Experimental Investigation and Modeling of Coalescence and Agglomeration for Spray Drying of Solutions

Abstract

Binary drop collisions were investigated with polymer solutions of a wide range of viscosities. Two experimental setups were constructed for producing droplet chains with liquids of low viscosities from 2 to 60 mPa · s and for liquids of high viscosities from 116 to 729 mPa · s. The low-viscous liquids were composed of three different aqueous solutions: sucrose with solids content from 20 to 60 %, PVP (1-Ethenyl-2-pyrrolidone) K17 with solids content from 5 to 35 %, and PVP K30 with solids content from 5 to 25 %. The high-viscous liquids were produced by PVP K17 solutions with solids content from 45 to 55 %. For the low-viscous liquids vibrating orifice drop generators were applied for drop generation, while for the high-viscous liquids a special drop generator based on pulse jetting was developed. Two drop trains were produced and the drops were directed toward each other at a certain angle so that individual droplet pairs collide with a desired velocity. For recording the collision events two high-speed cameras were used. One camera was positioned perpendicular to the collision plane recording the outcome of the collision, and the second camera was aligned parallel to the collision plane to assure that the droplet chains were exactly in one plane. Time-resolved series of pictures were used to analyze the dynamics of droplet collisions. Here, relative velocities between 0.5 and 4 m/s and impact parameters in the interval from 0 to 1 for equal-sized droplets have been investigated. For lower viscosities coalescence, reflexive separation, stretching separation, and bouncing were observed as outcomes of binary drop collisions. For this viscosity range, a model is proposed to predict the beginning of reflexive separation, using a viscosity-dependent correlation for the critical Weber number for head-on collisions. The existing model for stretching separation is found to be functional for a wide range of viscosities. The model constants could be uniquely related to the ratio of surface tension to dynamic viscosity for the different liquids considered. Thereby the shift of the triple point, where coalescence, bouncing and reflexive separation clustered together on the collision map, could be predicted. Experimental data for formation of satellite droplets were also provided. For the regime of high viscosities, the model for stretching separation was examined, and it is found that the model constants are dependent on the liquid viscosity. Additionally, collisions of droplets of different viscosities were investigated using the fluorescence technique. Penetration, or encapsulation, was documented, as well as delay of coalescence.