Liquid Fluidized Beds Research Articles

Simulation of structural phenomena in mixed‐particle fluidized beds

AbstractA statistical mechanical simulation was developed and used to predict the macroscopic and microscopic behavior of fluidized beds. In particular, the model was used to study structural phenomena in liquid fluidized beds that contain a distribution of particle sizes and/or densities. Bed structural phenomena, such as density‐based and size‐based segregation, classification, expansion, inversion, and solids dispersion have all been successfully simulated. Experimental data for the segregation of glass beads of different sizes and different densities agree well with predictions made from the simulation (without adjustable parameters). Classification and dispersion simulations also show good agreement with experimental data.

98/00796 Characterization of combustion behavior of fuels with the help of sensor technology

Liquid fluidized beds of angular particles exhibit channelling. Unsteady-state tracer injection experiments do not allow accurate detection of channelling. Two new, more sensitive, techniques to estimate channelling were developed: local conductivity measurements within the bed and cross-sectional averafe conductivity measurements from steady-state liquid tracer injections into the bed. The complete fluidization velocity can be estimated from local conductivity measurements and from the fractal dimension of unsteady-state tracer injections both into the bed and upstream of it. Jet mixing is a practical and effective method for reducing channelling in liquid fluidized beds without increasing particle entrainment.

Heat Transfer in Liquid-Fluidized Beds affected by Column Wall.

Heat transfer from column wall to liquid-fluidized beds was investigated experimentally. The diameter ratio of particle to column was changed from 0.089 up to 0.332, using 11 sizes of particles and two sizes of columns. It was found from the experiment, that the critical diameter ratio of particle to column is 0.2, below this value a good fluidizing condition is maintained for any flow rate and heat transfer coefficient changes smoothly from the minimum fluidization to the liquid singlephase flow. Based on the experimental data, a correlation was derived to predict heat transfer coefficient for the case affected by column wall.

Characteristics of Particle Mixtures in a Liquid-Fluidized Bed.

Behaviour of particle mixtures was investigated in a liquid-fluidized bed experimentally. In the experiment, two kinds of particles of different diameter, and of the same or different materials, were charged in a fluidization column and fluidized by water. Based on the observation of fluidization, a flow pattern map was proposed using the Archimedes number ratio and the terminal velocity ratio, which classifies the two patterns, i.e. separation and homogeneous mix. Measured mean void fractions of particle mixtures were agreed well with the values calculated from those for each particle independently.

Pulsed adsorbent transport in a countercurrent adsorptive reactor

AbstractSelective countercurrent transport of dense adsorbent particles through pulsed multistage, liquid fluidized beds of light catalyst particles is attractive as the basis for new countercurrent adsorptive reactors. Its applications are expected to demand operation at high solids flows, and thus at high pulsation frequencies and volumes. The effect of high pulsation frequencies and volumes on the flows of liquid and solids is studied by model and experiment. The key parameter determining contactor behavior is the product of pulsation frequency and volume: the pulsed back flow. Its theoretical maximum (flooding condition) equals the liquid flow rate. High pulsation frequencies and volumes greatly affect the degree of liquid and solids mixing. The degree of liquid mixing increases exponentially with increasing pulsed back flow from plug‐flow behavior (Pe = 10–100/stage) to complete mixing near the flooding condition. The solids mixing at low‐to‐moderate solids transport could be described with complete mixing at this stage. At higher solids transport, the extent of mixing slightly decreases toward two tanks‐in series per stage. The relations established allow for a preliminary design of a pulsed multistage fluidized‐bed reactor at any low and high solids transport rate, as well as for the estimation of corresponding operating conditions.

Effect of wall on void fraction of liquid-fluidized beds

Void fractions of liquid fluidized beds were measured using water as the fluidizing liquid. Particles of glass, ceramics, and chromium were tested, using a diameter range from 1.87 mm to 8.34 mm. Fluidization columns having a diameter of 51, 34, 21, and 8 mm were used. A void fraction measurement was performed for each particle-column combination. Based on the experimental data, a new correlation was derived that enabled over 95% of the data to be predicted to within ±7%. Existing correlations are also reviewed and compared with the results.

