Length-selective separation of cellulose fibres by hydrodynamic fractionation

Abstract

A hydrodynamic fraction device (HDF) was developed based on the principle of spontaneous 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 1mm. 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.