Abstract
The micromixing efficiency in the rotor–stator spinning disk reactor is studied for increasing injection flow rates of limiting reagent. Using both the Villermaux–Dushman method and second Bourne reaction, an increase in segregation index was found with increasing injection flow rate. Mechanistic models describing micro- and mesomixing were reviewed and implemented to describe the effects of increasing segregation index with increasing injection flow rate. A simplified turbulent-dispersion model was found to describe this trend effectively. A new parameter, based on a logistic function, was proposed to couple mesomixing by turbulent-dispersion and micromixing by engulfment on basis of the ratio of the characteristic times for turbulent-dispersion and engulfment. For injection flow rates where micromixing was calculated to be dominant, the engulfment model was implemented. An experimental relation was fitted to describe the dependency of the local energy dissipation rate on the turbulent diffusion coefficient in the rotor–stator spinning disk reactor. For the Villermaux–Dushman test reaction and the second Bourne reaction, discrepancies were found in the local energy dissipation rates experienced by the reaction zone. These discrepancies were analyzed on basis of the reaction times of both test reactions. The effect of the reaction time was accounted for by introducing a time-dependent local energy dissipation rate, which could be used to study the energy dissipation rate profile in the rotor–stator spinning disk reactor.
Highlights
The micromixing efficiency in the rotor–stator spinning disk reactor is studied for increasing injection flow rates of limiting reagent
This is an expected behavior since with increasing flow rate, locally there is a higher concentration of acid that takes longer to disperse in the mesomixing region (Baldyga and Bourne, 1992; Chen et al, 1993; Baldyga et al, 1993, 1994; Bałdyga and Pohorecki, 1995; Bourne, 2003; Akiti and Armenante, 2004; Vicum et al, 2004; Jasinska et al, 2013)
Since the reaction rates of the Villermaux–Dushman method are faster than the ones from the second Bourne reaction, the first one provides more information of the local reaction zone. From these experimental results it has been calculated that the experienced local energy dissipation rate εVexDp differs as much as 20 times higher for the Villermaux–Dushman method than the estimated εloc, based on the minimum observed segregation index
Summary
To study mixing in chemical reactors, it is a general consensus to subdivide the mixing in a cascade of macromixing, mesomixing and micromixing (Mao and Yang, 2017). Mesomixing is defined as mixing at the scale above micromixing, but below macromixing. It represents the coarse-scale turbulent exchange between fresh feed and its surroundings (Baldyga et al, 1993; Villermaux and Falk, 1994; Bałdyga and Pohorecki, 1995; Bałdyga et al, 1997). Mesomixing can influence the reaction zone, which is located near the feed point for fast reactions, sometimes resulting in a plume of fresh feed This plume of fresh feed can form a spatial structure above the micromixing scales and can be limiting in the mixing cascade, if feed rates are high (Bourne, 2003). Bałdyga and Bourne described micromixing by a different set of mechanistic models (Baldyga and Bourne, 1989b) They considered mixing to be described by three different steps. The third step is molecular diffusion, which leads to mixing on the molecular level
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