Take a gas-particle fluidized bed into space. Accelerate it toward the gas outlet end and the large particles move against the gas flow, right through the dispersed particles toward the gas inlet.
Now shut down the gas pump and accelerate the system toward the gas inlet and the small particles pack against the gas outlet. The large particles cannot make it past the packed bed.
Now cycle throught the above procedure again and again.
How is this different that shaking a can of mixed nuts on earth?
Gravity obviously plays a role in the BNE. The large particles move against the force of gravity.
The exact same separation, however, is possible with zero gravity using gas particle fluidization.
This is another proof that the BNE is merely a transport problem and a simple one at that.
A packed bed has an almost infinite resistance or "viscosity" and can accelerate the big particle to the other end of the canister. (Somehow everyone overlooked this basic fact.)
A fluidized or otherwise dispersed bed has a relatively low resistance or "viscosity" and cannot stop the big particle from moving to the other end of the canister.
That's the BNE in a nutshell. Not even 4 dozen words.
Understanding the BNE doesn't help solve the original problem of separation of particle mixtures, i. e., urea from P2O5 in fertilizers, but that doesn't mean the concept of transport property "switching" is useless either.
It works for heat as well as momentum transfer.
For example, a bed of packed glass beads has a very high resistance to heat transfer. A hot coil inside a packed bed is very well insulated from the outside.
It's a great way to store heat.
But just flick a switch and turn on a blower and the fluidized bed will move orders of magnitude more heat from the coil than the [mechanical] energy necessary for the blower.
It's the fastest most compact way to heat air.
The useful spinoff is the low cost, convenience and efficiency storing and retrieving heat in paraffins.
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