Separation and recirculation of bulk crystals to potentially mitigate sodium salt scaling in black liquor evaporators
Erik Karlsson, Mathias Gourdon and Lennart vamling, Chalmers University of Technology, Gothenburg, Sweden
Black liquor contains high concentrations of sodium carbonate and sodium sulfate. In black liquor concentrators, these salts begin to crystallize and have the potential to rapidly form scales on the surfaces of heat exchangers. The highest potential for scaling is when a supersaturated liquor begins to nucleate, which occurs when no crystals are available in the bulk. A mitigation strategy used in some mills is to recirculate thick liquor, which increases the bulk crystal concentration. However, a more efficient solution would be to separate bulk crystals and add them selectively to mitigate scaling directly.
This study investigated the separation of bulk crystal from black liquor using a hydrocyclone. Separation was shown to be possible and was proven for a dry solids content of up to 65%, which corresponded to a viscosity of 20 mPa·s at 120°C. The two most important parameters relating to separation were viscosity and flow velocity. The cyclone Reynolds number encompasses both of these parameters and can be used to estimate the degree of separation. An implementation strategy for bulk crystal recirculation has also been developed and discussed. The recirculation of bulk crystals is most beneficial when operating close to critical solids and when reseeding crystals after cleaning.
Black liquor evaporation, Crystallization fouling, Hydrocyclone separation, Scaling, Sodium salts
Nordic Pulp & Paper Research Journal (NPPRJ) is an international scientific magazine covering science and technology for the areas of wood or bio-mass constituents, pulp and paper and including new fiber-based materials, recovery and by-products from pulping processes, bio-refining and energy issues. Articles meeting required scientific standards are accepted from any continent. Open Access: Articles not available through Gold Open Access may be purchased by credit card as a pdf file, to be downloaded within 24 h after ordering, at a price of 22 EUR per article. (Value added tax may apply depending on your location, which you enter at payment logout.) Such articles have Green Open Access, i.e. authors may share articles on their personal non-commerical homepage immediately after publication on the website and download them to non-commercial hosting platforms one year after publication in a full issue of NPPRJ. ISSN: 0283-2631