Spruce Hemicellulose for Chemicals Using Aqueous Extraction: Kinetics, Mass Transfer, and Modeling

Abstract

Pressurized hot water extraction of hemicelluloses from spruce sapwood was studied at 120–170 °C using a batchwise-operated cascade reactor, which enables precise sampling as well as very accurate and rapid temperature control. The extraction was performed under identical conditions for two different chip sizes, a 1.25–2.0-mm sieved fraction and handmade 10-mm cubic blocks, to evaluate the influence of chip size on the overall extraction kinetics. The results showed that the extraction rate increases significantly with temperature and that the pH decreases during the extraction, as a result of the liberation of acid groups. The concentration of hydronium ions in the liquid phase was observed to have a linear correlation with conversion depending, however, on the chip size, which shows that the mass transfer of the acid groups differs significantly from that of the bulky hemicelluloses. It also shows that significant amounts of acetyl groups are liberated inside the chips before the hemicelluloses enter th...