Analysis of fouling rates in black liquor evaporators. A case study of Stora Enso Skutskär pulp mill

Abstract

Abstract Fouling and scaling is a common problem in most black liquor evaporation plants. These scales stick to the heat transfer surface area in the effects, which in turn disturbs the evaporation. In order to maintain the evaporation plant in a good condition and to avoid production losses, efficient removal of these scales are necessary. Optimizing the black liquor evaporation process will reduce the amount of energy needed, which will enhance the production and make it more lucrative and environmentally sound. Stora Enso Skutskär pulp mill has experienced problems with scaling, particularly in effect 3C, where dT often increases rapidly which forces the mill to stop and perform a wash. This thesis aims to examine the evaporation plant at Skutskär. In an analytical way evaluate process data and investigate the performance of the effects with a focus on 3C, 2C, 1C and the black liquor thickener. Additionally, a study to see if best practice guidelines on operating conditions and washing at the mill are being followed to see if any improvements could be made. The process data was collected from the last two years with information about the temperatures of the black liquor and the condensed steam, the DS in and out of the effects and the mass flow in to effect 3C. To see changes in the process data over time and in that way be able to evaluate the washes and how the plant operates in general, a MATLAB script was used. With plots provided by the MATLAB script, an evaluation could be conducted. The washing cycles were identified by the change in BPE, dT and the DS. A wash could be assumed to happen when these parameters went down to close to zero or deviated together simultaneously. The washes were found to be effective for 3C, but not quite as effective for 2C and 1C. On the other hand, the last two were not in need of cleaning whenever a wash cycle was initiated. Most of the scales in effect 3C seemed to be water soluble but there appeared to be some scales that did not dissolve during the wash cycles. These scales are most likely calcium carbonate scaling. The process data was also evaluated in terms of how reliable they seemed, and one notation was made with the DSout from effect 3C. This value could deviate considerably from time to time, implying that one or maybe more sensors are either incorrectly calibrated, or scaling has been built up on those sensor areas. There only appeared 7 wash cycles for the black liquor thickener in total, which through conversations with staff, it emerged that this is not the case. The data from the black liquor thickener appeared to be interpolated and hence has to poor temporal resolution to analyse correctly. Keywords: falling film evaporators, evaporator cleaning, fouling, scaling, crystallization on black liquor evaporators, calcium carbonate, sodium carbonate.