Mirko Peglow

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.

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

Abstract 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. Morl, Wirbelschicht-Spruhgranulation, Springer Verlag, 2000, ISBN 3-540-66985-X.].

The cell average technique for solving multi-dimensional aggregation population balance equations

In this paper, the cell average discretization [Kumar, J., Peglow, M., Warnecke, G., Heinrich, S., & Morl, 2006a. Improved accuracy and convergence of discretized population balance for aggregation: The cell average technique. Chemical Engineering Science, 61, 3327–3342] is extended to solving multi-dimensional population balance equations. Similar to the one-dimensional case, the scheme is based on an accurate prediction of certain moments of the population. The formulation is quite simple to implement, computationally not expensive and highly accurate. Numerical diffusion is a common problem with many numerical methods when applied on coarse grids. The presented technique nearly eliminates numerical diffusion and predicts four moments (zeroth, first, first cross and second) of the distribution function with high accuracy. The technique may be implemented on any type of grid. The accuracy of the scheme has been analyzed by comparing analytical and numerical solutions of three test problems. The numerical results are in excellent agreement with the analytical results and show the ability to predict higher moments very precisely. Additionally, an extension of the proposed technique to higher dimensional problems is discussed.

Chapter 2 Fluidized bed spray granulation

Publisher Summary Many attempts can be found to describe the particle formation in fluidized bed granulation in terms of population balances. Usually population balances describe the temporal change of particle property distribution. The influence of operating conditions on particle-size enlargement has been investigated by various authors. For example, Watano observed that the moisture content in solids is one of the most important particle properties to control the granulation process. Alongside the granulometry and the pneumatics, the particle growth process is also strongly influenced by the thermal conditions in the fluidized bed. Knowledge of the microprocesses of liquid injection, spreading, deposition, and evaporation, as well as the interactions with the gas–particle flow, is still limited. Nevertheless, some work was done to calculate the temperature and humidity distributions in such liquid sprayed fluidized beds. However, this chapter concerns the pneumatic behavior, the particle growth, the heat and mass transfer, as well as different apparatus configurations regarding the fluidized bed spray granulation by using simple analytical models. Granulation should be understood as a layered growth of particles. Typical product examples in the chapter explain the applicability of this technology for a broad range of particle processes. Using derived approximations, plant engineers are able to do rough calculations for a scale-up of the process.

STUDIES OF STEAM DRYING IN A FLUIDIZED BED

ABSTRACT In general, drying processes are described by the quantity of air humidity of the exiting gases. This approach is not possible however by the drying medium of water in steam drying, since the air humidity naturally possesses a constant value of 100%. This paper presents a model which represents the drying processes on the basis of the observation of temperature profiles of the material and energetic balancing of all components involved as well as the wall of the apparatus. The modeling differentiates three intervals: the condensation phase, the 1st drying period and the 2nd drying period. In addition, a validation of the model on the basis of experiments in an experimental plant DN100 belonging to the university is dealt with. The satisfying concurrence of the theoretical and practical results shows that, with the help of the theoretical model, discontinuous steam drying processes can be theoretically described with sufficient accuracy.

Unsteady and steady-state particle size distributions in batch and continuous fluidized bed granulation systems

A two-dimensional balance model of populations distributed by property was developed using the ordinates of particle diameter and particle density. This model makes it possible to calculate the spectrums of granulation changing over time in the fluidized bed during batch operation and in addition to calculate the time-dependent size distribution of the product granulate for continuous operation. A physics-based model was devised for the submechanism of the growth of the granulates. On the one hand, the population balance model is divided into the particle balance around the granulator and, on the other hand, when granulation is continuous, particles are constantly removed from the fluidized bed. To this end, a separator model was used that subdivides the extractor device into two areas of balancing in accordance with the geometry of the apparatus and the properties of the particle. In the case of continuous operation, the results when the product is classified using a classifier tube inside the fluidized bed are discussed. The outcome is the influence of the various particle flows on the particle diameter and the product mass withdrawn. In particular, the problematic nature of the start-up is dealt with, i.e. reaching stable points of steady operation. The simulation results are validated using results from continuous tests on a semi-industrial plant.

A simplified model of SO2 capture by limestone in 71 MWe pressurized fluidized bed combustor

A mathematical model of SO2 capture by uncalcined limestone particles with solid attrition under pressurized fluidized bed combustion conditions was developed based on the shrinking unreacted-core model. Since the thickness of the product layer is sufficiently much smaller than the particle size, a flat surface model was employed. The difference in SO2 capture behavior between continuous solid attrition and intermittent attrition was investigated. The reaction rate for intermittent solid attrition was found to be lower than that for continuous attrition mode under low SO2 concentration conditions. A simple mathematical expression to calculate reaction rate of SO2 capture per unit external surface area of limestone is proposed. The present simplified mathematical model of SO2 capture by single limestone particle under periodical attrition conditions was applied to the analysis of a large-scale pressurized fluidized bed combustor. By giving the period of attrition as a parameter, the experimental results agreed well with the model results. From the vertical concentration profile of SO2 concentration, the emission of SO2 was found to be governed by the balance between SO2 formation rate from char and SO2 capture by limestone at the upper surface of the dense bed. A simplified expression to estimate SO2 emission from pressurized fluidized bed combustors was proposed.

