Effect of Operating Conditions on Enrichment and Separation

The operating conditions that can be varied in a foam column are the superficial gas velocity G, the bubble size R, the column height L, feed flow rate F, the feed concentration Cf and the mode of operation. In addition, the separation will also be influenced by the viscosity of the feed and the extent of bubble coalescence in the foam column.

Protein enrichment depends on the total amount of protein selectively adsorbed at the gas-liquid interface as well as on the liquid hold-up in the foam. Smaller liquid hold-ups result in a larger interfacial area per unit volume of the liquid and therefore in larger enrichment. At higher superficial gas velocities, more liquid is entrained by the gas bubbles from the liquid pool leading to higher liquid hold-ups in the foam column and consequently to smaller enrichment. As the bubble size increases, a larger proportion of the liquid that is entrained by the foam is distributed in the film, resulting in a faster drainage rate. On the other hand, an increase in the bubble size results in a decrease in the interfacial area per unit volume. Because of the above two opposing effects, there exists an optimum bubble size at which enrichment may be maximum (Narsimhan and Ruckenstein, 1986) for one component protein solution as shown in Figure 4. In addition, this maximum is found to be more pronounced at smaller superficial gas velocities. Narsimhan and Ruckenstein (1986) have developed a population balance model to account for the bubble size distribution in the description of drainage and coalescence in a foam bed. Their model was able to predict the change in the bubble size distribution as a result of coalescence. The results indicated collapse of the foam bed for broader inlet bubble size distribution with a coefficient of variation above a critical value. In the case of a mixture of proteins, however, the separation efficiency would depend on the preferential adsorption of one protein over the other components as can be seen from eqns [22] and [23]. As expected, the separation efficiency is higher for the protein which adsorbs the most at the gas-liquid interface with a higher value of r. As a result, the separation efficiency

Bubble size (cm)

Figure 4 Effect of the inlet bubble size on the enrichment for ^ = 10_2P, ^.s = 10~4sP, and Co = 10~7 gmol mL~1. (Reproduced with permission from Narsimhan and Ruckenstein, 1986a.)

Bubble size (cm)

Figure 4 Effect of the inlet bubble size on the enrichment for ^ = 10_2P, ^.s = 10~4sP, and Co = 10~7 gmol mL~1. (Reproduced with permission from Narsimhan and Ruckenstein, 1986a.)

would be higher for larger values of Langmuir adsorption parameter Kt as can be seen from eqn [5]. An increase in the viscosity of the feed would result both in a larger amount of liquid entrained by the foam as well as slower liquid drainage leading to larger liquid hold-up. Also, an increase in the viscosity of the feed would tend to stabilize the foam resulting in lower bubble coalescence. Both these effects will result in lower protein enrichment. Bubble coalescence in a foam column leads to: (i) an increase in the protein concentration due to internal reflux with subsequent increase in the surface concentration; (ii) a decrease in the liquid hold-up because of increased liquid drainage rates as a result of larger bubble sizes; and (iii) a decrease in the total surface area because of larger bubble sizes. The first effect results in more protein adsorption per unit area at the gas-liquid interface. The second effect leads to higher surface area per unit volume of the liquid. The third effect leads to a decrease in the total amount of protein adsorbed at the interface. Consequently, the first two effects lead to an increase in the enrichment and separation whereas the second and third effects lead to lower recovery. The second effect may be predominant since coalescence was found to result in an increase in protein enrichment as well as recovery (Uraizee and Narsimhan, 1995). The separation efficiency, as one would expect, depends on the relative surface activities of proteins in a binary mixture. For larger values of Langmuir isotherm constant K (more surface active), the separation efficiency increases. In fact, the calculations show that the separation efficiency increases linearly with the ratio K2/K1 (Uraizee and Narsimhan, 1997). However, the separation efficiency was found to decrease rapidly with the feed concentration of the protein (Uraizee and Narsimhan, 1997).

