Behaviour of extractives in a bark steam dryer

The bark extractives from steam drying of bark residues at a pulp mill were studied in order to evaluate their role in the formation of deposits in the dryer. The lipid compositions of bark, condensate and deposit samples taken from a back-pressure steam dryer plant were analyzed by gas chromatography and mass spectrometry. The main lipid groups released from bark and found in the condensate were free fatty acids and resin acids, dominated by unsaturated C18 acids and dehydroabietic acid respectively. The release of these acid groups was 0.3% of the dry bark feed, corresponding to 50 wt% of the free fatty acid and resin acid content of the bark feed. It was assumed that these acids contribute by their tackiness to the formation of deposits at various sites of the dryer. The deposit samples taken from the dryer contained the same lipid compound groups as the bark raw material. However, their extract and lipid contents varied considerably, 2.9–40% and 0.5–18% on a dry ash-free basis, respectively, depending on their site in the dryer. The lipid composition of the deposit on a recycling fan was close to that of the condensate containing large amounts of fatty acids and resin acids. The deposit on heat exchanger tubes consisted of bark dust containing degraded bark suberin, and of fatty acids and resin acids carried with the steam.