Foam fractionation of proteins and enzymes. II. Performance and modelling Farooq Uraizee and Ganesan Narsimhan Department of Agricultural Engineering, Purdue University, West Lafayette, IN Introduction In Part I of this review, we introduced the topic and covered various applications in the separation of proteins and enzymes. In this part we focus on the factors that affect the performance of foam fractionation columns and their effect on the separation efficiency as well as on the modelling of the hydrodynamics of foam.
Effect of parameters on the performance of foam fractionation column Various investigators 1-7have found that the enrichment (the ratio of the concentration of the product to that of the feed) is maximum at the isoelectric point. It has also been observed 3 that some substances cannot be foamed at a pH other than pI because of poor foam stability. Because of the nature of the adsorption isotherm of proteins and enzymes at the gas-liquid interface, enrichment tends to be lower at higher concentrations.4'5'8'9 At sufficiently low concentrations, however, an optimum concentration is observed 1,6,1°at which the enrichment is maximum. Higher gas flow rates have b e e n f o u n d 1'4'6 to yield lower enrichments because of more liquid entrainment in the foam. Higher enrichments have been obtained 1 for larger bubble sizes because of lower liquid holdup resulting from faster rates of drainage. Larger pool heights have been found to yield higher enrichments 1'4'9 because of higher surface concentration at the gas-liquid interface resulting from increased residence time of the bubbles in the liquid pool. As the foam height is increased, the liquid holdup at the top of the foam is found to decrease, leading to higher enrichments 9 and lower separations.l,4,6 In the case of unstable enzymes such as streptokinase 11 and urease, 1 however, the enzyme activity is found to decrease at larger foam heights. Because of more adsorption, higher operating temperature has been found to yield higher protein concentration in the foam.l'9 Increasing the concentration of impurities such as
Address requests to Dr. Narsimhan at the Department of Agricultural Engineering, Purdue University, West Lafayette, IN 47907
© 1990 Butterworth Publishers
Na2SO 4 and ethanol decreases the enrichment. 1,9 At
ethanol concentrations above 1% (v), however, enrichment was found to increase because of the larger bubble sizes. 5 Solid impurities reduce protein enrichment because of their scavenging action. 9 Initial addition of water-soluble nonionic emulsifier was found to increase the separation. ~2At higher emulsifier concentrations, however, there is a pronounced decrease in the separation efficiency. 12Effects of several other impurities have also been investigated. 12 The most common mode of operation of foam fractionation employed by various investigators is the single-stage, semibatch mode. However, continuous 4,5 and multistage9 columns have also been evaluated. In a continuous column, increase in the feed rate has been found to decrease the enrichment. 4'5'9 There was little effect of the feed position on enrichment when the feed was introduced into the liquid pool. 4 Introduction of feed into the foam region, however, increased the efficiency considerably because the feed stream acted as a reflux. 4 Understandably, the multistage column yielded higher enrichment than a single stage. 9
Modelling of the foam column As pointed out earlier, it is necessary to quantify the effect of various operating variables on the performance of the foam fractionation column for the successful large-scale operation. Earlier investigators 13-15 predicted the foam density by assuming equal-sized bubbles and by accounting only for the gravity drainage of the liquid from the plateau borders. They either assumed the plateau border walls to be rigid (infinite surface viscosity) or accounted for its surface mobility through an adjustable parameter. The assumption of rigid plateau border walls was found to grossly underestimate the rate of liquid drainage. The effect of surface viscosity on liquid drainage was later accounted f o r 16,17 in the calculation of exit foam densities. The liquid holdup profile in a foam column was predicted 18 by considering the liquid drainage from the plateau border as well as from thin films. The change in bubble size distribution due to interbubble gas diffusion has been quantified.l%22 In spite of extensive studies on the staEnzyme Microb. Technol., 1990, vol. 12, April
:315
Literature Survey bility of isolated thin liquid films, very few attempts 23,24 have been made to couple the hydrodynamics of the foam bed to the instability of thin films in order to predict coalescence and subsequent foam collapse. The effects of (i) bubble size distribution, (ii) bubble coalescence as a result of the rupture of thin films caused by van der Waals forces-mediated growth of thermal perturbations, 24 and (iii) interbubble gas diffusion have been accounted for in a comprehensive population balance model. 25 The simplifying assumption of equalsized bubbles has been shown 25 to be valid for narrow inlet bubble size distributions, especially at higher superficial gas velocities and larger inlet bubble sizes. These models assume adsorption equilibrium of the surface-active component at the gas-liquid interface. Such an assumption, though valid for small-molecularweight surfactants, is inapplicable in case of proteins and enzymes because of their slow rates of adsorption. A model for the hydrodynamics of protein-stabilized foam has recently been proposed 26 which accounts for (i) the kinetics of adsorption of proteins and (ii) the dependence of surface viscosity on surface pressure. The protein enrichment w a s f o u n d 2v to be a strong function of the residence time of the bubbles in the liquid pool. The assumption of adsorption equilibrium has been s h o w n 27 to grossly overpredict enrichment excepting for high feed concentrations and large pool heights.
