Internal Loop Airlift Reactor Research Articles

Pros and cons of airlift and bubble column bioreactors: How internals improve performance

Gas fermentation is a promising technology of high commercial interest, particularly for capturing CO2 and CO from industrial off-gases to reduce greenhouse gas emissions and replace fossil fuels for bulk chemical production. Therefore, evaluating promising bioreactor settings ab initio is a crucial step. Whereas alternate configurations may be tested in laborious scale up studies, the procedure may be accelerated by in silico studies that accompany or even partially replace wet-lab work once the models are validated. In this context, the current study compares various pneumatically agitated reactor types – bubble column reactor (BCR), annulus- and center-rising internal-loop airlift reactor (AR-IL-ALR and CR-IL-ALR), and external-loop airlift reactor (EL-ALR) – to identify advantages and disadvantages for the given application based on computational fluid dynamics (CFD) models. Process performance is optimized by introducing internal structures to guide the flow. Despite a significant increase in the mass transfer coefficient (kLa) through internal modifications, the CR-IL-ALR still exhibited the poorest performance. The optimized AR-IL-ALR demonstrated good mixing and, after introducing an open-cone shaped internal in the head part and a conical bottom, superior mass transfer, achieving an enhancement over 10 % in the mass transfer coefficient to 315 1/h. This study thereby outlines the potential of internal structures for process improvement, as well as the value of a priori in silico design of reactor configurations.

Characteristics of Bubbly Flow, Mass Transfer, and CO2 Absorption in a Two-Stage Internal Loop Airlift Reactor Assembly with a Diamond-Shaped Guide Baffle

Characteristics of Bubbly Flow, Mass Transfer, and CO₂ Absorption in a Two-Stage Internal Loop Airlift Reactor Assembly with a Diamond-Shaped Guide Baffle

Investigation of hydrodynamic performance in a staggered multistage internal airlift loop reactor

The multistage internal airlift loop reactor (MIALR) has shown promising application prospects in gas–liquid–solid reaction systems. However, traditional MIALRs have a global circulation with strong interstage liquid-phase exchange. This paper proposes a staggered multistage internal airlift loop reactor (SMIALR) that incorporates special guide elements to create a staggered flow. Both experiments and computational fluid dynamics-population balance model simulations were conducted to investigate the hydrodynamic performances of MIALR and SMIALR. The results demonstrate that SMIALR exhibits a local circulation at each stage. Bubbles have a longer residence time in SMIALR, resulting in a 28.35%–55.54% increase in gas holdup and a 7.27%–13.69% increase in volumetric mass transfer coefficient. The gas–liquid mass transfer coefficient of SMIALR was improved by increasing the gas–liquid interfacial area. Additionally, the radial distribution of solids was found to be more uniform. This study offers insights for optimizing MIALR and provides a theoretical foundation for the design and scale-up of SMIALR.

Marangoni effect on hydrodynamics and mass transfer behavior in an internal loop airlift reactor under elevated pressure

AbstractThe Marangoni effect on the hydrodynamics and mass transfer was studied in an internal loop airlift reactor with the addition of ethanol under elevated pressure. The overall gas holdup αg increased by ~10% and the gas–liquid volumetric mass‐transfer coefficient kla increased by ~20% on average for each one bar increase in pressure. When the mole fraction of ethanol xE increased from 0 to 0.012, αg increased by ~110% and kla increased by ~50%. However, when xE increased from 0.012 to 0.11, αg decreased by ~40% and kla decreased by ~25%. This volcano‐shaped relationship between both hydrodynamics and mass transfer behavior and ethanol concentration was determined by the Marangoni effect. Semi‐empirical correlations of αg and kla were proposed by including the Marangoni effect in a wide range of ethanol concentration and operating conditions. These correlations gave good predictions of αg and kla compared with extensive experimental data in the literature.

