Abstract
dc:descriptionLiquid-liquid extraction processes are often found in industrial applications when a bulk phase needs to be purified from dissolved components. The strategy of extraction consists of dissolving the impurities into a second, carrier phase, with optimal extraction performance being guaranteed by maximizing both contact interface area and mass transfer rate. Providing the carrier phase in the shape of a swarm of dispersed droplets serves the purpose of generating a large interface between both fluids; at the same time, the buoyancy-driven flow of the droplets within the continuous medium induces internal fluid motion driven by momentum transfer at the drop surface. This convective transport greatly enhances mass transfer and increases the efficiency of an extraction column dramatically, compared to fluid motion solely governed by diffusion. However, understanding mass transfer vitally depends on a proper description of the precise flow field inside and outside of the droplets. Nuclear magnetic resonance (NMR) velocity imaging can give quantitative information about drop shape and velocities and is able to distinguish between different chemical compounds that will become relevant for the direct investigation of mass transfer. The potential of pulsed field gradient (PFG) NMR for the investigation of flow processes has been demonstrated by application to fluid motion subject to vastly different geometries and therefore is an exceptionally suitable technique for non-invasively monitoring the drop’s internal fluid dynamics and its change with time. In this thesis, fast NMR imaging techniques are combined with velocity encoding in order to generate statistical and imaging information about the oscillation and internal vortex patterns of ‘levitated’ drops. It is a well-known fact that the efficiency of mass transfer between the two phases is determined by convective transport made possible through circulation occurring both inside and outside of the droplets. The internal dynamics of the drops can generally be divided into different regimes. While small droplets behave like rigid spheres, larger droplets feature pronounced internal dynamics. The rigidity of the interface and therefore the internal behaviour of the drops are mostly characterized by the drop size, composition and purity of the system. In the present work, the two limit cases were studied: silicone oil (rigid interface) and toluene (mobile interface) drops are generated with a special pipette (that guarantees the reproducibility of the experiment and also allows to control the drop size) at the bottom of a glass tube filled with D2O. When the drop rises to the centre of the NMR resonator, it is ‘stopped’ by the counter flow of the continuous D2O phase. Velocity patterns in this free scheme can be studied. 1D, 2D, 3D NMR images and flow measurements of 3-4 mm diameter silicone oil drops and 4-5 mm equivalent diameter toluene drops in 4.7 T and 11.7 T magnets were performed and the velocity distributions analysed. In all cases, where experiments were performed in a controlled way and under strict purity conditions, a stable pattern was revealed for times of hours and even days, these patterns being characterized by the inherent impurities of the system. The latter was particularly well observed by the occurrence of a rigid cap next to a vortex region inside toluene drops with an overall mobile interface. It could also be shown that given a sufficient concentration of organic impurities during the experiment, the size if the rigid cap will grow with respect to the mobile surface as expected from the theory. Silicone oil drops, on the other hand, revealed a similar internal circulation pattern but with a much reduced velocity range. Interesting results are obtained when the glass measurement cell is not perfectly symmetric or when its orientation is not perfectly vertical. In this situation, axially symmetric results are no longer observed. In conclusion, the feasibility of applying PFG-NMR techniques for the non-invasive investigation of internal fluid dynamics in levitated drops was demonstrated. The technique is both reasonably fast and allows the visualization of three-dimensional velocity fields for providing input data to interface models and to allow a comparison with numerical simulations.
Degree
thesis:*- Grantor dc:publisher
- Shaker
- Year dc:date
- 2006
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Amar, Andrea M.
- Contributors dc:contributor
-
- Blümich, Bernhard
Subjects
dc:subject × 8Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng
Identifiers
dc:identifier.*- OAI identifier oai:identifier
- oai:publications.rwth-aachen.de:62283