Collisional particle pressure measurements in solid–liquid flows

Experiments were conducted to measure the collisional particle pressure in both cocurrent and countercurrent flows of liquid–solid mixtures. The collisional particle pressure, or granular pressure, is the additional pressure exerted on the containing walls of a particulate system due to the particle collisions. The present experiments involve both a liquid-fluidized bed using glass, plastic or steel spheres and a vertical gravity-driven flow using glass spheres. The particle pressure was measured using a high-frequency-response flush-mounted pressure transducer. Detailed recordings were made of many different particle collisions with the active face of this transducer. The solids fraction of the flowing mixtures was measured using an impedance volume fraction meter. Results show that the magnitude of the measured particle pressure increases from low concentrations (<10% solid volume fraction), reaches a maximum for intermediate values of solid fraction (30–40%), and decreases again for more concentrated mixtures (>40%). The measured collisional particle pressure appears to scale with the particle dynamic pressure based on the particle density and terminal velocity. Results were obtained and compared for a range of particle sizes, as well as for two different test section diameters.In addition, a detailed analysis of the collisions was performed that included the probability density functions for the collision duration and collision impulse. Two distinct contributions to the collisional particle pressure were identified: one contribution from direct contact of particles with the pressure transducer, and the second one resulting from particle collisions in the bulk that are transmitted through the liquid to the pressure transducer.

Flow Structure of the Solids in a Three-Dimensional Liquid Fluidized Bed

The motion of solids in a cylindrical liquid fluidized bed was experimentally characterized by means of a noninvasive radioactive particle tracking technique (RPT) for two monosized bed inventories, spherical glass beads (3 mm GB, density = 2,500 kg/m{sup 3}) and nonspherical poly-(vinyl chloride) particles (5.5 mm PVC, density = 1,300 kg/m{sup 3}). RPT-monitoring of a single radioactive solid tracer mimicking faithfully the characteristics of the solids revealed the detailed solids flow features by determining the tracer instantaneous 3-D Lagrangian trajectory and then its local instantaneous velocities. Transverse solids dispersion coefficients were an order of magnitude lower than their axial counterpart. Time, longitudinal, and circumferential averaging of local velocities in the fully developed region of the fluidized bed revealed a weak gulf streaming characterized by axial velocities positive (respectively negative) in the core and negative (respectively positive) near the vessel walls for the GBs (respectively for the PVCs). The measured mean circumferential ensemble-averaged radial velocity was essentially zero in the fully developed region. The solids flow turbulence field was anisotropic, but the less dense the particles, the closer to isotropy was the solids flow. Computed shear stress profiles showed that PVC particles exhibited a higher shear stress than GB particles. Themore » dynamic solids flow structure inside a 3-D liquid fluidized bed can be viewed as a core-annulus structure with axially-dispersed plug-flow and radial dispersion in each of the upward and downward zones.« less

Modeling of multi‐solid liquid fluidized bed

AbstractA dynamic model of the liquid fluidized bed containing two or more solid particle species of different size and density is presented. The model incorporates the particle mass transport mechanisms of the convection and the dispersion. The movement of the upper interface of the bed subject to a change in the liquid velocity is specified using the mass balance constraint. The particle velocities and dispersion coefficients are evaluated using correlations. The model is capable of describing the bed expansion, concentration profiles of the individual particle species, the bulk density profile, and the occurrence of layer inversion.

Mixing and segregation in water fluidised-bed bioreactors

The complete segregation model, already successfully tested for binary-solid liquid fluidised beds of smooth rigid particles, has been applied to fluidised-bed bioreactors. Qualitative comparison with reported experimental behaviour has shown the capability of the model to predict solid mixing and segregation in these specific types of fluidised beds.

