Ireneusz Zbicinski

DRYING KINETICS AND PARTICLE RESIDENCE TIME IN SPRAY DRYING

ABSTRACT A unique experimental equipment for extensive trials on the spray drying kinetics and particles residence time involving “in situ” analysis of the properties of continuous and dispersed phases was designed, built, and tested. Advanced experimental techniques (including laser techniques) to determine current parameters of spray drying process (temperature, humidity, moisture content) and current structure of spray (particle size distribution, particle velocities, etc.) were employed. Full scale spray drying tests of bakers yeast and maltodextrin enabled identification of the effect of process parameters on drying kinetics and spray residence time in the tower. Quantitative relationship describing spray drying kinetics as a function of atomization ratio and drying agent temperature were determined. The experimental results proved that spray residence time was controlled by atomization ratio and airflow rate. Drying kinetics in spray drying process is presented for the first time in the literature.

Conditions for Accurate CFD Modeling of Spray-Drying Process

The paper presents concluding results of extensive experimental and theoretical research on confident CFD modeling of spray drying. An earlier developed experimental method to determine spray-drying kinetics in a lab scale allowed us to find a critical material moisture content and to determine generalized spray-drying curves. The generalized drying curves, identical in shape in the laboratory and pilot plant units, were used in the CFD model of spray drying process. Extensive simulations for spray drying of 10, 30, and 50% of initial solid content of maltodextrin proved high accuracy of the predictions of discrete (particle size distribution, particle moisture content, particle velocity, spray temperature) and continuous-phase parameters (gas temperature and humidity). Maximum error of the predictions of discrete-phase parameters was on the level of 20%, which is probably close to the current capacity of the CFD technique for modeling of spray-drying process. Comparison of experimental measurements and theoretical results shows that incorporation of realistic spray-drying kinetics into the CFD model and correct definition of initial drying and atomization parameters enable reliable simulations of spray-drying process.

Equipment, technology, perspectives and modeling of pulse combustion drying

The paper presents the analysis of potential and real benefits of pulse combustion process applied in drying. The phenomenon of pulse combustion, its mechanism, pulse combustors designs, advantages and disadvantages of this technology were described and reviewed. Pulse combustion applications in industry were analyzed and evaluated. Experimental investigations carried out at the Faculty of Process and Environmental Engineering, Technical University of Lodz, on development of pulse combustion spray drying system were also presented. Laser techniques were used to characterize precisely flowfield in a drying chamber, spray structure and evaporation kinetics. An example of computational fluid dynamics (CFD) calculations of pulse combustion spray drying process was also given. The prospects of future development of this technology were discussed.

Spray Drying Tower Experiments

Abstract The article presents a full set of spray drying experiments for selected products performed in a co-current spray drying tower developed at Lodz Technical University. The experiments enabled identification of process and atomization parameters (feed properties, feed rate and feed temperature, drying agent temperature, air flow rate, atomization ratio, etc.) on drying and degradation kinetics, spray structure, particle residence time, and final product properties. Drying agent temperature measurements showed, in all cases, the initial increase of gas temperature in the spray envelope caused by the spray expansion and then a decrease induced by liquid evaporation and heat losses to the environment. PDA analysis confirmed that the initial velocity of particles was a function of a diameter and also the function of the distance from the axis. Practically an identical particle size distribution was observed in each cross-sectional area of the dryer. Negative values of particle velocity in the vicinity of the axis and at the edge of the spray envelope were found which proved that recirculation of particles appeared in the column. Analysis of final product properties showed that for agglomerate-like materials a decrease of bulk density with an increase of air temperature was related to morphological changes that occurred during drying and affected the shape of particles, surface structure, etc. The experiments proved that air/liquid ratio for two-fluid atomization and gas temperature were the most decisive factors controlling drying and degradation process rate and final product properties.

