M. Mezhericher

Theoretical Drying Model of Single Droplets Containing Insoluble or Dissolved Solids

A literature survey of published single-droplet drying models has exposed their various shortcomings. An advanced theoretical model of drying of single droplets, containing either insoluble or dissolved solids, has been developed and numerically solved. The validation of the developed model has been accomplished by comparison of the predicted temperature and mass histories with published experimental data for slurries of silica and skim milk solution under different conditions. The results of simulations showed a good agreement between the calculated curves and experimental points in all case studies.

Theoretical Models of Single Droplet Drying Kinetics: A Review

During the last decades, growing attention has been given to theoretical and experimental studies of drying behavior of single droplet containing solids. This research interest is motivated by the need for fundamental understanding of the drying phenomena in extensively used technological processes like spray drying, fluidized bed drying, pneumatic drying, etc., at drop-wise and particulate levels. The present literature review summarizes the developed theoretical models of single droplet drying kinetics, discovers their benefits and deficiencies, and identifies prospects for future research.

Modeling of Droplet Drying in Spray Chambers Using 2D and 3D Computational Fluid Dynamics

Steady-state and unsteady-state 2D axisymmetric and 3D calculations of heat and mass transfer processes in cylinder-on-cone spray chamber have been performed. The theoretical model of the process is based on two-phase Eulerian-Lagrangian approach for gas-droplet flow. In steady-state, two cases are considered: flow of hot air only (no liquid spray) and heat and mass transfer between water spray and hot air. For unsteady-state, droplet-droplet interactions in the water spray are included. It has been found that 2D axisymmetric model is suitable for fast and low-resource consumption numerical calculations and it predicts values of velocity, temperature, and vapor mass fraction in the spray chamber with reasonable accuracy. However, due to its restrictions, 2D axisymmetric model fails to predict asymmetry of flow patterns and presence of the transversal air flow and it cannot provide an actual three-dimensional picture of particle trajectories inside the spray chamber. In the case when the above characteristics are important, the utilization of 3D model is essential.

Droplet–Droplet Interactions in Spray Drying by Using 2D Computational Fluid Dynamics

A 2D axisymmetric model of the spray drying process is presented. The two-phase flow theoretical model is based on a combined Eulerian-Lagrangian approach and takes binary interactions (coalescence or bouncing) between spray droplets into consideration. Validation of the model (incorporated in FLUENT 6.3.26) demonstrated good agreement and consistency with the literature data. The results of transient simulations showed that droplet–droplet interactions displace the region of heat and mass transfer from the central core toward the periphery of the drying chamber. It was also found that insulation of the spray dryer can substantially affect temperature and humidity patterns, whereas its influence on the velocity flow field is less marked.

Modeling of Particle Pneumatic Conveying Using DEM and DPM Methods

The results of numerical modeling of horizontal pneumatic conveying of polyethylene pellets are presented. Two different techniques have been applied to track the particle trajectories: discrete element method (DEM) and discrete particle model (DPM). Particle-particle and particle-wall collisions in DEM are treated using a soft-spheres approach. In contrast, a hard-spheres modeling of particle-wall interactions is realized in DPM. The unsteady numerical calculations simulated the behavior of particles in a wide range of flow regimes, beginning from dilute and ending with dense particle flows. The advantages of DPM are fast computations and reduced requirements of computer resources owing to larger time step and smaller amount of stored information than DEM. However, the results of calculations show that DPM is more appropriate for modeling of dilute particle flow (particle volume fraction less than 10%), whereas the DEM approach can be successfully utilized for simulation of various flow regimes between low and large particle concentrations.

The Influence of Thermal Radiation on Drying of Single Droplet/Wet Particle

A literature review shows lack of the detailed research and discovers contradictions regarding influence of thermal radiation on the drying process of a single droplet, containing solids. The present study shows that for droplet initial diameters <0.25 mm and ambient temperatures <750°C, as like as for diameters <2.5 mm and ambient temperatures up to 200°C, the effect of thermal radiation is negligible. At the same time, for droplets with initial diameter greater than 0.5 mm and temperatures of the drying agent higher than 400°C, thermal radiation plays a substantial part in the drying process.

