MODELLING OF SPRAY DRYER

Abstract

In this thesis, a mathematical model has been developed for analysis, design and simulation of a co-current spray dryer. The model considers several important features of drying process as described below. (i) Velocity of particles in all the three directions. (ii) Drop/particle size ranges from 20 to 240 μm. (iii) Particle shrinks as a shrinking balloon in the constant rate zone, and after reaching critical moisture the diameter of particle becomes constant (iv) Dependence of physical properties and other constitutive parameters on temperature. (v) Porosity of particles. In the model, drying medium is assumed to be saturated on the surface of the particle, intraparticle thermal gradients being small are neglected, and nozzle zone and any thermal degradation are not considered. The model comprises a set of non-linear coupled ordinary differential equations, and are solved by a fourth order Runga-Kutta method during numerical simulation. Model has been validated with the data available in the literature. Spray drying of milk is taken as a system for the purpose of analysis and simulation. An empirical correlation has been developed for the decrease in drying rate after the attainment of critical moisture content. This correlation is based on the data available in the literature and is a polynomial function of moisture content of particle. Such a correlation does not exist in the literature, and it facilitates numerical simulation process. Optimum temperature range for drying of milk is observed to be 55- 65°C, which avoids reverse transport zone in drying process and thermal degradation of milk. It is suggested that the lower humidity of drying medium, and the feed temperature higher than the wet bulb temperature of the drying medium should be used. Detailed results of numerical simulation have also been presented.