Foam fractional process is one of assorted methods of adsorbent bubble separation techniques based on surface activity.
Foam fractional process is considered as a cost-efficient method of protein enrichment and has the possible to be an attractive replacing for one or more of the dearly-won initial stairss in an overall purification strategy.
Foam fractional process works good with low concentrations ( & lt ; 1 % wt ) while other separation techniques such as ultrafiltration are merely economical when runing at high concentration.
2.1.1. Properties of froth
Foams are scatterings of gaseous bubbles in a little sum of liquid, in which the gas volume is much greater than that of the liquid, with surface-active reagents. It is reported that froth cell sizes scope from 50 Aµm to several millimeters, and froth densenesss vary from really low to about 700 g.dm-3, beyond which gas emulsions instead than froths are found ( Morrison and Ross, 2002 ) . Lamellae, which is a all right liquid bed between spread bubbles, scope in thickness from 10 to 1000 nanometer.
Foam plays an of import function in a assorted peculiar applications. One of froth merchandises popularly employed in world is fire retardent froth used in snuff outing fires because its low denseness ensures that it floats upon firing oil. Similarly, froth can be widely used in separation procedures, take froth floatation and foam fractional process as illustration. However, in some other industrial procedures such as distillment and dissolver tripping, the great copiousness of froth demands to be prevented.
Morrison and Ross ( 2002 ) stated that froth belongingss depend on chemical composing and belongingss of adsorbed surface movies, and therefore on the thermodynamics of colloidal solutions. In add-on, there are many factors which affect froth belongingss including the surface rheology, size distribution of the cells, surface tenseness of the liquid and external force per unit area and temperature. The rheological features of froths include: ( 1 ) froths are extremely syrupy, ( 2 ) froth exhibit shear cutting in which viscousness decreases with increasing shear rate like polymer solutions and liquefied polymers, ( 3 ) froth exhibit output points at which it deforms elastically and reverts to its original form or place, ( 4 ) froths appear to steal at a solid boundary ( Morrison and Ross, 2002 ) .
2.1.2. Foam construction
Much of scientific plants on foams represents the geometry of froths. Plateau ‘s Torahs are proved by utilizing step theory to analyze surface country minimisation and depict the assemblies of bubbles and movies in froths. Two of these Torahs are demonstrated as followers:
Along an border, three and merely three liquid gill can every bit run into at angles of 120o.
At the intersection point of four bubbles, these four borders meet at the tetrahedral angle of 109o28A?16A? ( 109.47o ) .
If the bubbles are non inclined to those angles, so bubbles rearrange themselves to conform to Plateau ‘s Torahs.
For many old old ages, the construction of froth was accepted as following Lord Kelvin ‘s speculation. In his plants in 1887, Kelvin proposed that the body-centre-cubic ( b.c.c ) construction as the optimal agreement to fulfill Plateau ‘s Torahs. He pointed out that the polyhedron which divides infinite into many cells of equal volume with minimal surface country and without nothingness is bitruncated three-dimensional honeycomb called Kelvin construction ( or Kelvin cell ) . This is polyhydron which has truncated borders crossing at the vertices of 14-sided polyhydron ( tetradecahedron ) with six plane faces and eight hexangular faces ( Morrison and Ross, 2002 and Weaire and Phelan, 1994 ) . Figure 2.1 illustrates this agreement.
A froth construction proposed by Kelvin would hold both the lowest possible energy and the least surface country. However, Kelvin cells have merely four-sided and hexangular faces, while experimental observations of froth construction has proved that the gill in world are by and large pentangular ( Morrison and Ross, 2002 ) . Although Kelvin construction has revealed the job in the polyhedron signifier of the cells, it was widely accepted until the find of the Weaire-Phelan construction in 1994.
