Model validation for low and high superficial gas velocity bubble column flows

In the present work, gas-liquid flow dynamics in a bubble column are simulated with CFDLib using an Eulerian-Eulerian ensemble-averaging method in a two-dimensional Cartesian system. The two-phase flow simulations are compared to experimental measurements of a rectangular bubble column performed by Mudde et al. [1997. Role of coherent structures on Reynolds stresses in a 2-D bubble column. A.I.Ch.E. Journal 43, 913-926] and a cylindrical bubble column performed by Rampure et al. [2003. Modeling of gas-liquid/gas-liquid-solid flows in bubble columns: experiments and CFD simulations. The Canadian Journal of Chemical Engineering 81, 692-706] for low and high superficial gas velocities, respectively. The objectives are to obtain grid-independent numerical solutions using CFDLib to reconcile unphysical results observed using FLUENT with increasing grid resolutions [Law, D., Battaglia, F., Heindel, T.J., 2006. Numerical simulations of gas-liquid flow dynamics in bubble columns. In: Proceedings of the ASME Fluids Engineering Division, IMECE2006-13544, Chicago, IL], and to validate computational fluid dynamics (CFD) simulations with experimental data to demonstrate the use of numerical simulations as a viable design tool for gas-liquid bubble column flows. Numerical predictions are presented for the local time-averaged liquid velocity and gas fraction at various axial heights as a function of horizontal or radial position. The effects of grid resolution, bubble pressure (BP) model, and drag coefficient models on the numerical predictions are examined. The BP model is hypothesized to account for bubble stability, thus providing physical solutions.     

Liquid phase axial mixing in bubble columns operated at high pressures

Liquid phase axial mixing was measured in a 100 mm i.d. bubble column operated in the pressure range of 0.1-0.5 MPa. Water, ethanol and 1-butanol were used as the liquid phase and nitrogen as the gas phase. The temperature and superficial gas velocity were varied in the range of 298-323 K and 0.01-0.21 m/s, respectively. The axial dispersion coefficient increased with an increase in the gas density due to pressure. The temperature had surprisingly a small effect. A CFD model was developed for the prediction of flow pattern in terms of mean velocity and eddy diffusivity profiles. The model was further extended for the prediction of residence time distribution and hence the axial dispersion coefficient (D_L). The predictions of axial dispersion coefficient agree favorably with all the experimental data collected in this work as well as published in the literature. The model was extended for different gas-liquid systems. The predicted values of axial dispersion coefficient were found to agree very well with all the experimental data.     

Estimation of liquid phase mixing due to solids in a turbulent bed contactor

The mixing in two-phase gas-liquid and three-phase gas-liquid-solid system (turbulent bed contactor) is evaluated through residence time distribution (RTD) studies in terms of Peclet number. RTD experiments are conducted for various gas and liquid velocities, and number of stages for two- and three-phase systems. Since the mean residence time is very short in both the systems, a mixed flow tank with exponential decay RTD is used in series. After deconvolution, the RTD of the system is obtained. The experimental RTD curves are satisfactorily compared with the axial dispersion model and Peclet numbers are evaluated for all the experiments. The axial dispersion coefficients are calculated from Peclet numbers. With this study, it is thought that liquid phase mixing may be controlled by changing the quantity of solid particles in the bed.     

Gas hold-up and liquid mixing at low and intermediate gas velocities I. Air-water system

Gas hold-up and liquid phase dispersion experiments have been carried out in a 0.06 m bubble column at varying liquid and gas velocities. The results obtained show that the coefficient of liquid mixing varies with the flow regime. The isotropic turbulence theory of Baird and Rice (Chem. Eng. J., 9 (1975) 171) was used to provide dimensionally consistent correlations for the chain bubbling, bubbly and churn turbulent flow regimes. The gas hold-up was determined to increase with gas velocity in the chain bubbling and bubbly flow regimes. The results obtained from this study also show that the Froude number represents a useful criterion for mapping flow regimes in vertical bubble columns.     

