Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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Segregation of Granular Particles by Mass, Radius, and Density in a Horizontal Rotating Drum
Arntz, M.M.H.D. ; Beeftink, H.H. ; Otter, W.K. den; Briels, W.J. ; Boom, R.M. - \ 2014
AIChE Journal 60 (2014)1. - ISSN 0001-1541 - p. 50 - 59.
size segregation - particulate systems - flow - simulation - cylinders - mixtures - dynamics - solids - behavior - motion
The impact of particle properties on segregation and mixing of bidisperse granular beds in a rotating horizontal drum have been studied by discrete element method (DEM) simulations. Bidispersities in radius, density, and mass have pronounced influences on the stationary mixing pattern, although they hardly affect the granules’ flow regime. At 50% fill level, all beds mix well for a Froude number of 0.56, corresponding to a flow regime intermediate to cascading and cataracting, while segregation occurs both at lower (rolling and cascading regime) and higher (cataracting/centrifuging regime) Froude numbers. These observations are explained qualitatively by noticing that the angular drum velocity dictates the flow regime, which in turn determines the effectiveness and direction of four competing (de)mixing mechanisms: random collisions, buoyancy, percolation, and inertia. A further dozen particle properties have been varied, including the friction coefficients and elastic modulus, but these proved inconsequential to the steady-state degree of mixing.
The influence of end walls on the segregation pattern in a horizontal rotating drum
Arntz, M.M.H.D. ; Otter, W.K. den; Beeftink, H.H. ; Boom, R.M. ; Briels, W.J. - \ 2013
Granular Matter 15 (2013)1. - ISSN 1434-5021 - p. 25 - 38.
reversible axial segregation - granular-materials - mixtures - simulation - cylinders - dynamics - motion
The influence of end walls on segregation of bidisperse granular beds in a short rotating horizontal drum is studied by a discrete element method. Whereas non-closed periodically continued drums segregate radially, all simulations of drums with end walls resulted in axial segregation with two bands at low friction between the particles and the end-wall, and three bands at high friction. Various simulations show irregular transitions between two approximately equally stable states, with rapid oscillations preceding the conversions. The formation of two axial bands decreases the energy dissipation by the bed, whereas neither radial segregation nor axial segregation into three bands reduced the power absorption at constant angular velocity. Roughening up the end-walls also increased the rate of axial segregation.
Repeated segregation and energy dissipation in an axially segregated granular bed
Arntz, M.M.H.D. ; Otter, W.K. den; Beeftink, H.H. ; Boom, R.M. ; Briels, W.J. - \ 2010
Europhysics Letters 92 (2010)5. - ISSN 0295-5075 - p. 54004 - 54004.
long drum mixer - horizontal rotating cylinder - simulations - dynamics - mixtures - patterns - model - core
Discrete element simulations were used to study the segregation behaviour in a bed of bidisperse granules in a rotating drum. In the final state the large particles ended up in the upper part of the bed near the vertical walls. In order to arrive at this state, the system went through two cycles of structural changes, on top of which fast oscillations were observed between an axially segregated and a somewhat more mixed state. These oscillations were sustained by different angles of repose near the vertical walls and in the middle of the bed. Concomitantly with the structural changes, the system's energy dissipation went through two cycles after which it settled in the state requiring the least work of all traversed states, suggesting that the granular bed strives for minimal dissipation.
Modeling of particle segregation in a rotating drum
Arntz, M.M.H.D. - \ 2010
University. Promotor(en): Remko Boom; W.J. Briels, co-promotor(en): W.K. den Otter; Rik Beeftink. - [S.l. : S.n. - ISBN 9789085858027
vermenging - korrels - vaten - rotatie - simulatiemodellen - modelleren - mixing - granules - drums - rotation - simulation models - modeling
Mixing of granular solids is a processing step in a wide range of industries. The fundamental phenomena in granule mixing are still poorly understood, making it difficult to a priori predict the effectiveness of mixing processes.
While mixing of granules is easy when the particles are homogeneous in size, shape and density and other properties, in practice they are not. With such a mixture, homogenizing is far more complex, since the heterogeneous particles tend to segregate, and special care has to be taken in the design of the mixing process to avoid this.
