Fluidization of fine bulk solids can be an effective alternative to handling solids in a contact bed manner. Air or another gas can be used for powder aeration either in a fully fluidized state (liquid-like properties) or only in a localized fashion with small amounts of gas flow. Powder fluidization is common with pulverized fuel handling, drying operations, and reaction processes.
The advantages for handling powders and bulk solids via fluidization instead of traditional deaerated gravity discharge include:
Elimination or reduction of arching (bridging) or ratholing problems
Increased discharge rate from a silo or hopper
Blending of solids and reduction in mixture segregation
Increased gas-solids interaction for drying, agglomeration, or reactions
Typical bulk solids suitable for fluidization have a fine particle size, a low permeability, a tight particle size distribution, and low cohesive strength. Geldart’s fluidization chart is a common reference used to assess if a bulk solid is a good candidate for fluidization (Geldart zones A and B). Many solids can be fluidized, though powders prone to segregate or cake are often not good candidates.
To take the guess-work out of powder fluidization suitability, Jenike & Johanson can perform the following tests:
Minimum fluidization velocity – performed with 8 in. (200 mm) dia. rig
Fluidization discharge assessment – performed with 22 in. (560 mm) dia. rig
Air-assisted discharge – localized powder aeration vs. full fluidization
These tests can be run with various gases (e.g., nitrogen, carbon dioxide, etc.) and over a range of temperatures. During these tests, observations are made to note settlement behavior, channeling of the solids, formation of a solid plug, and bed expansion.
Fluidization testing apparatus
(click to enlarge)
Example of fluidization testing
(click to enlarge)
Minimum powder fluidization velocity
(click to enlarge)
