Chemical Separations with Liquid Membranes - American Chemical

Chapter 7 Preparation and Splitting of Emulsions for Liquid Membranes 1

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J.Draxler ,C.Weiss ,R. Marr , and G. R. Rapaumbya

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0642.ch007

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Institut für Thermische Verfahrenstechnik und Umwelttechnik, Technische Universität Graz, Inffeldgasse 25, A-8010 Graz, Austria Ecole Centrale Paris, Laboratoire de Chimie Nucléaire et Industrielle, Grande voie des Vignes, F-922295 Chatenay Malabry, France 2

An apparatus for the preparation of emulsions and various devices for the electrostatic splitting of emulsions were improved by investigating theflowpattern using computationalfluiddynamics software. In the case of electrostatic splitting it, an attempt is made to combine the flow pattern with the electric forces acting on the droplets. Although to date only single phase calculations were made, the experimental results show that considerable improvements could already be achieved. In the case of emulsion preparation a much simpler and therefore cheaper device gave the same droplet size distribution. As for the electrostatic splitting, the influence of various geometries and inlet positions was investigated for a given electric field. The preparation and splitting of emulsions are the key parameters in an emulsion liquid membrane process. A very stable emulsion which avoids any loss of emulsified droplets is a prior condition for the feasibility of the process. However, the more stable the emulsion, the more difficult to split it. So both steps are dependent on each other and have to be optimized, also with regard to cost ^timization. In the present work, we try to calculate theflowpattern in the two steps using CFD (computational fluid dynamics) software in order to improve the design of the two steps. Preparation of Emulsions There are many suitable devices available, such as high pressure homogenizers or devices using the rotor/stator principle. In the liquid emulsion membrane process often very acidic stripping phases are used in the emulsified phase. Together with the high shear forces in those devices this leads to high corrosion problems which makes the process very expensive. For this reason, a new low pressure homogenizer was developed which met all the requirements for the liquid emulsion membrane process 0097-6156/96/0642-0103$15.00/0 © 1996 American Chemical Society

In Chemical Separations with Liquid Membranes; Bartsch, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


CHEMICAL SEPARATIONS WITH LIQUID MEMBRANES

Figure 1. Low-pressure Homogenizes a whole homogenizer b detail of nozzle 1 continuous phase in 2 dispersed phase in 3 emulsion out



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(7). This homogenize! is shown in Figure 1. It was developed empirically and the objective of the present work was to improve this device by means of CFD. This type of homogenizer was used in all of our emulsion liquid membrane plants. No corrosion problems were observed after several months of operation. At a pressure difference of only 500 kPa, a mean droplet diameter of 1-2 pm could be obtained. Although this device worked very satisfactorily, there were two major drawbacks: (1) The radial suction of the dispersed phase by the axial flow of the continuous phase was not sufficient, so that the dispersed phase had to be injected by a pump; and (2) The rnanufacturing of the end-section with many edges, intended to enhance turbulence and assumed to be the reason for the good performance, is quite complicated and makes the homogenizer expensive. All the following calculations were made assuming a single phase flow. At first the velocity vectorfield,the pressure distribution, the mean turbulence fluctuations (local root mean square) and the turbulent kinetic energy dissipation were calculated for the existing type of homogenizer. The calculations were made for a slice of an axial symmetricfluiddomain. Figure 2A shows the velocity vectorfieldand Figure 2B the mean turbulence fluctuations. From these calculations, it could be concluded that the turbulence in the endsection is not as high as expected and that therefore this expensive part of the homogenizer cannot be a dominant factor in the good performance. The highest turbulencefluctuationsoccur at the periphery of the free liquid jet coming out of the inlet nozzle for the continuous phase. A few millimeters behind this nozzle is a second nozzle and between these two nozzles the radial inlet for the dispersed phase is located. It can be seen that the second nozzle is too small and also that the distance between these two nozzles is too small. The free liquid jet coming out of the first nozzle is broken at the second nozzle, so the pressure in the inlet for the dispersed phase is not low enough to suck in a sufficient amount of the dispersed phase. Based on these results, an new homogenizer was devised without the endsection with the edges and with improved geometry for the inlets of both phases. The calculations for this new design (Figure 3) show that the free liquid jet is not broken any longer due to a wider second nozzle. Therefore the pressure in the inlet region is much lower so self-suction of the dispersed phase could be expected. The experimental results confirmed the expectations based on the calculations. Figure 4 shows the dependence of the sucked-in dispersed phase on theflowrate of the continuous phase for the old and new geometries, respectively. It can be seen that the suction of the dispersed phase is much higher with the new geometry, so no extra pump is needed to achieve the desired phase ratio of about 3:1 (continuous : dispersed phase). The drop size distribution was about the same for both devices. Using the following conditions a mean diameter of around 1.5 um and a Sauter mean diameter of around 2.5 um could be obtained: continuous phase: dispersed phase: water + 0.15% Na-dodecylsulfate Shellsol Τ flow rate: 1001/h flow rate: 301/h pressure difference: 500 kPa These results were obtained by pumping the continuous phase only once through the nozzle. Recycling a part of the formed emulsion severalstimesthrough the nozzle would give even smaller drops and a narrower droplet size distribution.




