Mechanical dewatering and drying processes are an integral part of papermaking. Mechanical dewatering, i.e. suction dewatering or pressing, has low specific energy consumption, which is why it is preferred compared to drying. Higher dry content after the press means less steam consumption and thus less energy consumption in the drying section. The dewatering also has a direct effect on the properties of the final paper. Surface roughness, density gradients and out-of-plane deformation (e.g. cockling) are just a few examples of important paper properties that are influenced by the dewatering.
Key issues for the Cluster are the avoidance of rewetting, i.e. reabsorption of water from the forming fabric and the press felt, and the interplay between press felt and wet web in the press section. A particular focus is put on the reduction of dewatering non-uniformity, which could negatively influence the properties of the final paper. Non-uniformities are analyzed using high-speed infrared thermography and a newly developed on-line surface roughness measurement equipment.
The Cluster has access to advanced laboratory equipment to simulate various aspects of the dewatering process. Apart from lab-scale trials, the Cluster demonstrate new concepts on the FEX pilot paper machine and assist in implementing the results at the participating companies.
Goal
The overall goal is to improve the dewatering in the wire and press section of a paper machine with respect to energy consumption and paper properties. Dewatering will be made more efficient by a substantial reduction in rewetting, for example through the application of novel process concepts, or improved forming fabrics and press felts. Furthermore, a focus is put on the reduction of the non-uniformity in dewatering as this may result in a substantial, but unnecessary overdrying of some parts of the paper web, causing also non-uniform paper properties.
Challenge
To investigate drying non-uniformities at commercial machine speeds is difficult. Available QCS systems have limited MD and CD resolution, which limits their usefulness to study time-dependent and small-scale variations in dryness. To estimate in-plane moisture variations with sufficient spatial resolution, the Cluster uses a novel approach based on high-speed infrared thermography. By linking the temperature information to the local moisture content, new insights in the dynamics of drying can be gained. The fundamental work within this area is carried out by a research student who is fully funded by the Swedish Energy Agency.
It is well known that the surface roughness of a paper or board product is influenced both in the press- and the drying section. It is however difficult to measure exactly where in the process the roughness occurs. To be able to get access to the paper surface and follow the generation of surface roughness, the Cluster is developing an online version of the OptiTopo.
Value
The Cluster strives towards more uniform press felts, as press felt non-uniformity is considered to be the main origin of the variation in dewatering. An improvement in dryness in general, and a more uniform dewatering in particular, will lead to more efficient production processes. A 2% increase in dryness after the press section, which is a realistic goal, will reduce the drying energy consumption by approx. 8%, which is a huge improvement by industrial standards.
Improvements in dewatering uniformity are also likely to reduce variations in paper properties that give rise to unwanted phenomena such as cockle, waviness and two-sidedness. A more uniform dewatering will consequently also lead to more uniform paper properties.
A significant part of the research is carried out as “case studies” together with the participating companies. The methodology based on case studies was chosen in order to guarantee a fast and efficient transfer of the results to mill level.
For whom
The main target group for this Cluster is paper-, board- and pulp manufacturers as well as suppliers of machinery, wires and felts.