Rob
Ansys Employee

I would tend to use Eulerian for risers, not least because of the parcel loading. Or, more recently, I'd use Ansys Rocky as it's a DEM code: Fluent handles the flow and Rocky the particles. 

If you use 22R2 the DDPM model has had some work done to help with higher volume fractions. I also suggest increasing the max number of steps on the DPM panel. If you're losing that many parcels as incomplete you have a lot of unaccounted mass to worry about: I assume you are tracking the particles as transient?