The effect of the physical form and size of raw materials in pressurized hot water extraction of birch

In order to study how the physical form and size of wood raw material affect the xylan dissolution in hot water extraction (HWE), three different birch materials were used. Industrial chips, compressed chips and sawdust were extracted in a 30 L apparatus in batch mode at 160°C up to 5 h. The total extraction dissolved about 30% of all starting materials, slightly more from the sawdust than from the chips. Higher proportion of the native xylan could be extracted from the sawdust (77%) than from the chips (67%). Higher amounts of polymeric material, consisting mainly of xylan, can be produced from birch sawdust than from birch chips. For sawdust, the extraction time of 30 min or less is enough for production of high-molar-mass xylan, whereas extraction time of 1 h is required for the chips. Compression of the chips is beneficial for the release of higher amounts of high-molar-mass material with Mw about 10 kDa. (10 refs.)