Removal of toluene from air streams using a gas–liquid–solid three‐phase airlift loop bioreactor containing immobilized cells

Toluene, a kind of volatile organic compound (VOC), is widely used as a solvent (paints and coatings, gums, resins, rubber) as well as a reagent (medicines, dyes, perfumes) and is one of the components of gasoline. Over the more recent decades, many studies have led to the development of biological methods to treat toluene. This paper presents the results of a study on the treatment of airborne toluene using a laboratory‐scale gas–liquid–solid three‐phase airlift loop bioreactor containing immobilized cells. Based on the optimum operating conditions such as the temperature of 28–30 °C, pH of 7.0–7.2, and an empty bed residence time (EBRT) of 39.6 s, a continuous bioprocess showed that this immobilized airlift loop bioreactor had a steady‐state performance within 15 days, the outlet concentrations of toluene were lower than the national emission standard in China (GB 16297‐1996), and the chemical oxygen demand and NH₄⁺‐N of the effluent also satisfied the primary discharge standard in China (GB 8978‐1996). In addition, this immobilized airlift loop bioreactor had a good ability to tolerate shock loads, while the maximum elimination capacity of toluene was 168 g m^?3 h^?1 which was higher than those not only in biofilters and biotrickling filters but also in the airlift bioreactor with free microorganisms. Copyright © 2005 Society of Chemical Industry     

Effect of impeller type on continuous‐flow mixing of non‐Newtonian fluids in stirred vessels through dynamic tests

Mixing of non‐Newtonian fluids with axial and radial flow impellers is prone to a significant extent of nonideal flows (e.g., dead zones and channelling) within the stirred reactors. To enhance the performance of the continuous‐flow mixing of pseudoplastic fluids with yield stress, close‐clearance impellers were utilised in this study. We explored the effects of various parameters such as the type of close‐clearance impeller (i.e., the double helical ribbon (DHR) and anchor impellers), impeller speed (25–500 rpm), impeller pumping direction, fluid rheology (0.5–1.5% xanthan gum solution), fluid flow rate (3.20–14.17 L min^?1) and the locations of outlet (configurations: top inlet–top outlet, top inlet–bottom outlet) on the dynamic performance of the mixing vessel. The performance of the DHR impeller was then compared to the performance of various types of impellers such as axial‐flow (Lightnin A320) and radial‐flow (Scaba 6SRGT) impellers. The dynamic tests showed that the DHR impeller was the most efficient impeller for reducing the extent of nonideal flows in the continuous‐flow mixer among the impellers employed in this study. In addition, the mixing quality was further improved by optimising the power input, increasing the mean residence time, decreasing the fluid yield stress, using the up‐pumping impeller mode and using the top inlet–bottom outlet configuration. © 2011 Canadian Society for Chemical Engineering     

Treatment of the refinery wastewater in a gas–liquid–solid three‐phase flow airlift loop bioreactor

Aerobic treatment of refinery wastewater was carried out in a 200 dm³ gas–liquid–solid three‐phase flow airlift loop bioreactor, in which a biological membrane replaced the activated sludge. The influences of temperature, pH, gas–liquid ratio and hydraulic residence time on the reductions in chemical oxygen demand (COD) and NH₄‐N were investigated and discussed. The optimum operation conditions were obtained as temperature of 25–35 °C, pH value of 7.0–8.0, gas–liquid ratio of 50 and hydraulic residence time of 4 h. The radial and axial positions had little influence on the local profiles of COD and NH₄‐N. Under the optimum operating conditions, the effluent COD and NH₄‐N were less than 100 mg dm^?3 and 15 mg dm^?3 respectively for more than 40 days, satisfying the national primary discharge standard of China (GB 8978‐1996). Copyright © 2005 Society of Chemical Industry     

A net draft tube slurry airlift bioreactor for 2,4‐D (2,4‐dichlorophenoxyacetic acid) pesticide biodegradation

Biodegradation of the pesticides 2,4‐D have been investigated in a net draft tube airlift bioreactor at different concentrations (10–160 ppm) and air flow rates (0.75–4 L min^?1), employing acclimatised activated sludge. A modification for the glass draft tube airlift bioreactor was proposed in which the draft tube is made of non‐woven polypropylene textile net. The experiments were carried out as well in a bubble column (BC) for comparison. The results confirmed that immobilising the activated sludge on the net of the draft tube could enhance the substrates mass transfers and improve the biodegradation rate. A mathematical model was developed to simulate the biodegradation process. The simulated results were in good agreement with the experiments.     

