Loop Airlift Reactor Research Articles

3‐D Simulations of an Internal Airlift Loop Reactor using a Steady Two‐Fluid Model

AbstractThree‐dimensional (3‐D) simulations of an internal airlift loop reactor in a cylindrical reference frame are presented, which are based on a two‐fluid model with a revised k‐ϵ turbulence model for two‐phase bubbly flow. A steady state formulation is used with the purpose of time saving for cases with superficial gas velocity values as high as 0.12 m/s. Special 3‐D treatment of the boundary conditions at the axis is undertaken to allow asymmetric gas‐liquid flow. The simulation results are compared to the experimental data on average gas holdup, average liquid velocity in the riser and the downcomer, and good agreement is observed. The turbulent dispersion in the present two‐fluid model has a strong effect on the gas holdup distribution and wall‐peaking behavior is predicted. The CFD code developed has the potential to be applied as a tool for scaling up loop reactors.

Local Hydrodynamics Modeling of a Gas−Liquid−Solid Three-Phase Airlift Loop Reactor

A three-dimensional (3D) transient model was developed to simulate the local hydrodynamics of a gas-liquid-solid (GLS) three-phase airlift loop reactor (ALR) using the computational fluid dynamic (CFD) method, where the multiple size group (MUSIG) model was adopted to determine the bubble size distribution. Model simulation results such as the local time-averaged gas holdups and axial liquid velocities were validated by experimental measurements such as conductivity probes and laser doppler anemometer (LDA), under varied operating conditions, e.g., superficial gas velocities or initial solid loadings and at different locations of the ALR. Furthermore, the local transient hydrodynamics such as gas holdups, solid holdups, liquid velocities, as well as bubble size distribution were predicted reasonably by the developed model.

Epoxidation of Cyclohexene with Oxygen in Ultrasound Airlift Loop Reactor

Epoxidation of cyclohexene to cyclohexene oxide was studied in a new type reactor–the ultrasound airlift loop reactor. The influences of ultrasound intensity, molar ratio of isobutyraldehyde to cyclohexene and oxygen gas flow rate on the conversion of cyclohexene and selectivity of cyclohexene oxide were investigated and discussed, and the optimal operation condition was found, under which 95.2% conversion of cyclohexene and 90.7% selectivity of cyclohexene oxide were achieved. The ultrasonic airlift loop reactor utilizes the synergistic effect of sonochemistry and higher oxygen transfer rate. Possible reaction mechanisms were outlined and the reason of ultrasound promotion of epoxidation reactionwas analyzed.

Removal of Di- n-butyl Phthalate Using Immobilized Microbial Cells

The biodegradation of di- n-butyl phthalate (DBP) using immobilized microbial cells was carried out in an internal airlift loop reactor with ceramic honeycomb supports. A strain that is capable of degrading DBP was isolated from the activated sludge and identified as Bacillus sp. using 16S rDNA sequential analysis. Bacillus sp. could be rapidly attached onto the ceramic honeycomb supports. The immobilized cells could effectively degrade DBP in batch and continuous experiments. When the influent concentration of DBP was 50mg.L -1, the effluent DBP reached less than 1mg.L -1 with 6h hydraulic retention time (HRT) in continuous experiment. The immobilized microbial cells could grow and accumulate through the biodegradation of DBP, and the rate of degradation is accordingly increased. The possible pathway of DBP biodegradation using immobilized cells was tentatively proposed.

Modeling of oxygen mass transfer in a gas–liquid airlift reactor

AbstractThis article focuses on the physical modeling and numerical simulation of mass transfer in two‐phase bubbly flow in an airlift internal loop reactor. The objective of this article is to show the ability of computational fluid dynamics (CFD) to correctly simulate mass transfer in such a bubbly reactor. The modeling of two‐phase bubbly flow is based on the so‐called two‐fluid model derived from Reynolds‐averaged Navier–Stokes equations in two‐phase flow. From the hydrodynamic perspective, the flow is steady state. Given the steady‐state distributions of phases, interfacial area, and velocity field in the whole volume of the airlift, mass transfer is computed and the evolution of oxygen concentration in the two phases is predicted. Numerical simulations are discussed after comparison to experimental data. The simulations are validated in terms of oxygen concentration in the liquid vs. time. Then, different points are discussed, in particular, the perfectly mixed reactor assumption in the liquid phase and the spatial and temporal heterogeneity of oxygen concentration in the gas arising from oxygen impoverishment in bubbles in the downcomer. This leads to heterogeneity of the transfer driving force between the gas and the liquid. Bubble age or residence time in the airlift loop reactor has been calculated to show the weak renewal of oxygen in bubbles in the downcomer. This discussion generates questions on the estimation of a global mass transfer coefficient from experiments in such a heterogeneous airlift reactor. © 2007 American Institute of Chemical Engineers AIChE J 2007

