Length-selective separation of cellulose fibres by hydrodynamic fractionation

A hydrodynamic fraction device (HDF) was developed based on the principle of sponta- neous fibre network formation and subsequent segregation of fibres and fines in cellulose pulp. Separation is most successful in the so-called “annulus plug flow” regime, which is demonstrated for various combinations of fibre concentration and Reynolds number. In this regime, fibres form a network in the channel center, surrounded by fluid with relative low concentration of fibres and large concentration of fines. As in flow channel separation, wall bounded fluid — containing the fines fraction — is removed from the main flow via side-channels. Long fibres that form a network exit via the main channel. Via an array of experiments we demonstrate precise fractionation of cellulose pulp at a typical cut size of 1 mm. Also, we show that higher Reynolds numbers lead to a dispersion of the fibre network, and consequently a lower sharpness of cut — increasing the fibre concentration leads to a lower yield of shorter fibres. While variation of geometrical parameters did not affect the separation performance, the design of the HDF clearly impacts its capacity