Analyses Of Droplet Size Distribution And Design Of Spray Dryers Based On The Trajectories Of Particles In Drying Continuum

The dissertation systematically discussed the method determining droplets size distribution through the atomizer, the movement states of particles and dynamical properties of the flowing air in the spray dryer as well as building their mathematical models, and calculating heat and mass transfer between droplets and drying gas according to the Marshall Equation. By solving the dynamics equation of the particles in the chamber, we completed the design of spray drying equipment and the development of designing program. In addition, we verified the reliability of the program by contrast with the data of some references.In this article we brought forward a means determining droplet size distribution called plate sampling. That's to say, we experimented on partial material passing the centrifugal atomizer and therefore different sizes droplets came into being. These droplets descended on sample plates by the action of air drag force and gravitation. So we obtained the relation between radial distances and weight percentage corresponding droplets on each plate. Then the program supposed several groups of droplets with different diameters. By simulating their movement trajectories flying in still air, we could get the relation between particle diameters and distances in the radial direction. Finally the droplet size distribution is determined by means of mathematical calculation to deal with these two relations.Simulating droplets movement and drying process, we introduced the time-increasing method to determine the reference coordinates, namely movement coordinates, according to particles position hi the beginning of each time step. Particles movement trajectories were followed step by step since they left the atomizer, until the material to dry was completely dried. During each time step, the trajectories of droplets depended on weight and drag forces by drying air. At the same time the gas flow field and droplets size and mass would change because of heat and mass exchange between droplets and air, which simultaneously had significant effect on particles forces and trajectories. However, the influence could be neglected as long as the time step was enough small. So droplets places and velocities at the end of this time step would be determined by particles situation parameters at the beginning.The drying process was divided into two phases such as constant speed phase (the first phase) and decreasing speed one(the second phase). During the first phase vapor pressure on the droplet surface always equaled to saturation vapor pressure at the temperature of droplets surface.Nevertheless during the second phase the droplets surface no longer kept wet so that vapor pressure was lower than corresponding saturation vapor pressure. Anyway, the drying impetus was all the same pressure difference between vapor pressure of gas and droplets surface. From the point of view of heat transfer, however heat from drying air was mostly used to droplet evaporation, there were two different schemes about heat transfer discussed here. One option was during the constant speed phase the heat of drying air was all expended on evaporation and heat & mass transfer would reach instantaneous equilibrium, but during decreasing speed phase, heat from air also caused increasing droplets temperature besides water evaporation. The other thought heat was all used to evaporation and the equilibrium existed in heat & mass exchange during the two phases.This article simulated droplets trajectories and calculated some equations about movement, heat & mass transfer until the average moisture content met our request. Thus, we could determine the diameter and height of spray dryer by the interrelated program.Reasonable agreement with the data of samples in correlative references was reached so that it was verified the theoretical simulation of the program was credible, of which the condition was same as the samples. So the discussed model in this dissertation was of practical value used to design as well as check and optimization of to equip...