Makro‐ und Mikromischen im Rührkessel

Abstract

Herrn Professor Dr. Hanns Hofmann zum 65. GeburtstagAusgehend von der Turbulenztheorie wurde theoretisch eine Beziehung zur Berechnung der Mikromischzeit als Funktion der spezifischen Leistung, der kinematischen Viskositat, der Schmidt-Zahl und des Segregationsgrades hergeleitet. Ein Vergleich der theoretisch gefundenen Mikromischzeit mit den in der Literatur vorliegenden Angaben zur Makromischzeit ergibt, darj fur sehr kleine Behalterdurchmesser die Mikromischzeit der geschwindigkeitsbestimmende Schritt ist. Dagegen bestimmt die turbulente Makromischzeit den Mischvorgang fur grorjere Ruhrwerke mit technisch relevanten Durchmessern. An Hand von experimentellen Ergebnissen der turbulenten Geschwindigkeitsverteilungen in verschiedenen Ruhrkesseln werden fur groBe Schmidt-Zahlen Mikromischzeiten Omikro fur unterschiedliche Ruhrwerksgeometrien und Ruhrorgane sowie fur verschiedene lokale Positionen der Eduktzufuhr angegeben. Die mitgeteilten Werte fur die turbulente Mischzeit gelten jedoch nur bei geometrisch optimalen Ruhrbedingungen und konnen bei ungunstiger Anordnung die erforderliche Mischzeit uberschreiten. Bei Ablauf einer chemischen Konsekutiv-Reaktion ist mit einer wesentlichen Verschlechterung der gewunschten Produktausbeute zu rechnen.Macro-and micromixing in stirred tanks. Macromixing time, based on turbulence theory, is calculated theoretically as a function of the specific power input, the kinematic viscosity, the Schmidt number, and the degree of segregation. A comparison of the micromixing time with data for macromixing found in literature reveals that the micromixing time governs the reaction for very small vessel diameters. If the vessel diameter is greater than the critical size than the turbulent macromixing time controls the mixing process. The micromixing time Omikro is calculated by means of the experimental results of the turbulent velocity distributions in different stirred vessels for several geometrical arrangements of stirred tanks and different positions of feed input. The calculated data for turbulent mixing times are only valid for geometrically optimized stirred vessels. The necessary mixing time may increase or, if a chemical reaction of 2nd order takes place, the desired yield of the product may decrease considerably if the geometrical conditions are not taken into account.

Mesomixing in semi‐batch reaction crystallization and influence of reactor size

“…The constant C varies from 3.5-17.3 in the literature for the physical properties of water (Corrsin, 1964;Baldyga and Bourne, 1999;Geisler et al, 1988). The most common value seems to be 12.7 (Baldyga and Bourne, 1986), which is used in this study if nothing else is stated.…”

Experimental and theoretical investigations of the coating of capsules with titanium dioxide

“…(2) [21] and the micro-mixing time in dependence of the degree of segregation for a Schmidt number Sc > 100 (Sc = /D AB ) by Eq. (3) [20]. The Schmidt number gives the ratio between the kinematic viscosity and the diffusion coefficient D AB .…”

“…To assess the macro-and micro-mixing time Geisler et al [20] defined a time-dependent degree of segregation I s = exp(−t/ D ), where t is the time and D a characteristic time, which describes the extent of mixing between 1 completely separated (t = 0) and 0 completely mixed (t → ∞). The macro-time of mixing for a degree of segregation of about 0.01-0.05 can then be assessed by Eq.…”

Paints and Coatings

“…The horizontal design was intended to eliminate the influence of gravitational forces. Numerous design features have been optimized [401,402,[405][406][407][408][409][410][411][412][413][414][415]. The most important developments are illustrated in Fig.…”