SLURRY DRYING IN GAS-PARTICLE CONTACTORS: FLUID-DYNAMICS AND CAPACITY ANALYSIS

The fluid-dynamic characteristics and evaporation capacities of several substrates (pure water, a mixture of water and carbohydrates, and apple pulps with maltodextrines) for conical spouted bed, fluidized bed and pneumatic dryers was compared.

For fluidized and spouted beds, the presence of fluids increased the fluid-dynamics parameters over the values when only inert particles were present in the beds. In both cases it was proposed correlations to predict those effects. In the spouted bed drier, increasing the load of inert particles resulted in a slight increase of the water evaporation capacity, while in the fluidized bed drier the effect was more marked. For fluidized and spouted beds, under similar operation conditions, the maximums of the suspension drying capacities are lower than the values of water evaporation, but for pneumatic drying this effect was the opposite. In general terms, the maximal capacities of fluidized bed dryers are higher than those observed in the spouted bed dryers.     

Comparisons of particle cluster diameter and concentration in circulating fluidized bed riser and downer using computational fluid dynamics simulation

The information of particle cluster dynamics is necessary for improving the performance of a circulating fluidized bed system. The main objective of this study is to compare the particle cluster diameters and concentrations from computational fluid dynamics simulation results between circulating fluidized bed riser and downer. The calculation methodologies are based on the concept of kinetic theory of granular flow and statistics. The mathematical model was verified by using the experimental dataset from literature and used for computing the particle cluster dynamics. In the circulating fluidized bed riser and downer, a dense and dilute core-annulus flow structures were obtained, respectively. The particle cluster in the circulating fluidized bed riser possessed more heterogeneity movements than that in the circulating fluidized bed downer. This can be explained by the system flow direction. About the particle cluster dynamics, the particle cluster diameters and concentrations in the circulating fluidized bed riser were higher than the ones in the downer. The calculated values were comparable to the empirical correlations. This confirms the validity of the calculation methodologies. Particle cluster dynamics and its example application inside circulating fluidized bed riser and downer were also discussed.     

LIQUID-SOLID MASS TRANSFER IN A THREE PHASE DRAFT TUBE FLUIDIZED BED REACTOR

Liquid-solid mass transfer coefficients in a three phase draft tube fluidized bed reactor have been measured using spherical ion exchange particles. The particle diameters ranged from 655 to 1119μm and solids volume fractions of approximately 5 and 10% were employed in water at 28°C. The experimental data can be successfully correlated using a Reynolds number derived using Kolmogoroffs theory of isotropic turbulence, although it is doubtful whether isotropic turbulence actually prevails in the fluidized bed over the range of conditions employed. Comparison with correlations determined for bubble columns and gas-liquid fluidized beds is performed. A model which considers the draft tube reactor as comprising two distinct fluid mechanical regions is developed to explain the apparently lower values of mass transfer coefficients obtained in a draft tube as opposed to conventional fluidized bed reactor.     

Fluidization of elongated particles—Effect of multi-particle correlations for drag,lift, and torque in CFD-DEM

Having proper correlations for hydrodynamic forces is essential for successful CFD-DEM simulations of a fluidized bed. For spherical particles in a fluidized bed, efficient correlations for predicting the drag force, including the crowding effect caused by surrounding particles, are already available and well tested. However, for elongated particles, next to the drag force, the lift force, and hydrodynamic torque also gain importance. In this work, we apply recently developed multi-particle correlations for drag, lift and torque in CFD-DEM simulations of a fluidized bed with spherocylindrical particles of aspect ratio 4 and compare them to simulations with widely used single-particle correlations for elongated particles. Simulation results are compared with previous magnetic particle tracking experimental results. We show that multi-particle correlations improve the prediction of particle orientation and vertical velocity. We also show the importance of including hydrodynamic torque.     

Numerical simulation of horizontal jet penetration in a three-dimensional fluidized bed

The introduction of reactant gas as a jet into a fluidized bed chemical reactor is often encountered in various industrial applications. Understanding the hydrodynamics of the gas and solid flow resulting from the gas jet can have considerable significance in improving the reactor design and process optimization. In this work, a three-dimensional numerical simulation of a single horizontal gas jet into a cylindrical gas-solid fluidized bed of laboratory scale is conducted. A scaled drag model is proposed and implemented into the simulation of a fluidized bed of FCC particles. The gas and particles flow in the fluidized bed is investigated by analyzing the transient simulation results. The jet penetration lengths of different jet velocities have been obtained and compared with published experimental data as well as with predictions of empirical correlations. The predictions by several empirical correlations are discussed. A good agreement between the numerical simulation and experimental results has been achieved.     

