Effects of Different Draft Baffles on the Hydrodynamics in Internal-Loop Airlift Reactors

In this paper, a 2-D airlift reactor was developed. The streamline and hydrodynamic parameters were measured in a 2-D airlift loop reactor(ALR)with different draft baffles. Three regimes were observed under different conditions. Particle image velocimetry(PIV)measurement showed that the liquid velocity distribution in horizontal direction presented different profiles in the three regimes. The length, the height and the spacing of draft baffles were applied in the experiments to optimize the ALR structure. It was found that the draft tube structure is of great importance in determining the hydrodynamics of ALRs. Additionally, the experimental results may serve as a step to the further optimization and design of ALR.

Oilfield produced water treatment in internal-loop airlift reactor using electrocoagulation/flotation technique

Oilfield produced water is large quantities of salty water trapped in underground formations and subsisted under high temperatures and pressures that are brought to the surface along with oil during production. Produced water(PW) contains a lot of pollutants such as hydrocarbons and metals, this water must be treated before disposal. Therefore, different techniques are being used to treat produced water. Electrocoagulation is an efficient treatment technique involving the dissolution of anodes and formation of electro-coagulants, while the simultaneous generation of H_2 bubbles at the cathode leads to the pollutant removal by flotation. Electrocoagulation(EC)method is one of the most promising and widely used processes to treat oilfield produced water. In the present work, a conventional internal-loop(draught tube) airlift reactor was utilized as electrocoagulation/flotation cell for PW treatment by inserting two aluminum electrodes in the riser section of the airlift reactor. The EC airlift reactor was operated in a batch mode for the liquid phase. Different experimental parameters were studied on the oil and turbidity removal efficiencies such as current density, initial pH, electrocoagulation time, and air injection.The experimental results showed that mixing of the oil droplets in the PW was accomplished using only the liquid recirculation resulted by H_2 microbubbles generated by EC process which enhanced the oil removal. The experimental results further showed that the EC time required achieving ≥ 90% oil removal efficiency decreases from 46 to 15 min when operating current density increases from 6.8 to 45.5 mA·cm^(-2). This reactor type was found to be highly efficient and less energy consuming compared to conventional existing electrochemical cells which used mechanical agitation.

Hydrodynamics and Mass Transfer in a Modified Three-phase Airlift Loop Reactor

A modified internal-loop airlif 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.

HYDRODYNAMIC STUDIES ON LOOP REACTORS (Ⅱ) AIRLIFT LOOP REACTORS

Hydrodynamics of airlift loop reactors was studied in detail experimentally andtheoretically.An internal airlift loop reactor was designed and set up for this study.An instru-mentation system based on the electrochemical method was adapted to measure local gas holdup andliquid velocity.A two-dimensional two-fluid model based on the first principles was established andimplemented to model the flow in airlift loop reactors.A corrected turbulent model was incorporatedin the simulation.The shear rate,shear stress and energy dissipation are evaluated from the flowfield.The numerically predicted results and experimental data obtained from this work as well as thesereported in literature are analyzed and compared.

Treatment of Dimethoate Aqueous Solution by Using Ultrasonic Airlift Loop Reactor

Ultrasonic airlift loop reactor （UALR） shows potential and wide application for wastewater treatment. In this paper the performance and efficiency of UALR in dimethoate degradation were presented. The effects of O3 flow rate, ultrasonic intensity and initial concentration of dimethoate on degradation rate were investigated. UALR imposed a synergistic effect combining sonochemical merit with high O3 transfer rate. The results showed that UALR not only increased degradation rate, but also was better than the simole sum of degradation by O3 and ultrasound separately. Under the operation conditions of O3 flow of 0.34 m^3·h^-1, ultrasonic intensity 3.71 W.cm^-2, and initial concentration of dimethoate at 20 mg·L^- 1, the degradation rate of dimethoate increased to 80%. UALR seems an advisable choice for treating organic wastewater and this process may have wide application prospect in industry.

Epoxidation of Cyclohexene with Oxygen in Ultrasound Airlift Loop Reactor

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

Modeling of Continuous Cross-flow Microfiltration Process in an Airlift External-loop Slurry Reactor

New modified combination mathematical models including the pores blocking models and the cake layer models were developed to describe the continuous cross-flow microfiltration in an airlift external loop slurry reactor. The pores blocking models were created based on the standard blocking law and the intermediate blocking law, and then the cake layer models were developed based on the hydrodynamic theory in which the calculation method of porosity of cake layer was newly corrected. The Air-Water-FCC equilibrium catalysts cold model experiment was used to verify the relevant models.Results showed that the calculated values fitted well with experimental data with a relative error of less than 10%.

