Controlling the morphology of ceramic and composite powders obtained via spray drying – A review

Spray drying is one of the most convenient methods for drying suspensions (slurries) and for granulation of materials. Spray dried powders have good flowability, narrow size distribution and controllable morphology. Morphology of powder particles (also called granules or microspheres) strongly affects the use and handling of powders. This review discusses the latest research on parameters that affect morphology and size of granules obtained by spray drying: atomization parameters, properties of sprayed slurry, mass transfer etc. The formation of hollow and dense granules is extensively reviewed. Granule size is affected by droplet size, slurry concentration and initial particle size. Morphology mostly depends on size distribution of initial ceramic particles, agglomeration tendency in the slurry and mechanical strength of the shell of a granule during the drying process compared to capillary force of the suspension liquid. Polymer additives (e.g. binders and lubricants) change the properties of granule shell and the evaporation of moisture; thus, polymer additives significantly affect morphology.     

Pore-Level Modeling of Isothermal Drying of Pore Networks Accounting for Evaporation,Viscous Flow,and Shrinking

Abstract

Simulation results of pore-level drying of non-hygroscopic, non-rigid, liquid-wet porous media are presented. Two- and three-dimensional pore networks represent pore spaces. Two kinds of mechanisms are considered: evaporation and hydraulic flow. The process is considered under isothermal conditions. Capillary forces thus dominate over viscous forces and the drying is considered as a modified form of invasion percolation. Liquid in pore corners allows for hydraulic connection throughout the network. During drying, liquid is replaced by vapor by two fundamental mechanisms: evaporation and pressure gradient–driven liquid flow. The development of capillary pressure as menisci turn concave induces shrinkage of the matrix, which contributes to the pressure gradient that drives liquid toward the surface of the network. Using Monte Carlo simulation, we find evaporation and drainage times; the shortest calculated indicates the controlling mechanism. Here we report distributions of liquid and vapor as drying time advances. For the calculation of transport properties, details of pore space and displacement are subsumed in pore conductances. Solving for the pressure field in each phase, vapor and liquid, we find a single effective conductance for each phase as a function of liquid saturation. Along with the effective conductance for the liquid-saturated network, the relative permeability of liquid and diffusivity of vapor are calculated.     

A Study of Surface Wetting When Coating Paper

Abstract

The objective of this work will be to look at basic micro-level simulations of liquid state and movement. Defining liquid movement at fiber-coating boundaries is essential when modeling surface wetting of paper fibers. Drying studies have shown that chemical additives in base paper or coating color may reduce or increase quality, productivity, and energy efficiency considerably. The latest question is, Which are the factors that are significantly influencing liquid movement at fiber-coating boundaries? A phenomenon of less liquid drainage at lower paper moisture content is studied in this work together with the fiber hornification process. Fiber hornification is a complex change in the physicochemical properties of the fiber surface and the state of boundary molecules. Another important objective is to show how hornification may be accounted for in basic calculations. This while, printing properties of paper (mottling, etc.), may then be connected to the formation of the base paper and its drying history, explaining in more detail the importance of microlevel physicochemical property changes at fiber surfaces.     

From Laboratory Experiments to Design of a Conveyor-Belt Dryer via Mathematical Modeling

Abstract

A conveyor-belt dryer for picrite has been modeled mathematically in this work. The necessary parameters for the system of equations were obtained from regression analysis of thin-layer drying data. The convective drying experiments were carried out at temperatures of 40, 60, 80, and 100°C and air velocities of 0.5 and 1.5 m/sec. To analyze the drying behavior, the drying curves were fitted to different semi-theoretical drying kinetics models such as those of Lewis, Page, Henderson and Pabis, Wang and Singh, and the decay models. The decay function (for second order reactions) gives better results and describes the thin layer drying curves quite well. The effective diffusivity was also determined from the integrated Fick's second law equation and correlated with temperature using an Arrhenius-type model. External heat and mass transfer coefficients were refitted to the empirical correlation using dimensionless numbers (J _h , J _D  = m · Re ⁿ ) and their new coefficients were optimized as a function of temperature. The internal mass transfer coefficient was also correlated as a function of moisture content, air temperature, and velocity.     

Analysis of Multiphase Flow and Heat Transfer: Pressure Buildup in an Unsaturated Porous Slab Exposed to Hot Gas

This study develops a mathematical model for coupled heat and mass transfer in an unsaturated porous slab exposed to a flowing hot gas. Effects of the initial saturation conditions on associated variables, i.e., total pressure, temperature, moisture content, and multiphase flow, are studied. The Newton-Raphson method based on a finite volume technique is applied. This study emphasizes the influence of initial saturation level and gravitational effect in heat and multiphase flow phenomena associated with this system. Gravity enhances the downward flow of liquid within the porous slab. Pressure buildup occurs near the interface between the wet and the dry zone. However, it appears that the order of magnitude to the total pressure is small. This study explains the fundamental mechanism of multiphase flow that involves heat and mass transfer in a heated unsaturated porous slab.     

Simulation of Drying Using a Kiln Model

A mathematical model was developed for simulating a convective batch lumber drying process. The model incorporates mass and heat transfer relationships within the lumber stack, as well as thermodynamic properties of the wood and drying air. It takes into account the change of air properties along the stack and its effect on the mass and heat transfer parameters. The model relies on a drying rate function that is an empirical correlation based on single-board tests. A drying rate function for western hemlock (Tsuga heterophylla) lumber was developed. The drying rate function was obtained based on experiment results from 500 small boards dried over a range of conditions used in commercial practice. The model was first validated against data available in the literature and then against large batches of hemlock dried in a laboratory kiln. In both cases, the model output was in good agreement with the average moisture content, the drying rates, and the temperatures measured in the larger batches.     

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.     

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 baker's 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.     

Effects of Air Velocity,Air Temperature,and Berry Diameter on Wild Blueberry Drying

Abstract:

In order to design, manufacture, and commission a commercial dryer to dry individually quick frozen (IQF) wild blueberries (Vaccinium angustifolium), The Nova Scotian Fruit Company completed a series of experiments to characterize the effect of air velocity, air temperature, and packed bed depth on drying. Based on previous experience with forced air packed bed drying systems at air temperatures up to 65°C, the experiments focused on measuring the effect of air temperature and velocity during the first few hours of drying. The data collected suggest that drying occurs solely in the falling rate period. These data were used to successfully design, build, and commission a commercial dryer with a tenfold increase in production capacity over previous equipment.     

Drying Kinetics of Some Building Materials

Abstract

Moisture is one of the most deteriorating factors of buildings. The deteriorating effect of moisture occurs mainly during the drying phase, not in the wetting phase. Environmental factors, such as air temperature, air humidity, and air velocity affect drying. An experimental air dryer of controlled drying air conditions was used to investigate the drying performance of 4 stone materials, 2 bricks, and 6 plasters. Drying kinetics was examined at 3 air temperatures, 5 air humidities, and 3 air velocities. A first-order kinetics model was obtained in which the drying time constant was a function of the drying conditions, and the equilibrium material moisture content was described by the modified Oswin equation. The parameters of the proposed model were found to be affected strongly by the material characteristics.