Study of dynamic multi-dimensional temperature and concentration distributions in liquid-sprayed fluidized beds

A physically based model was developed for heat and mass transfer processes in liquid-sprayed fluidized beds. Such fluidized beds are used for granulation, coating and agglomeration. The complex correlations of a number of microprocesses, spraying, wetting, drop deposition, heat transfer, drying and mass transfer were studied, and transient three-dimensional distributions of air humidity, air temperature, particle wetting efficiency, liquid film temperature, particle temperature, local liquid loading and local evaporation rate were calculated. For the evaluation of the model, the stationary spatial air temperature distributions were measured at a fluidized bed pilot plant of the institute. The fluidized bed of monodisperse wood- or glass beads was sprayed with clear water. Conclusions are drawn on the relevance of particle dispersion, spraying and drying to simulating temperature and concentrations distributions.     

Numerical Spray Model of the Fluidized Bed Coating Process

In this study, a new model for the batch top-spray fluidized bed coating process is presented. The model is based on the one-dimensional (axial) discretization of the bed volume into different control volumes, in which the dynamic heat and mass balances for air, water vapor, droplets, core particles, and coating material were established. The coupling of the droplet phase's mass and heat transfer terms with the gas and solid phases was established by means of a droplet submodel in which droplet trajectories were individually simulated.

The model calculation method combines a Monte Carlo technique for the simulation of the particle exchange with the first-order Euler's method for solving the heat and mass balances, enabling the prediction of both the dynamic coating mass distribution and the one-dimensional (axial) thermodynamic behavior of the fluidized bed during batch operation. The simulation results were validated using experimental two-dimensional spatial air temperature and air humidity distributions, which were measured in a fluidized bed pilot reactor using a scanning probe.

Sensitivity analysis was carried out to study the effect of controllable process variables, such as fluidization air and atomization air properties, as well as the properties of the spraying liquid upon the simulated dynamic temperature and humidity distributions. Also, the effects of relevant process variables on growth rate uniformity and process yield were studied. Based on these sensitivity studies it was concluded that nozzle parameters, such as air pressure and positioning with respect to the bed, are as important as the fluidization air properties (humidity, temperature, and flow rate) for the coating growth rate uniformity and process yield.     

Numerical simulation of temperature and concentration distributions in fluidized beds with liquid injection

In this paper a numerical simulation study of dynamic behavior of a fluidized bed with liquid injection is presented. A continuum model has been developed taking into account the mass and energy balances of solid, gas as well as liquid to describe the temperature and concentration distributions in gas-solid-fluidized beds. The model considers the deposition efficiency of the liquid droplets as well as the influence of the spray nozzle region. For solving the non-linear partial differential equations with discrete boundary conditions a finite element method is used. Numerical computations have been done with two different schemes of time integration, a fully implicit and a semi implicit scheme. The complex correlations of mass and liquid flow rates, mass and heat transfer, drying, and transient two-dimensional air humidity, air temperature, particle wetting, liquid film temperature and particle temperature were simulated. The model was validated with transient measurements of the air temperature and air humidity at the outlet of a fluidized bed with water injection.     

Numerical Spray Model of the Fluidized Bed Coating Process

In this study, a new model for the batch top-spray fluidized bed coating process is presented. The model is based on the one-dimensional (axial) discretization of the bed volume into different control volumes, in which the dynamic heat and mass balances for air, water vapor, droplets, core particles, and coating material were established. The coupling of the droplet phase's mass and heat transfer terms with the gas and solid phases was established by means of a droplet submodel in which droplet trajectories were individually simulated.

The model calculation method combines a Monte Carlo technique for the simulation of the particle exchange with the first-order Euler's method for solving the heat and mass balances, enabling the prediction of both the dynamic coating mass distribution and the one-dimensional (axial) thermodynamic behavior of the fluidized bed during batch operation. The simulation results were validated using experimental two-dimensional spatial air temperature and air humidity distributions, which were measured in a fluidized bed pilot reactor using a scanning probe.

Sensitivity analysis was carried out to study the effect of controllable process variables, such as fluidization air and atomization air properties, as well as the properties of the spraying liquid upon the simulated dynamic temperature and humidity distributions. Also, the effects of relevant process variables on growth rate uniformity and process yield were studied. Based on these sensitivity studies it was concluded that nozzle parameters, such as air pressure and positioning with respect to the bed, are as important as the fluidization air properties (humidity, temperature, and flow rate) for the coating growth rate uniformity and process yield.     

