Foam fractionation is a

chemical process In a scientific sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by an outside force, and involves a chemical reaction of some ...

in which

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, t ...

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...

s are preferentially separated from a

liquid solution In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...

using rising columns of

foam Foams are materials formed by trapping pockets of gas in a liquid or solid. A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the ...

. It is commonly used, albeit on a small scale, for the removal of organic waste from aquariums; these units are known as "

protein skimmer A protein skimmer or foam fractionator is a device used to remove organic compounds such as food and waste particles from water. It is most commonly used in commercial applications like municipal water treatment facilities and public aquariums. Sm ...

s". However it has much broader application in the chemical process industry and can be used for the removal of surface active contaminants from waste water streams in addition to the enrichment of bio-products.

History

Whilst

s have been common place in aquaria for many years, it was not until the 1960s that a concerted effort was made by Robert Lemlich of the

University of Cincinnati The University of Cincinnati (UC or Cincinnati) is a public research university in Cincinnati, Ohio. Founded in 1819 as Cincinnati College, it is the oldest institution of higher education in Cincinnati and has an annual enrollment of over 44,0 ...

to characterise a model of adsorptive bubble separation processes, of which foam fractionation is one example. Until the mid-2000s, there was very little further development of foam fractionation or attempts to understand the underlying physics of the process. Many workers were satisfied with empirical descriptions of specific systems rather than attempt a mechanistic model of the process, and it is possibly for this reason that the adoption of the technology has been slow despite its enormous potential. Foam fractionation is closely related to the allied process of

froth flotation Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in mineral processing, paper recycling and waste-water treatment industries. Historically this was first used in the mining industry, wher ...

in which hydrophobic particles attach to the surface of bubbles which rise to form a pneumatic (i.e. rising) foam. In this way, relatively hydrophobic particles can be separated from relatively hydrophilic particles. Froth flotation is typically used to separate coal particles from ash or particles of valuable minerals from

gangue In mining, gangue () is the commercially worthless material that surrounds, or is closely mixed with, a wanted mineral in an ore deposit. It is thus distinct from overburden, which is the waste rock or materials overlying an ore or mineral body ...

material. It was research into the froth phase of froth flotation conducted at the

University of Newcastle, Australia The University of Newcastle (UON), informally known as Newcastle University, is a public university in Newcastle, New South Wales, Australia. Established in 1965, it has a primary campus in the Newcastle suburb of Callaghan. The university al ...

, specifically into the prediction of liquid fraction and liquid flux in a pneumatic foam, that enabled a preliminary mechanistic description of foam fractionation. The synergies between foam fractionation and froth flotation have been explored in a 2009 special issue of the Asia Pacific Journal of Chemical Engineering.

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 Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations. It is also used in chemistry to supply energy to reactions ...

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: #The target molecules adsorb to a surface, rather than absorb by travelling into the bulk of one phase from another, and #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 Ostwald ripening is a phenomenon observed in solid solutions or liquid sols that describes the change of an inhomogeneous structure over time, i.e., small crystals or sol particles dissolve, and redeposit onto larger crystals or sol particles ...

, one can engineer an internal source of reflux within the column. As in many chemical processes, there are competing considerations 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: #The target molecule adsorbs to a bubble surface, and #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.Stevenson P 2007 Hydrodynamic theory of rising foam, ''Minerals Engineering'' 20, p.282

Applications

#Enrichment of the solutions of biomolecules in pharmaceutical and food technologies. #Stripping of surface-active contaminants from streams of waste water. #Stripping of non-surface-active contaminants from streams of waste water (such as metal ions) with the help of one or more assistant surfactants. #The removal of frother downstream of froth flotation operations (known as frother stripping).

Notes

Fractionation