Weaire and Phelan ( 1994 ) proposed the construction in which there are two sorts of cells. One is a dodecahedron ( 12-sided polyhedron ) with pentangular faces, and the other 1 is a tetradecahedron ( 14-sided polyhedron ) with two hexangular and 12 pentangular faces. Although two sorts of cells have different figure of faces ( different form ) , they both have equal volume. The froth made of Weaire-Phelan construction has the lower surface country ( 0.3 % less than that of Kelvin construction ) , therefore Weaire-Phelan cells are soon accepted to depict the formation of froth cells. Figure 2.2 illustrates Weaire-Phelan construction.
Figure 2.1. Bitruncated 14-sided polyhydron ( Kelvin construction ) proposed by Kelvin. Picture was adapted from ( 1 )
Figure 2.2. Weaire-Phelan construction with pentangular faces and hexangular faces. Picture was adapted from ( 1 )
when Denis Weaire and Robert Phelan found a construction with 0.3 % less surface country than Kelvin ‘s construction.
The thin liquid movies shacking between interstitial infinite of next bubbles are called Plateau boundary lines. Plateau boundary line is one of basic elements of froths. It is composed of polyhedral gas bubbles with vertices are at the junction of movie gill. Significant sums of liquid in froths are contained in Plateau boundary lines which provide a path for drainage of the froth. Figure 2.3 illustrated the conventional representations of Plateau boundary lines.
Figure 2.3. Conventional 3-dimensional Plateau boundary line in which three movie lamellae meet at an angle of 1200. The movie thickness vitamin D was exaggerated, and A is the Centre of the radius of curvature Rz. Picture was adapted from ( 2 ) .
Nguyen ( 2002 ) suggested two estimate for the form presentation of Plateau boundary line, including a round and triangular cross subdivision. The round cross-section estimates are inappropriate for the existent form of Plateau boundary lines encountered in froth. The triangular estimates show the effects of shear viscousness of the gas-liquid interface on the liquid flow in the Plateau boundary lines.
Several premises are employed to pattern the form of the Plateau boundary line cross-section, including: ( a ) bubbles at any transverse subdivision of the froth bed are unvarying in size ; ( B ) existent cross-section has the form of three equal round discharge with a radius R, separated by three liquid movies ; ( degree Celsius ) the thickness of liquid movies is strikingly smaller than the radius of the round discharge ( Nguyen, 2002 ; Uraizee and Narsimhan, 1992 ) . Figure 2.4 shows the cross subdivision of Plateau boundary lines between next bubbles.
Figure 2.4. Conventional solid cross-section of the Plateau boundary lines with three equal round discharge joined together at three points. The image was adapted from ( 3 )
Uraizee and Narsimhan ( 1992 ) stated that the drainage of liquid from thin movie gill into next Plateau boundary lines due to the Plateau boundary line suction and disjoining force per unit area whereas the gravitation is the major cause of the drainage of liquid from interstitial web of Plateau boundary lines. Everett ( 1988 ) besides mentioned that the gravitative forces run outing the liquid in Plateau boundary lines are intensified by surface tenseness forces which can be observed that the surfaces of neighbouring junctions are aggressively curved.
2.1.3. Foam stableness
Schramm ( 2005 ) supported that foam stableness involves sustainability against two procedures including movie cutting and movie rupture ( coalescency ) . Film cutting is the phenomenon in which assorted gas bubbles approach closely, and the slender movies between them become dilutant. However, the bubbles do non really reach straight each other, and there is no alteration in entire surface country although the thickness of interstitial movies lessening. Coalescence is the status when the liquid movies prostration and the bubbles fuse together to organize a individual bigger bubble. In coalescency, the entire surface country of bubbles will cut down.
The major factors which play an of import portion in foam stabilization include: surface tenseness, surface snap, surface viscousness and disjoining force per unit area. A figure of conditions affecting froth stableness for movie cutting and bubble coalescency were proposed by Schramm ( 2005 ) as follows:
Low surface tenseness – it enables the formation and sustainability of big interfacial country.
Low gravitation drainage – it decreases the rate of movie cutting.
Low capillary suction ( suction from Plateau boundary line ) – it decreases the rate of movie cutting.
High surface snap – it counteracts the consequence of surface disturbances.