The Influence of Draft Tubes on the Mass Transfer and Mixing Performance of a Pilot-Scale Internal-Loop Airlift Reactor

The hydrodynamic characteristics, mass transfer, and mixing performance of three different reactors, a bubble column reactor (BCR), a single-stage internal-loop airlift reactor (SSALR), and a four-stage internal-loop airlift reactor (FSALR), were investigated systematically through cold model experiments to explore the influence of draft tube configurations on the pilot-scale internal-loop airlift reactor (ILAR). The findings indicated that the BCR yielded a higher gas holdup and mass transfer coefficient due to its longer bubble residence time. Segmenting the draft tube improved the gas holdup in both the riser and downcomer, and the overall gas holdup in the downcomer increased by 9%. Compared with the SSALR, the mass transfer coefficient of the FSALR in the riser and downcomer increased by 10.2% and 9.3% on average, respectively. In addition, a higher liquid circulating velocity was obtained with the ILARs due to a higher gas holdup difference between the riser and the downcomer. Specifically, the liquid circulating velocity of the FSALR was 134.1% higher than that of the BCR and 15.8% higher than that of the SSALR. The mixing time of the ILARs was reduced due to more intense overall circulation. The mixing effect of the FSALR was the best. The mixing time was reduced by 70.2% and 51.3% compared with the BCR and SSALR for UG ranging from 4.0 cm/s to 9.1 cm/s, respectively. Empirical correlations were proposed for the gas holdup, liquid circulating velocity, mass transfer coefficient, and mixing time on the superficial gas velocity, and agreement with experimental data was satisfactory.

Cyanide effluent treatment by electrocoagulation using airlift reactor: Modeling and optimization by response surface methodology

The extraction processes of gold, silver, and various metals from ores consume 13% of 1.1 million metric tons of hydrogen cyanide produced worldwide annually. Therefore, developing efficient processes to treat the issuing effluents before releasing them into the environment is imperative. In this context, the present study aims to evaluate the effect of three factors: inter-electrodes distance (d), current density (J) and electrolysis time (t) on the cyanide removal efficiency by a combination between Electro-Coagulation process and Internal Loop Airlift Reactor. Employing an airlift reactor enables an effective dispersion of the coagulant without the need for additional stirring. The optimization of the process was conducted using Response Surface Methodology in conjunction with Box–Behnken Design. The statistical analysis results reveal that electrolysis time(t)and current density (J) have significant effects on reactor performance to reduce cyanide with a confidence level of 99%, and 97.5%, respectively. The multiple regression analysis method demonstrates the model's overall predictability as the coefficient of determination was 98 % for the cyanide removal efficiency. The investigation inside the RSM 2 and 3-dimensional graphs allowed us to define the optimal domains of the variables to reduce cyanide. At a current density of 68 A.m−2 and an inter-electrodes distance of 13mm during 57.5min of electrolysis time, 98.27% of cyanide removal is achieved. The knowledge obtained through this study using the airlift reactor operating in batch mode and the EC process for the treatment of cyanide effluent confirms EC's ability to successfully remove cyanide and might be useful for the transposition from batch to continuous mode.

Performance and mechanism of simultaneous nitrification and denitrification in zeolite spheres internal loop airlift reactor

An internal loop airlift reactor was constructed with zeolite spheres as biofilm carriers (ZS-ALR), and the performance and mechanism of nitrogen removal were investigated. The results indicated that the TN, NH4+-N and TOC removal efficiencies of ZS-ALR reached 96.12%, 100% and 94.54% under appropriate conditions (HRT of 6–8 h, aeration rates of 80–120 mL/min, C/N ratios of 4–6), and the highest TN removal rate constant was 0.01156 min−1. Further investigating the influence of ammonia-N concentrations on nitrogen removal and biofilm stability revealed that catabolism was important in TN removal, and the prominent genera for nitrogen removal included Sphaerotilus (42.20%), Flavobacterium (17.47%) and Fusibacter (6.14%). Meanwhile, the abundance of amoA, napA, narG and nosZ genes was markedly influenced by ammonia-N concentrations. The nitrogen removal of ZS-ALR was mainly through ammonia-N adsorption by zeolite spheres and simultaneous nitrification and denitrification by biofilm.