Conclusions Information with regard to the adsorption characteristics of proteins and enzymes at the gas-liquid interface is lacking. Extensive investigation of the kinetics of adsorption, adsorption isotherm, competitive adsorption of proteins and other surfactants, and the extent of possible denaturation of proteins and enzymes upon adsorption is necessary. Moreover, the surface viscosity and surface elasticity of the interfacial adsorbed protein layer should be characterized in order to quantify the drainage and stability of the foam. Since coalescence has a considerable effect on separation efficiency, more efforts towards the understanding of the effect of different variables on coalescence and foam stability is needed. Single-stage, semibatch, as well as continuous foam fractionation columns, have been
316
Enzyme Microb. Technol., 1990, vol. 12, April
evaluated in detail. Multistage columns with countercurrent as well as cross-flow modes of operation need to be evaluated. Efficient large-scale methods of collapsing foam should be investigated for the design of compact large-scale multistage systems.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
London, M., Cohen, M. and Hudson, P. Biochim. Biophys. Aeta 1954, 13, 111 Charm, S. E., Morningstar, J., Matteo, C. and Paltiel, B. Anal. Bioehem. 1966, 15, 498 Lalchev, Z., Dimitrova, L., Tzvetkova, P. and Exerowa, D. Bioteehnol. Bioeng. 1982, 24, 2253 Ahmad, S.I. Separation Sci. 1975, 10, 673 Schnepf, R. W. and Coaden, E. L. J. Biochem. Microb. Technol. Eng. 1959, 1, l Bader, R., Schutz, F. and Stacey, M. Nature 1944, 154, 183 Weijenherg, D. C., Mulder, J. J., Drinkenburg, A. A. H. and Stemerding, S. Ind. Eng. Chem. Process Design Devel. 1978, 17, 209 London, M. and Hudson, P. Arch. Biochim. Biophys. Acta 1953, 46, 141 Coehle,R. D. and Schugerl, K. Appl. Mierob. Biotechnol. 1984, 20, 133 Lalchev, Z. and Exerowa, D. Biotechnol. Bioeng. 1981, 23, 669 Holmstrom, B. Biotechnol. Bioeng. 1968, 10, 551 Ahmad, S. I. Separation Sci. 1975, 10, 689 Miles, Co. D., Shedlovsky, L. and Ross, J. J. Phys. Chem. 1945, 49, 93 Jacobi, W. M., Woodcock, K. E. and Grove, C. S. Ind. Eng. Chem. 1956, 48, 2049 Steiner, L., HHunkeler, R. and Hartland, S. Trans. Inst. Chem. Eng. 1977, 55, 153 Desai, D. and Kumar, R. Chem. Eng. Sei. 1983, 38, 1525 Desai, D. and Kumar, R. Chem. Eng. Sci. 1984, 39, 1559 Hartland, S. and Barber, A. D. Trans. Inst. Chem. Eng. 1974, 52, 43 Lemlich R. Ind. Eng. Chem. Fundamentals 1978, 17, 89 Monsalve, A. and Schechter, R. S. J. Colloid Interface Sci. 1984, 97, 327 Callaghan, I. C., Lawrence, F. T. and Melton, P. M. Colloid Polymer Sei. 1986, 264, 423 Krotov, V. V. Kolloidnyl Zhurnal 1984, 48, 913 Krotov V. V. Kolloidnyl Zhurnal 1984 48, 1170 Narsimhan, CO. and Ruckenstein, E. Langmuir 1986, 2, 230 Narsimhan, (3. and Ruckenstein, E. Langmuir 1986, 2, 494 Narsimhan, CO., Paper presented at the American Institute of Chemical Engineers, Summer National Meeting, August 16-19, Minneapolis, 1987 Uraizee, F. and Narsimhan, CO.,Paper presented at 62nd ACS Colloid and Surface Science Symposium, June 19-22, 1988, The Pennsylvania State University, University Park, PA
Effect of parameters on the performance of foam fractionation column Various investigators 1-7have found that the enrichment (the ratio of the concentration of the product to that of the feed) is maximum at the isoelectric point. It has also been observed 3 that some substances cannot be foamed at a pH other than pI because of poor foam stability. Because of the nature of the adsorption isotherm of proteins and enzymes at the gas-liquid interface, enrichment tends to be lower at higher concentrations.4'5'8'9 At sufficiently low concentrations, however, an optimum concentration is observed 1,6,1°at which the enrichment is maximum. Higher gas flow rates have b e e n f o u n d 1'4'6 to yield lower enrichments because of more liquid entrainment in the foam. Higher enrichments have been obtained 1 for larger bubble sizes because of lower liquid holdup resulting from faster rates of drainage. Larger pool heights have been found to yield higher enrichments 1'4'9 because of higher surface concentration at the gas-liquid interface resulting from increased residence time of the bubbles in the liquid pool. As the foam height is increased, the liquid holdup at the top of the foam is found to decrease, leading to higher enrichments 9 and lower separations.l,4,6 In the case of unstable enzymes such as streptokinase 11 and urease, 1 however, the enzyme activity is found to decrease at larger foam heights. Because of more adsorption, higher operating temperature has been found to yield higher protein concentration in the foam.l'9 Increasing the concentration of impurities such as
