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

Abstract

An analytical model for low-consistency repulping linking pulp material properties, consistency, temperature, and rotor and vat geometry is provided, which allows for accurate prediction of the time and energy required for repulping in both a 0.25 m 3 laboratory-scale repulper and a 15 m 3 industrial-scale repulper. The model assumes that all deflaking work is done by the repulper rotor in the rotor-swept volume by turbulence generated by the rotor and that no deflaking occurs in the rest of the vat. Rotor shaft power is split linearly between the breakup of waste paper and dissipation by turbulence. Comparing the model predictions and experimental data for different pulp types, vat fill levels, and pulp suspension consistencies yields a correlation of R 2 ¼ 0.99 between all experimental results and the model predictions given the condition of fully turbulent (Reynoldsindependent) repulper rotor operation. The efficiency of the laboratory repulper is reduced from that predicted by the model for very low vat fill levels. This is due to a loss of effectiveness of the baffles at these low levels as indicated by solid body motion of the suspension and reduced rotor power number. This indicates that thorough mixing is a requirement to maximize repulping 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.