Many industries rely on fluidization, a process that consists of suspending granular materials in a gas (often compressed air) or a liquid. Why? For which applications? And with which filtration devices?
What is fluidization?
Fluidization refers to giving a solid powder the properties of a liquid by exposing it to an upward-flowing fluid—either liquid or gas—that counterbalances the settling velocity of the particles. Under these conditions, several laws and properties of liquids become applicable to the fluidized bed.
To be fluidizable, a powder must exhibit good dispersibility and flowability: it must not be too angular, too fine, or too moist. Its particle-size distribution must not be too broad (a ratio of 1 to 20 at most). Finally, the powder must show good attrition resistance, meaning resistance to surface wear.
Each powder behaves differently during fluidization, depending in particular on its density and particle size. In general, the smaller the particle size, the more cohesive the powder becomes, and the harder it is to set in motion.
Fluidization was first used in 1926, when the Winkler gasification process was developed to produce syngas from powdered coal. The process expanded significantly after the development of catalytic petroleum cracking in the 1940s.
Image source: Wikipedia
Why use fluidization?
Fluidization allows a powder to behave like a liquid and to move more easily—properties that make it attractive for many industrial applications.
It enables, for example, the performance of exothermic chemical reactions while controlling temperature rise by using a fluidized bed as a thermal ballast. If particles that react exothermically with the gas are injected into a fluidized bed of particles, the heat released by the reaction is distributed across all particles present.
The non-reactive particles in the fluidized bed absorb the heat released by the few reactive particles. This makes it possible to carry out chemical reactions at an almost constant and homogeneous temperature, avoiding hot spots and thermal runaway—both detrimental to reactor performance and likely to create unwanted particle agglomerates.
Using a fluidized bed that behaves like a liquid also facilitates feeding particles into the reactor or removing them from it. As a result, the number of feed and discharge points can be reduced, ultimately lowering costs. Homogeneous mixing is also easier to achieve by adding solid particles to a fluidized bed rather than to a conventional powder.
Fluidization: which industrial applications?
This gas–solid contact mode supports numerous applications involving physical or chemical processing of materials. It is used in the pharmaceutical industry (paracetamol, vitamins, etc.), the food industry (cereals, powdered sugar, coffee, flour, cocoa powder, hydrated pet food, starches, milk powder, etc.), the construction sector (sand, sulfur, pulverized fly ash, soda, coal dust, glass beads), metallurgy, and industries working with mineral materials.
Drying of all types of particles—such as cereals—is likely the most widespread application in terms of the number of reactors worldwide. Fluidization is employed to move pharmaceutical powders, flours, and cement, as well as to empty silos and tankers. Numerous chemical syntheses are carried out in fluidized beds, and a significant share of global energy production (electric and thermal) is generated through combustion of coal, waste, or unconventional fuels in fluidized-bed boilers or furnaces.
Fluidization is used in the manufacturing of many chemical and plastic products (pigments, carbon black, titanium dioxide, insecticides, calcium carbonate, detergents, PVC, natural rubber, polyethylene, epoxy and polyester powder coatings, etc.).
Mineral industries also use fluidization in drying processes or metallurgical treatments such as limestone calcination, sulfide reduction, chlorination of certain metal oxides, or reduction of iron oxides.
Fluidization: what filtration solutions?
Specific ranges of filter media and materials for fluidization and powder-handling units are available on the market. Three types of materials are preferred: metal powders, metal meshes, and porous plastics.
They can be manufactured in complex shapes (panels, cones, domes, etc.) to be integrated into silo fluidization cones or other installations.
Metal powder
For applications requiring localized fluidization and aeration, or for refurbishing existing silos or hoppers, filter elements made from metal powder are recommended.
They provide a simple, ready-to-use solution for powder handling. Available in various sizes, these aeration units allow low-pressure fluidization air to be introduced into the material either at discharge or just before. They are used, for example, for aerating or drying gypsum or fly ash.
Expert advice
Sintered metal-powder aeration pads can be used at high operating temperatures, up to 600 °C. They also offer excellent corrosion resistance.
Example of using metal-powder aeration panels
Steel mesh
Multilayer stainless-steel mesh fluidization media are available in 316L stainless steel and other alloys (304 stainless steel, Hastelloy®, Inconel®, Monel®). They are generally supplied as flat panels, in single-piece standard dimensions, with no size limitation for plates welded edge to edge.
They can also be delivered in custom dimensions and shapes, and fitted with perforations, end fittings, or rings. Tubes or fluidization cones for hopper bottoms can also be manufactured from steel mesh. Robust and self-supporting, the fabricated shapes generally do not require complex or costly support structures or assembly bands.
Image source: Porvair
Expert advice
For applications requiring a tightly controlled efficiency rating, a high-precision fine filter mesh (down to 2 µm nominal filtration) can be sintered onto the fluidization media. This is particularly useful to reduce clogging, for example when the fluidization gas flow is not constant.
These filter media are particularly well suited to high-temperature environments, up to 540 ºC, and to situations requiring excellent chemical resistance or strong abrasion resistance, such as silo discharge cones, reactors, and fluidized dryers.
It is also possible to design a custom filtration media to meet specific requirements regarding material, strength, flow rate, thickness, micron rating, and operating environment.
Resistant to mechanical abrasion, these filter media do not shed fibers. Easy to clean, they can be sterilized for use in the food and pharmaceutical industries.
Polymer
Polymer-based supports are manufactured from HDPE (high-density polyethylene) or PP (polypropylene) materials suitable for food and pharmaceutical applications. Their uniform pore structure provides a homogeneous fluidization surface.
They are self-supporting due to their semi-rigid nature, reducing the need for external support structures such as those required for mesh and felt media. They can be supplied as ready-to-install fluidization cone liners, or as flat sheets for use as tank linings or for further processing by the end user.
While they can be easily cleaned for reuse, their low cost — compared to stainless steel — also makes frequent replacement a viable option when preferred over cleaning. Naturally hydrophobic, polymer media release very few extractables and do not shed fibers.
Expert advice
Polymers are generally the most economical choice for working environments with temperatures between –70 and 80 °C.
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