APPLICATION OF STEAM EXPLOSION TO THE FRACTIONATION AND RAPID VAPOR-PHASE ALKALINE PULPING OF WHEAT-STRAW

The utilization of steam explosion technology for the production of ce llulose pulps was evaluated at a bench scale using wheat straw as raw lignocellulosic material. Steam explosion was used either as a pretrea tment method to achieve the fractionation of the straw into its consti tutive polymers, or as a rapid pulping method for the production of un bleached chemical pulps from alkali-impregnated straw. In the fraction ation process straw was pretreated by steam explosion at temperatures comprised between 205 and 230 degrees C, and a reaction time of 2 min. The exploded fiber was washed three successive times to yield a hemic ellulosic sugars-rich solution. The recovered fiber was delignified by alkali at 160 degrees C for 60 min. The alkali lignin was recovered b y filtration after acidification of the black liquor. The resulting fi ber was screened to separate the fines from the pulp. The latter was b leached and viscose-grade cellulose obtained. By-products of the proce ss were lignin, and molasses rich in hemicellulose-derived oligomers. The optimization of the process led to the following results at a stea m explosion severity of log(10)(R-0) = 3.80: viscose-grade cellulose p ulp yield = 70% of the potential; lignin recovery = 70% of the Klason lignin present in the original straw; hemicellulose sugars = 55% of th e potential pentosan, recovered as molasses. The production of chemica l pulp from wheal straw was studied using a conventional soda process and a two-stage cooking sequence, consisting of straw impregnation wit h the caustic liquor followed by rapid (less than or equal to 4 min) s team explosion treatment (160-215 circle C) of the impregnated materia l following withdrawal of the excess impregnation liquor. The impregna tion/steam treatment sequence for wheat straw pulping shortens the tot al processing time (impregnation + cooking) to less than 15 min. Unble ached IRSP pulps, with yields of 33-34% (screened and fines removed) s how strength properties comparable both to those of conventional unble ached wheat straw soda pulps and hardwood Kraft pulps, refined to simi lar freeness values (around 300 ml CSF). (C) 1998 Elsevier Science Ltd . All rights reserved.