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PULP MILL OPTIMISATION WORLD PULPPAPER44 development of a specific action plan for implementing the APC system. The implementation phase involved the deployment of the pulp tracking function in the DCS followed by the development of online soft sensors to model the Kappa number. Subsequently step tests were carried out on the process to develop mathematical models for use in the model predictive controller this was then commissioned tuned and handed over to the mill operators. The APCs are currently in the sustain phase in which ABB helps Mondi maintain the optimal performance of the APCs by performing quarterly audits remote monitoring and online retuning. A similar approach was followed for the implementation of the APC in the washing area. THE RESULT This work was carried out in phases between August 2011 and December 2013 by which time the system had been completely installed and tested. During this process the pulp mills engineers were kept fully engaged to make sure they were confident in using the APC and were able to adapt their working and training practices to make best use of it. Once the entire system was up and running it was possible to arrive at some quantitative measures as to how well the objective had been fulfilled Variations in the Kappa number in the digester were reduced by 56 percent meaning that the APC The first step was to audit and analyse the control loops then develop an advanced process control action plan based on model predictive control and soft sensors been added and just the right volume of white liquor or alkali injected means that as much waste as possible has been squeezed out of the process. And as the maximum amount of highest quality cellulose has been produced the quality and quantity criteria have been met as well. THE BROWN STOCK WASHING PROCESS This is the second key area where optimisation can be achieved through an APC system. As with the digester the data is gathered on a continuous basis so that soft sensors can give predictive readings of what is taking place. The aim in this stage of the process is to control the levels in the numerous filtrate tanks so that just the right amounts of fresh water are added for each ton of brown stock to be washed also known as the dilution factor. If an optimal value for this variable can be continuously solved for then the maximum quantity of the cooking chemicals can be recovered and the minimum amount of energy used in the evaporators. The alkali losses will also be reduced. This is achieved by the brown stock washing see Figure 2 which computes the optimal dilution factor at each washing stage based upon pulp conductivity measurements at various locations in the process. At the same time it ensures that the levels in the filtrate tanks are within their specified ranges. As with the digester the information gathered throughout the operation is presented to the control room in the form of key performance indicators. THE IMPLEMENTATION METHODOLOGY The approach followed to achieve Mondis objectives was based on ABBs Advanced Services philosophy diagnose implement and sustain Figure 3. The diagnose phase involved performing a detailed audit of the process and control loops at Mondi Swiecie which led to the Figure 3. APC implementation approach