Identification of thermal zones and population balance modelling of fluidized bed spray granulation

Air temperature measurements in a fluidized bed of glass beads top sprayed with water showed that conditions for particles growth were fulfilled only in the cold wetting zone under the nozzle which size and shape depended on operating conditions (liquid spray rate, nozzle air pressure, air temperature, and particles load). Evolution of the particle size distribution during agglomeration was modelled using population balance and representing the fluidized bed as two perfectly mixed reactors exchanging particles with particle growth only in the one corresponding to the wetting zone. The model was applied to the agglomeration of non-soluble glass beads and soluble maltodextrin particles spraying respectively an acacia gum solution (binder) and water. Among the three adjustable parameters, identified from experimental particle size distributions evolution during glass beads agglomeration, only one describing the kinetics of the size distribution evolution depended on process variables. The model allowed satisfying simulation of the evolution of the particle size distribution during maltodextrin agglomeration.     

Three-dimensional numerical study of heat and mass transfer in fluidized beds with spray nozzle

The numerical computations of temperature and concentration distributions inside a fluidized bed with spray injection in three-dimensions are presented. A continuum model, based on rigorous mass and energy balance equations developed from Nagaiah et al., is used for the three-dimensional simulations. The three-dimensional model equation for nozzle spray is reformulated in comparison to Heinrich. For solving the non-linear partial differential equations with boundary conditions a finite element method is used for space discretization and an implicit Euler method is used for time discretization.The time-dependent behavior of the air humidity, air temperature, degree of wetting, liquid film temperature and particle temperature is presented using two different sets of experimental data. The presented numerical results are validated with the experimental results. Finally, the parallel numerical results are presented using the domain decomposition methods.     

FLUID BED ENCAPSULATION OF PARTICLES: PRINCIPLES AND PRACTICE

The fluid bed encapsulation process consists of spraying a coating solution into a fluidized bed of solid particles. After several cycles of wetting-drying, a continuous film is formed. This paper provides an overview of the physical and transport phenomena involved in this process, especially in the so-called Wurster mode. Both qualitative and quantitative results are presented for several industrial applications. The main parameters affecting the process are flow-rate and pressure of the spraying liquid, composition and rheology of the coating solution, flow-rate and temperature of the fluidizing air. The particle size can range from 0.1 to several millimeters in equivalent diameter. The characteristics of the particle surface are important in determining adhesion of the liquid coating as well as its encapsulation properties.     

A Cheap,Fast and Simple Way to Measure Sorption Isotherms of Granular Materials

ABSTRACT

A set-up and a method are described with which sorption isotherms of granular materials can be measured. The set-up uses a recirculating air flow through a fluidized bed of granular materials. The measuring procedure starts with drying the granular material completely. A known amount of water is added to the fluidized bed. At equilibrium between the amount of water in the granular material and the circulating air the air humidity is measured. The method is demonstrated using silica gel.     

Modeling of the Spray Zone for Particle Wetting in a Fluidized Bed

In a spray agglomeration process the particle wetting influences the agglomerate growth and particle dynamics in the granulator. The mass of binder liquid that is deposited on single particles affects the amount of energy dissipation during particle contacts. For the agglomeration of colliding particles the whole impact energy has to be dissipated due to viscous and capillary adhesion forces in the liquid film and plastic deformation of the material. Therefore, a detailed knowledge of the particle wetting is necessary to model the agglomeration process. This contribution uses a coupled DEM‐CFD approach to describe the spray zone of a two‐fluid nozzle in a fluidized bed agglomerator. Droplets modeled as discrete elements showed the formation of a spray zone with a conical shape. Simulations of the spray zone and the wetting of single particles are in good agreement with experimental results.     

FILTERS AND FILTRATION HANDBOOK 4th Edition (1997) By T. Christopher Dickenson Publisher: Elsevier Advanced Technology The Boulevard,Langford Lane,Kidlington, Oxford OX5 1GB,U.K.

Abstract

Revolving air flow was generated by oblique holes on air distribution board in a fluidized bed dryer. Such a revolving air flow shows a large scale velocity fluctuation in radial as well as tangential directions. This turbulent flow of air can fluidize the inert particles with superior performance than vibrated fluidized bed. The revolving fluidized bed is simpler in fabrication and easier in operation. For two types of inert particles tested, 4 mm glass beads and 4 mm × 5 mm Teflon cylindrical extrudates, the volumetric heat transfer coefficients were found to increase with the liquid feed flowrate and air flow velocity but decrease with the air inlet temperature, height of static bed, and liquid concentration. The revolving fluidized bed gives an increase of volumetric heat transfer coefficient by 1 kW/m³ K, and represents a 15–25% enhancement from ordinary fluidized bed operated at a bed height of 60 mm, bed diameter of 140 mm, superficial air flow velocity of 3.5 m/s, liquid feed (Soya milk) flowrate of 20 mL/min at a concentration of 6.7%, and a temperature ranges of 80–140°C using Teflon extrudates as inert particles.     