Hydrodynamics of three-phase fluidization of homogeneous ternary mixture in a conical conduit——Experimental and statistical analysis

Hydrodynamics of conical fluidized bed differ from that of columnar beds by the fact that a velocity gradient exists along the axial direction of the bed.The gas–liquid–solid fluidized bed has emerged in recent years as one of the most promising devices for three-phase operations.Such a device is of considerable industrial importance as evident from its wide applications in chemical,refining,petrochemical,biochemical processing,pharmaceutical and food industries.To explore this,a series of experiments have been carried out for homogeneous well-mixed ternary mixtures of dolomite of varying compositions in a three-phase conical fluidized bed.The hydrodynamic characteristics determined included the bed pressure drop,bed fluctuation and bed expansion ratios.The single and combined effects of operating parameters such as superficial gas velocity,superficial liquid velocity,initial static bed height,average particle size and cone angle on the responses have been analyzed using response surface methodology(RSM).A 25 full factorial central composite experimental design has been employed.Analysis of variance(ANOVA) showed a high coefficient of determination value and satisfactory prediction second-order regression models have been derived.Experimental values of bed pressure drop,bed fluctuation and bed expansion ratios have been found to agree well with the developed correlations.     

Prediction of Pressure Drop and Holdup for Homogeneous Ternary Mixtures of Spherical Glass Bead Particles in a Three-Phase Fluidized Bed

Hydrodynamic characteristics, viz. bed pressure drop and gas holdup, have been studied for ternary mixtures of homogeneous regular particles in a co-current three-phase fluidized bed. For this, a series of experiments have been carried out in a 5-cm diameter column with air as the gas phase, water as the liquid phase, and ternary mixtures of glass beads (1.54, 1.3, and 1.1 mm) as the solid phase. The dependence of bed pressure drop on the average particle diameter, superficial gas velocity, and initial static bed height has been discussed. Based on the dimensional and statistical analyses, correlations have been developed with the system parameters, for both bed pressure drop and gas holdup. Experimental values of bed pressure drop and gas holdup have been found to agree well with those calculated from developed correlations.     

MASS AND MOMENTUM TRANSFER WITH NON-NEWTONIAN FLUIDS IN FLUIDIZED BEDS

A new model for non-Newtonian fluid flows through fluidized beds is developed by extending the Richardson-Zaki concept. In the proposed model, the existing correlations for an isolated single sphere are simply extended to those for a fluidized bed by incorporating a voidage function. The model is found to be in a good agreement with other correlations for flows of a non-Newtonian fluid through multi-particle systems. The model is also applied to interpret mass transfer in fluidized beds.     

MASS AND MOMENTUM TRANSFER WITH NON-NEWTONIAN FLUIDS IN FLUIDIZED BEDS

A new model for non-Newtonian fluid flows through fluidized beds is developed by extending the Richardson-Zaki concept. In the proposed model, the existing correlations for an isolated single sphere are simply extended to those for a fluidized bed by incorporating a voidage function. The model is found to be in a good agreement with other correlations for flows of a non-Newtonian fluid through multi-particle systems. The model is also applied to interpret mass transfer in fluidized beds.     

Impacts against the wall of a scaled-up fluidized bed

A scaled-up fluidized bed permitted the accurate determination of impact parameters, due to the relative improvement of the impact measuring system. The mean perpendicular kinetic energy per impact and the mean impact frequency per unit area, calculated by the equations derived by Bordet et al., did not agree with the experimentally determined values in the scaled-up fluidized bed. When these equations were used to calculate the same parameters for the original fluidized bed as applied in an MSF/FBE, again considerable deviations were found with the scaled-down experimental values.The Maxwell-Boltzmann velocity distribution calculated for the fluidized bed system appeared not to be valid when the values of comparatively low velocity impacts were included. The mean perpendicular velocity at impact was about a factor 10 lower than the superficial fluid velocity at a porosity of 0.8. The porosity of the fluidized bed measured as a function of the superficial fluid velocity appeared to be in good agreement with the equation derived by Richardson and Zaki.     

气固脉冲流化床流体力学特性的研究

在φ７０ｍｍ的流化床内，采用聚氯乙烯、玻璃珠和不规则天然刚玉等Ｂ类、Ｄ类颗粒，测定了０￣５．０Ｈｚ脉冲频率下气固流化床的基本流体力学特性，探讨了影响床层流化特性的一些主要因素，并根据实验数据对脉冲流化床的临界流化速度和临界流化压降的无因次准数式进行了关联。     

Behaviour of gas-fluidized beds of fine powders part III. Effective thermal conductivity of a homogeneously expanded bed

The effective thermal conductivity of the dense phase in expanded non-bubbling fluidized beds has been studied between minimum fluidization and minimum bubbling for 9 gas—solid systems. Except close to minimum bubbling, the effective thermal conductivity is not a function of bed voidage and is most sensitive to the gas phase thermal conductivity. The experimental results have been compared with 11 packed bed correlations and with suitable modification three of these can be used to accurately predict the effective thermal conductivity of a non-bubbling fluidized bed.     

