Optimisation de procédés innovants pour l'obtention de phycocolloïdes

Alginates are polysaccharides which are naturally present in the cell wall of brown algae and extracted because of their rheological properties. The alginate industry uses an extraction protocol, which central step, the carbonatation, is conducted in batch reactors. It is a high water, reagent and time demanding process. The main objective of this thesis is to optimize the alginate extraction process. A study of the existing process, focused on carbonatation step, enabled to highlight points which could be enhanced to optimize the extraction yield and the quality of extracted alginates. Stirring intensity and algae pieces size have been identified as parameters having an influence on the carbonatation reaction kinetics. A specific analysis on medullar and cortical tissues of Laminaria digitata highlighted specific extraction kinetics for each cell zone. These elements have been taken into account while building a model for carbonatation reaction kinetics. The opportunity of conducting the carbonatation step thanks to reactive extrusion process has been investigated. An experimental study enabled to optimize the process and to highlight its advantages compared to the existing batch process : continuous process, gain in time and in extraction yield, enhancement of rheological properties. Extracted alginates quality (purity, whiteness and rheological properties) was constant whatever extrusion conditions were. Only extraction yield showed a non linear dependence with regards to some process parameters : reagent concentration and ratio of algae and reactive solution feed rates. A carbonatation by reactive extrusion model for this process has been proposed. It combines a residence time distribution model, taking into account process and geometrical parameters, and the carbonatation reaction kinetics model.