Research program in paper manufacturing technology

The paper-making industry is an integral part of the forestry industry which, taken as a whole, provides the largest single surplus of foreign currency to the Swedish balance of trade. Paper-making machines are particularly complicated and require highly qualified labour for their operation. Consequently, the Swedish paper-making industry, for example STORA, SCA and MoDo, actively follow the development of technology for paper-manufacturing machines. Applied research into the technology of paper manufacturing technology, though not part of fundamental fluid mechanics, is being carried out at STFI, the Swedish Pulp and Paper Research Institute, in Stockholm. STFI is financed jointly by the Swedish forestry industry and NUTEK, the Swedish National Board for Industrial and Technical Development.

In a paper-making machine a suspension of cellulose fibres is turned into a wet mat of fibre by driving out most of the water. This part of the process is called forming. Subsequently, the fibre mat is pressed, dried { et cetera}. In traditional forming, most of the water is driven out of the suspension on a moving horizontal wire filtering net, a so called wire. The suspension is transfered to the wire by the means of a thin but broad jet from a head box . The water is then sucked out of the suspension through the wire. However, there are a number of drawbacks to this method. Hydrodynamic instabilities in the interface between the suspension and the air above it limit the speed at which the process can be carried out. Furthermore, one-sided dewatering makes the structure of the surface of the paper different on the two sides of the sheet of paper, which is not very suitable for, for example, printing paper.

These disadvantages can be eliminated to a great extent in modern twin-wire machines. In these the jet from the head box is directed into the space between two almost parallel wires, which are kept close together and at high tension. The pair of wires is then passed over one or more rollers or blades which makes the separation between the stream lines increase due to the centrifugal force. This leads to an increase in the pressure which drives the water out of the suspension. This method is successful in operation but the understanding of its basic mechanics is far from complete. A better understanding will almost certainly lead to considerable improvements in the method. The quality of the final product, measured by homogeneity and the isotropy of the fibres, is principally determined by the flow in the head box and the flow on and between the wires. Two of the primary goals of the paper-making industry are:

  • to decrease the costs for raw materials for a given quality of paper;
  • to increase the quality of the paper for a given suspension of fibres.

The next two sections give an indication of how these two goals can be achieved.

Flow in Head Boxes
The fibre suspension leaves the head box at high speed ( ~ 20 m/s )in the form of a thin ( ~ 10 mm ) but broad ( ~ 10 m )jet. Considerable demands have to be placed on the jet's homogeneity and stability. One result of these demands is that the outlet must be preceded by a contraction of the type that is used in wind tunnels to damp disturbances in the flow. However, the field of strain in the contraction tends to orient the fibres along the streamlines of the jet, which sometimes affects the isotropy of the paper adversely.

High rates of production require high jet velocities, but if the velocity is too high the jet becomes unstable, which leads to defects in the formed paper. The design of head boxes clearly involves several problems of optimization. Despite the fact that head boxes have been in use for a long time, no attempt has been made to solve these optimization problems using the basic methods of fluid mechanics. There can be no doubt that it is possible to make significant improvements.

A recently proposed idea for decreasing the costs for raw materials without lowering the quality, see point one above, is to manufacture printing paper composed of three layers. The middle layer could then be made from cheaper raw material of lower quality. On the other hand, expensive, high-quality raw material has to be used in the outer layers which determine the properties of the paper's surface, such as printability, { et cetera}. A head box for three-layered paper will, however, be complicated. Moreover, it is already known that, under certain circumstances, layered jets are more unstable than homogeneous jets.

Fundamental experimental and theoretical investigations of jets of fibre suspensions will be carried out within the Centre. The damping of turbulence in the contractions in entry boxes will be investigated experimentally and studied theoretically using Large-Eddy Simulation (LES), a method which has been developed in the last ten years or so. This method, which is very effective but rather difficult to handle, is being used to a considerable extent outside of Sweden, but its potential is only being realised to a rather limited extent within Sweden. The combined results of the experimental and theoretical investigations will raise the capability to optimize the design of head boxes. In a longer time perspective, pilot-scale trials will be carried out at S.T.F.I. Full-scale trials at one or more paper-manufacturing plants are being planned.

The fluid mechanics of forming between twin wires
In the fibre suspensions which are used in the manufacture of paper, the fibres are more or less stuck together in tangled lumps or flocs. Fewer or smaller flocs will in general lead to higher quality in the final product. Reduction of the number of flocs in the suspension will improve the quality of the paper without increasing the costs for raw materials, see point two above. When a pair of wires passes over a blade, the change in direction of its motion causes an increased separation between stream lines, as was pointed out in a previous section. As a result the suspension is locally subjected to a field of strain. Flocs with predetermined properties could be broken up, at least in principle, by adjusting the amplitude and geometry of this field of strain. This method is used in modern paper-making machines but, since the phenomenon's fundamental mechanics are largely unknown, its exploitation is based on empiricism.

The capability to effectively optimize the technology of forming in twin-wire machines requires quantitative knowledge of the influence of: the geometry of the blades, the permeability and structure of the wires, the concentration and distribution in size of the fibres, the mechanical properties of the fibres, et cetera. Fundamental studies of these phenomena have already been started at the Department of Mechanics in cooperation with S.T.F.I. These studies need to be expanded both experimentally and theoretically. On a longer time perspective, pilot-scale trials will also be carried out at S.T.F.I. and full-scale trials will be carried out at one or more paper-manufacturing plants.

Plans for future projects in
paper-manufacturing technology
Some examples of pertinent problems, which will be investigated once the Centre has been established for a year or two, are: the fluid mechanics of printing processes, hydrodynamic instabilities in coating processes, and the rheology of fibre suspensions and coating fluids.

FaxénLaboratoriet (FLA)
Last modified:1999-04-17