SESSION 12

MEASUREMENT AND INSTRUMENTATION

Chairmen: R. Semiat, M. Yamaguchi

CHARACTERIZATION OF DROPLETS IN LIQUID/LIQUID-EXTRACTION BY LASEROPTICAL MEASUREMENT

E. Hanno Schombacher, Thomas Wriedt, Klaus Bauckhage
University of Bremen
Department of Chemical Engineering/FBO4
Badgasteinerstr. 3
D-28359 Bremen

In the recent years increasing activities in the field of liquid/liquid-Extraction (LLE) Process modelization can be found in literature [1]. One prerequisite for further success in this field of research is the determination of process data like droplet size and velocity or mass transfer rates as a result of single-droplet experiments or determined from technical apparatus. Within this study improved laseroptical measurement techniques, Phase-Doppler-Anemometry (PDA) and Rainbow Refractometry (RBR), offering the ability of fast and reliable data acquisition will be proposed.

PDA can be regarded as a well established device for the determination of size and velocity of particles in two-phase or multiphase flows such as liquid/gas, solid/gas, solidlliquid and gas/liquid [2]. In standard applications the frequency f of a Doppler-burst received at a photodetector can be related to droplet velocity whereas the phase-difference measured between two Doppler bursts is proportional to the droplet diameter. One basic step for the adaption of the measurement technique to the specific conditions occuring in liquid/liquid-systems is the optimization of the Phase-Doppler setup by light scattering considerations. The systems under investigation were the recommended test systems Toluene/water und Butyl Acetate/water [3].

Using the so found optimum setup, experiments at single-droplets of the recommended test systems Toluene/water und Butyl Acetat/water [3] were performed. PDA exhibited satisfactory capabilities in terms of determination of single-droplet size and velocity. Additional simulations performed by an extended theory of Geometrical Optics show sensibility of the measurement technique to phenomena like droplet deformation, oscillation and acceleration. The theoretically obtained conclusions could be consolidated by additional experiments.

A second laseroptical technique, RBR [4], allows the determination of droplet composition within ternary two-phase systems. Concentrations can be concluded from the angular position of the primary monochromatic rainbow, which is sensitive to droplet refractive index.

In a first step, the performance of RBR in terms of determination of homogeneous concentrations was investigated. For different equilibrium concentrations of the recommended ternary test-system Toluene/water/Acetone droplets of well defined diameter were produced in order to detect from a linear CCD-scan.

The instationary mass transfer process of a third component from or to droplets is coming with the establishment of more or less distinctive concentration gradients in both the droplet phase and bulkphase. These gradients are of influence on the light scattering properties of the droplet and thus on the rainbow position . To investigate this effect, a combined diffusional mass-transfer and light scattering model is developed. From simulations can be concluded that the determination of concentration gradients by RBR under certain conditions is possible. This is underlined by mass-transfer experiments to pendant single-droplets.

1. Steiner, L.:
   Drop Population Balance - Werkzeug oder Flop.
   Chem.-Ing.-Tech. 68 (1996) 1 +2, p.141-146
2. Bauckhage, K.:
   The Phase-Doppler-Difference-Method, a New Laser-Doppler Technique for Simultaneous Size and Velocity Measurement - Part 1: Description of the Method.
   Part. Part. Sys. Charact. 5 (1988), p.16-22
3. Misek, T.; Berger, R.; Schröter, J.:
   Standard Test Systems for Liquid Extraction.
   European Federation of Chemical Engineering GB, Rugby 1985
4. Roth, N.; Frohn, A.:
   Theoretical and experimental studies on the influence of internal temperature gradients on rainbow refractometry.
   PARTEC 95 - 4th International Congress Optical Particle Sizing, Nürnberg 1995

INTERFACIAL INSTABILITIES IN LIQUID-LIQUID SYSTEMS

S.Wolf, J.Stichlmair
Technical University of Munich
Dep. Chemical Engineering
A, Arcisstrasse 21, D-80290
Muenchen, Germany
Tel. +49 89 28916505
E-mail: stefan.wolf@www.vta.mw.tu-muenchen.de
Fax: +49 89 28916510

The objective of the project is to quantify the influence of the Marangoni-effect on mass transfer rate and to include the shear stress at the interface for the development of criteria for the prediction of the onset of the interfacial convection. The dependence of the observed interfacial phenomena on parameters as mass transfer rate and bulk phase fluiddynamics will be investigated.

