Fundamentals of Paper Drying – Theory and Application from Industrial Perspective

No manufactured product plays a more significant role in every area of human activity than paper and paper products. Its importance in everyday life is obvious from its use in recording, storage and dissemination of information. Virtually all writing and printing is done on paper. It is the most widely used wrapping and packaging material, and is important for structural applications. The uses and applications for pulp and paper products are virtually limitless. Apart from the products and services that it provides, the paper and pulp industry is one of the major manufacturing industries in the world providing employment for vast number of people and contribute to national economy. The paper making process is essentially a very large dewatering operation where a diluted solution of pulp suspension with less than 0.5% fibre solid is used. The major sections of a paper machine consist of: forming section, press section and dryer section. In the forming section, the fibres present in the diluted pulp and water slurry form paper web through drainage by gravity and applied suction below the forming fabric. In the press section additional water in removed by mechanical pressure applied through the nips of a series of presses or rotating rolls and the wet web is consolidated in this section. Most of the remaining water is evaporated and inter-fibre binding developed as the paper contacts a series of steam heated cylinder in the dryer section. Water removal from the wet web to the final moisture level between 6% and 7% is a critical step of papermaking. Majority of the functional properties of paper are developed in this section. In spite of its key role in papermaking, large equipment size, and large capital and operating costs, drying is arguably the least understood papermaking operation. Books on papermaking technology generally devote fewer pages to drying than other papermaking operations such as forming, pressing or calendaring. A similar situation is found in papermaking courses, in which drying occupies a shorter time than the proportion of space it takes in a paper machine. Furthermore, a large portion of that time is devoted to the description of the equipment by its suppliers rather than to its operation by the papermakers. The dryer section of a paper machine removes between 1.1 and 1.3 kg of water per kg of paper compared to 200 kg and 2.6 kg in the forming and press sections respectively. It is significantly more expensive to remove this water than for any other section of the machine (Reese, 1988). The relative costs of dewatering are: forming section 10%; press section 12% and dryer section 78%. The dryer section is by far the largest consumer of