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WORLD PULPPAPER 43 ABBs APC solution maintains the chip level by optimally manipulating the digester bottom scraper speed and the pulp flow out of the digester length of time the chips are exposed to a given concentration of effective alkali and to keep the concentration of alkali the same throughout the digester vessel. The APC is also able to maintain a consistent production rate of pulp. Finally a log of the collected data is packaged into concise reports complete with measurements of key performance indicators. Another important variable in the continuous cooking process is the level of chips inside the digester. Variations in this chip level leads to non-uniform cooking disturbances in the overall liquor balance of the process and non- uniform pulp flow at the digester outlet. ABBs APC solution maintains the chip level by optimally manipulating the digester bottom scraper speed and the pulp flow or blow flow out of the digester. While the APC is handling these complex measurements making real- time predictions about the process and implementing optimal control actions the ABB Extended Automation System 800xA distributed control system DCS handles the basic controls such as liquor and chip flows temperature pressure etc. It also performs the vital job of controlling the H-factor that is the rate at which the lignin is being dissolved. As this is largely a function of temperature the amount of heat applied to the digester has to be closely controlled. A variance from the optimal of just a couple of degrees can make a big difference to the quality of the pulp. The obvious advantage of the APC and the DCS is that the outcome meets all of the customers requirements for this step of the pulping process. The fact that just the right amount of steam has is difficult to control for two reasons. Firstly because all the chips that are fed in have different moisture content and physical characteristics. Secondly the Kappa number cannot be physically measured in the digester but only after the chips have passed through it and entered the blow line. This is a problem because it is essential to know what the Kappa number is before this point. The pulp has to stay in the oven for just the right amount of time long enough to yield as much cellulose as possible but not so long as to break down its physical structure. Therefore to maintain a steady process with minimal variations in the quality of pulp the Kappa number has to be arrived at by taking continuous measurements of the various process variables before the chips enter the digester and feeding these numbers into a mathematical model or soft sensor that considers the multiple nonlinear process effects. As shown in Figure 1 this is what ABBs solution does. The advanced process control scheme employs a soft sensor based on ABBs Inferential Modelling Platform that yields soft measurements of the Kappa number from a series of process-variable measurements. These measurements are tracked using a tracking function that creates a virtual model of the chips on their journey through the digester. Along the way measurements are derived from the chips characteristics and these are fed into the model to predict the Kappa number in every zone of the digester. The soft sensor is deployed online and yields real-time virtual measurements of quality variables such as the Kappa number that are then used by a model predictive controller to optimise the cooking process to decide the Figure 2. Control overview washing APC