Mathematical modeling of mechanical dewatering in the press section - Paper Technology International 2023 - Paper Technology International 2023 - Flipbook - Page 85

Mathematical modeling of mechanical dewatering in the press section - Paper Technology International 2023

PAPERTECHNOLOGYINTERNATIONAL
Figure 5: Examples of effective material models describing different aspects of the fluid-fluid, fluid-structure and structure-structure
interaction which are used in the simulation.
To account for the compression of the paper web and the
press fabrics either a viscoelastic-plastic model of the fiber mat
[7] or an empirical model following compression curves from lab
measurement are introduced as effective material models. For
the study of thermal effects such as e.g., the evaporation and
condensation of water inside the pores or temperature dependent
changes of the viscosity of water we also included a thermal model
based on the heat transport in a partially saturated porous media.
Process simulation library
The effective material models described above are then
used as building blocks of the process simulation model describing
the process behavior in the entire press section. They enable
predictions of state variables like dry solid content, hydraulic
pressure or water saturation, while not resolving the microscopic
aspects of flow and compression. We use the Modelica modeling
language and follow a modular concept, starting from the individual
flow and material transport equations that are then combined to
larger units. These units describe, for example, a press fabric layer
or a layer of the paper web and can be combined to any requested
paper – fabric – roll cover sandwich. The different units interact
via connectors exchanging physical information about mechanical,
hydraulic, pneumatic or thermal states and flows as shown in Figure
6. We include process variables as e.g., machine speed, line load
and pressure shape (roll or shoe press) and are thus able to analyze
the dewatering performance of different press sections or compare
press fabrics for a defined press nip setup.
Application
To study the validity of our process model
we developed a simulation setup analogous to a
test setup in our experimental and pilot facilities.
The setup defines a roll press nip with a single felt
with defined initial saturation of the felt, constant
speed and constant line load. Two different press
fabrics were measured on the experimental setup
and simulated in the process simulation by changing
the press fabric parameters, e.g., permeability,
porosity and compressibility. These parameters were
obtained from lab data and small-scale simulations
as described in the sections above. As a measure of
the dewatering performance of the press fabric, the
cumulated water mass flow from the bottom of the
press fabric after the press nip is shown (Figure 7).
The results show that felt A had a significant better
dewatering performance compared to felt B. These results were also
confirmed by the experimental setup where the same trends could
be seen.
Figure 6: Schematic view of the process simulation model of a
double-felted press nip showing the modular approach for the
porous media layers as well as the press boundary conditions.
Figure 7: Results from the process simulation: Pressure profile
and dewatering behavior of two different press fabrics in a roll
press nip.
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