Application of the Fundamentals

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Towards Better Size Reduction Flowsheets: How the Application of Some Simple, yet Powerful Philosophies and Principles Can Improve both Process Operation and Product Quality 

Karl Jacob, The Dow Chemical Company

On the face of it, size reduction appears to be a relatively straightforward unit operation. Hit the desired material hard enough, use lots of energy, and make sure the device is sufficiently robust and we can make large stuff into small bits. Unfortunately, this is often not the case, resulting in poor grinding flowsheet performance and off-specification product. There are countless, complex articles in the literature which allow us to model in the most minute detail the selection function and daughter size fractions for various size reduction devices. However in many cases, it is simple, yet fundamental, concepts which can significantly improve both product quality and process operability. The most important concept is to look at the size reduction process holistically and understanding the connections between the various pieces of equipment and their limitations. For example, over grinding results not from inherently poor mill performance but rather poor selection of the screens associated with the mill. In addition, since many of the materials produced by comminution by chemical engineers are very high value-added products, we should divorce ourselves from the traditional fixation on energy use and conservation associated with size reduction processes. These are just a few ideas which we will examine and support with data from actual plant operations. 

Industrial Challenges in Particulate Flow and Attrition

Paul Mort, Procter & Gamble

Developing and optimizing processes that handle particulates either as intermediates or end products involves detailed understanding of particulate flows in process equipment.  We desire efficient processes – i.e., maximizing production throughput while minimizing the dissipated energy.   Aspects in this optimization include:  1) the process equipment and its integration within a process system; and 2) the properties of the particulate material that are relevant to the processing conditions, specifically the response of the material to the flow and stress fields that are imposed by the process equipment.   Attrition of particles under such stress fields poses additional challenges relevant to process efficiency and product quality.  This review seeks to elucidate the effect of attrition in relation to imposed flow fields, constitutive properties of the material (e.g., as a function of shear rate and packing dynamics), and the resultant stress and energy consumption.

Characterization of particulate flow and stress fields and material coupling therein is a multi-scale challenge.  Constitutive behavior of the particulate material depends on particle-particle and particle-boundary interactions; to the extent that such interactions depend on particle characteristics and material properties, these phenomena may be interrogated on a particle or particle-cluster scale.  A somewhat larger scale of scrutiny may be appropriate for analyzing bulk interactions in critical parts of the process, e.g., impeller design.  An even larger scale captures the unit operation, its flow fields and residence time distributions.  Integration of unit ops on a system scale is critical for ancillary process interactions such as feeder variations, intermediate storage, product and intermediate handling, recycle integration, etc.  Particle attrition has implications across these scales.

From an industrial perspective, a rheological description of granular flow is essential, e.g., in relating micro-scale particle transformations to system-scale optimization of production processes.  Continuum rheology is especially relevant to broad particle size distributions that occur as a consequence of attrition.  The success or failure of particulate product scale-up and process optimization depends on adequate understanding of material-process interactions.  For example, many processes are initially developed on a small batch scale in at laboratory or small pilot plant – scale-up to large batch operations can be technically challenging and expensive.  Similar motivation is evident in over 30 years of research sponsored IFPRI[1].  This presentation includes a summary of industrial challenges in flow and attrition supported by IFPRI, along with more recently-published research supported by jointly by IFPRI and the US National Science Foundation, grant NSF1010008.[2]

Fundamentals of Jet Milling and Hammermilling

Gary Liu, DuPont

Both jet milling and hammermilling are the most common size reduction operations in many industries. Fully understanding the fundamentals of these mills can assist the selection of the right type of milling equipment, as well as improving both their operability and energy efficiency. Here the milling mechanism, the importance of feeding, and the design of air systems for both mills are discussed. The selection of nozzle for jet mills, the classification principle, and the use of steam in jet milling are also deliberated.