(143d) Understanding the Mixing Mechanisms to Enable Sustainable Manufacturing of Bioinspired Nanomaterials

Nanomaterials have a vast potential in applications such as food, coatings, cosmetics, textile, transport, healthcare, and electronics, with a multibillion market. While many new nanomaterials are being discovered, their manufacturing is unsustainable and therefore their potential is not realised. Emulating natural designs, we have developed sustainable approaches to produce high value nanomaterials. To reach the market, it is crucial to develop a strategy to scale-up their manufacturing. With the example of bioinspired silica (BIS), we report on understanding the underpinning mixing mechanisms and develop a methodology for scaling-up a wide range of nanomaterial synthesis. Herein, we take a two-pronged approach: on one hand we investigated reactive mixing during BIS synthesis in a stirred tank reactor and on the other hand, we are investigating mixing using a multi-inlet vortex mixer (MIVM). Combining both approaches, we are generating mixing knowledge for BIS synthesis over a range of production scales.

To this end, a non-invasive colorimetric method coupled with novel image analysis was developed to monitor the reaction and the mixing (ACS Eng. Au 2023, 3, 17–27). Specifically, the technique involved image analysis of colour (pH) change by using a custom-written algorithm to produce a detailed pH map. The degree of mixing and mixing time were determined from this analysis for different impeller speeds and feed injection locations. Cross validation of the mean pH of selected frames with measurements using a pH calibration demonstrated the reliability of the image processing technique. The results suggest that the bioinspired silica formation is controlled by meso- and, to a lesser extent, micro-mixing. Based on the new data from this investigation, a mixing time correlation is developed as function of the Reynolds number. Further, we were able to correlate the effects of mixing conditions on the reaction and the product.

While these results provide valuable insights on mixing occurring during BIS synthesis, the stirred tank geometry can be limiting in terms of energy dissipation and hence mapping wider range is not possible. In order to address this issue, further work is performed using multi-inlet vortex mixers (MIVM). MVIM provide a wide range of flow conditions (Re = 10-10,000) and energy dissipation (ϵ = 10^-1 – 10³ W/kg), covering the lab-scales as well as plant scales, hence they are very useful in this research. Further, MIVM can accommodate unequal flow of reactants and can be used for mixing condition requiring flexibility in the inlet stream content and flow rate ratios.

The design also enables the separation of reactive components prior to mixing which is not possible with alternate confined impinging jet mixing geometries. The MVIM was characterised using the residence time distribution (RTD) and the Villermaux-Dushman reaction approaches. Next, the flow properties, the BIS formation process and the particle properties were investigated using a systematic experimental approach. These results from stirred tank reactor and MVIM (V-D and BIS synthesis) and their comparisons will be presented in order to develop mixing knowledge and scale-up rules.