Effect of liquid volume in the gas‐separator on the hydrodynamics of airlift reactors

Gas hold‐up and liquid circulation velocity measurements were made using a 167 dm³ external loop airlift reactor. The gas‐separator was of the open channel configuration. The reactor height was 2.5 m with riser and downcomer diameters of 0.19 m and 0.14 m respectively. The systems investigated were Newtonian air–water and air–glycerol with the superficial air velocity varying between 0.02 and 0.12 m s⁻¹. The ratio of the liquid volume in the gas‐separator to the liquid volume in the reactor (volume‐ratio) was varied from 0.0% to 37%, to find its minimum critical value for optimum operation of the airlift reactor. For the air–water system, discernible effects of the volume‐ratio on riser and, downcomer gas hold‐ups and liquid circulation velocity were observed at volume ratios ≤7%. Beyond this value, the volume‐ratio had no effect. For a viscous and foaming air–glycerol system the critical volume‐ratio was increased to 19%. New and simple correlations for predicting gas hold‐up in the riser, gas hold‐up in the downcomer, and liquid circulation velocity were developed with reasonable accuracy. © 1999 Society of Chemical Industry     

Improving docosahexaenoic acid production by Schizochytrium sp. using a newly designed high‐oxygen‐supply bioreactor

A sufficiently high oxygen supply is crucial for high‐cell‐density cultivation of aerobic microorganisms, including Schizochytrium sp. We, therefore, designed a novel bioreactor enabling high‐level oxygen supply, and its relevant process parameters and fermentation‐stage characteristics were investigated. The real‐time changes of pH and nonoil biomass were monitored as proxies for the consumption of nitrogen and lipid accumulation status, which was first applied to divided fermentation process with three stages. The experimental results showed that the biomass in this porous‐membrane‐impeller bioreactor was higher than in conventional bioreactor, while docosahexaenoic acid (DHA) percentage in total lipids was lower than in conventional bioreactor. A multistage control strategy is subsequently implemented for the porous‐membrane‐impeller bioreactor, and the maximum biomass, DHA concentration, DHA percentage in biomass and DHA productivity reached 151.0 g/L, 44.3 g/L, 29.33%, 369.08 mg/(L·h), respectively. This study thus provides a highly efficient and economic bioreactor for the production of DHA by Schizochytrium sp. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4278–4286, 2017     

Gas–liquid interfacial area in the oxygen absorption to oil‐in‐water emulsions in an airlift reactor

The present work includes an exhaustive study about gas–liquid interfacial area between gas phase and liquid heterogeneous medium generated in an airlift bioreactor. The system studied is composed by water, methyl ricinoleate (MR), and Tween‐80, since it is the base of the medium used for the production of γ‐decalactone, a peach‐like aroma compound of industrial interest that can be produced biotechnologically through the biotransformation of ricinoleic acid, by the yeast Yarrowia lipolytica. Experimental results allow describing the hydrodynamic behaviour of the gas phase into the biphasic medium.     

Hydrodynamic Modelling of Internal Loop Airlift Reactor Applying Drift‐Flux Model in Bubbly Flow Regime

A new model for the liquid circulation rates in airlift reactor (ALR) is presented. The model is based on the energy balance for the flow loop (riser, turn riser‐downcomer, downcomer, and turn downcomer‐riser) coupled with a drift flux theory of two‐phase flow gas‐liquid system, considering a bubbly flow regime. The predicted values of the liquid circulation rates by the developed model are compared with experimental results performed in a 22 dm³ internal loop airlift reactor and with the results obtained in the literatures. The proposed model predicted the experimental results very well. Slip velocity relationship based on the drift flux model was proposed; including the gas holdup, bubble size and the liquid physical properties. The predicted slip velocity was similar to that obtained from the literature. The study revealed that appropriate arrangements of internal bioreactor parts can positively influence the liquid circulation velocity at the same energy consumption. The proposed models are useful in the design; scale up and characterization of the internal loop airlift reactors, and provides a direct method of predicting hydrodynamic behaviour in gas‐liquid airlift reactors.     

