Understanding how particle size affects flow properties helps ensure reliable flow of materials between process steps
Many industries utilize dry solid materials and powders (collectively referred to as bulk solids in this article) for producing and manufacturing products. It is common for particle size to be a part of the specifications for raw, process intermediate and finished products. Depending on the process, the particle size of bulk solids can change via several means, including milling, grinding, agglomerating, crystallizing, screening and so on. These steps that involve a direct change to the material are generally the primary focus when designing a process. Often, the equally important aspect of ensuring that the material flows consistently and reliably between process steps is overlooked, leading to systems operating under design capacity throughout their lifetime.
Particle size has an impact on the flow properties of bulk solids, and hence on process performance. While there are several other factors — such as process conditions (temperature and relative humidity), chemical composition, moisture content and particle shape that impact a material’s flowability — particle size can be used as a first-pass indicator that there could be potential concerns.
Particle size distribution
Before discussing how particle size affects different flow properties of bulk solids, it is important to understand the methods used to describe and measure particle size. First, when reviewing the impact of particle size on flowability, it is helpful to describe particle size in terms of a distribution, because the average particle size (often referred to as d50, or the size at which half of the particles are smaller and half are larger) does not tell the full story. The minimum and maximum, as well as secondary peaks showing a concentration of coarse or fine particles, is significantly more helpful in identifying potential flow issues than a single average value. For example, if a material were to have a maximum particle size that is significantly larger than the average, handling equipment may need to be sized around the prevention of these particles mechanically interlocking or jamming together.
Particle size distribution can be measured via a variety of methods, including sieving, which is commonly used for coarser materials (generally greater than 500 microns), and laser diffraction, which is commonly used for finer material (generally less than 500 microns). It is important to note that discrepancies in results between different methods, and even between different instruments utilizing the same method, are common. With this in mind, keeping a record of the specific devices and conditions used for particle-size distribution testing is important [1].
Cohesive strength
The cohesive strength of a bulk solid is the relationship between the consolidating pressure and the yield strength of the material. A low cohesive strength indicates that a material is free-flowing, while a high cohesive strength indicates that a material is prone to forming ratholes and arches in bins or hoppers. These common flow problems can drastically increase the need for operator intervention to keep material flowing through a process. The particle size distribution of a bulk solid has a general effect on its cohesive strength. Typically, a material with a finer particle size equates to a greater cohesive strength, compared to the same material with a larger particle size [2].
Permeability
The permeability of a bulk solid is the resistance of airflow through a bed of material as a function of bulk density. A low permeability results in two-phase (air and solid) flow behaviors. These behaviors include air retention in a material bed, which can lead to material flooding uncontrollably though feeders, as well as extended charge time, due to the need to wait for the material to deaerate and settle in a storage bin or silo. Two-phase flow issues can manifest in other ways also, such as a limiting discharge rate through handling equipment, or dosing issues from unsteady flow. Two-phase flow issues almost always worsen as the particle size of a material is reduced [3]. The exception to the rule comes in cases where a fine material can be fluidized. In this situation, the solids act as a liquid. However, if the cohesion of a material is too high, fluidization will not be possible or will be unrealistic in a production setting.
Segregation
When dealing with a blend of different bulk solids, a difference in particle size can cause the individual components of the blend to separate; this is called segregation. Segregation can manifest in a variety of ways. The two most common modes of segregation are sifting and fluidization. In sifting segregation, as a pile of material is formed, there tends to be a concentration of fines in the center, while coarser particles roll to the sides (Figure 1). This kind of segregation can occur with a relatively narrow distribution (ratio of large to small particles of 1.4) and a mean particle size around 500 microns, so long as inter-particle motion is possible. Sifting segregation is typically reduced with a decrease in mean particle size, but it can still occur at a mean particle size as small as 100 microns if the distribution is wide enough [4].
FIGURE 1. In sifting segregation, fine particles concentrate closer to the center of the pile, while coarser particles roll to the sides
In fluidization segregation, fines are stripped from the blend via interactions with air. This is common during transfer of a blend where the solids are allowed to freefall. In this situation, the fines are concentrated at the top, and the coarse particles are concentrated on the bottom of the pile that is formed after settling. Fluidization segregation tends to be dominant over sifting segregation if the mean particle size of the blend is below 100 microns. Test methods to determine the potential of a material to segregate via sifting and fluidization mechanisms can be used to justify designing handling equipment that minimizes these effects. Test results can also be used to inform changes to the characteristics of a blend to reduce the likelihood for segregation [5].
Assessing properties
The particle size distribution of a bulk solid can provide insights into various flow properties. When assessing the flow behavior of a bulk solid, a finer particle size tends to indicate that the material will have a higher cohesive strength. This means that associated handling equipment will need to have larger outlet dimensions to prevent cohesive arches and ratholes and ensure reliable flow. For air and solid interactions, a finer particle size almost always indicates that the material will be less permeable. This means that the associated handling equipment may need to have larger outlet dimensions to account for a flowrate restriction due to two-phase flow interactions.
Additionally, extra storage capacity may need to be designed into a silo or bin due to settlement time. For segregation tendencies, the nominal particle size and distribution will play a major role in whether a blend will segregate. As long as there is an opportunity for interparticle motion, and as long as there is a wide enough particle-size distribution, segregation can occur. While particle size can be used as an early warning for potential flow issues, there are many other factors that contribute to the overall flowability of a material. Therefore, material testing at representative conditions to mimic a particular application is necessary to measure flow properties and allow for informed decisions to avoid flow problems.
Edited by Scott Jenkins
References
1. Barnum, R.A. and Prescott, J.K., Considerations for Steering Particle Size, Pharmaceutical Processing, March 2011.
2. Craig, D.A. and Hossfeld, R.J., Measuring Powder Flow Properties, Chem. Eng., September 2002, pp. 41–46.
3. Baxter, T.J., Powder Flow – When Powders Flow Like Water: Addressing Two-phase Flow Effects in Tablet Feed Systems, Tablets & Capsules, March 2009, pp. 26–32.
4. Williams, J.C. and Kahn, M.I., The Mixing and Segregation of Particulate Solids of Different Particle Size, Chem. Eng., London, Vol. 19 (1973); p. 269.
5. Prescott, J. K. and Hossfeld, R. J., Maintaining Product Uniformity and Uninterrupted Flow to Direct Compression Tablet Presses, Pharmaceutical Technology, June 1994, pp. 99–114.
Author
Joseph Karadizian is a project engineer at Jenike & Johanson (400 Business Park Drive, Tyngsborough, MA 01879; Phone: 978-649-3300; Website: www.jenike.com; Email: jkaradizian@jenike.com), an engineering consulting company specializing in bulk solids handling. Karadizian has been with Jenike & Johanson for more than a year and has worked on many projects involving critical solids-handling equipment to support reliable transport of material across all manufacturing industries. Karadizian received a bachelor’s degree from the University of Massachusetts-Lowell.