External Loop Airlift Reactor Research Articles

Experimental Study and CFD-PBM Simulation of the Unsteady Gas-Liquid Flow in an Airlift External Loop Reactor

The performance of external loop airlift reactor (ELALR) strongly depends on local prevailing hydrodynamics. In this work, the hydrodynamics characterization of the gas-liquid flow is performed in a laboratory scale ELALR. The bubbles plume and the velocity field of the riser are captured using Particle Image Velocimetry (PIV) technique and a three-dimensional CFD-PBM simulation is implemented to provide a full description of the flow structure. The effects of superficial gas velocity on the flow structure and the time-averaged liquid velocity are quantitatively investigated and the numerical results are in reasonable agreement with the PIV measurement. The results show that the gas-liquid two-phases flow in the riser is always unsteady, and the bubble plume meandering behavior is observed clearly. Gas holdup increases with the increase of superficial gas velocity, while the liquid axial velocity shows a trend of first increase and then decrease and the maximum is observed at superficial gas velocity of 30 mm/s. With the increase of superficial gas velocity, the probability density function curve becomes flatter, and changes from unimodal to bimodal. This work helps to gain in-depth understanding of the complex fluid hydrodynamics behavior inside an ELALR.

Global hydrodynamic of hybrid external loop airlift reactor: Experiments and CFD modelling

In this study, the mixing and mass transfer of air-water system were characterized in hybrid external loop airlift reactor using the computational fluid dynamics (CFD). This hybrid configuration involves the advantages of both airlift and torus reactors. To predict the flow in hybrid geometry, an Eulerian-Eulerian approach has been adopted with per-phase interaction of the standard k–ε turbulence model. To simplify the solution, the bubbles were considered to have approximately the same size. The global flow characteristics were experimentally determined by a conductimetric method. Gassing-out method was applied to determine gas-liquid mass transfer coefficients. The findings highlighted a uniform gas flow distribution in the riser tube, despite the riser included two different sections. It was noticed that beyond the superficial gas velocity, 0.0253 m/s, the fine rotational bubbles were dragged away by the liquid recirculation in the downcomer section. The STD k–ε turbulence model with per-phase effect, estimated correctly the hydrodynamic of homogenous and transition regimes except for high superficial gas velocity, Ug = 0.0703 m/s. For superficial gas velocities ranging from 0.0253 to 0.0703 m/s, there was a good agreement between simulated and experimental values; the error was less than 6%. Optimal conception of this hybrid geometry and its industrial scale use need further research and testing.

Influence of the geometry on the riser gas holdup in an external-loop airlift reactor: empirical and neural network modelling

Experimental studies on the effect of geometrical characteristics (restriction orifice free area and sparger configuration) on the riser gas holdup of an external-loop airlift reactor have been conducted in this paper. The impact of the addition of non-coalescing media, such as n-butanol, was also investigated. In general, the decrease in the restriction orifice free area led to a substantial increase (up to 90%) in the riser gas holdup. The addition of n-butanol effected gas holdup to a lesser extent, as it increased the gas holdup in the range of 4-15%, depending on the restriction orifice free area. On the other hand, the impact of the sparger type was observed only at lower gas velocities, for which the sinter plate increased the gas holdup up to 20% in comparison to the single orifice. The paper also presents an empirical correlation for the estimation of the riser gas holdup with an average relative error of 7%. Finally, a neural network model with an average relative error of 2.4% was proposed.