Hydrodynamic considerations on optimal design of a three‐phase airlift bioreactor with high solids loading

The hydrodynamic study of a three‐phase airlift (TPAL) bioreactor with an enlarged gas–liquid dual separator was carried out. Different lengths and diameters of the draft tube were tested to show how the design of the separator zone affects the hydrodynamic performance of the TPAL reactor. Ca‐alginate beads with entrapped yeast biomass at different loadings (0, 7, 14 and 21% v/v) were used in order to mimic the solid phase of conventional high cell density systems, such as those with cells immobilized on carriers or flocculating cells. Important information on multiphase flow and distribution of gas and solid phases in the internal‐loop airlift reactor (ALR) with high solids loading was obtained, which can be used for suggesting optimal hydrodynamic conditions in a TPAL bioreactor with high solids loading. It is finally suggested that the ALR with a dual separator and a downcomer to riser cross‐sectional area ratio (A_D/A_R) ranging from 1.2 to 2.0 can be successfully applied to batch/continuous high cell density systems, where the uniform distribution of solid phase, its efficient separation of particles from the liquid phase, and an improved residence time of air bubbles inside the reactor are desirable. Copyright © 2003 Society of Chemical Industry     

Optimization of Carbon Dioxide Fixation by Chlorella vulgaris Cultivated in a Membrane‐Photobioreactor

In this paper, an enclosed membrane‐photobioreactor was designed to remove CO₂ using Chlorella vulgaris. The performances of four reactors, which included the presented novel bioreactor, a draft tube airlift photobioreactor, a bubble column and a membrane contactor, were compared. The effects of the gas flow rate, light intensity, quality of the inner light source, and the characteristics of membrane module on CO₂ fixation were investigated. The results showed that the rate of CO₂ fixation in the membrane‐photobioreactor was 0.95–5.40 times higher than that in the other three conventional reactors under the optimal operating conditions     

Interrelation among Liquid Velocity,Impeller Speed and Gas Flow Rate in the Stirred Airlift Reactor

We investigate the liquid circulation velocity in the draft‐tube airlift reactor, with mechanical agitation in the internal column as a riser, at the impeller speed of up to 40 s^–1. An influence of the gas flow rate and the stirrer speed on the riser and downcomer gas hold‐up difference and on the liquid circulation velocity is also investigated. The character of the liquid circulation velocity changes depends on the relation between the gas flow rate and the impeller speed. A monotonic increase of the liquid circulation velocity with an increase of the gas velocity is observed at the impeller speed of lower than 15 s^–1. A distinct decrease of the liquid velocity is found at higher impeller speeds and at low gas velocities. The decrease is larger for higher impeller speeds. We observe the minimum on the curve of the liquid velocity dependence on the gas velocity followed by the monotonic increase of the liquid velocity with an increase of the gas flow velocity. The minimum of the liquid circulation velocity appears if the ratio of the gas flow number to the impeller speed is about 0.0006. The minima are shifted towards the higher gas velocities at higher impeller speeds. An experimental equation for the prediction of the liquid circulation velocity in the stirred airlift reactor is presented.     

Effect of aeration mode on the performance of center‐ and annulus‐rising internal‐loop airlift bioreactors

Extensive experimental studies on internal‐loop airlift reactors, including center‐rising (CR‐ALR) and annulus‐rising airlift reactors (AR‐ALR), have been reported in the literature. However, to the best of the authors’ knowledge, the effects of the aeration mode on the local hydrodynamics remain an under‐investigated area, especially for complex culture media. At present, it is difficult to select the best aeration mode for ALRs due to limited understanding of the pros and cons of the different modes. This study presents a detailed quantitative investigation of the overall gas holdup, local liquid velocity, liquid circulation time, shear rate distribution, and volumetric mass transfer coefficient in center‐ and annulus‐rising airlift bioreactors to better understand the effect of aeration mode on airlift bioreactor performance. Particle image velocimetry is employed to conduct local measurements. The results show that the overall gas holdup, liquid circulation time, and volumetric mass transfer coefficient are larger in the AR‐ALR than in the CR‐ALR. The local liquid velocity circulating into the downcomer of the AR‐ALR, which contributes to bubble entrainment and therefore to overall gas holdup, is higher than in the CR‐ALR. It was observed that a large circulation loop formed in the CR‐ALR, whereas two counter‐looping circulation cells appeared in the AR‐ALR. It was also found that the shear rate field was more uniform in the AR‐ALR than the CR‐ALR although the shear rates were similar in magnitude.     

