The Chinese Journal of Process Engineering

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Research progress of continuous generation of microbubbles by microdispersion

Bingqi XIE Caijin ZHOU Xiaoting HUANG Xiangdong MA Jisong ZHANG

The Chinese Journal of Process Engineering 2021, 21 (8): 865-876. DOI: 10.12034/j.issn.1009-606X.220341

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Microbubbles have been drawn more attentions due to their widely applications. At present, the preparation methods of microbubbles mainly include ultrasonic, electrolytic process, dissolved air flotation and microdispersion. Compared with traditional methods of microbubble generation, the microchannel technology has the advantages of high production efficiency, good controllability, excellent flexibility, which has been applied to produce the monodisperse microbubbles and drops. And the microchannels devices with different structures, such as co-flowing microfluidic, flow focusing, T-microchannel and venturi devices are an all-around introduced in this paper. In the process of gas-liquid membrane dispersion, the microbubbles size is affected by many factors, such as liquid flow velocity, liquid surface tension, liquid viscosity, the pore size, porosity, pore structure of membrane and gas flow velocity. So far, the mechanism of microbubbles formation is complicated, which is still not clear. Moreover, it is also critical important to rapidly and accurately measure the size and distribution of microbubbles due to the wide application of microbubbles. Traditionally, the size and distribution of microbubbles are measured by probes and laser particle analyzer, which is efficient and easy accessibility. However, the insertion of probes will affect the flow filed and the mechanism of lase particle analyzer is not clear. With the rapid development of digital image recognition technology, combination of high-speed camera and digital image recognition technology provides an effective, visual and accurate online microbubbles recognition method to measure microbubbles size. Furthermore, the application of deep learning technology in the recognition of microbubbles has drawn more attentions. In this work, the commonly characterization methods of microbubble size are summarized. In addition, the advantages and disadvantages of different methods of preparation microbubbles are also expounded and the current research status of microchannel method and gas-liquid membrane dispersion method are mainly introduced. On this basis, the future research directions of microbubbles prepared by microdispersion are prospected.

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Direct numerical simulation of mass transfer process of single free rising microbubbles under the influence of surface active materials

Chengxiang LI Yizhou CUI Xiaogang SHI Jinsen GAO Xingying LAN

The Chinese Journal of Process Engineering 2021, 21 (8): 877-886. DOI: 10.12034/j.issn.1009-606X.220240

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The international organization for standardization (ISO) has defined bubbles with the diameter between 1~100 μm as microbubbles in 2017. According to the above definition and the fact that microbubbles are easily affected by surface active materials, the mass transfer rate of single free rising microbubbles with various diameters under the influence of surface active materials was studied by direct numerical simulation. It was found that the velocity and mass transfer rate of the microbubbles were in good agreement with the theoretical results of creeping flow recommended by Clift et al. The adsorption of surface active materials on the surface of microbubble reduced the liquid-side mass transfer coefficient. However, for microbubbles that were greatly affected by surface active materials, the liquid-side mass transfer coefficient increased with the decrease of bubble size, which was different from the trend of clean microbubbles that decreased first and then increased. Therefore, in the application where the effect of surface active materials cannot be excluded, further reduction of the initial size of microbubbles can not only increase the specific surface area of the gas phase but also further increase the liquid-side mass transfer coefficient of the bubble, and the mass transfer capacity can be further enhanced.

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Microbubble enhanced Fe ²⁺ oxidation in phosphoric acid solution

Yaru WANG Yeqing LÜ Shaona WANG Hao DU

The Chinese Journal of Process Engineering 2021, 21 (8): 877-894. DOI: 10.12034/j.issn.1009-606X.220248

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n order to realize the efficient separation of Fe2+ in acidic media, hydrogen peroxide oxidation method is often used in industry to transform Fe2+ into Fe3+ with lower solubility to realize the precipitation of iron. Due to the low utilization rate of hydrogen peroxide and poor economy, it is urgent to develop a new Fe2+ low-cost and high-efficiency oxidation method. Based on the principle that reactive oxygen species can be generated during the microbubbles bursting, the microbubble enhanced Fe2+ oxidation technology had been developed in this study. The effect of the aeration head aperture, reaction temperature, and acid concentration on the oxidation efficiencies of Fe2+ and ?OH production were studied. The oxidation efficiency of Fe2+ can reach to 99% within 30 min under the optimized conditions (90℃, 30wt% H3PO4, 0.22 μm aeration head aperture). The oxidation effect of microbubble enhanced technology was comparable to the current H2O2 oxidation, and greatly reduced the economic cost of the process. Moreover, the mechanism of Fe2+ oxidation enhanced by microbubbles was studied in this work, and the main reactive oxygen species generated by microbubbles bursting were determined to be hydroxyl radicals, and the influences of aeration head aperture, reaction temperature and acid concentration on the generation of hydroxyl radicals were studied, so as to obtain the regulation rules of hydroxyl radicals in acidic media.

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