External Loop Airlift Research Articles

TRIPHASIC EXTERNAL-LOOP AIRLIFT REACTORS. HYDRODYNAMIC AND DISPERSION STUDIES

Considering that three-phase dispersions are common in chemical engineering and biotechnology, and with increasing airlift reactor applications to chemical and biotechnological processes, a experimental study was undertaken in order to investigate the liquid phase hydrodynamics and dispersion in an external-loop airlift reactor on laboratory scale. Experiments were conducted in order to find a relationship between relevant hydrodynamic and mixing parameters of a solid-suspended external-loop airlift reactor of laboratory scale, and operating variables such as gas velocity, solids loading and solids density, which should be the basis for extending the study to larger scale contactors. It was found that these parameters affect the hydrodynamic characteristics of the investigated external-loop airlift reactor and empirical correlations for gas holdup and liquid circulation velocity were obtained as functions on the above-mentioned operating variables. The lateral and axial mixing of dispersed particles in a circulating fluidized bed were measured by using the phosphor tracer technique. The dispersion model satisfactorily described the measured residence time distribution.

Experimental and Numerical Simulation for Gas−Liquid Phases Flow Structure in an External-Loop Airlift Reactor

A gas−liquid external-loop airlift reactor with a riser 0.47 m in diameter and 2.5 m in height and two external-loop down-comers 0.08 m in diameter and 2.5 m in height were used to investigate the gas−liquid two-phase flow structure. Local phase holdups were measured simultaneously by a microconductivity probe with air as the gas phase and water as the liquid phase over a wide range of operation conditions. Liquid flow velocity measurements were performed using the electrolyte tracer measurement (ETM) technique. The hydrodynamics near the sparger zone, riser disengagement zone (zone 1), junction zone (zone 2), and down-comer disengagement zone (zone 3) were systematically examined using the CFDs at the local scale and at the riser scale, respectively. The simulation results showed that zones 1, 2, and 3 exhibit three different flow regimes, which were the secondary mixed flow regime, the mixed flow regime, and the homogeneous bubble regime, respectively. It was also indicated that turbulent kinetic energy...

Experimental study and modeling on liquid dispersion in external-loop airlift slurry reactors

Abstract Liquid dispersion in an external-loop airlift slurry reactor was experimentally studied. The effects of the superficial gas velocity, concentration of fine particle, flowing resistance, and liquid level in the gas–liquid separator on the liquid dispersion coefficient were investigated. A liquid dispersion model was proposed based on Taylor dispersion equation to predict the liquid dispersion coefficient. According to this model, the dispersion coefficient and the combination factor u L 2 e L have a linear relationship. Validation of the liquid dispersion model together with the hydrodynamic model in our previous work was made by comparing the experimental and predicted results. The good agreement showed that the models could predict the liquid dispersion coefficient and the hydrodynamic behaviors of an airlift slurry reactor in a wide range of operating conditions with a satisfactory accuracy.

EFFECTS OF AERATED LIQUID LEVEL ABOVE DOWNCOMER ON HYDRODYNAMIC CHARACTERISTICS OF AN EXTERNAL-LOOP AIRLIFT REACTOR

Effects of the aerated liquid level above the downcomer on riser superficial liquid velocity, gas flow rates, and gas holdups in individual sections were investigated in an external-loop airlift reactor. The aerated liquid level is an important operating parameter, and the valve connected to the extension tube could adjust it. As the aerated liquid level was increased up to 0.158 m, which was slightly larger than the diameter of the riser, both the liquid circulation velocity and the gas flow rate in the extension tube increased, whereas the riser gas holdup, the downcomer gas holdup, and the gas flow rate in other individual sections except the extension tube decreased. A combination of the gas-liquid separation ability and hydraulic resistance of the head region and the surface aeration could explain the impact of the aerated liquid level on the hydrodynamic characteristics. However, the effect was negligible when the aerated liquid level was higher than 0.158 m.