High majority viscousness – it reduces the rate of movie cutting.
High surface viscousness – it reduces the rate of movie rupture.
High electric dual bed repulsive force – it increases disjoining force per unit area and reduces the rates of movie cutting and rupture.
High steric repulsive force – it reduces the rates of movie cutting and rupture.
Low scattering force attractive force – it decreases the rates of movie cutting and rupture.
Foam drainage, enhanced by Plateau boundary line suction, is a important component in the formation and stableness of froths. The prostration of the liquid drainage from movie gill leads to coalescency of bubbles. This rupture consequences in the redistributions of majority wetting agent and adsorbed wetting agent from the collapsed interface in the transitional part of Plateau boundary lines and thin movies. This leads to more surface assimilation at the gas-liquid interface due to enrichment of wetting agent in the majority. This position of liquid rupture was supported by Uraizee and Narsimhan ( 1992 ) . Lemlich ( 1968a ) demonstrated that two grounds for internal bubble coalescency are the diffusion of gas from little bubbles to bigger bubbles and the rupture of thin movies segregating the bubbles.
The distributions of the internal reflux were besides mentioned by Uraizee and Narsimhan ( 1992 ) that the surfactant concentrations will be the same in both thin movies and Plateau boundary lines if dispensed uniformly in both. Conversely, if the internal reflux is dispensed merely into the Plateau boundary lines, the surfactant concentration in the movies will non alter because the movies and Plateau boundary lines are divided. By and large talking, the enrichments in Plateau boundary lines and thin movies may be different depending virtually on the distributions of liquid in the thin movie. The existent enrichment in world will lie between the above two utmost state of affairss.
2.1.4. Preparation of froths
Modified with Weaire 1999
Foams are merely formed by blending and fomenting a gas and a liquid together in the same container. When a solution incorporating surface-active agents ( wetting agents ) is sparged with an air flow, froth can besides be produced. In pure liquids, when two bubbles approach closely together, coalescency occurs virtually instantly and there is no thin movie continuity between bubbles. When wetting agent is introduced to the liquid, the surface assimilation of surfactant enhances thin-film continuity between bubbles. It can be explained that when two gas bubbles approach, there exists a thin liquid movie gill which stabilises ( OR prevents ) the bubbles from tearing ( Schramm, 2005 ) .
To brace the froths in the solution, a foaming agent which may incorporate wetting agents, supermolecules, or all right solids is needed. Schramm ( 2005 ) suggested that a foaming agent is necessary to diminish the surface tenseness, increase the interfacial country with the least mechanical energy input with regard to economic issues, and forestall the thin-film prostration and bubble coalescency. Other good factors which besides promote the froth continuity include the addition of viscousness and surface snap.
2.1.5. Liquid hold-up
Liquid hold-up is the volumetric fraction of liquid in the froth and entire froth volume. The simplified equation of liquid hold-up can be shown as follows:
( Equation 1 )
In systems including foam fractional process column and froth floatation column, liquid hold-up is an of import factor used for determining interstitial liquid flow, abode clip distribution, bubble coalescency, or finding reflux ratio ( Yianatos et. Al, 1985 ) . Liquid hold-up is besides utile for the column design, scale-up or use of systems.
Due to the ( its ) importance in column operation, a great figure of measurement methods have been taken into history. Yianatos et. Al ( 1985 ) demonstrated that liquid hold-up was conventionally determined by comparing straight the tallness of aerated liquid with that of liquid without aeration, or by gauging from bed enlargement for fluidized two phase systems. Besides, the uses of radioactive tracers or electrical conduction were alternate methods to find liquid hold-up of froth columns by assorted plants. Yianatos et. Al ( 1985 ) themselves approached the liquid hold-up appraisal from conduction measurings based on construct of tortuousness. However, those old attacks were clip devouring, low cost effectual and inappropriate to use the measuring from laboratory graduated table to industrial works graduated table.