Gas holdup, bubble size distribution, and mass transfer in an airlift reactor with ceramic membrane and perforated plate distributor

AbstractThe airlift reactor is one of the most commonly used gas–liquid two‐phase reactors in chemical and biological processes. The objective of this study is to generate different‐sized bubbles in an internal loop airlift reactor and characterize the behaviours of the bubbly flows. The bubble size, gas holdup, liquid circulation velocity, and the volumetric mass transfer coefficient of gas–liquid two‐phase co‐current flow in an internal loop airlift reactor equipped with a ceramic membrane module (CMM) and a perforated‐plate distributor (PPD) are measured. Experimental results show that CMM can generate small bubbles with Sauter mean diameter d32 less than 2.5 mm. As the liquid inlet velocity increases, the bubble size decreases and the gas holdup increases. In contrast, PPD can generate large bubbles with 4 mm < d32 < 10 mm. The bubble size and liquid circulation velocity increase as the superficial gas velocity increases. Multiscale bubbles with 0.5 mm < d32 < 10 mm can be generated by the CMM and PPD together. The volumetric mass transfer coefficient kLa of the multiscale bubbles is 0.033–0.062 s−1, while that of small bubbles is 0.011–0.057 s−1. Under the same flow rate of oxygen, the kLa of the multiscale bubbles increases by up to 160% in comparison to that of the small bubbles. Finally, empirical correlations for kLa are obtained.

A turbulent mass diffusivity model for the simulation of the biodegradation of toluene in an internal loop airlift reactor

In this study, a three-dimensional computational fluid dynamics model is established in Ansys Fluent to describe the biodegradation of toluene in an internal loop airlift reactor (IALR). The Euler-Euler approach is used with the standard k-ε method to predict the hydrodynamics of the reactor, and the population balance model is used to describe the bubble size distribution in the reactor. The recently developed concentration variance c2¯ and its dissipation rate εc formulations are adopted to close the turbulent mass transfer differential equations, so that the turbulent mass diffusivity can be determined without using empirical methods. There is a good agreement between the simulated dissolved oxygen concentrations obtained by the proposed model and the experimental data reported in the literature. As the predicted turbulent mass diffusivity is found to be unevenly distributed and the calculated turbulent Schmidt number Sct is not a constant but varies, it may not be appropriate to use a constant Sct or an experimentally determined dispersion coefficient. In addition, the maximal shear stress is found in the top region of the IALR and the liquid flow are very uniform in the riser and downcomer.

Evaluación hidrodinámica de un reactor airlift de recirculación interna con fluidos newtonianos y adelgazantes: Experimentación vs Simulación CFD

The hydrodynamics of an internal-loop airlift reactor was numerically and experimentally characterized. The gas holdup, liquid velocity, shear rate, flow pattern and volumetric oxygen transfer coefficient (kLa) were evaluated as a function of the air velocity and medium rheology. Tap water and CMC solutions were used as Newtonian and non-Newtonian fluids, respectively. The standard  model was employed for modeling turbulence, and unsteady three-dimensional simulations with the Euler–Euler model were performed. Gas holdup, liquid velocity and kLa measurements were performed for validating simulations. An increase in bubble coalescence and a decrease in kLa was observed with CMC solutions. The presence of recirculation loops inside the riser for CMC solutions is reported, which was not observed with tap water. The higher the CMC concentration, the larger the recirculation region seems to be. Results show that recirculation loops play a substantial role in the reactor’s hydrodynamic performance, and it turns out that the gas holdup in the riser increases with increasing the CMC concentration.

