in Wiley InterScience (www.interscience.wiley.com).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.

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. © 2013 American Institute of Chemical Engineers AIChE J, 60: 50–59, 2014

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.

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-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.

A method using immobilised yeasts has been developed and successfully applied for production of alcohol-free beer. The influence of environmental conditions present during alcohol-free beer production on the flocculation and immobilisation of the yeast Saccharomyces cerevisiae var. uvarum was investigated in the present study. In wort, the cells developed flocculation at the end of exponential growth, according to the NewFlo phenotype. In defined medium, the flocculation capacity appeared to be temporary and was lost rapidly during the stationary phase. No increase in cell wall hydrophobicity at the onset of flocculation was observed in either medium. Low growth temperatures increased flocculation capacity approximately four-fold, compared to growth at high temperatures. The optimum temperature for flocculation was at 25°C with cells grown at low or high temperature.A novel method using carboxyfluorescein-stained cells was developed to analyse the initial adhesion of cells to carrier. This method also allowed rapid analysis of the effects of immobilisation to DEAEcellulose carrier during alcohol-free beer production process. It appeared that a high flocculation capacity stimulated adhesion to the DEAE-carrier.