Particles agglomeration in a conical fluidized bed in relation with air temperature profiles

Particles agglomeration is obtained by spraying liquid over solid particles fluidized by hot air. The growth mechanism depends on the operating parameters (geometry and process conditions) and initial materials, influencing drying conditions and agitation, leading either to agglomeration or coating or wet quenching. It is linked to air temperature and/or humidity distributions appearing in the well-mixing system of the fluidized bed due to the penetration of the sprayed liquid jet.In this study, air temperatures distributions in a conical fluidized bed of model particles (glass beads) top sprayed with water were measured varying the initial particles load (250, 500, 750 g), the fluidizing air inlet temperature (60-70-80 °C), the liquid feed rate (2.65, 5.33, ) and the relative air spraying pressure (1,2,3 bars). Three thermal zones were identified (heat transfer, isothermal, wetting-active), with sizes and shapes related to particles circulation patterns and drying and spraying conditions influenced by the operating parameters. Subsequent agglomeration trials, were carried out with glass beads and soluble maltodextrin particles agglomerated, respectively, with an acacia gum solution and water. They showed a relationship between the air temperatures distribution and the resulting growth mechanism. Particularly, controlled agglomeration was obtained for a wetting-active zone occupying 18-30% of the fluidized bed.     

Fluidized bed spray granulation: Investigation of the coating process on a single sphere

The coating and granulation of solid particles in a fluidized bed is a process which converts pumpable and atomizable liquids (solutions, slurries, melts) into granular solids in one step by means of drying. The solution to be processed is sprayed onto a fluidized bed. Particle growth can take place either via surface layering or agglomeration. In the case of surface layering the atomized droplets deposit a thin layer of liquid onto the seed particles. The solvent is then evaporated by the hot fluidizing, leaving behind the dissolved material on the surface. Although fluidized bed spray granulation and film coating have been applied in industry for several years, there is still a lack of understanding of the physical fundamentals and the mechanisms by which spherical granules are formed. Hence a new method was developed which allows the direct observation of the subsequent particle-forming mechanisms such as droplet deposition, spreading, wetting and drying. The authors present a laboratory scale apparatus in which a single freely suspended particle can be coated under well defined and constant coating and drying conditions. With this device, particle-growth-rate and the development of particle morphology were measured and investigated under various experimental conditions.     

A new fluidized bed coating process via photo‐initiated cationic polymerization

An ultraviolet (UV) photo‐polymerization particle coating process was developed by coupling the photo‐initiated cationic polymerization with the fluidized bed coating techniques. Unlike the conventional air‐suspension coating in the fluidized bed, the new process employs a UV curable composition instead of a solvent/water‐borne system as a coating material, which has a rapid curing rate and virtually no inhibition to oxygen and moisture. A modified fluidized bed coater equipped with a quartz window allows UV light to penetrate and to initiate the curing of photo‐sensitive polymerizable chemicals coated on the particles. A UV‐curable liquid composed of cycloaliphatic epoxide, oxetane, and triarylsulfonium cationic photo‐initiator was specifically formulated for the fluidized bed particle coating process. A systematic experimental approach including photo‐Differential Scanning Calorimetry, Fourier Transform Infrared Spectroscopy, and tackiness measurements has been developed to characterize the curing mechanism of the cationic UV curable formulations and to optimize the chemical compositions. The effects of the UV curable chemicals, viscosity of coating liquid, and the fluidization operating conditions on the physical properties of coated particles have been thoroughly investigated. Under optimized conditions, this novel process is very efficient as follows: particles can be coated very rapidly with ultra‐thin films of the cured chemicals, with little, if any, formation of particulate agglomeration. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Fluidized bed film coating of cohesive Geldart group C powders

The difficulty of coating cohesive Geldart group C powders in a conventional fluidized bed is attributed to strong inter-particle force between fine particles leading to poor fluidization behavior. Dry coating approach involving deposition of nanosize particles on the surface of group C powders is considered to reduce the interparticle force and improve the fluidization behavior of fine powders. Polymer film coating at an individual particle level is achieved on these pre-coated fine powders in a commercially available spouting fluidized bed (MiniGlatt). The effect of operating conditions such as inlet air temperature, polymer concentration, polymer weight ratio, water percentage in solvent and spray rate of coating solution on the quality of film coating are investigated. Experimental results demonstrate that the quality of film coating goes down as polymer concentration in coating solution goes higher, whereas the lower inlet air temperature is found to enhance polymer film generation and coating quality. It is also observed that the higher polymer weight ratio promotes agglomeration without affecting the coating quality to a great extent. An optimum water ratio in acetone-water solvent as well as spray rate can be optimized to achieve superior coating quality with acceptable agglomeration ratio.Graphical abstractDry coating approach involving deposition of nanosize particles on the surface of group C powders is considered to reduce the interparticle force and improve the fluidization behavior of fine powders. Polymer film coating at an individual particle level is achieved on these pre-coated fine powders in a commercially available spouting fluidized bed (MiniGlatt). The effect of operating conditions such as inlet air temperature, polymer concentration, polymer weight ratio, water percentage in solvent and spray rate of coating solution on the quality of film coating are investigated. Experimental results demonstrate that the quality of film coating goes down as polymer concentration in coating solution goes higher, whereas the lower inlet air temperature is found to enhance polymer film generation and coating quality.

