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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Continuous foam separation | 1/3 | https://en.wikipedia.org/wiki/Continuous_foam_separation | reference | science, encyclopedia | 2026-05-05T10:46:52.712893+00:00 | kb-cron |
Continuous foam separation is a chemical process closely related to foam fractionation in which foam is used to separate components of a solution when they differ in surface activity. In any solution, surface active components tend to adsorb to gas-liquid interfaces while surface inactive components stay within the bulk solution. When a solution is foamed, the most surface active components collect in the foam and the foam can be easily extracted. This process is commonly used in large-scale projects such as water waste treatment due to a continuous gas flow in the solution. There are two types of foam that can form from this process. They are wet foam (or kugelschaum) and dry foam (or polyederschaum). Wet foam tends to form at the lower portion of the foam column, while dry foam tends to form at the upper portion. The wet foam is more spherical and viscous, and the dry foam tends to be larger in diameter and less viscous. Wet foam forms closer to the originating liquid, while dry foam develops at the outer boundaries. As such, what most people usually understand as foam is actually only dry foam. The setup for continuous foam separation consists of securing a column at the top of the container of solution that is to be foamed. Air or a specific gas is dispersed in the solution through a sparger. A collecting column at the top collects the foam being produced. The foam is then collected and collapsed in another container. In the continuous foam separation process a continuous gas line is fed into the solution, therefore causing continuous foaming to occur. Continuous foam separation may not be as efficient in separating solutes as opposed to separating a fixed amount of solution.
== History == Processes similar to continuous foam separation have been commonly used for decades. Protein skimmers are one example of foam separation used in saltwater aquariums. The earliest documents pertaining to foam separation is dated back to 1959, when Robert Schnepf and Elmer Gaden, Jr. studied the effects of pH and concentration on the separation of bovine serum albumin from solution. A different study performed by R.B. Grieves and R. K. Woods in 1964 focused on the various effects of separation based on the changes of certain variables (i.e. temperature, position of feed introduction, etc.). In 1965, Robert Lemlich of the University of Cincinnati made another study on foam fractionation. Lemlich researched the science behind foam fractionation through theory and equations. As stated earlier, continuous foam separation is closely related to foam fractionation where hydrophobic solutes attach to the surfaces of bubbles and rise to form foam. Foam fractionation is used on a smaller scale whereas continuous foam separation is implemented on a larger scale such as water treatment for a city. An article published by the Water Environment Federation in 1969, discussed the idea of using foam fractionation to treat pollution in rivers and other water resources in cities. Since then, little research has been done to further understand this process. There are still many studies that implement this process for their research, such as the separation of biomolecules in the medical field.
== Background ==
=== Surface chemistry === Continuous foam separation is dependent on the contaminant’s ability to adsorb to the surface of the solvent based on their chemical potentials. If the chemical potentials promote surface adsorption, the contaminant will move from the bulk of the solvent and form a film at the surface of the foam bubble. The resulting film is considered a monolayer. As contaminants', or surfactants', concentration in the bulk decreases, the surface concentration increases; this increases surface tension at the liquid-vapor interface. Surface tension describes how difficult it is to extend the area of a surface. If surface tension is high, there is a large free energy required to increase the surface area. The surface of the bubbles will contract due to this increased surface tension. This contraction encourages the formation of a foam.
==== Foams ====
===== Definition ===== Foam is a type of colloidal dispersion where gas is dispersed throughout a liquid phase. The liquid phase is also called the continuous phase because it is an uninterrupted, unlike the gas phase.
===== Structure ===== As the foam is formed, it changes in structure. As the liquid foams up into the gas, the foam bubbles begin as packed uniform spheres. This phase is the wet phase. The farther up the column the foam travels, the air bubbles distort to form polyhedral shapes, the dry phase. The liquid that separates the flat faces between two polyhedral bubbles is called the lamellae; it is a continuous liquid phase. The areas where three lamellae meet are called plateau borders. When the bubbles in the foam are the same size the lamellae in the plateau borders meet at 120 degree angles. Since the lamella is slightly curved, the plateau region is at low pressure. The continuous liquid phase is held to the bubble surfaces by the surfactant molecules that make up the solution being foamed. This fixation is important because otherwise the foam becomes very unstable as the liquid drains into the plateau region making the lamellae thin. Once the lamellae become too thin they will rupture.
=== Theory ===
==== Young–Laplace equation ==== As vapor bubbles form in a liquid solvent, interfacial tension causes a pressure difference, Δp, across the surface given by the Young–Laplace equation. The pressure is greater on the concave side of the liquid lamellae (the inside of the bubble) with radius, R, dependent on the pressure differential. For spherical bubbles in a wet foam and standard surface tension γ°, the equation for the change in pressure is as follows:
Δ
P
=
2
γ
∘
R
{\displaystyle \Delta P={\frac {2\gamma ^{\circ }}{R}}}
As the vapor bubbles distort and take the form of a more complex geometry than a simple sphere, the two principal radii of curvature R1 and R2 would be used in the following equation: