Abstract

Numerical simulation of micro?mixing in a stirred reactor using the engulfment model coupled with CFD *

Xiaoxia DUAN, Xin FENG, Chao YANG*, Zai?Sha MAO Institute of Process Engineering, Chinese Academy of Sciences
Beijing 100190, China
Stirred tanks are widely encountered in process industries, and often used for complex and fast reactions such as reactive crystallization, nitrification and sulfonation. The yield and selectivity of desired products depend directly on the conditions of mixing on the molecular level[1]. The micro? mixing simulations are being developed with improvement of computer capacity and application of computational fluid dynamics. The engulfment model (E?model) based on the turbulence promoted molecular diffusion has been widely used[2]. Owing to the computational efficiency and model simplicity, the E?model compared with other micro?mixing models based on the probability density function is more suitable for industrial scale reactors[3,4]. However, the E?model does not contain the information about macro?mixing on the reactor scale and is only resolved in the Lagrangian frame perspective.
In this work, a CFD method combining with the E?model is presented and implemented to predict the micro?mixing effects on the course of parallel competing chemical reactions carried out in a semi?batch stirred tank with a 45° pitched blade turbine (PBTD). The new modeling approach describes the small?scale segregation of fluid by solving the transport equations of mixture fraction and its variance based on the CFD simulation of the macro?flow, and then calculates the representative turbulence kinetic energy and dissipation in the reaction zone via the weighted average of the mixture?fraction variance for the E?model. The model predictions of segregation indexes at different agitation speeds and feeding locations are compared with the experimental
results in satisfactory agreement (Fig.1).

0.35

0.30

0.25

exp.[3]

Modified E-model[3] E-model[3] simulation

0.20

0.15

0.10

0.05

0.00

0 50 100 150 200 250 300 350 400 450

N/rpm

Fig.1 Effect of agitation speed on XS for the feed location at the free surface

Corresponding authors: Chao Yang (chaoyang@ipe.ac.cn); Xin Feng (xfeng@ipe.ac.cn)

* The financial supports from 973 project (2010CB630904), 863 project (2012AA061503), the National Natural Science

Foundation of China (21306197) and the National Natural Science Fund for Distinguished Young Scholars (21025627) are gratefully acknowledged.

References

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