Hi All
Can anyone let me know will the bed hydrodynamics vary if I inject bubbles through a central nozzle at much higher pressure compared to the background fluidizing gas pressure?
The reason for asking this is because for the 2D bed that I am dealing with (1000mm x 250mm x 12mm) with a central jet of 4.5 mm, I am finding hard to inject bubbles at near Umf (i.e., 10-40 Umf). I could do so only when I keep the injection pressure at 5-6 times the pressure that I use to maintain the bed at uniform fluidization.
Regards
Vishwanath
Vishwanath,
The pressure just above the distributor will be equal to the ambient pressure (or pressure in the freeboard) plus the pressure drop Δp across the bed.
Ignoring pressure drop at the distributor, in order to inject the bubble, the injection pressure has to be equal to that pressure of the fluidization air. Actually, a little higher, since the pressure inside the bubble is always higher than outside it. This pressure difference is what causes the bubble to erupt at the bed surface.
To assist you in understanding the answer more fully, I have attached a sketch.
Thanks Sir. But actually, since I am unable to inject bubble at the same pressure as that of fluidizing air with the flow rates that I am dealing with. Actually, the minimum fluidization flow rate was around found to be 6 lpm (for Alumina powder of mean particle size 160 micron) at a fluidizing air pressure of 1.5 bar. I want to study the bubble characteristics at varied flow rates higher than 6 lpm up to 30 lpm.
Bubble injection is not happening at 1.5 bar especially at near 6 lpm. So, I could observe only at an injection pressure 5.15 bar which is almost 3 times of fluidizing air pressure. I think may it is due to tremendous resistance at the central orifice which is covered with a 56 mic wire mesh.
So, can there be such huge difference between fluidizing air and injection air pressures? Your response would be much appreciated.
You may remember that I qualified my statement with "Ignoring pressure drop at the distributor...."
The theoretical limit is primary air pressure + a delta pressure to account for the resistance in the path of the bubble. It cannot be less. (I point this out because I have seen many students suffer from the misconception that, because of the 'weightlessness' of the bed a pressure is not necessary to inject the bubble.) If you had to use a higher pressure, it will drop to the bed pressure (+ delta p) by equilibrium considerations. If at the orifice exit a higher pressure exists, it is converted to velocity and causes entrainment of the bed in the bubble's wake.
It will be a good idea to study the chapter on bubble dynamics in the new edition of Kunii and Levenspiel. Also, read the original paper of D. Geldart on his classification.
I missed answering your specific question. Yes, it is possible for a large pressure difference to exist between fluidizing air and air for bubble injection. What kind of distributor are you using? What is the thickness of the distributor plate? What is the diameter of the orifice?
Try this simple experiment. Drain the bed from the column and inject only the bubble air. Plot the upstream pressure vs. flow rate. You should be able to get an upward curving (called supralinear) line. The pressure you read for any desired flow rate + bed weight per unit area (when the bed is present) is the injection pressure you need for the study.
Thanks once again sir. I missed your message as I was working in the lab. I am using a sintered porous plate of pore size 10 mic as my distributor plate.
The diameter of the orifice at the centre of the bed bottom is 4.5 mm.
I need to study the bubble characteristics at different bed aspect ratios, injection flow rates and especially under wide ranges of vacuum pressures (down to 30 mbar).
It is not done before by anyone and thus, I need some suggestions from experts like you.
Any way, I will experiment the procedure that you have asked me to do.
Regards
Vishwanath
Dear Raghavan sir,
I understood your answer. The doubt is you are asking me to plot the upstream pressure vs flow rate, but the injection pressure will be fixed and only the flow rate will be varied. Oh! I understood. But I am observing that bubble initiation is happening but it is not able to pass through till the surface. In that case, how will this experiment helps me as I can just feel the air flow through the orifice, but it cannot guarantee for all the way till surface at that pressure and flow rate?
Regards
Vishwanath
The injection pressure is the pressure just before the injection orifice. You are perhaps referring to the air compressor receiver pressure as the "fixed" pressure. Remember that the pressure at injection will be equal to the freeboard pressure + bed weight per unit area (i.e., the [ρ g H] value where H is height of bed and ρ is the effective density of the bed) + pressure drop in the injection orifice. The pressure in the compressor receiver throttles down to this value. To understand this, visualize the analogous situation in an electrical circuit.
Your injection pressure simply cannot be 5.15 bar. If it indeed was so high, you should see the gas punching a hole through the bed, sucking up a lot of the alumina in its wake and producing a spout (a fountain) of particles.
A bubble should get larger as the surrounding bed pressure reduces during the ascent of the bubble. If the bubbles are disappearing before reaching the surface, it is because of the hydrodynamic conditions in the bed. The gas in the bubble is percolating between the particles and dissipating as interstitial gas.
Are you using fused alumina particles? Do they spherical or sand-like under a microscope? Do you have a supply of larger alumina particles? Or if you have (or can obtain) glass ballotini (which are spherical) of say 1 mm, perform the experiment with it. Start with a low bed height and progressively increase the height. Observe what happens. What particles have been used by other researchers? These kind of bubble visualization studies in a 2-D bed are as old as the hills. You should find plenty of information in the literature.
To understand all this, you really should read the background. In addition to Kunii and Levenspiel (II edition), you might also benefit by reading the books by Jack Howard or J.S.M. Botterill.
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