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Foam fractionation is one of various methods of adsorptive bubble separation techniques based on surface activity.
Foam fractionation is considered as a cost-effective method of protein enrichment and has the potential to be an attractive replacement for one or more of the costly initial steps in an overall purification scheme.
Foam fractionation works well with low concentrations (< 1%wt) while other separation techniques such as ultrafiltration are only economical when operating at high concentration.
2.1.1. Properties of foam
Foams are dispersions of gaseous bubbles in a small amount of liquid, in which the gas volume is much greater than that of the liquid, with surface-active reagents. It is reported that foam cell sizes range from 50 µm to several millimetres, and foam densities vary from very low to approximately 700 g.dm-3, beyond which gas emulsions rather than foams are found (Morrison and Ross, 2002). Lamellae, which is a fine liquid layer between dispersed bubbles, range in thickness from 10 to 1000 nm.
Foam plays an important role in a various particular applications. One of foam products popularly employed in reality is fire retardant foam used in extinguishing fires because its low density ensures that it floats upon burning oil. Similarly, foam can be widely used in separation processes, take froth flotation and foam fractionation as example. However, in some other industrial processes such as distillation and solvent tripping, the great abundance of foam needs to be prevented.
Morrison and Ross (2002) stated that foam properties depend on chemical composition and properties of adsorbed surface films, and thus on the thermodynamics of colloidal solutions. In addition, there are many factors which affect foam properties including the surface rheology, size distribution of the cells, surface tension of the liquid and external pressure and temperature. The rheological characteristics of foams include: (1) foams are highly viscous, (2) foam exhibit shear thinning in which viscosity decreases with increasing shear rate like polymer solutions and molten polymers, (3) foam exhibit yield points at which it deforms elastically and reverts to its original shape or position, (4) foams appear to slip at a solid boundary (Morrison and Ross, 2002).
2.1.2. Foam structure
Much of scientific works on foams represents the geometry of foams. Plateau's laws are proved by using measure theory to study surface area minimization and describe the assemblies of bubbles and films in foams. Two of these laws are demonstrated as following:
Along an edge, three and only three liquid lamellae can equally meet at angles of 120o.
At the intersection point of four bubbles, these four edges meet at the tetrahedral angle of 109o28¢16² (109.47o).
If the bubbles are not inclined to those angles, then bubbles rearrange themselves to conform to Plateau's laws.
For many previous years, the structure of foams was accepted as following Lord Kelvin's conjecture. In his works in 1887, Kelvin proposed that the body-centre-cubic (b.c.c) structure as the optimum arrangement to satisfy Plateau's laws. He pointed out that the polyhedron which divides space into many cells of equal volume with minimum surface area and without void is bitruncated cubic honeycomb called Kelvin structure (or Kelvin cell). This is polyhydron which has truncated edges intersecting at the vertices of 14-sided polyhydron (tetradecahedron) with six plane faces and eight hexagonal faces (Morrison and Ross, 2002 and Weaire and Phelan, 1994). Figure 2.1 illustrates this arrangement.
A foam structure proposed by Kelvin would have both the lowest potential energy and the least surface area. However, Kelvin cells have only quadrilateral and hexagonal faces, while experimental observations of foam structure has proved that the lamellae in reality are generally pentagonal (Morrison and Ross, 2002). Although Kelvin structure has revealed the problem in the polyhedron form of the cells, it was widely accepted until the discovery of the Weaire-Phelan structure in 1994.
Weaire and Phelan (1994) proposed the structure in which there are two kinds of cells. One is a dodecahedron (12-sided polyhedron) with pentagonal faces, and the other one is a tetradecahedron (14-sided polyhedron) with two hexagonal and twelve pentagonal faces. Although two kinds of cells have different number of faces (different shape), they both have equal volume. The foam made of Weaire-Phelan structure has the lower surface area (0.3% less than that of Kelvin structure), thus Weaire-Phelan cells are presently accepted to describe the formation of foam cells. Figure 2.2 illustrates Weaire-Phelan structure.
Figure 2.1. Bitruncated 14-sided polyhydron (Kelvin structure) proposed by Kelvin. Picture was adapted from (1)
Figure 2.2. Weaire-Phelan structure with pentagonal faces and hexagonal faces. Picture was adapted from (1)
when Denis Weaire and Robert Phelan found a structure with 0.3% less surface area than Kelvin's structure.
