带气体分布器表面曝气反应器的气液传质特性

在高径比较大的表面曝气反应器中,由表面吸入的气体较难分散到反应器的底部,为增强反应器的气液传质性能,需要将反应器底部供气和表面曝气的配置优化。在直径380 mm、液位为530 mm的表面曝气反应器中,表观气速为2.45,4.90,7.35 mm/s的条件下,实验测定了3种不同构型的双层桨组合的气液传质系数和搅拌功率。结果表明,表面曝气结合气体分布器供气能有效改善气液反应器内气体的整体混合状况,以较低的功耗达到气体分布均匀、增强气液传质性能的目的。     

Experimental and numerical study of gas hold-up in surface aerated stirred tanks

Although the distribution of gas hold-up in stirred tanks is a key factor to their design and operation, systematic experimental data on local gas hold-up of surface-aerated stirred tanks are not available in open literature. In this work, turbulent two-phase flow in a surface aeration stirred tank with a diameter of 0.380 m was investigated experimentally and numerically. The gas hold-up was measured with a conductance probe at various operating conditions. A surface baffle to improve the efficiency of surface aeration of a Rushton disk turbine was designed and tested. The experimental data suggest that the gas hold-up distribution in the surface aeration tank is very non-uniform, and the surface baffle improves the aeration rate particularly at a high agitation speed. A three-dimensional in-house computational fluid dynamic (CFD) two-fluid model with the standard k?A_p turbulence model was used to predict the gas-liquid flow, and the impeller region was handled using the improved inner-outer iterative procedure. Based on Kolmogoroff's theory of isotropic turbulence, a constitutive equation for surface aeration strength was proposed. The numerical prediction, in combination with the measurements, gives insight to the surface aeration performance of stirred tanks. It was found that the simulation reasonably predicted the gas hold-up distribution in the upper tank, but underestimated it in the region below the stirrer.     

Performance comparison of batch and continuous flow surface aeration systems

The oxygen transfer rate and the corresponding power requirement to operate the rotor are vital for design and scale-up of surface aerators. The aeration process can be analyzed in two ways such as batch and continuous systems. The process behaviors of batch and continuous flow systems are different from each other. The experimental and numerical results obtained through the batch systems cannot be relied on and applied for the designing of the continuous aeration tank. Based on the experimentation on batch and continuous type systems, the present work compares the performance of both the batch and continuous surface aeration systems in terms of their oxygen transfer capacity and power consumption. A simulation equation developed through experimentation has shown that continuous flow surface aeration systems are taking more energy than the batch systems. It has been found that batch systems are economical and better for the field application but not feasible where large quantity of wastewater is produced.     

Continuous‐Flow Surface Aeration Systems

An aeration process in an activated sludge plant is a continuous‐flow system. In this system, there is a steady input flow (flow from the primary clarifier or settling tank with some part from the secondary clarifier or secondary settling tank) and output flow connection to the secondary clarifier or settling tank. The experimental and numerical results obtained through batch systems can not be relied on and applied for the designing of a continuous aeration tank. In order to scale up laboratory results for field application, it is imperative to know the geometric parameters of a continuous system. Geometric parameters have a greater influence on the mass transfer process of surface aeration systems. The present work establishes the optimal geometric configuration of a continuous‐flow surface aeration system. It is found that the maintenance of these optimal geometric parameters systems result in maximum aeration efficiency. By maintaining the obtained optimal geometric parameters, further experiments are conducted in continuous‐flow surface aerators with three different sizes in order to develop design curves correlating the oxygen transfer coefficient and power number with the rotor speed. The design methodology to implement the presently developed optimal geometric parameters and correlation equations for field application is discussed.     

有无静态混合器时的机械搅拌气升式环流反应器传递特性研究

The mechanically stirred internal loop airlift reactors equipped with or without static mixers are devised for intensification of gas-liquid mass transfer rate. The influences of superficial gas velocity, agitation or static mixers on gas hold-up, mixing time, liquid circulating velocity and volumetric mass transfer coefficient have been investigated with tap water and carboxymethyl cellulose (CMC) aqueous solution. The experimental results indicate that mechanical agitation is more efficacious than static mixer in highly viscous media for improving mass transfer in airlift reactors. The empirical correlation of volumetric mass transfer coefficient with apparent viscosity, and energy consumption for mechanical agitation and aeration is developed.     

