Greener production of cellulose nanocrystals: An optimised design and life cycle assessment

Cellulose nanocrystals (CNCs) are usually prepared by sulphuric acid hydrolysis in industrial production, resulting in a large amount of alkaline and water consumption. In this paper, three acid-CNCs separation methods, including gravity settling, centrifugation and microfiltration, are used to directly separate sulphuric acid from hydrolysate mixture to reduce alkaline and water consumption. Microfiltration was selected as the best method for acid-CNCs separation. Using microfiltration, the highest sulphuric acid recovery ratio was about 65.0%, with an acid concentration range of 8-10 wt% in the pilot-scale CNCs production. Continuous centrifugation was introduced to replace gravity settling to separate CNCs from the neutralised mixture for faster production and reducing water consumption. The consumption of alkaline and water decreased by 63.0% and 68.0%, and the cost was reduced by 26%. The result of life cycle assessment (LCA) showed that the process with acid-CNCs separation using microfiltration achieved a reduction of 33.4% in aquatic ecotoxicity, 38.1% in terrestrial ecotoxicity, 47.9% in aquatic acidification, 55.8% in aquatic eutrophication, 40.0% in global warming potential and 47.8% in non-renewable energy consumptions. This study provided a promising approach for separating sulphuric acid from hydrolysate mixture and reducing consumption of alkaline and water. Such clean and economic process has great potential in the industrial production of CNCs.