditions while the temperature difference along the axis, particularly between a temperature reading right above the distributor plate and others at higher levels within the dense bed, simultaneously increases. This novel approach was thus proposed for the concurrent occurrence of these drifts to provide an opportune recognition of the onset of agglomeration in a bubbling gas-solid fluidized bed. The results demonstrated that it could effectively detect the defluidization condition minutes to hours before the complete defluidization state depending on the growth rate of agglomeration within the bed. Two pairs of detection thresholds for the timely recognition of agglomeration in bubbling fluidized beds of coarse silica sand particles were introduced according to the observations made in this study. The approach exhibited minimal sensitivity to variations in the superficial gas velocity (±10%), operating temperature (±100 °C), and bed inventory (±20%) while both legs of the in-bed differential pressure transducer were well below the splash zone and above the jetting zone formed in the vicinity of the distributor plate.

An in-depth examination of the hydrodynamics of gas-solid fluidized beds at high temperature and pressure is critical for their design and operation owing to the global trend of processing lower quality feedstocks, e.g., high ash content coal, biomass, and waste, in these units. Current knowledge on gas-solid fluidization, though, refers to ambient conditions and the hydrodynamic models based on these conditions by merely changing the gas properties, i.e., its density and viscosity, are generally employed to estimate the overall performance of gas-solid fluidized bed processes under extreme conditions. This strategy, however, overlooks possible modifications induced by the operating conditions on the structure and dynamics of fluidized particles, i.e., the level of interparticle forces. With the development of new processes adopting gas-solid fluidized beds under extreme conditions, a comprehensive review of the experimental and simulation studies of gas-solid fluidization at elevated temperatures and pressures and in the presence of interparticle forces is warranted. This review addresses the effects of temperature, pressure, and interparticle forces on the fluidization characteristics of gas-solid fluidized beds for a wide spectrum of particle systems, ranging from Geldart groups A, B, and D classifications, refer to the fluidization behavior at ambient conditions.

Interparticle forces (IPFs) are among the important parameters that can influence the hydrodynamics of a gas-solid fluidized bed and, hence, its overall performance. They are particularly critical at elevated temperatures, where their magnitude is typically greater than that at ambient conditions. Two simple and indirect approaches were developed in this study to quantify the magnitude of IPFs and its variation with temperature in a gas-solid fluidized bed. Quantification of thermally induced IPFs, i.e., the difference between the level of IPFs at a given temperature and that at ambient conditions, was attempted in the first approach. It was shown that the variation of IPFs with temperature is related to the difference between the experimental minimum fluidization velocity ( U mf , e x p ) and that predicted by a correlation ( U mf , p r e d ) based purely on hydrodynamic principles. The magnitude of thermally induced IPFs at a given temperature was then quantified with the help of the Ergun equation and the difference between U mf , e x p and U mf , p r e d . The second approach was developed for the quantification of absolute values of IPFs based on the difference between the loose bulk voidage ( e 0 ) and the minimum fluidization voidage ( e mf ). It was shown that the magnitude of IPFs directly affects the ratio of e mf / e 0 , which we called “dynamic Hausner ratio”. A phenomenological model was, hence, proposed where the magnitude of IPFs is expressed as a function of the dynamic Hausner ratio. The performances of these approaches were verified and qualitatively validated, while their limitations and opportunities were also discussed.

This article presents some observations of the effect of interparticle forces (IPFs) on solids motion in a gas-solid fluidized bed operated in the bubbling fluidization regime and at atmospheric pressure. The radioactive particle tracking (RPT) technique was adopted to observe the solids flow pattern and quantify spherical equivalent bubble size, distributions of upward and downward-moving clusters and idle and bubble-induced times, cycle frequency, and axial/radial solids diffusivities. The level of cohesive IPFs was increased and controlled in the fluidized bed with a polymer coating approach. Experimental results showed that the presence of IPFs could effectively modify the solids flow pattern in a bubbling gas-solid fluidized bed. The influence was more pronounced at low gas velocity, where the ratio of the magnitude of IPFs to hydrodynamic forces was high. At constant superficial gas velocity, beds with IPFs contained smaller bubbles indicating a higher tendency of gas entering the dense phase compared to the bubble phase. The different characteristic parameters of solids mixing showed that the favorable effect of IPFs on the division of the fluidizing gas between the bubble and dense phases was accompanied by reductions in the quality of global and local solids mixing.

