University of Tasmania
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A mathematical model for the simulation of paper drying energy consumption

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posted on 2023-05-27, 07:19 authored by Reardon, SA
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.


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