The Effects of Flow Velocity on Crystallization Fouling in Falling Film Black Liquor Evaporators

Paper i proceeding, 2011

In pulp mills, fouling can be a significant problem in the so-called black liquor evaporation plant. This is a multistage evaporation plant that actually works as a crystallizer because, as the black liquor becomes more concentrated, large amounts of sodium salts, particularly sodium carbonate and sodium sulphate, will saturate and precipitate from the solution. The precipitated crystals may form on the heat transfer surface or can adhere to it, or they can remain in the solution. Black liquor is a complex viscous alkaline solution of organic and inorganic components. Also, because the proportions of the constituents in the black liquor vary depending on wood raw material and cooking conditions, it is difficult to predict when and where the sodium salts will precipitate. In this paper, fouling measurements in a 4.5 m falling film research evaporator are presented. The results are mainly based on heat transfer measurements, both local and average, and these are presented in the form of fouling resistances. A number of experiments are presented, aiming at evaluating the effects of the flow velocity on the formation and removal of a crystalline fouling layer on the evaporator surface. The results show that the cleaning effect created by the physical shear of the falling film on the fouling layer is not predominant.