Effective Analysis of Different Gas Diffusers on Bubble Hydrodynamics in Bubble Column and Airlift Reactors towards Mass Transfer Enhancement (original) (raw)
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Processes, 2020
Many researchers have focused on multi-phase reactor development for improving mass transfer performance. However, solid particle addition in gas–liquid contactor for better oxygen mass transfer performance is still limited. Hence, this study aims to analyze the relative effect of different types of local solid media on the bubble hydrodynamic characteristics towards mass transfer enhancement in bubble columns (BCR) and airlift reactors (ALR). This was investigated by varying solid media types (ring, sphere, cylinder, and square), solid loadings (0%–15%), and superficial gas velocities (Vg) (2.6–15.3 × 10−3 m/s) in terms of the bubble hydrodynamic and oxygen mass transfer parameters. The result showed that bubble size distribution in BCR and ALR with additional plastic media was smaller than that without media addition, approximately 22%–27% and 5%–29%, respectively, due to the increase of the bubble breaking rate and the decrease of the bubble rising velocity (UB). Further, adding ...
Journal of Environmental Chemical Engineering, 2019
Modified airlift reactor (MALR) was developed for improving the oxygen transfer coefficient (K L a) by installing the slanted baffles in the riser compartment. MALR can enhance K L a value up to 97% compared to a regular reactor. However, insufficient analysis of the hydrodynamics and other oxygen transfer parameters of this new reactor has been performed to date. Therefore, this paper aims to analyse local gas-liquid dynamics and evaluate the oxygen transfer performance in MALR compared to regular reactors using clean water as a liquid phase. Air bubble distribution was employed in terms of bubble diameter, rising velocity, and interfacial area. Slanted baffles maintained the bubble size between 3.88 and 4.63 mm for the studied superficial gas velocity (Ug), which is smaller than that in airlift reactor (ALR) by approximately 0.2 mm. Bubble rising velocity (U B) is relatively increased with Ug, regardless of reactor classes. New reactor could extend the bubble residence in the water by lengthening the bubble stream path of the slanted baffles leading to decrease U B values about 39% and 52% compared to bubble column reactor (BCR) and ALR, respectively. These performances consequently resulted in an interfacial area almost two times higher than in regular reactors. Oxygen transfer improved in MALR with an extra amount of oxygen transfer efficiency of 1.57% and 0.63% over BCR and ALR, respectively. This resulted in the highest aeration efficiency (AE) compared to other examined reactors, following a trendline expression of AẼ 0.046 Ug −0.6. Insertion of slanted baffles produced 5% and 12.8% more dead zone volume than ALR and BCR, respectively. In conclusion, MALR can significantly enhance oxygen transfer performance due to the ability to maintain bubble size and extend the gas-liquid transfer period.
Mass transfer studies in shallow bubble column reactors
Chemical Engineering and Processing: Process Intensification, 2012
Mass transfer studies are carried out in a bubble column with an internal diameter of 14 cm and various static liquid heights. The mass transfer coefficient is evaluated by using an oxygen sorption method. A model considering the gas holdup flushing and the sensor response is used. The interfacial mass transfer area is determined according to the measured bubble size distribution. The liquid-side mass transfer coefficient is also estimated from the volumetric mass transfer coefficient and the interfacial mass transfer area found. Results show that the effect of static liquid height on gas-liquid mass transfer is primarily on the interfacial mass transfer area. The mass transfer process is also governed by the type of gas distributor used. A single nozzle distributor is not suitable for shallow bubble column operations due to the large initial bubbles and the large volume of dead zone generated. It is also found that the different dependence of the liquid-side mass transfer coefficient on the superficial gas velocity observed in the literatures is due to the different bubble rising regimes.
Comparison of Hydrodynamics and Mass Transfer in Airlift and Bubble Column Reactors Using CFD
Chemical Engineering & Technology, 2003
Computational Fluid Dynamics (CFD) is used to compare the hydrodynamics and mass transfer of an internal airlift reactor with that of a bubble column reactor, operating with an air/water system in the homogeneous bubble flow regime. The liquid circulation velocities are significantly higher in the airlift configuration than in bubble columns, leading to significantly lower gas holdups. Within the riser of the airlift, the gas and liquid phases are virtually in plug flow, whereas in bubble columns the gas and liquid phases follow parabolic velocity distributions. When compared at the same superficial gas velocity, the volumetric mass transfer coefficient, k L a, for an airlift is significantly lower than that for a bubble column. However, when the results are compared at the same values of gas holdup, the values of k L a are practically identical.
In this paper, a multi-scale approach is followed to study gas-liquid mass transfer in bubble columns. First, a single bubble of equivalent diameter d is considered. Its morphology and its gas to liquid relative velocity are related to the bubble diameter through the use of known correlations. Then, the gas-liquid mass transfer between the bubble and the surrounding liquid is studied theoretically. An equation describing the transport of the transferred species in the viscous boundary layer around the bubble is solved. In a second step, a bubble column of 6-10 m height is studied experimentally. The gas phase in the column is characterized experimentally by means of a gammametric technique. Finally, the two studies are linked, yielding a 1D mathematical model able to predict the gas-liquid mass transfer rate in a bubble column operated in the heterogeneous regime. 2005 Elsevier Ltd. All rights reserved.
