Application of the refractometer in the measurement and monitoring of brown stock washing

Brown stock kraft pulp washing removes soluble impurities from a pulp suspension, recovers cooking chemicals and incinerates organics for energy recovery (steam and electricity generation). The parameters used to describe the performance of washing or its effectiveness fall into two categories: wash loss, which describes the amount of washable compounds in the pulp suspension which could have been removed in washing, and the dilution factor which represents the net amount of water that is added during washing. The performance of brown stock washing has an effect on many sub-processes like evaporation, oxygen delignification, bleaching and waste water treatment. Washing results can have positive or negative effects on these sub-processes. It is important to find the correct optimal operating point for washing, because without effective washing of kraft pulp production is not economically viable. The starting point for this study was to develop a real-time measurement application for brown stock kraft pulp washing by which means it would be possible to measure washable compounds or their sum parameters in the liquid and pulp streams on a laboratory and mill scale. The next step was to study the feasibility of using a real-time refractometer, which measures dissolved dry solids, to measure wash loss and monitor the performance of brown stock washing systematically, and then be able to take control of the process by using the results of enough such measurements. In addition, different washers' operational performance were studied by performing stepwise tests and monitoring the results using refractometers. The results show that refractometer measurement works with enough accuracy under very demanding process conditions to reliably measure liquid and pulp streams' dissolved dry solids. Therefore, reliable wash loss measurement can be undertaken using refractometers, once calibration is carried out for the wood species and the installation's characteristics. By utilising the results from three refractometers' consistency and flow rate measurements, it was possible to create a measurement system able to calculate the effectiveness of a single washer accurately and in real time. These refractometers can also be used to determine a washer's optimal operating point. Adjusting a washer's operation towards more efficient operation succeeded in reducing the amount of wash loss to the oxygen delignification and bleaching stages, increasing the dry solids content to the evaporation stage and thus achieving significant economic savings. As continuation has considered to match the filtrate balances over the washers by using four or at least three real-time measurements. Moreover, utilising the refractometer in bleaching washing and calculating the wash loss effects on the bleaching costs has been considered. The work also involves utilising refractometers for very low concentrations in WWTP.