Hemicellulose extraction from aspen chips prior to kraft pulping utilizing kraft white liquor

Abstract

Pre-extraction of hemicelluloses before commercial pulping processes such as kraft pulping has been considered as one potential approach to generate fermentable sugars as feedstock for biofuels and bioproducts. In this study kraft white liquor with 18% active alkali and 25% sulfidity was used to partially extract xylan from commercial aspen chips. Reaction temperatures of 70–90 °C and reaction times of 1–4 h were explored. Amount of xylan extracted increased with increasing time and temperature. The extracted xylan was recovered using ultrafiltration. We were able to recover xylan at levels of up to 48 g kg−1 of dry original chips. Pre-extracted chips were used for subsequent kraft pulping. The ultrafiltration permeate was utilized as cooking liquor without any additional processing and without additional chemical addition. Due to the pre-extraction the lignin removal in subsequent kraft processes was significantly faster; offering the option of reducing cooking time and increasing mill throughput. Pulp yield remained in the same range as the control. Physical properties of pulps made from pre-extracted aspen chips showed similar strength properties as the control runs if compared at given lignin content. Pre-extraction of aspen chips using existing kraft white liquor appears to be a viable approach in an attempt to produce a side stream of fermentable sugars in an existing pulp mill.