Height Of Fluidized Bed Research Articles

Performance of industrially implemented turbulent contact absorbers – A comparative study

The current study aims to assess the performance and industrial feasibility of using three-phase fluidized bed absorption columns for post-combustion carbon dioxide (CO2) capture. The analysis is based on a mathematical model of a scaled-up fluidized bed absorption column that has been validated using pilot plant experimental data. The focus is on evaluating the energy efficiency of the fluidized bed column in comparison to packed bed technologies, which are currently widely used. The results of the analysis are evaluated in terms of three performance indicators (i.e., absorption efficiency, carbon capture rate, and energy performance index) and compared to packed bed columns. Simulation results indicate that the best column configuration is represented by a three-stage absorber and that the system performs optimally when particle diameter and density decrease with each section. Consequently, this leads to an increase in the total fluidized bed height and carbon capture rate by up to 25%. The energy performance index is maintained below 3.6, indicating an energy-efficient operation of the system. As such, the findings of this study suggest that the use of gas-solid-liquid three-phase fluidized bed absorption columns for post-combustion CO2 capture is a promising technology for the implementation of CCUS and for achieving of climate neutrality targets.

Comparison and validation of various drag models for fluidization characteristics of bubble fluidized beds with a high-speed particle image velocimetry experiment

Bubbling liquefaction of dense particles is one of the most common forms of industrial fluidization in gas–solid flow systems. Computational fluid dynamics and the discrete element method are important tools for studying dense gas–solid flows. In these methods, the momentum transfer between phases relies on a drag model, so a reasonable choice of drag model is crucial for accurately predicting the hydrodynamic behavior of dense gas–solid flows. This paper investigates the effect of different drag models on the flow behavior prediction of dense gas–solid flow for the “Small-Scale Challenge Problem-I” published by the National Energy Technology Laboratory in 2013. The gas–solid fluidization characteristics, such as instantaneous particle flow processes, particle velocity vector distributions, changes in the fluidized bed height, and average gas phase pressure drops, were compared for different drag models. A detailed validation analysis of each dominant drag model was carried out in conjunction with the experimental data. The results show that the drag model significantly affects the numerically predicted results of particles’ hydrodynamic behavior, especially in terms of the bed height variation and the remixing behavior of particles. These research results are expected to improve the predictive accuracy of gas–solid flow hydrodynamic behavior and provide guidance for designing and optimizing fluidized beds, which has theoretical and practical significance.

Influence of bed material density on fluidized bed flotation performance: A study on the flotation performance of quartz and alumina beds in the HydroFloat

This work investigated the influence of bed material density on fluidized bed flotation performance. Experiments were carried out in a HydroFloat cell to investigate the influence of bed material density on required bed masses and bed behaviour at a bed level of 15 cm and water rate of 1.19 cm/s. Bubble size image analysis was carried out under four different fluidization conditions: water only, water and frother (1 ppm), water, frother (1 ppm) and quartz, and water, frother (1 ppm) and aluminium oxide. Flotation experiments were undertaken to investigate the influence of the resulting bed density and characteristics on concentrate recovery and grade performance. The results confirm that there is an upper particle size limit on fluidization. Apparent bed density was higher for the denser bed material (aluminium oxide) even though bed behaviour remained identical with both being highly permeable. The product Cu grades for the quartz bed were lower (8.42 % and 15.82 %) compared to the alumina bed (23.81 % and 21.86 %). This was due to the elutriation of quartz into the concentrate with Si concentrations of 25.85 % and 15.69 % compared to 2.4 % in the feed. In contrast, Al concentrations were 2.44 % and 1.09 % in the concentrates compared to the 0.38 % in the feed. However, recovery performances for both bed materials were identical (39.72 % and 40.88 % at low collector concentrations and 98.93 % and 97.95 % after ethylenediaminetetraacetic acid (EDTA) cleaning and increased collector concentration for the quartz and aluminium oxide beds respectively). This indicates that apparent bed density may have only a slight influence on flotation recovery compared to other direct control variables such as fluidization water rate and bed height.