Multi-Scale Multiphase Modeling of Transport Phenomena in Spray-Drying Processes

Spray drying is an extensively used technology in process engineering for receiving small particles by rapid moisture evaporation from a spray of droplets. This contribution summarizes achievements and results of the comprehensive scientific research on multi-scale multiphase modeling of transport phenomena in spray-drying processes undertaken by our research group: (1) study of particle formation on the scale of an individual droplet; (2) modeling and simulation of droplet–droplet and particle–particle collisions in a spray; (3) study of gas-spray mixing; (4) 2D and 3D study of spray drying by an innovative multi-scale simulation tool coupled to a commercial CFD software. The proposed multi-scale multiphase model of transport phenomena in a spray-drying process has been developed based on a thorough analysis of previously published experimental and theoretical works. The content of this paper will be useful for both academia and industry; e.g., pharmaceutical, biotechnology, chemical, ceramics, materials, nutrition, and other applications of spray drying.

Three-Dimensional Spray-Drying Model Based on Comprehensive Formulation of Drying Kinetics

In our previous 2D axisymmetric study of the spray-drying process, it was shown that the introduction of comprehensive formulation of drying kinetics had a substantial influence on both predicted gas and particle flow patterns in a spray dryer. On the other hand, it was demonstrated that the concept of 2D axisymmetric modeling is suitable for fast and low-resource consumptive numerical calculations of the spray-drying process and that the predicted velocity, temperature, and vapor mass fraction are of reasonable accuracy. However, due to their restrictions, 2D axisymmetric simulations fail to predict asymmetry of flow patterns and the presence of the transversal air flow and cannot provide actual 3D representation of particle trajectories inside the spray chamber. Therefore, in the present study a novel 3D theoretical model of multiscale multiphase transport phenomena in a steady-state spray-drying process is proposed for predicting more realistic particles paths, residence times, temperatures, and moisture contents. A drying process of silica slurry in a short-form pilot-scale spray dryer fitted with a pressure nozzle atomizer is investigated. The simulated 3D drying behavior of the dispersed phase and flow patterns of air velocity, temperature, and humidity are compared with the previously published results of 2D axisymmetric modeling. A significant influence of both drying kinetics and number of utilized dimensions on the predicted particle trajectories and transport phenomena in the drying chamber is observed. Hence, a proper formulation of the droplet drying kinetics and realistic 3D flow modeling is crucial for accurate numerical representation of the actual spray dryer performance.

Heat and Mass Transfer and Breakage of Particles in Drying Processes

An advanced theoretical model of unsteady coupled heat and mass transfer and breakage of wet particles in two-stage drying processes is presented. The numerical simulations of drying of silica slurry droplets have shown that both temperature and mechanical stresses emerge in the wet particle during drying. It has been found that mechanical stresses play a substantial role at the beginning of the second drying stage, whereas the thermal stresses are much more considerable at the end of drying. In addition, tangential stresses in the crust of wet silica particles are predominant over the radial component (approximately five times greater). Compared to the proposed breakage criterion, the model predicts that the total stresses can be a reason for wet particle cracking/breakage and this depends on granule diameter, drying agent temperature, and size of primary particles. To prevent granule breakage at given drying conditions, the slurry droplets with primary particles as small as possible are recommended for drying.

Drying of Droplet Containing Insoluble Nanoscale Particles: Numerical Simulations and Parametric Study

To predict particle morphology, a theoretical description of drying of a single droplet containing insoluble nanoscale has been developed. Various possible morphological structures of the droplet/particle, depending on temperature, velocity, moisture content, porosity, and other droplet and drying agent properties, have been examined. Furthermore, the present contribution contains a qualitative description of the overall drying process and a mathematical model for the first stage of drying, including the transition period, which has been validated with good agreement to experimental data. The implementation and compatibility of the numerical solution using different solid diffusion coefficients is explained and compared. Moreover, the importance of choosing an appropriate solid diffusion coefficient is indicated. Finally, because the behavior of drying material and the characteristics of dry product are affected by the specifics of the drying process, an extensive parametric study has been conducted to determine the influence of different parameters on the drying kinetics and morphology of obtained dry particle.