The thin liquid films residing between interstitial space of adjacent bubbles are called Plateau borders. Plateau border is one of basic elements of foams. It is composed of polyhedral gas bubbles with vertices are at the junction of film lamellae. Significant amounts of liquid in foams are contained in Plateau borders which provide a route for drainage of the foam. Figure 2.3 illustrated the schematic representations of Plateau borders.
Figure 2.3. Schematic three-dimensional Plateau border in which three film lamellae meet at an angle of 1200. The film thickness d was exaggerated, and A is the centre of the radius of curvature Rz . Picture was adapted from (2).
Nguyen (2002) suggested two approximation for the shape demonstration of Plateau border, including a circular and triangular cross section. The circular cross-section approximations are inappropriate for the actual shape of Plateau borders encountered in foam. The triangular approximations show the effects of shear viscosity of the gas-liquid interface on the liquid flow in the Plateau borders.
Several assumptions are employed to model the shape of the Plateau border cross-section, including: (a) bubbles at any cross section of the foam bed are uniform in size; (b) actual cross-section has the shape of three equal circular arcs with a radius R, separated by three liquid films; (c) the thickness of liquid films is strikingly smaller than the radius of the circular arcs (Nguyen, 2002; Uraizee and Narsimhan, 1992). Figure 2.4 shows the cross section of Plateau borders between adjacent bubbles.
Figure 2.4. Schematic solid cross-section of the Plateau borders with three equal circular arcs joined together at three points. The picture was adapted from (3)
Uraizee and Narsimhan (1992) stated that the drainage of liquid from thin film lamellae into adjacent Plateau borders due to the Plateau border suction and disjoining pressure whereas the gravity is the major cause of the drainage of liquid from interstitial network of Plateau borders. Everett (1988) also mentioned that the gravitational forces draining the liquid in Plateau borders are intensified by surface tension forces which can be observed that the surfaces of neighbouring junctions are sharply curved.
2.1.3. Foam stability
Schramm (2005) supported that foam stability involves sustainability against two processes including film thinning and film rupture (coalescence). Film thinning is the phenomenon in which various gas bubbles approach closely, and the slender films between them become thinner. However, the bubbles do not actually contact directly each other, and there is no change in total surface area although the thickness of interstitial films decrease. Coalescence is the condition when the liquid films collapse and the bubbles fuse together to form a single bigger bubble. In coalescence, the total surface area of bubbles will reduce.
The major factors which play an important part in foam stabilisation include: surface tension, surface elasticity, surface viscosity and disjoining pressure. A number of conditions involving foam stability for film thinning and bubble coalescence were proposed by Schramm (2005) as follows:
Low surface tension - it enables the formation and sustainability of large interfacial area.
Low gravity drainage - it decreases the rate of film thinning.
Low capillary suction (suction from Plateau border) - it decreases the rate of film thinning.
High surface elasticity - it counteracts the effect of surface perturbations.
High bulk viscosity - it reduces the rate of film thinning.
High surface viscosity - it reduces the rate of film rupture.
High electric double layer repulsion - it increases disjoining pressure and reduces the rates of film thinning and rupture.
High steric repulsion - it reduces the rates of film thinning and rupture.
Low dispersion force attraction - it decreases the rates of film thinning and rupture.
Foam drainage, enhanced by Plateau border suction, is a significant element in the formation and stability of foams. The collapse of the liquid drainage from film lamellae leads to coalescence of bubbles. This rupture results in the redistributions of bulk surfactant and adsorbed surfactant from the collapsed interface in the transitional region of Plateau borders and thin films. This leads to more adsorption at the gas-liquid interface due to enrichment of surfactant in the bulk. This view of liquid rupture was supported by Uraizee and Narsimhan (1992). Lemlich (1968a) demonstrated that two reasons for internal bubble coalescence are the diffusion of gas from small bubbles to bigger bubbles and the rupture of thin films segregating the bubbles.
The distributions of the internal reflux were also mentioned by Uraizee and Narsimhan (1992) that the surfactant concentrations will be the same in both thin films and Plateau borders if dispensed uniformly in both. Conversely, if the internal reflux is dispensed only into the Plateau borders, the surfactant concentration in the films will not change because the films and Plateau borders are divided. Generally speaking, the enrichments in Plateau borders and thin films may be different depending virtually on the distributions of liquid in the thin film. The actual enrichment in reality will lie between the above two extreme situations.
2.1.4. Preparation of foams
Modified with Weaire 1999
Foams are simply formed by mixing and agitating a gas and a liquid together in the same container. When a solution containing surface-active agents (surfactants) is sparged with an air flow, foam can also be produced. In pure liquids, when two bubbles approach closely together, coalescence occurs virtually immediately and there is no thin film persistence between bubbles. When surfactant is introduced to the liquid, the adsorption of surfactant enhances thin-film persistence between bubbles. It can be explained that when two gas bubbles approach, there exists a thin liquid film lamella which stabilises (OR prevents) the bubbles from rupturing (Schramm, 2005).
