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Aiming at the fact that the function of gas distributor is often neglected in the case of high viscosity conditions, a novel spherical micro-orifice gas distributor was proposed and investigated experimentally in water system and non-Newtonian viscous system, respectively. The comparisons of gas–liquid dispersion and mass transfer performance between novel spherical micro-orifice gas distributor and traditional ring distributor were also done. Furthermore, the effects of gas distributors on the gas–liquid dispersion and mass transfer characteristics of three wide-viscosity-range impellers including LDB impeller, FZ impeller and MB impeller were also researched, and an impeller-distributor configuration with good gas–liquid dispersion and mass transfer performance was achieved. The results show that, compared with ring gas distributor, the bubbles dispersed by novel spherical micro-orifice gas distributor are smaller, and the gas holdup and volumetric mass transfer coefficient are relatively higher under the same power consumption per unit volume. Meanwhile, the significant effect of gas distributor geometry on gas dispersion was found for high-viscosity liquid, but not relevant for low-viscosity batches. In addition, compared with other impeller-distributor configurations, the configuration consisting of FZ impeller and novel spherical micro-orifice gas distributor can achieve greater global gas holdup and higher oxygen mass transfer rate, which is recommended especially in viscous system with high gas flow rate. The research is beneficial to promote the application of novel spherical micro-orifice gas distributor in stirred tanks.

Coaxial mixers as novel mixers have the characteristics of high flexibility and wide adjustability. Three kinds of coaxial mixers consisting of an outer anchor and different inner impellers (six 45° pitched-blade turbine, propeller and Rushton turbine) and single-impeller mixers were investigated and compared in viscous systems. Effects of rotation and pumping modes, outer impeller speed, inner impeller diameter, liquid viscosity, solid volume fraction and particle diameter on the just-suspension impeller speed and power consumption were also studied. Results showed that the coaxial mixers were more energy efficient than the single-impeller mixers, and had greater advantage in systems of higher viscosity and solid volume fraction. Different from single-impeller mixers, adopting up-pumping mode was more energy efficient for coaxial mixers when the anchor speed was relatively high. In addition, the coaxial mixers with the inner up-pumping propeller and six 45° pitched-blade turbine had obvious superiority compared with the combination of anchor and Rushton turbine in terms of power consumption. Based on the experimental data, a new correlation for predicting just-suspension impeller speed of coaxial mixers was proposed.

A novel method to determine the flooding/loading transition point (Nf) was proposed based on the measurements of the bottom pressure. Experiments were carried out in an aerated vessel stirred with a Rushton impeller in single-phase and two-phase systems. The results showed that the bottom pressure (P) in the single-phase system had a parabolic decrease with the increasing impeller speed (N), which followed the Bernoulli Effect; the flooding/loading transition was defined by a sharp change in the P–N curve. The data were well consistent with the results obtained by the global gas holdup method, as well as the existing literature data on flooding/loading transition.

As a kind of mixer with wide adaptability, coaxial mixer has a wide-application in the process industry. With the help of experiment and numerical simulation, the gas-liquid dispersion performance of four impeller combinations in viscous system was studied in terms of the global gas holdup, local gas holdup, local bubble size and liquid phase flow field. Wall-scraping anchor is the common outer impeller of the four impeller combinations, and the four inner impellers are Rushton turbine, six-straight-blade turbine (SBT), 45° six-pitched-blade turbine pumping downwards (PBTD) and 45° six-pitched-blade turbine pumping upwards (PBTU). The experimental results indicate that the global gas holdup of the Rushton impeller combination is the highest among the four impeller combinations and that of PBTU is the lowest. The local gas holdup of Rushton and PBTU combinations are all high, but the gas distribution with the Rushton combinations is more uniform. Besides, the local bubble size of the four impeller combinations has the same tendency with the local gas holdup. Under the same conditions, the flow field and distribution of gas holdup of the four impeller combinations in stirred tank were simulated numerically. Compared with the experimental results, the simulation results show a good agreement on the value and distribution of local gas holdup. Among the four coaxial mixers, the Rushton impeller combination is more suitable for gas-liquid dispersion.