A study on flow characteristics in a rotating screen blade extraction column

Studies have been carried out on the axial dispersion in the continuous phase and the hold-up of dispersed phase in a rotating screen-blade extraction column, by employing an axial dispersion model. Experiments on both single and two phase operations have been conducted with the plate spacing, the mesh size, the impeller speed (RPM), and the superficial velocities as the system parameters. By regression analysis of experimental data, empirical equations correlating the dispersion coefficients and the fractional hold-up of dispersed phase with the system parameters were obtained.     

开孔转盘塔分散相滞存率及液滴直径分布的研究

本文在无传质条件下研究了开孔转盘塔(RPDc)内乳液—水体系(W/O/W)的流体力学性能。测定了塔内分散相滞存率x,考察了塔几何条件及操作条件对分散相滞存率x及特性速度U_o的影响。采用因次分析法,建立了x及U_o与塔几何条件及操作条件间的经验关联式。并采用摄影法,对分散相液滴直径及其分布进行了初步研究。实验结果表明,分散相液滴直径的分布可用上限对数正态分布来描述。     

GRAVITY SETTLING OF DISPERSED PHASE DROP SWARMS IN EXTRACTION COLUMNS IN ABSENCE OF MASS TRANSFER

Taking into account the effect of velocity profiles of the continuous phase in interstices of drops,dragcoefficient and relative motion correlations for dispersed liquid-liquid two-phase flow in absence of mass transferwere developed in terms of the pseudo-fluid concept based on the simple similarity criteria and the mixtureviscosity model suggested by Ishii and Zuber.The present model was compared with the experimental data fromfive(different sources and also with seven other pertinent correlations available in literature.Fairly goodpredictions were obtained under wide ranges of the dispersed phase holdup and Reynolds number.The validity ofthe present model has also been checked against the experimental slip velocity data and holdup data obtained ina Karr reciprocating plate extraction column by the author of the present paper and satisfactory agreement isachieved、The results show that the equations of the motion of a multi-droplet system can be formulated in aform identical with those for a single dr     

开孔转盘塔(RPDC)分散相滞存率及液滴直径分布的研究

本文在无传质条件下研究了RPDC内乳液-水体系的流体力学性能。测定了塔内分散相滞存率x,考察了塔几何条件及操作条件对x及特性速度u_6的影响。采用因次分析法,建立了x及u_o与塔几何条件及操作条件间的经验关联式。并采用摄影法,对分散相液滴直径d_滴及其分布进行了初步研究。实验结果表明,d_滴的分布可用上限对数正态分布来描述。     

Modelling of liquid-liquid extraction columns: Use of published model correlations in design

Design of several liquid-liquid extraction columns — packed, pulsed-packed, pulsed-plate, Oldshue-Rushton columns and the rotating disc contactor — was attempted utilizing available correlations for drop size, holdup of dispersed phase, flooding velocities, mass transfer coefficients and axial mixing coefficients. Correlations in many cases were vaguely defined and often based on very limited data. Results indicated that for given flow rates and extraction efficiency, the height of a packed or Oldshue-Rushton column must be considerably greater than the predicted minimum heights of the other three columns, which were comparable considering the limited data utilized in the developed correlations. A critical evaluation of the correlations should be carried out to guide the further experimental effort required to confirm the utility of the axial dispersion model in liquid-liquid extraction column design. Extension of the theory to include drop size variation is highly desirable.     

“FORWARD MIXING” MODEL: APPLICATION TO PULSED PLATE EXTRACTION COLUMN OPERATION IN THE EMULSION REGION

The “Forward Mixing” model has been applied to data obtained from a 22 cm diameter pulsed plate extraction column. Measurements of drop size distributions, dispersed phase hold-up and concentration profiles for two systems (toluene-acetone-water and n-butanol-succinic acid-water) of quite different properties were made with the column operating in the emulsion region. Generated drop size distribution function parameters, size-dependent slip velocities and mass transfer coefficients, and continuous phase axial dispersion coefficients were accurate in predicting dispersed phase hold-up and extraction efficiencies (or the related plug flow number of transfer units). These parameters were correlated with phase superficial velocities and pulse velocities. The influence of continuous phase axial dispersion was much greater than the influence of drop size variation, and was not accurately predicted by most previous tracer-based correlations. An inlet dispersed phase distributor was beneficial to the performance with the high interfacial tension system.     

Hydrodynamic and mass transfer parameter correlations for the rotating disc contactor

The normal, fragmented manner in which various model hydrodynamic and mass transfer parameters are measured and correlated separately, in the vain hope that accurate predictions of extraction column performance will be eventually possible, is avoided here. The ‘forward mixing’ model parameters are all determined simultaneously, in experiments with two sizes of rotating disc contactor where all required measurements, including drop size distributions and continuous phase profile compositions, are made during solute transfer between phases. Accurate predictions from the derived correlations of dispersed phase hold-up fraction, drop size distributions and extraction efficiency are the result.     