Assessment of turbulent contact absorber hydrodynamics with application in carbon capture

Chemical gas–liquid absorption systems are one of the most employed technologies for CO2 removal from post-combustion processes, due to the high capture efficiency. This work presents a complex hydrodynamics model used to describe a novel CO2 capture technique that relies on the use of a three phase, gas–solid-liquid absorption column, in which the gas phase acts as the continuous phase, the liquid as the dispersed phase and the low-density solid particles are inert.To determine and analyze the hydrodynamic parameters of the process: minimum fluidization velocity, fluidized bed height, pressure drop, fluid retention, an air–water system was used. The developed hydrodynamics model was validated against experimental data from a pilot column (correlation coefficients above 0.97).For the study of the chemisorption process, the liquid phase consists of NaOH solution and the gas phase is a mixture of CO2 and air. The implementation of the hydrodynamic model along with the mass transfer model contributes to increasing the correlation coefficient for the outlet CO2 concentration to a value above 0.99. In this case, the increased value of the mass transfer area leads to a value up to 10 times higher of the transferred flow of carbon dioxide between the two phases, compared to regular packed bed columns.The model proves useful in the analysis of the influence of liquid phase and solid phase characteristics on the system’s performance. The results show that the capture rate is highly dependent on the fluidized bed height. Increasing the size or density of solid particles leads to a decrease in the height of the fluidized bed resulting in a 20 – 40% decrease in the carbon capture rate.

Chemical-looping combustion in packed-fluidized beds: Experiments with random packings in bubbling bed

Chemical-looping combustion (CLC) in packed-fluidized bed reactor was investigated. Experiments were carried out in a cylindrical laboratory-scale bubbling fluidized-bed reactor with an inner diameter of 78 mm and a hight of 1.27 m. Ilmenite concentrate particles in the size range 90–212 μm was used as oxygen carrying fluidizing solid. Two different types of random packings were used: aluminum silicate balls (ASB) with a diameter of 12.7 mm and bulk density of 1439 kg/m3 and 25 mm stainless steel thread saddles (RMSR) with bulk density of 204 kg/m3. The superficial gas velocity was 0.3 m/s. The fuels were CO and CH4. The bed temperature was 840 °C for CO and 940 °C for CH4. The height of the packed bed was kept constant at 1 m. The fluidized oxygen carrier bed height was varied from 2 cm to 40 cm. Results showed that fuel conversion in packed-fluidized beds is highly dependent on oxygen carrier bed height and the nature of the packing. Packed-fluidized beds with RMSR packing resulted in a significant improvement in fuel conversion, compared to a bubbling bed with no packing. With 30–40 cm bed height, CO conversion was ≈99.5% with RMSR packing and 91–96% without packing. The corresponding numbers for CH4 were ≈84% and ≈78%. Further, the RMSR packing has very high void factor (0.96). Thus, it should have limited effects on particle inventory, pressure drop and throughput. The most likely mechanism for improved fuel conversion is improved gas-solid mass transfer due to be reduced bubble size. The ASB packing has low void factor (0.43) and provided mixed results with respect to fuel conversion.

Determining the Parameters of the Fluid Layer with a Rigid Mobile Nozzle

Hydrodynamic problems of a three-phase fluidized bed are considered. The main attention is paid to experimental studies into the process of cooling industrial circulating water in a three-phase layer. An experimental setup was developed and assembled, and its brief description and operating principle are given. Adjustment experiments and experiments with a two-phase fluidized bed were carried out, the main task of which was to determine the dependence on the air velocity in the apparatus of the hydraulic resistance of a dry grate without a nozzle. A series of experiments is described that made it possible to establish the optimal cooling mode for circulating water in an apparatus with a three-phase fluidized bed. The experiment was carried out at both constant and variable irrigation density, and the experiments were repeated at different air velocities. The following parameters of the three-phase fluidized bed were recorded: hydraulic resistance, the rate of onset of fluidization, the rate of entrainment, and the height of the dynamic bed. The results of experimental studies into the fluidization rate and entrainment rate at various ball nozzles and the dependence of the dynamic height of a three-phase fluidized bed on the rate and density of irrigation are presented. Experimental data have been obtained on the pressure drop in a three-phase fluidized bed depending on the air velocity, the rate of the beginning of fluidization of the irrigated bed of the packing, and the rate of flooding of the three-phase fluidized bed as a function of the irrigation density. The analysis of hydrodynamic and thermal processes occurring in a three-phase fluidized bed is carried out, and the main technological parameters for the optimal operation of installations with the specified bed are obtained in relation to solving the problem of cooling the circulating water. The dependence of the expansion of a three-phase fluidized bed on air velocity and irrigation density has been investigated. On the basis of the performed experimental studies, empirical formulas for calculations are derived.