Characterization of packed-bed in the downcomer of a concentric internal-loop airlift bioreactor

A packed-bed basket containing corn cob powder was used for the adaptation of a concentric-tube airlift reactor for enzymatic reactions. The bed was inserted in the downcomer region of the bioreactor generating a decrease in liquid downflow. To characterize the liquid velocity through the packed-bed and calculate the residence time, the intrinsic permeability was determined using Darcy’s law, resulting in 1.275 × 10−9 m2 (40-mesh particle size), and the pressure difference was calculated using the riser gas holdup, reaching values up to 56.6% for water at 3.5 vvm. Thus, a superficial liquid velocity model was obtained to describe the liquid velocity at different airflow rates and bed lengths, covering a wide range of support masses commonly used in enzymatic reactions. The model also allows the variation of viscosity, caused by different reaction temperatures and the use of larger bed sizes for scaling. Using the dye tracer method, it was possible to visualize the flow through the bed and corroborate the residence times obtained by the model for a wide range of airflows. Therefore, it describes the use of a basket filled with potential enzyme support, contributing to the applicability of pneumatic systems as enzymatic bioreactors, particularly internal-loop airlift reactors.

Influence of Sparger Type on Mass Transfer in a Pilot-Scale Internal Loop Airlift Reactor

In a pilot-scale internal loop airlift reactor with a height of 5.5 m and a main column diameter of 0.484 m, the influence of three gas sparger structures (ladder distributor, tri-nozzle sparger and perforated plate) on the volumetric mass transfer coefficient kLa was investigated. It was found that the perforated plate produces the highest gas holdup difference and circulating liquid velocity between the riser and the downcomer. The perforated plate provides the most efficient mass transfer due to the more uniform gas distribution and higher circulating liquid velocity, followed by the ladder distributor and tri-nozzle spargers. Compared with the tri-nozzle sparger, the perforated plate increases the value of kLa by up to 16% at a superficial velocity of 0.15 m/s. Interestingly, the analysis of the liquid-phase mass transfer coefficient kL and specific area a with respect to gas velocity shows that the mass transfer rate is primarily controlled by a. By comparing the predictions of different mass transfer models, the slip velocity model based on penetration theory yields a satisfactory agreement with the experimental results within ±15% error. Meanwhile, empirical correlations regarding gas holdup and kLa were developed and were found to have good consistency with experimental values.

Removal of Phenol by Expanded Bed Airlift Loop Reactor

The exaggerated release of industrial wastes especially those containing phenol into the environment led to the contamination of both surface and groundwater supplies. In present work a synergistic and combined system technique between three operations, adsorption of phenol via (rice husk or granular activated carbon GAC as adsorbents) together with stripping by airflow and advance oxidation via hydrogen peroxide as the oxidation agent, to evaluate the possibility of using a proposed new design for internal airlift loop reactor for removing the phenol from wastewater. The experiments were set up in a cylindrical Perspex column consisted of a transparent outer column having a 15 cm inside diameter and 150 cm height that included an internal draught tube of 7.5 cm and extending vertically to 120 cm top contains a bed having a dimension (7.5 x 30 cm) filled with adsorbent materials (rice husk, granular activated carbon GAC) and a volume capacity 25 liters. The experiments were conducted under the influence of both of the following variables air flow rate(2-20) (L/min), treatment time(5-60 min), the molar ratio of hydrogen peroxide to phenol,(1:10, 1:15 and 1:20)). The results showed the success of the proposed design with obtaining a removal efficiency (83%),( 81%)when using GAC and the rice husk as adsorbent materials respectively, with minimum remediation time 60 minutes, airflow rate of 18 L/min, and molar ratio(20) hydrogen peroxide to phenol. This study demonstrated that the proposed synergistic system could be utilized for the remediation of contaminated aqueous systems.

Computational Fluid Dynamics Simulation of Hydrodynamics in a Two-Stage Internal Loop Airlift Reactor with Contraction-Expansion Guide Vane.

