Sparging

Abstract: Microbial fuel cells (MFCs) can be used to directly generate electricity from the oxidation of dissolved organic matter, but optimization of MFCs will require that we know more about the factors that can increase power output such as the type of proton exchange system which can affect the system internal resistance. Power output in a MFC containing a proton exchange membrane was comparedusing a pure culture ( Geobacter metallireducens) or a mixed culture (wastewater inoculum). Power output with either inoculum was essentially the same, with 4071 mW/m 2 for G. metallireducens and38 71 mW/m 2 for the wastewater inoculum. We also examinedpower output in a MFC with a salt bridge instead of a membrane system. Power output by the salt bridge MFC (inoculated with G. metallireducens) was 2.2 mW/m 2 . The low power output was directly attributed to the higher internal resistance of the salt bridge system ð19920 � 50OÞ comparedto that of the membrane system ð1286 � 1OÞ basedon measurements using imped ance spectroscopy. In both systems, it was observed that oxygen diffusion from the cathode chamber into the anode chamber was a factor in power generation. Nitrogen gas sparging, L-cysteine (a chemical oxygen scavenger), or suspended cells (biological oxygen scavenger) were used to limit the effects of gas diffusion into the anode chamber. Nitrogen gas sparging, for example, increasedoverall Coulombic efficiency (47% or 55%) comparedto that obtainedwithout gas sparging (19%). These results show that increasing power densities in MFCs will require reducing the internal resistance of the system, and that methods are needed to control the dissolved oxygen flux into the anode chamber in order to increase overall Coulombic efficiency. r 2005 Elsevier Ltd. All rights reserved.

Abstract: This experimental study is aimed at investigating the effect of liquid phase properties and gas distribution on bubble and hydrodynamic characteristics in bubble columns. With the various measuring techniques used, systematic measurements of bubble size, velocity and frequency and gas hold-up are possible. Bubble size distribution and shape factors which are rarely found in literature, are also available. Water–alcohol solutions are used to simulate the behaviour of industrial non-coalescing organic mixtures. The experimental results obtained with three different spargers in the coalescence inhibiting solutions are compared with data on standard coalescing air–water system. Evolutions of bubble characteristics and gas hold-up have been interpreted successfully by considering the simultaneous influence of the hydrodynamic regime of the gas–liquid flow and of the operating regime of the distributor. It has also been put into evidence that bubble frequency measurements are good tools to evaluate distributor efficiency. The influence of the distributor has been shown to be enhanced in non-coalescing media. Bubble shape and bubble size distributions are dramatically modified by addition of minute quantities of alcohol in water. Bimodal distributions can be observed even in the homogeneous regime with orifice nozzle spargers.