An experimental research to determine the fluidized bed regime of pepper in the fluidized bed drying process

This is an experimental study to determine that the minimum fluidization velocity, the fluidized bed height, and the pressure loss through the grain layer depend on the thickness of grain layers and moisture content of materials. The drying materials used in this study are fresh pepper and dried pepper of which moisture content is 54.2% and 12%, respectively. All the experiments are conducted with the thickness of the pepper layer ranging from 4mm to 44mm while the wind speed ranging from 5 m/s to 11m/s. From the result of experimental analysis, it can be seen that the minimum fluidization velocity of an air stream through fresh pepper ranges from 5.2m/s to 6,5m/s. The pressure drop of air stream through the fresh pepper is 225Pa while 147Pa for dry pepper, corresponding to the grain thickness of 37mm layer. By comparing the results determined from these experiments with computational models and some available correlations, the computational models of Fedorov, Baeyens-Geldart and Leva all match the experimental data collected from the pepper material. The correlation between the calculated and experimental results has a negligible error, the tendency to increase pressure loss depends on the thickness of grain layer is also quite similar. From the experimental data results obtained by determining the pressure drop through the pepper layers which depends on the thickness of both fresh pepper and dried pepper, the linear equations of the first order are established with high credibility.

Development of stochastic model of particulate coal fluidized bed expansion and axial structure

Particulate solids are in the state of fluidization at many stages of preparation and treatment of solid fuels. An effective drag force coefficient Cd is used to describe a mechanical contact between gas stream and an individual particle. The evaluation of the effective drag force coefficient is not limited by the force of hydraulic resistance but also includes a set of different forces. This set of forces is rather indeterminate, and a lot of empirical equations for effective drag force coefficient calculations can be found in the scientific papers. Choosing the applicable formula for calculation is often difficult. In addition, it requires taking into account the flow patterns around an individual particle. Thus, a comparative study to examine the most well-known drag force models with a uniform approach to account the flow patterns around an individual particle is important. The Markov chain approach is used as a mathematical basis for modeling of the flow patterns in a fluidized bed. The identification of the model parameters is completed and the complementation of transition matrices with the current physical properties of substances involved into the flow makes the model non-linear. The comparative study of the results obtained with using of the two correlations for drag force coefficient is performed. The stochastic model of fluidized bed expansion and axial structure has been proposed. The comparative analysis of two different scenarios of fluidized bed expansion using different drag force models has been performed. The authors developed and tested the model to describe fluidized bed expansion and axial structure on the basis of the Markov chain approach. The low certainty of physical drag coefficients models under conditions of flow patterns around an individual particle has been shown. The conducted research proves that consistent description of structure inhomogeneity of fluidized bed is possible using the nonlinear mathematical models based on mesoscale level of the object decomposition. Predictive efficiency of similar models is limited by low reliability of formula for calculation of drag force coefficient. Thus, it is possible to state the importance of further comparative research to check different formulas for calculation of gas-particle drag force coefficient in order to provide a reliable forecast of the fluidized bed height.

