Kinetics of fluidized bed melt granulation—II: Modelling the net rate of growth

Previous work [Tan, H.S., Goldschmidt, M.J.V., Boerefijn, R., Hounslow, M.J., Salman, A.D., Kuipers, J.A.M. (2004a). Building population balance for fluidized bed granulation: lessons from kinetic theory of granular flow. Powder Technology, 142, 103-109] shows that we can derive an aggregation kernel (equi-partition of kinetic energy (EKE)) on the basis of the kinetic theory of granular flow and use it effectively to describe the net granule growth in fluidized bed melt granulation (FBMG). In this paper, we incorporate the EKE kernel into a population balance model to extract the effective aggregation rate constant that accounts for the net granule growth for the series of FBMG experiments shown in Tan, et al. [(2004b). Kinetics of fluidized bed melt granulation I: effect of process variables, Chemical Engineering Science, to be submitted]. These extracted rate constants are subsequently expressed as a function of different operating condition. The results consistently show that the aggregation rate constant increases in direct proportion with binder spray rate, from where we conclude that the rate of granule formation is directly dependent on the amount of binder available for aggregation per unit time. The aggregation rate constant was also observed to increase with higher bed temperature when a higher viscosity binder was used, but showed a maximum value for a less viscous binder as a function of temperature. The aggregation rate was also seen to be faster when granulating using a larger droplet size and at a lower fluidizing air velocity. The observations in the rate constant plot can be effectively explained by the physical parameters in the EKE model and the sequence of rate events proposed in Tan, et al. [(2004b). Kinetics of fluidized bed melt granulation I: effect of process variables, Chemical Engineering Science, to be submitted].     

Coupled CFD-PBE Simulation of Nucleation in Fluidized Bed Spray Granulation

Fluidized bed spray granulation is used to produce porous granular particles from suspensions, solutions, and melts. It is the general aim of our work to provide a physics-based simulation tool for this process. For this the process-relevant mechanisms such as droplet deposition, dust integration, and drying are implemented using computational fluid dynamics (CFD) to generate growth kinetics of the granules and of the dust particles. The latter gives rise to the nucleation rate. This kinetics is applied to simulate the development of the particle size distribution of granules for a continuously operated fluidized bed spray granulation process using population balance equations of the granules.     

DRYING OF OILSEEDS

Drying of oilseeds is a post harvest operation required for safe storage of harvested seeds. Oilseeds have gained importance in India over the last few years. Fluidized bed (FB) drying and Spouted Bed (SB) drying of oilseeds were studied as potential dryers for these seeds. Experimental pilot fluidized bed dryers were developed and a 2 TPH capacity (mustard seed) FB dryer plant was designed and installed in the lab. Experimental units and a pilot spouted bed dryer were also developed. The dryer units and the plant have been extensively tested and satisfactory drying performance has been achieved. Drying data and drying characteristics have been generated for different oilseeds such as mustard, sunflower, soybean and groundnut etc. The drying rates in different regimes of drying characterized by constant rate and falling rate periods have been evaluated. The salient results of the work are briefly outlined.     

SOLAR ASSISTED DRIER FOR SEEDS

A high performance solar assisted seed dryer has been designed and constructed at a seed processing station in Hungary.

The dryer is arranged in a building specially designed for this purpose, the roof of it serves as solar air collector field. Two drying cells with individual fans of two RPM stages are formed in the building. Drying air preheated by the collector can optionally be distributed between the two drying cells. Material to be dried can be arranged either in a static bed or in containers depending on the requirements. Moist air leaving the bed streams out of the drying cells through openings of the side walls. As auxiliary air heater a natural gas generator is used.     

Inhibition and promotion: The effect of earth alkali metals and operating temperature on particle agglomeration/defluidization during incineration in fluidized bed

Some studies have demonstrated that earth alkali metals promote agglomeration; however, others have shown that they inhibit the generation of agglomeration. The earth alkali metals (Mg and Ca) may cause both inhibition and promotion of agglomeration/defluidization. Therefore, this study focuses on the effect of Mg, Ca and the operating temperature on the agglomeration/defluidization of sodium during incineration in a fluidized bed reactor. The results indicate that the added Mg and Ca inhibit agglomeration and increase the defluidization time. At low Na/Mg and Na/Ca mole ratios, Mg and Ca inhibit defluidization significantly. However, the inhibition reduces as the concentration of Na increases. When the mole ratio (Na/Mg and Na/Ca) exceeds two, the inhibition of Mg and Ca is not apparent. Under these conditions, the operation temperature is the main factor with regard to agglomeration/defluidization. When Mg and Ca are added to reduce the agglomeration/defluidization, both the concentration of Na and operating temperature must be considered.     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

ABSTRACT

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D _e ) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X _o ). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.     

