The present paper provides an overview of the intensification of mixing processes with complex fluids. First, mixing processes of rheologically complex fluids and their design methodology are discussed. An example of process intensification of mixing with rheologically complex fluids using a Couette–Taylor flow reactor is introduced. Second, the intensification of the mixing of a suspension with high concentration solids and a novel approach for solid–liquid separation using a stirred vessel are discussed. Finally, a perspective of the intensification of mixing processes with complex fluids is summarized.

In the present work, the formation of unmixed regions and material transport in a stirred vessel flow with an anchor impeller, which is common in mixing highly viscous fluids particularly with yield stress, have been investigated by experimental and numerical approaches. Newtonian fluid is employed as a working fluid and flow visualization experiments are carried out with the laser-induced fluorescence technique. Experimental unmixed regions are compared and analyzed using flow simulations with the Poincaré section. We discuss formation of peculiar dynamical systems structures in flows of a Newtonian fluid in an anchor agitator: a subharmonic Viking-helmet-like structure with two horns, a central bowl-shape island and an outer toroidal band. The effects of the Reynolds number on the dynamical structures have found to be minor, though there appears unsymmetrical formation of the horns that enhances mixing performance to some extent.

This study was focused on the intensification of alpha amylase activity by fungi. Submerged culture of Aspergillus oryzae as a non-Newtonian fluid with a complex rheology and morphology was used. The aim of this study was to enhance the hydrodynamics of the biological fluid for an improvement in cell-gas mass transfer. Therefore, we investigated the advantages of using a Swingstir^® agitator that features an angular motion. Results showed that mixing time, mixing energy, and power-law model parameters using the Swingstir^® all were lower than when a double Rushton turbine (DRT) was used at the same level of power consumption. The Swingstir^® can produces a homogeneous and periodic-flow distribution. Moreover, K_La could be controlled throughout the fermentation process. Also, the morphology of pellets agitated by Swingstir^® had a positive effect on mass transfer intensification, because the stress is applied by flexible shaft movement instead by the blade tip of impeller. Consequently, the alpha amylase activity and cell growth of a culture agitated by Swingstir^® were higher than that developed using a DRT.

Just-suspension conditions were studied in a dished base baffled cylindrical tank with internal diameter, T=155 mm using two variables of impeller-to-tank geometry, D=0.3T and 0.5T. The performance of the alternative geometries was studied using a conventional down-pumping 45° four pitched blade turbine (4PBT) impeller in comparison to the novel SATAKE SUPERMIX^® HR100 and HS604 impellers. The slurry comprised of 18.0, 75.3 and 195.5 µm PMMA particles in concentrations ranging from 5–40% by weight.

The performance of solid suspension was assessed in a dished base tank, which was found to be not as efficient as the flat base tank when used with the D=0.5T impellers. However, it is interesting to note that, when used with a suitable impeller, namely the HS604 impeller, the dished base tank was more superior to the flat base tank. This indicates that impeller-to-tank geometry is critical in determining the efficiency of solid suspension. Additionally, solid suspension performance in a dished base tank was markedly enhanced with D=0.3T impellers by approximately 60 to 80% reduction in the specific energy at just suspension (ε_js) compared with D=0.5T, resulting from the fluid flow pattern generated inside the stirred tank. This suggests that the dished base tank is not a poor option for solid suspension if it is used with the right impeller geometry.

We previously reported that a rotationally reciprocating impeller, which slowly rotates back and forth with a sinusoidal change in the rotational speed, can realize superior mixing performance even under moderate fluid flow. In the case of crystallization process, it is expected that the rotationally reciprocating mixing suppresses crystal brakeage caused by collision to impeller plate and inhomogeneous supersaturation due to notable mixing performance. In the present study, we studied reactive crystallization process of salicylic acid and investigated the effect of mixing conditions on the size and shape of crystals. For a steady rotational mixing, the largest crystal size was obtained at the appropriate mixing condition, and the size and shape of crystals are both affected by the rotational speed. Meanwhile, it is found that the size of crystals maintained roughly constant in the wide range of mixing condition of the rotationally reciprocating mixing. Additionally, no significant difference in the shape was observed even at different reciprocating conditions.

Liquid jet mixing is a common practice in industry when mechanical mixing becomes unfeasible due to the scale of mixing vessel required. The present study employed a flow visualization technique to investigate the agitation of a non-Newtonian fluid (0.5 wt% xanthan gum solution) using a submerged recirculating jet. Building on the work already published by the authors, the present study seeks to identify how the mixing performance is influenced by two factors: namely, jet-nozzle orientation (upward and downward facing) and mixing vessel aspect ratio (1 : 1 and 3 : 1). It was found that the nozzle orientation played a much larger role in the 1 : 1 setup compared to the 3 : 1 setup. The best performing setup for the 0.5 wt% xanthan gum solution was a 1 : 1 tank with an upward facing nozzle. It is postulated that this is because the suction in this setup has a net positive effect on the mixing performance; whereas, for other setups, the interplay between jet and suction has a deleterious effect.

Recently, fine bubbles (micro/nanobubbles) have been applied for industrial use. The present study investigates oil and salt cleaning using fine bubbles. As a result, it was confirmed that, in the removal of oleic acid and salt, the removal effect was increased depending on the total surface area of fine bubbles. In oil cleaning, it was thought that the large hydrophobic interaction field was formed by the generation of fine bubbles by introducing air, and consequently the attaching action to oleic acid, which is same hydrophobic material, was effectively accelerated. In microbubbles, it was estimated that the hydrophobic interaction bridged oleic acid oil drops and coalesced them together by microbubble self-shrinkage, and it resulted in the effective separation action. On the other hand, the shortening of salt cleaning time was confirmed in fine bubble water, in comparison to the time in water without fine bubbles.

Microbubbles with diameters smaller than 100 µm are currently attracting considerable attention because of their properties such as a large surface area per unit volume, low rise velocity, and self-pressurization due to surface tension. Because of these properties, they find wide applications in agriculture, aquaculture, and medicine. Makuta et al. pioneered the development of a microbubble generator using a hollow cylindrical ultrasonic horn. In this method, the gas supplied through the path in the horn creates a gas–liquid interface in water, and microbubbles with diameters smaller than 100 µm are easily generated by irradiation of the ultrasonic oscillation to the gas–liquid interface. However, a higher yield of microbubbles is required to enable their practical application. Therefore, we changed the orifice shape by replacing the detachable tip at the end of the hollow cylindrical ultrasonic horn and evaluated the yield of microbubbles from the dissolved oxygen concentration in water for different shapes of the orifice. We also investigated the microbubble generation ability of the generator under changing flow rates of the gas supply to the horn. We found that the microbubble generation ability improved with an increase in the number of orifices at the horn end.

The present study focused on a “tacit knowledge” on agitating motions in cooking. The study observed human agitating motion conducted by an expert confectionery hygiene mistress and non-expert female students. Mixing (whipping) state of fresh cream was evaluated by an overrun which indicates the amount of gas phase and rheology of fresh cream. In the motion analysis, three observation points were set at research subjects’ elbow, wrist and whisk. Mixing behaviors were visualized by food red and recorded by a high-speed camera. The expert confectionery hygiene mistress accomplished a peak value of the overrun in two times shorter period than the non-experts. In the non-experts’ motions, all motions of elbow, wrist and whisk were strongly synchronized and the time series of displacement showed relatively simple periodic motions. On the other hand, in the expert’s motions, phase difference of motions among elbow, wrist and whisk was observed and the time-series of displacement showed to be rather complex and chaotic. Furthermore, it has been found that the expert agitates with small motion putting snap on the whisk.