Radial pressure profiles in a cold‐flow gas‐solid vortex reactor

A unique normalized radial pressure profile characterizes the bed of a gas‐solid vortex reactor over a range of particle densities and sizes, solid capacities, and gas flow rates: 950–1240 kg/m³, 1–2 mm, 2 kg to maximum solids capacity, and 0.4–0.8 Nm³/s (corresponding to gas injection velocities of 55–110 m/s), respectively. The combined momentum conservation equations of both gas and solid phases predict this pressure profile when accounting for the corresponding measured particle velocities. The pressure profiles for a given type of particles and a given solids loading but for different gas injection velocities merge into a single curve when normalizing the pressures with the pressure value downstream of the bed. The normalized—with respect to the overall pressure drop—pressure profiles for different gas injection velocities in particle‐free flow merge in a unique profile. © 2015 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 61: 4114–4125, 2015     

基于介尺度PBM模型的生物反应器放大模拟及实验研究

对于通气搅拌式工业生物反应器的放大设计而言,精确预测气泡尺寸和体积传质系数非常重要,因此需要建立合适的气泡聚并和破碎模型,以保证反应器的高效操作。以5 L通气搅拌式生物反应器为对象,以气泡尺寸和体积传质系数的实验数据为基准,模拟并考察了两种聚并模型和四种破碎模型对生物反应器内流体流动行为以及传质能力的影响。结果表明,基于介尺度理论的修正聚并模型与考虑黏流剪切的破碎模型组合,所得模拟结果与实验数据吻合最好,这为大型生物反应器的桨型优化提供了模型基础。因为工业化生物发酵通常是在大型生物反应器中进行,搅拌桨型对生物反应器效能至关重要,故本研究在选定最优气泡聚并破碎模型的基础上,通过叶轮末端剪切力相等的放大原则将5 L通气搅拌式工业生物反应器放大到400 m³,同时考察了六斜叶圆盘搅拌桨、非对称式抛物线搅拌桨、布鲁马金式搅拌桨以及六直叶圆盘搅拌桨等桨型组合对气泡破碎能力和气体分散效果的影响,并通过综合对比气含率、体积传质系数等参数,得到400 m³通气搅拌式生物反应器的最优桨型组合。     

Suspension of solid particles in multi‐impeller three‐phase stirred tank reactors

Solids suspension characteristics in gas—liquid–solid three‐phase stirred tanks with multi‐impellers were experimentally examined. Minimum impeller speeds for ultimately homogeneous solid suspension have been measured stirred tank reactors. Three impellers were installed: two four‐pitched blade downflow disk turbines and one Pfaudler type impeller chosen to provide good gas dispersion and to accomplish off‐bottom suspension of solid particles, respectively. Gas dispersion causes an increase in particle sedimentation associated with a decrease in power consumption and as a result, minimum impeller speeds for ultimately homogeneous solid suspension increase with increasing gas flow rates. A correlation was developed to predict minimum impeller speeds for ultimately homogeneous solid suspension. The proposed correlation, which agrees satisfactorily with the experimental results, is expected to be useful in design and scale‐up.     

Model for a solid‐liquid airlift two‐phase partitioning bioscrubber for the treatment of BTEX

