A mathematical model forthe simulation of paperdrying energy consumption (1994)

A detailed analysis of the dryer section of a paper machine was undertaken
in this study. The mathematical model developed relates initial sheet
conditions, paper sheet properties, drying conditions and dryer section
. configuration to the variation of the moisture content and temperature in
the paper sheet as well as energy consumption per unit production.
The theoretical model was based on mass and energy balance equations
about a slice of paper composing pulp fibres, free and bound water, water
vapour and air. Time-varying boundary conditions, corresponding to the
movement of the paper sheet over the 40-50 drying cylinders, were
assigned and the equations were converted to finite difference form and
solved by computer using standard linear algebra methods.
The heat and mass transfer coefficients which control evaporation rates in
the dryer section were investigated experimentally. Drying trials and
boundary layer analysis were performed to establish the effects of dryer
fabric permeability and tension, air flow, cylinder surface temperature,
pulp furnish and sheet basis weight on paper drying rates.
A number of paper sheet properties were investigated as part of the study.
The pore size distribution, permeability and sorptive behaviour of machine
made newsprint were investigated experimentally and theoretically. Other
paper properties correlated from literature for the purposes of this study
included thermal conductivity, specific heat capacity, density, fibre
saturation point, diffusibility and shrinkage.
A number of simulations were performed on the two Australian Newsprint
Mills paper machines that formed the focus of this study. A comparison
with actual machine moisture content values showed the model's prediction
of moisture change during drying and specific steam consumption under
different operating conditions to be within 2% and 10% respectively.
A series of simulations were performed to demonstrate the effect of the
paper condition on drying, specifically, initial moisture content, sheet
thickness, sheet basis weight and initial temperature. These graphs
provided a useful insight into the effect on achievable machine speed and
specific steam consumption as a function of such variables. An alternative
perspective on the analysis was to specify the machine speed as constant, as often occurs in a production environment, and determine the necessary
change in each of pocket temperature, pocket humidity and cylinder
temperature to maintain this speed. In each case, increases in the drying
duty were handled most energy efficiently by increasing the temperature of
the pocket ventilation air.
Two dryer section configuration changes were also considered. ANM's
PM3 was examined from the perspective of an optimal design. The results
showed the current lengths of individual cylinder wraps and open draws to
be within lOcm of the optimal case from the viewpoint of drying capacity.
The second prospective change examined was that of PM2's conversion to
an initial dryer sub-section with seven foot diameter cylinders, in a bid to
improve heat transfer and hence drying rate and also improve runnability in
this critical wet section. The simulation predicted that this would lead to a
production increase of almost 20 rnlmin, just over 2%. This suggested the
project would not be feasible on the basis of increased drying rate alone
but would need to be co-justified by other factors.