A variety of spray pyrolysis (SP) techniques have been developed to directly produce ceramic powders from solutions. This paper reviews the current status of these processes in terms of the process parameters that enable the formation of particles with controlled morphology and composition. A model incorporating solute diffusion in the droplet and solvent evaporation from the droplet surface is presented to establish the critical parameters leading to solid particle formation. The model illustrates that solid particles can be obtained if solutes with high solubility and a large difference between the critical supersaturation and equilibrium concentration are used and if the process is designed to avoid solvent boiling. It is demonstrated that mixed metal oxide, non-oxide, and composite particles that are solid, hollow, porous, or fibrous can be produced by modifying the precursor characteristics, solution properties, and process parameters. The physical and chemical flexibility of SP processes offers numerous opportunities for the controlled synthesis of advanced ceramic powders and films. However, production rates are limited by the need to produce <5-[mu]m-diameter droplets and to avoid subsequent droplet coagulation. Developments in process controls, atomization, and system design are required for wider commercialization of SP-type processes.

Ceramic Powder Synthesis by Spray Pyrolysis

The evaporation state of solution aerosol thermolysis (SAT) was modeled to study the effect of various parameters on solid particle formation by solute precipitation. A comparison of the characteristic time constants for various processes demonstrated that droplet shrinkage and solute diffusion are the slowest processes, and that the fast processes, i.e., vapor diffusion and heat conduction in the gas phase and the liquid phase, can be assumed to have reached steady state. Differential equations for these faster processes were thus simplified and were solved numerically along with a modified solute diffusion equation, using an explicit first-order finite difference scheme. The computations were done until the solute concentration at the droplet surface reached the critical supersaturation. Then, if the solute concentration at the droplet center is higher than the equilibrium saturation, volume precipitation is proposed to occur. Solutes with a large difference between critical supersaturation and equilibr...

https://www.tandfonline.com/doi/pdf/10.1080/02786829308959653

Modeling of Solid Particle Formation During Solution Aerosol Thermolysis: The Evaporation Stage

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.

Theoretical Models of Single Droplet Drying Kinetics: A Review

Optimization of the spray-drying process for developing guava powder using response surface methodology

Biological materials contain a variety of individual soluble components. When cellular biological materials are immersed in osmotic solution, multicomponent mass transfer occurs, which ultimately leads to the loss of water from the food, or osmotic dehydration. Mass transfer of food constituents during osmotic dehydration may cause changes in food quality in terms of nutritional value, texture, color, and taste. The aspects that are important for mass transfer modeling are the driving force and the physical properties of cell units and tissue matrix of cellular material. The complex cell wall structure of cellular material acts as a semipermeable membrane, resulting in two simultaneous and countercurrent flows in the biological tissue: water transfer out of material tissue and transfer of the solute from the osmotic solution into the cellular tissue. When the cells remain intact, the chemical potential of water is the main local driving force for mass transfer towards the free intercellular spaces. At the...

Osmotic dehydration of foods: mass transfer and modeling aspects

A survey is given of heat and mass transfer around droplets in spray dryers and the diffusional transport inside them. A calculational model is developed which includes variable diffusion coefficients in the drying liquid and swelling or shrinking of droplets. Calculations for droplets containing soluble solids show how the drying histories of droplets are influenced by three extreme patterns of air circulation and spray dispersion, and by droplet inflation. The influence of these factors on the properties of spray-dried liquid foods are discussed. Furthermore diffusion equations for binary systems are surveyed and diffusion coefficients for super-saturated aqueous maltose solutions are reported.

/pdf/simulation-of-heat-and-mass-transfer-in-spray-drying-1nwv8bqyuy.pdf

J. van der Lijn

Papers

Simulation of heat and mass transfer in spray drying