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While there is a universal trend to replace fossil fuels at least partially, renewable fuels seem to impose new solutions. Hydrogen sulfide, typical for closed water ponds such as the Black Sea, seems to offer one namely, a new sulfide-driven fuel cell providing for exchange of OH− anions across the membrane by use of hydrogen sulfide in natural marine water. When tested in batch and continuous operation modes, this solution showed that the initial sulfide concentration needed to achieve results of practical value was within 200 to 300 mg dm−3. The predominating final products of the energy production process were sulfite and sulfate ions. Very low overpotentials and mass transfer resistances were observed. The mass balance and the electrochemical parameters showed about 30% efficiency in sulfate ions as the final product. Efforts should be made to enhance sulfide to sulfate conversion. The observed current and power density were comparable and even better than some of the results so far reported for similar systems. Three types of ion exchange membranes were tested. Comparison of their ion conductivity to literature data shows good performance. At higher initial sulfide concentrations polysulfides and thio-compounds were formed with considerably low current yield.

Gas dispersion has been analysed experimentally by measuring local gas hold-up in two lab-scale conventional turbine-agitated vessels loaded with viscoelastic fluids. The gas hold-up radial profiles obtained in various axial positions close and far from the impeller reveal high non-uniformity of gas dispersion. Nonuniformity is quantified and related to elasticity, shear dependent viscosity, shear rate, first normal stress difference coefficient and type of fluid. The low values of gas hold-up obtained for the vessel central area are interpreted as hindered gas dispersion caused by the flow pattern specific for the viscoelastic properties of the circulating fluid. The paper is complementary to the effort for collecting up data for benchmarking numerical CFD simulations (Kumar et al., 1997).On a Ctudik expkrimentalement la dispersion de gaz en mesurant la rktention de gaz dans deux rkservoirs agitks B l'kchelle de laboratoire aliment& avec des fluides viscoklastiques. Les profils de rktention de gaz radiaux obtenus dans diverses positions axiales B proximitk ou B distance de la turbine rkvble une forte nonuniformitk de la dispersion de gaz. Cette non-uniformitk est quantifike et relike B l'klasticitk, h la viscositt dipendante du cisaillement, B la vitesse de cisaillement, au coefficient de diffkrence des contraintes normales primaires et au type de fluide. Les faibles valeurs de rktention de gaz obtenus pour la region centrale des rkservoirs sont interprktkes comme une dispersion de gaz entravke en raison du profil d'kcoulement spkcifique induit par les propriktks viscoklastiques du fluide circulant. Cet article vient complkter des efforts pour rkunir des donnkes pour des simulations numkriques (Kumar et al., 1997).Keywords: mixing and gas dispersion, viscoelastic systems, gas hold-up, non-uniformity. recent discussion on mixing in stirred non-Newtonian A fluids, Tatterson (1991) quotes differences of the main mixing parameters for viscoelastic solutions, e g , mixing time, power draw, circulation time. Emphasising on gas dispersion, the analyses (see Chhabra, 1993, Dekee et al., 1986) reveal significant viscoelastic effects on the behaviour of dispersed fluid particles. Bearing on the complexities of stirred flow, it has been realised that viscous and elastic flow properties influence the quality of gas dispersion in agitated reactors. However, analyses of the behaviour of structural macroparameters in stirred gas-liquid systems, such as local gas hold-up or bubble size distribution, are lacking. Apart from the studies of total gas hold-up in such type of fluids by Ranade and Ulbrecht (1978) and fluid flow mobility limitations noted by Wichterle and Wein (1981) and later by Nienow and Elson (1988), phenomenological descriptions of gas and liquid volumetric distribution in agitated non-Newtonian fluids are scarce. Regarding viscoelastic mixing flow patterns, the specific loops caused by shapepreserving forces in viscoelastic fluids (Ide and White, 1974; Hbcker et al, 1981) and the different discharge angles obse...

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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 fluidfoil 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 lowshear 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' correlation of k L a in stirred tanks is tested in the presence of surfactin.

In this work, the gas-liquid mass transfer in a lab-scale fibrous bed reactor with liquid recycle was studied. The volumetric gas-liquid mass transfer coefficient, k L a, is determined over a range of the superficial liquid velocity (0.0042-0.0126 m.s ), surface tension (35-72 mN/m), and viscosity (1-6 mPa.s). Increasing fluid velocities and viscosity, and decreasing interfacial tension, the volumetric oxygen transfer coefficient increased. In contrast to the case of co-current flow, the effect of gas superficial velocity was found to be more significant than the liquid superficial velocity. This behavior is explained by variation of the coalescing gas fraction and the reduction in bubble size. A correlation for k L a is proposed. The predicted values deviate within ± 15 % from the experimental values, thus, implying that the equation can be used to predict gas-liquid mass transfer rates in fibrous bed recycle bioreactors.