I would treat it like water, or non-cohesive soil perhaps... A Manuf can probably provide the unit density, or even get the weight of a bag and the volume of contents. Sorry never worked with those before, had to do a google search just to see what they are exactly.
JRGse:
Do the pellets clump up and stick together when they get wet? What do they weigh, lbs./cu.ft.? Dump a bag or a cubic yard of the pellets on a slab, what is the angle of repose? You should treat these like any fluid/granular material solid, product in a bin or silo or bottom dump truck or railcar. I suspect it flows pretty well as long as you keep it dry, and you could have a kinda gluey mess if it gets damp. Dig into a stored pile of this, at the plant, what was the angle of repose of that pile, and do the pellets flow to fill in the scooped out hole? What exactly are you trying to do, trying to determine. How much dust does the product leave behind during the handling, as this relates to clogging up hopper gates, racks on truck gates, etc. Various Ag. organizations, local Ag. Uni. or the pellet manufacturer might have most of this info. Otherwise, the storage and handling ought to be about like any other dried grain product.
In addition to dhengr's comments, investigate whether these pellets expand when they get moist, perhaps even merely when their environment changes from dry-air to humid-air.
Some time back, I took a 2-day class in the design of bins & hoppers. It was pointed out there that in a lot of cases, you'd just have to do testing of the material if you wanted any reasonably close numbers for design properties.
One idea that can be useful for designing smaller silos and the like- under normal silo flow, the product exerts both an outward pressure force and a vertical drag force on the silo walls. As a vastly simplified approach, you can treat the stuff like a liquid to evaluate the outward pressure, then assume the entire contents is supported by friction to get the drag. That should be conservative, and if it doesn't lead to a lot of material waste in the design, there's not a lot of motivation to refine those assumptions.
