Flow properties of coarse and fine sugar powders

Abstract

© 2017 Wiley Periodicals, Inc.The densification dynamics and flow properties of powdery to granular varieties of commercial sugars were obtained. Samples of granular sugar (GS), refined (RS) sugar, a raw type VHP sugar (VS), and icing sugar (IS) were characterized concerning particle morphology, size distribution, loose and tapped bulk densities. Primary particles of IS sugar have a mean size (D50) of 77 μm and they agglomerate into larger size granules with intraparticle voidage. The RS, GS and VS sugars have D50 equal to 286 μm, 537 μm, and 696 μm, respectively. The RS sugar revealed some irregular and rough agglomerates, while the other samples were composed of flat and well-defined crystals. After full compaction, the initial bulk volume of the IS sugar was reduced by 25%, of the RS sugar by 18%, of the GS and VS sugars by about 10%. The IS sugar was the sample with poorest flowability, based on a Hausner ratio equal to 1.33. Compared to the others, this sugar variety presented a higher bed porosity and a smaller tapped density. In spite of its irregular shaped particles, the RS sugar compacted easily and achieved the same tapped density of samples with well-defined granular particles. A slower compaction kinetic was observed for the coarser sugar—VS (D50 = 696 μm). Finally, the static and dynamic repose angles increased from the granular (GS and VS) to the powdery sugars (RS and IS), but for the latter ones the values were not significantly different in spite of the mean sizes decreased from 206 μm in the RS sugar to 77 μm in the IS. A more relevant difference was observed in the dynamic repose angle for the powdery sugar and the difference between the dynamic and static repose angles increased as the mean particle size was reduced. According to their repose angles, the flowability would be classified as good (GS and VS) or fair (RS and IS). The densification dynamics of all sugars were fitted with good accuracy to an empirical equation. Practical applications: Powder flow is influenced by the presence of fines contained in the bulk powder, by the particles' size distribution and by their morphological characteristics. To meet the different needs of customers, a number of sugar varieties are marketed, including the granulated white sugar, the brown raw sugars and the powdery refined sugars. Every sugar variety behaves differently in operations such as packing, transport, and discharge from silos and hoppers. As sugar manufacturing plants deal with materials in a broad range of sizes and shapes, data of basic flow indicators for different sugar varieties obtained under standardized and comparable conditions may be useful for predicting the performance of a processing plant and monitoring the product quality attributes and for designing equipment as well.