Treatment of phenolic wastewater in a novel multi-stage external loop airlift reactor using activated carbon

In this work, a novel multi-stage external loop airlift reactor has been designed to remove phenol from wastewater by means of its adsorption onto the surface of activated carbons. The multi-staging has been achieved by hydrodynamically induced continuous bubble generation, breakup and regeneration. The continuous rupture and bursting of bubbles creates localized turbulence and recirculation, which helps in faster transfer of trace pollutants to the active sites of the solid adsorbents. Thus, the pollutants get adsorbed to the surface of the solid adsorbents due to continuous agitation. Different operating parameters that affect the performance of the reactor are the superficial gas velocity, liquid circulation velocity, concentration of the pollutant present in wastewater, contact time and the carbon loading. The results show that the removal time as well as the activated carbon loading for this system were quite lower as compared to simple batch adsorption systems. The kinetic data were fitted to the models of intra-particle diffusion, pseudo-second-order and Lagergren, and followed more closely the Lagergren pseudo-first-order model. Langmuir and Freundlich models were used to study the adsorption isotherms. A correlation has been developed to predict the efficiency of percentage removal of phenol and found to be highly significant from statistical point of view.

Electrocoagulation/electroflotation in an external-loop airlift reactor—Application to the decolorization of textile dye wastewater: A case study

A 20 L external-loop airlift reactor was used as an electrochemical cell in order to carry out water depollution using batch electrocoagulation (EC) without mechanical agitation, pumping requirements or air injection. Mixing and complete flotation of the pollutants were achieved using only the overall liquid recirculation induced by H 2 microbubbles generated by water electrolysis. A red dye from the Moroccan textile industry was used in a case study to validate this innovative application of airlift reactors. Experimental results showed that the axial position of the Al electrodes and the residence time in the separator section were the key parameters to achieve good mixing conditions, to avoid bubbles/particles recirculation in the downcomer and to prevent floc break-up/erosion by hydrodynamic shear forces. Such optimum conditions corresponded to an optimum liquid overall recirculation velocity that was correlated to current, electrode position and dispersion height. Operation time required to achieve 80% COD and 80% color removal efficiencies was modeled as a function of current density. Similarly, specific energy and electrode consumptions were correlated to current, electrode gap and conductivity, which provided the necessary tools for scale-up and process optimization. Operation time and removal efficiencies were similar to those reported in conventional EC cells, but specific energy and electrode consumptions were even smaller without the need for mechanical agitation, pumping requirements and air injection, which could not be achieved in other kinds of conventional gas–liquid contacting devices.

Mass transfer characteristics of a novel multi-stage external loop airlift reactor

This paper reports on the experimental investigation carried out to evaluate the interfacial area, a, and liquid-side volumetric mass transfer coefficient, kLa, to characterize a multi-stage external loop airlift reactor (with contraction and expansion disks), which has been conceived, designed, and fabricated as an equipment to remove trace pollutants from wastewater. In addition, it has versatile use as a gas–liquid contactor in chemical process industries. In the experimental range of gas and liquid flows investigated, the maximum interfacial area and volumetric mass transfer coefficient obtained was in the order of 300 m2/m3 and 0.05 s−1, respectively. Correlations developed for predicting the interfacial area and liquid-side volumetric mass transfer coefficient have been found to be encouraging and highly significant from statistical analysis.

Permeabilization of Phospholipid Bilayer Membranes Induced by Gas‐Liquid Flow in an Airlift Bubble Column

The permeability of 5(6)-carboxyfluorescein (CF) through the phospholipid bilayer membranes was measured by using the system in which the CF-containing phospholipid vesicles (liposomes) were suspended in the gas-liquid flow in an external loop airlift bubble column. The airlift was operated at various superficial gas velocities UG up to 2.4 cm/s at 25 and 40 degrees C. The CF-containing liposomes composed of POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) had the nominal diameters of 50, 100, and 200 nm. The 50- and 100-nm liposomes were stable at 40 degrees C for 5 h even at a high UG value of 2.4 cm/s based on the observed turbidity of the liposome suspension in the airlift. On the other hand, the 200-nm liposomes were stable at a low UG value of 1.4 cm/s, although a progressive decrease in size of the liposomes was implied at the high UG value of 2.4 cm/s. The permeability coefficient PCF of CF through the lipid membrane of the 100-nm liposomes was significantly increased with increasing UG at a high temperature of 40 degrees C, while at a low temperature of 25 degrees C the PCF value was little dependent on UG. As a typical result on the above liposomes, the PCF value (9.2 x 10(-11) cm/s) at 40 degrees C and UG = 2.4 cm/s in the airlift was more than 15 times larger than that at 25 degrees C in the static liquid corresponding to UG = 0. In addition, the dependence of the PCF value on UG at 40 degrees C became more significant with increasing the size of liposomes suspended. The results obtained revealed that the permeability of the liposome membranes could be regulated by suspending the liposomes in the gas-liquid flow in the airlift without modulating the membrane composition of liposomes.