Novel external-loop CO2-lift reactor for desilication of alkaline/surfactant/polymer produced water from oil well

Desilication is one of the most important pretreatment processes that prevent membrane fouling. In addition, it also increases the membrane flux during the filtration of alkaline/surfactant/polymer produced water. We developed a novel external-loop gas-lift reactor to enhance the filtration efficiency. In the modified reactor, CO2 was pressurized through a porous titanium sintered pipe (mean pore size: 1μm) to form uniform bubbles. The fine CO2 bubbles reacted with the highly soluble silicate in water to form polysilicic acid sediments. A two-phase gas-liquid flow was maintained with high mixing. Furthermore, a liquid-falling tube was used to enhance the liquid circulation velocity as the tube separated the gas-liquid phases. A blister type draft tube was introduced in the riser to increase the gas residence time. The well-known Euler model of computational fluid dynamics and the population balance model (PBM) were used to simulate and optimize the reactor. Compared to the original reactor R-1, the novel reactor R-3 had improved the liquid circulation velocity and gas holdup by 369.8% and 25.4%, respectively. The novel reactor R-3 decreased the silica content from 452.3 to 96.7mg·L−1 in ASP produced water; the removal efficiency of the novel reactor R-3 was 6.3% greater than the conventional external-loop air-lift reactor. The novel reactor should be used in industrial water treatment plants, because it shows better compatibility and efficiency in treating the large amount of oil and silicate in wastewater.

Predicting hydrodynamic parameters and volumetric gas–liquid mass transfer coefficient in an external-loop airlift reactor by support vector regression

For modeling, design and scale-up of the airlift reactors, it is crucial to estimate hydrodynamic parameters and volumetric gas–liquid mass transfer coefficient for different flow regimes. Prediction of these variables had begun by applying empirical power low correlations and later evolved in use of the artificial neural networks (ANN) as the best option available in the literature. The objective of this study was to present the support vector regression (SVR) model that predicts the gas holdup, downcomer liquid velocity and volumetric gas–liquid mass transfer coefficient values in the external-loop airlift reactor better than ANN. Furthermore, to demonstrate the applicability of the SVR model, it was used on the different literature data sets with wide-ranging databanks. The statistical error analysis revealed that the proposed generalized SVR model had more precisely prediction than ANN with an average absolute relative error (AARE) of 2.17%, 1.32% and 9.64% for gas holdup, downcomer liquid velocity and volumetric gas-liquid mass transfer coefficient values, respectively.

Hydrodynamics of an external-loop airlift reactor with inserted membrane

Abstract The objective of this study was to investigate the hydrodynamics of an external-loop airlift membrane reactor (ELAMR). The ELAMR was operated in two modes: without (mode A) and with bubbles in the downcomer (mode B), depending on the liquid level in the gas separator. The influence of the gas distributor’s geometry and various alcohol solutions on the hydrodynamics of the ELAMR was studied. Results for the gas holdup and the downcomer liquid velocity are commented with respect to an external-loop airlift reactor of the same geometry but without the membrane in the downcomer (ELAR). Due to the presence of the membrane in the downcomer, acting as the local hydrodynamic resistance, the gas holdup in the riser of the ELAMR increases maximally by 16%, while the liquid velocity in the downcomer decreases up to 50%. The values of the gas holdup and liquid velocity predicted by the application of empirical power law correlations and a feed forward back propagation neural network (ANN) are in very good agreement with experimental values.

Enhanced hydrodynamics in a novel external-loop airlift reactor with self-agitated impellers

In the present work, self-agitated impellers, as a novel type of internals, have been proposed for improving hydrodynamic and mass transfer characteristics of external-loop airlift reactors. The influence of inserted self-agitated impellers, in the riser section, in various liquid phases and with different sparger types, on main hydrodynamic parameters, was studied. The results show that the insertion of impellers led to significant bubble breakage and decrease in mean bubble size, particularly in pseudoplastic liquid. Obtained riser gas holdup values were up to 47% higher, in comparison to the configuration without impellers. Higher improvements were obtained with single orifice, given that this is the least effective gas distributor. Even though impellers represent an obstacle to the flow, relatively low reduction (about 10%) in downcomer liquid velocity was observed, for all investigated cases. Having this in mind, the benefit of inserting self-agitated impellers for improving the performance of external-loop airlift reactors was apparent.