Particle population modeling in fluidized bed-spray granulation—analysis of the steady state and unsteady behavior

Owing to its intensive mass and heat transfer ratios and its coupling of the process stages of drying, shaping and homogenization as well as classification, continuous fluidized bed-spray granulation drying has gained acceptance as a thermal treatment process for granular solids. In this study, a balance of the particle populations is completed for a continuous fluidized bed-spray granulation with external classification. Thus, it ought to be possible to describe the particle size distributions changing over time in the fluidized bed and in the product flow [Powder Technol. 82 (1995) 37; H. Uhlemann, L. Mörl, Wirbelschicht-Sprühgranulation, Springer Verlag, 2000, ISBN 3-540-66985-X.].     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D_e) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X_o). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.     

Analysis of the start-up process in continuous fluidized bed spray granulation by population balance modelling

A physics-based population balance model is constructed for continuous fluidized bed spray granulation with internal and external separations. A balance area around the granulator and around the separator is described, including all input and output particle and mass flows. A simplified growth and attrition model is developed for the diameter change of the particles in the granulator. The population balances facilitate the calculation of the particle size distributions changing over time in the fluidized bed and in the product flow. It is demonstrated that an unsteady start-up phase occurred in every case, which possibly leads to instability (oscillating behaviour). This may be regulated by controlling the overall nuclei balance.     

Drying and Heat Transfer Characteristics for a Novel Fluidized Bed Dryer

Abstract

The use of a fluidized bed dryer with a lateral air flow and mechanical agitation to the drying of sludge from a wastewater treatment plant was investigated. Experimental curves of moisture content vs. drying time, as well as heat transfer coefficients and the size characteristics of the products, were determined at temperatures between 80°C and 110°C, a stirring rate of 55 rpm and air velocity of 0.9 m/s for 3 kg sludge batches with initial moisture contents of 0.55 and 0.65 (d.b.). Experimental drying kinetics were compared with values derived from three models based on Fick's second law, namely: the constant diffusivity model, the simplified variable diffusivity model, and the modified quasi-stationary model.     

An Experimental and Modeling Investigation on Drying of Ragi (Eleusine corocana) in Fluidized Bed

Experimental investigation on drying of ragi (Eleusine corocana) in a fluidized bed has been attempted covering the operating parameters such as temperature, flow rate of the drying medium, and solids holdup. The drying rate was found to increase significantly with increase in temperature and marginally with flow rate of the heating medium and to decrease with increase in solids holdup. The duration of constant rate period was found to be insignificant, considering the total duration of drying and the entire drying period was considered to follow falling rate period. The drying rate was compared with various simple exponential time decay models and the model parameters were evaluated. The Page model was found to match the experimental data very closely with the maximum root mean square of error (RMSE) less than 2.5%. The experimental data were also modeled using Fick's diffusion equation and the effective diffusivity coefficients were estimated. The effective diffusion coefficient was found to be within 5.7 to 14 × 10^-11 m²/s for the range of experimental data covered in the present study with RMSE less than 5%.     

Investigating the dynamic behaviour of fluidized bed spray granulation processes applying numerical simulation tools

Particulate processes involve different kinetic processes such as formation of nuclei, their subsequent growth and breakage. In addition, external product classification can play a significant role. A reliable prediction and thorough understanding of potential sources for instability is not only of scientific interest but also an important issue for better process design and process control.The application of these nonlinear dynamics is concerned with fluidized bed spray granulation. Self-sustained oscillations may rise in processes with external product classification.Therefore, focus of this work is on process stability which can be influenced by classifying, milling and recycling of particles and by the production of internal and external seeds.First a brief introduction to the model is given, that couples the particle population state with thermodynamic-, fluiddynamic- and granulation process for a unit with non-classifying product discharge and a screening and milling unit in the seed recycle and builds the core of the software package AVA^®FBSim^®, used for the experiments.     

