Micromixing Performance Of Micro-impinging Stream Reactors And Its Effects On The Co-precipitation Process Of Nickel-based Metal Composites

Micromixing,as the final stage of mixing,plays an important role in a lot of chemical processes because it occurs at the molecular scale.For rapid co-precipitation processes,the quality of product is controlled by both the micromixing and the intrinsic kinetics of precipitation.However,the micro-mixing performance of traditional reactors is usually slow,and the specific influence mechanism of micromixing on the co-precipitation process is not clear enough,which hence limits the applications co-precipitation method for large-scale production of nano or micro metal composites.Based on the above analysis,a flexiable micro-impinging stream reactor（MISR）with a simple structure was constructed by stainless steel capillaries and commercial T-junctions.It not only has superior mass and heat transfer rates similar to the microchannel reactors,but also can further strengthen the micromixing performance through the accelerated impinging of the fluids,so that the micro-mixing time can be close to or even smaller than the nucleation induction time.Therefore,the effects of micromixing on the co-precipitation process could be investigated specifically.The major findings of this work are as follows:（1）With the aid of the commonly used iodide-iodate parallel competition reaction system,the micromixing time of MISR with different tube diameters under various operating conditions was experimentally studied.Results showed that the higher the fluid linear velocity was,the shorter the micro-mixing time was.Within the working range of the pump,when the linear velocity of the fluid was the same,the micromixing time of the MISR with a larger diameter was shorter.However,converting the linear velocity of the fluid into the volumetric flow,the volumetric flow required by the MISR with a larger diameter was much larger for achieving the same micromixing time.The micromixing time of MISR obtained by experiment was in the range of 0.07 ms～1.27 ms.（2）The flow field of MISR was simulated by CFD,and the impinging zone of the reactor was determined based on the analysis of the flow field.Combined with the E-model,the micromixing time could be studied without the aid of probe reactions,which not only saves calculation time,but also avoids debates caused by the unclear kinetics of the probe reactions and errors caused by additional simulation of reactions.The reliability and rationality were verified by comparison with the data obtained from the experiment and the traditional simulation approach.（3）MISR was applied to preparing LaNiO₃ and Co doped Ni（OH）₂materials with co-precipitation method and the effects of micromixing on the co-precipitation process of the materials were systematically studied.The nucleation induction time of LaNiO₃ perovskite and Co doped Ni（OH）₂materials was estimated as about 0.1 ms and 0.18 ms,respectively.The effects of micromixing on the co-precipitation process were studied at three different stages based on the scale of micromixing time:larger,close to and less than the nucleation induction time.Results showed that for LaNiO₃ perovskite material,with the micromixing time approaching the nucleation induction time,the shape of the precursor,morphology and crystallinity of calcined products and the electrochemical performance of the material after reduction would be improved.However,with the micromixing time continues to decrease to be less than the nucleation induction time,the quality of the product would deteriorate due to the agglomeration caused by excessive turbulence and impinging.In the preparation process of Co doped Ni（OH）₂ composite,the relationship between the micromixing time and the co-precipitation process was also in accordance with the LaNiO₃ co-precipitation system.（4）The traditional stirred tank reactor was also applied to prepare the two kinds of materials under the same reaction conditions.The comparison of material properties with those prepared by MISR reflected the advantages of MISR in the process of co-precipitation,and further indicated the leading role of micromixing in the process of co-precipitation.The properties of materials prepared under similar micromixing conditions would be close when the influence of other process conditions was excluded.（5）The electrochemical properties of the two supercapacitor electrode materials were investigated.In the three-electrode test system,the LaNiO₃ after N₂H₄·H₂O reduction and Co doped Ni（OH）₂ materials,which were prepared under the optimal condition in MISR with co-precipitation method,showed an initial specific capacitance at the current density of 1 F·g^-1 as 129.8 and 1548F·g^-1,respectively,and displayed a capacitance retention rate as 200.8%after3000 cycles and 106.0%after 1000 cycles,respectively.In the two-electrode system,the initial specific capacitance of the two devices at the current density of 0.5 F·g^-1,which were composed of the LaNiO₃ after reduction and the Co doped Ni（OH）₂ materials,was 12.6 and 30.6 F·g^-1,respectively,and the corresponding cycling stability was 87.6%after 3000 cycles and 75.6%after1000 cycles,respectively.