Predicting flow behavior without lab testing
Q. Is there a simple way to predict particle flow behavior without extensive laboratory testing?
There is. Back in the early 1970s, Derrick Geldart correlated the fluidization behavior from more than a dozen researchers and found that particles can be classified into four flow regimes. His work is now commonly referred to as the Geldart Classification Regime Model, as shown in Figure 1. By comparing the Sauter mean particle size, which can be calculated by the reciprocal summation of a particle weight fraction of bin i divided by the particle size of that bin (see the equation below) or to the difference between the particle and gas density (ρparticle - ρgas), he was able to classify particles into four distinct behaviors when fluidized. He labeled these behaviors Geldart Groups A through D.
Geldart Group A: Group A particles typically range in size from 30 to 220 microns and are considered aeratable and easily fluidized.
Geldart Group B: Group B particles typically range in size from 220 to 2,000 microns and are also fluidized easily but tend to have large bubbles, which can be problematic for some applications.
Geldart Group C:
Group C particles are typically small particles less than 30 microns and have cohesive properties and are difficult to fluidize. For example, let's assume we have a powder with a particle density of 1,000 grams per cubic centimeter (g/cm3
), indicated by the dotted line in Figure 1. The gas densities are typically an order of magnitude smaller than particle densities, so we can expect the difference in these densities to be approximately 1,000 g/cm3
Geldart Group D: Group D particles are larger than 2,000 microns and tend not to fluidize but have a spouting behavior. In other words, the bed doesn't fluidize since you tend to get one large, particle-laden jet in the center of the bed. This fluidization result can be an issue from some applications but works really well for particle coating applications, which is the foundation of the Wurster process.
If I wanted to develop a fluid-bed process, my target would typically be for Group A or B. Most fluidized beds are Group A where the catalyst is easily synthesized by spray drying. Group B particles are common in combustors, gasifiers, or pyrolysis units where the powders are obtained from mining and crushing of minerals such as sand, rutile, or lime. For most fluidized-bed applications, I certainly want to avoid particles smaller than Group A or particles larger than Group B. Cohesive spouting beds are challenging.
The Geldart classification diagram isn't just limited in understanding fluidized-bed behavior. The diagram can be helpful in looking at other operations in powder and bulk processing. A powder that's classified as a Group C powder should be expected to be difficult to handle in hoppers, feeders, and dense-phase conveying lines. This group's cohesive nature makes it prone to funnel-flow and plugging as well as to compaction.
Group D powders, and in some cases, Group B powders, generate large bubbles or spouts. These particles also have large arching diameters, which may require a larger hopper opening at the bottom and additional considerations with feeders.
Thus, Geldart's simple particle characterization diagram provides a quick way to get an initial understanding of potential processing issues with your powder and bulk materials. The diagram helps in determining initial designs, safeguards, cost estimates, and more. However, it may be a poor substitute to just seeing what the result looks like in the lab under similar operating conditions and scale. In short, I use the Geldart classification diagram as a start to the design process but would not rely on the model alone to make final design decisions.