Non-isobaric bubble columns with variable gas velocity

When designing bubble columns two major uncertainties are encountered, viz. the reliable estimation of parameter values and the application of the pertinent design model. This paper presents a design model which on the base of the dispersion model accounts for the opposite effects of gas shrinkage and expansion caused by absorption and reduced hydrostatic head, respectively. As the holdup of the phases can be assumed to be axially independent the gas velocity is variable. Therefore, the differential equations are non-linear and were solved by the method of Lee. Computed profiles show that the gas concentration may run through a minimum. The calculations reveal that the neglect of a variable gas velocity may yield serious errors even at isobaric conditions. The influence of gas phase dispersion is marked in large diameter bubble columns. Worked examples are given for the case of a fast and slow reaction in the liquid phase which are compared with the predictions of a simpler model.     

Liquid phase mixing in trayed bubble column reactors

The compartmentalization of conventional bubble columns by perforated trays constitutes a very effective method to reduce the liquid backmixing. The effect of tray design and operating conditions on the overall liquid mixing was studied in a bench-scale trayed bubble column. The extent of liquid backmixing in the column was investigated in light of liquid-phase tracer response experiments. In average, a three fold reduction in the liquid backmixing was achieved in the trayed column as compared to the column without the trays. Moreover, the tray open area and the superficial liquid velocity were found to have the strongest effects on the liquid backmixing. The N-CSTR with Backmixing Model was found to match the experimental tracer response curves better than the Axial Dispersion Model.     

Liquid mixing by large gas bubbles in bubble columns

Liquid mixing by large gas bubbles of spherical cup was investigated for co- and counter-current contact of air-water system with bubble columns of 5 and 10 cm dia. The results obtained are that for the column of 5 cm dia., the longitudinal dispersion coefficient ranges from 5 to 20 cm²/sec for superficial gas velocity from 0·07 to 8 cm/sec and that for the one of 10 cm in diameter it ranges from 9 to 45 cm²/sec for that from 0·035 to 8 cm/sec. Liquid mixing under the coexistence of large and small bubbles was also investigated and it was found that the gas holdup was fairly well explained by an equation derived on the assumption that the mixture of small bubbles and liquid behaves independently of large bubbles. The expansion model was applied to the experimental results on the longitudinal dispersion coefficient and it was observed that there should be the lower limit in the holdup of small bubbles where this model can be applied.     

乙炔与氯化氢在低温下混合的安全性研究

乙炔与氯化氢混合气体中含有由游离氯与乙炔反应生成的氯乙炔和二氯乙炔组分时,存在着易燃、易爆的安全隐患,这两种组分的产生原因主要是混合温度高。笔者提出了将乙炔和氯化氯气体先分别在-14℃下进行除水后再混合的预防措施,可使气体混合工段生产的安全性在原来的基础上提高70%。     

Determination of flooding gas velocity and liquid hold-up at flooding in packed columns for gas/liquid systems

A new model of suspended bed of droplets for describing the vaour or gas the vapour or gas velocity at the flooding point in packed of columns for rectification and absorption under vacuum and normal pressure is presented metallic, ceramic and plastic packings with diameters of 8–90 mm as well on sheet metal and gauze packings, in a wide range of liquid and vapour loads. Approximately 650 literature measurements and own data were evaluated. The mean relative error in determining the gas velocity at flooding point is less than ±5%. On the basis of the double layer model, a new equation was derived for the hold-up at flooding point, which is needed for the calculation of the flooding gas velocity. An example of calculations for sample applications is also included.     

Dynamics of the gas phase in bubble columns

The conventional two-bubble-class model for bubble column reactors has been modified to account for interactions between the two bubble classes. It is shown that the modified model is more consistent with the observed residence time distribution in the gas phase than the conventional model. The results indicate that there is an intensive cross-mixing between the bubble phases. This study also reveals that the modified two-bubble-class model cannot be distinguished from the single-bubble-class model solely on the basis of the residence time distribution.     

