Pneumatic Modulation Technology

Knowledge Gap  --  Much of the research into this technology is decades old.  Firstly, the concept that vibration help powders flow better is hardly a ground breaking announcement.  The concept that vibration helps powders pack together more tightly is similarly well known.  Understanding why the same action can be used to achieve opposing goals does not get much thought. 

Targeted Application  --  There are many devices on the market that can be attached to the walls of a steel silo to vibrate the silo's contents or to emit pulsed air blasts and while they may have high unit energy consumption these devices have proven to be very effective.  In fact if your problems are isolated to small steel silos, then these alternative devices are probably more cost effective.  Our interest is focused on large silos, domes and storage bunkers that are constructed of concrete, which typically range in diameter from 20 ft to 200ft.  Here the solution affecting a given volume of powder needs to be cost effective** when scaled by orders of magnitude and needs to be physically compatible with the containers concrete walls which may range from a few inches of composite on a dome to 3 ft thick on a bunker.

**   Note: High pressure air blasting can be very expensive on per sqr.ft basis.  e.g. using an array of air blasters at 2.5 cf/pulse  once every 10 minutes @ 100psi on a 200ft sqr bunker, placed on an 24" grid, consumes ~ $1,400,000/Yr of energy.  Not surprisingly, few web sites address this large scale cost issue because on the small scale the operating costs are hidden in the existing plant-air operating costs.  Few people stop to calculated the high cost of 100+psi air, the use of which can swamp the cost of these devices.
The same facility using 3 TEI Fluidizers 24/7 would consume only $500/Yr to run the fluidizers and conserve ~$115,000/Yr of power on reduced blower duty.

Research  --  Our PM Technology drawn on several effects.  Firstly, many fine powders behave as a thixotropic liquid (specifically a Bingham type fluids), where there apparent viscosity is a function of its recent shear history.  This is much the same as ketchup, in that it behaves as a gel when left to sit, and when the liquid is sheared by a sharp whack to the bottom of the bottle it momentarily thins to water an may splash out every-where, then it quickly thickens to a more gel like consistency and resists further pouring. 

Many people attribute the similar behavior in powders to the breaking of cohesive bonds that exist between individual particles.  Thus, vibrating powders while simultaneously aerating them achieves a drop in viscosity and prevents the particles from packing together. While this helps explain why the working angle of air-slides can be greatly reduced, allowing airslide-chutes to reach twice as far for a given drop; it however; does not fully explain the ability for PM Technology to fluidize a bed of particles using only 50% of the air flow normally required.

Our research of PM and non-PM fluidized beds revealed several interesting phenomena when one's fingers were used to feel the passage of air in its journey from the distributor-fabric through a couple of feet of powder to the surface.  With non-PM aeration it was possible to feel air bubbles and note the coalescence to courser bubble voids towards the surface (these can also be seen using a clear wall).  In some cases it was possible to feel these bubbles organizing themselves to form rat-holes.  The introduction of  low frequency PM aeration showed a high efficiency at breaking up any macro-structures caused by bubble organization such as rat-holes and channeling.  Whereas the use of higher frequency PM aeration has a very different 'feel', as the bubbles fell more like the  fizz of champagne than a course gurgle.   The capacity of PM aeration to either form finer aeration bubbles or to hinder their coalescence will result in greater 'gas hold up' and greater air-to-particle contact, which helps explain the greatly reduced air flow demands when using PM Technology. 

Our work also correlates well with the research of others (Ref. 1) in the use of sonication to increase the efficiency of bubbling through a variety of fluids.  Who concluded that; "by subjecting the liquid phase to low frequency vibrations the bubbles are shown to suffer significant breakage, induced by resonance. When the vibration is properly tuned, the interfacial area is found to increase by a factor of 1.8 to 2.4, depending on the properties of the liquid.  Resonance-induced bubble breakage phenomena has a great potential for improving the rates of chemical processes involving fast reactions, with minimal energy input. "

The influence of the vibration frequency on the number of bubbles issuing from the orifice (@ constant flow rate ) can be perceived through the images shown below (Ref. 1)  "Using this method, the mass transfer and the gas hold up in a bubble column can be increased considerably".
 

Air - water system
gas flow rate: 1.65x10-7 m3/s

0 Hz:

 

30 Hz:

 

200 Hz:

 

 
Air - paraffin oil (EDM oil 110 - Shell)
gas flow rate: 1.79x10-7 m3/s

0 Hz:

 

38 Hz:

 

130 Hz:

 

200 Hz:

 

All experiments above use the nozzle injection (experimental setup 1) system shown below.

Different injection systems:
System: air - paraffin oil (EDM oil 110 - Shell)
 

Injection system 1
gas flow rate: 1.79x10-7 m3/s

0 Hz:

 

200 Hz:

 

 

Injection system 2
gas flow rate: 5.33x10-6 m3/s

0 Hz:

 

225 Hz:

 

 

    

Experimental setup 1:

upward oriented single injection nozzle (pipette)

Experimental setup 2:

downward oriented single injection tube

 

Problems in Eden  --  Unfortunately, the laws of physics are  many and the issue of sonic wave reflection is one of the realities that need to be mastered before implementing successful PM Technology in an airslide environment.  Wave reflection has two adverse effects that become evident after a short time, and this relates to the reinforcement and cancellation that occurs when the wave troughs and peaks collide.   When they reinforce the airslide develops zones of super-fluidization that results in lowering the bed's viscosity in that area, and when they cancel the reverse occurs.  The air will of course tend to migrate to any path of least resistance.  The end result is that single frequency PM aeration generates only short-term gains that soon reverse and if continued will lead to the formation of large rat-holes. 
But we found a solution !

Success  --  Our PM Technology recognizes the different benefits derived from both high and low frequencies and that combining a complex wave structure with constantly variable frequencies the benefits on airslide efficiency can continue indefinitely.

 

 

References:

1]    Utilisation of bubble resonance phenomena to improve gas-liquid contacting
R. Krishna, J. Ellenberger, M.I. Urseanu,  Department of Chemical Engineering, University of Amsterdam
and, F.J. Keil,  Technical University of Hamburg-Harburg