Hydrodynamics and bubble behaviour in a three-phase two-stage internal loop airlift reactor

Local hydrodynamics of a gas–liquid–solid system,such as bubble circulation regime,gas holdup,liquid velocity and axial profile of solid concentration,are studied in a two-stage internal loop airlift reactor.Empirical correlations for gas holdup and liquid velocity are proposed to ease the reactor design and scale-up.Different bubble circulation regimes were displayed in the first(lower) and second(upper) stages.Increasing superficial gas velocity and solid loading can promote regime transition of the second stage,and the gas holdup of the second stage is higher than that of the lower stage.In addition,the effects of solid loading on bubble behaviour are experimentally investigated for each stage.It is found that bubble size in the downcomer decreases with the presence of solid particles,and bubble size distribution widens under higher superficial gas velocity and lower solid loading.

Analysis and Measurement of Mass Transfer in Airlift Loop Reactors

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

Experimental and Numerical Study of Gas-Liquid Flow in a Sectionalized External-Loop Airlift Reactor

The external loop airlift reactor(ELALR)is widely used for gasliquid reactions.It’s advantage of good heat and mass transfer rates compared to conventional bubble column reactors.In the case of fermentation application where a medium is highly viscous and coalescing in nature,internal in riser helps in the improvement of the interfacial area as well as in the reduction of liquidphase back mixing.The computational fluid dynamic(CFD)as a tool is used to design and scaleup of sectionalized external loop airlift reactor.The present work deals with computational fluid dynamics(CFD)techniques and experimental measurement of a gas holdup,liquid circulation velocity,liquid axial velocity,Sauter mean bubble diameter over a broad range of superficial gas velocity 0.0024≤UG≤0.0168 m s 1.The correlation has been made for bubble size distribution with specific power consumption for different plate configurations.The effects of an internal on different mass transfer models have been completed to assess their suitability.The predicted local mass transfer coefficient has been found higher in the sectionalized external loop airlift reactor than the conventional ELALR.

CFD Simulation of Scale Influence on the Hydrodynamics of an Internal Loop Airlift Reactor

In this work, the overall gas hold-up in the riser and down-comer of three internal airlift reactors with a working volume of 10.5, 32 and 200 l at the range of temperatures 18℃ - 21℃, under atmospheric pressure was simulated using Com-putational Fluid Dynamics (CFD). The range of superficial gas velocity was 0.5 - 3 cm/s. The three reactors geometrically were similar to each other. CFD simulation and experimental data showed that the gas hold-up in the riser and down-comer increased by increasing the reactor scale. It was concluded that the simulated data were in good agreement with the experimental ones obtained from the literature.

Influences of top clearance and liquid throughput on the performances of an external loop airlift slurry reactor integrated mixing and separation

A new developed external loop airlift slurry reactor, which was integrated with gas–liquid–solid three-phase mixing, mass transfer, and liquid–solid separation simultaneously, was deemed to be a promising slurry reactor due to its prominent advantages such as achieving continuous separation of clear liquid from slurry and cyclic utilization of solid particles without any extra energy, energy-saving, and intrinsic safety design. The principal operating parameters, including gas separator volume, handling capacity, and superficial gas velocity, are systematically investigated here to promote the capabilities of mixing, mass transfer, and yield in the pilot external loop airlift slurry reactor. The influences of top clearance and throughput of the clear liquid on flow regime and gas holdup in the riser, liquid circulating velocity, and volumetric mass transfer coefficient with a typical high solid holdup and free of particles are examined experimentally. It was found that increasing the gas separator volume could promote the liquid circulating velocity by about 14.0% at most. Increasing the handling capacity of the clear liquid from 0.9 m3·h-1 to 3.0 m3·h-1 not only could increase the output without any adverse consequences, but also could enhance the liquid circulating velocity as much as 97.3%. Typical operating conditions investigated here can provide some necessary data and guidelines for this new external loop airlift slurry reactor to upgrade its performances.

The Preparation of 2-Chloro-5-chloromethylpyridine in an Airlift Loop Reactor

A new process for the direct chlorination of 2-chloro-5-methylpyridine to yield 2-chloro-5-chloro-methylpyridine in an airlift loop reactor (ALR) has been studied. Five main reaction conditions including TR, na/ns, cp, Qg and dD/dR were optimized. The average molar yield and purity of 2-chloro-5-chloromethylpyridine obtained were 79% and 98.5% respectively under the optimum operating conditions. Finally, the efficiency for the preparation of 2-chloro-5-chloromethylpyridine with ALR and stirred tank reactor (STR) respectively was compared.

MASS TRANSFER AND HYDRODYNAMICS IN AN AIRLIFT REACTOR WITH VISCOUS NON-NEWTONIAN FLUID

In an internal loop airlift reactor of 55L working volume,the gas-liquid volumetric oxygenmass transfer coefficient k_Lα,gas holdup ε_G and liquid circulation time t_c were measured with the sol-ution of carboxymethyl cellulose(CMC)to simulate the performance of a reactor with highly viscousbroth.Electric conductivity and oxygen probes were used to measure the local gas holdup,liquidcirculation time and oxygen mass transfer coefficient in the individual sections of the reactor(riser,downcomer and the gas-liquid separating section at the top of the reactor)and the total reactor,respectively.The values of k_Lα for the riser,downcomer and separation sections of the reactor were alsoestimated and compared with that for the total reactor.The results show that,both k_Lα and ε_G in-crease but t_c decreases with increasing gas velocity.Correlations and comparisons with works reportedin the literature are also presented.Data show that the methods developed for k_Lα measurements inthe individual section

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.

