Design of impeller blades for efficient homogeneity of solid-liquid suspension in a stirred tank reactor

- A new type of punched rigid-flexible impeller was proposed.
- Punched rigid-flexible impeller can improve solid-liquid homogeneous degree.
- Punched rigid-flexible impeller can reduce the power consumption.
- Homogeneity ξ was calculated for solid-liquid mixing performance evaluation.

Computational fluid dynamics (CFD) was used to investigate the hydrodynamics of solid-liquid suspension process in a stirred tank with rigid impellers, rigid-flexible impellers and punched rigid-flexible impellers. The effects of impeller type, impeller speed, flexible connection piece length, impeller spacing, particle size, and aperture size/ratio on the mixing quality were investigated. Results showed that the degree of solid-liquid homogeneity increased with an increase in impeller speed. A long flexible connection piece was conductive to solid particles suspension process. The solid particles could not obtain enough momentum to suspend to the upper region of stirred tank with small impeller spacing. Larger particle size resulted in less homogenous distribution of solid particles. The optimum aperture ratio and aperture diameter of punched rigid-flexible impeller were 12% and 8 mm, respectively, for solid particles suspension process. It was found that punched rigid-flexible impeller was more efficient in suspending solid particles compared with rigid impeller and rigid-flexible impeller at the same power consumption. In addition, less impeller power was consumed by punched rigid-flexible impeller compared with rigid impeller and rigid-flexible impeller at the same impeller speed.

The punched rigid-flexible impeller consists of a rigid impeller and six flexible connection pieces. A certain number of holes exist on the surface of the rigid impeller blades. The flexible connection pieces were connected to the blades of rigid impeller. The flexible connection pieces can cause a series of disturbances on the flowing medium and transfer the impeller energy through the form of wave. A series of high-speed jet flows and a plurality of vortices can be generated in the flow field due to the existence of apertures. These conditions are beneficial to improve the solid particles suspension quality in the solid-liquid mixing process.31