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Flow visualization and characterization of multiphase flows have been the quest of many fluid mechanicians. The process is fairly straight forward only when there is good optical access (i.e., the vessel is not opaque or there are appropriate viewing ports) and the flow is transparent, implying a very low volume fraction of the dispersed phase; however, when optical access is not good or the fluid is opaque, alternative methods must be developed. Several different noninvasive visualization tools have been developed to provide high-quality qualitative and quantitative data of various multiphase flow characteristics, and overviews of these methods have appeared in the literature. X-ray imaging is one family of noninvasive measurement techniques used extensively for product testing and evaluation of static objects with complex structures. X-rays can also be used to visualize and characterize multiphase flows. This paper provides a review of the current status of X-ray flow visualization and details various X-ray flow visualization methods that can provide qualitative and quantitative information about the characteristics of complex multiphase flows. Flow visualization and characterization of multiphase flows have been the quest of many fluid mechanicians. The process is fairly straight forward only when there is good optical access (i.e., the vessel is not opaque or there are appropriate viewing ports) and the flow is transparent, implying a very low volume fraction of the dispersed phase; however, when optical access is not good or the fluid is opaque, alternative methods must be developed. Several different noninvasive visualization tools have been developed to provide high-quality qualitative and quantitative data of various multiphase flow characteristics, and overviews of these methods have appeared in the literature. X-ray imaging is one family of noninvasive measurement techniques used extensively for product testing and evaluation of static objects with complex structures. X-rays can also be used to visualize and characterize multiphase flows. This paper provides a review of the current status of X-ray flow visualization and details various X-ray flow visualization methods that can provide qualitative and quantitative information about the characteristics of complex multiphase flows.

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To examine the potential use of synthesis gas as a carbon and energy source in fermentation processes, Rhodospirillum rubrum was cultured on synthesis gas generated from discarded seed corn. The growth rates, growth and poly-beta-hydroxyalkanoates (PHA) yields, and CO oxidation/H(2) evolution rates were evaluated in comparison to the rates observed with an artificial synthesis gas mixture. Depending on the gas conditioning system used, synthesis gas either stimulated or inhibited CO-oxidation rates compared to the observations with the artificial synthesis gas mixture. Inhibitory and stimulatory compounds in synthesis gas could be removed by the addition of activated charcoal, char-tar, or char-ash filters (char, tar, and ash are gasification residues). In batch fermentations, approximately 1.4 mol CO was oxidized per day per g cell protein with the production of 0.75 mol H(2) and 340 mg PHA per day per g cell protein. The PHA produced from R. rubrum grown on synthesis gas was composed of 86% beta-hydroxybutyrate and 14% beta-hydroxyvalerate. Mass transfer of CO into the liquid phase was determined as the rate-limiting step in the fermentation.

This study compares the power demand and gas-liquid volumetric mass transfer coefficient, kLa, in a stirred tank reactor (STR) (T = 0.211 m) using different impeller designs and schemes in a carbon monoxide-water system, which is applicable to synthesis gas (syngas) fermentation. Eleven different impeller schemes were tested over a range of operating conditions typically associated with the "after large cavity" region (ALC) of a Rushton-type turbine (D/T = 0.35). It is found that the dual Rushton-type impeller scheme exhibits the highest volumetric mass transfer rates for all operating conditions; however, it also displays the lowest mass transfer performance (defined as the volumetric mass transfer coefficient per unit power input) for all conditions due to its high power consumption. Dual impeller schemes with an axial flow impeller as the top impeller show improved mass transfer rates without dramatic increases in power draw. At high gas flow rates, dual impeller schemes with a lower concave impeller have kLa values similar to those of the Rushton-type dual impeller schemes but show improved mass transfer performance. It is believed that the mass transfer performance can be further enhanced for the bottom concave impeller schemes by operating at conditions beyond the ALC region defined for Rushton-type impellers because the concave impeller can handle higher gas flow rates prior to flooding.

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