喷雾塔内雾粒粒径对传质行为的影响

本文研究了喷雾塔内的两相混合及雾粒粒径对传质行为的影响.建立了描述粒子群在塔内运动和传质规律的数学模型,这一模型既考虑了两相的返混,也考虑了粒径的影响,即所谓粒子群的向前混合.通过稳态浓度剖面法求解模型方程,得到了两相的返混准数Pe_x、Pe_y及粒子的真空传质系数K_(oco)用线性最小二乘法关联得到计算这些参数的经验方程.这些关联式可用于计算塔内的真实浓度剖面,并能适用于不同结构和尺寸的喷嘴.     

Drop-Size Distribution and Dispersed Phase Hold-up in a Large Rotating Disc Contactor

Abstract

The effect of drop size and size distribution and the dispersed-phase hold-up on the performance of an extraction column are the most important hydrodynamic characteristics, because under steady operating conditions, drop size and hold-up are proportional to the interfacial area. Correspondingly, the efficiency of mass transfer is a function of drop size as well as hold-up.

A number of experimental investigations using the rotating disc contactor (RDC) have reported the measurement of dispersed-phase hold-up, drop size, and size distribution. However, most of the published data are for very small RDCs of <7.5-cm diameter. All the correlations introduced to describe the column hydrodynamics give unreliable results when applied to large-scale RDC operation and with different systems to those studied. Therefore, RDC hydrodynamics in the absence of mass transfer have been studied on a 450-mm-diam column, 4.3-m high, and the results obtained have been compared with those reported previously from small extraction columns. Wide divergences have been found. The results of this study have been correlated to predict the drop size in each compartment. Agreement has been within 10% of the experimental data. When the results of this and previous studies are analyzed together by including the column dimensions, the agreement between predicted and experimental results is generally within 15%.     

High flux mass transfer: Comparison of liquid-liquid extraction column and drop (Handlos-Baron) model predictions with rotating disc contactor performance

High flux mass transfer measurements have been made in a rotating disc contactor and the results compared with model predicted results. Extraction column and drop model equations for single solute transfer were modified to include the influence of the interphase convective, or drift, flux, previously neglected. The Handlos-Baron drop model was utilized and found to predict the correct trends with changes in drop size. Continuous phase axial dispersion measurements were carried out by pulse tracer injection and by concentration profile measurements at low flux, when simultaneous measurements were made of the continuous phase mass transfer coefficient. When these values were used in the high flux model, high flux extraction efficiencies were accurately predicted but concentration profiles were not.     

Extraction of rare earth metals with a multistage mixer-settler extraction column

Extraction behavior of rare earth metals within a mixer-settler extraction column has been analyzed with the stage efficiency calculated from mass transfer coefficients and interfacial area. The mass transfer coefficient within the dispersed drops is determined from a rigid sphere model by taking into account the residence time distribution of drops, and the coefficient around the drops is calculated by Ranz-Marshall's correlation with the terminal settling velocity of a rigid sphere. The interfacial area of dispersed drops is calculated by the use of correlations for the drop diameter and the holdup of dispersed phase in the mixer-settler extraction column. The calculated results for the separation of samarium and gadolinium with a five-stage mixer-settler extraction column are compared with the experimental results at various agitation speeds and flow ratios between two phases. The extraction behavior in the multistage column is explained by a model based on the hydrodynamics and the mass transfer within the mixer. Effects of the pH value in aqueous phase, the extractant concentration in organic phase and the number of stages on the extraction behavior in the mixer-settler column are also shown.     

Hydrodynamics,axial mixing and mass transfer in open turbine rotating disc contactor (OTRDC)

An experimental study of hydrodynamics, axial mixing and mass transfer has been carried out in a newly developed liquid-liquid extraction contactor, namely the open turbine rotating disc contactor (OTRDC). It has been established that the OTRDC can be operated with larger holdups of the dispersed phase, larger interfaces and, hence, more efficient mass transfer than the conventional RDC. In correlating axial mixing data, a combined model has been applied in which both the forward mixing due to drop size distribution and the backmixing of droplets are taken into account. The RTD curves of dispersed phase predicted by the model agree well with the experimental data. Comparison of experimental mass transfer data with those predicted by the proposed axial mixing model and the theoretical single drop model shows that they are in good agreement.     

大型萃取塔液体分布器性能和设计方法的研究

本文用界面张力极低的丁醇-水体系研究了管式液体分布器孔径和喷口流速对分散相液滴直径分布和液滴平均直径的影响。对适宜喷口速度的选择进行了定量的研究。在此基础上,编制了调整孔径和孔分布以保证分散相沿塔截面均匀分布的计算机程序,可用于石油化工系统大型萃取塔管式分布器的优化设计。     

An improved stagewise model of countercurrent flow liquid—liquid contactors

An improved stagewise model has been developed to represent more accurately the physical processes occurring in a liquid—liquid extraction column. The influence of drop size distribution is more realistically represented in this model. The model equations have been solved numerically with a range of parameter values in order to predict the extent of the influence of drop size distribution on extraction rates. Drop size distributions, measured in a turbine-agitated column section and inserted into the model equations, gave predictions of a considerable influence of size distribution on extraction column performance.     

