Internal Loop Airlift Reactor Research Articles

Transferência de oxigênio em reatores de leito fluidizado com circulação em tubos concêntricos em meios bifásico e trifásico com variação da relação entre diâmetros

A eficiência do reator de leito fluidizado com circulação em tubos concêntricos depende das condições hidrodinâmicas que influem na transferência de oxigênio ao biofilme. Este trabalho investigou a influência da relação entre diâmetros dos tubos e da concentração de meio suporte (areia), sobre o coeficiente global de transferência de oxigênio (K La). Os ensaios - em reatores de 2,6 m de altura, com diâmetro externo de 250 mm e internos de 100, 125, 150 e 200 mm - empregaram vazões de ar até 2.500 L.h-1 e concentrações de até 150 g.L-1 de areia. O K La aumentou ligeiramente com 30 g.L-1 e diminuiu para concentrações maiores, confirmando relatos da literatura em condições semelhantes. Um modelo para K La em meio bifásico foi ajustado para as diversas relações ensaiadas entre a área externa e a interna, postulando-se uma redução na razão entre a transferência na fase líquida e o diâmetro da bolha com o aumento da vazão de ar.

CFD simulation of hydrodynamics and mass transfer in an internal airlift loop reactor using a steady two-fluid model

Steady state simulation of the hydrodynamics and mass transfer in bubbly flow in an axisymmetric internal airlift loop reactor (IALR) with the special treatment of interphase decoupling and outlet boundary conditions is performed, which is based on a Favre averaging two-fluid model for multiphase flow. Different models of mass transfer coefficient are compared and validated with the literature data. A model for describing the mass transfer for a wide range of superficial gas velocity is chosen and the predicted mass transfer coefficient agrees well with the experimental data in the literature. The numerical procedure can be used as a tool for the design and scale-up of IALRs.

A novel three‐phase airlift reactor without circulation of solids

AbstractA novel configuration of a three‐phase internal loop airlift reactor is proposed. The draft is divided in two sections: (top) gas–liquid contact section; (bottom) liquid–solid contact section. Solids particles are fluidized in the bottom section by liquid circulation. The main advantage compared with classic airlift or three‐phase fluidized bed is the reduced stress on particles as interference with bubbles is prevented. Experiments with silica sand (325 µm diameter) were carried out to characterise the hydrodynamics of the airlift. The influence of superficial gas velocity, overall solids hold‐up and sparger height were assessed. A theoretical analysis was proposed to derive simple design criteria.

Oxygen transfer in three scales of concentric tube airlift bioreactors

Oxygen transfer was evaluated in three internal-loop airlift reactors (ALRs) of different working volumes (2, 5, and 10 dm 3) and similar geometric configuration utilizing eight Newtonian and five non-Newtonian fluids. The effects of the superficial gas velocity ( U GR ) and liquid viscosity ( μ L ) had opposite effects on the volumetric oxygen transfer coefficient ( k L a). However, they presented the same orders of magnitude showing that the viscosity effect on oxygen mass transfer cannot be neglected. A correlation for k L a based on dimensional analysis considering the effects of the geometric parameters, the physical properties of the fluid, and the operational conditions presented a very good fitting to the experimental data. The correlation shows that the influence of the reactor internal diameter on k L a was considered positive. Therefore, in larger-scale reactors an appropriate oxygen transfer can be reached under smaller aeration conditions.

Identification of regime transitions in an inner-loop airlift reactor using local bubble-induced pressure fluctuation signals

The local bubble-induced pressure fluctuation signals, which were extracted by Zhang's method [15], were used to identify the flow regime in an inner-loop airlift reactor. For this purpose, local energy ratio and chaotic analysis were employed to extract useful information in detecting the flow regime transition. It was found that local energy ratio was able to indicate the two transition points in the downcomer and two transition points in the riser. Correlation dimension and Kolmogorov entropy can also detect two transition points in the downcomer, but only one transition points in the riser. Since the local energy ratio was easy to calculate, it had a great potential for detecting flow regime transition in an internal-loop airlift reactor or other multiphase systems.

