Through drying of paper from mechanical and chemical pulp blends: Transport phenomena behavior

Printing and heavier grades of paper, often made from pulp blends, are dried by a mature process, cylinder drying, which has a much lower drying rate than through air drying. Analysis of the possible extension of through drying to such semipermeable sheets requires knowledge of the basic characteristics of this process when applied to such grades. Both momentum transport and drying rate aspects of transport phenomena in through air drying were investigated for sheets made from mechanical pulp - chemical pulp blends. The first determination of air permeability for moist paper from TMP and blends of TMP and kraft pulp is reported. The porosity, bulk and specific surface of paper from such blends are also reported. For paper from such blends the mechanical pulp exerts a disproportionate influence through control of the macrostructure by the stiff mechanical pulp fibres and control of the microstructure by its high fines content. Changes in sheet structure affect the momentum transport sensitively but the heat and mass transfer behavior determining drying rate is much less affected. Progress was made on the fundamental Re-f-d(p) basis of analysis of through drying. The highly nonlinear dependence of momentum transport properties on pulp blend composition and on moisture content was related to the corresponding changes in the macro- and micro-pore structure available for air through now.