Development of microbubble aerator for waste water treatment using aerobic activated sludge

In large-scale waste water treatment plants, the aerobic biochemical reactor is the most important process, where the oxygen supply into the microorganisms often limits the overall waste water treatment rate. On the other hand, several kinds of microbubble distributors have been developed to enrich the oxygen dissolution in water. Therefore, the application of microbubbles for a waste water treatment system was investigated in this study.The oxygen absorption performance of typical microbubble generators was compared with typical bubble generators. To evaluate each bubble generator, the liquid-phase volumetric oxygen transfer coefficient, gas hold-up and power consumption per unit liquid volume were measured in a bubble column attached to each bubble generator. All the microbubble generators allowed the oxygen to dissolve faster than the typical aerators. The spiral liquid flow type microbubble generator had the highest oxygen transfer coefficient even at a low air flow rate although it used more energy than the typical distributors.To improve an industrial waste water treatment system, a novel aeration system utilizing a spiral liquid flow type microbubble generator was proposed in this study. The present system has some advantages such as compact size, portability and fast oxygen dissolution rate. To ensure the performance for organic waste water treatment, the effects of the aeration rate, dissolved oxygen concentration and device properties on the specific consumption rate of model organic waste were investigated. For the novel aeration system, the most suitable conditions to treat organic waste were found.     

Simultaneous measurement of gas hold-up and mass transfer coefficient by tracer dynamic technique in “Emulsair” reactor with an emulsion-venturi distributor

Hydrodynamics and gas-liquid mass transfer have been investigated in an “Emulsair” reactor with cocurrent downflow of gas and liquid. This consists of a cylindrical tank with conical bottom topped by an emulsion-venturi as the gas-liquid distributor in which the gas is self-aspired by action of the kinetic energy of the liquid recirculation. An original tracer dynamic technique using the CO₂-N₂/water system that enables the simultaneous measurement of overall gas hold-up and overall volumetric mass transfer coefficient has been developed and validated using conventional techniques, such as volume expansion for gas hold-up and dynamic oxygenation for mass transfer. It has been shown that gas hold-up and K_La_L can be deduced from the moments of experimental response curves using a CO₂ pulse in the gas feed. Experimental results have proved that hydrodynamics and mass transfer in the Emulsair reactor are strongly influenced by the flow regime in the divergent. Two different regimes have been observed depending on the liquid recirculation flow rate: annular and homogeneous bubbling flows. In both regimes, self-aspired gas flow rate, gas hold-up and K_La_L have been reported to increase with the liquid flow rate. The operation effectiveness, estimated from the gas-to-liquid flow rate ratio, has been shown to pass through a maximum around 0.59 as a function of liquid recirculation. A comparison with the literature proved that this maximum is higher than those observed for other kinds of gas-liquid reactors equipped with a venturi. Correlations for mass transfer estimation have been derived and are in agreement with the literature.     

STUDY OF DOWNFLOW LIQUID JET LOOP REACTOR

1 INTRODUCTIONLiquid jet loop reactor(JLR)may be upflow(U-JLR)or downflow reactors(D-JLR)in design.The major differences between the two are the location of the nozzle andthe direction of the fluid flow.A large number of investigations on U-JLR havebeen published,but D-JLR with nozzles positioned on the top portion of the reac-tor was not much studied until recently.Up to now,only a few experimentalstudies on the hydrodynamics and mass transfer of D-JLR have been carried out[1-4].     

Gas hold-up and bubble diameters in a gassed oscillatory baffled column

In this paper attempts are made to address how bubble behaviour in a batch oscillatory baffled column (OBC) contributes to the overall measured enhancement in mass transfer. A CCD camera is used to measure the bubble size distribution and the gas hold-up in the OBC. The experimental results of Sauter mean diameter and gas hold-up are correlated as a function of the power dissipation and superficial gas velocity, in order to allow for comparisons with published correlations for the mass transfer coefficient. In general, an increase in the oscillatory velocity causes an increase in the hold-up and a decrease in the Sauter mean diameter. Furthermore, we are able to show that the changes in the gas hold-up contribute more than the mean bubble size to the control of the mass transfer coefficient.     