In view of the practical need for better understanding and control of solids mixing, the work in this thesis has two closely coupled objectives. The first objective is to obtain a better understanding of segregation mechanisms. This insight should enable the enhancement of mixing and at the same time suppress segregation, or vice versa, namely the deliberate and controlled segregation of a mixture. The second objective is to provide guidelines for mixing operations that can be derived from insights extracted from the data on mixing behaviour at different rotational velocities and fill levels. From this perspective, we here report an extensive numerical study of mixing and segregation in a bed of bidisperse granules in a rotating horizontal drum, which is the simplest relevant geometry in industrial practice.
Two types of segregation can occur: fast radial segregation during which smaller or denser particles accumulate along the axis of rotation; and slow axial segregation with fully segregated bands of small and large particles perpendicular to the rotating axis, with in general particle bands of large particles adjacent to the end walls. This thesis reports on both radial and axial segregation phenomena in a horizontally rotating drum.
While visual observation of the particle bed was used as a qualitative observation technique to determine the degree of mixing/segregation, in parallel a more quantitative method was developed as well, which was based on calculating the entropy over the systems. By subdividing the system with a lattice, calculating the entropy of mixing in each cell of the lattice, and summarizing them over the system, a measure for the degree of overall segregation was obtained. By using different grids (a 3D mesh, a 2D set of slices perpendicular to the axis, or 2D bars parallel to the axis), different types of segregation could be distinguished.
The radial segregation dynamics were investigated in semi-2D (very short) drums, which inhibits axial segregation. Diagrams were prepared that visualise the mixing behaviour as function of the Froude number (rotational speed) for systems with different bidisperse systems. It was found that while almost all systems showed radial segregation at low Fr (rolling regime), and most showed inverted radial segregation at high Fr (cataracting or centrifuging regime), at Fr ≈ 0.56 all systems became radially mixed. This could be understood by assuming a percolation mechanism. In the moving layer on top of the load, smaller particles percolate in between the moving larger particles, down to the centre of the load, as long as the motion is not too fast. The same phenomenon is inverted at high speeds. In between, the flowing layer is expanded in such a way that many large voids are present, which makes the percolation mechanism less selective on the particle size. The little segregation that occurs is negligible, since the two phenomena described above work in different directions. Surprisingly this transitional Fr number is the same for all investigated systems.
Since axial segregation is always preceded by radial segregation, it is logical to also study axial segregation. This was done by studying longer drums, which allows axial segregation to develop along the axis. Axial segregation was found for most systems; its occurrence is mostly dictated by differences in size.
It was found that for drums that have intermediate length, surprising dynamic behaviour results. The axial segregation developed with low and high frequency oscillations. While the low frequency oscillations could be understood as the development and migration of segregated areas in the system, the higher frequency oscillations, with a period of 10 to 20 revolutions, were not identified before. This oscillatory behaviour is probably coupled to the use of intermediately sized drums, as this behaviour has not been seen with very long drums. We ascribe the oscillations to the influence of the (vertical) end walls, which expose the adjacent particles to different forces than those particles inside the drum load. These differences induce an axial flow in the system. The particles adjacent to the vertical walls tend to be lifted higher than the particles far away from the vertical walls. This creates a concave profile of the load surface throughout the drum, inducing the particles (in the rolling regime) to follow a path away from the vertical walls towards the centre of the drum. Once past the centre, the particles flow back to the vertical walls in response to the locally convex bed profile.
Even in this particular flow profile the percolation mechanism is of importance: smaller particles percolate through the flowing layer and end up deeper inside the bed, while the larger particles accumulate on top of the flowing layer and are conveyed back to the vertical walls. Due to the percolation of the small particles the final end configuration must clearly be a banding configuration of large-small-large particle bands. Prolonged rotation of the bed increases the concave form of the flowing layer. This induces fast oscillations and a sudden mixing of a part of the large particle band with the small particle band, giving fast mixing and leading to a configuration, in which a small-particle band is formed below the large-particles bands. Subsequently segregation into three bands (large-small-large) slowly occurs again, after which the asymmetry in the angel of repose further increases. The configuration, in which larger particles accumulate on top of the bed adjacent to the end walls, coincides with a minimum in energy dissipation, which is not present when the systems segregates radially or axially into three pure bands.
