Axial dispersion in a rectangular bubble column

This paper presents the results of an experimental study on the gas holdup and the liquid phase axial dispersion coefficient in a narrow packed and unpacked rectangular bubble column. In both cases the gas and liquid flow rates were varied and the data were obtained by employing standard tracer technique. The gas holdup and the axial dispersion coefficient for both the packed and unpacked columns were found to be dependent on the gas and liquid flow rates. For given gas and liquid velocities and a given packing size in the case of the packed column, the rectangular column gave significantly higher dispersion coefficients than a cylindrical column of the equivalent cross sectional area. This result agrees very well with the one predicted by the velocity distribution model. The correlations for the Peclet number, the axial dispersion coefficient, and the fluid holdup for both the unpacked and packed bubble columns are presented.     

CFD modeling of flow,macro-mixing and axial dispersion in a bubble column

The flow pattern in a bubble column depends upon the column diameter, height, sparger design, superficial gas velocity and the nature of gas–liquid system. In this paper, the effect of some of these parameters have been simulated using Computational Fluid Dynamics (CFD). The relationship of these parameters with the interphase force terms has been discussed. A complete energy balance has been established. Using this methodology, the flow patterns reported by Hills (1974), Menzel et al. (1990), Yao et al. (1991) and Yu and Kim (1991) have been simulated. Excellent agreement has been shown between the CFD predictions and the experimental observations. The above model has been extended to homogenization of an inert tracer. In order to confirm this model, mixing experiments were carried out in a 200 mm i.d. bubble column. A radioactive tracer technique was used for the measurement of mixing time. Tc-99m (^99m Tc), in the form of sodium pertechnate salt, was used as the liquid phase tracer. Good agreement has been shown between the predicted and the experimental values of mixing time. The model was further extended for the estimation of axial dispersion coefficient (D_L). Excellent agreement between the simulated and the experimental values of the axial dispersion coefficient confirms the predictive capability of the CFD simulations for the mixing process.     

Calculation of flow fields in two and three-phase bubble columns considering mass transfer

The dimension of bubble column reactors is often based on empirical correlations. Very popular is the axial dispersion model. However, the applicability of these models is limited to the experimental conditions for which the dispersion coefficients are measured, because backmixing depends strongly on the columns dimension and the flow regime. This paper presents a numerical method for the calculation of the three-dimensional flow fields in bubble columns based on a multi-fluid model. Therefore, the local bubble size distribution is considered by a transport equation for the mean bubble volume, which is obtained from the population balance equation. For comparison with experimental results, the axial dispersion coefficients in the liquid and gas phase are calculated from the instationary, three-dimensional concentration fields of a tracer. The model is then extended to include mass transfer between the gas and liquid phase. Increasing mass transfer rates significantly influence the flow pattern. For several applications, a dispersed solid phase is added. For the calculation of three-phase gas-liquid-solid flow, the solid phase is considered numerically by an additional Eulerian phase.     

THERMAL DISPERSION AND HEAT TRANSFER IN NONISOTHERMAL BUBBLE COLUMNS†

Local axial and radial temperatures were measured at steady-state conditions in a 0.078-m-I.D. bubble column heat exchanger. Nitrogen and water superficial velocity ranges were 0-0.6 m/s and 0-0.02 m/s, respectively. Average column pressures were 3.0, 5.1, and 7.1 atm. The axial temperature profile varied significantly with all conditions encountered. Radial temperature profiles were found to be nearly constant, indicating very good radial mixing.

An axial thermal dispersion heat transfer model, capable of representing nonisothermal systems, was employed to characterize the measured bubble column temperature profiles. Thermal dispersion was apparent from large temperature changes in the entrance of the bubble column. Heat transfer coefficients depended on the gas and liquid flow rates. However, the thermal dispersion coefficients depended on linear gas velocity and were a weak function of liquid flow rates. The thermal dispersion coefficients obtained in this study were found to be consistent with other investigations. In addition, they were compared to the mass dispersion coefficients obtained by other studies and found to be in good agreement     

Multicompartment hydrodynamic model for slurry bubble columns

A core-annulus multicompartment two-dimensional two-bubble class model accounting for slurry recirculation and coupled with catalyst transport was developed as a part and parcel of the analysis of the behavior of slurry bubble column reactors at high gas throughputs corresponding to the churn turbulent flow regime. The model analyzed the contributions of bubble-induced turbulence closures, bubble coalescence and breakup phenomena, and catalyst axial distribution as the resultant of sedimentation, advection via liquid-solid slip, per-compartment axial dispersion and core-annulus lateral exchange of catalyst by bubble-induced turbulence. The model was also used to analyze the effects of catalyst loading, gas density and superficial velocity, and column diameter and vessel aspect ratio on the hydrodynamics of slurry bubble column reactors, namely, the per-compartment phase holdups and interstitial velocities, pressure gradient, bubble coalescence and break-up rates, and loci of velocity inversion for the gas and slurry profiles.     