The Effect of Bed Temperature on Mass Transfer between the Bubble and Emulsion Phases in a Fluidized Bed

The rate of interphase mass transfer between the bubble and emulsion phases of a bubbling fluidized bed is of primary importance in all models for fluidized bed reactors. Many experimental studies have been reported, however, all these investigations have been carried out in fluidized beds operated at room temperature. In this work, the effect of the bed temperature on the interphase mass transfer is reported. Single bubbles containing argon – used as a tracer – were injected into an incipiently fluidized bed maintained at the required temperature. The change in argon concentration in the bubble was measured using a suction probe connected to a mass spectrometer. The effects of bed particle type and size, bubble size, and bed temperature on the mass transfer coefficient were examined experimentally. The interphase mass transfer coefficient was found to decrease with the increase in bed temperature and bubble size, and increase slightly with increase in particle size. Experimental data obtained in this study were compared with some frequently used correlations for estimation of the mass transfer coefficient.     

Hydrodynamics of a superheated steam vacuum fluidized bed

Hydrodynamics of a superheated steam vacuum fluidized bed was experimentally studied. In these experiments, eight different types of large particles (1970–7430 μm) were used. In all cases, a behavior similar to that found in an air fluidized bed was observed. The minimum fluidization velocity was found to be increasing with decreasing operating pressure. In the case of employing superheated steam, the minimum fluidization conditions are established at a lower velocity than using air as the fluidizing medium. These tendencies are attributed to the variation of the mean free path of molecules. On the other hand, the experiments showed that the bed voidage in the minimum fluidization conditions is almost insensitive to the variation of the operating pressure. Several equations were developed to predict the minimum fluidization velocity. The values provided by these equations were compared with the experimental data as well as with the predictions of the correlations presented in the technical literature.     

磷矿颗粒流态化特性的实验研究

在高1 m、内径42mm的流化床中,对粒径54-600 μm、密度2 252-2 665 kg/m3的磷矿颗粒的流态化特性进行实验研究.实验结果表明:磷矿颗粒粒径和密度对磷矿颗粒流态化行为有较大影响,床层膨胀比随着磷矿颗粒粒径的增大而逐渐减小.当磷矿颗粒属于Geldart B类颗粒时,流化较好;而当颗粒平均粒径为82 ...     

Entrainment of particles from a pilot scale fluidized bed

Particles entrained from a pilot scale fluidized bed of sand having a mean size of 102 μm have been captured and analysed at superficial air velocities up to 1.3 m/s. Variation of bed temperature over the range 300 to 445 K and insertion of a tube bundle in the splash zone had little effect on the measured entrainment fluxes. Existing correlations for predicting entrainment from fluidized beds gave widely divergent results, and generally differed markedly from the experimental data. Of the correlations tested, the graphical saturation carrying-capacity correlation of Zenz and Weil gave the best agreement with the experimental data. The size distribution of particles carried over differed widely from the size distribution in the bed at low gas velocities but approached the in-bed distribution at higher velocities.     

SOME ASPECTS OF THEORETICAL COMPARISON OF FISCHER-TROPSCH REACTORS

The reactor systems used for the Fischer-Tropsch synthesis, fixed bed, fluidized bed and slurry bed, are compared on the basis of space time yield (STY) and level of conversion obtainable under the same set of feed and operating conditions. The slurry bed and fluidized bed reactor were compared on the basis of a first order reaction model. The performance of these two reactors was found to be comparable at low values of WHSV, but at higher values of WHSV, the fluidized bed reactor gave higher conversions and STY. A power law kinetic expression was used to compare the performance of the slurry bed and fixed bed reactors. Higher conversions and STY were obtained from the fixed bed with varying WHSV. This may be due to the omission of the intra and inter phase mass transfer resistances in the modelling of the fixed bed reactor.     

Correlations for mass transfer between fluidized spheres and a liquid

Data on mass transfer between the solid particles and the liquid in a fluidized bed have been analyzed. After a convenient selection, various correlations have been derived. They show that for a given fluidized system, the Sherwood number is independent on the Reynolds number.     

Experimental verification of scaling relationships for fluidized bed

To verify the scaling relationships for fluidized beds, an atmospheric fluidized bed combustor and a cold bed constructed to operate at the same values     

Experimental and computational study of the bed dynamics of semi‐cylindrical gas–solid fluidized bed

With computational fluid dynamics (CFD) it is possible to get a detailed view of the flow behaviour of the fluidized beds. A profound and fundamental understanding of bed dynamics such as bed pressure drop, bed expansion ratio, bed fluctuation ratio, and minimum fluidization velocity of homogeneous binary mixtures has been made in a semi‐cylindrical fluidized column for gas–solid systems, resulting in a predictive model for fluidized beds. In the present work attempt has been made to study the effect of different system parameters (viz., size and density of the bed materials and initial static bed height) on the bed dynamics. The correlations for the bed expansion and bed fluctuations have been developed on the basis of dimensional analysis using these system parameters. Computational study has also been carried out using a commercial CFD package Fluent (Fluent, Inc.). A multifluid Eulerian model incorporating the kinetic theory for solid particles was applied in order to simulate the gas–solid flow. CFD simulated bed pressure drop has been compared with the experimental bed pressure drops under different conditions for which the results show good agreements.