BACKGROUND: Airlift solid–liquid two‐phase partitioning bioreactors (SL‐TPPBs) have been shown to be effective for the treatment of gas streams containing benzene, toluene, ethylbenzene and o‐xylene (BTEX). The airlift SL‐TPPB is a low‐energy system that utilizes a sequestering phase of solid silicone rubber beads (10%v/v) that will uptake and release large amounts of BTEX in order to maintain equilibrium conditions within the system. This increases mass transfer from the gas phase during dynamic loading periods and improves degradation performance. This study discusses the development and analysis of a steady‐state, tanks‐in‐series mathematical model, arising from mass balances on BTEX and oxygen in the gas, aqueous and polymer phases to predict the performance of the airlift SL‐TPPB over various gas flow rates and BTEX loadings. RESULTS: An estimability analysis on model parameters determined that the parameters to which model output is most sensitive are those that affect biological activity, which were targeted for estimation. The developed tanks‐in‐series model was able to predict the removal of BTEX components and dissolved oxygen concentrations over various inlet loadings (20, 60 and 100 mg L^?1 h^?1) and gas flow rates (2,3 and 4 L min^?1) that resulted in a range of system performance from effective BTEX treatment to oxygen limiting conditions. CONCLUSIONS: The model developed, with estimated parameters, provides a valuable tool to determine operating conditions that will result in favourable performance of the airlift SL‐TPPB. Copyright © 2009 Society of Chemical Industry     

Mixing of medium viscosity liquids in a stirred tank with fractal impeller

Mixing of viscous liquids in a stirred tank is a daunting task. The present paper explores the possibility of using a fractal impeller for mixing of viscous liquids in a stirred tank. The analysis includes power consumptions characteristics, mixing characteristics and the flow patterns in the stirred tank. Ultrasonic velocity profiler (UVP) was used to measure the local velocities in the stirred tank. Fractal impeller found to exhibit different power consumption characteristics than known for conventional impellers. For the range of viscosities 0.58–0.192 Pa s, mixing time found to be directly proportional to the power consumption per unit mass. The normalised mean radial velocity profiles were found to be independent of fluid viscosities studied in the present work.     

Effect of a gas–liquid separator on the hydrodynamics and circulation flow regimes in internal‐loop airlift reactors

The role of the gas–liquid separator on hydrodynamic characteristics in an internal‐loop airlift reactor (ALR) was investigated. Both gas holdup and liquid velocity were measured in a 30 dm³ airlift reactor with two different head configurations: with and without an enlarged separator. A magnetic tracer method using a neutrally buoyant magnetic particle as flowfollower was used to measure the liquid velocity in all sections of the internal‐loop airlift reactor. Average liquid circulation velocities in the main parts of the ALR were compared for both reactor configurations. At low air flow rates the separator had no influence on gas holdup, circulation velocity and intensity of turbulence in the downcomer and separator. At higher superficial air velocities, however, the separator design had a decisive effect on the hydrodynamic parameters in the downcomer and the separator. On the other hand, the gas holdup in the riser was only slightly influenced by the separator configuration in the whole range of air flow. Circulation flow regimes, characterising the behaviour of bubbles in the downcomer, were identified and the effect of the separator on these regimes was assessed. © 2001 Society of Chemical Industry     

Computational Fluid Dynamics Study of Flow Induced by a Grooved High‐Shear Impeller in an Unbaffled Tank

A detailed hydrodynamic characterization in the transitional flow regime of two variants of the Norstone Polyblade® high‐shear impeller (HSI), of industrial relevance, is presented. The study was carried out on a simulated Newtonian fluid using computational fluid dynamics (CFD). Measurements of power number were carried out to validate the simulation results. Hydrodynamic parameters considered of key importance in powder dispersion processes (i.e., viscous dissipation and effective circulation) were used to assess the impellers' performance. Furthermore, their performances were compared with reported data for two ring‐style HSIs of two and four rings, and power number measurements of a sawtooth (Cowles‐type) impeller.     

On the accuracy of Landweber and Tikhonov reconstruction techniques in gas‐solid fluidized bed applications

As electrical capacitance tomography technique needs a sophisticated reconstruction, the accuracy of two of the most widely used reconstruction techniques (Landweber and Tikhonov) for gas‐fluidized bed applications were assessed. For this purpose, the results of two‐fluid model simulations were used as an input of reconstruction. After finding the optimum reconstruction parameters for the studied system, it is found that both techniques were able to obtain the radial profile and overall value of average volume fraction very well. Conversely, both methods were incapable to determine bubble sizes accurately especially small bubble sizes, unless the Landweber technique with inverted Maxwell concentration model is applied. The probability distribution of the reconstructed results was also smoother in transition between the emulsion and bubble phases compared to the reality. Finally, no significant differences in noise immunity of these two techniques were observed. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4102–4113, 2015