The effect found implies that the end walls are important in the dynamics of axial segregation. This effect is studied further by varying the end wall properties. The above mentioned fast and slow oscillations vanish in systems that have smoother end walls, while also the rate of segregation decreases; nevertheless the same axially segregated three band (large-small-large) state of mixing resulted finally. Reducing the friction further to completely smooth end walls however changed the final configuration into a two-banded system. Systems with no end wall at all, simulated through periodic end walls, only gave radial segregation over the (considerable) simulated time span. We expect here that as long as there is still a driving force for axial segregation, the absence of the induction of axial flow by the end walls make the transition very slow or impossible. The formation of two axial bands lowers the energy dissipation by the bed, whereas neither radial segregation nor axial segregation into three bands reduced the power absorption at constant angular velocity.
While the oscillatory behaviour is relevant in its own right, their study also allows shedding some light on the fundamental mechanisms underlying the segregation mechanisms, and especially the transition from radial to axial segregation. The fact that this is dependent on not only the properties of the granular materials, but also on the geometry and design of the drum, implies that these findings have relevance to the design and operation of processes.
Granular mixing and segregation in a horizontal rotating drum: a simulation study on the impact of rotational speed and fill level
Arntz, M.M.H.D. ; Otter, W.K. den; Briels, W.J. ; Bussmann, P.J.T. ; Beeftink, H.H. ; Boom, R.M. - \ 2008
AIChE Journal 54 (2008)12. - ISSN 0001-1541 - p. 3133 - 3146.
size segregation - radial segregation - transverse plane - rotary kilns - flow - cylinders - solids - particles - dynamics - model
The rich phase behavior of granular beds of bidisperse hard spherical particles in a rotating horizontal drum is studied by Discrete Element Method (DEM) simulations. Several flow regimes and various forms of radial segregation, as well as mixing, are observed by systematically varying the operational parameters of the drum, i.e. fill level and angular velocity, over a wide range. Steady states after several dozen revolutions are summarized in two bed behavior diagrams, showing strong correlations between flow regime and segregation pattern. An entropy method quantifies the overall degree of mixing, while density and velocity plots are used to analyze the local properties of the granular bed. The percolation mechanism may provide a qualitative explanation for the distinct segregation processes, and for the transient mixing in nonradially segregated beds. Initially blockwise segregated beds are found to mix before radial segregation sets in. High fill fractions (>65%) show the most intense segregation
A multicomponent reaction-diffusion model of a heterogeneously distributed immobilized enzyme
Roon, J.L. van; Arntz, M.M.H.D. ; Kallenberg, A.I. ; Paasman, M.A. ; Tramper, J. ; Schroën, C.G.P.H. ; Beeftink, H.H. - \ 2006
Applied Microbiology and Biotechnology 72 (2006)2. - ISSN 0175-7598 - p. 263 - 278.
kinetically controlled synthesis - cephalexin - acylase
A physical model was derived for the synthesis of the antibiotic cephalexin with an industrial immobilized penicillin G acylase, called Assemblase. In reactions catalyzed by Assemblase, less product and more by-product are formed in comparison with a free-enzyme catalyzed reaction. The model incorporates reaction with a heterogeneous enzyme distribution, electrostatically coupled transport, and pH-dependent dissociation behavior of reactants and is used to obtain insight in the complex interplay between these individual processes leading to the suboptimal conversion. The model was successfully validated with synthesis experiments for conditions ranging from heavily diffusion limited to hardly diffusion limited, including substrate concentrations from 50 to 600 mM, temperatures between 273 and 303 K, and pH values between 6 and 9. During the conversion of the substrates into cephalexin, severe pH gradients inside the biocatalytic particle, which were previously measured by others, were predicted. Physical insight in such intraparticle process dynamics may give important clues for future biocatalyst design. The modular construction of the model may also facilitate its use for other bioconversions with other biocatalysts
Partial purification and characterization of two aminotransferases from Lactococcus lactis subsp. Cremoris B78 involved in the catabolism of methionine and branched-chain amino acids
Engels, W.J.M. ; Alting, A.C. ; Arntz, M.M.T.G. ; Gruppen, H. ; Voragen, A.G.J. ; Smit, G. ; Visser, S. - \ 2000
International Dairy Journal 10 (2000). - ISSN 0958-6946 - p. 443 - 452.
Effect of environmental conditions on flocculation and immobilisation of brewer's yeast during production of alcohol-free beer.
Iersel, M.F.M. van; Meersman, E. ; Arntz, M. ; Rombouts, F.M. ; Abee, T. - \ 1998
Journal of the Institute of Brewing 104 (1998). - ISSN 0046-9750 - p. 131 - 136.
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