The fluidization behavior of bubbling fluidized beds of coarse particles was investigated between 700 and 1000 °C for superficial gas velocities ranging from 0.6 to 1.5 m/s. The objective of the study was to highlight modifications in the bed behavior with the operating temperature at conditions under which the role of hydrodynamic forces (HDFs) or interparticle forces (IPFs) was dominant. To this end, the experimental work was divided into two phases. In the first phase, the influence of temperature on the hydrodynamics of a bubbling fluidized bed of coarse particles for which HDFs were dominant was investigated. In the second phase, the surface characteristics of the fluidized particles were primarily modified through the formation of eutectics resulting from a chemical reaction between the bed material and alkali/alkali earth metal based reagents that were introduced into the bed throughout the periods of solid fuel combustion at elevated temperatures. This then triggered changes in their fluidization characteristics with increasing temperature while IPFs were present in the bed. Experimental results revealed that the flow dynamics of a bubbling bed of coarse particles at high temperature was principally influenced by the variation of the gas density with temperature when IPFs did not play a discernible role. Nevertheless, with the presence of different levels of IPFs in the bed, a multiplicity of behaviors was realized at elevated thermal levels. Consequently, the physical and/or physico-chemical changes in the fluidized particles due to an increase in temperature and the variation in the physical properties of the fluidizing gas should be seriously considered when attempting to successfully design and operate gas–solid fluidized beds at high temperature.

Particle agglomeration is a common phenomenon in a large number of industries. The presence of a liquid bridge between colliding wet particles is the principal source of agglomeration in such processes as wet granulation and thermal conversion of fuels in a high temperature gas–solid fluidized bed reactor. A comprehensive agglomeration model for wet particles must incorporate both liquid capillary and viscous contributions, as well as the liquid bridge volume effect. Darabi et al. (Chem. Eng. Sci. 64 (2009) 1868–1876) developed a coalescence model for a binary collision of identical wet particles. The model provides reasonable results for wet particles with a low viscosity liquid layer, i.e., under capillary limiting conditions. However, application of this model for collisions of particles coated with a high viscosity liquid layer, i.e., under viscous limiting conditions, results in unreliable outcomes. The current study presents an improved version of Darabi et al.’s model, which (i) corrects viscous energy loss equations in the original model, thus providing reasonable outcomes under both capillary and viscous limiting conditions with highly wetting systems, (ii) accounts for the contribution of capillary force in the approach stage when estimating the initial particle speed for the separation stage, (iii) accounts for the collision of particles having different initial speeds with respect to the laboratory reference frame, (iv) employs a more reliable correlation to estimate the liquid bridge rupture distance, and (v) evaluates the collision outcome in three steps based on the values of kinetic energy parameters. We compared experimental particle-plate collision data with the predictions from agglomeration models and it showed that considering the liquid bridge volume effect and the liquid bridge rupture distance can be essential for the accurate prediction of collision outcomes. The results of sensitivity analyses with the improved agglomeration model revealed that the agglomeration outcome under capillary limiting conditions is most sensitive to the thickness of the coating layer, particle inertia, liquid surface tension, and dry restitution coefficient. Under viscous limiting conditions, proper measurement or estimation of the thickness of the coating layer, liquid viscosity, asperity height, and particle inertia are critical for evaluating the particle collision outcome.

The influence of interparticle forces (IPFs) on the hydrodynamics of a gas–solid fluidized bed was experimentally investigated with the help of a polymer coating approach. The results showed that the presence of IPFs in the bed can considerably change the hydrodynamic parameters. The tendency of the fluidizing gas passing through the bed in the emulsion phase increased with IPFs in the bubbling regime. The performance of a fluidized bed reactor was then studied through simulation of a reactive catalytic system using three different hydrodynamic models: (a) a simple two-phase flow model, (b) a dynamic two-phase flow model, and (c) a dynamic two-phase flow model, integrating the effects of superficial gas velocity and IPFs. The simple two-phase flow model was found to underestimate the reactor performance for catalytic reaction most likely due to the oversimplified assumptions involved in this model. Also, the simulation results showed that modification of the bed hydrodynamics due to IPFs resulted in a better performance for a bubbling fluidized bed reactor. This suggests that the hydrodynamic models should take into account the effects of superficial gas velocity and variation in the ratio of the magnitude of IPFs/hydrodynamic forces, due to any operational reason, to yield a more reliable evaluation of the performance of the fluidized bed reactor.