To stabilise the foams in the solution, a foaming agent which may contain surfactants, macromolecules, or fine solids is needed. Schramm (2005) suggested that a foaming agent is necessary to decrease the surface tension, increase the interfacial area with the least mechanical energy input with respect to economic issues, and prevent the thin-film collapse and bubble coalescence. Other beneficial factors which also promote the foam persistence include the increase of viscosity and surface elasticity.
2.1.5. Liquid hold-up
Liquid hold-up is the volumetric fraction of liquid in the foam and total foam volume. The simplified equation of liquid hold-up can be shown as follows:
In systems including foam fractionation column and froth flotation column, liquid hold-up is an important factor used for ascertaining interstitial liquid flow, residence time distribution, bubble coalescence, or determining reflux ratio (Yianatos et. Al, 1985). Liquid hold-up is also useful for the column design, scale-up or manipulation of systems.
Due to the (its) importance in column operation, a great number of measurement methods have been taken into account. Yianatos et. al (1985) demonstrated that liquid hold-up was conventionally determined by comparing directly the height of aerated liquid with that of liquid without aeration, or by estimating from bed expansion for fluidized two phase systems. Besides, the usages of radioactive tracers or electrical conductivity were alternative methods to determine liquid hold-up of foam columns by various works. Yianatos et. al (1985) themselves approached the liquid hold-up estimation from conductivity measurements based on concept of tortuosity. However, those previous approaches were time consuming, low cost effective and inappropriate to apply the measurement from laboratory scale to industrial plant scale.
The liquid hold-up measurement presently is conducted by the use of nuclear scintigraphic imaging technique proposed by Lockwood et al. (2000). The nuclear scintigraphic imaging technique is actually employed in medical studies to specify the development process of disease in the body.
Lockwood et al. (2000) proved by experiments that the volume fraction of water in the foam solution are unchanged with the increase of column height. This view is opposite to that previously used in the study of foam drainage, which stated that liquid holdup decreases with height as believed by Brown et al. (1990).
Uraizee and Narsimhan (1992) and Noble et al. (1998) proposed (agreed) that the liquid hold-up in the foam column increases when the gas flow-rate is high. This results in the low enrichment due to adsorption of protein into the foam phrase and high protein recoveries, because of the removal of protein from the bulk into foam liquid. Conversely, the low gas flow-rate leads to the reduction of liquid hold-up because the drainage residence time increases and slower liquid drainage rate from the films into the Plateau borders.
2.2. Surfactant theory
2.2.1. Introduction of surfactant
Surface-active agents (surfactants) are amphiphilic molecules which contain hydrophobic and hydrophilic parts, i.e. they have one part that has an affinity for non-polar molecules and the other one that has an affinity for polar molecules such as water. In non-aqueous systems, the polar group (hydrophilic part) is known as the oleophobic group, and the non-polar group (hydrophobic part) as oleophilic.
Their amphiphilic property provides them the surface activity which can considerably decrease the interfacial tension, and ability to solubilise themselves in solution. Because of their surface activity, surfactants are widely employed in various practical applications ranging from detergents, foaming agents, to inks, paints, or crude oil recovery.
2.2.2. Classification of surfactants
Surfactants are classified according to the nature and structure of their polar (hydrophilic) group: anionic, cationic, non-ionic and amphoteric (electroneutral) (see Figure 2.5).
Figure 2.5. Schematic representation of classified types of surfactants. The picture adapted from (4)
Royal Society of Chemistry (2003) listed the comparisons between classifications of surfactants as follows:
Anionic surfactants are molecules in which the hydrophilic part contains an electro-negative atom (such as sulphate, phosphate) and a cation (alkaline metal or amines). They are the most commonly used surfactants.
Cationic surfactants are molecules that contain a long chain hydrocarbon as the hydrophobic part with quaternary ammonium nitrogen as hydrophilic part. The counterion is mostly a halide ion (Chloride or bromide).
Non-ionic surfactants are molecules that comprise a chain of ethoxy groups as hydrophilic part. This is the second to anionics in surfactant applications.
Amphoteric surfactants (zwitterions) consist of a long hydrocarbon chain as hydrophobic part attached to a hydrophilic part containing both positive and negative changes.
Cetylpyridinium chloride (CPC) which is employed and experimentally conducted in this work is a cationic surfactant.
