An experimental investigation of single and multiple pass high temperature, high pressure paper drying

Abstract

The primary drying process used in the paper industry is conventional drying which employs heated, rotating cylinders to evaporate moisture from the paper. One process which may improve energy efficiency is the use of higher temperature, higher pressure drying. The present experimental investigation considers the effects of high temperature, high pressure single and multiple pass drying on moisture removal. An apparatus was designed and constructed for the testing of wet paper specimens under different drying conditions. All handsheets were prepared from unbleached pulp with a dry basis weight of 100 g/m (20.5 lb/1000 ft ) were dried at different combinations of drying temperatures (93, 149, 204 'C (200, 300, and 400 'F)), contact pressures (0.7, 1.4, and 2.1 MPa (100, 200, 300 psi)), and contact times (30, 40, and 60 msec). The handsheets were dried with both single and multiple passes (up to six passes through the test facility). The percent of the original moisture content removed was determined from the experimental data and used to compare the moisture removal for different drying conditions. The percent of moisture removed ranged from 6% to almost 60%,depending upon the particular drying conditions and number of passes. The results indicate that increases in drying temperature most dramatically increase the amount of moisture removed, while increases in contact pressure do not significantly increase the amount of moisture removed. The percent of moisture removed increases with contact time, but less than proportionally to the change in contact time. Surface roughness decreases as drying temperature and contact pressure increase. As the number of passes increased, the moisture removed also increased, but in decreasing percentages and did not approach a maximum limit with up to six passes. Alternating the drying side during multiple passes increased the amount of moisture removed. Accurate correlations for single pass and multiple pass drying were also developed. Further study of high temperature, high pressure drying with multiple passes should be pursued to further quantify the effects of variations related to multiple passes. Additionally, other factors such as paper properties, pulp type, felt type, and basis weight should be studied.