* Corresponding author. Tel.: # 44-0207-594-5591; fax: # 44-0207- 594-5629. E-mail addresses: g.lye@ucl.ac.uk (G. J. Lye), d.stuckey@ic.ac.uk (D. C. Stuckey). Chemical Engineering Science 56 (2001) 97}108 Extraction of erythromycin-A using colloidal liquid aphrons: Part II. Mass transfer kinetics G. J. Lye, D. C. Stuckey* Department of Chemical Engineering, Imperial College, Prince Consort Road, London SW7 2BY, UK The Advanced Centre for Biochemical Engineering, Department of Biochemical Engineering, University College London, London WC1E 7JE, UK Received 13 July 1999; received in revised form 10 April 2000; accepted 8 May 2000 Abstract An emerging technique for the recovery of microbial secondary metabolites, such as antibiotics, is the use of colloidal liquid aphrons (CLAs) in pre-dispersed solvent extraction (PDSE) processes. Knowledge of the extraction kinetics and the limiting mass transfer resistances will be vital for e$cient process design and operation. In this work, the rates of erythromycin extraction using CLAs and conventional aqueous}organic, two-phase systems have been investigated. The CLAs used were formulated from 1% w/v Softanol 120 in decanol and 0.5% w/v SDS in water. The rate of erythromycin extraction with CLAs dispersed in well-mixed systems was found to be extremely rapid with equilibrium being achieved within 15 s or less. Overall erythromycin mass transfer coe$cients, K  , were typically 6.310 ms for extraction experiments, and 1.010 ms for stripping experiments. The rapid rates of erythromycin transfer were attributed to the small size of the dispersed CLAs, typically 5 m diameter, and hence the large interfacial area available for mass transfer of around 1510 m m. Experiments over a range of Reynolds numbers indicated that both the extraction and stripping processes were probably under mixed control of an interfacial resistance and boundary-layer di!usion. To investigate the in#uence of the surfactants used for aphron formulation on erythromycin extraction, further experiments were performed using aqueous}organic, two-phase systems in a non-dispersive stirred cell. The measured K  values were of the same order of magnitude as in the case of experiments with dispersed CLAs. When present individually, or together with SDS, the non-ionic Softanol surfactant was seen to retard erythromycin extraction rates in all cases. In contrast, the presence of SDS was found to enhance K  values by a factor of 2}4 compared to surfactant-free systems. This may be attributed to a speci"c interaction between individual SDS and erythromycin molecules and/or the generation of interfacial turbulence at a microscopic level. Further investigations are underway to elucidate the mechanism responsible. The concentration of either surfactant was also found to signi"cantly a!ect the measured K  values. The prediction of K  values using existing correlations, and the implications of the experimental results on contactor design and operation are also discussed.  2001 Published by Elsevier Science Ltd. All rights reserved. Keywords: Erythromycin; Aphrons; Lewis cell; Mass transfer; Surfactants; Marangoni phenomena 1. Introduction Colloidal liquid aphrons (CLAs) are micron-sized sol- vent droplets surrounded by a thin aqueous "lm which is stabilised by a mixture of non-ionic and ionic surfactants. They were "rst described by Sebba in 1972, who postu- lated that the surfactant molecules in the aqueous "lm formed three distinct interfaces (a monolayer and a sep- arate bilayer) in order to account for the stability of these solvent droplets when dispersed in a continuous aqueous medium (Sebba, 1987). A number of investigators have subsequently studied the application of CLAs for the pre-dispersed solvent extraction (PDSE) of a range of organic molecules. These include antibiotics (Lye & Stuckey, 1994) and organic pollutants such as dichloro- benzene (Wallis, Michelsen, Sebba, Carpenter & Houle, 1985) and 3,4-dichloroaniline (Lye, Poutiainen & Stuckey, 1994b). Solute extraction is driven by a fa- vourable partitioning of the target molecule between the aqueous feed solution and the solvent core of the 0009-2509/01/$ - see front matter  2001 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 0 0 ) 0 0 1 4 1 - X