The liquid hold-up measuring soon is conducted by the usage of atomic scintigraphic imagination technique proposed by Lockwood et Al. ( 2000 ) . The atomic scintigraphic imagination technique is really employed in medical surveies to stipulate the development procedure of disease in the organic structure.
Lockwood et Al. ( 2000 ) proved by experiments that the volume fraction of H2O in the foam solution are unchanged with the addition of column tallness. This position is opposite to that antecedently used in the survey of foam drainage, which stated that liquid armed robbery decreases with tallness as believed by Brown et Al. ( 1990 ) .
Uraizee and Narsimhan ( 1992 ) and Baronial et Al. ( 1998 ) proposed ( agreed ) that the liquid hold-up in the foam column additions when the gas flow-rate is high. This consequences in the low enrichment due to surface assimilation of protein into the foam phrase and high protein recoveries, because of the remotion of protein from the majority into foam liquid. Conversely, the low gas flow-rate leads to the decrease of liquid hold-up because the drainage abode clip additions and slower liquid drainage rate from the movies into the Plateau boundary lines.
2.2. Surfactant theory
2.2.1. Introduction of wetting agent
Surface-active agents ( wetting agents ) are amphiphilic molecules which contain hydrophobic and hydrophilic parts, i.e. they have one portion that has an affinity for non-polar molecules and the other 1 that has an affinity for polar molecules such as H2O. In non-aqueous systems, the polar group ( hydrophilic portion ) is known as the oleophobic group, and the non-polar group ( hydrophobic portion ) as oleophilic.
Their amphiphilic belongings provides them the surface activity which can well diminish the interfacial tenseness, and ability to solubilise themselves in solution. Because of their surface activity, wetting agents are widely employed in assorted practical applications runing from detergents, frothing agents, to inks, pigments, or crude oil recovery.
2.2.2. Categorization of wetting agents
Wetting agents are classified harmonizing to the nature and construction of their polar ( hydrophilic ) group: non-ionic detergent, cationic, non-ionic and amphiprotic ( electroneutral ) ( see Figure 2.5 ) .
Figure 2.5. Conventional representation of classified types of wetting agents. The image adapted from ( 4 )
Royal Society of Chemistry ( 2003 ) listed the comparings between categorizations of wetting agents as follows:
Anionic wetting agents are molecules in which the hydrophilic portion contains an electro-negative atom ( such as sulfate, phosphate ) and a cation ( alkalic metal or aminoalkanes ) . They are the most normally used wetting agents.
Cationic wetting agents are molecules that contain a long concatenation hydrocarbon as the hydrophobic portion with quaternate ammonium N as hydrophilic portion. The counterion is largely a halide ion ( Chloride or bromide ) .
Non-ionic wetting agents are molecules that comprise a concatenation of ethoxy groups as hydrophilic portion. This is the 2nd to non-ionic detergents in surfactant applications.
Amphoteric wetting agents ( zwitterions ) consist of a long hydrocarbon concatenation as hydrophobic portion attached to a hydrophilic portion incorporating both positive and negative alterations.
Cetylpyridinium chloride ( CPC ) which is employed and by experimentation conducted in this work is a cationic wetting agent.
2.2.3. Properties of wetting agents
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 nowadays at higher concentrations, surfactant molecules sums of a big figure of molecules known as micelles. In micelles, the wetting agents hide their hydrophobic “ dress suits ” in the inside of the sum, go forthing hydrophilic ( water-soluble Attic ) “ caputs ” exposed the aqueous medium ( see Figure 2.6a ) . At even higher concentration, wetting agents continue to organize long columns packed into hexangular arrays. These columns have hydrophobic insides and hydrophilic surfaces ( see Figure 2.6b ) . The formation of micelles is employed as a good case of thermodynamically stable lyophilic colloidal scattering ( Schramm, 2005 ) .
( a )
( B )
Figure 2.6. Conventional illustrations of micelle formation from surfactant molecules
The image adapted from ( 5 ) and ( 6 )
Once concentration of wetting agents at an adsorbed monolayer is high plenty, it can take to the lessening of interfacial tenseness. Besides, extremely concentrated surfactant consequences in the addition of interfacial viscousness which promotes the opposition to movie cutting and movie rupture. ( Schramm, 2005 ) .