Comparison of cutting-oil emulsion treatment by electrocoagulation–flotation in bubble column and airlift reactors

ABSTRACTSeparation of nanoscale oil droplets in the cutting-oil emulsion by electrocoagulation–flotation (ECF) was carried out in a bubble column reactor (BCR) and an external-loop airlift reactor (ALR). Under the batch operation, aluminium electrode provided the highest efficiency of 99% and required the shortest separating time compared to iron and graphite electrodes. The separation performance was also affected by the electrode gap and current density due to the amount of produced aluminium ions and turbulence by bubble motions. Additionally, the ECF efficiency obtained from the ALR was similar to that of the BCR. However, the ALR was preferable owing to its lower energy consumption, less electrode sacrifice, and less sludge production. Similar results were acquired under the continuous mode; nevertheless, the highest efficiency of only 85% was achieved from both reactors. It was found that the efficiency declined with increasing flow rates. According to the results suggested by the residence time distribution (RTD), the ALR was more effective at higher flow rates since the plug flow condition can be retained. On the other hand, an increase in flow rate also provoked the bypass flow to the down-comer of the ALR, resulting in the presence of a dead zone and reduction in the treatment efficiency.

Simple model for gas holdup and liquid velocity of annular photocatalytic external-loop airlift reactor under both bubble and developing slug flow

Based on the momentum conservation approach, a theoretical model was developed to predict the superficial liquid velocity, and a correlation equation was established to calculate the gas holdup of an annular external- loop airlift reactor(AELAR)in the bubble flow and developing slug flow pattern. Experiments were performed by using tap-water and silicone oil with the viscosity of 2.0 mm2/s(2cs-SiO)and 5.0 mm2/s(5cs-SiO)as liquid phases. The effects of liquid viscosity and flow pattern on the AELAR performance were investigated. The predictions of the proposed model were in good agreement with the experimental results of the AELAR. In addition, the comparison of the experimental results shows that the proposed model has good accuracy and could be used to predict the gas holdup and liquid velocity of an AELAR operating in bubble and developing flow pattern.

Volumetric gas-liquid mass transfer coefficient in an external-loop airlift reactor with inserted membrane

The effects of the inserted membrane in the downcomer of an external-loop airlift reactor, the gas sparger type (single orifice and sinter plate) and added alcohol (ethanol, n-butanol, or n-hexanol) on the volumetric gas-liquid mass transfer coefficient (kLa) were studied. Due to the presence of the membrane in the downcomer, kLa did not change significantly; the differences were smaller than 10%. The highest values of the kLa were obtained using the sinter plate. It was found that the addition of small amounts of alcohol increased the mass transfer. Using our experimental results and the data of other authors, the feed-forward back propagation neural network for prediction of kLa in external-loop airlift reactors with alcohol solutions was proposed.

Computational fluid dynamics simulation of hydrodynamics in the riser of an external loop airlift reactor

Local hydrodynamics in the riser of an external loop airlift reactor (EL-ALR) are identified and the performances of three drag models are evaluated in computational fluid dynamics simulation. The simulation results show that the Schiller–Naumann drag model underestimated the local gas holdup at lower superficial gas velocity whereas the Tomiyama drag model overestimated that at higher superficial gas velocity. By contrast, the dual-bubble-size (DBS)-local drag model gave more reasonable radial and axial distributions of gas holdup in all cases. The reason is that the DBS-local drag model gave correct values of the lumped parameter, i.e., the ratio of the drag coefficient to bubble diameter, for varying operating conditions and radial positions. This ratio is reasonably expected to decrease with increasing superficial gas velocity and be smaller in the center and larger near the wall. Only the DBS-local drag model correctly reproduced these trends. The radial profiles of the axial velocity of the liquid and gas predicted by the DBS-local model also agreed well with experimental data.

Oxygen transfer in a pressurized airlift bioreactor.

