Flow Characteristics of Eucalypt and Radiata Pine Suspensions in a Pressure Screen with Smooth 1 MM Holes and 150 Micron Contoured Slot Apertures, 1998 Pulping Conference Proceedings

The flow behavior of eucalypt and radiata pine kraft pulp in a 200mm diameter pressure screen with 1mm smooth holes and 150mm contoured slots for three different rotors is reported. Specifically, reject thickening, fiber passage ratio, accept and reject pressure differential, and specific energy data are presented for a range of feed concentrations between 0.02% and 3.8%, rotor speeds of 1030 and 2060rpm and accept flow rates between 200 and 650 L/min. This corresponds to superficial aperture passing velocities between 0.19 and 2.4m/s.

Reject thickening data are shown to correlate well with the plug flow model of screening for volumetric rejects rates of 0.1 to 0.5, for all three rotors, for both pine and eucalypt, for different accept flow rates, and for different rotor speeds. Using the plug flow model, reject thickening can be described by a single parameter, the fiber passage ratio. Passage ratio is inversely correlated with reject thickening and relates to ease of fiber passage through the apertures. Passage ratio increases (less thickening) with increasing rotor speed, decreasing fiber length, and increasing aperture size. Rotor type, rotor clearance, basket profile and feed concentration also have interactive effects.

Pressure difference data are shown to be affected significantly by the thickening occurring in the screen and the initial fiber concentration. The pressure difference from feed to accepts and feed to rejects generally increases with feed fiber concentration, and both increases and decreases with volumetric rejects rate. Thickening and pressure difference data combine to form screening specific energy. Specific energy decreases first with decreasing rejects rate and then increases due to thickening. Similarly, specific energy first decreases with increasing feed fiber concentration and then increases as the internal screen fiber concentration reaches a critical concentration where the screen permanently blocks.