Ray Cocco, Particulate Solid Research Inc., says:
The onset of bubbling is commonly called the minimum bubbling velocity (umb). If we have a packed but unconstrained bed of powder and begin a flow of gas through that powder with increasing superficial gas velocity, at some point the gas has enough drag force or air friction to fully suspend the powder. This is called minimum bubbling fluidization, and the superficial gas velocity that this occurs at is the minimum fluidization velocity (umf).
For Geldart-classified group A powders or powders that are between approximately 30 and 150 microns in size as defined by the Geldart Classification Regime Model, the onset of fluidization results in smooth, homogenous fluidization. There are no bubbles. Only by further increasing the gas velocity do bubbles form. Thus, the minimum fluidization velocity is less than the minimum bubbling velocity.
For larger particles such as those characterized as Geldart-classified group B or D, the bubbling starts at the same time as when the bed becomes fluidized. In other words, the minimum fluidization velocity is similar to the minimum bubbling velocity. To hypothesize why the minimum bubbling velocity is different for the newest batch, it’s best to know if you have a Geldart-classified group A or B/C powder. As a sidenote, Geldart-classified group C powders are smaller and more cohesive than Geldart-classified group A powders and tend to channel rather than form bubbles. Channeling is when the gas moves through the bed as a continuous, axial chain of localized bubbles.
If you have Geldart-classified group B or D powders, the difference in the minimum bubbling velocity is due to changes in the emulsion phase of the fluidized bed or the nonbubbling part. The onset of bubbles is the result of this emulsion phase not being able hold any additional gas. As a result, the additional gas leaves the bed in the form of bubbles. Therefore, the differences in your batches are related to the differences in the particles holding onto the gas.
In one case, the differences in your batches can be due to particle density differences where heavier particles may show bubbles earlier than the lighter particles. Particle size also has the same effect. Larger particles will have a higher minimum bubbling velocity and will also probably have larger bubbles.
What if the particle density and size are the same? In this case, you’ll need to look closer at the individual particles. The best way to look at this is from an aerodynamic perspective or drag-force perspective.
Particles can be influenced by aerodynamics much like larger objects. For example, automobiles have been made more aerodynamic through redesign over time. Being more aerodynamic means the vehicle experiences less drag when traveling against the wind. The shape and smoothness of the automobile allow the air (gas) to better slip past the vehicle because there’s less resistance.
In a fluidized bed, a smooth particle’s relationship to air is the same. The more aerodynamic the particle, the less likely it can hold the gas in the emulsion phase. If the particles hold less gas, the emulsion will hold less gas, and bubbles will form at lower-than-expected gas velocities. The gas has to go somewhere, and if it isn’t in the emulsion, it will be as bubbles.
So, what makes a particle less aerodynamic? That would be the particle’s shape and roughness. The less spherical the particles or the flatter the particle’s shape, the more drag or the more hold that particle has on the gas. The same is true with surface roughness although this isn’t as straightforward. Generally, the rougher the surface, the more hold that particle will have on the gas. The more hold on the gas, the more that gas stays in the emulsion, and the onset of bubbles with increasing gas velocity is delayed.
If the density and particle size are similar in the two batches you’re comparing, I’d use a microscope to examine the particle shape and surface roughness in each batch to find any inconsistencies. Another method would be to put each batch in a sealed packed bed where the pressure drop across the bed can be accurately measured against the flow of gas through the bed. The sample with the fewer spherical particles, more rough particle surfaces, or both will have a higher pressure drop, because these particle characteristics mean that the particles will have a better “hold” on the gas.
For Geldart-classified group A powders, this all becomes more complicated. The conditions noted above still apply, however, the characteristic of the bed itself is also a factor. Geldart-classified group A powders have a lower permeability than group B or D powders. Thus, it isn’t just about holding onto the gas, it’s about introducing the gas into the bed, itself, for these powders. This is where fines need to be considered. Fines are typically defined as particles smaller than 44 microns or 325 mesh. The fine particles have a much higher surface-to-volume ratio, and the drag force is a surface phenomenon. Thus, on a weight basis, fines will drag much more gas into the bed than larger particles will. In addition, fines tend to move with the gas (Stokes flow) whereas larger particles tend to be less directed by the gas.
Yet, would having 5 weight-percentage (wt%) fines in your batch be too small of an amount to do anything to the bed behavior? The answer to this question has to do with the differences in the particle size distribution based on weight and differences in the number of particles. Having a particle size distribution with 5 wt% fines could actually mean having a distribution of 30 (num%) of particles on a per number basis. On a per mass basis, you’ll have many more fine particles than the weight-percentage number indicates. Though with a fluidized bed, the number of particles matters more for Geldart-classified group A powders. So, a little bit of fines on a weight basis results in many individual particles dragging the gas into the bed.
In fact, you can see this behavior for yourself by adding a little bit of fines to a fluidized bed of Geldart-classified group A powder. Once added, the bed density will go down, as evidenced from the bed height going up. Add enough fines and the bed height could easily double. This means the emulsion is holding onto more gas, and the onset of bubbles will be delayed.
Thus, for Geldart-classified group A powders, the differences in your fines levels between the batches could also be a factor. Geldart-classified group B powder is much less sensitive to the level of fines, and group D powders just don’t care and the fines just end up being blown out.
In summary, if you’re concerned about why the bubbling behavior is different between your two batches, I recommend that you first check the particle size and size distribution as well as the particle density (or tapped bulk density, which can be found using a procedure defined in ASTM 7481-18 Standard Test Methods for Determining Loose and Tapped Bulk Densities of Powders Using a Graduated Cylinder). After doing so, you may then want to look at the particles under a microscope to identify differences in shape or surface roughness (or see if the pressure drop across a packed bed is different). For Geldart-classified group A powders, I recommend that you also check the fines levels.
Ray Cocco is the president at Particulate Solid Research Inc.