A review on hydrodynamic parameters and biofilm characteristics of inverse fluidized bed bioreactors for treating industrial wastewater

Abstract Wastewater treatment processes play crucial role in keeping the human health and environment at proper hygiene. Currently, biological wastewater treatment methods are dominant in the effluent treatment process because of their cost-effectiveness in the treatment. However, the rapid increase in population has created a need for the implementation of highly efficient wastewater treatment methods. In the recent past, the treatment of wastewater through fluidized-bed bioreactor has proven as one of the best processes from the currently available methods of chemical and biochemical engineering fields. This review aims to explore and compile the methods of fluidization used for the treatment of wastewater in the recent past, majorly by the inverse fluidization method. In the inverse fluidization process, the density of solid particles utilized in the fluidization is lower than that of the liquid and compared to an up-flow fluidized bed bioreactor, and it is a very effective system for biological wastewater treatment. This review discusses the developments made in recent years with regard to aerobic inverse fluidized bed bioreactors, with more emphasis placed on the reported applications on treating industrial wastewaters. The influence of hydrodynamic parameters (minimum fluidization velocity, phase holdup, bed expansion, and bed height, pressure drop, superficial gas velocity, liquid velocity, hydraulic retention time, biofilm carriers and ratio of volume of settled bed to the volume of reactor) are highlighted in the discussion, and also the effect of these parameters on the biofilm and its subsequent effect on wastewater is included. Recent trends of application of aerobic inverse fluidized bed bioreactors for the removal of COD and scope of future study elements have been discussed in this manuscript.

Mixing and segregation behavior in an air dense medium fluidized bed with binary mixtures for dry coal beneficiation

Particle mixing and segregation behavior in an Air Dense Medium Fluidized Bed with binary mixtures of solid particles were investigated for dry coal beneficiation. Magnetite mixed with coal/gangue/sand particles belonging to Geldart B/D group were tested individually for the bed density adjustment. The effects of operating parameters including particle density ratio, particle size ratio, mixture composition, superficial gas velocity, and fluidized bed height on the mixing and segregation pattern were examined. The results demonstrated that the segregation becomes more severe with increasing density difference of binary mixtures. An increase in particle size ratio may also lead to partial segregation. Mixing and segregation of binary systems are almost independent of lower excess gas velocity and initial bed height when it is over 15 cm. Moreover, a mixing index was employed to evaluate the mixing and segregation performance, and the criteria for good mixing to achieve the bed density adjustment were identified.

ЭКСПЕРИМЕНТАЛЬНЫЕ ИССЛЕДОВАНИЯ ПЕРСПЕКТИВНЫХ СПОСОБОВ ИНТЕНСИФИКАЦИИ ТЕПЛОПЕРЕДАЧИ В ТРУБЧАТОМ ТЕПЛООБМЕННИКЕ

Актуальность работы обусловлена необходимостью повышения интенсивности теплоотдачи в аппаратах трубчатого типа химической и нефтехимической промышленности. Аппараты данного типа, как правило, громоздки, характеризуются высокой металлоемкостью, занимают большие производственные площади. Интенсификация теплоотдачи позволит снизить габариты теплообменного оборудования, уменьшить потери теплоты в окружающую среду и повысить технико-экономические показатели производства. Экспериментально полученные критериальные уравнения могут быть использованы при разработке интенсифицированных теплообменников. Цель: экспериментально определить зависимость критерия Nu от критерия Re в лабораторных теплообменникахтипа «труба в трубе», реализующих следующие способы интенсификации теплоотдачи: 1) использование сетчатого и спирального турбулизаторов; 2) использование псевдоожиженного слоя сферических металлических частиц; 3) вращение теплообменной трубы за счет кинетической энергии потока теплоносителя. Методы: экспериментальное определение зависимости критерия Nu от критерия Re в лабораторных интенсифицированных теплообменниках типа «труба в трубе», инструментальное определение температуры и расхода жидкости, высоты псевдоожиженного слоя; видеосъемка. Результаты. Экспериментально определена зависимость критерия Nu от критерия Re в лабораторных интенсифицированных теплообменниках типа «труба в трубе». Для исследованных турбулизаторов при 10000<Re<25000 получены эмпирические критериальные зависимости в пределах погрешности ±5%. Исследование способа интенсификации теплоотдачи за счет использования псевдоожиженного слоя в вертикальной теплообменной трубе показало, что наилучшие гидродинамические условия для интенсификации теплоотдачи достигаются в псевдоожиженном слое из свинцовых сферических частиц диаметром d=0,002м. Для псевдоожиженного слоя сферических свинцовых частиц d=0,002м при 5000<Re<15000 получена эмпирическая критериальная зависимость в пределах погрешности ±6%. Установлено, что во вращающейся охлаждаемой горизонтальной теплообменной трубе с постоянной частотой 2,0 об/с повышается критерий Nu в среднем 1,24 раза в интервале 10000<Re<40000. Анализ показал, что увеличение интенсивности теплоотдачи в исследованных теплообменниках обеспечено за счет улучшения перемешивания потока жидкости, а также за счет турбулизации пристеночного слоя. Результаты экспериментальных исследований позволяют сделать вывод, что рассмотренные методы интенсификации процесса теплоотдачи являются перспективными для дальнейшего исследования и использования на химическом и нефтехимическом производстве.