2.2.3. Properties of surfactants
In an aqueous solution, when dissolved at dilute concentrations, a surfactant adsorbs onto surfaces or interfaces and significantly alters the interfacial free energy. However, when present at higher concentrations, surfactant molecules aggregates of a large number of molecules known as micelles. In micelles, the surfactants hide their hydrophobic "tails" in the interior of the aggregate, leaving hydrophilic (water-soluble ionic) "heads" exposed the aqueous medium (see Figure 2.6a). At even higher concentration, surfactants continue to form long columns packed into hexagonal arrays. These columns have hydrophobic interiors and hydrophilic surfaces (see Figure 2.6b). The formation of micelles is employed as a good instance of thermodynamically stable lyophilic colloidal dispersion (Schramm, 2005).
Figure 2.6. Schematic illustrations of micelle formation from surfactant molecules
The picture adapted from (5) and (6)
Once concentration of surfactants at an adsorbed monolayer is high enough, it can lead to the decrease of interfacial tension. Besides, highly concentrated surfactant results in the increase of interfacial viscosity which promotes the resistance to film thinning and film rupture. (Schramm, 2005).
The concentration above (at) which surfactant molecules considerably approach together to form aggregates in an aqueous solution is known as critical micelle concentration (CMC). The CMC is an important property of the surfactant since it relates several other properties namely the surface tension, conductivity, osmotic pressure and temperature (Kraft point TK and cloud point). Further reading of CMC can be approached from Schramm (2005).
The determination of CMC can be carried out by conducting surface tension measurements based on various concentrations of surfactant. Figure 2.7 illustrates the surface tension curve as a function of surfactant concentrations. When present at a concentration below CMC, surfactant acts without any considerable change in surface tension as well as interfacial properties. Then, the increase of concentration results in that of surface tension since the surfactant molecules approach the surface and lower the interfacial free energy. When the surfactant concentration reaches equally or above the CMC, i.e. when the surface is saturated with surfactant molecules, the surface tension appears to be constant with the further increase of concentration (Kruss, 2010). The CMC, which is the intersection of two straight lines, can be obtained from the plot shown in Figure 2.7.
Figure 2.7. Schematic representation of relationship of surfactant concentration and surface tension. The picture is adapted from (7).
Modified with Morrison, pp.246
The principle of foam fractionation as well as other kinds of selective separation techniques is adsorption process. The adsorption in foam fractionation depends on the adsorption of waste molecule at the gas-liquid surface. Adsorption is basically the adhesion and accumulation of concentration of components at the phase boundary of a system in which the attraction of adsorbate molecules (gas or liquid) to adsorbent molecules (liquid or porous solid) occurs.
Lemlich (1968b) stated that that under equilibrium conditions, the adsorption equation, called Gibbs adsorption isotherm, gives quantitatively relationship for surface tension and surface excess in the surfactant system:
is surface tension (kg.s-2)
R is gas constant = 8.314 J.K-1mol-1
T is the thermodynamic temperature (K)
is surface excess (mol.m-2)
is activity of the ith component (mol.m-3)
For an interface, surface excess of a component can be defined as the extra amount per unit area of the solute is present at or near the surface which is in equilibrium with the adjoining phase containing the solute (Academy of Chromatography, 2010). Surface excess is an algebraic quantity and may be positive (excess) or negative deficiency) (IUPAC, 1997).
The disadvantage of the Equation 2.1, however, is the deficiency of activity coefficients and the difficulty in accurately estimating minor changes in surface tension (Lemlich, 1968b). It was suggested by Wall (2007) and Lemlich (1968b) that the activity coefficient of the surfactant can be assumed to be constant and equal to the concentration of the bulk solution at concentrations below the critical micelle concentration (CMC). Thus, for an ideally simple equilibrium system of two components, activity coefficient ai in Equation 2.1 will be substituted by the concentration of the bulk solution Ci or C. The simplified equation can be expressed as follows (Lemlich 1968b):
Where n is the ionic charge of the surface- active agent. For Cetylpyridinium chloride (CPC) which is a cationic (positively charged) surfactant, n = 2 (???).
Figure 2.5 represents a plot of variation of surface excess against concentration. The curve indicates that at low concentration, the surface excess increases markedly illustrated by an inclined straight line through the origin. However, when the concentration reaches the certain value, the surface excess appears to be considerably unchanged showed by a virtually horizontal line.
Figure 2.8. The correlation curve between bulk concentration C and surface excess G. The picture was adapted from Lemlich (1968b).