Effect of some solid properties on gas–liquid mass transfer in a bubble column
Chemical Engineering and Processing, 2011
The knowledge about the effects of solids on gas-liquid systems and the respective physical mechanisms are not yet totally clarified. In this work, the effect of the solids on the mass transfer characteristics in a bubble column was studied experimentally for the systems air/water/expandable polystyrene (EPS) beads and air/water/glass beads. Volumetric liquid side mass transfer coefficient, k L a, was determined under different solid concentrations (up to 30 vol.%), superficial gas velocities (up to 2.7 mm/s) and mean diameters (1100, 770 and 591 m for EPS and 9.6 m for glass beads). The presence of EPS solids affects negatively k L a being this effect more pronounced for the smaller particles. Also, a decrease in k L a occurs when the solid loading increases. Experiments done with large polystyrene particles (d p ≥ 591 m) contaminated with very fine EPS particles (d p ∼ = 0.1 m) indicate that very fine particles play an important role on gas-liquid mass transfer. Mass transfer experiments in a hollow glass spheres three-phase slurry showed a dual effect of solids loading on k L a, contrarily to what happens with the previous particles. These results can be associated with the different surface properties of the particles studied. An empirical correlation for k L a on the experimental variables was developed.
International Journal of Chemical Reactor Engineering, 2000
Hydrodynamics of two external loop circulating bubble columns with open channel gas separators were investigated experimentally. The hydrodynamics were studied under the effects of three operating parameters and one design parameter. The operating parameters were superficial gas velocity in the riser, U GR , gas-liquid dispersion property, σ, and efficiency of gas separation, T VR . The design parameter was the scale-up factor of the riser to downcomer cross-sectional area A D /A R . The gas-liquid dispersion property was varied by using various concentrations of additives like antifoam, alcohol and wettable suspended solids. All experiments were conducted using compressed air and tap water from the university main supply system. Time-dependent hydrodynamics variation related to air-water quality was minimized by using consistent air-water samples in order to satisfy experimental reproducibility. The results show that for any gas-liquid-additive system, there is an optimum volume of the liquid in the gas separator, T VRO , that gives the minimum gas recirculation in the downcomer, ε GD . At any A D /A R and T VR , the effects of antifoam and suspended solids were found to reduce the gas holdup, while the alcohols were found to increase gas holdup. At T VRO , the efficiency of the gas separator was improved significantly by increasing the scale-up factor, A D /A R . Liquid circulation was increased in the antifoam system, but reduced in the presence of wettable suspended solids. The results of this study indicate that the design and operating conditions were better at scale-up factor A D /A R =0.55 and volume ratio T VR >20%, than that at A D /A R =0.25 for all the additives investigated.
Experimental Study of Volumetric Mass Transfer Coefficients in Slurry Bubble Column Reactor
Scaling up of Fisher-Tropsh requires suitably accurate estimations of hydrodynamic and mass transfer parameters as a function of column size and slurry concentration. The present study focuses on gas holdup, volumetric mass transfer and local mass transfer coefficients in a slurry bubble column employing a C 9-C 11 Iraqi paraffin oil as a liquid phase, alumina and silica particles as a solid phase and oxygen was used as a gas phase because the properties of the paraffin oil are similar to those of Fisher-Tropsch wax under actual operating conditions in the industrial slurry bubble column and alumina and silica serve as carrier for the active cobalt catalyst utilized in the Fischer-Tropsch process. The experimental work was carried out using two rectangular (0.1 m length, 0.02 m width and 0.95 m height) and (0.1 m length, 0.02 m width and 2.6 m height) slurry bubble columns. The superficial gas velocity was varied in the range of (1-6 cm/s) for short column and (7-12 cm/s) for long column. For all experiments the height of liquid phase was maintained at (75 cm) for short column and (160 cm) for long column from the gas distributor and solid loading varied in 0, 5, 10, 15% vol. The experimental results showed that the gas holdup increases linearly with superficial gas velocity at both homogeneous and heterogeneous regimes but the rate of increasing is slower at the heterogeneous flow and also the volumetric mass transfer coefficient increases with superficial gas velocity for both regimes. And the results showed that the gas holdup and volumetric mass transfer coefficient decreased with increasing solid loading.
Effect of contaminants on mass transfer coefficients in bubble column and airlift contactors
Chemical Engineering Science, 2003
In this work, the e ects of surface-active contaminants on mass transfer coe cients kLa and kL were studied in two di erent bubble contactors. The oxygen transfer coe cient, kL, was obtained from the volumetric oxygen transfer coe cient, kLa, since the speciÿc interfacial area, a, could be determined from the fractional gas holdup, , and the average bubble diameter, d32. Water at di erent heights and antifoam solutions of 0.5 -100 ppm were used as working media, under varying gas sparging conditions, in small-scale bubble column and rectangular airlift contactors of 6.7 and 0:85 × 10 −3 m 3 capacity, respectively. Both the antifoam concentration and the bubble residence time were shown to control kLa and kL values over a span of almost 400%. A theoretical interpretation is proposed based on modelling the kinetics of single bubble contamination, followed by sudden surface transition from mobile to rigid condition, in accordance with the stagnant cap model. Model results match experimental kL data within ±30%. ?