Computational Fluid Dynamics in research into sustainable paper production methods

The Modellfabrik Papier (FOMOP) research cluster is investigating how industrial paper production can be made more climate-friendly and energy-efficient. Our research teams are using methods of computational fluid dynamics (CFD) for this purpose. So far, this method is not yet widely used in basic research into paper production, though it offers a number of advantages.

The laws of fluid mechanics are omnipresent in our world. Analyzing and predicting them is quite complex. “If you want to describe how air flows through a straight piece of pipe, the flow process in such simple geometries can still be calculated relatively easily on paper with a pencil and a calculator,” says Steffen Flaischlen, group leader in the FOMOP research project. But this simple method reaches its limits at the slightest sign of curvature. When it comes to more complex situations involving additional parameters such as temperature or pressure, researchers, process engineers and mechanical engineers rely on computational fluid dynamics (CFD).

CFD analyzes flow patterns and heat and mass transfer and provides insights into their effects on a system or product. CFD uses calculations to model and visualize various flow properties, such as temperature, pressure, velocity, and density, without the need for actual measurements in real operating environments. What is used in aerodynamics, heat transfer or even biomedicine is a helpful research tool in the research and development of innovative solutions for sustainable paper production. It sheds light on the physical processes inside a paper machine that are difficult to see from the outside and can only be observed with great effort or merely locally in the course of experiments, if at all. For example, in the dry section.

The challenge of the dryer section

This is where the majority of the energy required for industrial paper and cardboard production is used. In the dryer section, the remaining water that could not be removed mechanically in the upstream press section is thermally extracted from the paper web. Using heat and dry air, the water that is bound between and in the cellulose fibers is evaporated over several drying cylinders. A good 60 percent of the total energy consumption occurs during the drying process to obtain the final dry content of paper.

CFD as a research tool

One approach of the FOMOP research is to minimize this high energy demand in the dryer section in the conventional manufacturing process. “To do this, we first need to understand the complex flow process and the behavior of the water vapor-air mixture in the dryer hood of a paper machine. The computer-aided numerical method helps us to describe the phenomena and to calculate the flow behavior in the pockets and in the environment of drying cylinders,” explains research group leader Johannes Lunewski.

Making flow processes visible

This analysis helps to define the initial situation. “We first create a geometric model of the area to be described, which is simulated in a second step.” The flow simulation process requires thorough planning to ensure that model, grid size and numerical settings are selected correctly in order to obtain reliable and meaningful results.

Better process understanding

Advanced CFD simulations allow for the calculation of multiphase flow processes. For example, when chemical reactions take place in the air flow – as in the dryer section of a paper machine – or when the air flow also contains water or water vapor. This significantly increases the complexity of a numerical calculation. But the advantage pays off: the insight gained into the processes in the dryer section provides an improved understanding of the process and helps to find possible new approaches to energy saving more quickly and reliably. “This way, we can explore optimization measures without interfering with ongoing production and without investing a lot of time and money in physical experiments,” explains Lunewski.

CFD for realistic research

The simulation makes it possible to virtually test different variants and individual measures such as structural changes or changes to input parameters like temperature and moisture content. In this way, different optimization approaches in the dryer section are theoretically tested and their effects on the operating behavior of a dryer hood are evaluated on different size scales.

“CFD identifies optimization potential without interfering with ongoing operations, indicates savings potential and enables the evaluation of individual measures virtually, initially with minimal experimental effort.”

Paper production without water?

A radically different approach being pursued by FOMOP research involves the idea of eliminating the use of water in paper production. The function of water as a means of transporting fibers must be replaced by another medium – for example, by air. This concept is still pioneering. “In order to understand how paper fibers behave in an air flow instead of a water flow, we rely on numerical flow simulation. This helps us to better understand the underlying fluid mechanical effects,” says research group leader Steffen Flaischlen. Another advantage: ”With the help of CFD, we can predict how the fibers used will behave in an air flow, taking into account various physical properties and conditions.”

The challenge here concerns the size of the fibers. “We therefore have to create simplifications that allow us to correctly represent the behavior of the very small fibers in the right size ratio to a large paper machine and obtain reliable results,” explains Flaischlen. This knowledge is needed to digitally explore the possibilities of dry paper laying and to support the development of new, disruptive solutions for dry paper laying.

Accelerating technology development

CFD has become a standard optimization and design tool in many industrial sectors. The method is also increasingly being used in the process industry. The methods of computational fluid dynamics offer a whole range of advantages for research into climate-neutral paper production. This is particularly true where insights into the highly automated, multi-stage manufacturing process in many areas of a paper machine are difficult. “Where measurement data cannot be taken, or only locally at certain points, CFD helps to provide more precise insights,” says Flaischlen. “We can even describe and visualize phenomena that cannot be measured in real paper machines, or only with considerable effort.”

While CFD cannot replace experimental research, it does complement the scientific approach with a numerical methodology that provides highly reliable information at an early research stage and creates an additional level of knowledge when preparing experiments or test stands.