Influence of mass transfer and acid prehydrolysis on the process of flax shives catalytic oxidation into vanillin and pulp

The influence of mass transfer intensity on kinetics of catalytic oxidation of flax shives with oxygen to aromatic aldehydes and pulp was studied. The process was carried out in an autoclave in an aqueous-alkaline medium in the presence of CuO as the catalyst. Oxidation of flax shives into vanillin, syringaldehyde, and pulp was shown to proceed as a completely diffusion-controlled process under the studied conditions. Depending on the process conditions, it can be limited by stages of oxygen transfer through the gas–liquid interface as well as by oxygen transfer through the bulk of a Newtonian liquid phase and non-Newtonian suspension of the reaction mass. The results obtained on the influence of stirring speed and volume of the reaction mass on the rates of oxygen consumption and vanillin accumulation were described by a simple well-known model connecting the intensity of mass transfer and the stirring power density in the bulk of the liquid phase. It was shown that, despite the distinctly diffusion-controlled conditions of flax shives oxidation with oxygen, the vanillin yield was very close (70–83%) to the theoretical maximum of vanillin yield in the processes of lignin oxidation.