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CHEMICAL SEPARATIONS WITH LIQUID MEMBRANES


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In our further work we will try to account for the dispersed phase and calculate a two phase flow. The goal will be to predict a mean droplet size for given operating conditions and physical properties.


Splitting of Emulsions For emulsion liquid membranes mostly water-in-oil-emulsions are used. For this type of emulsions the electrostatic coalescence is the method of choice. This is a well known method used in crude oil desalting. However, the emulsions in the liquid membrane process are much more stable, therefore this process had to be improved. These improvements concerned mainly electrical parameters, but also the flow regimes in order to avoid the formation of spongy emulsions. Although a myriad of different emulsions has been split, not only for the liquid membranes process but also for many other applications, no general design criteria could be found. The efficiency of the splitting depends very much on the physical properties of the components and on the phase ratio. Sometimes, a high frequency, a homogenous electric field, a countercurrent flow, or a high temperature is favourable; but also the contrary might be true. In our present work we try to combine existing correlations for droplet-droplet coalescence efficiency with the flow pattern in different splitting devices with the main objective to compare different splitting designs for a given emulsion. As a starting point the following conditions were chosen: • An emulsion consisting of 99.5% continuous organic phase with a density of 934 kg/m and a viscosity of 4.6 mPa.s at 20°C and 0.5% tap water. This low water content was chosen because initially all of the calculations were based on a single phase flow and this small amount of dispersed phase was assumed to be negligible. • Apparatus: Parallel plate coalescer with 7 electrodes and a height of 0.26m, a length of 0.27 m and a width of 0.17 m. This type was chosen because existing gravity settlers could be easily equipped with such electrodes and because of the homogenousfieldbetween the electrodes. • Voltage: 5 kV/cm • Frequency: 10 kHz • Throughputs: 501/h and 1001/h, respectively. In afirststep the velocity pattern was calculated for different inlet heights and different throughputs. These velocity vectors are shown in Figure 5. Now for each cell the droplet diameter was calculated when the droplet starts to settle (one droplet in each cell). This point is given when the upward velocity of the continuous phase is balanced by the sinking velocity of the droplets according to Stokes law: 3

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V,drop

Ap.g.d

with d

(1)

where v and Vdrop are the velocity of the continuous phase and the droplet velocity, respectively, Δρ the density difference between the two phases, g the acceleration due to gravity, d the droplet diameter, and T| the viscosity of the continuous phase. c

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Figure 5. Velocity Vector Field in a Parallel Plate Coalesces



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These droplet size distributions are shown in Figure 6. It can be seen that the average diameter of the droplet when they start to sink is around 165 um with the inlet at the bottom (a). This value is shifted to around 100 pm and 35 pm with the inlet in the middle (b) and at the top (c), respectively. These differences are due to the velocity pattern of the continuous phase. When the inlet is at the bottom, the continuous phase flows up and the droplets must flow down countercurrently. When the inlet is at the top, the continuous phase flows partially down and this cocurrent movement promotes the settling of small droplets. At a higher flow rate (100 1/h, inlet at bottom) the droplet size is shifted to higher values (Figure 6d). According to these numerical results, settling starts earlier with the inlet at the top than at the bottom. To calculate the time till settling starts we need a correlation of the droplet growth in dépendance of the electric field. There are several such correlations available in literature. We used the relation of Williams and Bailey (2), which gives the growth pattern of a droplet d(t) for the assumption that droplets coalesce only in pairs: d(t)

(2)

where d(t) is the droplet diameter at time t, do the initial droplet diameter, 6c the dielectric constant of the continuous phase, Ε the electricfieldstrength, and x

Chemical Separations with Liquid Membranes - American Chemical

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