Analysis and Measurement of Mass Transfer in Airlift Loop Reactors

Inter-phase mass transfer is important to the design and performance of airlift loop reactors for either chemical or biochemical applications, and a good measurement technique is crucial for studying mass transfer in multiphase systems. According to the model of macro-scale mass transfer in airlift loop reactors, it was proved that the airlift loop reactor can be regarded as a continuous stirred tank reactor for measuring mass transfer coefficient. The calculated mass transfer coefficient on such a basis is different from the volumetric mass transfer coefficient in the macro-scale model and the difference is discussed. To describe the time delay of the probe response to the change of oxygen concentration in the liquid phase, a model taking into account the time constant of response is established. Sensitivity analysis shows that this model can be used to measure the volumetric mass transfer coefficient. Applying this model to the measurement of volumetric mass transfer coefficient in the loop reactor, results that coincide with the turbulence theory in the literate were obtained.

Mathematical models for macro-scale mass transfer in airlift loop reactors

Inter-phase mass transfer is an important issue for design and development of airlift loop reactors of high performance in either chemical or biochemical applications. In this work, the axial dispersion in both gas and liquid phases was taken into account for modeling the macro-scale mass transfer in airlift loop reactors. Finite difference method was used to numerically solve the differential equation system of the mass transfer model. For oxygen, the numerical results showed that the solute concentration of the gas phase can be treated as constant and the flow pattern of the liquid phase as a plug flow. Based on the conclusion obtained from numerical solution, the mass transfer model was simplified and the analytical solution of the simplified model was obtained. Comparison between the numerical and analytical solutions showed that the simplification of the model was reasonable and there was almost no influence on the calculated results. Experimental measurements on the mass transfer rate were carried out to verify the mathematical model. The comparison between the experimental and calculated results showed that the mass transfer model has satisfactory prediction ability and can be used to describe the mass transfer process in airlift loop reactors.

Flow mixing model of liquid phase in an internal airlift loop reactor

Residence time distribution (RTD) analysis of liquid phase was conducted in an internal airlift loop reactor (AL) and a bubble column (BC) with the tracer response technique. These data were simulated and compared through several flow mixing models. The modeling results of two-parameter model indicated that there were higher ratio of full mixing zones and lower ratio of bypass flow in AL than in BC. Then a completely mixed-plug flow parallel combined (four-parameter) model was established. Modeling results show that it is more precise and more obvious than two-parameter model.

LOCAL LIQUID SIDE MASS TRANSFER MODEL IN AIRLIFT LOOP REACTOR AND SELF-ASPIRATED REVERSED FLOW JET LOOP REACTOR

ABSTRACT Combining Higbie's penetration theory and Kolmogoroff's theory of isotropic turbulence, a small-scale isotropic mass transfer model for the local liquid side mass transfer coefficients in a gas-liquid two-phase airlift loop reactor and a self-aspirated reversed flow jet loop reactor has been developed. Good agreement with the experimental data suggests a general applicability of the proposed model in these reactors.

Mathematical modeling of the residence time distribution in loop reactors

The axial dispersion model was applied to the riser and down-comer, respectively, of an airlift loop reactor and a mathematical model was developed to describe its residence time distribution (RTD). A numerical solution was first obtained for the case where there is no gas–liquid separator. A tracer impulse was considered to flow in an infinite tube comprising alternating riser, gas–liquid separator and down-comer. The tracer concentration at a point in the reactor is the sum of all the tracer concentrations at the corresponding points in the infinite tube. Based on this model, an analytical solution was obtained for the mathematical model. The comparison between the analytical solution and experimental data shows that the model can be used to simulate the RTD in airlift loop reactors. By using the analytical solution in a loop reactor with partial recirculation, the solution with a recycle factor is obtained. A comparison with data in the literature shows that the solution with the recycle factor can be used for the RTD in a loop reactor with partial recirculation.

Mechanical stress on suspended particles in two- and three-phase airlift loop reactors and bubble columns

The effect of power input, fluid phase viscosity and solids loading on the mechanical stress on suspended particles was examined. Experiments were carried out in an airlift loop reactor and a bubble column operated in two- and three-phase mode. The disintegration of a shear sensitive floc system was observed with an optical in-line particle system analyser and information about the mechanical stress was obtained by means of mathematical analysis of the raw data. The volumetric power input has been derived to be the governing factor and a linear dependence between volumetric power input and resulting mechanical stress was observed. The addition of a solid phase leads to a drastical change of mechanical stress on the particles with a dominating increase of the stress at high solids loadings. Likewise higher fluid phase viscosity leads to higher mechanical stress on the floc system.

