Different Modes of Operation of a Foam Column

Figure 1 depicts the different modes of operation of a foam fractionation column. The simplest mode is the production of a protein-rich concentrate phase from a dilute aqueous protein solution. This can be operated as semi-batch mode (Figure 1A), in which a pool of protein solution is maintained at the bottom

Table 1 Foam fractionation of proteins

Proteins separated

Experimental set-up

Reference

Choline esterase

Batch

Schultz, 1937; Bader etat., 1944

Pepsin, rennin

Batch

Andrewsand Schultz, 1945

Sodium cholate

Batch

Bader etat, 1944

Apple proteins

Semi-batch

Davis etat, 1949

Bovine serum albumin

Batch

Schnepfand Gaden, 1959

Gehle and Schugerl, 1984

Bovine serum albumin

Continuous

Ahmad, 1975a,b

Brown et al., 1990

Uraizee and Narsimhan, 1996

Potato proteins

Batch with recycle

Weijenberg et al, 1978

Catalase, amylase

Batch

Charm et al, 1966

Streptokinase

Batch

Holmstrom, 1968

Lysozyme, human serum albumin

Batch

Lalchev and Exerowa, 1981

Acid phosphatase

Batch

London and Hudson, 1953

Urease, catalase

Batch

London et al., 1954

Bovine serum albumin-DNA, lysozyme-DNA

Batch

Lalchev etal, 1982

Placental proteins

Continuous

Sarkar etal, 1987

Figure 1 Different modes of operation of a foam fractionation column.

enriching mode (Figure 1D), the feed stream is introduced into the liquid pool and part of the top product that is obtained by collapsing the foam is refluxed into the column. Protein-rich reflux flows down countercurrently through the foam resulting in further enrichment of protein in the top product. In the combined mode (Figure 1E), the feed is introduced into the foam and the external reflux is used. Part of the column above the feed acts as an enricher, whereas the bottom part of the column acts as a stripper.

It is reasonable to assume that the residence time of the bubbles through the liquid pool is sufficiently large for protein adsorption to reach close to equilibrium so that the surface concentration of protein at the gas-liquid interface can be assumed to be close to the equilibrium value. Also, if bubble coalescence in the foam bed is negligible, the concentration of protein in the interstitial liquid can be expected to be the same as that in the liquid pool. For simple mode of operation of the foam column with a continuous feed stream consisting of a dilute protein solution, the top product concentration cD, is related to the pool concentration cB via:

of a column and is sparged with an inert gas which forms the foam. The foam is continuously removed at the top of the column, sent to a foam breaker and the top product collected. Since the most surface-active protein is preferentially removed from the solution, the solution would progressively get depleted in that protein as time progresses. As a result, the pool would get enriched in other components in the case of mixtures. In continuous operation, a feed stream of protein solution is introduced into the pool and the bottom product withdrawn (Figure 1B). Sparging of gas bubbles mixes the liquid pool well enough so that the bottom product is at the same composition as the liquid pool. In addition, the continuous foam column can also be operated in stripping, enriching or combined modes. In the stripping mode, the object is to remove, almost completely, protein from a dilute solution. In this mode, the feed is introduced into the foam and trickles down countercurrently through the rising foam (Figure 1C). The protein concentration in the liquid below the feed-point falls with foam depth, due to it being adsorbed on to the rising bubble surface. There is a net upflow of solution through the foam maintained by entrained up-flowing liquid from the pool. If the foam column is deep enough the protein adsorbed on the bubble surface rF, will be in equilibrium with the feed liquid concentration cF, and the pool liquid concentration will be very low. Consequently, the bottom product is stripped of more protein than that in the simple mode of operation. In the cD — cB # '

6GTb dD

where G is the gas flow rate, D is the top product flow rate, d is the bubble size and rB is the equilibrium surface concentration of protein at the gas-liquid interface corresponding to the pool concentration. In the above equation, the first term on the right-hand side is the contribution to the protein concentration from the bulk interstitial liquid before the foam is collapsed and the second term is the contribution from the adsorbed protein at the gas-liquid interface which is recovered into the bulk upon collapse of the foam. A mass balance around the column now gives the following equations for the top product concentration cD and the bottom product concentration cB respectively (Lemlich, 1968):

6GrB

and:

cB cF

6GrB

where cF is the feed concentration, cB is the pool concentration, F is the feed flow rate, and B is the bottoms flow rate. In the case of binary mixture of two proteins, the separation efficiency S, defined as the ratio of the two enrichments, is given by:

simple mode of operation. The separation ciency for a binary mixture is given by:

effi-

6GY1(Cb1 B

6.59Gr2(cF2) 1

6.59Gr1(cF1) 1

where the subscripts 1 and 2 refer to components 1 and 2 and r,- (cBj) is the equilibrium surface concentration of component i corresponding to the bulk concentration cB,i. It can easily be seen that the separation ratio is greater than unity if component 2 is more surface active than 1. Also, in the above equation factor 6 arises because the area per unit volume of spherical bubbles of diameter d is 6/d. If the dodecahedral shape of the bubbles in the foam is to be accounted for, factor 6 is to be replaced with 6.59. For a Langmuir adsorption isotherm, the surface concentration of proteins is related to the bulk concentration via:

where K is the equilibrium constant, c is the bulk concentration and aj is the area occupied by a protein molecule.

In the stripping mode, the feed stream is introduced into the foam (Figure 1C). For a long stripping column, the protein concentration of downflowing interstitial liquid will approach that of entrained liquid in the foam. The two concentrations will approach each other at the feed level. Therefore, the protein concentration of the interstitial liquid at the top can be taken to be the feed concentration. Therefore, mass balance around the foam column yields (Lemlich, 1968):

and:

6.59GrF ~Bd

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Solar Panel Basics

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