Effects of Sparging Gas Properties and Substrate Size on Enzymatic Hydrolysis of Waste Paper in an Ultrasonic External Loop Airlift

AbstractOffice paper shredded into various sizes was used as a model substrate for waste paper in the present work, aimed at developing an efficient utilization for waste cellulosics. The enzymatic hydrolysis of the office paper was carried out in an external loop airlift bubble column reactor equipped with an ultrasonic horn system, by sparging with water‐saturated or unsaturated gas. The enhanced hydrolysis was characterized by a two‐stage time course when either water‐unsaturated air or nitrogen was bubbled into the reaction suspension in the airlift reactor, without ultrasonic irradiation. In contrast, a much lower enhanced effect was observed when either saturated air or nitrogen was employed. The total sugar productivity with sparging by either saturated air or nitrogen was lower than in the case of sparging by unsaturated gas. This was ascribed to a shear stress acting on the surface of the paper in the gas sparging section in the riser. The critical superficial gas velocity for complete circulation of the substrate was reduced as its size was decreased. The smaller paper size for sparging with unsaturated air resulted in a more enhanced hydrolysis with a more remarkable two‐stage time course. In the case of sparging with saturated air, the continuous ultrasonic irradiation was effective for enhancing the enzymatic hydrolysis. The time courses of the glucose and total sugar concentrations were successfully analyzed and simulated, based on the kinetic model proposed previously. The variation in the paper size, sparging gas humidity and ultrasonic irradiation exerted an effect on the apparent rate constant and ultimate total sugar concentration, but had no effect on the apparent Michaelis, competitive inhibition and equilibrium constants.

Effect of Suspended Liposomes on Hydrodynamic and Oxygen Transfer Properties in a Mini‐Scale External Loop Airlift Bubble Column

AbstractThe circulating liquid velocity, gas holdup, bubble size distribution, and liquid phase oxygen transfer coefficient were measured in a mini‐scale external loop airlift bubble column (MELBC) with the liquid volume suspending enzyme‐free liposomes of varying diameters. These values were compared to those for liposome‐free MELBC, normal bubble column (NBC), and a larger scale airlift column. The liposomes suspended in the MELBC are incorporated into the liquid film around the bubbles, leading to the development of a foam layer, where the incorporated liposomes exert negligible effect on the oxygen transfer in the film.

CFD Simulation of Flow and Axial Dispersion in External Loop Airlift Reactor

In the present work, a CFD simulation, using two-fluid model, has been performed to obtain the flow pattern and the gas hold-up in external loop air-lift reactors (EL-ALR). An agreement was observed between the predicted and the experimental data available in the published literature. The validated CFD model has been extended for the simulation of mixing and the axial dispersion coefficient. CFD predictions of mixing time are well in agreement with the experimental values published in literature, and the results of axial dispersion coefficient also show good agreement with the experimental values published in literature except in the case of Vial et al. (2005), where the CFD model underpredicts. A systematic numerical study was undertaken to understand the relative contribution of convection and eddy diffusion to the mixing and axial dispersion.

Flow behavior and mass transfer in three-phase external-loop airlift reactors with large particles

The flow behavior and mass transfer in a three-phase external-loop airlift reactor can be improved by adding large particles. The mass transfer and liquid dispersion behavior for a three-phase external-loop reactor with large particles are studied in terms of the effect of the diameter and loading of the large particles on the liquid dispersion coefficient and mass transfer coefficient. The results showed that increasing the diameter or loading of the large particles tend to decrease dispersion and intensify mass transfer, and that an increase in the diameter of the large particles remarkably decreases the particle loop rate, while the effect of fine particles is much less notable.

