INITIAL DELIGNIFICATION AND CELLULOSE DEGRADATION OF CONVENTIONAL ANDETHANOL-ASSISTED OZONATION

It has been proposed that the initial regime of pulp ozonation is a di ffusion controlled process, due to the low solubility of ozone and its very fast reaction with lignin. However, whether the process is gover ned by diffusion on a fibre wall scale (macro diffusion control), on a (micro)fibril scale (micro diffusion control), or by reaction with li gnin located on the fibre surface followed by diffusion control is an open question. In order to address this, a 31 kappa Hemlock kraft pulp impregnated with acidified (pH = 2.0) water or acidified (pH = 2.0) e thanol-water (70% w/w) was ozonated at high consistency in a different ially operated packed bed reactor. The cellulose degradation and pulp delignification rates were determined from the change in pulp viscosit y and kappa number with ozonation time. Transmission electron microsco py (TEM) was used to visualize the lignin distribution of KMnO4 staine d fibre walls. The TEM pictures clearly show reacted lighter regions a t exposed fibre surfaces, which are sharply separated from unreacted d arker areas inside the fibre wall. Therefore, these pictures support t he macro diffusion theory. The macro diffusion theory was used to desc ribe the progress of delignification and cellulose degradation as a sh arp reaction front moving through the fibre wall from the external fib re surface towards the lumen. In agreement with this so called diffusi on controlled shrinking core model, the present experimental results s how that the delignification and cellulose degradation increase propor tionally with the square root of the ozonation time. The effects of oz one partial pressure and ethanol-water impregnation on the delignifica tion and cellulose degradation rates are also described.