Case Studies

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On the Origin of High Powder Cohesion after Milling: Micro-Scale Examination and Fundamental Approach to Reduced Cohesion

Rajesh Davé, New Jersey Institute of Technology

Micronization of powders leads to their very poor flow, packing, and dispersion, not to mention increased sticking tendency. Most of these problems are usually attributed to fine particle size, which results in higher cohesion as compared to particle weight. This is usually represented via granular Bond number, which increases with size reduction. However, in many cases, there is an additional factor, which further contributes to higher cohesion. This factor is related to the altered surface characteristics arising from micronization. Such behavior is usually prevalent in crystalline materials, where milling leads to exposing of higher surface energy sites or defects. This additional factor adds to the poor flow and handling of micronized powders. Unfortunately, this aspect is poorly investigated in literature and hence, poorly understood. In this work, fundamental investigation is conducted to shed light on this problem as well as its mitigation. Towards that goal, simultaneous micronization and surface coating of crystalline particles with nano silica is carried out to. The purpose is to examine the role of surface energetics and the influence of dry coating on both creation of nano-scale surface roughness and passivation/stabilization of high surface energy sites, and hence reduced cohesion. Pharmaceutical powders were used as test materials and a fluid energy mill was used to micronize the particles to several sizes under 30 µm with or without simultaneous nano-silica coating. Powder flow property and dispersibility were characterized using FT4 powder tester and Rodos/Helos laser diffraction particle sizer. Surface energy was characterized using a next generation Inverse Gas Chromatography instrument. Total surface energy as well as Lifshitz –van der Waals (LW) dispersion component of surface energy were measured as a function of milling intensity. In addition, surface energy heterogeneity was also examined, revealing the micro-scale changes that occur during milling of crystalline materials. Impact of dry coating with nano-silica on surface energetics and powder cohesion was investigated. It was found that dry coating leads to reduction in cohesion and granular Bond number, hence improved flowability due to both reduced LW dispersive component of surface energy and creating nano-scale surface roughness.

Attrition of Limestone Particles Related to the Fluidized Bed Capture of SO₂ and CO₂

John Grace, University of British Columbia

Limestone is the least expensive solid sorbent for the in situ capture of both sulphur dioxide and carbon dioxide in fluidized beds, but attrition is a major issue.  This paper will review experimental and modelling findings at the University of British Columbia related to the attrition of limestone.

Application of Attrition-Resistant Materials for Novel Clean Energy Technologies

Atish Kataria, RTI International

At RTI’s Energy Technology Division, we are developing cleaner energy solutions, a number of which utilize fluidized bed reactors (FBR) at their core. RTI has been developing process-specific fluidizable materials for a variety of technology applications including coal gasification, biomass conversion, CO₂ capture and utilization, and syngas cleanup and conversion. We have developed significant expertise in designing highly attrition-resistant particles while imparting chemical reactivity and scaled up the manufacturing up to 100 ton scales. This presentation will share our experience and expertise in developing attrition-resistant sorbents for two applications: a) high-temperature syngas desulfurization, and b) CO₂ capture from coal-derived flue gas. The ZnO sorbent employed for high-temperature desulfurization process is inherently a hard material and the critical research challenge was to optimize the balance between reactivity (lowering hardness) and attrition resistance. On the contrary, the PEI-based CO₂ sorbents are supported on fluidizable silica support shells which are soft. RTI has addressed this limitation by designing a process that minimizes attrition.

Predicting and Controlling Attrition of Active Pharmaceutical Ingredients during Agitated Drying

Brenda Remy, Bristol-Myers Squibb

Scale-up of agitated drying processes to minimize particle size changes in Active Pharmaceutical Ingredients (API) can be challenging. Particle attrition problems due to agitated drying are often discovered upon the initial scale-up from the lab to the plant. Traditional laboratory drying equipment has not successfully reproduced the degree of attrition observed at scale. Factors such as API characteristics, batch size, dryer geometry, residual solvent content, and the defined agitation protocol can impact the extent of attrition during drying. The development of new scale-down tools can provided a platform to study the mechanism for particle attrition during API manufacturing in a material sparing way. This work characterizes the propensity for particle attrition to occur by employing a laboratory agitated vessel that has been modified to measure impeller torque and reproduce the range of compressive and shearing forces observed during scale-up. A workflow for assessing risk of API attrition at manufacturing scale is described. The application of this methodology for several different APIs will be highlighted.