Modeling of Air Flow in an Industrial Countercurrent Spray-Drying Tower

This article presents both theoretical and experimental determination of the hydrodynamics of drying air in an industrial countercurrent spray dryer. Air velocity measurements were performed in selected areas of the tower to determine the flow pattern in the dryer and to collect data to validate the computational fluid dynamics (CFD) modeling and verify the results. For no-swirl operation mode, 3D CFD calculations showed high instability of the air flow in the dryer. A bent, pillow-shaped flow above the drying air inlet, which promotes deposition of particles in this area of the wall, was detected. CFD calculations also proved that when drying air was introduced to the tower tangentially; from 20° to 30° in relation to the initial air flow configuration in the spray-drying tower, the air flow was stabilized and air velocity near the wall increased, which might reduce wall deposition of particles. Comparison of experimental and theoretical results showed that the CFD model of countercurrent spray-drying process developed in this study can be used for a reliable estimation of tower performance.

CFD Model of Particle Agglomeration in Spray Drying

A 3D CFD model of the agglomeration of droplets and particles in a counter-current spray-drying process was developed and verified. An original discrete phase model was elaborated, with an agglomeration module taking into account hydrodynamic segregation of particles, droplet coalescence, and droplet shrinkage for accurate calculations of mass balance of the discrete phase. The characteristic drying curves were applied to the model of particle moisture evaporation, which included the coupling of particle agglomeration with heat, mass, and momentum transfer between the discrete and continuous phases. Two agglomeration zones were observed in the tower: wet particle agglomeration in the atomization zone, and “dry agglomeration” above the air inlets, due to the intensive mixing of particle streams. A comparison of the calculated particle size distributions and experimental data obtained from particle dynamics analysis (PDA) measurements proves the accuracy of the developed methodology. The elaborated model allows the final PSD of the powder in the spray towers to be predicted.

CFD Model of Apple Atmospheric Freeze Drying at Low Temperature

An Internet-controlled atmospheric freeze dryer was designed, built, and tested in the Department of Heat and Mass Transfer, Technical University of Łódź. A film sublimation-based CFD model was developed and verified using Fluent 6.1 commercial CFD software. The model enables the simulation of phase change and water vapor diffusion process within porous media. Transport of non-condensable species can be calculated with species transport inbuilt model of Fluent 6.1 and used to predict sublimation rate under given conditions. Results were compared with AFD experimental data of 10-mm Idared apple cubes. The viability of applying the film sublimation model to atmospheric freeze-drying process was demonstrated. Higher mass flux was found on the leading edge, relatively uniform mass flux within the porous zone illustrates that vapor diffusion dominates atmospheric freeze drying process at low temperature (below 0°C). CFD results for apple cubes show a predomination of diffusional resistance of porous tissue.

Spray Drying of Probiotics: Process Development and Scale-Up

Spray drying is an alternative method to freeze drying of Bifidobacterium bifidum biomass during the production of solid dosage forms. In order to develop a new drying process for probiotics and to establish the generalized mathematical model for the proposed technology, both experimental and theoretical studies on B. bifidum biosuspension have been carried out. These studies focused on drying kinetics, kinetics of cell death due to thermal shock and growth of osmotic pressure; on defining the influence of process parameters on product quality; and on identifying the limitations of process parameters due to material characteristics. Based on experimental data and modeling results, recommendations for organization of industrial process have been provided.

Pulse Combustion Drying: Aerodynamics, Heat Transfer, and Drying Kinetics

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Continuous and Discrete Phase Behavior in Countercurrent Spray Drying Process

The behavior of continuous and discrete phases in countercurrent spray drying process is described in the article. Literature and our own experimental results on spray drying in the countercurrent system are presented and the mechanism of moisture evaporation and hydrodynamics of the discrete phase are discussed. Negative values of particle velocity were found in the drying chamber, which confirmed the existence of recirculation zones where agglomeration of particles took place. An increase of mean particle size with the distance from the nozzle caused by agglomeration in recirculation zones in the column was also observed. High sensitivity of the discrete-phase flow pattern to small changes in drying and atomization parameters and the nozzle position in the dryer was found. Analysis of the results proved that due to complicated hydrodynamics of the continuous and disperse phases, a narrow range of stable operating parameters, and significant changes in final product properties at slight changes of drying parameters, the countercurrent spray drying process was difficult to control and scale up.