Multiphase flows are classified as either "dispersed", involving bubbles, drop-lets and particles, or "separated", such as free surface and stratified flows. In STAR-CD we provide models to cover the whole spectrum of flow types, including the VOF method for free surface flows, the Lagrangian particle method for dispersed flows and the Eulerian multiphase method for general multiphase flows. "Eulerian" includes high volume fraction flows, and here we describe some progress in this area, stimulated by our involvement with partners in an industrially-driven, Brite/Euram program sponsored by the EU.

Liquid-liquid extraction column

Liquid-liquid extraction is often used in the petrochemical industry to promote mass transfer between two fluids. We worked with Total to model an extraction column. To provide maximum contact between the two fluids, a counter-current flow arrangement was used, shown in Figures 1 & 2. The heavier fluid is introduced through a central inlet at the top of the column. A distributor screen distributes the fluid into the column. The lighter fluid enters the column through the central inlet at the bottom. Perforated trays are placed horizontally in the column to provide further contacts between the two fluids in similar fashion to a distillation column. The two fluids can leave the column via the bottom or the top outer annuli.

The STAR-CD simulation accurately captured the flow inside this column, which is quite intricate. Figure 1 shows the collection of the heavier fluid on the trays, the rolling-off at the tips and the cascade down the column.

Break-up and coalescence of drops and bubbles

Industrial processes such as emulsification involve break-up and coalescence. Drops and bubbles break into smaller ones and coalesce into larger ones and we end up with a spectrum of drops or bubbles of different sizes. Modeling these complex processes accurately is a challenge for the CFD technologist.

Within the Brite/Euram program, advanced modeling of break-up and coalescence of drops and bubbles was investigated and a new, so called "S-gamma" model was developed. In this, transport equations are solved for the moments of an assumed particle size distribution function. The break-up and coalescence models govern changes in the particle size distribution. Both processes involve overcoming the surface energy of the particle due to turbulence or the shear flow effects. Surface energy is represented by surface tension, which is affected by surfactant concentration in the system. Transport equations were derived and solved for surfactant concentration in the bulk liquid and on the surface of the particles. Figures 3 and 4 are experimental results from the Laboratoire de Génie Chimique of Toulouse showing break-up and coalescence of droplets with the flow.

We worked with Unilever to apply the technology to a pin stirrer for food processing. The goal was to calculate changes of droplet diameter with time in an emulsification process. A liquid-liquid mixture (40 vol.% oil in water) is stirred vigorously in the vessel.

The break-up of the oil into small diameter droplets can be seen in figure 5. STAR-CD’s predictions agreed well with experiments, with details to be published soon.

Conclusions

Eulerian multiphase modeling in STAR-CD is advancing well. New technology is being developed for future versions of STAR-CD applicable to high volume fraction flows encountered in equipment such as stirred tank emulsifiers, liquid-liquid extraction and settling systems, and gas-liquid stirred tanks.

*The examples show here are part of the outcome of joint work with partners in a Brite Euram program involving Imperial College, Institut National Polytechnique Toulouse, University of Twente, and industrial partners Unilever, ICI/Huntsman, and Total.

*Images provided by Laboratoire du GénieChimique,Toulouse