Kinetics of fluidised bed melt granulation I: The effect of process variables

This paper is the first of a series to study the influence of operating conditions on the kinetics of fluidised bed melt granulation. First, we identify the rate processes responsible for the net growth in granule size in a top-sprayed fluidised bed granulator and propose a sequence of events based on these rate processes. The overall kinetics during the process is identified to be a combination of particle aggregation, binder solidification and granule breakage. By conducting experiments in a small-scale modified commercial fluidised bed granulator, the influence of various operating conditions (binder spray rate, bed temperature, atomising pressure, fluidising air velocity) on the granule growth behaviour was examined. The results indicate the granule growth rate to be directly dependent on the relative amount of binder sprayed into the bed, which essentially determines the speed of the aggregation process. The overall granule growth rate is observed to increase relatively with increased bed temperature for a more viscous PEG4000, while a maximum growth is seen for a lower viscosity PEG1500. A larger droplet size was also seen to have increased the overall growth rate, even though a smaller droplet seems to be able to induce a faster initial growth. The results also reveal the increase in fluidising air velocity to reduce the overall granule growth rate. The final granule size distribution was also observed to become narrower with increased bed temperature and fluidising air velocity. These observations are effectively explained using the proposed sequence of rate events.     

Modeling the Drying of Grass Seeds (Brachiaria brizantha) in Fluidized Beds

A modified three-phase model is developed to simulate the drying of Brachiaria brizantha in fluidized beds. In this new model, the constitutive equation of drying kinetics is formulated including both the constant rate and the falling rate mechanisms; the seed shrinkage is taken into account during all drying operation and the transition between bubbling to slugging regime is delineated for estimating the bubble velocity and size. Such modifications improve the mathematical model to better simulate the drying of coarse particles in fluidized beds. The best estimation of the five adjustable model parameters, which are required to define heat and mass transfer mechanisms between interstitial gas and seed particles and to specify the heat loss from dryer walls to ambient air, is attained by incorporating an optimization routine into the computer model program. Having been specially designed to supply data for this model, experiments are performed in a bath laboratory-scale fluidized bed. Additional data are generated to validate the model and program routines. Results show a good agreement between simulated and experimental data, validating the approach used to describe drying kinetics and particle shrinkage.     

Fluidized bed spray granulation: Analysis of the system behaviour by means of dynamic flowsheet simulation

Nowadays, software tools for the flowsheet simulation of industrial processes are commonly used for design, simulation, balancing, troubleshooting and optimization purposes. Most of the tools are applicable to fluid processes only and cannot be effectively used for processes which involve solids.In this contribution we want to present the conceptual design of a new system applicable for the dynamic flowsheet simulation of complex solids processes. This system is developed as an enhancement to the existing simulation program.The novel software is able to simulate the unsteady behaviour of complex circuits of granulation processes. The transient behaviour during the start-up and changing of the process or material parameters can also be examined.As flowsheet examples, a typical spray granulation process with different schemes consisting of fluidized bed granulators, screens, mill and splitters was used. The mathematical model of the fluidized bed granulator is described by a one-dimensional population balance equation and coupled with heat and mass transfer and simple fluid dynamics.Received simulation results have shown that the proposed concept of the dynamic flowsheet simulation of granulation processes can be used effectively and has the potential to be generalized for other types of solids processes.     

ANALYSIS OF MECHANICALLY AGITATED FLUID-PARTICLE CONTACT DRYERS

Incorporation of agitation to spouted and fluidized bed dryer result in significant increases in the drying capacity (Q_s ), although product retention persist, which is reduced by increasing the air flow. The physical phenomena occurring in these dryers with several liquid substrates was analyzed and the residence time distributions (RTD) were obtained by the use of dye tracers. The residence time (τ) was found to be a function of the rate of agitation (n) and reaches a minimum at n = n _opt, which was characteristic for each type of substrate, and where maxima also appeared for the drying capacity (Q_s = Q _{s max}) and the heat transfer coefficient (Nu _p = Nu _{p max}). The RTD can be modeled by series of consecutive dryers and a modified Vanderschuren and Delvosalle model can be employed to calculated moisture of the dry product.