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Figure: SEM images of Aluminum-1 particles coated to investigate the effect of polymer concentration. (a, b) Spray rate 2.11 ml/min, polymer concentration 4%, inlet air temperature 40 °C, polymer weight ratio 4% and water in solvent 0%; (c, d) spray rate 2.11 ml/min, polymer concentration 16%, inlet air temperature 40 °C, polymer weight ratio 4% and water in solvent 0%.     

Batch top-spray fluid bed coating: Scale-up insight using dynamic heat- and mass-transfer modelling

A mathematical model was developed for batch top-spray fluid bed coating processes based on Ronsse et al. [2007a, b. Combined population balance and thermodynamic modelling of the batch top-spray fluidised bed coating process. Part I—model development and validation. Journal of Food Engineering 78, 296-307; Combined population balance and thermodynamic modelling of the batch top-spray fluidised bed coating process. Part II—model and process analysis. Journal of Food Engineering 78, 308-322]. The model is based on one-dimensional discretisation of the fluid bed into a number of well-mixed control volumes. In each control volume, dynamic heat and mass balances were set up allowing the simulation of the contents of water vapour, water on core particles and deposited coating mass as well as fluidisation gas, particle and chamber wall temperature. The model was used to test different scale-up principles by comparing simulation results with experimental temperature and humidity data obtained from inorganic salt coating of placebo cores in three pilot fluid bed scales being a 0.5 kg small-scale (GEA Aeromatic-Fielder Strea-1), 4 kg medium-scale (GEA Niro MP-1) and 24 kg large-scale (GEA MP-2/3). Results show good agreement between simulated and experimental outlet fluidisation air temperature and humidity as well as bed temperature. Simulations reveal that vertical temperature and humidity gradients increase significantly with increasing scale and that in fluid beds as the simulated 900 kg (RICA-TEC Anhydro) production-scale, the gradients become too large to use the simple combined drying force/relative droplet size scale-up approach without also increasing the inlet fluidisation air temperature significantly. Instead, scale-up in terms of combinations of the viscous Stokes theory with simulated particle liquid layer profiles (obtained with the model) is suggested. In this way, the given fluid bed scale may be optimised in terms of low agglomeration tendency for a given process intensity across scale.     

Drying of water treatment process sludge in a fluidized bed dryer

The drying characteristics of water treatment process (WTP) sludge were investigated with a fluidized bed. The equilibrium moisture ratio of WTP sludge increased with relative humidity and decreased with temperature of drying air. However, equilibrium moisture ratio of WTP sludge was more sensitively dependent on relative humidity than temperature of drying air. When the sludge was dried in a batch fluidized bed, the drying rate of sludge decreased as the moisture ratio of sludge in the bed decreased. The periods of constant drying rates were apparently not observed on the drying rate curves. In addition, the maximum drying rates were increased with bed temperature and superficial air velocity. As the fluidized bed was operated continuously, the degree of drying of WTP sludge increased with bed temperature but was weakly dependent on superficial air velocity. However, the drying efficiency was decreased with bed temperature and relatively insensitive to superficial air velocity and increased with feed rate of sludge.     

FLUID1ZED BED SPRAY GRANULATION AND FILM COATING A NEW METHOD FOR THE INVESTIGATION OF THE COATING PROCESS ON A SINGLE SPHERE

ABSTRACT

The coating and granulation of solid particles in a fluidized bed is a process which converts pumpable and atomizable liquids (solutions, slurries, melts) into granular solids in one step through drying. Although this process has been applied in industry for several years, there is still a lack of understanding the physical fundamentals and the mechanisms by which spherical granules are formed. Hence a new method was developed which allows the direct observation of the subsequent particle-forming mechanisms such as droplet deposition, spreading, wetting and drying. The authors will present a laboratory scale apparatus in which a single freely suspended particle can be coated under well defined and constant coating and drying conditions. With this device, particle-growth-rate and the development of particle morphology were measured and investigated under various experimental conditions.