Synthesis gas is readily obtained by gasifying coal, oil, biomass, or waste organics and represents an abundant, potentially inexpensive, feedstock for bioprocessing. The primary components of synthesis gas, carbon monoxide and hydrogen, can be converted into methane, organic acids, and alcohols via anaerobic fermentations. Bioconversion of synthesis gas is an attractive alternative to catalytic processing because the biological catalysts are highly specific and often more tolerant of sulfur contaminants than inorganic catalysts. However, because the aqueous solubilities of carbon monoxide and hydrogen are low, synthesis-gas fermentations are typically limited by the rate of gas-to-liquid mass transfer. Consequently, a major engineering challenge in commercial development of synthesis-gas fermentations is to provide sufficient gas mass transfer in an energy-efficient manner. This paper reviews recent progress in the development of synthesis-gas fermentations, with emphasis on efforts to increase the efficiency of gas mass transfer. Metabolic properties of several microbes able to ferment synthesis gas are described. Results of synthesis-gas fermentations conducted in various bioreactor configurations are summarized. Recent results showing enhancement of synthesis-gas fermentations using microbubble dispersions are presented, and studies of the mass-transfer and coalescence properties of microbubbles are described.

Synthesis-gas fermentations have typically been gas-to-liquid mass-transfer-limited due to low solubilities of the gaseous substrates. A potential method to enhance mass-transfer rates is to sparge with microbubble dispersions. Mass-transfer coefficients for microbubble dispersions were measured in a bubble column. Oxygen microbubbles were formed in a dilute Tween 20 solution using a spinning disk apparatus. Axial dispersion coefficients measured for the bubble column ranged from 1.5 to 7.2 cm2/s and were essentially independent of flow rate. A laser-diffraction technique was used to determine the interfacial area per unit gas volume, a. The mass-transfer coefficient, KL, was determined by fitting a plug-flow model to the experimental, steady-state, liquid-phase oxygen-concentration profile. The KL values ranged from 2.9 x 10(-5) to 2.2 x 10(-4) m/s. Volumetric mass-transfer coefficients, KLa, for microbubbles with an average initial diameter of 60 microns ranged from 200 to 1800 h-1. Enhancement of mass transfer using microbubbles was demonstrated for a synthesis-gas fermentation. Butyribacterium methylotrophicum was grown in a continuous, stirred-tank reactor using a tangential filter for total cell recycle. The fermentation KLa values were 14 h-1 for conventional gas sparging through a stainless steel frit and 91 h-1 for microbubble sparging. The Power number of the microbubble generator was determined to be 0.036. Using this value, an incremental power-to-volume ratio to produce microbubbles for a B. methylotrophicum fermentation was estimated to be 0.01 kW/m3 of fermentation capacity.

Biomass-derived synthesis gas can be readily converted into fuels and chemicals by anaerobic microorganisms. However, synthesis -gas fermentations typically exhibit low volumetric productivities due, in part, to low cell densities, production of unwanted by -products, and slow transfer of the synthesis gas into the liquid phase. Engineering approaches to improve bioreactor productivities are discussed, and recent advances in this area are summarized. Particular emphasis is placed on the use of bioreactor design to increase biocatalyst concentrations, development of metabolic models to study pathway regulation and the use of microbubble dispersions to enhance synthesis-gas mass transfer.Synthesis gas, which consists primarily of carbon monoxide (CO), and hydrogen (H 2 ), is produced by the partial oxidation of an organic feedstock at high temperature in the presence of steam. Although coal and petroleum have historically been the most commonly used feedstocks for synthesis-gas production, several new gasification plants have recently been based on biomass (7). Biomass offers several advantages over the traditional feedstocks. First, it has a much lower sulfur content than many coals. Synthesis gas produced from wood chips at the GE gasification plant in Schenectady, NY contained 28 ppm H 2 S (2), compared to 1-2% for coal-derived synthesis gases (3). Purification steps to remove sulfur from coal-derived synthesis gas are energy-intensive and add significantly to the product costs (4). Second, biomass materials are more reactive and thus require lower gasification temperatures and/or residence times. Fluidized-bed coal gasifiers are typically run at 1000°C using residence times of 0.5-3.0 h. By comparison, a temperature of 850°C was sufficient to gasify biomass using a residence time of 30 s to 5.0 min (5). Third, gasification of wood waste has the potential to solve a disposal problem while producing a valuable product. When wood waste is in

Gas-to-liquid mass transfer is commonly the rate-limiting step in industrial fermentations. Microbubble sparging has been shown to give extremely high volumetric masstransfer rates, even for low power-to-volume ratios. Microbubbles are surfactantstabilized bubbles having a radius on the order of 25 µm. The extremely high surfacearea-to-volume ratios of microbubbles can result in rapid changes in their size, internal pressure, and gas composition. Consequently, an unsteady-state modeling approach is needed to adequately describe microbubble mass transfer. A dynamic model of a single microbubble immersed in an infinite pool of stagnant liquid was developed and solved numerically. The model accounts for mass-transfer resistances of the surfactant shell and surrounding bulk liquid, bubble shrinkage, changes in the gas pressure and composition inside the bubble, and changes in the liquid-phase concentration profile of the transferred gas surrounding the bubble. The model was used to explore a variety of dynamic phenomena associated with microbubble mass transfer. The presence of a nontransferred gas in the microbubble was predicted to significantly reduce the masstransfer rate, indicating that microbubble sparging is better suited to gases with a high consumable fraction. The instantaneous mass-transfer coefficient was predicted to change significantly with time, but the time-averaged coefficient was constant enough to justify the measurement of average mass-transfer coefficients for microbubbles. Average mass-transfer coefficients predicted by the model agreed well with values measured experimentally and calculated using literature correlations.

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