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

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

Experimental Study on the Hydrodynamics and Mass Transfer Characteristics of Airlift Loop Reactors (ALR)

The promoting effect of ultrasonic wave on the hydrodynamics and mass transfer characteristics of the airlift loop reactor was studied. The effect of the airlift reactor and ultrasonic wave on the reactor’s gas holdup, liquid circulation velocity, mixing time and overall volumetric mass transfer coefficient respectively with and without the presence of ultrasonic wave is empathetically examined and compared. The experiment has proven that the incorporation of ultrasonic wave has no effect on the gas holdup but has the tendency to gradually decrease the liquid circulation velocity and increase the overall volumetric mass transfer coefficient; the effect on the mixing time is relatively complex. At low gas velocity, low powered ultrasonic wave promotes the radial mixing of fluid; with the increase of ultrasonic power, ultrasonic vibration obstructs the radial mixing of fluid. Therefore, there exists an optimal ultrasonic power. Moreover, the effect of ultrasonic wave on the mixing time gradually decreases with the increase of the superficial gas velocity. Correlations were also proposed for the hydrodynamics and mass transfer characteristics of the reactor.

高效气提环流式生物脱氮反应器在污水厂扩建改造中的应用

西北某污水处理厂扩建改造一期工程处理规模为5000m^(3)/d,采用高效气提环流式生物脱氮反应器(DIAB)工艺,所用反应器基于厌氧-缺氧-好氧(AAO)推流式生物反应器与升流式厌氧污泥床(upflow anaerobic sludge bed,UASB)反应器特点进行改良,由多级环状结构构成,利用三相分离器结构设计实现水、气、污泥分离,利用曝气尾气作为动力来源,实现混合液的无动力回流。总水力停留时间为21 h,其中厌氧区、缺氧区和好氧区停留时间分别为1.5、8.5 h和11 h,混合液悬浮固体(MLSS)质量浓度为5500 mg/L,平均BOD5污泥负荷为0.1 kg BOD5/(kg MLSS·d),混合液回流比为300%。污水厂采用该工艺完成改造运行后实际出水水质可稳定达到一级A标准。扩建工程单位处理成本为0.454元/m^(3)、生物处理区占地面积为0.161 m2/m^(3),与相似规模传统AAO工艺相比较处理成本和占地面积分别减少了24.3%和25.8%,在出水水质、运行成本方面均有明显改善,与相似规模循环式活性污泥(CASS)工艺相比较占地面积相似,但处理成本减少约20%,出水水质优于CASS工艺,尤其在脱氮方面表现出色。DIAB工艺脱氮效率及碳源利用率较高,脱氮效果好,碳源投加及回流设备用电电耗低,适用于土地、工期受限,以及进水低碳氮比条件下污水处理厂的扩建、应急建设。

二级环流反应器中收缩-膨胀隔板的性能优化

通过在二级环流反应器两级之间安装具有收缩-膨胀形状的隔板,可以有效减小两级之间的液相回流流量,但同时会增加流体流动阻力。为了在减小液相回流流量的前提下减小流动阻力,本研究对隔板进行优化设计。首先通过模拟计算获得不同隔板下的压降和液相回流流量,随后对数据进行拟合,得到压降、液相回流流量与结构参数的关联式。计算结果表明当表观气速较小时,隔板宽度对压降影响较大,高度的影响较小。表观气速增大时,隔板高度对压降的影响变大。在所有气速下,隔板宽度是影响液相回流流量的决定因素。最后通过多目标优化,获得了在不同表观气速下最优隔板的Pareto前沿。

气升式环流反应器在生化处理中的中试试验

生化处理是炼油废水处理过程中的关键环节,目前的生化处理工艺以氧化塘、SBR为主,但好氧反应区域存在气液混合不均匀、发泡尺寸较大等问题,进而导致氧气利用率低、能耗高。为提高生化反应效率,缩短曝气时间,本文设计了气升式环流反应器(ALR),通过安装微细气泡曝气盘配合曝气环产生分形气泡以强化气-液-固三相传质和混合,将二级生化处理单元中的好氧反应区和沉淀区集成在同一个反应器内,并开展了中试试验。研究对比了ALR和SBR对气浮出水的处理效果,衡量了两者的技术经济性指标。试验结果表明:在ALR与SBR在油含量、COD和氨氮去除效果相当的情况下,ALR的污泥负荷提升了10倍,COD容积负荷提升了5倍;同时,ALR的吨水处理的鼓气量减少了57%,吨水处理占地面积仅为SBR的50%左右,吨水处理的能耗降低了22.9%,解决了传统生化处理工艺占地面积大、能耗高等问题,为未来生化反应器的升级改造提供了重要参考。