Address requests to Dr. Narsimhan at the Department of Agricultural Engineering, Purdue University, West Lafayette, IN 47907
© 1990 Butterworth Publishers
Na2SO 4 and ethanol decreases the enrichment. 1,9 At
ethanol concentrations above 1% (v), however, enrichment was found to increase because of the larger bubble sizes. 5 Solid impurities reduce protein enrichment because of their scavenging action. 9 Initial addition of water-soluble nonionic emulsifier was found to increase the separation. ~2At higher emulsifier concentrations, however, there is a pronounced decrease in the separation efficiency. 12Effects of several other impurities have also been investigated. 12 The most common mode of operation of foam fractionation employed by various investigators is the single-stage, semibatch mode. However, continuous 4,5 and multistage9 columns have also been evaluated. In a continuous column, increase in the feed rate has been found to decrease the enrichment. 4'5'9 There was little effect of the feed position on enrichment when the feed was introduced into the liquid pool. 4 Introduction of feed into the foam region, however, increased the efficiency considerably because the feed stream acted as a reflux. 4 Understandably, the multistage column yielded higher enrichment than a single stage. 9
Modelling of the foam column As pointed out earlier, it is necessary to quantify the effect of various operating variables on the performance of the foam fractionation column for the successful large-scale operation. Earlier investigators 13-15 predicted the foam density by assuming equal-sized bubbles and by accounting only for the gravity drainage of the liquid from the plateau borders. They either assumed the plateau border walls to be rigid (infinite surface viscosity) or accounted for its surface mobility through an adjustable parameter. The assumption of rigid plateau border walls was found to grossly underestimate the rate of liquid drainage. The effect of surface viscosity on liquid drainage was later accounted f o r 16,17 in the calculation of exit foam densities. The liquid holdup profile in a foam column was predicted 18 by considering the liquid drainage from the plateau border as well as from thin films. The change in bubble size distribution due to interbubble gas diffusion has been quantified.l%22 In spite of extensive studies on the staEnzyme Microb. Technol., 1990, vol. 12, April
:315
Literature Survey bility of isolated thin liquid films, very few attempts 23,24 have been made to couple the hydrodynamics of the foam bed to the instability of thin films in order to predict coalescence and subsequent foam collapse. The effects of (i) bubble size distribution, (ii) bubble coalescence as a result of the rupture of thin films caused by van der Waals forces-mediated growth of thermal perturbations, 24 and (iii) interbubble gas diffusion have been accounted for in a comprehensive population balance model. 25 The simplifying assumption of equalsized bubbles has been shown 25 to be valid for narrow inlet bubble size distributions, especially at higher superficial gas velocities and larger inlet bubble sizes. These models assume adsorption equilibrium of the surface-active component at the gas-liquid interface. Such an assumption, though valid for small-molecularweight surfactants, is inapplicable in case of proteins and enzymes because of their slow rates of adsorption. A model for the hydrodynamics of protein-stabilized foam has recently been proposed 26 which accounts for (i) the kinetics of adsorption of proteins and (ii) the dependence of surface viscosity on surface pressure. The protein enrichment w a s f o u n d 2v to be a strong function of the residence time of the bubbles in the liquid pool. The assumption of adsorption equilibrium has been s h o w n 27 to grossly overpredict enrichment excepting for high feed concentrations and large pool heights.