鼓泡塔反应器分布板的开孔率和液体返混

考察鼓泡塔内鼓泡现象,发现气流经过小孔的速度（称为小孔气速）需大于某一临界值。计算并比较了气流通过多孔板的干板压力降和表面能损耗,提出计算此临界气速的公式和确定最大开孔率的方法。 用脉冲注入法测定鼓泡塔液体停留时间分布,提出非整数串连混合器模型,推导出其停留时间分布曲线方程式,计算结果与实验数据相符。并验证了Towell的轴向分散系数的计算方法。     

快速流化床中气体轴向混合特性

在内径90mm、高8m的快速循环流化床试验装置上,用非理想脉冲示踪、双探头检测、计算机实时采样分析的方法,对湍流流态化、快速流态化和气力输送不同流动状态的气体混合程度分别进行了测量,结果表明均存在不同程度的轴向混合.轴向扩散系数主要与由气体速度和固体循环量决定的床层空隙度有关;用拟均相一维扩散模型对结果进行处理,时域拟合法求取模型参数,并将轴向扩散系数进行了关联,得到如下统一关联式D_e=0.1953ε~(-4.1197)     

Settling velocities of particles in bubbly gas-liquid mixtures

The axial dispersion-sedimentation model is commonly used to describe the axial concentrations of solids in three phase bubble columns at low liquid velocities. When the two parameters of this model, the particle settling velocity and the solids axial dispersion coefficient, are uncoupled by the use of various assumptions, physically unrealistic values of these parameters often result. Direct experimental measurements of solids settling rates in bubbly gas-liquid mixtures were carried out. The measured mean settling velocities decreased slightly with gas flow rate and were equal to or slightly less than the single particle free settling velocity in the liquid alone. Solids axial dispersion coefficients were also obtained from the solids settling rate distribution data, and gave values considerably less than the experimental liquid axial dispersion coefficient.     

Liquid phase axial mixing in a bubble column with viscous non-newtonian liquids

This paper presents some new data for the liquid phase axial dispersion coefficient in a bubble column with highly viscous non-Newtonian liquids (μ_L > 0.03 Pa · s). The data were obtained in a 0.15 m diameter column operating in the slug flow regime, and the dispersion measurements were conducted using heat aas a tracer. The experimental results show that the dispersion coefficients increase with both gas and liquid velocities and quantitatively they are about three times higher than those obtained for the air-water system. The results are explained based on a known hydrodynamic model of vertical gas-liquid slug flow.     

A SIMPLE EXPERIMENTAL TECHNIQUE TO MEASURE GAS PHASE DISPERSION IN BUBBLE COLUMNS

A new technique to measure gas phase dispersion in bubble columns is presented. This technique is fairly simple to implement, accurate and inexpensive as compared to conventional methods used in the literature. Nitrogen is used as a tracer and a step change is effected by switching from air to the tracer, nitrogen. The gas phase concentration of oxygen is monitored by means of a polarographic oxygen sensor. The system studied is air-water and the experimental results compare well with the literature data. The technique presents a simple way to measure gas phase axial dispersion in bubble columns and can also be applied to other types of reactors     

A SIMPLE EXPERIMENTAL TECHNIQUE TO MEASURE GAS PHASE DISPERSION IN BUBBLE COLUMNS

A new technique to measure gas phase dispersion in bubble columns is presented. This technique is fairly simple to implement, accurate and inexpensive as compared to conventional methods used in the literature. Nitrogen is used as a tracer and a step change is effected by switching from air to the tracer, nitrogen. The gas phase concentration of oxygen is monitored by means of a polarographic oxygen sensor. The system studied is air-water and the experimental results compare well with the literature data. The technique presents a simple way to measure gas phase axial dispersion in bubble columns and can also be applied to other types of reactors     

Errors caused by tracer solubility in the measurement of gas phase axial dispersion

Gas phase dispersion studies in bubble columns have been previously analyzed with the axial dispersion model neglecting the solubility of the tracer gas. In the present study, the effect of the solubility of the tracer gas on the accuracy of dispersion measurements is examined using the axial dispersion model with interphase transfer. Expressions for the first and second moments have been derived assuming the liquid phase to be well mixed. These moments are found to be functions of the Stanton number, Peclet number, Henry's constant, ratios of gas to liquid holdups and velocities. It is found that neglecting the interphase transfer can lead to significant deviations from the expected moments, and thus to errors in the Peclet number, even for a relatively insoluble gas like helium in water.     

多级鼓泡塔中液相返混系数与交换速度的研究

采用示踪方法对高2 000 mm,内径282 mm多级筛板鼓泡塔内液相返混系数进行测量研究,并通过扩散-返混模型以及RTD曲线给出鼓泡塔内筛板上下二侧液体交换速度,同时考查了表观气速、开孔率等因素对轴向扩散系数与液体交换速度的影响.根据实验得出鼓泡塔内轴向返混系数以及液体交换速度与表观气速、开孔率有很大关系,均随表观气...     