Airlift bioreactors (ALBs) offer advantages over conventional systems, such as simplicity of construction, reduced risk of contamination, and efficient gas-liquid dispersion with low power consumption. ALBs are usually operated under atmospheric pressure. However, in bioprocesses with high oxygen demand, such as high cell density cultures, oxygen limitation may occur even when operating with high superficial gas velocity and air enriched with oxygen. One way of overcoming this drawback is to pressurize the reactor. In this configuration, it is important to assess the influence of bioreactor internal pressure on the gas hold-up, volumetric oxygen transfer coefficient (k(L)a), and volumetric oxygen transfer rate (OTR). Experiments were carried out in a concentric-tube airlift bioreactor with a 5 dm(3) working volume, equipped with a system for automatic monitoring and control of the pressure, temperature, and inlet gas flow rate. The results showed that, in disagreement with previous published results for bubble column and external loop airlift reactors, overpressure did not significantly affect k(L)a within the studied ranges of pressure (0.1-0.4 MPa) and superficial gas velocity in the riser (0.032-0.065 m s(-1)). Nevertheless, a positive effect on OTR was observed: it increased up to 5.4 times, surpassing by 2.3 times the oxygen transfer in a 4 dm(3) stirred tank reactor operated under standard cultivation conditions. These results contribute to the development of non-conventional reactors, especially pneumatic bioreactors operated using novel strategies for oxygen control.

Hydrodynamics and mass transfer study of oil-water micro-emulsion in a three phase external loop airlift reactor

In this study, the impact of operating conditions, liquid properties and loading of a solid phase on the hydrodynamics and mass transfer coefficient in an airlift reactor with an external loop was investigated. The gas phase sparged through the reactor was air. Oil-in-water micro emulsions containing diesel, kerosene, heavy naphtha and light naphtha made up the liquid phase with an oil concentration of 5%. The impact of the solid phase was studied with solid loadings of 0, 0.25 and 0.5 wt%. The results showed that light naphtha which had the lowest density, viscosity and surface tension had the highest mass transfer and gas holdup and the lowest liquid circulation velocity. Solid addition decreased the gas holdup and mass transfer while increasing the liquid circulation velocity. This effect was considerable on the parameters above. In addition, a correlation based on dimensionless numbers was gained to predict the Sherwood number. Good agreement was observed between the correlated and experimental values.

Hydrodynamics and Mass Transfer of Oily Micro-emulsions in An External Loop Airlift Reactor

This study reports an experimental investigation on hydrodynamics and mass transfer characteristics in a 15.6×10−3 m3 external loop airlift reactor for oil-in-water micro-emulsions with oil to water volume ratio (ϕ) ranging from 3% to 7% (by volume). For comparative purposes, experiments were also carried out with water. Increase in ϕ of micro-emulsion systems results in an increment in the gas holdup and a decrease in the volumetric gas-liquid oxygen transfer coefficient and liquid circulation velocity, attributed to the escalation in the viscosity of micro-emulsions. The gas holdup and volumetric mass transfer coefficient for micro-emulsion systems are significantly higher than that of water system. Two correlations are developed to predict the gas holdup and oxygen transfer coefficient.

Numerical Simulations for Hydrodynamics of Air-Water External Loop Airlift Reactor Flows With Bubble Break-Up and Coalescence Effects

The external loop airlift reactor (ELALR) is a modified bubble column reactor that is composed of two vertical columns interconnected with two horizontal tubes and is often preferred over traditional bubble column reactors because it can operate over a wider range of conditions. In the present work, the gas-liquid flow dynamics in an ELALR were simulated using an Eulerian–Eulerian ensemble-averaging method with bubble breakup and coalescence effects in a three-dimensional system. The population balance models (PBM) of Luo and Svendsen (1996, “Theoretical Model for Drop and Bubble Breakup in Turbulent Dispersions,” AIChE J., 42, pp. 1225–1233) and Prince and Blanch (1990, “Bubble Coalescence and Breakup in Air-Sparged Bubble Columns,” AIChE J., 36, pp. 1485–1499) were used to simulate the bubble breakup and coalescence effects, respectively. The bubble breakup and coalescence closure models were implemented into CFDLib, a multiphase flow source code developed by Los Alamos National Laboratory, and validated with experiments. The computational fluid dynamics (CFD) simulations were then compared to experimental measurements from a 10.2 cm diameter ELALR for superficial gas velocities ranging from 1 to 20 cm/s. From this work, the 3D PBM simulations of an external loop airlift reactor were generally comparable with the 3D single bubble size simulations. However, the 3D PBM simulations have closer agreement with experimental findings than the single bubble size simulations especially regarding the length of gas bubbles in the downcomer.

