Solid Dissolution into a Vertical Falling Film under Industrial-like Conditions

Abstract

During the evaporation of black liquor, a residual stream in pulp mills, scales form on heat transfer surfaces due to the crystallization of sodium carbonate and sodium sulfate salts. As a result, falling film evaporators need regular cleaning to remove these water-soluble scales, and therefore, knowledge about the dissolution process is important. In this work, dissolution of the aforementioned salts was tested experimentally in a pilot evaporator close to the industrial scale. Dissolution was diffusioncontrolled and could be described by film theory, where the concentration difference between the saturated wall and an assumed perfectly mixed bulk was the driving force of the process. The measured mass transfer coefficient could be predicted to within 30% accuracy using the Chilton−Colburn heat and mass transfer analogy together with a standard heat transfer correlation. ■ INTRODUCTIONHeat and mass transfer between a solid wall and falling liquid film is of interest in many industrial applications. For instance, falling film evaporation is often used to efficiently concentrate different liquids. Other applications of falling film technology are crystallizers and condensers. In this study, the dissolution of scales consisting of double salts of sodium carbonate and sodium sulfate are investigated. These scales are formed by crystallization fouling during the black liquor evaporation process, the most steam-consuming part of a pulp mill. Regular cleaning is needed to maintain operation. Because these salts are easily dissolved in water, the normal procedure for cleaning is to recirculate condensate or weak black liquor. Previous research has mainly focused on how to prevent fouling by optimizing operating conditions rather than on how to efficiently remove scales. 1−3 Cleaning the evaporators' surfaces is also an important part of the operation, but the cleaning process is based on trial and error due to a lack of fundamental knowledge. 4 The aim of this work was to gain an understanding of the process of scale dissolution into a falling film of water and to determine how this dissolution can be modeled under industrial-like conditions. Emphasis was placed on making a simple and robust model that can be implemented in industry. The parameters of interest were the solvent temperature, flow rate, and inlet salt concentration. First, dissolution experiments were conducted on a relatively large scale, and then, a dissolution model was developed and used to fit a mass transport coefficient to the experiments. Another aim of this work was to compare the mass transport coefficient using the Chilton−Colburn analogy with heat transfer.■ THEORY Solid Dissolution. The dissolution process, i.e., the mass transfer from a solid surface to a liquid, can be controlled by both reaction and diffusion. 5 In the case of a salt, the reaction involves breaking bonds in the crystal lattice to generate free ions. When dissolution is diffusion-controlled, the limiting step is the diffusion of the substance from the solid−liquid interface ...

Cleaning of Water-Soluble Scales in Black Liquor Evaporators – A Modeling Approach

“…Salt Dissolution into a Falling Film. Karlsson et al 16 showed that the mechanism for scale removal is a diffusioncontrolled dissolution. Furthermore, the mechanism can be modeled by film theory when using water or salt solution as solvent.…”

“…The diffusion film is significantly thinner than the total thickness of the falling film, which implies turbulent conditions. Karlsson et al 16 demonstrated that the flow regime was sufficiently turbulent for the model to work at relatively low specific mass flow rates (Γ ≈ 0.2 kg/ms).…”

“…The composition of the scales was not investigated. Additional details on the experimental procedure are given in Karlsson et al 16 Dissolution Experiments. In this study, 6 experiments were performed using black liquor at 20% and 41% DS as solvents.…”

“…The investigated range of specific mass flow rates was 0.2 to 0.4 kg/ms. In this case the range was limited by insufficient wetting for the lower flow rates and sputtering at the higher flow rates, effects that were found more sensitive for black liquor than water used by Karlsson et al 16 The wash liquor was preheated to the desired temperature prior to the dissolution experiment and pumped into the evaporator. After it had passed through the evaporator, it was collected in 1 to 5 dm 3 plastic containers.…”

Numerical simulation of the behavior of high‐viscosity fluids falling film flow down the vertical wavy wall

“…For the liquid film attached on the rotating drum surface, as the speed of the rotating drum was increased, the film thickness increased at the film rising side and at the minimum film thickness point . Karlsson reported the film thickness of vertical falling film evaporators for solid dissolution could even reach up to 3.8 mm at the initial stage under industrial‐like conditions.…”