The attainment of fast mixing is a requirement to avoid transport limitations in processes with fast kinetics such as precipitation at high supersaturations. In this work, the mixing performance of different micro mixers is characterized experimentally and using computational fluid dynamics (CFD).

First, mixing is characterized using a set of competitive-parallel chemical reactions [1]. The mixing efficiency of a wide-angle Y-mixer and a Roughton mixer are compared by measuring the reaction yield at different flow rates, flow rate ratios and reaction time constants [2]. It is found that the Roughton mixer achieves a better mixing performance compared to the Y-mixer and that the mixing efficiency is not affected by the flow rate ratio.

Secondly, the two mixing devices are modeled using computational fluid dynamics (CFD). A closure method using a presumed probability function and the turbulent mixer model is employed to account for micromixing on sub-grid scale [3, 4]. The chemical reaction set is implemented in the CFD code and applied to predict the reaction yield of the mixing experiments which in turn is used to validate the CFD model [2]. Experimental results and model predictions are in good agreement for all mixer geometries and operating conditions. CFD is used to calculate absolute mixing times based on the residence time in the segregated zone and it is shown that mixing times of less than one millisecond can be achieved in the Roughton mixer. CFD provides insight in local concentrations and reaction rates and serves as a valuable tool to improve or to scale-up mixers.

Based on these results the CFD model can be employed for the prediction of the particle size distribution (PSD) in precipitation processes when incorporating the population balance equation (PBE) in the model. However, for most substances nucleation and growth kinetics are unknown, especially at high supersaturations, and the application of a combined CFD-PBE model for real processes might be limited. Therefore, in the third part of this study the mixer setup is used to determine nucleation and growth kinetics of L-asparagine at high supersaturations, i.e. true kinetics which are not affected by transport limitations. The method is based on measuring the PSD obtained at different residence times using a Coulter Multisizer. Nucleation rates are calculated from the change of absolute particle number and growth rates from the change of particle size over time.

References

[1] B.K. Johnson, R.K. Prud'homme, Chemical processing and micromixing in confined impinging jets, AIChE Journal 49 (2003) 2264-2282.

[2] C. Lindenberg, J. Schöll, L. Vicum, J. Brozio, M. Mazzotti, Experimental characterization and multi-scale modeling of mixing in static mixers, accepted for publication on Chem. Eng. Sci. (2008).

[3] J. Baldyga, W. Orciuch, Barium sulphate precipitation in a pipe – an experimental study and CFD modelling, Chem. Eng. Sci. 56 (2001) 2435-2444.

[4] L. Vicum, M. Mazzotti, Multi-scale modeling of a mixing-precipitation process in a semibatch stirred tank, Chem. Eng. Sci. 62 (2007), 3513-3527.