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Energy and paper recycling : an investigation of repulping Saville, Frank

Abstract

Fibers from waste paper are recovered through repulping. Repulping is accomplished using machines called repulpers. Repulpers are large machines which use a high-speed rotor mounted in a vat to disintegrate waste paper for the recovery of fibers. Repulpers consume a significant amount of energy to recover fibers from waste paper. For the work in this thesis, a 0.25m³ laboratory repulper was built for the purpose of determining which variables affect the specific energy (energy/mass) required for repulping. Scale replicas of 3 commercial repulper rotors were constructed to test the effect of rotor geometry on repulping specific energy. It was found that the flake content as a function of specific energy follows the form dF⁄dE=-λF where F is flake content, E is specific energy, and λ is a rate constant. The rate constant λ varies with pulp type, temperature, consistency, repulper volume, 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. An analytical model for repulping linking pulp material properties, consistency, temperature, and rotor and vat geometry is provided which allows for the accurate prediction of the time and energy required for repulping in both the 0.25m³ laboratory scale repulper and a 15m³ industrial repulper. The model assumes that all work to deflake is done by the repulper rotor in the rotor swept volume by turbulence and that no deflaking occurs in the rest of the vat. CFD simulations of the flow field produced by each rotor and high-speed film of each rotor indicate that the rotors tested in this thesis all produce strong trailing vortices akin to those produced by common mixing impellers like the Rushton turbine. Uniform mixing is important for efficient repulping. Solid body motion of the suspension in the repulper makes for poor repulping energy efficiency. Repulping time and energy savings can be accomplished by increasing the suspension consistency and the rotor swept-volume/vat volume ratio by either increasing rotor size or reducing vat volume all while ensuring complete mixing and circulation in the vat.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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