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.
    Chemical Engineering Science 05/2013; 95:313–322. DOI:10.1016/j.ces.2013.03.022 · 2.34 Impact Factor
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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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    ABSTRACT: The wettability of porous powders is typically described by the solid/liquid contact angle and finds importance in the fields of catalysis, pharmaceuticals and separation techniques. The measurement of contact angles of porous particles with liquids is notoriously difficult and different techniques produce highly variable results. A variety of catalyst supports were used to investigate several methods recommended in the literature to determine their applicability for the measurement of the wettability of highly porous particles. The catalyst supports consisted of SiO2, Al2O3, TiO2 and ZrO2.The following methods were investigated: the capillary rise method, thin layer wicking and the sessile drop method on particles either compressed in pellets or dispersed upon glass slides. It has been found that methods based on the capillary rise cannot be used to measure contact angle of highly porous particles and this experimental observation has been confirmed by the mass balance of liquid in the bed of particles. Multiple measurements carried out with water and paraffin indicate that only the sessile drop method on particles dispersed on a glass slide gives consistent results. Using this method, no differentiation was found in terms of wettability of paraffin oil for Al2O3, SiO2, and ZrO2 particles although TiO2 was measurably different but largely wettable. In the case of water, the contact angles varied significantly and were found to be θAl2O3=91°,θSiO2=75°,θTiO2=132°,θZrO2=49°θAl2O3=91°,θSiO2=75°,θTiO2=132°,θZrO2=49°.
    Powder Technology 01/2013; 233:52–64. DOI:10.1016/j.powtec.2012.08.032 · 2.35 Impact Factor
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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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    ABSTRACT: Bubble columns are used in a large number of applications in chemical engineering. The important variables that affect the gas holdup, bubble dynamics and flow regime in a bubble column are gas and liquid velocities, liquid viscosity, liquid surface tension, design of the gas distributor, solid concentration and column diameter. Experiments have been performed in a 15 cm diameter co-current slurry bubble column with liquid phase as water and air as the gas phase. Glass beads of mean diameter 35 μm have been used as solid phase. Solid loading up to 9% has been used. The superficial gas velocity varies from 1.0 to 16.28 cm/s and superficial liquid velocity varies from 0 to 12.26 cm/s. Effects of liquid height, liquid velocity, gas velocity and solid concentration over gas holdup for both two and three phase co-current flows have been studied. For batch case the liquid height didn’t affect the gas holdup. The gas holdup increases with increase in gas velocity for both two and three phase co-current columns. For two phase and three phase flow up to 1% solid loading; at low superficial gas velocity i.e. in the homogeneous regime, the increase in liquid velocity doesn’t show any change in the gas holdup. For higher gas velocities i.e. in the heterogeneous regime, increase in liquid velocity decreases the gas holdup rapidly. Above 1% solid loading, liquid velocity effect over gas hold-up is negligible. With increase in solid concentration for co-current bubble column the gas holdup slightly increases or remains constant up to 5% loading; beyond this loading there is a significant decrease in gas holdup
    Procedia Engineering 01/2012; 42:782–794. DOI:10.1016/j.proeng.2012.07.470
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