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:
The next two sections give an indication of how these two goals can
be achieved.
Flow in Head Boxes 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 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
FaxénLaboratoriet (FLA)
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.
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.
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.
Last modified:1999-04-17