Modeling and evaluation of evaporator cleaning

Paper in proceeding, 2014

To sustain an efficient operation of the black liquor evaporation plant, extensive cleaning is often needed to remove water soluble scales that are rapidly formed on the heat transfer surfaces in the part of the plant with the highest concentrations. The soluble scales consist of sodium carbonate – sodium sulfate crystals and are normally formed at concentrations above ca 50% dry solids content (DS). To have an efficient cleaning, the cleaning time should be sufficient to dissolve all scales, and, at the same time, be as short as possible to minimize disturbances and down time. Today, however, very little fundamental knowledge exists about how to efficiently remove scales and the cleaning process is based on trial and error. In this paper, cleaning of industrial falling film evaporators was evaluated using a modelling approach. The cleaning performance using black liquor as wash liquid was evaluated for the whole range of DS, from 0% (i.e. condensate) to 45%. It was found that the feed rate of wash liquid and evaporation rate were the two most important parameters for the cleaning time, while sump volume and recirculation flow rate were less important. The limiting process for the cleaning was mainly the discharging of salt from the evaporator unit and not the dissolution itself.