Enhanced fluidization of nanosized TiO2 by a microjet and vibration assisted (MVA) method

A microjet and vibration assisted (MVA) fluidized bed was developed to enhance the fluidization quality of nanosized TiO2 particles. Two commercially available TiO2 nanopowders, P25 and P90, were tested. Fluidization quality was assessed by determining the non-dimensional bed height as well as the non-dimensional pressure drop. Smooth fluidization of the TiO2 nanopowders was observed in the MVA system. The non-dimensional bed height for the nanosized TiO2 in the MVA system optimized at about 5 and 7 for P25 and P90 TiO2, respectively, at a resonance frequency of 50 Hz. The non-dimensional pressure drop was also determined and showed that the MVA system exhibited a lower minimum fluidization velocity for both of the TiO2 types as compared to fluidization that employed only vibration assistance. Additional experiments were conducted with the MVA to characterize the synergistic effects of vibrational intensity and gas velocity on the P25 and P90 fluidized bed heights. Mathematical relationships were developed to correlate vibrational intensity, gas velocity, and fluidized bed height in the MVA. To test the mathematical model, 18 additional experimental points were gathered for both P25 and P90 under varying conditions. The actual and model-predicted non-dimensional bed heights were found to correlate well, with less than 14% error between values. The non-dimensional bed height in the MVA system is comparable to previously published P25 TiO2 microjet assisted (MA) fluidization employed with the addition of an alcohol that was used to minimize electrostatic attraction within the system. However, the MVA system achieved similar results without the addition of a chemical, thereby expanding the potential chemical reaction engineering and environmental remediation opportunities for fluidized nanoparticle systems. Fluidized bed stability was greatly improved when the dual effects of vibration and microjet assistance were used.

Pellet Softening Process for the Removal of the Groundwater Hardness; Modelling and Experimentation Modelling and experimental of groundwater shortening by a pellet reactor

A lab scale pellet reactor (PR) was designed and fabricated to carry out extensive investigations on the removal efficiency of the hardness of groundwater. The groundwater of 2200 – 2600 mg/L hardness was collected from Abdulla Ibnalhassan wells area located at the west desert of Al-Shinafiyah district (70 km to the southwest of Al-Dewaniyah city, Iraq). Both hydrodynamic parameters of the pellet reactor (porosity and fluidized bed height) and the parameters of calcium carbonate crystallization process (calcium carbonate equilibrium, pellet size, and density) were modeled and compared with the experimental results of the lab scale pellet reactor. The comparison showed that fair agreement between modeled and measured results was observed. The removal efficiency of both calcium and magnesium ions were 62.5-99% and 83-99% respectively. The removal efficiency was found to be strongly dependent on pH and the ratio of NaOH solution flow rate to the groundwater flow rate in the pellet reactor.