Mechanisms of biofilm detachment in fluidized bed reactors

Biofilm detachment in liquid fluidized bed biological reactors was investigated to point out how different mechanisms influence the process. Erosion due to liquid shear and abrasion due to collisions of particles were considered as possible mechanisms of biomass detachment in liquid fluidized beds. A dimensional analysis technique allowed the identification of the significant parameters affecting the process. The influence of these parameters was established on a lab-scale reactor. An empirical model was proposed to correlate the experimental data and to analyze the effect of some characteristic quantities, such as particle Reynolds number, biomass fraction, liquid shear stress and solid concentration, on the detachment rate. Detachment rate strongly increased with fluid velocity while, owing to modifications in biofilm structure and morphology during the biological growth, it slightly decreased with liquid shear stress.

Characterization of channelling in multiphase systems. Application to a liquid fluidized bed of angular Biobone particles

Liquid fluidized beds of angular particles exhibit channelling. Unsteady-state tracer injection experiments do not allow accurate detection of channelling. Two new, more sensitive, techniques to estimate channelling were developed: local conductivity measurements within the bed and cross-sectional averafe conductivity measurements from steady-state liquid tracer injections into the bed. The complete fluidization velocity can be estimated from local conductivity measurements and from the fractal dimension of unsteady-state tracer injections both into the bed and upstream of it. Jet mixing is a practical and effective method for reducing channelling in liquid fluidized beds without increasing particle entrainment.

Prediction of optimum operating conditions of liquid fluidized bed systems

AbstractAn important advantage of fluidized beds applied in various chemical and physical operations is the high rate of heat transfer between the bed and the heat transfer surfaces. However, the design of these systems for optimum conditions remains uncertain and essentially empirical. In this study a model for bed voidage and a model for heat transfer are presented which can be used to predict the maximum attainable heat transfer coefficient and the corresponding bed voidage of liquid fluidized bed systems with good accuracy.

Fully developed travelling wave solutions and bubble formation in fluidized beds

It is well known that most gas fluidized beds of particles bubble, while most liquid fluidized beds do not. It was shown by Anderson, Sundaresan & Jackson (1995), through direct numerical integration of the volume-averaged equations of motion for the fluid and particles, that this distinction is indeed accounted for by these equations, coupled with simple, physically credible closure relations for the stresses and interphase drag. The aim of the present study is to investigate how the model equations afford this distinction and deduce an approximate criterion for separating bubbling and non-bubbling systems. To this end, we have computed, making use of numerical continuation techniques as well as bifurcation theory, the one- and two-dimensional travelling wave solutions of the volume-averaged equations for a wide range of parameter values, and examined the evolution of these travelling wave solutions through direct numerical integration. It is demonstrated that whether bubbles form or not is dictated by the value of Ω = (ρsv3t/Ag) 1/2, where ρs is the density of particles, vt is the terminal settling velocity of an isolated particle, g is acceleration due to gravity and A is a measure of the particle phase viscosity. When Ω is large (> ∼ 30), bubbles develop easily. It is then suggested that a natural scale for A is ρsvtdp so that Ω2 is simply a Froude number.

On-line flow visualization in multiphase reactors using neural networks

The success of the radioactive particle tracking system (RPT) developed at the École Polytechnique (Montréal) and pplied to the study of particle motion in a variety of chemical reactors (three-phase fluidized bed, gas spouted bed and liquid fluidized bed) has motivated us to continue improving this technique (in terms of accuracy and resolution) and to apply it to new reactor types. Our goals are: (i) to enhance the original search location algorithm in order to permit on-line flow visualization and (ii) to extend RPT to very fast solids flows, such as those encountered in circulating fluidized beds (particle velocities higher than a few m s −1). The potential of neural networks for on-line and real-time visualization of particle movements in multiphase reactors will be illustrated. The original least-squares search location algorithm (Larachi et al., 1994) has been replaced with an enhanced algorithm which uses a three-layer feedforward neural network. The results obtained from the two algorithms for particle tracking in a three-phase fluidized bed reactor are compared. The RPT system employs 8 NaI(TI) scintillation detectors to study the movement of solid particles in chemical reactors. The performance of the system was investigated using particles containing the radioisotopes 46Sc (γ-ray energy 1005 keV), 99Mo (γ-ray energy 140 keV) and 198Au (γ-ray energy 412 keV). The three-dimensional spatial resolution was measured in empty and water-filled tubes, simulating highly diluted and dense media. The best results were obtained using 198Au with which the particle can be located to within 7 mm in water and 9 mm in air 100 times s −1.

