FRACTIONATION OF POPULUS-TREMULOIDES AT THE PILOT-PLANT SCALE - OPTIMIZATION OF STEAM PRETREATMENT CONDITIONS USING THE STAKE-II TECHNOLOGY

Fractionation of a prototype hardwood, Populus tremuloides, was optimized in a 4 t/h pilot plant STAKE II unit located in Sherbrooke, Quebec. The aim was to find operational zones which maximized the recoveries of hemicelluloses (pentosans), lignin and cellulose. A phenomenological approach, based on the definition of a severity parameter R(o) = t*exp[(T - 100)/14.75] which combines time, t (min), and temperature, T (degrees-C), to express the severity of a given pretreatment, was developed. Performance curves, using the R(o) parameter, have been experimentally determined. These indicated that the maximum recovery of pentosans is 65% of the potential at log10R(o) = 3.8. Under these conditions, lignin recovery by caustic extraction was about 80% of total lignin. Cellulose was completely recovered at this severity but needed to be bleached to achieve its natural coloration. At severities both greater and smaller than log10R(o) = 3.8 pentosans recovery decreased. The cellulose derived from the steam fractionation process was rapidly depolymerized as severity increased, whilst its enzymatic digestibility was close to theoretical at severities beyond log10R(o) = 3.2. The results obtained suggest that steam explosion processes are fundamentally hydrolytic in nature.