The first part of the experiments was carried out in a diffusion cell where the stability behavior of several liquid-liquid systems was investigated. Marangoni-convection was detected by using a holographic interferometer. Holographic interferometry is an optical method based on the recording and reconstiuction of the distribution of phase and amplitude of coherent lightwaves on a holographic plate. The experiments were carried out with drops and with flat interfaces. Parameters, e.g. as investigated systems, direction of mass transfer, concentration difference and flow rate of the phases, were varied during the investigations. It has been shown that these parameters have a strong influence on the occurence and on the form of the Marangoni-convection. Many systems showed interfacial instabilities in both directions of mass transfer.

Additional concentration gradients can be established in the bulk phases by interfacial instabilities. If density instabilities and Marangoni-convection occure at the same time, the result will be chaotic concentration profiles across the whole measuring cell.

To obtain more data about interfacial convection, studies are made in a flowing channel with the system water / acetone / toluene now. Measurements of the velocity profiles of the flowing phases are accomplished by laser-doppler-anemometer (LDA). The concentration gradients at the inlet and the outlet are determined by a holographic interferometer.

During cocurrent flow of the phases, the intensity of the Marangoni-convection is the strongest at the first contact of the phases since the concentration difference is the highest. At the outlet, the instabilities are less violent because of the smaller concentration gradient and may even disappear at low concentrations.

The analysis of the holographic interference pattern by a picture evaluation systems reveals that interfacial instabilities lead to locally different mass transfer rates and to concentration profiles that are totally different from those profiles predicted by the penetration model.

LASER GRATING VELOCIMETRY FOR LIQUID-LIQUID SYSTEMS

R. Semiat
Chemical Engineering Faculty,
Technion, Institute of Technology
Haifa 32000

Measurements of the local rise velocity of large drops of 3-7 mm in diameter and the local interfacial velocity of a liquid jet in liquid-liquid systems were successfully performed using the Laser Grating Velocimeter technique. The LGV was applied to the determination of the drop rise velocity distribution of a population of 300 drops and axial distribution of the jet interfacial velocity in order to demonstrate the capability and effectiveness of the technique.

FINITE ELEMENT ANALYSIS OF FULLY-DEVELOPED TURBULENT TWO-PHASE FLOW DISTRIBUTION IN TRIANGULAR CONDUITS

Tel) +82-2-880-7210
Fax) +82-2-889-2688
E-mail) parkgc@plaza.snu.ac.kr

In order to analyze phase distribution mechanisms in flow channels, the mathematical model has been upgraded by relaxing the homogeneous secondary flow assumption. We derived the analytical model of turbulent two-phase flow in triangular ducts using the two-fluid conservation equations of continuity and momentum for fully developed, adiabatic, incompressible two-component two-phase flows. Therefore, we used two continuity equations for vapor and liquid, and six momentum equations for x, y, z-direction for vapor and liquid; respectively. We assumed "no slip" boundary conditions on all solid surface. We used the Prandtl mixing length model of Drew derived for triangular conduit, to specify the turbulent stress tensors. The variables that are associated with each triangular element are the axial liquid and vapor velocities, the lateral liquid and vapor secondary velocities, the void fraction and the static pressure.

Standard finite element approximations, based on the Galerkin formulation, were used. Therefore no upwind finite element schemes are used. Rather the phasic viscosities were artificially increased to ensure numerical stability. This is equivalent to upwinding and allows the test functions to be identical to the shape functions.

FEMOTH code was tested for fully developed turbulent single-phase flows in equilateral triangular duct. The FEMOTH code predicted three identical secondary velocities in a half triangular duct due to the geometrical symmetry. It was also found, after sensitivity studies, that turbulence is an important parameter for the secondary flow fields.

FEMOTH code was then tested for fully developed turbulent two-phase flow in equilateral and isosceles triangular ducts. The FEMOTH code predicted qualitatively similar secondary flow patterns for the turbulent two-phase flow in the equilateral triangular duct as in the corresponding single-phase flows. By varying parameters in two-phase turbulent model, it was demonstrated that the ratio of shear-induced to bubble-induced turbulence determines the structure of the secondary flow fields for fully developed turbulent two-phase flows in triangular duct. It was found that the slip ratios between the phasic axial velocities are strongly dependent on the void distribution.

FEMOTH code also predicted the phase distribution in the equilateral and isosceles triangular ducts. It was found that turbulence and secondary flow patterns are important flow parameters for the phase distribution in the channel. However, due to the lack of two-phase turbulence data on triangular ducts in the literature, the FEMOTH predictions of turbulent two-phase flow were qualitative than quantitative. The FEMOTH code is undergoing further development for the upwinding schemes and turbulent models.