Scaling Up of the Geometrically Similar Unbaffled Circular Tank Surface Aerators

The present investigation is concerned with the development of scale‐up equations for the oxygen transfer coefficient, k, and the power number, P_O, for unbaffled circular tank surface aerators under geometrically similar conditions, with the purpose of designing energy efficient aerators. It has been found for the present aerators that k and P_O are uniquely related to a parameter, X, governing the theoretical power per unit volume and which is defined as Fr^4/3Re^1/3, where Fr and Re are the impellers' Froude and Reynolds numbers, respectively. Empirical correlations between k and X, as well as P_O with X are developed for the range of experiments conducted. Based on such experimental results, procedures to design energy efficient aeration systems have been demonstrated and it was found that smaller sized aerators are more energy efficient and economical when compared to bigger sized tanks, while aerating the same volume of water, by maintaining the same input power to the tanks irrespective of their size. It has been also demonstrated that substantial amounts of energy can be saved if the aeration tanks run at higher input power.     

Gas—liquid dispersion and mixing characteristics and heat transfer in a stirred vessel

Properties of gas—liquid dispersion and mixing of seven types of impellers were studied and compared in a stirred vessel with aeration. New correlations for the properties including critical dispersion impeller speed, dispersion regime, power consumption, gas hold-up, discharge flow number and discharge efficiency have been developed. The fluid/wall heat transfer was also studied with several types of dual impeller combinations. There is a critical impeller speed which determines how aeration changes the heat transfer coefficient. Operating conditions influence heat transfer interactively by three factors, which can be expressed by proper dimensionless variables.     

Gas-liquid reactions in packed beds: The effectiveness of static hold-up in presence of gas side resistance

The interfacial area in a packed bed may be divided into two parts, i.e. the interfacial area corresponding to the dynamic hold-up and that corresponding to the static hold-up. The effectiveness of the latter for gas absorption with a chemical reaction can become quite low due to the depletion of the liquid phase reactant in the static hold-up. As a result, the effective interfacial area may become substantially smaller. This aspect has been investigated earlier for liquid phase controlled absorption with a chemical reaction. In this paper, a procedure is developed for calculating the effectiveness of the interfacial area of the static hold-up for the case of significant gas-side mass transfer resistance for a second-order chemical reaction. It is shown that the gas-side mass transfer resistance increases the effectiveness of the static hold-up. The results are presented in a convenient graphical form. It is shown that under certain conditions this effectiveness is quite sensitive to the magnitude of the gas-side mass transfer resistance. The analysis presented here is based on the film theory.     

Gas hold-up and volumetric mass transfer coefficient in bubble columns under foam control

Experimental measurements of the gas hold-up and volumetric mass transfer coefficient have been made for baffle columns (BCs) reacting various foaming liquids under mechanical and chemical foam control. The gas hold-up and the volumetric mass transfer coefficient in a mechanical foam-control system (BCs with rating-disk mechanical foam-breakers) were larger than those in a chemical foam-control system (BCs with an antifoam agent added). Correlations for the gas hold-up and the volumetric mass transfer coefficient in BCs under foam control are presented. Comparison of the volumetric mass transfer coefficient between the mechanical foam-control system and the chemical foam-control system in terms of the specific power input also demonstrated higher mass transfer performance and saving power requirements for the mechanical foam-control system.     

Electrochemical study of liquid-solid mass transfer in packed bed electrodes with upward and downward co-current gas-liquid flow

Hydrodynamic and mass transfer parameters (pressure drop, gas and liquid hold-up, liquid-solid mass transfer coefficients) have been measured for porous electrodes with upward or downward co-current gas-liquid flow by means of several electrochemical techniques. The influence of the most important parameters (packing diameter, gas and liquid flow rates) and of the hydrodynamic flow regimes, has been studied. It is found that in the trickle flow regime the limiting current densities depend only on the liquid flow rates (with no measurable influence of the gas). In the upward flow configuration, the strong turbulence generated by the ascending gas bubbles leads to a sharp increase of current densities with the gas flow rate. A comparison between both configurations is presented.     

