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 = t∗exp[T − 100)/14·75] which combines time, t (min), and temperature, T (°C), to express the severity of a given pretreatment, was developed. Performance curves, using the R0 parameter, have been experimentally determined. These indicated that the maximum recovery of pentosans is 65% of the potential at log10Ro = 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 log10Ro = 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 log10Ro = 3·2. The results obtained suggest that steam explosion processes are fundamentally hydrolytic in nature.