Background

AssiDomän Frövi produces 350.000 tons of board per year on one 7 m wide and 250 m long board machine (see figure). The board is composed of four plies formed in separated headboxes. Most of the used pulp is bleached and unbleached sulphate pulp produced within the plant. The bleached pulp is used on the topside for a bright and white surface.

The process section are:

1. Wire section. The pulp is produced in the sulphanate plant and stored in intermediate silos, whose capacity corresponds to several days' production. From the silos, the pulp is fed into the pulp preparation where it is refined and mixed in two tanks for each ply. From the headboxes the pulp is fed onto the wires, one for each ply, where the first dewatering takes place. In the headboxes, the pulp concentration is about 0.3% and after the wire is about 20%.
2. Wet pressing In order to form the basic paper board, the four plies are pressed together. By doing so, the water content is reduced to 60%.
3. Drying To remove more water, the board is dried in several stages. The drying takes places by passing the board over a large number of drying cylinders, whose temperature is 110-130 degrees. The moisture content is now about 8%.
4. Calendering To achieve a flat smooth surface, the board is pressed together by two hot calenders.
5. Coating After the calendering, the topside of the board is covered with a white coating, containing mainly of pigment and a binder. This gives the board a fine printing surface.
6. Final finish Here the board is given its final finish by light calendering and after-drying
7. Reeling At the end of the machine, the board is reeled on large tambours, each weighting up to 50 tonnes. Finally, the produced board is divided into smaller rolls, or cut into sheets, depending on customer demands.

The process equipment is controlled by local regulators within an integrated digital control system (DCS). There are some 600 local controllers involved in the board machine systems. The primary physical variables , basis weight and moisture, are measured on-line by traversing sensors and controlled by a dedicated computer. All available process information from on-line measurements and laboratory tests are stored in a database within an advanced mill wide information system called Info. However, most of these on-line measured variables are irrelevant to the customers. Their main concern is the quality whose specification includes nominal and extreme values for variables such as bending stiffness, resistance against delamination, printability, curl and twist, etc. On-line sensors for these variables are not available and the operators rely on the laboratory tests for their control actions. The continuously moving web in the paper machine is wound up at the end of the machine on a tambour. After approximately 55 minutes the operator initiates an automatic change of tambours. Samples for laboratory testing are only available from last few meters of board of each tambour. Some 20 quality variables are analyzed in the lab at different positions in the cross-machine directions. Then, the operator has to compare laboratory values to the nominal values and deviation limits in the product specifications and take the opportune decisions. Besides, he has to consider the dynamic properties of the process. The effect of some control actions is almost immediate, others have an effect within 10 minutes, and still others within an hour. There are also inherent time constants of about 2 and 16 hours, via the short and long water circulations around the machine, respectively. Moreover, the influence of a given control action is not constant over time, nor linear. The process behaviour is different for different grades, and production levels, and wood raw material. A given control action may improve one quality variable, and have an adverse affect on another. Hence, the control problem is very difficult.

In such a complex industrial process, simulation tools are extremely useful since they can contribute to higher product quality and production efficiency in several ways:

• Validation of proposed process modification. Today many modifications in the paper industry are giving unexpected sideresults that often lead to secondary and even tertiary modifications. This of course means production disturbance and losses as well as quality upsets. If the modification could be tested in advance in a simulator, much of this trial and erro could be eliminated.
• Testing of control strategies. A rebuild of the process element or its control structure often requires a rather long time of control programming and tuning during production, causing production loss or quality upsets. A dynamic simulator of the process and its controls would allow for a troughout study of different control strategies, and efficient and well tuned controllers directly from the start of the new equipment.
• Process understanding. A board machine is a very complex system. Some 20 quality variables must be kept within customer specified limits using 100 process settings. A simulator is one way of maintaining the quality knowledge within the organization among process, qualitu and control engineers, thus refining the description of quality that is built into the simulation model. We have particular interest to model the water, fiber and chemical balances of the wet end, as well as the drier and its subprocesses.
• Operator training. The board machine at Frövi is operated by six crews. In practise they behave quite differently, when facing a process or quality problem. Consequentely they are more or less efficient in correcting the problem. There is a huge economical potential to make them act in the same and efficient way. A simulation tool can take a snapshot of the process at a given instance, test the outcome of several different control actions, and learn the best behaviour, would be a more efficient way of training not only the operators but also the production engineers and technicians.