Flowability definition

Particle behavior is a function of particle size, density, surface area, and shape. These interact in a complex manner to give the total particle behavior pattern [28], The shape of a particle is probably the most difficult characteristic to be determined because there is such diversity in relation to particle shape. However, particle shape is a fundamental factor in powder characterization that will influence important properties such as bulk density, permeability, flowability, coatablility, particle packing arrangements, attrition, and cohesion [33-36], Consequently it is pertinent to the successful manipulation of pharmaceutical powders that an accurate definition of particle shape is obtained prior to powder processing. 

Figure 1. Top and side views of a resin/glass prepreg showing the definition of flowable resin height. (Data are from Ref. 4.)...

The bulk density of a powder is obtained by dividing its mass by the bulk volume it occupies. The volume includes the spaces between particles as well as the envelope volumes of the particles themselves. The true density of a material (i.e., the density of the actual solid material) can be obtained with a gas pycnometer. The bulk density of a powder is not a definite number like true density or specific gravity but an indirect measurement of a number of factors, including particle size and size distribution, particle shape, true density, and especially the method of measurement. Although there is no direct linear relationship between the flowability of a powder and its bulk density, the latter is extremely important in determining the capacity of mixers and hoppers and providing an easily obtained valuable characterization of powders. 

In addition, the flowability of a bulk solid is a function of the bulk solids flow properties and the design parameters of the handling equipment. For example, poor flowing bulk solids can be handled reliably in properly designed equipment, and good flowing bulk solids may develop flow problems in improperly designed equipment. As such, a more accurate definition of flowability is the ability of powder to flow in a desired manner in a specific piece of equipment. 

A precise definition of the flowability of a powder is only possible with several numbers and curves, derived from a family of yield loci of the powder (measured with a shear cell) - see section 4 for further detail. Jenike23 proposed a simpler classification, according to the position of one point of the failure function (at a fixed value of the unconfined yield strength, say 5 lbf (22.3 N) with the Jenike shear cell, i.e. 3112 Pa or 65 lbf/ft2) with respect to the flow factor line (straight line through the origin, at a slope l///where//is the flow factor) - see Fig. 8 for a schematic representation of this. 

Conversely, Ehlermann and Schubert (1987) sustained that compressibility results from materials of different composition cannot be compared and that flowability characterization through compressibility must be made specifically for each food variety. Moreover, confined uniaxial compression is a simple compression test that provides an approximate measure of the flowability of powders. Therefore, it is not suitable for silo design but may prove to be a convenient method for process control in any food laboratory (e.g., to evaluate particle cohesion). Table II offers a range value definition for flowability classification by comparing flow function (ratio between the maximum consolidation stress and unconfined yield stress) with compressibility. 

Besides the common types of conveyor, CEMA standards also address classifications and definitions of bulk materials. CEMA 550, for example, presents an alphanumeric method for classif5dng bulk density, particle size, flowability, abrasiveness, and handling characteristics of solids. The identifier contains, in order ... 

The common definition of flowabihty is the ability of a powder to flow from a specific item of plant equipment at a desired degree of flowability. Flowability is generally quantified by a range of mobility, from free flow to non-flow which can, at times, be measured in a laboratory with specific flow property instrumentation. A bulk powder may be described as being free flowing, cohesive or non-flowing, and thus powder flowability tends to become an integral part of the description of bulk powder property measurement. Correlations between powder bulk properties in terms of both micro-scaled particulate factors as well as macro-... 

Yokoyama et al. (1982) measured the tensile strength with the swing method of the Hosokawa cohetester, for a range of particle sizes (1-90 urn) of different densities (0.928-6.01 g/cm ) over a range of moisture percentages (0.06-15.2% w/w), to evaluate the potential floodability of powders. Yokoyama et al. defined their dimensionless floodability, as opposed to Carr s definition of flowability, as the ratio of the cohesive force (C) - measured by cohetester - to the gravity force on particles m g). 

Schulze, D. (1996a) Flowability of bulk solids - Definition and measuring techniques. Part I. Powder Bulk Eng., 10(4, Part 1), 45-61. [See Schwedes, J. (2003)]... 

Schulze, D. Flowability of Bulk Solids - Definition and Measuring Techniques, Part I and II. Powder and Bulk Engng. 10(1996)4,45/61 6,17/28... 

Viscosity is a measure of flowability at definite temperatures. The flow properties of oils influence the rale of production of an oil-well, the transport of crude oil and refined products in pipelines and the performance of an oil as a lubricant in a machine. 

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