Gas Hold-Up in Slurry Bubble Columns: Effect of Column Diameter and Slurry Concentrations

AIChE Journal (Impact Factor: 2.75). 02/1997; 43(2):311 - 316. DOI: 10.1002/aic.690430204


To study the influence of particle concentration on the hydrodynamics of bubble-column slurry reactors operating in the heterogeneous flow regime, experiments were carried out in 0.10, 0.19, and 0.38-m-dia. columns using paraffinic oil as the liquid phase and slurry concentrations of up to 36 vol. %. To interpret experimental results a generalization of the “two-phase” model for gas–solid fluid beds was used to describe bubble hydrodynamics. The two phase identified are: a dilute phase consisting of fast-rising large bubbles that traverse the column virtually in plug flow and a dense phase that is identified with the liquid phase along with solid particles and entrained small bubbles. The dense phase suffers backmixing considerably. Dynamic gas disengagement was experimented in the heterogeneous flow regime to determine the gas voidage in dilute and dense phases. Experimental data show that increasing the solid concentration decreases the total gas holdup significantly, but the influence on the dilute-phase gas holdup is small. The dense-phase gas voidage significantly decreases gas holdup due to enhanced coalescene of small bubbles resulting from introduction of particles. The dense-phase gas voidage is practically independent of the column diameter. The dilute-phase gas holdup, on the other hand, decreases with increasing column diameter, and this dependence could be described adequately with a slight modification of the correlation of Krishna and Ellenberger developed for gas–liquid systems.

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    • ". The variation of the gas holdup with time has two different stages: in the first stage, both large bubbles and small bubbles disengaged from the liquid which resulted in a fast decrease in the gas holdup, and in the second stage, all large bubbles have disengaged and only the remaining small bubbles disengaged from the liquid, which gave a slower decrease in the gas holdup. The method of determining the volume fractions of large bubbles and small bubbles was similar to that used by Krishna et al. (1997) and Yang et al. (2010), as shown in Fig. 2 "
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    ABSTRACT: The effect of liquid viscosity on the hydrodynamic behavior in a bubble column was investigated by experimental study and numerical simulation with a coupled CFD–PBM (population balance model) model. The total gas holdup and volume fractions of small and large bubbles were determined by the dynamic gas disengagement method. In the low viscosity range, the total gas holdup and the volume fractions of small and large bubbles were almost independent of the liquid viscosity. In the high viscosity range, an increased viscosity gave a decrease in the total gas holdup and volume fraction of small bubbles and an increase in the volume fraction of large bubbles. The simulation captured these features and showed that when the liquid viscosity was <10 mPa s, the viscosity had negligible effect on the bubble breakup rate and daughter bubble size distribution, while a further increase of the liquid viscosity significantly decreased the bubble breakup rate and increased the probability of equal breakup. The influence of liquid viscosity on bubble coalescence was less important than its effect on bubble breakup for the hydrodynamic behavior. With the use of the multiple bubble breakup and coalescence models and the use of the bubble size distribution to describe the interphase forces, the coupled CFD–PBM model could describe the dependences of the total gas holdup and volume fractions of small and large bubbles on liquid viscosity in both the homogeneous and heterogeneous regimes.
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    • "Interfacial area between gas and liquid in multiphase catalytic reactors also depends on the type and wettability of particles suspended in liquid [5] [6]. The hydrophobic particles adhere to the bubble surface and cause additional resistance to the film drainage and prevent coalescence leading to smaller bubbles and larger interfacial area. "
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    • "Most published studies have shown that increasing the superficial gas velocity leads to increase in the gas holdup [6] [34] [36] [37] [38]. The dependence of the gas holdup on the superficial gas velocity has been defined by the following power-law expression [39]: Experiments were performed for liquid velocity up to U l = 16.04 cm/s. "
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