Publication: Pulp & Paper International
Pulp makers tailor their production to market demands
Pulp mills have been hit hard by the recession and by environmental pressures. One answer is to use better controls to customize production.By Torbjorn Brattberg and Torbjorn Herngren
One of the main challenges facing the pulp and paper industry in recent years has been the increasingly fierce competition in a market where there has been significant overcapacity. Environmental issues, too, have been playing a role of growing importance. To meet this challenge, the industry has been focusing on improved productivity and higher quality.
Customers in the pulp market are increasingly demanding specific grades of pulp in small batches that have to comply with very narrow quality limits. There is also a growing demand for elemental-chlorine-free (ECF) and totally-chlorine-free (TCF) pulps.
The pulp suppliers have responded in several ways. New technologies have been introduced in the field of delignification and bleaching as well as in the field of quality control and management.
Pulp mills are also cooperating more closely with their customers to improve their competitiveness. Instead of simply producing pulp and then selling it, the modern pulp mill has become customer-oriented, producing customized pulp. This could be as simple as special labeling, pulp having a high tear resistance due to a special recipe, the supply of TCF pulp, or even the use of raw materials from forests ecologically managed to ensure sustainable development.
One tool that helps to achieve this goal of customized pulp is the Pulp Quality Footprint, or PQF, system developed by ABB. PQF is a portfolio of software products that provides the quality control needed by a modern pulp mill. It tracks the pulp from the woodyard right through the mill to the customer. All quality information is displayed on-line in the control room. It also provides the final product documentation to enhance after-sales customer support.
PQF at Vallvik
PQF is being used in three Swedish pulp mills. One of these is the Vallvik mill, a producer of ECF and TCF kraft pulp, and now part of the AssiDomon Group.
Vallvik has been producing Robur pulp since 1907 and Robur Flash pulp since 1974. The mill is situated on the river Ljusnan in central Sweden. A major upgrading project was carried out between 1989 and 1992. The work included the adoption of high environmental standards and technology for producing TCF pulp, as well as a rationalization program. Among specialists, the mill is famous for its single control room.
Recently a successful mill trial using peracetic acid in TCF bleaching was carried out. Results look very good, with brightness levels of more than 88 % ISO achieved.
As part of the upgrading project covering the digester, delignification, bleaching and recovery operations, the Vallvik mill installed a PQF system, which has been in service since 1992. It is used to track 50 important variables such as the lignin content, brightness, dryness, viscosity and basis weight.
Several factors lay behind the installation, namely the need to improve mill performance, the increasing demand for customized pulp, a greater focus on quality and the growing competition in the pulp market.
One consequence of the introduction of PQF was that Vallvik could meet customers' demands for compliance with the ISO 9000 standard for product identification and traceability. Further, it was possible to improve the pulp quality as a result of the tracking of the pulp through the fiber line. PQF also constitutes a valuable tool in Vallvik's product and process development.
Communications a key factor
An important feature of the PQF at Vallvik is that it forms part of the information technology aimed at improving communications.
At Vallvik, as in other mills, the customer's paper quality is largely determined by the variables of pulp production, such as the proportion of sawmill chips used, kappa number and washing efficiency.
Vallvik must validate the impact of these variables on a specific paper grade and paper machine. Communications are the key to success here; communications between the technical staff of the pulp mill and those in the paper mills. Huge amounts of data are accumulated in pulp and paper production, and these must be brought together in the same computer. This will give the technical staff and others a very efficient customizing tool. It is also possible to integrate customer specifications into the same database used by the PQF.
Vallvik believes that in the near future the PQF database will also be integrated into sales management systems, to enable better selling. One consequence of this marriage of databases would be closer relations between sales staff and purchasers, who would support closer ties. Similarly, closer relations would be established between the technical staff of the pulp mill and its customers.
Vallvik believes that these objectives will be met in the coming years and that the continued use of PQF will play an important role in their realization.
The technology of PQF
In simple terms the PQF system records the production history of a specific unit or "slice" of pulp, as it makes its way along the fiber line, and stores it in a database. This history starts with the wood and chips used for this specific slice and usually concludes with the weight of the pulp bales included in the slice. Its timescale ranges from 24 to 48 hours, and corresponds to the period from the feeding of chips into the digester to the baling of the dried pulp or its usage in an associated paper mill.
Figure 1 illustrates the basic principles of the PQF. As a specific slice of pulp proceeds through the fiber line, data are collected and labeled for this slice. For example, the proportion of sawmill chips prior to the feeding into the digester is stored in the database. When the same chips have come half-way through the digester, the residual alkali content is recorded. When the chips leave the digester as pulp, the kappa number corresponding to the lignin content is recorded. More data are recorded as the slice passes through the screen room, and the oxygen delignification and bleaching processes.
The final data stored in the database could be the weight of the slice. In addition, as the slice passes a laboratory sampling point, it is time-stamped. As soon as the test results become available, they are stored in the database for the slice with the corresponding time stamp. A built-in process model of the pulp mill takes into account any changes that are made in the production rates and levels in buffer tanks. The pulp is usually divided into 150-200 slices, with each slice corresponding to 2-10 tons of pulp, or 5-10 minutes of production.
Besides the Pulp Tracking function, the PQF includes a Pulp Laboratory function, which allows a precise manual input of the results of pulp tests to a database for current quality values. This database feeds information to the Pulp Tracking database. The Pulp Laboratory function can be separate from or integrated with the Pulp Tracking function.
Grade and variable track records
The PQF has a grade tracking display which allows operators, the quality department and management to see what is taking place in the various production units of the fiber line, including the digester, oxygen delignification and bleaching units.
The staff can study aspects such as the production swings between pulp grades, and the twilight zone between two pulp grades, and take corrective action.
There is a variable tracking display which is primarily intended to show how close a parameter is to its set value. It is particularly valuable during the development of new pulp grades and trial runs.
From the PQF's Mill Profile display, which normally covers the complete fiber line, the user can locate pulp lying close to the specification limits. Up to four quality profiles in any combination can be studied at the same time. The display presents the current specification limits along with the mill profile. It is therefore a valuable tool that helps to provide better overall control of the production and to classify the pulp.
ABB's Advant Stations are normally used to run the mill model and the database and as operator interface with the PQF software. The Advant Station complies with modern standards and can thus be connected to other computers and control systems - apart from being integrated into the distributed control system of ABB. Its ergonomically designed displays improve overall efficiency and increase operator recognition of important information. A better working environment and reduction of operator fatigue are other important features.
The Advant Information Mangement Station also helps to speed up decision-making and in many installations it forms an integral part of the whole mill network, linking together the control and administrative systems.
Torbjorn Brattberg is manager of technology and quality with AssiDomon's Vallvik mill. Torbjorn Herngren is manager of pulp mill applications with ABB Industrial Systems.
© 1994 Miller Freeman, Inc. All rights reserved.