Carr flowability

FIGURE 9 Correlation between actual mass flow rates and the Carr flowability indices of 12 samples of microcrystalline cellulose that had differing particle size distributions. 

ASTM D 6393-99 (1999) Carr Flowability Iruiices. Ammcan Society for Testing Materials, International, West Conshohocken, PA. www.astm.org. 

H.G. Brittain, unpublished flowability results obtained using a model PT-D Powder Characteristics Tester manufactured by the Hosokawa Micron Company. The Carr flowability index was determined from the individual indices derived from measurements of the angle of repose, compressibility, angle of spatula, and cohesion. 

The classification system introduced by Carr [29,30] was used to evaluate the flow properties of the sorbitol powders. In Carr s system, a flowable powder is defined as free flowing and will tend to flow steadily and consistently. This is to be contrasted with a floodable powder, which will exhibit an unstable, discontinuous, and gushing type of flow. The parameters in Carr s system include the angle of repose, angle of spatula, compressibility, cohesion, and dispersibility. Based on these parameters, flowability and floodability indices are calculated to determine the handling properties of bulk solids. 

Table 6 Properties of bulk solids to determine flowability with the Carr method ...

It would be desirable to have simple tests capable of characterising the fluidisation behaviour or flowability of particulate materials on the basis of their bulk properties. To this end, Carr19 developed a system to characterise bulk solids with respect to flowability. Table 6 summarises the properties which are determined. In Carr s method a numerical value is assigned to the results of each of these tests, and is summed to produce a relative flowability index for that particular bulk material. Given the extensive use of these empirical techniques in academia and industry, a brief review on the subject is reported here. Nevertheless, it should be emphasised that these techniques allow measurements of the flow-ability or cohesion of materials solely in their stationary or compressed status and at ambient conditions. A direct relationship between these... 

The Hausner ratio varies from about 1.2 for a free-flowing powder to 1.6 for cohesive powders. Carr index classifications for flowability [2] are listed in Table 15. 

TABLE 15 Carr Index Classification and Powder Flowability... 

Given that the bulk volume associated with the particle mass is a mixture of air and solid material, the bulk density value is highly dependent on sample history prior to measurement. Calculation of the tapped density can then be achieved by tapping the bulk powder a specified number of times (to overcome cohesive forces and remove entrapped air) to determine the tapped volume of the powder. The tapped and bulk density values can be used to define the flowability and compressibility of a powder using Carr s index and the Hausner ratio. 

Some work has shown a direct correlation between shape factor and the flow properties of powders. The flowability of fine powders, as measured by a shear-cell as well as by Carr s method, was found to increase with increasing sphericity, where the sphericity is indicated by a shape index approaching one, as measured by an image analyzer. Huber and co-workers derived an equuation in which flow rate was correlated to the volume specific surface as measured by laser diffractometry. Reasonable predictions were made for individual powders as well as binary and ternary mixtures. 

Other classification systems are used less frequently. Carr " also devised a system to classify materials as to their floodability. He defines the floodability of a material as its tendency to flow like a liquid because of the natural fluidization of a mass of particles by air. In order to so classify a material, the flowability is determined utilizing the method just described. This value is equivalent to a measurement Carr calls the angle of fall, angle of difference, and dispersibility. Though referred to in any of the papers mentioned here, this system is much less utilized then the flowability measurements. Geldart reported on a characterization system of powders according to their ability to aerate and later Molerus modified this system. In a more recent symposium this method of powder classification was examined. ... 

Flowability 10.8-11.7g/s for corn starch 30% for corn starch (Carr compressibility index). Corn starch is cohesive and has poor flow characteristics. 

Another, this time indirect, method of assessing flowability was proposed by Carr36. This was based on measuring four properties of the powder (angles of repose and spatula, compressibility and uniformity coefficient, all as defined by Carr36) and, by awarding points (out of 25) for each, the sum total was then a measure of flowability and called Carr s Index . A commercial piece of equipment is available (Hosokawa Powder Characteristics Tester) for measuring the properties necessary for Carr s Index. 

Peleg (1978) mentions that the characteristic compressibility can be used as a parameter to indicate flowability changes, because compressibility (Equations 13 15) has been related to cohesion C. The more compressible a material is, the less flowable it will be (Carr, 1965). This relationship has been found from experiments on limestone, powdered sugar, semolina, and flours at different particle sizes, size distributions, and moisture contents. 

---