The role of evaporative dewatering in impulse pressing

• The objective was to link heat transfer to water removal mechanisms, specifically evaporative dewatering, in impulse pressing experiments. Two independent laboratory devices - a platen press and a shoe press - were used to impulse press fiber webs made from softwood bleached kraft (SBK) over a range of basis weights (20-200 g/m2). In the platen press experiments, evaporative dewatering was evaluated by comparing the measured heat input to the amount of heat theoretically required to vaporize the water in excess of unheated wet pressing. Evaporative dewatering was found to be most pronounced for lightweight sheets, and enhanced dewatering can be attributed to this mechanism and other high-temperature mechanisms for basis weights above 60 g/m2. In the shoe press experiments, a composite sheet of five 30-g/m2 webs was used to evaluate profiles for resulting dryness, density, and water retention value (WRV) in the thickness direction. Evaporative effects were confined principally to the layer directly in contact with the heated surface. Application: A better understanding of the water removal mechanisms in impulse pressing process may accelerate the industrial application of impulse technology.

Nyckelord

Dewatering

Drying

Evaporation

High temperature effects

Pressing (forming)

Softwoods

Impulse pressing

Bleached pulp

Bleached Pulps

Drainage

High Temperature Tests

Press Drying

Softwood Pulps

Water Removal

Water Retention

Wet Pressing

Wet Webs

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