A dynamic model of fluidized-bed flotation

Abstract The concept of fluidized-bed flotation (FBF) (e.g. HydroFloatTM) was recently introduced to improve the recovery of coarse particles (425 to 1180 µm). In spite of encouraging results from the first industrial applications dating back more than a decade, FBF still remains an emerging technology in the global mining industry. This paper presents a dynamic model of a FBF cell based on first principles describing volume conservation balances and macro hydrodynamic conditions. It was specifically developed to address process design, monitoring, and control problems. The model relies on the drift-flux theory to predict the flow of three populations (free particles, attached particles and bubbles) throughout the proposed vertical mixer-in-series framework. A calibration of empirical parameters on literature data demonstrates the ability to reproduce the effect of operating parameters (airflow rate, fluidization water flow rate and bed height) on steady-state recovery as long as the equipment operates at low turbulence.

Повышение эффективности охлаждения оборотной воды в испарительных охладителях с псевдоожиженным слоем

The article describes the results from experimental investigations into the hydrodynamics of a three-phase fluidized bed. Calculation dependences of the three-phase fluidized bed height on the spraying density, which can be used for achieving more efficient cooling of circulating water in evaporative coolers, are given. The optimal conditions for stable operation and the technological parameters characterizing the water cooling process in a three-phase fluidized bed are determined. An experimental setup for carrying out full-scale experiments on evaporative cooling of circulating water in a three-phase fluidized bed is constructed, and the actual potential of the evaporative water cooling technology under real conditions is determined. It is shown that the calculated and experimental data are in good agreement with each other, and validity of the accomplished investigations has been confirmed. A mathematical model of the circulating water evaporative cooling process in a three-phase fluidized bed is proposed. Based on the numerical implementation of the proposed model, the dependences of cooled water and spent humid air temperatures at the evaporator outlet on relative humidity ф, spraying ratio ц, air velocity W, and ambient air and inlet water temperatures to and tE1 have been established. An algorithm and a program have been developed for determining the temperature of outlet water tH2 and humid air tm at the outlet of the evaporative cooler. A linear dependence of the circulating water evaporative cooler’s energy coefficient on the spraying coefficient has experimentally been established for the considered cooler type. A dependence of the evaporated moisture fraction in the considered circulating water evaporative cooler on the cooled water temperature at the cooler inlet and on the spraying coefficient is established. An evaporative air cooling method is proposed, which is embodied by using an evaporative chamber with a three-phase fluidized bed and a regenerative rotating heat exchanger.

Optimization fluidization characteristics conditions of nickel oxide for hydrogen reduction by fluidized bed reactor

We evaluated the optimal conditions for fluidization of nickel oxide (NiO) and its reduction into high-purity Ni during hydrogen reduction in a laboratory-scale fluidized bed reactor. A comparative study was performed through structural shape analysis using scanning electron microscopy (SEM); variance in pressure drop, minimum fluidization velocity, terminal velocity, reduction rate, and mass loss were assessed at temperatures ranging from 400 to 600 °C and at 20, 40, and 60 min in reaction time. We estimated the sample weight with most active fluidization to be 200 g based on the bed diameter of the fluidized bed reactor and height of the stocked material. The optimal conditions for NiO hydrogen reduction were found to be height of sample H to the internal fluidized bed reactor diameter D was H/D=1, reaction temperature of 550 °C, reaction time of 60 min, superficial gas velocity of 0.011 m/s, and pressure drop of 77 Pa during fluidization. We determined the best operating conditions for the NiO hydrogen reduction process based on these findings.

Investigation of phases interactions and modification of drag models for liquid-solid fluidized bed tapered bioreactors

Fluidized bed bioreactors have been increasingly applied to wastewater treatment, since they offer advantages when compared to other reactors, such as high mass transfer and low excess of sludge per unit of reactor volume. Numerical techniques such as Computational Fluid Dynamics (CFD) can be applied to study these multiphase reactors in detail. However, in most of the available literature it is possible to find only models developed to predict gas-liquid or gas-solid interactions, even though the dynamics in anaerobic bioreactors is dominated by interactions between the liquid and solid phases. The aim of the present study is to evaluate the effect of the drag force together with the influence of the lift force and particles collisions on the prediction of bioparticles fluidization. Moreover, a modification to the drag model will be presented in order to achieve better agreement with experimental data. Simulations were performed with a two-phase, transient and turbulent approach, applied to a symmetrical three-dimensional geometry. Results show a redistribution of the bioparticles along the radial direction due to the lift force. Moreover, the use of different restitution coefficient values did not significantly affect the height of the fluidized bed. It was also observed that the usual drag models are unsuitable for predicting the height of the liquid-solid fluidized bed in the case considered, i.e. with bioparticles. A modification of the drag force with a dimensionless number provided good agreement with the experimental data under different operational conditions.

