Foam Fractionation - Design Considerations

Design Considerations

Robert Lemlich showed how foam fractionation columns can be operated in stripping, enriching, or combined modes (depending on whether the feed is sent to the top, bottom or middle of the column), and can be operated with or without an external reflux stream at the top of the column. It helps to think of the process as similar to a gas-liquid absorption column. The differences are that:

1. The target molecules adsorb to a surface, rather than absorb by travelling into the bulk of one phase from another, and
2. The foam autogenously provides the packing within the column.

Just as in gas-liquid absorption, the adoption of reflux at the top of the column can engender multiple equilibrium stages within the column. However, if one can control the rate at which the bubble size changes with height in the column, either by coalescence or Ostwald ripening, one can engineer an internal source of reflux within the column.

As in many chemical processes, there are competing condiderations of recovery (i.e. the percentage of target surfactant that reports to the overhead foamate stream) and enrichment (i.e. the ratio of surfactant concentration in the foamate to the concentration in the feed). A crude method of moving upon the enrichment-recovery spectrum is to control the gas rate to the column. A higher gas rate will mean higher recovery but lower enrichment.

Foam fractionation proceeds via two mechanisms:

1. The target molecule adsorbs to a bubble surface, and
2. The bubbles form a foam which travels up a column and is discharged to the foamate stream of foam fractionation.

The rate at which certain non-ionic molecules can adsorb to bubble surface can be estimated by solving the Ward-Tordai equation. The enrichment and recovery depend on the hydrodynamic condition of the rising foam, which is a complex system dependent upon bubble size distribution, stress state at the gas-liquid interface, rate of bubble coalescence, gas rate inter alia. The hydrodynamic condition is described by the Hydrodynamic Theory of Rising Foam.