Prediction of drop size in a pulsed and non-pulsed disc and doughnut solvent extraction column

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Highlights

  • Sauter-mean drop diameters in a pulsed disc and doughnut (PDD) column are compared.

  • Cumulative drop size distribution is predicted using the Weibull function.

  • A unified correlation is proposed for d32 in a PDD column.

Abstract

Recently a number of industrial scale pulsed disc and doughnut (PDD) solvent extraction columns have been operating with no pulsation. However most of the published research studies in the literature that describe and predict the performance of PDD columns were developed for pulsing conditions. In this study the Sauter-mean drop diameter, d32, was measured and correlated under pulsing and non-pulsing conditions using a 75 mm diameter PDD column. Under non-pulsing conditions, the results show that the d32 slightly decreased with increasing dispersed phase velocity, while there was no noticeable change in d32 with continuous phase velocity. Under pulsing conditions, the d32 decreased with increasing pulsation intensity from zero. The cumulative drop size distribution in disc and doughnut columns was found to be predicted well using the Weibull function. A new unified correlation was proposed in this study to predict the experimental d32 data of the PDD column used in this study, as well as published experimental data which was obtained using different systems and column geometries, over a wide range of pulsation rates including no pulsation.

Introduction

Liquid–liquid extraction is an important separation process. Various types of solvent extraction columns have been used for a range of applications in the chemical, petroleum, nuclear, hydrometallurgical industries and other areas for many years. Disc and doughnut solvent extraction columns (DDC) are attractive from both safety and economic stand points, in particular its simplicity of design, less space consumption, higher throughput and no internal moving parts (Movsowitz et al., 1997). In the column, pulsing may be introduced by compressed air (pulsed disc and doughnut column, PDD column) in order to enhance the efficiency of the column.

Knowledge of the hydrodynamic parameters including drop size is of fundamental importance in the design of liquid–liquid extraction columns. Drop size affects the dispersed phase holdup, the residence time of the dispersed phase, and the allowable throughputs. Furthermore, a combination of the Sauter mean diameter together with the dispersed phase holdup gives a measure of the interfacial area of contact between the phases, which is important for the prediction of mass transfer performance in the column. It is therefore important to understand the drop size distribution in the column under different operating conditions and for different systems and geometries. It is also important to be able to predict the drop size distribution as a function of the column geometry, operating conditions and solvent physical properties.

There have been many studies reporting the drop size in solvent extraction columns, including rotating disc (Mao and Slater, 1994), Kühni (Dongaonkar et al., 1991), Wirz-II (Rinconrubio et al., 1994), perforated-plate (Kumar and Hartland, 1986), Karr (Smith et al., 2008) and packed (Torab-Mostaedi et al., 2011b) columns. Jahya (2002) and Torab-Mostaedi et al. (2011a) studied the drop size in a PDD column at relatively high pulsation rates.

To date, most of these studies have been conducted at relatively high pulsation intensities as low pulsation intensity and non-pulsation were thought to be of little practical importance due to poor mass transfer performance. However recently a number of industrial scale PDD columns have been operating with no pulsation. As the correlations and models for drop size from the literature have been developed for pulsing conditions only it is important to investigate how the performance of these columns change when operated under non-pulsing conditions. It is also important to develop a new model that can predict the drop size over the full range of pulsation conditions seen in practice, including low pulsing intensity and non-pulsing conditions.

In this study, the drop size of the dispersed phase was measured over a wide range of operating conditions, including pulsing and non-pulsing conditions, using a 75 mm diameter disc and doughnut column. The experimental data was then compared to literature correlations. An empirical correlation was developed to predict the mean drop size based on the experimental data and the literature data for different liquid systems.

Section snippets

Previous studies

The equilibrium droplet size distribution in a solvent extraction column is usually represented as the Sauter mean diameter (d32) and is calculated from the following equation:d32=ni=1nidi3ni=1nidi2

In the absence of pulsation or at low levels of pulsation, the breakup of drops is controlled by the ratio of buoyancy to interfacial tension forces. A limiting value of the drop size under such conditions may be predicted from (Chang-Kakoti et al., 1985):d32=C1γΔρg1/2where the constant, C1, is a

Equipment

The pilot scale PDD column used in this study consists of a 1.0 m long QVF® precision bore glass column with an internal diameter of 72.5 mm, enclosing a stack of alternating stationary discs and doughnuts. A t-piece is located on top of the main column section to act as the organic phase outlet, and below the column an expanded glass hemispherical section provides an area for settling of the dispersed droplets before exiting from the bottom of the column. The fluid was pulsed by the motion of

Drop size distribution

Fig. 2 shows the typical droplet size distribution under different pulsation intensity. Broad and multimodal drop size distribution is observed at no pulsation condition (f = 0 Hz) which corresponds to the mixer-settler region. The selection of drop diameter range will affect the results of drop size distribution. In this work, the selected drop size ranges are 0.1 mm for f = 1.5 Hz and 0.25 mm for f = 0 Hz. With an increase of pulsation intensity, the drop size distribution tends to narrow and a single

Correlation of drop diameter, d32

In order to design industrial solvent extraction PDD columns, it is important to be able to predict the drop size via correlations that cover the range of operational parameters and physical properties relevant to the particular system. As described previously, there are no existing correlations for predicting d32 in a PDD column that can be used for both non-pulsing and pulsing conditions.

Based on the published research (Kumar and Hartland, 1996) and analysis above, the d32 is determined to be

Impurities of operation of pulsed disc and doughnut columns

This work clearly shows that as the pulsation energy input is decreased, for constant continuous and dispersed phase velocities, the drop size increases and the drop size distribution is broader. The inclusion of data and models to predict this is an important development and enables the prediction of an operating window for this type of column. Operations using these columns and trying to maximize throughput and recovery have found that they can do this by reducing pulsation but maintain

Conclusions

The conclusions from this study may be summarized as follows:

  • (i)

    The droplet size distribution in PDD column is broad and multimodal at low pulsation conditions (Mixer settler region) and it tends to narrow and become single modal with increasing pulsation intensity (Emulsion region).

  • (ii)

    The cumulative drop size distribution in PDD column can be predicted well using the Weibull function.

  • (iii)

    Under non-pulsing conditions, the d32 in a PDD column is not obviously affected by continuous phase velocity, and

Conflict of interest statement

The authors declare the following competing financial interest.

Funding

The authors would like to acknowledge the funding provide by the Australian Research Council through Linkage grant LP130100305 and BHP Billiton, Olympic Dam, for this project, and would also like to thank the Particulate Fluids Processing Centre for the resources provided for this project.

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