Global circulation and liquid back mixing adversely affect the continuous production of a multistage internal airlift loop reactor. A contraction-expansion guide vane (CEGV) is proposed and combined with a two-stage internal loop airlift reactor (TSILALR) to suppress the liquid back mixing between stages. A computational fluid dynamics (CFD) simulation is conducted to evaluate the performance of the CEGV in the TSILALR. The bubble size distribution and turbulent flow properties in the TSILALR are considered in the CFD simulation by using the population balance model and RNG k-ε turbulence model. The CFD model is validated against the experimental results. The deviations in the gas holdup and mean bubble diameter between the simulation and experimental results are less than 8% and 6%, respectively. The streamlines, flow pattern, bubble size distribution, and axial liquid velocity in the TSILALRs with and without the CEGV at superficial velocities of 0.04 and 0.08 m/s are obtained by CFD simulation. It has been shown that the CEGV generated local circulation flows at each stage instead of a global circulation flow in the TSILALR. The average global gas holdup in the TSILALR with a CEGV increased up to 1.98 times. The global gas holdup increased from 0.045 to 0.101 and the average axial velocity in the riser decreased from 0.314 to 0.241 m/s when the width of the CEGV increased from 50 to 75 mm at the superficial gas velocity of 0.08 m/s.

Investigation of hydrodynamics and mass transfer in an internal loop airlift slurry reactor integrating mixing and separation

Based on newly invented internal loop airlift slurry reactor integrating mixing and separation, hydrodynamics and mass transfer under different solid loadings and superficial gas velocities were systematically investigated, and mixing and mass transfer characteristics were studied. Additionally, some empirical models for these critical parameters are proposed. A phenomenon of internal fluid relay circulations in the riser of the internal loop airlift reactor at the high superficial gas velocity was firstly discovered by visual observation and was then verified both by the picture sequences and theoretical analysis, and some new thoughts for structural optimization were also proposed. It was found that the axial solid concentration deviation can be more than 30% and the solid particles were inclined to exist near the wall. Surprisingly, the particle size distributions were found to be the same in the whole reactor. This slurry reactor will be popular in industry with a further structural optimization.

Analysis of Rapid Start-up and Mixed Nutritional Nitrogen Removal Performance of Complete Autotrophic Granular Sludge

A rapid start of the completely autotrophic nitrogen removal over nitrite (CANON) process based on granular sludge and efficient nitrogen removal under mixotrophic conditions are important steps in a continuous flow reactor for CANON engineering applications. In this study, an aged CANON granular sludge was mechanically crushed to 0.3 mm as inoculum in an airlift internal-loop reactor (AIR) to achieve simultaneous COD removal and mixotrophic denitrification of the single-stage granular sludge. The system achieved stable partial nitrification by controlling DO after 26 days of startup. Granulation and anaerobic ammonia oxidation were then promoted by shortening the HRT to increase the ammonia nitrogen load to 5.65 kg ·(m3 ·d)-1. The total nitrogen removal rate reached 58% on the 68th day. Subsequently, the C/N ratio of influent was increased from 0 to 0.25 and 0.5, which promoted the synergistic growth of AOB, AMX, and heterotrophic microorganisms. The removal rates of ammonia and total nitrogen were 95% and 85% respectively, and the removal of COD reached approximately 80%. The activity of NOB such as Nitrospira was effectively inhibited as the COD concentration was increased. q(NH4+-N) and q(TN) were stable at 0.4 g ·(g ·h)-1 and 0.34 g ·(g ·h)-1, respectively, while q(NO3--N) was approximately 0.02 g ·(g ·h)-1. Microbial diversity was observed using MiSeq high-throughput sequencing. It showed that organic carbon had no significant effect on the abundance of Nitrosomomas and Candidutus_Kuenenia, while increasing the abundance of Candidutus_Brocadia and Denitratisoma in the sludge. This study provides ideas for the rapid start of continuous flow CANON granular sludge process to treat wastewater with low C/N ratio.

