Nanocelluloses are seen as the basis of high-performance materials from renewable sources, enabling a bio-based sustainable future. Unsurprisingly, research has initially been focused on the design of new material concepts and less on new and adapted fabrication processes that would allow large-scale industrial production and widespread societal impact. In fact, even the processing routes for making nanocelluloses and the understanding on how the mechanical action fibrillates plant raw materials, albeit chemically or enzymatically pre-treated, are only rudimentary and have not evolved significantly during the past three decades. To address the challenge of designing cellulose comminution processes for a reliable and predictable production of nanocelluloses, we engineered a study setup, referred to as Hyper Inertia Microfluidizer, to observe and quantify phenomena at high speeds and acceleration into microchannels, which is the underlying flow in homogenization. We study two different channel geometries, one with acceleration into a straight channel and one with acceleration into a 90°bend, which resembles the commercial equipment for microfluidization. With the purpose of intensification of the nanocellulose production process, we focused on an efficient first pass fragmentation. Fibers are strained by the extensional flow upon acceleration into the microchannels, leading to buckling deformation and, at a higher velocity, fragmentation. The treatment induces sites of structural damage along and at the end of the fiber, which become a source for nanocellulose. Irrespectively on the treatment channel, these nanocelluloses are fibril-agglomerates, which are further reduced to smaller sizes. In a theoretical analysis, we identify fibril delamination as failure mode from bending by turbulent fluctuations in the flow as a comminution mechanism at the nanocellulose scale. Thus, we argue that intensification of the fibrillation can be achieved by an initial efficient fragmentation of the cellulose in smaller fragments, leading to a larger number of damaged sites for the nanocellulose production. Refinement of these nanocelluloses to fibrils is then achieved by an increase in critical bending events, i.e., decreasing the turbulent length scale and increasing the residence time of fibrils in the turbulent flow.

Understanding separation of poly-disperse particle suspensions according to the particles size is of great importance to product quality. Previous experimental studies of suspension flow through coiled tubes report different results for spherical and elongated particles, e.g., larger and thus heavier elongated particles are faster than smaller ones. We use Euler-Lagrange simulations, as well as experiments, to measure the residence time distribution of fibres with different size in coiled tubes with different curvatures. Fluid flow through the coiled tubes was simulated as toroidal flow, i.e., the pitch of the tube was neglected. Fibres are one-way coupled to the fluid, and their movement in the cross section, as well as their orientation is predicted based on the assumption of an infinitely dilute suspension. We find that in coiled, dilute suspension flow of fibres the ratio of particle settling velocity to the secondary flow speed determines the fibre motion in the tube cross section. For low Reynolds number and thus larger effect of gravitation, fibres are found to concentrate in distinct orbits. Long fibres form flocs propagating through the torus whilst small fibres are well mixed and thus retained in the tube. We found that fibre-fibre interaction and the formation of flocs and not fibre-fluid interaction is key to the size based separation.

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Flotation of cellulose pulp suspension in paper industry is primarily used for separation of ink particles from cellulose fibres. Entrainment, an unwanted phenomena well described in the field of mineral flotation, also leads to a removal of fibres with the flotation froth. We find that the entrainment phenomena can be used for the separation of long fibres from a fibre pulp suspension, and hence for pulp fractionation. Specifically, we use a 2D bubble column to investigate the influence of (i) bubble size, (ii) wash rate and (iii) stirring on the separation of long fibres from cellulose pulp suspension. Separation of fibres from fibre pulp suspension is tested for mechanical pulp and chemical pulp. We find that size selective recovery yields best result for (i) large bubbles, and (ii) additional washing due to the increase of small particle drainage. However, both strategies lead to a reduction of the total recovery rate. Stirring significantly improved the total recovery and benefited the selective separation. Best results are achieved with small bubbles for chemical pulp. For mechanical pulp, fractionation is more challenging due to lower froth stability, but still fibres with a reduced amount of smaller fraction can be recovered.

Cellulose fibrils are the structural backbone of plants and, if carefully liberated from biomass, a promising building block for a bio-based society. The mechanism of the mechanical release—fibrillation—is not yet understood, which hinders efficient production with the required reliable quality. One promising process for fine fibrillation and total fibrillation of cellulose is cavitation. In this study, we investigate the cavitation treatment of dissolving, enzymatically pretreated, and derivatized (TEMPO oxidized and carboxymethylated) cellulose fiber pulp by hydrodynamic and acoustic (i.e., sonication) cavitation. The derivatized fibers exhibited significant damage from the cavitation treatment, and sonication efficiently fibrillated the fibers into nanocellulose with an elementary fibril thickness. The breakage of cellulose fibers and fibrils depends on the number of cavitation treatment events. In assessing the damage to the fiber, we presume that microstreaming in the vicinity of imploding cavities breaks the fiber into fibrils, most likely by bending. A simple model showed the correlation between the fibrillation of the carboxymethylated cellulose (CMCe) fibers, the sonication power and time, and the relative size of the active zone below the sonication horn.

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Euler-Lagrange (EL) simulations of particulate suspension flow are an important tool to understand and predict multiphase flow in nature and industrial applications. Unfortunately, solid-liquid suspensions are often of (mathematically) stiff nature, i.e., the relaxation time of suspended particles may be small compared to relevant flow time scales. Involved particles are typically in the size range from µm to mm, and of non-spherical shape, e.g., elongated particles such as needle-shaped crystals and/or natural and man-made fibres. Depending on their aspect ratio and bending stiffness, those particles can be treated as rigid, or flexible. In this paper we present a recent implementation into the open-source LIGGGHTS ® and CFDEM ® software package for the simulation of systems involving stiff non-spherical, elongated particles. A newly implemented splitting technique of the coupling forces and torques, following the ideas of Fan and Ahmadi (J. Aerosol Sci. 26, 1995), allows significantly larger coupling intervals, leading to a substantial reduction in the computational cost. Hence, large-scale industrial systems can be simulated in an acceptable amount of time. We first present our modeling approach, followed by the verification of our code based on benchmark problems. Second, we present results of one-way coupled CFD-DEM simulations. Our simulations reveal segregation of fibres in dependence on their length due to fibre-fluid interaction in torus flow. 586

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