Study of Black Liquor Evaporator Fouling at Södra Cell Värö

Abstract

The growing concern for global climate change has put pressure on industry to reduce its carbon dioxide emissions by reducing energy use. At a pulp mill the single largest energy consumer is the evaporation plant. If there is unwanted accumulation of unwanted substances on the heat transfer surfaces, referred to as fouling, the efficiency decreases. This master’s thesis aims to investigating possible reasons for problems with black liquor fouling in the evaporation plant at Södra Cell Värö. An evaporation plant consists of several evaporation stages called effects and the effects studied in this thesis are effect 1B and effect 2. The method used consists of studying the flow rate and composition of the black liquor in and out of effect 2 and 1B in the evaporation plant. A study was also made to look at process parameters in the process control system relevant to fouling to find correlation with the fouling problems. The most apparent findings are that the fouling in effect 1B is rapid and can be explained by salt precipitation of a mixture of burkeite and dicarbonate. As this precipitation takes place, the soap content in the black liquor has often been high. In effect 2 the fouling is not as rapid as in effect 1B. In effect 2 the wetting degree is low and this could be an explanation for the fouling taking place in the effect. The calcium content of the black liquor is also high but does not seem to be the major contributor to the fouling problems. The results from the process parameter analysis show no clear correlation to the fouling problems.