CFD-DEM Numerical Simulation and Experimental Validation of Heat Transfer and Two-Component Flow in Fluidized Bed

Abstract Based on the improved computational fluid dynamics and discrete element method (CFD-DEM), heat transfer and two-component flow of biomass and quartz sand have been studied from experiments and numerical simulation in this paper. During experiments, the particle temperature and moving images are respectively recorded by infrared thermal imager and high speed camera. With the increase of the velocity, the mixing index (MI) and the cooling rate of the particles are rising. Due to larger heat capacity and mass, the temperature of biomass drops slower than that of quartz sand. Fictitious element method is employed to solve the incompatibility of the traditional CFD-DEM where the cylindrical biomass are considered as an aggregation of numerous fictitious sphere particles arranged in certain sequence. By the comparison of data collected by infrared thermal imager and the simulated results, it can be concluded that experimental data is basically agreement with numerical simulation results. Directly affected by inflow air (25℃), the average temperature of particles in the bed height area (h >30 mm) is about 3 degrees lower than that of the other heights. When the superficial gas velocity is larger, the fluidization is good, and the gas temperature distribution is more uniform in the whole area. On the contrary, bubbles are not easy to produce and the fluidization is restricted at lower superficial gas velocity. Gas-solid heat transfer mainly exists under the bed height of 10 mm, and decreases rapidly on fluidized bed height. The mixing index (MI) is employed to quantitatively discuss the mixing effectiveness, which first rises accelerate, then rising speed decreases, finally tends to a upper limit.

Bubbling and Slugging of Geldart Group A Powders in Small Diameter Columns

Bubbling and slugging were investigated in a 0.05 m internal diameter bed of Geldart group A powders. The slugging characteristics were assessed in terms of the operating parameters. Neither bed height nor particle properties affect the excess gas velocities at the onset of slugging, as predicted by the Stewart criterion. Slugging characteristics (length and frequency) and fluidized bed properties were determined. The bottom zone of the bed is freely bubbling, and the bubble sizes can be predicted by dB = 0.047H0.37, where dB is the frontal diameter of the bubble and H is the fluidized bed height. Mostly, wall slugs occur higher in the bed when dB exceeds about 50% of the column diameter. The slug frequency varies with the height in the bed but reaches an equilibrium value of 1 Hz for bed heights exceeding ∼1 m. Three distinct zones appear in a deep bed of group A powders. The design and scale-up should combine the bubbling and slugging modes within their corresponding regions in the bed.

Experimental characterization of axial fuel mixing in fluidized beds by magnetic particle tracking

A magnetic particle tracking (MPT) system is applied to a bubbling fluidized bed to study how axial mixing and segregation of fuel are influenced by the fuel density and operational conditions (fluidization velocity, bed height and pressure drop across the gas distributor). The MPT system is used to determine the vertical distribution of the tracer particle in a fluid-dynamically down-scaled cold unit resembling a 0.74×0.74m2 fluidized bed reactor operating at 800°C. This work uses a tracer particle of 10mm in diameter, corresponding to a fuel particle of 44mm. Different tracer particles are applied with solids density representing biomass, biomass char and that of the average bulk. The MPT system yields a spatial accuracy in the order of 10−3m and a time resolution of 10−3s.For the operational range investigated, three fuel segregation regimes can be identified from the MPT measurements: 1) A flotsam regime which occurs at low fluidization velocities and for low density tracer particles, 2) A transition regime over which an increase in fluidization velocity results in the presence of fuel particles at the bed surface decreases rapidly, and 3) A fully developed mixing regime in which the presence of tracer particle at the bed surface and the splash zone remains constant with fluidization velocity. The transition velocities between the regimes depend on bed height and density of the tracer particle.