鼓泡床反应器内流动与传质行为的研究进展

总结了有关浆态鼓泡床反应器内流动、混合用气液传质特性的研究成果,详细地介绍了鼓泡床反应器内气含率、液速、液体轴向扩散系数、传质系数的测量方法,阐述了鼓泡床反应器性能的主要影响因素,如系统压力、温度、气体表观气速、液体性质及固含率等对流动、液相混合和传质特性的影响,并对鼓泡床反应器的应用前景进行了详述.     

Axial gas phase dispersion in a molten salt oxidation reactor

Gas phase axial dispersion characteristics were determined in a molten salt oxidation reactor (air-molten sodium carbonate salt two phase system). The effects of the gas velocity (0.05–0.22 m/s) and molten salt bed temperature (870–970 °C) on the gas phase axial dispersion coefficient were studied. The amount of axial gas-phase dispersion was experimentally evaluated by means of residence time distribution (RTD) experiments using an inert gas tracer (CO). The experimentally determined RTD curves were interpreted by using the axial dispersions model, which proved to be a suitable means of describing the axial mixing in the gas phase. The results indicated that the axial dispersion coefficients exhibited an asymptotic value with increasing gas velocity due to the plug-flow like behavior in the higher gas velocity. Temperature had positive effects on the gas phase dispersion. The effect of the temperature on the dispersion intensity was interpreted in terms of the liquid circulation velocity using the drift-flux model.     

Modelling churn-turbulent bubble columns—I. Liquid-phase mixing

A new two-region phenomenological model for liquid-phase mixing in churn-turbulent bubble columns has been proposed. A gas-rich region rises rapidly through the column transporting liquid to various points in the system while a relatively stagnant, gas-lean region is vigorously agitated by the passage of the gas-rich region. Estimation of most of the model parameters by physical reasoning reduces the proposed model to a one-parameter model. This unknown parameter, which describes liquid exchange between the gas-rich and gas-lean regions, can be obtained by matching the model predictions to the experimentally measured tracer response. The comparison of the proposed model's tracer response and that of the axial dispersion model also allows this exchange parameter to be calculated from the available axial dispersion coefficient correlations. The proposed model is computationally much superior to the axial dispersion model.     

Features of the motion of gel particles in a three-phase bubble column under foaming and non-foaming conditions

Features of the motion of gel particles in a three-phase bubble column with non-foaming and foaming gas–liquid systems,determined by using experiments of radioactive particle tracking(RPT),have been compared.The tracer used is a gel particle which resembles typical immobilized biocatalyst.The tracer trajectory is analyzed to extract relevant information for design purposes.The solid velocity field,turbulence parameters,dispersion coefficients,mixing times and flow transitions are determined and compared.The presence of foam significantly affects many quantified parameters,especially within the heterogeneous flow regime.The hydrodynamic stresses are reduced in the presence of foam,especially close to the disengagement.The dispersion coefficients also decrease,and the solid mixing time is only slightly affected by the presence of foam.Gas holdup,inferred both from RPT experiments and from gamma ray scanning,is higher for foaming systems and leads to a shift in the transition gas velocity towards higher values.     

PHASE DISTRIBUTION AND MIXING IN A JET BUBBLE COLUMN

This paper describes the results of an experimental study to evaluate phase holdups and RTD for a jet bubble column. The experimental data were obtained in a 61 cm diameter jet bubble column with a conical inlet. Air and water were used as a two-phase system. The ranges of gas and liquid velocities examined were 0 to 9 cm/sec and 0 to 0·6 cm/sec respectively, both based on the cylinder diameter. The experimental data indicate that in the conical section of the column, the gas holdup first decreases with an increase in distance away from the cone inlet, achieves a minimum and then increases until it reaches a somewhat constant value within the cylinder. Gas holdup varies radially with the maximum at the center and the minimum near the wall. Radially-averaged gas holdup increased with gas velocity and remained essentially unchanged with liquid velocity. The RTD measurements were correlated by a two-dimensional dispersion model. The axial dispersion coefficient increased linearly from the cone inlet to the cylinder. It also increased with the gas velocity. The radial dispersion coefficients were considerably smaller than the axial dispersion coefficients.     

On axial dispersion in packed beds with fluid flow: Über die axiale dispersion in durchströmten festbetten

In the present work a theoretical analysis of the dependence between the axial dispersion of mass and the flow nonuniformity in packed beds is given. Three types of flow nonuniformity are defined: micro-, meso- and macroscopical. In order to describe the influence of microscopical flow nonuniformity on axial dispersion in packed beds the flow channels are approximated by equal-sized cylindrical capillaries. A step function is used for the velocity profile and the fluid in the outer region of each capillary is assumed to be stagnant. The relationship derived in this manner can describe most results of tracer experiments well; above all, it helps in understanding the differences observed between dispersion of gases and dispersion of liquids. Anomalously high dispersion coefficients, obtained during dispersion of gases through beds of fine-grained particles, are attributed to the mesoscopical flow nonuniformity and are described in a model. Finally, the dependence between macroscopical flow nonuniformity and axial dispersion in packed beds is discussed. It turns out that the influence of channelling on axial dispersion is rather insignificant. In this manner, the results of tracer experiments can be brought into accordance with velocity profiles proposed in the literature. The essential differences between the present analysis and previous works are pointed out.