No abstract

A receding interface model of the drying of single drops of slurries of sodium sulfate decahydrate has been developed to describe the drying characteristics of this material and to estimate the drying rates of particulate slurries. The simultaneous heat and mass transfer rate equations have been solved numerically, and the results obtained have been compared with those obtained experimentally by drying single drops suspended on the tip of a glass filament.Single drops of slurries, 1.0 to 1.5 x m dia., were suspended on the tip of a flexible glass cantilever inserted in a vertical wind tunnel. A 50pm dia. nickel wire passed through the center of the glass beam and the outer surface was coated with a thin film of copper, thereby forming a thermocouple that measured the temperature of the core of the drop; the deflection of the beam gave the loss in weight during drying. In this way the instantaneous drying rate and drop temeprature were determined and compared with those predicted by the receding interface model. In all cases the agreement between the predictions of the model and experimental results was good. SCOPEOne of the prerequisites for the optimum design of processes involving evaporation of a spray is an understanding of the controlling mechanisms in the heat and mass transfer processes during drying. Although a great deal of fundamental research had been carried out on the evaporation of pure liquid drops (WhytlawGray and Patterson, 1932;Langstroth et at. 1950;Ranz and Marshall, 1952;Pasternak and Gauvin, 1960), experimental data on the drying of drops containing solids is limited. This can be attributed in part to the complexities in analyzing the heat and mass transfer processes after a solid crust has formed. Heat is transferred by convection from the drying medium to the outer surface of the crust and then by conduction through the solid portion of the crust to the interior. Evaporation occurs and moisture diffuses through the pores in the crust into the surroundings. As the particle dries, the crust increases in thickness, resulting in an increase in the resistance to heat and mass transfer. This invariably decreases the core temperature, initially causing a reduction in the partial pressure driving force. However when the temperature of the core of the drop is considerably below that of the drying medium, the core temperature afterwards increases and the drying rate consequently increases. The transfer process is therefore highly complex and difficult to model mathematically.The present study was initiated to further the understanding of the mechanisms involved in the drying of drops containing solids, and more specifically to formulate a mathematical model to predict the drying rate under conditions that might be encountered in spraydrying equipment. CONCLUSIONS AND SIGNIFICANCEThe filament thermocouple developed for this investigation provided valuable insight into the drying process not normally obtainable from a simple mass balance.

Experimental coalescence time and time distribution studies of the coalescence of a single droplet a t a plane interface are presented for four hvo-component-two-phase systems. The results have been analyzed by dimensional analysis and statistical analysis, and a correlation of coalescence time and coalescence time distribution with the physical properties of the system is proposed.

Correlations of the collision frequency and the coalescence frequency of an agitated dispersion are combined with models relating drop sizes and hold‐up to agitator speed, to predict the ambivalence range and phase inversion composition of liquid‐liquid dispersions. Our model was tested by comparing predicted and experimental phase invesrion compositions of the system toluene‐water ad carbontetrachloride‐water. In addition, the predicted results have been compared ith published results for the system kerosene‐water, and in all cases, agreemet between predicted and experimental resutls are excellent.

The phase equilibria of the systems a r e discussed:(i) n-butanol-acetic acid-water and n-heptane (ii) Water-acetic acid-benzene and carbon tetrachloride (iii) Water-methanol-p.cresol and methyl naphthalene have been studied and a correlating procedure has been developed that utilize5 binary and ternary experimental data to predict the composition of the quaternary equilibrium mixtures. Allowance is made for the different salting effects and the agreement between the experimental and predicted quaternary data is in all cases very good.