wow, this is a huge question. You need to get a copy of Kister's Distillation book and scour the sections on tray efficiency, performance and on and on.
Depending on what the column is doing, you may need as much a 3X as many trays as you've estimated. Start by googling tray efficiency and then keep going. Read papers from the Distillation Group - there's a ton of info out there. Check GPSA, call a vendor, even better, try modelling the same tower to check actual efficiency.
In any case, I would rely on Hysys outputs - but only at conceptual stage of the project. I would never establish my proposed design based on the results from Hysys/Tray sizing, or on any other commercially available tool.
For designing/revamping distillation columns, one needs to go to proper address: tray/packing vendors, such as Sulzer, Koch-Glitsch, ACS, etc.
"We don't believe things because they are true, things are true because we believe them."
