Variables affecting specific energy requirements for repulping

Abstract

A 0.25 m 3 laboratory repulper was built for the purpose of determining which variables (pulp type, rotor tip speed, pulp suspension temperature, rotor design, rotor to extraction plate clearance, power to vat volume ratio, and pulp suspension consistency) affect the specific energy (kW-h/ton) required for repulping. Scale replicas of 3 commercial repulper rotors were constructed to test the effect of rotor geometry on repulping specific energy. The pulp types used included kraft aspen, C-flute corrugated cardboard, office printing paper, and unbleached paper towel. The energy required for repulping was found to vary with pulp type, temperature, consistency, and rotor design. It was found that a given material at a given temperature and consistency requires a unique quantity of energy to be repulped independent of the rate of energy addition. It was also found that repulper rotors show Reynolds independence at rotor tip speeds greater than 12 m/s. It was found that the specific energy consumption in a 0.25 m 3 laboratory repulper was identical to that for a 15 m 3 repulper given the comparison was made with identical pulp types, temperatures, consistencies, and rotor designs.

Energy and paper recycling: Modelling the time and energy requirements for low consistency batch repulping

“…Although the recovery of waste paper clearly benefits the environment, further environmental benefit could be had by reducing the energy required for repulping as repulping itself is an energy-intensive process. [1] Repulping is accomplished in machines called repulpers. Repulpers consist of a vat with a bottom-or side-mounted rotor and an extraction plate underneath the rotor.…”

“…This leads to the term flake content being used to quantify the amount of remaining waste paper in a repulping process. Flake content is defined by Equation (1).…”

“…Deflaking in terms of specific energy is known to follow the form of Equations (2a, 2b). [1,2] dF dt ¼ ÀkF ð2aÞ…”

“…where t is time, k is a rate constant (s À1 ), E is specific energy (energy/unit mass), l is a rate constant in energy (kg/J), and F is flake content as defined by Equation (1). Given a repulper having a constant rotor speed, and hence constant rotor shaft power, and in batch operation, l can be obtained from k using the following relation:…”

“…[2,3] Saville et al showed that the rate constants k and l in Equation (2) depend upon the repulper feed material toughness; the area under the stress-strain curve. [1] However, a theory-based link between material properties and the time and energy required for repulping is yet to be found.…”