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in Wiley Online Library (wileyonlinelibrary.com) Both a new modeling approach and new experimental data for the sediment build-up in centrifuges are presented. In semibatch apparatus, the suspension is continuously fed to the centrifuge, separating the particles inside the rotor and discharging the clarified liquid. The solid phase is removed once the capacity of the centrifuge is reached. The solids fraction of the sediment depends on the rheological properties of the cake. The sediment growth and consolidation throughout the process can be calculated using a pseudo two-dimensional approach that takes into account particle-size dependent settling, sediment compressibility, the centrifugal force field, and the geometry of the bowl. The predictions of the separation behavior and the particle-size distributions of the sediment and overflow are compared with experimentally obtained results, showing improved accuracy when compared to simpler models. The model presented is applicable to all solid-bowl centrifuges without conveying systems. V C 2013 American Institute of Chemical Engineers AIChE J, 59: [3843][3844][3845][3846][3847][3848][3849][3850][3851][3852][3853][3854][3855] 2013

The classification of dispersed particles below 1 lm is a difficult task due to the high surface area-to-volume ratio. Tubular-bowl centrifuges offer high centrifugal numbers, which enable the separation and classification of fine particles, biological cells and cell debris. This work presents the classification of two fine products with a mean particle size below 1 lm. Polydisperse silica and polystyrene were split successfully into a fine and a coarse fraction by a semi-continuous tubularbowl centrifuge. The fine fractions exhibited narrow particle size distributions. An optimization of the process could be achieved by a comprehensive understanding of the flow patterns, which are accessible with computational fluid dynamics. The axial and tangential velocity profiles were calculated for rotational speeds up to 40 000 rpm and throughputs ranging from 0.1 to 2 L/min. IntroductionVarious nanoparticles enhance the durability, processability and convenience of everyday products such as invisible sunscreens, coatings for displays, grinding materials, and additives for products in the chemical and construction industry [1,2]. New fields of applications have been developed due to new particle properties, e.g. superparamagnetic composites for drug delivery in cancer treatment and the recovery of valuable proteins in fermentation processes in the field of biotechnology [3]. Most of the effects originate only from the size of the particles. Coarse particles could impair the properties of the final product, e.g. rough surfaces of coatings or occlusion of the vasculature in the human body.It is possible to control the particle size during production at laboratory scale, but it is difficult to avoid reasonable amounts of particles with an undesired size in bulk products. Among the most important manufacturing processes are spray pyrolysis and grinding in stirred-media ball mills [4][5][6][7].The continuous classification of particles above the size of 1 lm [8][9][10][11] and the batchwise classification of small amounts of nanoparticles with microchannel technologies [12][13][14][15][16] are well developed, but for bulk products there is no apparatus available to classify nanoparticles. The required separating forces for the classification of fine particles and nanoparticles could be achieved in high-speed centrifuges. Tubular-bowl centrifuges offer high centrifugal numbers at throughputs of up to 10 L/min and capacities of up to 10 L. This type of centrifuge has already been used for the separation of fine particles [17,18], bacteria and viruses [19,20].The aim of this work is the determination of the classification efficiency for submicron particles and the evaluation of the flow patterns at different process parameters by computational fluid dynamics (CFD). The classification efficiency is investigated by mixing different size fractions of a polystyrene and subsequent centrifugation. The separation efficiency of the centrifuge allows splitting of the mixture into the initial fractions. Furthermore, the screening process...

This work contributes to the field of solid-liquid separation in focusing on the screening of colloidal particles in a semicontinuous process. The aim of the study is to provide the basics for a continuous classification process in industry. After a short introduction to the field of particle screening, a review of the possibilities of classification in the gas or liquid phase is presented, including analytical methods, devices for the fractionation on the laboratory scale as well as apparatus for screening on the pilot and industrial scale. The results show the dispersion, stabilization and classification of fine kaolin and polystyrene particles. Kaolin was used to demonstrate the possibility of removing the fraction of fine particles, polystyrene particles were used to investigate the removal of the coarse fraction. The diameter of the particles used in this study ranges from 250 nm to 3 lm.