Effect of Melt Urea Level on VibroPriller Performance

That is an interesting question. I believe the following: liquid (melt) urea having various levels in the container will exert more or less pressure on the outlet nozzle(s). The standing waves phase does not particularly matter, since we assume "constant" velocity of flow at the nozzle(s), constant visosity, etc. All that matters to the induction of droplet separation is the applied frequency producing oscillations in sound pressure (time dependent pressure variation), oscillations within the droplets that result in a reproducible closing of the meniscus, etc. What does matter is the average pressure, and I can see two or three things happening:

(1) higher average pressure (liquid level) will result in the system operating at a different point on the (non-linear) dynamic curve. If the system at all average pressures is operating in the single valued portion of the curve, then small changes in prill size (slightly larger) is expected, and if the level fluctuates, then the size distribution will be wider at the 98% control band. However if the average pressure is already near a dynamice instability point there may be a sudden change to a bifurcation of droplet size, one larger, one smaller. Even higher flows will result in a much broader distribution of random sizes, and so on it goes until one sees a three-cycle size distrubution.

(2) if the acoustic coupling is affected by depth of immersion (assuming the acoustic probe does not move), then perhaps there will be more sound pressure, and this affects tuning. You could experiment with sound pressure (decibel level) to see the effects on size distribution yourself, and this all is controlled by things like nozzle geometry and other factors, but I hypotheize in a high limit of sound pressure, odd things like sub-droplets may begin to form.

(3) vessel level must not be allowed to have any effect on temperature. If temperature changes widely, then the density and surface tension change, affecting droplet size, and also the acoustic speed changes somewhat with temperature in this manner, resulting in possible changes in acoustic pressure that are not desirable.

Me experience with droplet (prill) operations had to do with rare earth metal salts molten and being chilled with liquid nitrogen, the acoustic device was a glass rod coupled through the glassware in contact with a capillary nozzle. The materials had a completely different set of molten properties than what you are working with, but could be controlled within 1% easily for items to be dosed into high pressure metal halide lamps (stadium, TV studio lighting), the gent that invented this did so back during WWII, I think, and it was his company, quite a wonderful group of people to work with.

I could not correct one spelling error I found right after posting: I meant to say viscosity.

One additional point: If the nozzles are designed such that flow barely occurs at each nozzle (capillary), then the back-pressure should help control the dynamics of droplet formation so that less effect of level is seen, and more dependence on acoustic frequency and pressure, thus assuring uniformity of run. This should be done on a lab or pilot scale test initially to allow for better modeling of the larger production system.

if not up for re-engineering, then get some good level controls, even something simple like an over-flow weir device to keep constant head pressure, and pump recycle the part of the melt that overflows.