Trickle flow of dense particles through a fluidized bed of others

Dense (or large) particles fall through liquid fluidized beds of light (or small) particles. In this work, a new contactor is described in which continuous, countercurrent transport of dense particles in a stationary, liquid fluidized bed of light particles is exploited to obtain selective and continuous transport of the dense phase. The system is refered to as the ‘trickle flow fluidized-bed reactor’. This sytem is evaluated in its application as a countercurrent adsorptive reactor. By selecting a suitable adsorbent as the dense phase, and a catalyst as the light phase, simultaneous reaction and countercurrent product removal may be achieved within one contactor. For a systematic design and optimization of such an adsorptive trickle flow fluidized-bed reactor, a predictive hydrodynamic model is required. In this work, the relation between the volume fractions and the fluxes of the three phases is investigated. A laboratory scale trickle flow fluidized-bed reactor has been constructed for experimental studies on hold-up and flux. A multi-component transport model to predict the volume fractions, fluxes and operating conditions is developed. The model is validated with the experimental hold-up and flux data from the trickle flow fluidized-bed reactor. To apply this concept as a countercurrent adsorptive reactor, a high countercurrent adsorbent flow, a high hold-up of the dense adsorbent and light catalyst are required, together with a stable operation. Hence, the reactor should operate at a minimum liquid fraction of approximately 50 vol% and a free area fraction of the supporting plate between 0.3 and 0.5. It is demonstrated that the system is very sensitive to the bulk density of the multi-component suspension. At a high hold-up of the dense particles, the bulk density of the bidisperse suspension may increase to a level which causes a complete wash-out of the light particles. Therefore, the difference in density between the light catalyst and dense adsorbent particles should be as small as possible.

Mechanisms of biofilm detachment in fluidized bed reactors

Biofilm detachment in liquid fluidized bed biological reactors was investigated to point out how different mechanisms influence the process. Erosion due to liquid shear and abrasion due to collisions of particles were considered as possible mechanisms of biomass detachment in liquid fluidized beds. A dimensional analysis technique allowed the identification of the significant parameters affecting the process. The influence of these parameters was established on a lab-scale reactor. An empirical model was proposed to correlate the experimental data and to analyze the effect of some characteristic quantities, such as particle Reynolds number, biomass fraction, liquid shear stress and solid concentration, on the detachment rate. Detachment rate strongly increased with fluid velocity while, owing to modifications in biofilm structure and morphology during the biological growth, it slightly decreased with liquid shear stress.

97/00735 Oil sludge recycling by fluidized-bed pyrolysis

Liquid fluidized beds of angular particles exhibit channelling. Unsteady-state tracer injection experiments do not allow accurate detection of channelling. Two new, more sensitive, techniques to estimate channelling were developed: local conductivity measurements within the bed and cross-sectional averafe conductivity measurements from steady-state liquid tracer injections into the bed. The complete fluidization velocity can be estimated from local conductivity measurements and from the fractal dimension of unsteady-state tracer injections both into the bed and upstream of it. Jet mixing is a practical and effective method for reducing channelling in liquid fluidized beds without increasing particle entrainment.

Effect of Wall on Void Fraction of Liquid-Fluidized Beds.

Void fractions of liquid fluidized beds were measured using water as the fluidizing liquid. Particles of glass, ceramics and chromium were tested, and their diameter range was from 1.87mm to 8.34mm. Fluidization columns having a diameter of 51, 34, 21 and 8mm were used. A void fraction measurement was performed for each particle column combination. Based on the experimental data, a new correlation was derived which enabled over 95% of the data to be predicted within ±7%. Existing correlations were also reviewed and compared with the results.

Propagation of voidage waves in a two-dimensional liquid-fluidized bed

Propagation of voidage waves in a two-dimensional liquid-fluidized bed