多级鼓泡塔内液体速度分布的实验研究

液体循环流动是多级鼓泡塔重要流体力学特征之一,文中在内径为282 mm,高2000 mm的鼓泡塔内,采用不同类型的筛板将普通鼓泡塔分割成双级气液鼓泡塔.采用Pavlov管测液速的方法考察了不同筛板、不同表观气速下该鼓泡塔中上下二侧的液体速度分布.根据实验结果得出了液体速度在塔中心处最大,且与表观气速有关,随着表观气速的...     

Liquid phase mixing in bubble columns with Newtonian and non-Newtonian fluids

A hydrodynamic model for the liquid phase in bubble columns is developed. The proposed model for fully developed turbulent Newtonian and non-Newtonian fluids is based on an energy balance and the mixing length theory. The predictions of the model are in reasonably good agreement with data on liquid velocity at the column axis and the axial dispersion coefficient. The liquid velocity data in an inverted conical bottom gas—liquid column contactor have also been measured. They are correlated by the proposed model reasonably well.     

Shape and terminal velocity of single bubble motion: a novel approach

The relative terminal rising velocity of a single gas bubble, moving into a liquid phase, is determined by its size, by the interfacial tension, by the density and viscosity of the surrounding liquid. Both shape and velocity are strongly interacting. Along the years several methods have been presented for solving the problem of bubble deformation and relative rising velocity, at least in connection with some specific regimes of motion and/or shape of bubble. In this work, a new approach is proposed.     

Elongated gas bubble dissolution under a turbulent liquid flow

Mass transfer between an elongated homogeneous gas bubble under a turbulent liquid flow in a duct is investigated experimentally. Elongated gas bubble dissolution is encountered in bioengineering tubular photobioreactors. Such reactors are interesting because they are compact, they have a low contamination risk and a low mechanical stress for a liquid phase containing fragile microalgae cells. It is demonstrated from experimental mass transfer measurements, that the interface of an immobilised elongated bubble can be approximated to a flat plane. Measured mass transfer experimental data, estimated using this simplification, appear to be well fitted by Sh_L = 1.76 × 10⁻⁵ × Re^1.506 × Sc^0.5, a correlation from Lamourelle and Sandall [8], given for a turbulent liquid flow in wetted-wall columns. A formula drawn from this hypothesis is proposed for mass transfer prediction in photobioreactors. For different applications, it is suggested that the results obtained for the studied geometry could be used to build mass transfer feedback control systems.     

兑卤在光卤石区反应平衡时液相组成的计算

在K~+、Na~+,Mg~++//Cl~--H_2O水盐体系中,利用E点和F点母液兑卤析盐过程计算平衡母液组成时发现,E、F点母液总物料中的MgCl_2与反应平衡时液相中MgCl_2的含量近似相等,掘此可以很快算出反应平衡的液相组成。在察尔汗盐湖进行的盐田兑卤生产光卤石的中试中,应用该方法计算的结果与实测的母液数据基本相符。该公式可以指导生产实践。     

Analysis of dispersion coefficient of bubble motion and velocity characteristic factor in down and upflow bubble column reactor

Dispersion coefficient of bubble motion based on velocity distribution theory has been analyzed in up and downward gas-liquid two-phase contactor. The intensity of dispersion of phase depends on motion of the dispersed phase and the characteristics of velocity distribution. In this paper the effects of operating and geometric variables on the dispersion coefficient of bubble motion and the characteristic factor of velocity distribution have been analyzed within the range of column diameter 0.10-2.5 m, superficial liquid velocity, 0.04-0.21 m/s and superficial gas velocity 0.41-3.16 mm/s. From the different developed model of longitudinal dispersion coefficient of liquid, comparison of dispersion coefficient of bubble motion and characteristic feature of velocity distribution in down and upflow two-phase contactor has been reported. Also the functionalities of dispersion coefficient of bubble motion and velocity characteristic factor have been developed with operating variables. The condition for dispersion based on velocity pattern has also been discussed in the present work. The present analysis on the dispersion coefficient of bubble motion and velocity distribution factor associated with the knowledge of the liquid phase dispersion in two-phase contactor can give insight into a further understanding and modeling of multiphase reactor in industrial applications.