Brown et al. (1990) measured enrichment and recovery in a continuous foam concentration column for bovine serum albumin (BSA). In their experiments, foam was generated by sparging nitrogen gas through a glass frit. As a result, the foam consisted of nonuniform size distribution of bubbles. They compared the experimental data with predictions based on a model similar to the one described above but neglecting drop coalescence. Their experimental data showed a decrease in the protein enrichment with superficial gas velocity. The model predictions agreed fairly well for the highest feed concentration of 0.1 wt% as shown in Figure 5. The experimental enrichments were found to be larger than the model predictions (even for the largest bubble size in the foam) with the deviation being larger at lower feed concentrations. This was believed to be due to the fact that drop coalescence in the foam column became increasingly important at lower feed concentrations as confirmed by experimental measurements of bubble size with the height of the column.

Uraizee and Narsimhan (1996) also observed a decrease in enrichment with gas velocity for foam concentration of BSA in their continuous foam con

Figure 6 Comparison of experimental results with model predictions for BSA; feed concentration 0.1 wt%, bubble diameter

gas velocity 2.6x10 3ms 1, foam height

1.3 x 10~1 m, F= 2 x 10 m s , pH 4.8, ionic strength 0.1 M. (•) Experimental data. (A) Model predictions accounting for kinetics of adsorption as well as coalescence. (—) Model predictions accounting only for kinetics of adsorption. ( ♦) Model prediction accounting only for coalescence assuming equilibrium surface concentration is shown in the inset. (Reproduced with permission from Uraizee and Narsimhan (1996).)

1.0 4—i—i—i-1—i—i—T—i-E—i—i—i—

0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Superficial gas velocity (cm s"1)

Figure 5 Effect of superficial gas velocity on protein enrichment for cF = 0.1 wt%. F = 0.02 cms-1, /= 0.1 M, pH = 7, z = 5 cm. The curves refer to model predictions for different bubble sizes. (Reproduced with permission from Brown etal., 1990.)

centration experiments in which the foam was generated by sparging nitrogen through a capillary bundle thus resulting in a foam of uniform bubble sizes. In their experiments, the residence time of the bubbles in the liquid pool was varied by varying the pool height. Interestingly, protein enrichment was found to increase with pool height at sufficiently high pool heights, thus indicating the importance of kinetics of adsorption of protein on to the gas-liquid interface on enrichment. At low pool heights, however, they observed an increase in protein enrichment with a decrease in pool height due to excessive bubble coalescence in the foam. Their model, which accounted both for the kinetics of protein adsorption as well as coalescence, was able to explain the increase in protein enrichment due to bubble coalescence at small pool heights and an increase in enrichment with pool heights at larger pool heights. A comparison of the experimental data with their model predictions is shown in Figure 6.

Ahmed (1975) observed an increase in the separation efficiency of albumin with the superficial gas velocity with the value reaching a plateau at sufficiently high gas velocities. Schnepf and Gaden (1959) and Ahmad (1975) reported a maximum protein enrichment at the isoelectric point of the protein which can be explained by the maximum protein adsorption at the interface due to the absence of electrostatic energy barrier for adsorption. However, this maximum was found to be considerably less pronounced at higher protein concentrations. Protein enrichment was also influenced by the change in the bubble size at different pH (Brown et al., 1990). Separation efficiency of albumin was found to decrease dramatically as the foam height increased from 3 to 17 cm (Ahmed, 1975). Even though enrichment increased with foam height because of internal reflux resulting from increased drop coalescence, the top product flow rate was also found to decrease dramatically due to faster drainage. As a result, the protein separation was less at higher foam heights. Ahmed (1975) also found that the introduction of the feed stream into the foam instead of liquid pool increased the separation efficiency because the foam column was operated in the combined mode with an enricher and stripper.

In conclusion, the main attractive features of foam fractionation are its low capital and operating costs. Therefore, it can be employed as a first step for preconcentration/separation before more expensive separation methods can be used. More work is needed to establish the applicability of foam frac-tionation as a viable separation method for mixtures of proteins and to develop new processes based on this technique. Few experimental data are available on the adsorption isotherm and kinetics on to gas-liquid interface for mixtures of proteins. More importantly, it is necessary to probe denaturation (if any) of proteins and enzymes when subjected to foaming.

See also: II/Flotation: Bubble-Particle Capture; Froth Processes and the Design of Column Flotation Cells; Historical Development.

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