Mass-transfer effects on droplet phenomena and extraction column hydrodynamics revisited

The effect of solute mass transfer between the two liquid phases on the drop size distribution and holdup profiles of the dispersed phase in a multistage extraction column is experimentally investigated in this article. Experimental results of the drop size distribution, dispersed-phase volume fraction (holdup), and concentration profiles are obtained for both directions of mass transfer. The drop size distribution is measured by a photomicrographic technique, the holdup profile is measured by an ultrasonic technique, and the concentration profiles are measured by refraction index measurements. A strong influence of the mass-transfer direction on drop size and holdup has been found. The results are in qualitative agreement with the observations of other investigators. Mass transfer of butyric acid from toluene (dispersed) to water (continuous) produced larger drop sizes and lower values of the dispersed-phase holdup than for the equilibrated toluene—water system. For mass transfer of butyric acid from water (continuous) to toluene (dispersed) smaller drop sizes and higher holdup values are observed than for the equilibrated toluene-water system. This behavior significantly affects the performance of the extractor and indicates the necessity for more studies to determine the physics of the phenomenon and to model these processes appropriately.     

Determination of holdup by air purge in φ50 mm discs and doughnuts pulsed extraction column

在柱径为50 mm的折流板脉冲萃取柱中,首先利用吹气法研究了硝酸水溶液和30％TRPO-煤油水溶液体系的单相流时均摩擦压降Δp_f特性.实验结果表明当A′ω≠0,且u_c＝0时,Δp_f可以忽略;而当Aω＝0时,Δp_f与u_c满足Noh模型的线性关系.在Noh模型的基础上,给出了计算Aω≠0时单相流时均摩擦压降的模型,实验结果与模型预测相一致.在单相流摩擦压降的基础上,又研究了连续相分别为硝酸溶液和30％TRPO-煤油溶液体系的两相流时均摩擦压降Δp_f特征.实验结果表明：两相流时均摩擦压降Δp_f可用于计算分散相存留分数,其近似由连续相所对应的Noh单相流模型求得,并可用于折流板脉冲萃取柱分散相存留分数的确定.     

Hydrodynamic and mass transfer performance of multiple upcomer extraction trays

The multiple upcomer (MU) extraction tray was specifically developed for aromatics extraction processes. Hydro-dynamic and mass transfer experiments have been carried out with n-butanol/succinic acid/water as an experimental system in a MU tray extraction column of 0.1 m diameter. The dispersed phase hold-up, drop size and overall efficiency were measured and the hydrodynamic and mass transfer parameters were correlated. The flooding velocities and the overall efficiency predicted from the correlation for this column agreed well with the experimental data.     

φ50 mm折流板脉冲萃取柱吹气法测量存留分数

在柱径为50 mm的折流板脉冲萃取柱中,首先利用吹气法研究了硝酸水溶液和30％TRPO-煤油水溶液体系的单相流时均摩擦压降Δp_f特性.实验结果表明当A′ω≠0,且u_c＝0时,Δp_f可以忽略;而当Aω＝0时,Δp_f与u_c满足Noh模型的线性关系.在Noh模型的基础上,给出了计算Aω≠0时单相流时均摩擦压降的模型,实验结果与模型预测相一致.在单相流摩擦压降的基础上,又研究了连续相分别为硝酸溶液和30％TRPO-煤油溶液体系的两相流时均摩擦压降Δp_f特征.实验结果表明：两相流时均摩擦压降Δp_f可用于计算分散相存留分数,其近似由连续相所对应的Noh单相流模型求得,并可用于折流板脉冲萃取柱分散相存留分数的确定.     

转轮式分级器切割粒径的预测模型

针对转轮式分离器,建立不同结构参数及操作参数下的切割粒径预测模型有助于指导其设计和运行。基于相似理论以及对颗粒分离过程的分析,提出一种考虑结构参数的半经验模型,确定了模型参数,并通过实验以及文献报道的数据验证了模型的可靠性。与前人工作相比,此模型在不同的转轮分离器结构下显示出较好的通用性;此外,模型可以复现切割粒径随叶轮转速增大而先减小后增大的非单调关系,这是原有的半理论模型不能实现的。对不同结构转轮分离器性能的系统分析表明,较大的分离器入口面积及较小的导流角度可确保入口产生强旋流气流,进而使切割粒径对转速的敏感段在低转速区;而较小的分离器入口面积,或较高的入口风速可以提高对细颗粒的分离能力。对运行参数的研究表明,增大颗粒流率会减小切割粒径;调整叶轮转速可以灵活地改变切割粒径,适应不同的工作需求。