June 2, 2016
Nearly everyone of my generation will assume that an article entitled, "fluidized beds," would be about water beds. Water beds were popular in the late 1970s through the early 1990s, but they are presently rare. Sales of waterbeds peaked at about 22% of all mattress sales in the United States in 1987.
The modern water bed was invented by Charles Prior Hall, who patented his invention in 1971 (see figure). An essential element of his design was an electric heater designed to heat the water to a temperature between 85 and 105°C. As his patent states, "This heating obviates the cold, clammy feeling normally inherent in fluid filled envelopes."
As can be imagined, water beds have had a long history. Scottish physician, Neil Arnott, invented a water bed in 1832 to aid in the treatment of a woman having life-threatening bedsores. Arnott's bed was of simple construction, consisting of a tub of water, covered with a rubber-impregnated canvas, and overlaid with light bedding. Arnott was also a popularizer of physics, having written a general interest book on optics and astronomy in 1864. He also funded a scholarship for experimental physics at the university of London.
Science fiction author, Robert A. Heinlein designed a waterbed after being bedridden with pulmonary tuberculosis in 1934. His design, as he recalled in his collection of essays and short stories, Expanded Universe, tackled many of the engineering issues for such a bed, including temperature control, leak mitigation, and floor loading, but he never built one. A waterbed is mentioned in his 1961 novel, Stranger in a Strange Land.
While all this is enlightening, a fluidized bed is a technology of a different sort. In its usual form, it's a container filled with a granular material through which a pressurized gas is pumped. A liquid stream can be used, also. Under proper gas flow conditions, the granular material, which is fluidized, will behave as if it were a fluid; that is, it can be easily poured and pumped.
It's easy to see how fludization can occur. As gas flow is increased, the particles will start to levitate. At a critical pressure, the levitating force on a particle will just equal its downward gravitational force, so the particle will be suspended and will be free to move as a fluid.
Aside from the fact that fluidized solids are easily conveyed, fluidized beds are technologically important for two other reasons; namely, there is excellent heat transfer from a heat source or heat exchanger immersed in the bed, and the mixing in a fluidized bed keeps its temperature uniform throughout. The intimate contact between the granular particles and the gas or liquid medium provides excellent heat transfer across the bed, notably to a heat source or a heat exchanger.
Since the media in a fluidized bed behaves just like a liquid, objects with a density higher than that of the bed will sink, and lower density objects will float. For granular particles fluidized with air, the bed density is quite nearly the product of the volume fraction of the granular medium and its density.
While investigating such density segregation in a fluidized bed, a team of Japanese and Australian researchers discovered an hitherto unknown phenomenon. The interdisciplinary team of two chemists, a mechanical engineer, and a mathematician are from Okayama University (Okayama, Japan), Osaka University, (Osaka, Japan), the University of Melbourne (Parkville, Australia), and the Swinburne University of Technology (Hawthorn, Australia). They discovered that a bed in its apparently "fixed bed" regime at low gas flow rates still has the property that dense objects sink.[3-4]
It appears that the presence of spheres, such as the one shown above, locally fluidizes a supposedly rigid bed, allowing the formation of voids and percolation bubbles that enable the spheres to sink at a rate slower than for a fluidized bed, but deeper than expected. The final depth to which a sphere sinks varied with the sphere density and air velocity, and spheres with smaller densities sank more deeply than denser spheres when the sphere density was close to the powder bed density (see graph).
Although the process is slow, it has the ability to segregate particles by density within the fixed bed.[3-4] This might be an advantage over a standard fluidized bed, which can just sort floating and sinking particles. As shown in the graph, you need to be within 85% of the air velocity required for fluidization to see this effect.
|Fig.1 U.S. Patent No. 3,585,356, "Liquid support for human bodies," by Charles P Hall, June 15, 1971. (Via Google Patents.)|
- Charles P Hall, "Liquid support for human bodies," U.S. Patent No. 3,585,356, June 15, 1971.
- Lives of the fellows: Munk's Roll : Volume III : Neil Arnott, Royal College of Physicians.
- Jun Oshitani, Toshiki Sasaki, Takuya Tsuji, Kyohei Higashida, and Derek Y. C. Chan, "Anomalous Sinking of Spheres due to Local Fluidization of Apparently Fixed Powder Beds," Phys. Rev. Lett., vol. 116, Document No. 068001 (February 11, 2016), DOI:http://dx.doi.org/10.1103/PhysRevLett.116.068001.
- Anomalous sinking of spheres in apparently fixed powder beds discovered, Osaka University Press Release no. 2016-2-1, May 2, 2016.
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