(510g) Towards Green Manufacturing of Biologically Inspired Nanomaterials: Degree of Mixing and Mixing Time Analysis

Bioinspired silica materials have received an unmatched attention in recent times owing to their green synthesis which offers a scalable, sustainable and economical method to produce high value silicas suitability for wide ranging applications including catalysis, environmental remediation, biomedical and energy storage.^(1,2) In order to scale-up bioinspired silica synthesis, it is critically important to understand how mixing affects the reaction at different scales. In particular, successful scale-up can be achieved if mixing time is measured, modelled and kept constant across different production scales.⁽³⁾

In this study, in order to identify the key mixing mechanisms controlling the reaction as well as to measure mixing times under various process conditions, a new image analysis technique was used for reactions performed in a transparent stirred vessel. The reaction pH is one of the key parameters in silica formation and hence pH could be used to both monitor the reaction and the mixing. Specifically, the technique involved video recording of colour (pH) change using bromothymol blue as the pH indicator. This was then followed by using a custom-written MATLAB algorithm to analyse the acquired images, each of which was divided into discrete grids to produce a detailed 2-colour pH map. The degree of mixing and mixing time were determined from this analysis for different shaft speeds and acid 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 higher spatial resolution revealed important details on mixing mechanisms and the identification of dead zones, a limitation inherent in other measurement techniques. Additionally, the method offers simplicity and cost efficiency as only a high-speed camera and scripting software such as MATLAB and Python for image analysis are required. The results suggest that the bioinspired silica formation is controlled by meso- and, to a lesser extent, micro-mixing. Further, we were able to correlate the effects of mixing conditions on the reaction and the product. These results are provide valuable insights in the process and pathways to their scale-up for commercial manufacturing.

(1) S. V. Patwardhan, J. R. H. Manning and M. Chiacchia, Curr. Opin. Green Sus. Chem., 12, 110-116, 2018

(2) S. V. Patwardhan and S. S. Staniland, Green Nanomaterials: From Bioinspired Synthesis to Sustainable Manufacturing of Inorganic Nanomaterials, IoP Publishing, Bristol, 2019.

(3) J. Bałdyga and J. R. Bourne, Turbulent mixing and chemical reactions, Wiley, 1999.