Phenomenological Simulation Model for the Prediction of Hydrodynamic Parameters of an Internal Loop Airlift Reactor

Airlift reactors are pneumatically agitated reactors finding wide applications in chemical and biotechnology industries as well as in the treatment of wastewater. The design and scale-up of an airlift reactor is difficult and largely based on empirical approaches. The bubble size, distribution, and concentration are important parameters that influence the gas holdup and interfacial area. However, the influence of bubble size and distribution are not taken into account by any of the empirical correlations. In the present investigation, a phenomenological approach has been developed to predict the gas holdup in an internal loop airlift reactor. This approach considers the effect of bubble dynamics such as bubble breakup and bubble coalescence in the column and employs a population balance approach to determine bubble concentration in the column. From the knowledge of number and size of the bubbles, gas holdup in the reactor was obtained. A reasonably good agreement was observed between the simulation model ...

Gas holdup, bubble behavior and mass transfer in a 5 m high internal-loop airlift reactor with non-Newtonian fluid

Gas holdup, bubble behavior, interfacial area and gas–liquid mass transfer in a 5 m internal-loop airlift reactor with non-Newtonian fluid were studied in the superficial gas velocity ( U g) range of 2–12 cm/s. Air and aqueous CMC solutions of 0–0.45 wt% were used as the gas and liquid phases, respectively. It was found that increased U g or CMC concentration led to a wider bubble size distribution and an increase in the bubble Sauter diameter. The volumetric mass transfer coefficient increased with an increase in U g and a decrease in CMC concentration. In the air–water system, k l a/ α g was found to be independent of U g and was 0.2 1/s, and a constant liquid-side mass transfer coefficient ( k l) was found in the heterogeneous regime. However, in the air–CMC solution system, the influences of the superficial gas velocity and liquid viscosity were much more complicated: k l a/ α g was not constant and was affected by the superficial gas velocity and CMC concentrations; the interfacial area increased with an increase in U g and a decrease in CMC concentration; k l increased more significantly with increasing U g, and no obvious trend was found for the influence of CMC concentration on k l.

Region-dependent mass transfer behavior in a forced circulation airlift loop reactor

Abstract The contribution of local regions to global mass transfer holds the key to optimization and scale-up of a reactor. Extensive study has been conducted to investigate gas–liquid mass transfer occurring in the internal airlift loop reactor, but mostly restricted to global mass transfer performance. A cold model forced circulation internal airlift loop reactor was employed and divided into six regions in which dissolved oxygen concentration in slurry and mass transfer interfacial area were measured respectively. Different models were utilized to calculate volumetric mass transfer coefficient. Contributions of individual region to global mass transfer performance were calculated and compared. It was found that mass transfer coefficient and mass transfer interfacial area of individual region increases with increasing superficial gas velocity and slurry feed flowrate. The feed affected region has the greatest mass transfer coefficient and volumetric mass transfer coefficient, contributing more than 30% to global mass transfer in most operating condition. Mass transfer interfacial area is close in the gas distributor region, feed affected region and the gas-slurry separator region. In the present work, circulating bubbles are rare and contribute negligibly to the global mass transfer. Global volumetric mass transfer coefficient is close to that of the gas-slurry separator region, ranging from 0.02 to 0.1 1/s. Comparison of kLa is made between this work and literatures, suggesting a great improvement of mass transfer due to external liquid circulation.

Gas Hold-Up and Mass Transfer in Three Geometrically Similar Internal Loop Airlift Reactors Using Newtonian Fluids

Global gas hold-up and volumetric oxygen transfer coefficient (kLa) were evaluated in three internal-loop airlift reactors (ALR) with different working volumes (2, 5, and 10 dm3) and similar geometric configuration utilizing eight Newtonian fluids. Superficial gas velocity in the riser (UGR) was ranged from 0 to 0.112 ms-1. The effects of the superficial gas velocity (UGR) and liquid viscosity had contrary effects on kLa. However, they presented the same order of magnitude showing that the viscosity effect on the oxygen mass transfer cannot be neglected. Two correlations based on dimensional analysis relating global gas hold-up and kLa to the geometric parameters, physical properties of the fluids, and operational conditions were proposed. Very good fittings between calculated and experimental data of and kLa were obtained. The influence of the internal diameter of the reactor on kLa was considered positive showing that in large-scale reactors an appropriate oxygen transfer can be reached under smaller aeration conditions, due to higher internal diameter.

Hydrodynamics and batch biodegradation of phenol in an Internal Loop Airlift Reactor

Determination of Residence Time Distribution (RTD), oxygen volumetric mass transfer coefficient (kLa) and mixing pattern in an Internal Loop Airlift Reactor (ILALR) for both living and nonliving systems have been investigated in this paper. Optimal operating condition is found at a gas flow rate of 2 l min−1. The estimated kLa values for nonliving and living system at 27°C are 0.004 s−1 and 0.017 s−1 respectively. Batch biodegradation of phenol with an initial concentration of 300 mg l−1 by the mixed microbial culture occurs at a very high degradation rate (0.16 mg l−1 h−1).