Glucose Oxidation Catalyzed by Liposomal Glucose Oxidase in the Presence of Catalase-Containing Liposomes

A catalase-containing liposome (CAL) was prepared and characterized in terms of stability during storage and catalysis of the decomposition of hydrogen peroxide (H2O2) that was initially added or produced in the oxidation of glucose catalyzed by the glucose oxidase-containing liposomes (GOL). The reactors used were a test tube and an external loop airlift bubble column as the static liquid and circulating liquid flow systems, respectively. The free catalase (CA) at low concentrations was unstable during storage at 4 degrees C as a result of dissociation of the tetrameric CA subunits. On the other hand, the deactivation of the CA activity in the CAL was depressed because of the high CA concentration in the CAL liposome. The CAL effectively catalyzed the repeated decompositions at 25 degrees C with 10 mM H2O2 added initially, whereas the free CA was significantly deactivated during the repeated reactions. The high stability of the CAL was attributed to the moderately depressed reactivity, which was essentially derived from the diffusion limitation of the CAL membrane to H2O2 in the liquid bulk. In the GOL-catalyzed prolonged oxidation of 10 mM glucose at 40 degrees C in the static liquid in a test tube, both the free CA and CAL could continuously catalyze the decomposition of H2O2 produced. This was because the glucose oxidation rate was small due to the limited reactivity of the GOL to glucose with its low permeability through the GOL membrane. In the glucose oxidation catalyzed by the GOL with the free CA or the CAL in the airlift, much larger oxidation rates were observed compared to those in the test tube because the permeability of the GOL membrane to glucose was increased in the gas-liquid two phase flow in the airlift. The GOL/CAL system in the airlift operated in an acidic condition, which was preferable to the GO activity, gave the largest oxidation rate with negligible accumulation of the H2O2 produced. On the other hand, the GOL/free CA system gave an oxidation rate smaller than that of the GOL/CAL system even under the acidic condition due to an unfavorable interaction of the free CA molecules with the GOL membranes leading to the decreased reactivity of the GOL.

Continuous bioremediation of phenol‐polluted air in an external loop airlift bioreactor with a packed bed

AbstractAn external loop airlift bioreactor with a small amount (99% porosity) of stainless steel mesh packing inserted in the riser section was used for bioremediation of a phenol‐polluted air stream. The packing enhanced volatile organic chemical and oxygen mass transfer rates and provided a large surface area for cell immobilization. Using a pure strain of Pseudomonas putida, fed‐batch and continuous runs at three different dilution rates were completed with phenol in the polluted air as the only source of growth substrate. 100% phenol removal was achieved at phenol loading rates up to 33 120 mg h−1 m−3 using only one‐third of the column, superior to any previously reported biodegradation rates of phenol‐polluted air with 100% efficiency. A mathematical model has been developed and is shown to accurately predict the transient and steady‐state data. Copyright © 2006 Society of Chemical Industry

Optimal Preparation of Immobilized Liposome-Bound Cellulase for Hydrolysis of Insoluble Cellulose in an External Loop Airlift Bioreactor

Immobilized liposome-bound cellulase (ILC) was optimally prepared for the ILC-catalyzed hydrolysis of insoluble cellulose in an external loop airlift bioreactor. The liposomes with mean diameters of 200, 100, and 50 nm were used to prepare three kinds of ILCs, i.e., ILC(200), ILC(100) and ILC(50), respectively. The activity and stability of ILC(100) were examined with soluble cellulose (CMC) in addition to the insoluble substrate of cellulose powder (CC31) in a shaking flask as well as the airlift bioreactors. The experiments were carried out with 45 degrees C and pH 4.8 being found to be optimal for the activity. The activity of ILC(100) was stable in either airlift or shaking flask bioreactor during the five times repeated hydrolyses of CC31 corresponding to a total reaction time of 240 h. This confirmed that the cellulase molecules were covalently bonded to the liposomes covalently bound to the chitosan gel beads. Nevertheless, the activity of ILC(100) with CMC steadily decreased throughout the repeated reactions, suggesting an adverse effect of CMC on the ILC(100) activity. Among the three ILCs, ILC(50) was found to be the most stable and productive biocatalyst during the repeated hydrolyses of insoluble CC31 in the airlift bioreactor. More than 70% of the initial activity of ILC(50) was retained even after the six times repeated reactions for 288 h. Conversely, the ILC(200) was found to be the most unstable catalyst. Such a difference in stability among these ILCs was suggested to be caused by the difference in physical stability of their liposome membranes to the liquid shear stress due to the rising bubbles and circulating liquid as well as that in the amount of the cellulase molecules unstably incorporated in the membranes. ILC(50) was thus shown to have the most potential for an efficient hydrolysis of insoluble cellulose in a practical airlift bioreactor.

