The dryer section of a paper machine

Abstract

Currently, the paper producing industry is a significant energy consumer and contributes to the excessive greenhouse emissions. In order to become carbon neutral by 2050, it is evident that the paper industry must become more sustainable as well. Most of the energy is consumed in the dewatering process. Therefore, optimizing this process is key for efficient operation and reducing the carbon footprint. Since the late 1950's, there have been several models proposed to describe the dryer section of a paper machine. However, the majority did not capture internal transport phenomena resulting in inadequate models for heavier paper grades. In this work, a comprehensive physical-numerical model is developed that describes the internal transport phenomena in multicylinder paper drying. The model solves a set of two differential equations describing the moisture content and temperature in the thickness direction of the sheet. The model can be solved by imposing time-varying boundary- and initial conditions. The model includes unfelted and double-felted cylinders. Correlations for multiple thermodynamic properties are proposed and evaluated. Heat and mass transfer coefficients at the open surface are determined using the Chilton-Colburn analogy. Furthermore, the sorptive characteristics are fully accounted for. In order to find the most applicable thermodynamic properties, the model is validated against data gathered in a field survey, carried out as part of this project. In this field survey, a paper grade of 203 [g/m^2] running at 403 [m/s] was investigated. Temperature measurements were performed in order to determine the conditions per cylinder. A final moisture content and two temperature profiles were used as test criteria to validate the model. Furthermore, the survey gave direct insight on the state of the dryer section: Cylinder 23 was found to be in the flooded state and the warm-up time was too long. The model was found to slightly overestimate the final moisture content: the calculated final wet-basis moisture content at the end of the pre-dryer section was 0.235 [kg/kg], which is 0.022 [kg/kg] above the measured final moisture content. The calculated temperature profiles closely followed measured values. The effect of changing three operating conditions was evaluated. This involved fixing the relative humidities in the pockets, felting the first 10 cylinders and varying the machine speeds. By fixing the relative humidities in the pockets, the final moisture content increased by almost 20%. However, a 60% reduction in volumetric flow rate over the fans could be achieved. Felting the first 10 cylinders results in a 50% reduction in the warm-up length and a 8.9% decrease in the final moisture content. Lastly, it was found that for an increased machine speed, the final moisture content increases as well, due to reduced heat transfer and drying time. This Master Thesis Project was carried out by the author at the company Kadant Johnson in the Netherlands.