(93f) CFD Approach for the Design of Microcapsules Based On the Formation of Drops Using Viscous Non-Newtonian Fluids Through Fan Jet Nozzles

Currently a number of applications such as cosmetics, pharmaceutical or food industry rely on the use of microcapsules to bring the costumer the appropriate feeling, release the drug at the proper rate or transport it across the body, provide good taste or texture to the food, the proper essence etc. New applications require a better controlled design of the particles that involves the use of high viscosity non Newtonian liquids which must be handled at low temperatures so that the microcapsules and the content of them survive the process. The design of these microcapsules is a tedious and expensive process and a systematic approach is presented in order to reduce the design time and effort.

In the present paper we use a CFD modeling approach for the design of the microcapsules being able to predict drop size formation for a large number of experimental conditions involving flow rates and liquid properties. Close attention is paid to the non-Newtonian behavior of the solution which, in the end, is responsible for the droplet generation and size. Thus special emphasis is placed upon the experimental characterization of the liquid phase, mainly its rheology, and how the shear stress generated by the geometry and the air flow focusing the liquid through the nozzle results in a particle size. The experimental set up consists of a device develop within the group¹  were the liquid is fed from a pressurized tank into a 350 mm diameter nozzle and forced to exit an orifice just below the nozzle by regulating the pressure in the tank. Air is also fed into the nozzle chamber and its pressure can also be controlled. The generated thread which exits the nozzle orifice breaks into droplets of specific size which are gelated in a solution of barium chloride (2% wt) placed downstream. Both air and liquid pressures are regulated by a control panel which controls a pneumatic cabinet. The cabinet is fed of compressed air from a pressurized bottle. The obtained particle solution is then filtered and washed with two volumes of distilled water and particles are collected in a flask with distilled water.  After that, particles are placed into an ultrasound sonicator and sonicated for 40 minutes. Then microcapsules are ready for particle analysis. We simultaneously record the thread/cone dispersion and the droplet formation using high speed video techniques.

We first develop an empirical model for predicting the Sauter mean diameter as function of the pressure drop between the liquid and the gas flows and the apparent viscosity of the liquid so as to calibrate the experimental device and understand the phenomenon as well as to establish the range of the operating variables. Next, based on the experimental experience we developed a Finite Element based model using the commercial package COMSOL, involving the creation of the complex geometry, which is capable of reproducing the mean drop size generated by the equipment, in the range of 20 – 60 mm.  In this sense we are a step closer to define the operating conditions and the properties of the liquid phase in order to design the proper microcapsules.

References

(1)     Cervero, JM; Nogareda, J;del Valle, EMM; Galan, MA; Chem. Eng. J. 174. 2-3   699-708