Measurement of the local intensities of segregation with the tomographical dual wavelength photometry

Abstract

The newly developed tomographical dual wavelength photometry enables the measurement of the local intensity of segregation at a multitude of points inside the stirred vessel. This is done by injecting a mixture of an inert and a reacting dye into the vessel. The inert dye serves as a tracer for the macromixing, whereas the vanishing of the reacting dye shows the micromixing. The concentration fields of the dyes are measured simultaneously by transluminating the vessel from three directions with superimposed laser beams of different wavelength. The light absorption by the dyes is measured with CCD-cameras and these projections are used for the tomographic reconstruction of the concentration fields. Low Reynolds number measurements with a Rushton turbine show better macro-and micromixing for a dye injection closer to the stirrer shaft compared to a position closer to the main vortex. Keywords: micro-and macromixing, laminar mixing, optical tomography.he various aspects of micro-and macromixing have been T addressed by a wealth of scientists. The first suggestion for a differentiation between mixing on the macro-and microscale came from Danckwerts (1958). He introduced the scale of segregation as a measure of the size of the regions of non-homogeneity. The quality of the non-homogeneity between the regions can be described by the intensity of segregation which is defined as the mean square deviation of the composition from the mean for a given time t compared to the one for t = 0.The direct local determination of these two values is difficult, because this would require measurements on the molecular scale. Kappel (1976) and Hiby (1979) developed therefore measurement techniques to determine the global intensity of segregation. This was done by adding a discolouration agent to the coloured contents of a mixing vessel. The decreasing colour concentration was measured over time and compared to the initial concentration. This ratio can be used to determine an overall intensity of segregation versus time.Bourne and Baldyga (1989) and Villermaux et al. (1994) used a different approach to determine the micromixing time. They injected an agent into the mixing vessel which underwent a spontaneous reaction. The product R of this reaction reacted with the limiting educt by a fast consecutive reaction to a product S. In the ideal case of instantaneous *Author to whom correspondence may be addressed. E-mail address: dms~c36.uni-hannover.de micromixing only the product R would appear. The final product ratio S/R+S was therefore linked to the ratio between the time of the consecutive reaction and the micromixing time. This method allows the determination of the micromixing time inside a specific area of the vessel, if the micromixing is fast enough to avoid a spread of the reaction cloud over the whole vessel. In the case of laminar mixing the micromixing is rather slow, so that the spatial resolution of this technique is rough.During the laminar mixing of high viscous non Newtonian liquids phenomena like flow reversal or the form...

Visualisieren des Mikro‐ und Makromischens mit Hilfe zweier fluoreszierender und chemisch reagierender Farbstoffe

“…Da der Ablauf einer chemischen Reaktion von der lokalen Mischgüte abhängt, ist die Angabe eines über den Behälterinhalt gemittelten Werts für die Mischgüte oder die Mischzeit nur von begrenztem Nutzen. B U C H M A N N und M E W E S [7] verwenden die tomographische Zweiwellenlängentomographie, um die dreidimensionalen Konzentrationsfelder eines inerten und eines reagierenden Farbstoffs simultan im gesamten Rührbehälter zu messen. Die Makround Mikromischung wird für Rührbehälter vermessen, die mit unterschiedlichen Rührertypen ausgestattet sind.…”

Flow Visualization in Stirred Vessels

“…The technique has been used to investigate the¯ow in various process vessels for various two-phase systems 22,23,42 ,50,86,122,145,146,185,200,215,221,22 3 . The tomographic technique for the reconstruction of the¯ow inside vessels has also been applied recently with a visual technique, using two dyes 24,25,184,186 .…”

Reactive PLIF Method for Characterisation of Micromixing in Continuous High-Throughput Chemical Reactors

“…Micromixing can be assessed using physical and chemical methods. Physical methods are based on the incorporation of a non-reactive tracer in the fluid streams and either: (a) measurement of its concentration at different locations, including reactor outlet, as a function of time (e.g., Nadeau, et al [4]) or (b) visualisation of pathlines inside the reactor by optical methods (e.g., Buchmann and Mewes [5], Buchmann and Mewes [6]). Thus, while the physical methods are related to the degree of the homogeneity of the fluid, chemical methods are based on test-reactions, where product yield or reagent consumption are used to infer the micromixing degree.…”