Single drop granule formation for regime separated granulation

Heather N Emady, Purdue University

Abstract

Wet granulation is the process of adding a liquid binder to a fine powder in order to get larger granules for improved particle properties. This process is used in a variety of industries, including pharmaceuticals, food, agricultural chemicals, and detergents. In typical granulation equipment, many of the granulation rate processes (wetting and nucleation, consolidation and growth, and breakage and attrition) occur simultaneously, making it notoriously difficult to control and predict product properties, such as size and shape. Recently, a new granulation approach, regime separated granulation, has been proposed as a way to physically separate the different rate processes to get dramatically better control of the product granule attributes. The first and most important stage in any regime separated granulation process is nucleation, where new granules are formed by the addition of liquid to the powder bed. Drop controlled nucleation, where one drop forms one granule, is the most desirable operating regime. In order to apply regime separated granulation in practice, more knowledge is necessary regarding the mechanisms by which granules are formed from drop impact and penetration into a static powder bed. Single drop granule experiments with a syringe and a dish filled with powder are used to simulate drop controlled nucleation in static beds. Sixteen model inorganic powders were used in single drop granule experiments, with surface mean particle sizes ranging from 1.51 to 261 μm and bed porosities from 0.33 to 0.878. Three different binder fluids (distilled water and silicone oils with viscosities of 9.3 and 96 mPa·s) were released onto these powder beds from heights between 0.5 and 30 cm. From high speed camera videos of drop impact and penetration, three different granule formation mechanisms were identified: Tunneling, Spreading, and Crater Formation. Tunneling occurred for loose, cohesive powder beds. Powder aggregates were sucked into the drop which then tunneled into the beds. For coarser powders, granules were formed by a Spreading mechanism at a drop height of 0.5 cm. At higher drop heights, from 5 to 30 cm, the drop formed a crater in the bed surface and deformed elastically in the crater, coating the drop in a layer of powder before penetrating into the bed by capillary action. All granules formed had narrow size distributions on the order of the drop size, but dramatically different shapes were observed. A new shape factor, the vertical aspect ratio, V.A.R. (the ratio of the granule's projected area diameter to its maximum vertical height), was proposed as a more accurate descriptor of granule shape than currently used descriptors such as the horizontal aspect ratio. The Tunneling mechanism produced round granules (V.A.R. in the range of 1.05-1.7), which were insensitive to liquid properties and drop release height. The Spreading mechanism produced flat disks (V.A.R. in the range of 1.42-2.73), while the Crater Formation mechanism produced granules of varying shapes that were generally rounder than those produced from Spreading (V.A.R. in the range of 1.11-2.47). To quantify the conditions under which each mechanism will occur, dimensional analysis was performed and a new regime map was created that plots the powder bed porosity (ϵ) against the modified granular Bond number (Bog*), which is a ratio of the capillary force acting on a particle to the gravitational force acting on a particle. Tunneling occurred for Bog* > 65000 for all values of bed porosity, while Spreading and Crater Formation occurred when Bog* < 65000 for all values of bed porosity below the minimum fluidization porosity, ϵmf. Above ϵmf, the free-flowing powders in the Spreading/Crater Formation regime should form round granules in an Engulfing regime, with a mechanism similar to Tunneling. The Spreading and Crater Formation regimes can be distinguished from each other with the Weber number (We), which is the ratio of impact energy to liquid surface tension. Crater Formation occurred for We ≥ 36, while Spreading occurred for lower values of We. It was hypothesized that the Tunneling mechanism would occur when the capillary and surface tension forces exceeded the weight of a powder aggregate in contact with the drop. To confirm this hypothesis, force balances were derived for a drop in contact with a single particle and separately for a drop in contact with an aggregate to predict when a particle or aggregate will be sucked into the drop. The force ratios derived for each case were compared to the experimental Bog* force ratio used in the regime map. The aggregate model predicted the Tunneling boundary better than the single particle model, but it still under predicted the experimentally determined Tunneling criterion given by the Bond number. (Abstract shortened by ProQuest.)

Degree

Ph.D.

Advisors

Litster, Purdue University.

Subject Area

Chemical engineering

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