In this study, a polymer coating approach was applied to increase and adjust the level of cohesive interparticle forces (IPFs) in a gas–solid fluidized bed. This novel approach is based on coating spherical inert particles with a polymer material having a low glass transition temperature followed by using the coated particles in a gas–solid fluidized bed. Since the level of artificial IPFs inside the bed depends on the temperature of the coated particles, it was simply controlled by the temperature of the inlet air. Accordingly, the system temperature was gradually varied near and slightly above the glass transition temperature of the polymer, between 20 and 40 °C, to investigate the influence of IPFs on the fluidization behavior of the bed at different superficial gas velocities, covering fixed bed state, bubbling, and turbulent fluidization regimes. The study of hydrodynamics was carried out through the visual observation of bed height, the measurement of bed pressure drop, and the recording pressure signals in the windbox and dense bed. Experimental results indicated that enhancing the level of IPFs in the bed can alter the fluidization behavior of the bed from Geldart (Geldart, 1973) group B behavior to Geldart group A and even Geldart group C behaviors, result in a fixed bed with a looser structure that can hold more gas inside, increase the characteristic fluidization velocities, such as minimum fluidization velocity and transition velocity from a bubbling to turbulent fluidization regime, increase the tendency of the fluidizing gas passing through the emulsion phase in the bubbling regime, and result in a noticeably larger bubble size at gas velocities slightly higher than the bubbling to turbulent transition velocity of the bed without IPFs.

This work shows that simultaneous measurements of temperature and pressure signals for a bubbling gas-solid fluidized bed can be considered as a simple and effective early detection technique of defluidization conditions. The modification of the hydrodynamics of the bed due to the presence of interparticle forces (IPFs) was primarily investigated using different measurement techniques (i.e., pressure transducers, optical fiber probe, and Radioactive Particle Tracking). Different levels of IPFs were attained in the bed with the help of a polymer coating approach at near-ambient temperature (30–40 o C) in a 15 cm ID fluidized bed. Experimental results showed that by increasing the degree of IPFs in the bed, larger bubbles were noted at gas velocities well above the minimum fluidization velocity, the emulsion phase voidage increased, and the mean value of the in-bed differential bed pressure drop and the axial solids mixing decreased. The high temperature defluidization tests (800–1000 o C) as the second part the experimental campaign were conducted in a 20 cm ID fluidized bed reactor. It was found that the temperature difference between the bottommost thermocouple (located 5 cm above the distributor) and the others located in the dense bed was continuously increasing when the bed was approaching defluidization. Simultaneously, the mean value of the differential pressure signals was successively decreasing from its regular value under normal conditions. A combination of these two conditions was considered as the monitoring method for the early detection of defluidization. It was found that this approach was effectively capable of predicting the onset of defluidization minutes to hours before complete defluidization, allowing time to apply counteracting strategies. Experimental results of the first part of the work clearly demonstrated why the simple integrated approach discovered in the second part of the study can be efficiently used for timely recognition of defluidization conditions.

This article reports the results obtained from an extensive experimental campaign aimed at investigating the effect of interparticle forces (IPFs) on the local flow structure of a gas–solid fluidized bed. A polymer coating approach was used to enhance and control the degree of cohesive IPFs in a gas–solid fluidized bed. In this work, the local transient solids concentration (bed voidage) was carefully measured with the help of an accurate optical fiber probe at different temperatures and gas velocities covering both bubbling and turbulent fluidization regimes. Also, the Radioactive Particle Tracking (RPT) technique was employed to track the trajectory of a tracer mimicking the behavior of solid particles in two systems, one with the least amount of IPFs in the bubbling regime and the other with the highest amount. Experimental results showed that by increasing the level of IPFs in the bed the fixed bed and emulsion phase voidage in the bubbling regime increased and demonstrated higher capacities in holding gas inside their structures. In addition, the emulsion phase fraction increased, the tendency of the fluidizing gas passing through the bed in the emulsion phase enhanced in the bubbling regime, the frequency of the bubble/emulsion phase cycle decreased, and the meso-scale transition from bubbling to turbulent fluidization regime delayed until reaching higher superficial gas velocities.

In this study, silicon microchannel heat sink (MCHS) performance using nanofluids as coolants was analyzed. The nanofluid was a mixture of nanoscale Cu particles and pure water with various volume fractions. Based on theoretical models and experimental correlations, the heat transfer and friction coefficients required in the analysis were used. In the theoretical model, nanofluid was treated as a single-phase fluid. In the experimental correlation, thermal dispersion due to particle random motion was included. The microchannel heat sink performances for a specific geometries with Wch = Wfin = 100 µm and Lch =300 µm is examined. In this study, flow in laminar and turbulent regimes using the theoretic and experimental relations was investigated; moreover an artificial neural network (ANN) was used to simulate the MCHS having laminar flow with different circumstances and after that, the best geometry and volume fraction of nanofluid could be found based on minimum thermal resistance.