Integrated anaerobic/aerobic biodegradation in an internal airlift loop reactor for phenol wastewater treatment

Anaerobic and aerobic biodegradation were integrated in an internal airlift loop reactor (IALR) by adding porous microbial carriers. In this bioreactor, aerobic activated sludge was suspended in the liquid bulk, while the anaerobic microbes were attached within the core of carriers. The integrated IALR was applied to the treatment of synthetic phenol wastewater. After 50 days’ acclimation according to co-substance strategy, the influent COD decreased from 3,700 mg/L to 400 mg/L (phenol removal rate was over 99%) with the residence time of 24 h. High performance could be achieved under the operation condition of superficial gas flow rate higher than 0.07 cm/s, temperature beyond 15°C and the microbial carrier volume fraction larger than 5%. Integration of anaerobic/aerobic biodegradation in IALR enhanced the synergetic effects between aerobic and anaerobic degradation; therefore, it has great potential in the treatment of phenol wastewater and other wastewater containing hard biodegradable organics.

Investigation of the Gas Layer Height in a Multistage Internal-Loop Airlift Reactor

A novel multistage internal-loop airlift reactor was realized by using a novel interstage internal. In this reactor, a gas layer is formed below the internal and has important effects on the hydrod...

Hydrodynamics of a Multi‐Stage Internal Loop Airlift Reactor

AbstractThe local hydrodynamic properties in a multi‐stage internal loop airlift reactor were investigated in this study. The gas‐liquid two‐phase flow hydrodynamic properties, including gas holdup, bubble velocity, bubble diameter, and liquid circulation velocity at various stages were measured by dual electrical resistivity probes and conductivity cells. Detailed studies on the gas holdup, bubble velocity, bubble diameter, and liquid circulation velocity were conducted with respect to various values of superficial gas. The Zuber and Findlay drift flux model was used to represent the variation of slip velocity with total gas‐liquid velocity at various stages and the model fits the data well.

Sensitivity Study on Modeling an Internal Airlift Loop Reactor Using a Steady 2D Two‐Fluid Model

AbstractThe sensitivity study of bubbly flow in an internal airlift loop reactor is presented using a steady Reynolds averaging two‐fluid model. Comparative evaluation of different drag formulations, drag coefficient correlations, turbulence effect on the drag coefficient, outlet slip velocity, and bubble size is performed and the respective influence to the simulation results is highlighted. It is found that a complicated drag formulation may not result in reliable predictions. All the drag coefficient correlations underpredict the gas holdup if the influence of turbulence on the drag coefficient is not well incorporated. Fortunately, the global hydrodynamics is not sensitive to the outflow slip velocity for a wide range, so a steady two‐fluid model can be used to simulate the bubbly flow when the flow field is fully developed. The correct estimation of bubble size with properly selected correlations play an important role in successful simulation of gas‐liquid bubbly flow in airlift loop reactors.

Origin of pressure fluctuations in an internal-loop airlift reactor and its application in flow regime detection

The origin of pressure fluctuations in an internal-loop airlift reactor (ILALR) and its application in the flow transition detection are investigated. It is found that pressure fluctuations can be characterized as global pressure fluctuations and local pressure fluctuations by frequency domain analysis and wavelet analysis. The global pressure fluctuations generated by gas compression in the gas plenum and flow fluctuations in the gas-supply system have almost a linear attenuation in the downcomer and almost no attenuation in the riser, especially in heterogeneous flow regime. However, it is found that the pressure wave from bubble eruption at bed surface has little impact on the wall pressure fluctuations. The global pressure fluctuations may be explained by Sasic's model. The local bubble-induced pressure fluctuations generated by bubble passage, coalescence and breakage can be determined by bubble passage frequency bandwidth and lower coherence. After extracting the local bubble-induced pressure fluctuations from the origin wall pressure fluctuations, it is shown that the Hurst exponent of the local pressure fluctuations increases faster in the homogeneous flow regime than in the heterogeneous flow regime, which can be employed to indicate the flow regime transition.

Biodegradation of phenol and m-cresol in a batch and fed batch operated internal loop airlift bioreactor by indigenous mixed microbial culture predominantly Pseudomonas sp.