Core/shell Fe3O4@Al2O3-PMo magnetic nanocatalyst for photocatalytic degradation of organic pollutants in an internal loop airlift reactor

In the present work, core/shell nanocomposite of phosphomolybdic acid immobilized on magnetic alumina (Fe3O4@Al2O3-PMo) has been synthesized and characterized by XRD, FE-SEM, VSM, UV–vis spectroscopy and BET analysis methods. The photodegradation of azo dye Cibacron Brilliant Yellow 3G-P (CBY) was used to assess the photocatalytic activity of synthesized magnetic Fe3O4@Al2O3-PMo photocatalyst in a three-phase airlift photocatalytic reactor under UV irradiation. Various parameters affecting the photocatalytic activity include photocatalyst dose, initial dye concentration, initial pH, air flowrate and photocatalyst recycle has been investigated. Relatively, the synthesized Fe3O4@Al2O3-PMo photocatalyst revealed good degradation efficiency (>90 %). Furthermore, the results proved that integration of PMo with Fe3O4@Al2O3 magnetic nanocomposite leads to an improvement in photocatalytic conversion due to reduced recombination of photo-generated electron-hole pair. Fe3O4@Al2O3-PMo photocatalyst displayed significant stability and recyclability after five reaction cycles.

Effect of Surface Tension on Hydrodynamics and Mass Transfer Coefficient in Airlift Reactors

AbstractThe hydrodynamics and volumetric mass transfer coefficient play important roles in the design and scale‐up of airlift reactors. The effect of surface tension on hydrodynamics and volumetric mass transfer coefficient in internal loop airlift reactors was investigated. With reduction of the surface tension of the fluid, the hydrodynamic parameters raised, namely, gas phase holdup, flow regime transition point, and interfacial area, whereas the bubble diameter as well as the liquid velocity decreased and the volumetric mass transfer coefficient increased. Empirical correlations are proposed for gas‐phase holdup and volumetric mass transfer coefficient in terms of dimensionless numbers and can be applied in the design of airlift reactors.

Rapid Achievement of Nitrifying Micro-granular Sludge and Its Nitritation Function

The rapid achievement of nitrifying micro-granular sludge and its nitritation function was studied in a continuously operated internal-loop airlift reactor seeding with floccular sludge. Results showed that the sludge micro-granulation was almost realized within three weeks by gradually reducing the hydraulic retention time from 5 h to 2.5 h. The color of the sludge first changed from yellowish-brown to creamy white, and then changed to pale yellow during the micro-granulation process. The settleability of the sludge first changed from good to bad, and then recovered to good. The value of the sludge settling velocity (SV) at SV5 and SV30 were both equal to 4%-5%, while SVI30 and SVI5 were both around 12-13 mL·g-1. The average size of the obtained nitrifying micro-granular sludge was 134 μm on day 27. Nearly 70% of the nitrifying micro-granular sludge was maintained in a relatively narrow range of 59-163 μm, thus indicating the largely homogeneous diameter distribution of these micro-granules. After sludge micro-granulation, the nitritation function was achieved within one week by progressively increasing the influent NH4 concentrations from 50 mg·L-1to 200 mg·L-1. The NO2- accumulation ratio and the nitritation loading rate reached up to 90% and 1.34 kg·(m3·d)-1, respectively. The high level of residual NH4 concentration in the effluent, or the low ratio of dissolved oxygen (DO) to NH4+-N concentrations (0.03-0.09), should be the primary cause of the rapid achievement of nitritation in the micro-granular sludge reactor.

Hydrodynamic studies in sectionalised external loop air lift reactors

ABSTRACT External loop air lift reactor (EL-ALR) is widely used for multiphase reactions. They offer lower pressure drop and good heat and mass transfer rates compared to conventional bubble column reactors. In the case of fermentation applications where a medium is highly viscous and coalescing in nature, sectionalisation of riser helps in the improvement of the interfacial area as well as the reduction in liquid-phase backmixing. The present work deals with the measurement of the gas hold-up, mixing time, liquid dispersion, Sauter mean bubble diameter over a broad range of superficial gas velocities in sectionalised EL-ALR. Effects of electrolytes and surfactants have also been studied. Different plate designs have been studied for the sectionalisation. The correlations have been proposed for the gas hold-up, liquid circulation velocity, Sauter mean bubble diameter for superficial gas velocities. Abbreviations: EL-ALR: external loop air lift reactor; IL-ALR: internal loop air lift reactor; ADC: analog to digital converter; DAC: digital to analog converter.