A discretized model for tracer population balance equation: Improved accuracy and convergence

Abstract A new discretization for tracer population balance equation is developed. It is compared to the modified discretized tracer population balance equation of Peglow et al. [Peglow, M., Kumar, J., Warnecke, G., Heinrich, S., Morl, L., Hounslow, M. J. (2006). Improved discretized tracer mass distribution of Hounslow et al. American Institute of Chemical Engineers , 52 , 1326–1332]. The new formulation provides excellent prediction of the tracer mass distribution in all test cases. Furthermore, the new formulation is more efficient from a computational point of view, it takes less computational effort and is able to give a very good prediction for a coarser grid. Additionally, it is independent of the type of grid chosen for computation. For finer grids, both formulations tend to produce the same results. The performance of the new formulation is illustrated by the comparison with various analytically tractable problems. Moreover, the new formulation preserves all the advantages of the modified discretized tracer population balance equation of Peglow et al. [Peglow, M., Kumar, J., Warnecke, G., Heinrich, S., Morl, L., Hounslow, M. J. (2005). Improved discretized tracer mass distribution of Hounslow et al. American Institute of Chemical Engineering (published online: 29 December 2005)] and provides a significant improvement in predicting tracer mass distribution and tracer-weighted mean particle volume during aggregation process.

FLUIDIZED BED SPRAY GRANULATION: ANALYSIS OF HEAT AND MASS TRANSFERS AND DYNAMIC PARTICLE POPULATIONS

Abstract - A model was developed taking into consideration the heat and mass transfer processes in liquid-sprayed fluidized beds. Such fluidized beds (FB) are used for granulation, coating and agglomeration. Conclusions are drawn on the relevance of particle dispersion, spraying and drying to temperature and concentrations distributions. In extension, the model was coupled with a population balance model to describe the particle size distribution and the seeds formation for continuous external FBSG (fluidized bed spray granulation) with non-classifying product discharge and a screening and milling unit in the seeds recycle. The effects of seeds formation on the stability of the process is discussed. Keywords : Fluidized bed; Granulation; Population balance. INTRODUCTION The FBSG is a process used for the production of granular high-quality, free-flowing, low-dust and low-attrition solids originating from liquid products, e. g. solutions, suspensions, melts and emulsions. The advantage is the coupling of the wetting, drying, particle enlarging, shaping, homogenisation and separation processes and the the production in a single processing step (Uhlemann and Morl, 1999). Especially for large production units a continuous operation of the FBSG is desirable. The continuous granulation process presents, unlike to the batch-operation, the advantage to operate the plant under stationary condition at high throughputs. The stationary operation point is reached, provided constant granulate spectrum beside constant mass flows and constant thermal conditions, whereby initially fed granulates have to be removed at all. Sometimes this unsteady phase lasting up to a few hours. The aim of the following examinations is to study the stability behaviour of the FB by using of a one-dimensional population balance which is coupled with the heat and mass transfers in such liquid sprayed gas-solid fluidized beds to calculate the time dependent particle size distributions of the fluidized bed and of the product particles and the temperature and humidity progressions. Depending on the seeds formation mechanisms– overspray (non deposited dried drops), attrition, separation – a narrow or wide particle size distribution is obtained. Additionally, this work also introduces a mathematical model for the spatial temperature and humidity distributions, which are evaluated by measurements of the stationary spatial air temperatures at a semi-industrial fluidized bed pilot plant of the institute.

On two-compartment population balance modeling of spray fluidized bed agglomeration

Abstract The present work focuses on the modeling and analysis of a spray fluidized bed granulation (SFBG) process based upon the concept that particles are communicating between the two compartments at some steady state mass flow rate. A numerical technique for solving the proposed two-compartment model (2CM) is developed and validated against some newly derived analytical solutions. Moreover, the inverse technique for extracting the rate constant of one-compartment model (1CM) is extended to 2CM. A correlation of aggregation rate constant of 2CM with the rate constant of conventional 1CM under some restrictions is investigated and it is found that the 1CM cannot be used, in general, to predict results of 2CM. Furthermore, it is observed that the existence of two zones in SFBG is responsible to certain extent for time dependent behaviour of aggregation rate constant. Finally, influence of compartment sizes and particles residence times on particle size distribution is investigated.