Indunstning av CTMP-avlut - en studie av inkrustering och värmeöverföring vid fallfilmsindunstning

Stora Enso Skoghall bleached kraft pulp and CTMP. CTMP is an abbreviation for Chemo Thermo Mechanical Pulp. Stora Enso has invested in a new black liquor evaporation plant at Skoghall mill. Two evaporation effects for CTMP-effluent are integrated with the black liquor evaporation plant. Until now all of the CTMP-effluent was released to recipient after sedimentation and treatment in an aerated lagoon. Skoghall plans to evaporate one third of the total CTMP-effluent in order to decrease their pollution of oxygen demanding substances etc. Evaporation is a new method for treatment of CTMP-effluent and is only implemented at a few mills nowadays. These mills have experienced problems with scaling resulting in a low availability. Scales from CTMP-evaporation differs from scales formed during black liquor evaporation being more organic. They are mainly composed of extractives (resin) and fragments of fibers. The purpose of this thesis was to find operating conditions and, if necessary, find new solutions to obtain a high availability for the CTMP-evaporation at Skoghall. In the study, evaporation experiments were carried out in a new pilot evaporator. Two cases were compared, one with untreated CTMP-effluent and one with CTMP-effluent by adding white and green liquor. Previous studies have shown that alkalinisation decreases the scaling tendency. An important part of the experiments was to find washing procedures enabling scale removing. The results are based on measurement data, liquor and scale analyses and observations during evaporation and washing. In a comparison between the two cases the alkalinised CTMP-effluent formed less scales. Scales from the alkalinized experiment where dark, hard and brittle and consisted mainly of calcium carbonate. The scales from the untreated experiment where slightly yellow, soft and sticky and consisted mainly of resin. After the experiment with alkalinised CTMP-effluent, acid washing was performed with satisfying results. Washing with dispersing agent, after the untreated experiment, showed an excellent result even though the amount of scales where larger. Dispersing agents are normally used as additives in pulp mills to prevent resin scales. Adding dispersing agent continuously to untreated CTMP-effluent, before evaporation, is probably a better pre-treatment t hen alkalinisation in order to prevent scales