Contribution to the study of hydrodynamics and gas–liquid mass transfer in a two- and three-phase split-rectangular airlift reactor

Hydrodynamics and gas–liquid mass transfer in a rectangular airlift loop reactor with an internal baffle has been studied in this work. Measurements were carried out for two- and three-phase systems using air–water and air–water–polystyrene particles. The effects of the superficial gas velocity and solid loading up to 15% (v/v) were examined and correlations describing the hydrodynamics and gas–liquid mass transfer behaviours were proposed. The experimental results of mass transfer were compared with those of the literature. It was found in all cases that the gas hold-up in the riser and downcomer and volumetric mass transfer coefficient increased with increasing the superficial gas velocity, while the liquid velocity in the downcomer increased with U gr until 0.033 m s −1 and then decreased to reach a plateau value for U gr > 0.04 m s −1. It was also found that the liquid velocity and the volumetric mass transfer coefficient decreased only with the initial addition of solid for U gr higher than 0.02 m s −1 and that the gas hold-up in the downcomer was significantly influenced by the presence of solid for a concentration above 10% (v/v), while the gas hold-up was almost not affected in the riser. The aeration efficiency was also calculated and was found to be in the range 2.54–4.41 kg O 2/kW h.

Three‐phase airlift internal loop reactor: correlations for predicting the main fluid dynamic parameters

AbstractThree‐phase airlift loop reactors have many industrial applications. Examples include applications in the chemical, metallurgical, biochemical and minerals‐processing industries; fluid dynamics in such systems is a critical factor affecting efficiency. The experimental work was carried out at a pilot‐plant scale in a tank with a cylindrical shape and conical bottom (height 1.25 m; diameter 0.42 m), water and air were used as liquid and gas phases, and for different solid phases, different loads of glass spheres (diameters: 0.25 and 1 mm; density 2.6 g cm−3), and polystyrene cylinders (diameter and length 3 mm; density 1.0 g cm−3) were introduced. Air was injected through the bottom of the tank by means of 12 nozzles (diameter 1 mm each). An internal draft tube riser was tested on different configurations as its diameter was varied (44, 82, 125 and 240 mm) as was its height (1050 and 630 mm). Corresponding liquid velocities in the adjacent annular downcomer were determined by a thermal tracer technique, solid holdups were determined by conductivity methods, riser overall gas holdups were deduced from the liquid level, and riser gas holdup from manometer readings. Several adjusted correlations were considered in a method to predict the main fluid dynamic parameters (solids and gas holdups, and superficial liquid velocities). Copyright © 2005 Society of Chemical Industry

Oxygen Transfer in a Baffled Rectangular Air‐Lift Loop Reactor

AbstractThe objective of this study is to characterize mass transfer in a rectangular air‐lift loop reactor in two‐phase flow. In a previous work, it has been shown that the reactor presents a complex gas flow pattern. Therefore, first, the global mass transfer volumetric coefficient kLa was measured in two‐phase flow, by three methods (two based on the liquid phase mass balance, one based on the gas phase mass balance). Then, second, a localized analysis was implemented in order to obtain more information about the phenomena governing the gas phase flow.

Critical gas velocity in a three‐phase internal loop airlift reactor with low‐density particles

AbstractIn the present investigation the effects of the addition of organic additives (propanol, benzoic acid, iso‐amyl alcohol and carboxymethyl cellulose) on the critical gas velocity, (Usg)c, in an internal airlift loop reactor with low‐density particles (Nylon‐6 and polystyrene) were reported. Whereas the (Usg)c was reduced by adding the above additives, it increased with solids loading and density of the particles. The draft tube‐to‐reactor diameter ratio (DE/D) in the range of 0.5–0.6 gave minimum (Usg)c values. The proposed dimensionless correlation predicted the experimental data well. Copyright © 2004 Society of Chemical Industry

Hydrodynamic and mass transfer study in a rectangular three-phase air-lift loop reactor

This study concerns a three-phase rectangular air-lift reactor. The first part deals with the reactor’s gas–liquid two-phase flow hydrodynamics. Parameters, such as the gas hold-up, the liquid velocity, the mixing parameters, and the solid hold-up, were quantified in the presence of two different plastic solids—a regular shape solid called P1, and an heterogeneous shape solid, P4. No fundamental influence of the solids on the studied parameters was noticed, except for the gas hold-up. Indeed, a bubble coalescence phenomenon was highlighted for high solid concentrations ( ε s > 16%). Moreover, a comparison of these two materials led to the conclusion that P4 exhibited better hydrodynamic performances. In a second set of experiments, the oxygen mass transfer was characterised in three-phase flow, with both kinds of plastic materials. The k La coefficient was deduced from a mass balance on the gas phase. It was found to be weakly influenced by the presence, the nature or the quantity of solid.

