Continuous foam separation

This process is commonly used in large-scale projects such as water waste treatment due to a continuous gas flow in the solution.

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.

Grieves and R. K. Woods[3] 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[4] of the University of Cincinnati made another study on foam fractionation.

Continuous foam separation is dependent on the contaminant’s ability to adsorb to the surface of the solvent based on their chemical potentials.

The farther up the column the foam travels, the air bubbles distort to form polyhedral shapes, the dry phase.

When the bubbles in the foam are the same size the lamellae in the plateau borders meet at 120 degree angles.

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.

[6] As vapor bubbles form in a liquid solvent, interfacial tension causes a pressure difference, Δp, across the surface given by the Young–Laplace equation.

For spherical bubbles in a wet foam and standard surface tension γ°, the equation for the change in pressure is as follows: 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:[1] As pressure grows inside the bubbles, the liquid lamellae shown in the figure above will forced to move toward plateau borders causing a collapse of the lamellae.

In this case, the following equation can be applied where a is the activity of the surfactant, R is the gas constant, and T is the absolute temperature: In order solve for the area on the foam surface occupied by one adsorbed molecule, As, the following equation can be used where NA is the Avogadro constant.

[9] The foam produced during wastewater treatment can either be recycled back into the activated sludge tank within a waste treatment plant, the bacterial organisms that live there have been found to break down ABS when allowed enough time, or extracted and collapsed for disposal.

[10] Foam separation has also been found to decrease the chemical oxygen demand when used as secondary treatment technique for wastewater.

[11] The removal of heavy metal ions from wastewater is important because they accumulate easily in the food chain, ending in animals such as swordfish that humans eat.

Foam separation can be used to remove heavy metal ions from wastewater at low costs, especially when used in multistage systems.

When purifying proteins from solution on an industrial scale, the most cost efficient method is desired.

However, an increased velocity of the gas being pumped into the system has been shown to lead to a decrease in the enrichment ratio.

However, in recent years foaming for protein and pharmaceutical extraction has gained increased interest for researchers.

The process is stationary (or in steady state) as long as the volume of liquid is constant as a function of time.

As long as the process is in steady state, the liquid will not overflow into the foaming column.

As the foam rises and becomes drained of the liquid, it gets diverted into a separate container to collect the foamate.

Foam separators for different types of applications use the basic set up shown in the diagram, but can vary with placements and addition of equipment.

[17] The variation of flow rates on the gas input as well as other equipment settings has effects on the optimization of the parameters.

The isoelectric point is one factor that must be taken into consideration, when surfactants have neutral charges they are more favorable for adsorption to the liquid-gas interface.

pH offers a unique problem for proteins due to the fact that they will denature in pHs that are too high or low.

The diagram depicts the accumulation of surfactant molecules at the liquid-vapor interface causing a contraction of the surface to form a foam.
This schematic depicts a basic wastewater treatment plant that utilizes foaming as an extraction technique. The foamate can both be extracted and disposed of if it is being used to remove heavy metal, or it can be returned to the activated sludge tank if it contains detergents that the organisms in the tank can degrade over time.
The basic continuous foam separator contains a feed flow in, a feed flow out, and a gas flow in. The foam column rises and gets diverted into a separate vessel to be collected.