Research Article Surfactant Effects on Aeration Performance of Stirred Tank Reactors The effect of surfactants on aeration performance in stirred tank reactors (STR) at high rates of foaming is studied. The volumetric oxygen transfer coefficient (k L a) and foaming activity estimated as foaming height (H f ) were determined. Biotechnology of lipopeptide biosurfactants from aerobic organisms, e.g., Bacillus subtilis were addressed. Using model solutions of known foam-generating properties, high-molecular weight surfactin and low-molecular weight sodium dodecyl sulphate (SDS), as well as impellers of different types, with flat and fluid- foil blades, clues on the concentration dependence of STR oxygen transfer and foaming as well as options for foam reduction in the presence of biosurfactant were sought. In response to a two-fold decrease of surface tension by surfactin, k L a values decreased up to 30 % but remained within the range expected for the mixing system in water; the experiments with SDS showing stronger dependence on surfactant concentration and surface tension. Mixing of surfactant media by a standard six-blade disc turbine (RT) imposed rate limitations on gassing. A low- shear impeller Narcissus (NS) could be used to avoid bulk foam outflow, while preserving k L a values that remained unchanged. The ‘power per unit volume’ cor- relation of k L a in stirred tanks is tested in the presence of surfactin. Keywords: Agitation, Gas-liquid systems, Mass transfer, Surfactant, Foaming Received: October 24, 2007; revised: July 22, 2008; accepted: August 12, 2008 DOI: 10.1002/ceat.200700401 1 Introduction Stirred tank reactors are key processing vessels in biotechnol- ogy and the rate of oxygen delivery determines, in many cases, the efficiency of aerobic treatment. Depending on the media involved, mass transfer is influenced by liquid properties, such as surface tension, density, viscosity, etc. Most frequently, the systems are foaming because of the presence of cell proteins and/or surface active products of a specific nature, e.g., biolog- ical surfactants. In cases of surfactant products, important variables are product concentrations and foaming. To improve mass transfer efficiency, data on the gas-liquid mass transfer coefficient and the range of foaming activity is needed. An important case study is seen in biosurfactant production, in particular those related to bio-production of lipopeptides [1]; some of them, such as iturin, surfactin, and fengycin are highly surface active. Due to their biological and physical chemical properties, they are subject to significant commercial interest [2–6]. Among them, surfactin exhibits extremely high foaming capacity [7]. Combined with biodegradability, its properties render the substance as a highly demanded product [2–7]. In order to increase the production prospects, the fer- mentation rate has to be increased. Referring to the fact that most biosurfactant-producing organisms are aerobic, develop- ments have to focus on aeration. Nevertheless, specific oxygen transfer rates and their relationships with surfactant concen- tration and foaming conditions at agitation are largely un- known [8]. So far, only one study of a similar problem has come to the authors’ attention. Sheppard and Cooper [9] have studied the effect of the cell-free production medium upon oxygen transfer in a cyclone tower. Being fluid flow sensitive, the process was carried out at constant gassing rate, at no agi- tation or excessive foaming. No pure biosurfactant or foaming aspects were considered. The paper delivered important data on the effect of nutrient constituents, pH, and liquid volume on aeration in the presence of surfactant. However, as far as seen also from their analysis, the process is accompanied by some difficulties: On the one hand, this is the extent of foam- ing. On the other hand, the lipopeptide accumulates at the gas-liquid interface and changes the system transport proper- ties. Both aspects invoke further analysis of the lipopeptide ef- © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com Martin Martinov 1 Frederique Gancel 2 Philippe Jacques 2 Iordan Nikov 2 Serafim Vlaev 1 1 Institute of Chemical Engineering, Bulgarian Academy of Sciences, Sofia, Bulgaria. 2 Polytech’ Lille, Département IAAL ProBioGEM, Université des Sciences et Technologies de Lille, Cité Scientifique, Villeneuve d’Ascq, France. – Correspondence: Prof. S. D. Vlaev (mixreac@bas.bg), Institute of Chemical Engineering, Bulgarian Academy of Sciences, Acad.G. Bonchev Str. Bl. 103, 1113 Sofia, Bulgaria. 1494 Chem. Eng. Technol. 2008, 31, No. 10, 1494–1500