Conclusions Information with regard to the adsorption characteristics of proteins and enzymes at the gas-liquid interface is lacking. Extensive investigation of the kinetics of adsorption, adsorption isotherm, competitive adsorption of proteins and other surfactants, and the extent of possible denaturation of proteins and enzymes upon adsorption is necessary. Moreover, the surface viscosity and surface elasticity of the interfacial adsorbed protein layer should be characterized in order to quantify the drainage and stability of the foam. Since coalescence has a considerable effect on separation efficiency, more efforts towards the understanding of the effect of different variables on coalescence and foam stability is needed. Single-stage, semibatch, as well as continuous foam fractionation columns, have been
316
Enzyme Microb. Technol., 1990, vol. 12, April
evaluated in detail. Multistage columns with countercurrent as well as cross-flow modes of operation need to be evaluated. Efficient large-scale methods of collapsing foam should be investigated for the design of compact large-scale multistage systems.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
London, M., Cohen, M. and Hudson, P. Biochim. Biophys. Aeta 1954, 13, 111 Charm, S. E., Morningstar, J., Matteo, C. and Paltiel, B. Anal. Bioehem. 1966, 15, 498 Lalchev, Z., Dimitrova, L., Tzvetkova, P. and Exerowa, D. Bioteehnol. Bioeng. 1982, 24, 2253 Ahmad, S.I. Separation Sci. 1975, 10, 673 Schnepf, R. W. and Coaden, E. L. J. Biochem. Microb. Technol. Eng. 1959, 1, l Bader, R., Schutz, F. and Stacey, M. Nature 1944, 154, 183 Weijenherg, D. C., Mulder, J. J., Drinkenburg, A. A. H. and Stemerding, S. Ind. Eng. Chem. Process Design Devel. 1978, 17, 209 London, M. and Hudson, P. Arch. Biochim. Biophys. Acta 1953, 46, 141 Coehle,R. D. and Schugerl, K. Appl. Mierob. Biotechnol. 1984, 20, 133 Lalchev, Z. and Exerowa, D. Biotechnol. Bioeng. 1981, 23, 669 Holmstrom, B. Biotechnol. Bioeng. 1968, 10, 551 Ahmad, S. I. Separation Sci. 1975, 10, 689 Miles, Co. D., Shedlovsky, L. and Ross, J. J. Phys. Chem. 1945, 49, 93 Jacobi, W. M., Woodcock, K. E. and Grove, C. S. Ind. Eng. Chem. 1956, 48, 2049 Steiner, L., HHunkeler, R. and Hartland, S. Trans. Inst. Chem. Eng. 1977, 55, 153 Desai, D. and Kumar, R. Chem. Eng. Sei. 1983, 38, 1525 Desai, D. and Kumar, R. Chem. Eng. Sci. 1984, 39, 1559 Hartland, S. and Barber, A. D. Trans. Inst. Chem. Eng. 1974, 52, 43 Lemlich R. Ind. Eng. Chem. Fundamentals 1978, 17, 89 Monsalve, A. and Schechter, R. S. J. Colloid Interface Sci. 1984, 97, 327 Callaghan, I. C., Lawrence, F. T. and Melton, P. M. Colloid Polymer Sei. 1986, 264, 423 Krotov, V. V. Kolloidnyl Zhurnal 1984, 48, 913 Krotov V. V. Kolloidnyl Zhurnal 1984 48, 1170 Narsimhan, CO. and Ruckenstein, E. Langmuir 1986, 2, 230 Narsimhan, (3. and Ruckenstein, E. Langmuir 1986, 2, 494 Narsimhan, CO., Paper presented at the American Institute of Chemical Engineers, Summer National Meeting, August 16-19, Minneapolis, 1987 Uraizee, F. and Narsimhan, CO.,Paper presented at 62nd ACS Colloid and Surface Science Symposium, June 19-22, 1988, The Pennsylvania State University, University Park, PA