Removal of Real Textile Dyes by Electrocoagulation/Electroflotation in a Pilot External-Loop Airlift Reactor

This paper studied the efficiency of electrocoagulation/electroflotation in removing colour from real textile wastewater by using aluminum and iron electrodes in an innovative pilot external-loop airlift reactor of 150 L. The reactor was designed to operate in batch and continuous modes. The real effluent contained 90% of disperse dye and 10% of reactive dye. A complete flotation of pollutants with acceptable mixing was achieved in this reactor using only the overall liquid recirculation induced by H2 microbubbles. The treatment of these discharges was easier using electrodes of iron rather than aluminum. The optimal initial pH was 10 for both aluminum and iron electrodes. By using iron electrodes, the maximum decolourisation efficiency and COD reduction efficiency reached respectively 96% and 65% for 90 minutes of treatment. Similarly, by using aluminum electrodes, the maximum decolourisation efficiency reached 90%, COD reduction reached 51% for 120 minutes of treatment. In the case of an initial pH slightly different to 10, the required time to reach 90% ranged from double to triple.

Hydrodynamics and mass transfer study of aliphatic alcohols in airlift reactors

Gas holdup and gas–liquid mass transfer coefficient were considered in an external airlift reactor. Air was sparged through some aliphatic alcohols (methanol, ethanol, n-propanol, and n-butanol) with different concentrations (0–1%, v/v). It was observed that gas holdup and mass transfer coefficient increased with increasing the number of carbons in alcohols. Furthermore, an increment in alcohols concentration increased gas holdup and mass transfer coefficient. The same behavior was observed in external and internal loop airlift reactors although gas holdup and mass transfer coefficient values were less than those of internal airlift reactor. According to the experiments, two correlations for gas holdup and mass transfer were developed.

Production of polygalacturonases by Aspergillus oryzae in stirred tank and internal- and external-loop airlift reactors

The production of endo- and exo-polygalacturonase (PG) by Aspergillus oryzae was assessed in stirred tank reactors (STRs), internal-loop airlift reactors (ILARs) and external-loop airlift reactors (ELARs). For STR production, we compared culture media formulated with either pectin (WBE) or partially hydrolyzed pectin. The highest enzyme activities were obtained in medium that contained 50% pectin in hydrolyzed form (WBE5). PG production in the three reactor types was compared for WBE5 and low salt WBE medium, with additional salts added at 48, 60 and 72h (WBES). The ELARs performed better than the ILARs in WBES medium where the exo-PG was the same concentration as for STRs and the endo-PG was 20% lower. These results indicate that PG production is higher under experimental conditions that result in higher cell growth with minimum pH values less than 3.0.

Research on Mass Transfer Coefficient Measurement Model of External-Loop Airlift Reactor

A mathematical model considering the inter-phase mass transfer both in the down-comer and the riser of an external-loop airlift reactor was established in this paper. The calculated global volumetric mass transfer coefficient based on the assumption of continuous stirred tank reactor (CSTR) was different from the local volumetric mass transfer coefficients by the newly proposed mathematical model and the difference was discussed. The effects of mass transfer in the down-comer, the hydrodynamic pressure and the experimental time on the mass transfer coefficient measurement model have been studied in detail. And it was also proved that only the global volumetric mass transfer coefficient, but not the local volumetric mass coefficient, can be obtained from a time-concentration curve in the external-loop airlift reactor.

Sparger type influence on the gas holdup of an external-loop airlift reactor with diluted alcohol solutions

Sparger type influence on the gas holdup of an external-loop airlift reactor with diluted alcohol solutions