Studies in multiple impeller agitated gas-liquid contactors

Experiments have been performed to study the effect of the density and the volume of the tracer pulse on the mixing time for two impeller combinations in the presence of gas in a 0.3 m diameter and 1 m tall cylindrical acrylic vessel. The tall multi-impeller aerobic fermenters, which require periodic dosing of nutrients that are in the form of aqueous solution, is a classic case under consideration. Conductivity measuring method was used to measure the mixing time. Two triple impeller combinations; one containing two pitched blade downflow turbines as upper impellers and disc turbine as the lowermost impeller (2 PBTD-DT) and another containing all pitched blade downflow turbines (3 PBTD) have been used. Other variables covered during experiments were the density and the amount of the tracer pulse, the impeller rotational speed and the gas superficial velocity. Fractional gas hold-up, Power consumption and mass transfer coefficient have also been measured for both the impeller combinations. Influence of aeration and impeller speed on the mixing time has been explained by the interaction of air induced and impeller generated liquid flows. Three different flow regimes have been distinguished to explain the hydrodynamics of the overall vessel (i.e., multiple impeller system). A compartment model with the number of compartments varying with the flow regimes have been used to model liquid phase mixing in these flow regimes. A correlation for the prediction of the dimensionless mixing time in the loading regime has been proposed in order to account the effect of the density and the amount of the tracer pulse on the mixing time. Correlations have also been proposed to predict fractional gas hold-up and k_La.     

污水好氧生物处理工艺中供气系统的优化与节能分析

对于污水好氧生物处理技术而言,供气系统包括曝气方式/装置、气体输送管线、曝气器(气体释放头)、反应器/反应池以及结合运行程序的监控仪表等,以节能降耗作为考察目标,本文全面分析了供气系统中的常见曝气方式的适用范围、影响因素、优化控制与节能途径.以鼓风曝气方式为主要考察对象,分析了空气从风机到曝气池释放的全过程耗能及节能途...     

INFLUENCE OF NON-NEWTONIAN FLOW BEHAVIOUR ON MASS TRANSFER INBUBBLE COLUMNS WITH AND WITHOUT DRAFT TUBES†

Gas hold-up and mass transfer were examined in a column with and without a draft tube. It was found that the introduction of a draft tube increases the gas hold-up but decreases the volumetric mass transfer coefficient in Newtonian fluid systems. For non-Newtonian fluid systems, both parameters were increased by the presence of the draft tube. Empirical correlations are proposed for the gas hold-up and the volumetric mass transfer coefficient in the bubble column with Newtonian and non-Newtonian fluid systems. The correlations are in general agreement with the data in this work and in the literature. They should be useful for design and scale-up purposes. It was also found that introduction of an ancillary impeller improves the mass transfer in non-Newtonian fluids due to the break-up of large bubbles.     

Solid effects on hydrodynamics and heat transfer in an external loop airlift reactor

The effect of a solid presence on global hydrodynamic parameters and heat transfer in an external loop airlift reactor has been experimentally investigated. Results obtained in both two- and three-phase flow are presented in this study. Two different external loop airlift reactor sizes have been used and local hydrodynamic characteristics including local gas hold-up and bubble velocity have been obtained in two-phase flow. Optical and ultrasound probes have been used to obtain this information, respectively. It was found that an increase of solid hold-up leads to a decrease of liquid velocity and heat transfer coefficient. Measured in a two- and three-phase reactor using a horizontal-heating probe, a correlation of the average gas hold-up and heat transfer coefficient is proposed. Correlation parameters are identified in homogeneous and heterogeneous flow regimes, which have been derived from the gas slip velocity concept. The experimental liquid velocity and gas hold-up in the riser have been represented in a satisfactory way by a hydrodynamic model, either in the absence or in the presence of solid particles.     

Hydrodynamics and ozone mass transfer in a tall bubble column

In the paper, major hydrodynamic parameters such as gas hold-up, phase velocities and axial dispersion as well as the ozone mass transfer coefficients in the liquid phase have been investigated in a tall bubble column for co-current, counter-current and semi-batch modes of operation. The major emphasis has been placed on evaluation of the dynamic characteristics of the combined system of experimental column and measuring sensors, which was applied in the subsequent determination of the axial dispersion and ozone mass transfer coefficients in the liquid phase. The ozone mass transfer coefficients have been estimated using two treatment methods of the recorded changes of ozone concentration in the liquid and gas phases with time during ozone absorption or stripping to an inert gas.     

气-液-液萃取的流体力学研究

在液-液萃取过程中,提高分散相的表面更新速率可有效提高萃取的传质效率。研究发现,在萃取过程中使用气体搅拌可以增加液液之间的接触面积,促进液相内的湍动和循环。本文研究了气-液-液三相下油滴的流动形态,并对不同填料的流体力学性能进行了测定。实验结果表明,气相速度的增加可导致气含率、液含率的增加,从而提高分散相在填料萃取塔中的停留时间,在一定的速度范围内明显降低萃取的表观传质单元高度,极大地强化传质效果。通过与散装填料对比,发现规整填料更利于强化萃取效果,其液泛速度平均增加25%。