Mechanics of paper pressing

点击翻译按钮获取中文摘要

Papermachines are comprised of three major components: the forming section, the press section, and the dryer section. The wet mat of paper leaving the forming section has a consistency of around 15% (consistency is the ratio of dry weight to wet weight of the paper), and in the press section the consistency is increased to about 50%. The press section dewatering occur by bringing together at high pressure a sandwich comprised of the paper and a dry press felt, which is a flexible porous medium. Under the action of the applied pressure water is squeezed from the paper and becomes trapped in the press felt. Paper pressing is important because water extraction in the press section is orders of magnitude more efficient than in the dryer section. Each kilogram of water extracted through mechanical pressing of wet paper saves about 2 megajoules of energy in the dryer section. Pressing can also affect the mechanical properties of the finished paper, as pressing increases the sheet density and increases the inter-fiber bond strength.In this research we will study two different aspects of paper pressing -- press felt break-in and rewetting. Press felt break-in is the period of a few days after the installation of a new press felt during which the water absorption characteristics of the felt slowly increase to approach their steady state. Rewetting refers to the possibility that water that has been squeezed from the paper into the felt may return to the paper as the applied pressure is reduced. Our research will include both experimental studies of pressing and the development of a mechanistic model of paper pressing.