Fines mobility and distribution in streaming fibre networks: experimental evidence and numerical modeling

The motion of flocculated fibres in a streaming suspension is governed by the balance of the network strength and hydrodynamic forces. With increasing flow rate through a channel, (1) the network initially occupying all space, (2) is then compressed to the centre, and (3) ultimately dispersed. This classical view neglects fibres-fines: we find that the distribution of these small particles differs in streaming suspensions. While it is known that fibre-fines can escape the fibre network, we find that the distribution of fibre-fines is non-homogenous in the network during compression: fibre-fines can be caged and retarded in the streaming fibre network. Hence, the amount of fibre-fines is reduced outside of a fibre network and enriched at the network’s interface. Aiming on selectively removing fibre-fines from a streaming network by suction, we identify a reduction of the fines removal rate. That documents a hindered mobility of fibre-fines when moving through the network of fibres. Additionally, we found evidence, that the mobility of fibre-fines is dependent on the fibre-fines quality, and is higher for fibrillar fines. Consequently, we suggest that the quality of fibre-fines removed from the suspension can be controlled with the flow regime in the channel. Finally, we present a phenomenological model to compute the length dependent fibre distribution in an arbitary geometry. For a fibre suspension channel flow we are able to predict a length-dependent fibre segregation near the channel’s centre. The erosion of a plug of long fibres was however underestimated by our model. Interestingly, our model with parameters fitted to streaming fibre suspension qualitatively agreed with the motion of micro-fibrillated cellulose. This gives hope that devices for handling flocculated fibre suspensions can be designed in the future with greater confidence.

On the development of a continuous methodology to fractionate microfibriallated cellulose

The focus of this study is the development of a methodology to mechanically separate or fractionate micro-fibrillated fibre suspensions (MFC) into different size classes. We extend the principle outlined by Madani et al. (2010) and create a continuous separation in an annular gap undergoing spiral Poiseuille flow (solid body rotation superimposed on pressure driven flow). Achieving hydrodynamic stability of this flow was the main scientific challenge for scale-up. This work is presented in two different studies. In the first study, we perform a series of batch-wise centrifugation tests to develop the criteria for motion of the individual classes of particles which compose a Eucalyptus MFC suspension. Here, we suspend the MFC in a weak gel and demonstrate a linear reduction in average particle size with increasing centrifugal force; motion is initiated in heavier particles before the lighter ones. In the second study, we use this batch-wise data to design a continuous prototype and we successfully demonstrate a continuous separation with performance similar to that achieved in the batch-wise tests.