IAL-CHS (internal airlift loop - ceramic honeycomb supports) reactor used for biodegradation of 2,4-dichlorophenol and phenol

The internal airlift loop reactor with ceramic honeycomb supports (IAL-CHS) was applied for biodegradation of 2,4-dichlorophenol (2,4-DCP) and phenol. A strain of DCP-degrading bacteria isolated from activated sludge, Achromobacter sp., was rapidly immobilized onto the ceramic honeycomb supports. The immobilized cells effectively biodegraded 2,4-DCP alone and together with phenol in batch and continuous-flow experiments. For example, 2,4-DCP was biodegraded from an influent concentration of 50 mg/L to less than 1 mg/L with a 6-h hydraulic retention time (HRT) in continuous flow tests. The immobilized biomass grew and accumulated through 2,4-DCP biodegradation, and the rate of degradation increased accordingly.

Optimization of eicosapentaenoic acid production by Phaeodactylum tricornutum in the flat panel airlift (FPA) reactor

Biomass and eicosapentaenoic acid (EPA) productivities were investigated in a flat panel airlift loop reactor ideally mixed by static mixers. Growth with ammonium, urea and nitrate as nitrogen source were performed at different aeration rates. Cultures grew on ammonium but the decay of pH strongly inhibited biomass increase. On urea biomass productivity reached 2.35 g L−1d−1at an aeration rate of 0.66 vvm (24 h light per day, 1000 μmol photon m−2s−1). Aeration rates between 0.33 vvm and 0.66 vvm and maximal productivities on urea were linearly dependent. Productivity on nitrate never exceeded 1.37 g L−1d−1. In the range of maximum productivity photosynthesis efficiency of 10.6% was reached at low irradiance (250 μmol photon m−2s−1). Photosynthesis efficiency decreased to 4.8% at 1000 μmol photon m−2s−1. At these high irradiances the flat panel airlift reactor showed a 35% higher volume productivity than the bubble column. At continuous culture conditions the influence of CO2concentration in the supply air was tested. Highest productivities were reached at 1.25% (v/v) CO2where the continuous culture yielded 1.04 g L−1d−1(16 h light per day, 1000 μmol photon m−2s−1). The average EPA content amounted to 5.0% of cell dry weight, that resulted in EPA productivities of 52 mg L−1d−1(continuous culture, 16 h light per day) or 118 mg L−1d−1(batch culture, 24 h light per day).

High-carbohydrate wastewater treatment by IAL-CHS with immobilized Candida tropicalis

A novel immobilized-cell bioreactor was developed and utilized for treatment of high-carbohydrate wastewater and production of single-cell protein (SCP) simultaneously. The system is the internal airlift loop reactor with ceramic honeycomb support for biofilm (IAL-CHS), and the microorganisms tested was Candida tropicalis, which has a high protein yield. The optimum temperature for C. tropicalis was 35–40 °C, and bioreactor experiments were performed at 35 °C. The internal airlift loop reactor with ceramic honeycomb supports allowed a large and dense accumulation of biomass, but also generated significant SCP. Mass-transport resistance did not limit the overall COD-removal kinetics and oxygen mass transfer may have been accelerated to the biofilm. Fermentation of carbohydrates to organic acids lowered the pH below the optimum range of 4.5–6.5, and pH below 4.5 retarded or stopped the oxidation reactions for the organic acids. Since organic-acid oxidation is necessary for attaining COD removal from the wastewater, pH control may be essential for effective treatment in some cases with high-carbohydrate wastewaters. The addition of 0.35 g/L urea could increase the COD removal rate. Continuous wastewater was carried out and the organic loading reached 15 kg COD/(m 3 day).

High-carbohydrate wastewater treatment by IAL-CHS with immobilized Candida tropicalis

A novel immobilized-cell bioreactor was developed and utilized for treatment of high-carbohydrate wastewater and production of single-cell protein (SCP) simultaneously. The system is the internal airlift loop reactor with ceramic honeycomb support for biofilm (IAL-CHS), and the microorganisms tested was Candida tropicalis, which has a high protein yield. The optimum temperature for C. tropicalis was 35–40 °C, and bioreactor experiments were performed at 35 °C. The internal airlift loop reactor with ceramic honeycomb supports allowed a large and dense accumulation of biomass, but also generated significant SCP. Mass-transport resistance did not limit the overall COD-removal kinetics and oxygen mass transfer may have been accelerated to the biofilm. Fermentation of carbohydrates to organic acids lowered the pH below the optimum range of 4.5–6.5, and pH below 4.5 retarded or stopped the oxidation reactions for the organic acids. Since organic-acid oxidation is necessary for attaining COD removal from the wastewater, pH control may be essential for effective treatment in some cases with high-carbohydrate wastewaters. The addition of 0.35 g/L urea could increase the COD removal rate. Continuous wastewater was carried out and the organic loading reached 15 kg COD/(m3 day).