Foam fractionation
Encyclopedia
Foam fractionation is a chemical process
in which hydrophobic molecule
s are preferentially separated
from a liquid solution
using rising columns of foam
. It is commonly used, albeit on a small scale, for the removal of organic waste from aquarium
s; these units are known as "protein skimmer
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.
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
to characterise of model 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
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
material. It was research into the froth phase of froth flotation conducted at the University of Newcastle, Australia
, 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
.
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:
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:
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.
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 somebody. Such a chemical process commonly involves a chemical reaction of some sort...
in which hydrophobic molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s are preferentially separated
Separation process
In chemistry and chemical engineering, a separation process, or simply a separation, is any mass transfer process used to convert a mixture of substances into two or more distinct product mixtures, at least one of which is enriched in one or more of the mixture's constituents. In some cases, a...
from a liquid solution
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
using rising columns of foam
Foam
-Definition:A foam is a substance that is formed by trapping gas in a liquid or solid in a divided form, i.e. by forming gas regions inside liquid regions, leading to different kinds of dispersed media...
. It is commonly used, albeit on a small scale, for the removal of organic waste from aquarium
Aquarium
An aquarium is a vivarium consisting of at least one transparent side in which water-dwelling plants or animals are kept. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, marine mammals, turtles, and aquatic plants...
s; these units are known as "protein skimmer
Protein skimmer
A protein skimmer or foam fractionator is a device used mostly in saltwater aquaria to remove organic compounds from the water before they break down into nitrogenous waste...
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 protein skimmerProtein skimmer
A protein skimmer or foam fractionator is a device used mostly in saltwater aquaria to remove organic compounds from the water before they break down into nitrogenous waste...
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
University of Cincinnati
The University of Cincinnati is a comprehensive public research university in Cincinnati, Ohio, and a part of the University System of Ohio....
to characterise of model 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
Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in several processing industries...
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
Gangue
In mining, gangue is the commercially worthless material that surrounds, or is closely mixed with, a wanted mineral in an ore deposit. The separation of mineral from gangue is known as mineral processing, mineral dressing or ore dressing and it is a necessary and often significant aspect of mining...
material. It was research into the froth phase of froth flotation conducted at the University of Newcastle, Australia
University of Newcastle, Australia
The University of Newcastle is an Australian public university that was established in 1965. The University's main and largest campus is located in Callaghan, a suburb of Newcastle in New South Wales...
, 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
Asia Pacific Journal of Chemical Engineering
The Asia-Pacific Journal of Chemical Engineering is a peer-reviewed scientific journal published by John Wiley & Sons on behalf of Curtin University of Technology. Until 2006 it was known as Developments in Chemical Engineering and Mineral Processing: The Australasian Research Journal and published...
.
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 refluxReflux
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...
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
right|thumb|300px|Basic schematic of the Ostwald ripening process Ostwald ripening is an observed phenomenon in solid solutions or liquid sols which describes the change of an inhomogeneous structure over time...
, 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:
- 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.
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).