The concentration above ( at ) which surfactant molecules well approach together to organize sums in an aqueous solution is known as critical micelle concentration ( CMC ) . The CMC is an of import belongings of the wetting agent since it relates several other belongingss viz. the surface tenseness, conduction, osmotic force per unit area and temperature ( Kraft point TK and overcast point ) . Further reading of CMC can be approached from Schramm ( 2005 ) .
The finding of CMC can be carried out by carry oning surface tenseness measurings based on assorted concentrations of wetting agent. Figure 2.7 illustrates the surface tenseness curve as a map of surfactant concentrations. When nowadays at a concentration below CMC, surfactant Acts of the Apostless without any considerable alteration in surface tenseness every bit good as interfacial belongingss. Then, the addition of concentration consequences in that of surface tenseness since the wetting agent molecules approach the surface and lower the interfacial free energy. When the surfactant concentration reaches every bit or above the CMC, i.e. when the surface is saturated with surfactant molecules, the surface tenseness appears to be changeless with the farther addition of concentration ( Kruss, 2010 ) . The CMC, which is the intersection of two consecutive lines, can be obtained from the secret plan shown in Figure 2.7.
Figure 2.7. Conventional representation of relationship of surfactant concentration and surface tenseness. The image is adapted from ( 7 ) .
Modified with Morrison, pp.246
The rule of foam fractional process every bit good as other sorts of selective separation techniques is adsorption procedure. The surface assimilation in foam fractional process depends on the surface assimilation of waste molecule at the gas-liquid surface. Adsorption is fundamentally the adhesion and accretion of concentration of constituents at the stage boundary of a system in which the attractive force of adsorbate molecules ( gas or liquid ) to adsorptive molecules ( liquid or porous solid ) occurs.
Lemlich ( 1968b ) stated that that under equilibrium conditions, the surface assimilation equation, called Gibbs surface assimilation isotherm, gives quantitatively relationship for surface tenseness and surface surplus in the surfactant system:
( Equation 2.1 )
is surface tenseness ( kg.s-2 )
R is gas changeless = 8.314 J.K-1mol-1
T is the thermodynamic temperature ( K )
is surface extra ( mol.m-2 )
is activity of the ith constituent ( mol.m-3 )
For an interface, surface surplus of a constituent can be defined as the excess sum per unit country of the solute is present at or near the surface which is in equilibrium with the bordering stage incorporating the solute ( Academy of Chromatography, 2010 ) . Surface surplus is an algebraic measure and may be positive ( extra ) or negative lack ) ( IUPAC, 1997 ) .
The disadvantage of the Equation 2.1, nevertheless, is the lack of activity coefficients and the trouble in accurately gauging minor alterations in surface tenseness ( Lemlich, 1968b ) . It was suggested by Wall ( 2007 ) and Lemlich ( 1968b ) that the activity coefficient of the wetting agent can be assumed to be changeless and equal to the concentration of the majority solution at concentrations below the critical micelle concentration ( CMC ) . Therefore, for an ideally simple equilibrium system of two constituents, activity coefficient Army Intelligence in Equation 2.1 will be substituted by the concentration of the majority solution Ci or C. The simplified equation can be expressed as follows ( Lemlich 1968b ) :
( Equation 2.2 )
Where N is the ionic charge of the surface- active agent. For Cetylpyridinium chloride ( CPC ) which is a cationic ( positively charged ) wetting agent, n = 2 ( ? ? ? ) .
Figure 2.5 represents a secret plan of fluctuation of surface surplus against concentration. The curve indicates that at low concentration, the surface surplus additions markedly illustrated by an inclined consecutive line through the beginning. However, when the concentration reaches the certain value, the surface extra appears to be well unchanged showed by a virtually horizontal line.
Figure 2.8. The correlativity curve between bulk concentration C and surface extra G. The image was adapted from Lemlich ( 1968b ) .