An internal loop airlift reactor (ILALR) is developed and studied for biodegradation of phenol/ m-cresol as single and dual substrate systems under batch and fed batch operation using an indigenous mixed microbial strain, predominantly Pseudomonas sp. The results showed that the culture could degrade phenol/ m-cresol completely at a maximum concentration of 600 mg l −1 and 400 mg l −1, respectively. Batch ILALR study has revealed that phenol has been preferentially degraded by the microbial culture rather than m-cresol probably owing to the toxic effect of the later. Sum kinetic model evaluated the interaction between the phenol/ m-cresol in dual substrate system, which resulted in a high coefficient of determination ( R 2) value >0.98). The fed batch results showed that the strain was able to degrade phenol/ m-cresol with maximum individual concentrations 600 mg l −1 each in 26 h and 37 h, respectively. Moreover for fed batch operation, degradation rates increased with increase in feed concentration without any lag in the degradation profile.

Bubble circulation regimes in a multi-stage internal-loop airlift reactor

Abstract Specially designed internals were used in a novel multi-stage internal-loop airlift slurry reactor to decrease liquid backmixing and provide for a homogeneous suspension of solid particles. There was three bubble circulation regimes with different bubbling characteristics in the downcomer of the bottom stage: bubble-free regime (BFR), transition regime (TR) and complete bubble circulation regime (CBCR). The effects of the unaerated liquid level and superficial gas velocity on the bubble circulation regimes were investigated. The unaerated liquid level significantly affected the regime transition from the TR to CBCR, but had a negligible effect on the transition from the BFR to TR. The axial profile of the gas holdup in the downcomer was non-uniform in the BFR, but almost uniform in the CBCR. With increasing superficial gas velocity, the liquid circulation velocity increased in the BFR, remained approximately constant in the TR, and then increased in the CBCR. A mechanism for the regime transitions that was based on the measured gas holdup, liquid circulation velocity and bubble penetration depth was given.

Modeling of enzymatic reaction in an airlift reactor using an axial dispersion model

AbstractThis work was focused on modeling of biochemical processes in a 40-L internal-loop airlift reactor. Due to different mixing in the specific zones of the reactor four main sections, bottom, riser, separator and downcomer, were recognized. Each zone was modeled by an adequate mixing model: bottom and separator sections by the model of ideally-stirred reactor; riser and downcomer sections by the model of plug-flow reactor with axial dispersion. In the model, the effects of mass transfer, hydrodynamics, and reaction kinetics were taken into account. The model of the reactor was experimentally verified by the aerobic enzymatic oxidation of glucose to gluconic acid. Simulations are in good agreement with experimental data.

Simultaneous nitrification–denitrification achieved by an innovative internal-loop airlift MBR: Comparative study

This study assessed the performance of different single-stage continuous aerated submerged membrane bioreactors (MBR) for nitrogen removal. Almost complete nitrification was achieved in each MBR irrespective of operating mode and biomass system. Denitrification was found to be the rate-limiting step for total nitrogen (T-N) removal. The MBR with internal-loop airlift reactor (ALR) configuration performed better as regards T-N removal compared with continuous stirred-tank reactor (CSTR). It was demonstrated that simultaneous nitrification and denitrification (SND) is the mechanism leading to nitrogen removal and the contribution of microenvironment on SND is more remarkable for the MBRs with hybrid biomass. Macroenvironment analyses showed that gradient distribution of dissolved oxygen (DO) level in airlift MBRs imposed a significant effect on SND. Higher mixed liquor suspended solid (MLSS) concentration led to the improvement in T-N removal by enhancing anoxic microenvironment. Apparent nitrite accumulation coupled with higher nitrogen reduction was accomplished at MLSS concentration exceeded 12.6 g/L.

Hydrodynamics and mass transfer in a modified three-phase airlift loop reactor

A modified internal-loop airlift reactor (MIALR) with a continuous slurry phase was studied to investigate the local hydrodynamic characteristics, including gas holdup, bubble size, bubble rise velocity and local mass transfer properties. Based on the analysis of geometrical construction and fluid properties of gas and slurry, MIALR was divided into six flow regions. In these flow regions, the local hydrodynamic characteristics were investigated over a wide range of operating variables. Furthermore, a new method was developed to measure the dissolved oxygen concentration. The volumetric mass-transfer coefficient in six flow regions was also calculated for comparison.