Showing papers on "Mass transfer coefficient published in 2008"

Carbon Monoxide Mass Transfer for Syngas Fermentation in a Stirred Tank Reactor with Dual Impeller Configurations

Abstract: 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.

Dissolved oxygen transfer to sediments by sweep and eject motions in aquatic environments

Abstract: Dissolved oxygen (DO) concentrations were quantified near the sediment-water interface to evaluate DO transfer to sediments in a laboratory recirculating flume and open channel under varying fluid-flow conditions. DO concentration fluctuations were observed within the diffusive sublayer, as defined by the time-averaged DO concentration gradient near the sediment-water interface. Evaluation of the DO concentration fluctuations along with detailed fluid-flow characterizations were used to quantify quasi-periodic sweep and eject motions (bursting events) near the sediments. Bursting events dominated the Reynolds shear stresses responsible for momentum and mass fluctuations near the sediment bed. Two independent methods for detecting bursting events using DO concentration and velocity data produced consistent results. The average time between bursting events was scaled with wall variables and was incorporated into a similarity model to describe the dimensionless mass transfer coefficient (Sherwood number, Sh) in terms of the Reynolds number, Re, and Schmidt number, Sc, which described transport in the flow. The scaling of bursting events was employed with the similarity model to quantify DO transfer to sediments and results showed a high degree of agreement with experimental data.

CO2 absorption into amine solutions: a novel strategy for intensification based on the addition of ferrofluids

Abstract: BACKGROUND: In this paper a novel approach for increasing the mass transfer coefficient in gas/liquid mass transfer is reported and applied to the industrially important system of CO2 absorption. The approach makes use of a ferrofluid additive to the liquid phase. To demonstrate this strategy, a wetted wall column has been built and experiments have been conducted on the CO2/methyl diethanolamine (MDEA) system. This reaction system allows the absorption to be carried out in the transition regime between slow and fast regimes, so that the mass transfer coefficient (kL) and interfacial area (ai) can be measured independently, in the presence and absence of ferrofluids. A surfactant-coated aqueous magnetic fluid was prepared and shown to be stable in MDEA solutions. RESULTS: The experimental results, with this fluid, show that there is an enhancement in mass transfer in the presence of ferrofluids, the extent of which depends on the amount of ferrofluid added. The enhancement in mass transfer coefficient was 92.8% for a volume fraction of the fluid of about 50% (solid magnetite volume fraction of about 0.39%). Experiments were also carried out to further enhance the mass transfer rates by employing a periodic oscillating magnetic field. Under the conditions employed, there was no further impact of magnetic field on mass transfer rates. CONCLUSIONS: Magnetic nanoparticles are able to significantly enhance gas/liquid mass transfer rates when added to the liquid phase. The reasons for this striking effect are under investigation. It is possible for significantly higher enhancements to be produced by the action of an external field, but this requires special fluids to be formulated which are not affected by the chemical solvent used. These results have significance for the absorption of CO2 in industrial absorbers using amine solutions.  2008 Society of Chemical Industry

Gas-Liquid Mass Transfer Coefficient in Stirred Tank Reactors

Abstract: Volumetric gas—liquid mass transfer coefficient (kLa) data available in the literature for larger tanks (T = 0.39 m to 2.7 m) have been analyzed on the basis of relative dispersion parameter, N/Ncd. It was observed that at a given superficial gas velocity (VG), kLa values were approximately the same irrespective of geometric configuration (size of the tank, type and size of the impellers, type of the sparger, etc.) at a particular N/Ncd. A single correlation based on N/Ncd is presented which shows satisfactory agreement with the kLa data of different workers.

Volatile organic compounds absorption in a cross-flow rotating packed bed.

Abstract: A cross-flow rotating packed bed (RPB) process was evaluated for its absorption of some volatile organic compounds (VOCs) into water, including isopropyl alcohol, acetone, and ethyl acetate. The experimental results showed that the mass transfer coefficient (KGa) increased with increasing rotational speed, liquid rate, and gas rate, and thus an empirical correlation of KGa was proposed for the cross-flow RPB for the first time. It was found that this correlation could reasonably estimate our experimental KGa data as well as those reported in literatures. Although the mass transfer coefficient was lower than that in a countercurrent-flow RPB, a cross-flow RPB is believed to be capable of handling a higher gas rate because of its flow pattern.

Influence of liquid viscosity and surface tension on the gas-liquid mass transfer coefficient for solid foam packings in co-current two-phase flow

Abstract: The gas–liquid mass transfer coefficient and other hydrodynamic parameters such as liquid holdup and frictional pressure drop are presented for gas and liquid moving in co-current upflow and downflow through solid foam packings of 10 and of 40 pores per linear inch (ppi). The effect of increasing the liquid viscosity on the mass transfer coefficient in co-current upflow is quantified and correlated to the frictional pressure drop, a measure of the frictional energy dissipation: k L a GL ɛ L ( S c L / S c water ) 0.69 = 2.05 × 1 0 − 4 P f 0.8 (mL3 mP−3 s−1). The gas–liquid mass transfer coefficient in co-current downflow is correlated to the liquid velocity and the Schmidt number using the correlation proposed by Sherwood and Holloway [Sherwood, T. and Holloway, F., 1940, Performance of packed towers—liquid film data for several packings, Transactions of the American Institute of Chemical Engineers 36: 39–70]: k L a GL ɛ L D L − 1 = 3.7 ( u L ρ L μ L − 1 ) 1.16 ( S c L ) 0.5 (mL mP−3). The results for the gas–liquid mass transfer coefficient in co-current upflow were correlated with a similar equation, where the influence of the gas velocity is included, similar to the correlations for packed beds of spherical particles proposed in Fukushima and Kusaka [Fukushima, S. and Kusaka, K., 1979, Gas–liquid mass transfer and hydrodynamic flow region in packed columns with cocurrent upward flow, Journal of Chemical Engineering of Japan 12 (4): 296–301]: k L a GL ɛ L D L − 1 = 311 u G 0.44 ( u L ρ L μ L − 1 ) 0.92 ( S c L ) 0.5 (mL mP−3). In this study the liquid Schmidt number dependency of the gas–liquid mass transfer coefficient points to the penetration theory describing the rate of mass transfer for gas–liquid flow through solid foam packings.