Through-Drying of Tissue Paper - Experiments and Modelling

Sammanfattning: In through-drying of paper, the energy required for drying is supplied by hot air, which is forced through the paper structure. The area for heat and mass transfer is the internal surface area of the paper, which is often many times larger than the external web surface area (xy-plane). Therefore, the drying rates achieved in through-drying are often several times higher than in contact drying processes, such as cylinder and so-called Yankee drying, where the area for heat and mass transfer is essentially the web surface area. The through-drying process confers superior quality on the product compared with conventionally dried products. The main traits of a through-dried paper product are the high bulk (low density), high softness and high water absorbency. The drawback of the through-drying process is the relatively high energy demand, as the paper enters the drying section at a higher moisture ratio than in conventional drying. The process also requires a substantial amount of electric energy to operate the fan system. Because of the high pressure drop associated with drying of heavy paper grades, through-drying is used in the paper industry exclusively for the drying of lightweight paper grades (usually below 40 g/m2), such as tissue and towel. Studies of commercial through-drying systems have shown that the specific energy demand (energy/mass of evaporated water) is higher for through-drying than for other paper drying processes. The aim of the work presented in this thesis was to increase our fundamental understanding of the through-drying process, by experimental studies, as well as through mathematical modelling, and to use this knowledge to study the energy utilization of the process.