Hydrodynamics of a novel multi-stage external loop airlift reactor

In the present investigation a novel multi-stage external loop airlift reactor with hydro-dynamically induced continuous bubble generation, breakup and regeneration has been proposed. The system has been designed to operate with relatively large sized bubbles, so that interfacial circulation can be induced in the liquid–bubble interfaces and faster transfer of components can take place by turbulent diffusion through the interface of the bubbles and also due to the physical rupture and reformation of the bubbles. The system was also designed to operate in three stages operating in series so that in each stage completely deaerated liquid could be brought in contact with freshly generated bubbles. Detailed studies on the gas holdup and liquid circulation velocity have been carried out with respect to various values of superficial gas as well as liquid velocities. The gas holdup of the proposed multi-stage system is 45% higher than the single stage system, which results in better mass transfer characteristics. Empirical correlations describing the performance of the proposed reactor have been presented in this paper.

Volatile Organic Chemical Mass Transfer in an External Loop Airlift Bioreactor with a Packed Bed

A stainless steel mesh packing with 99.0% porosity was installed in the riser section of an external loop airlift bioreactor (ELAB) to develop a new bioreactor that is a combination of a traditional ELAB and a packed-bed bioreactor. The gas holdup and mass-transfer rates of three volatile organic chemicals (VOCs) were studied in this ELAB, both with and without the packing. When the packing was used, the overall volumetric mass-transfer coefficient increased to values of 0.005 and 0.004 s-1, an average increase of 65.1% and 33.4% for toluene and benzene, respectively. The packing increased gas holdup and decreased bubble size in the bioreactor, both contributing to the improvements in the mass-transfer rates. A difference was observed between absorption and desorption rates of VOCs, which was explained by the change in gas bubble sizes in the presence of VOCs. A dynamic spatial model was used to predict the transient concentration distribution in the ELAB with and without a packed bed. The mass-transfer c...

Solid effects on hydrodynamics and heat transfer in an external loop airlift reactor

The effect of a solid presence on global hydrodynamic parameters and heat transfer in an external loop airlift reactor has been experimentally investigated. Results obtained in both two- and three-phase flow are presented in this study. Two different external loop airlift reactor sizes have been used and local hydrodynamic characteristics including local gas hold-up and bubble velocity have been obtained in two-phase flow. Optical and ultrasound probes have been used to obtain this information, respectively. It was found that an increase of solid hold-up leads to a decrease of liquid velocity and heat transfer coefficient. Measured in a two- and three-phase reactor using a horizontal-heating probe, a correlation of the average gas hold-up and heat transfer coefficient is proposed. Correlation parameters are identified in homogeneous and heterogeneous flow regimes, which have been derived from the gas slip velocity concept. The experimental liquid velocity and gas hold-up in the riser have been represented in a satisfactory way by a hydrodynamic model, either in the absence or in the presence of solid particles.

Hydrodynamic and oxygen mass transfer in an external loop airlift bioreactor with a packed bed

A stainless steel mesh packing with 99.0% porosity has been inserted in the riser section of an external loop airlift bioreactor (ELAB). The hydrodynamic characteristics and oxygen mass transfer rates of the ELAB, both with and without packing, were compared. The packing increased the overall volumetric oxygen mass transfer coefficient by an average factor of 2.45 compared to the unpacked column. The packing increased gas holdup, decreased bubble size, and decreased liquid circulation rates in the bioreactor, all of which contributed to the dramatic improvement in the oxygen mass transfer rates. A dynamic, spatial model was used to predict the transient, oxygen concentration distribution in the ELAB with and without a packed bed. This model was compared to simulating the ELAB as a completely stirred reactor and demonstrated improved prediction of the cyclical changes in liquid oxygen concentrations. The oxygen mass transfer coefficient was determined as a best fitting parameter of the model and at higher gas superficial velocities was found to increase to values approaching 0.021 s −1 using the small amount of packing. Finally, simplified correlations were developed to predict the oxygen mass transfer coefficient in the ELAB with and without the packed bed.

External-loop airlift magnetically stabilized bed—minimum stabilization and fluidization conditions

Experimental study of an airlift with a magnetically stabilized bed in the riser bottom has been performed. External magnetic field allows easy control of magnetized bed structure and liquid circulation rate. Minimum stabilization and fluidization conditions have been determined experimentally and by a three-line graphical method. Semi-empirical data correlations of sections of the experimental curves have been performed. Scaling relationships known from non-magnetic airlift are applicable too, but with the assumption that the magnetic field affects the loop friction coefficient only.