A Deethanizer makes a separation between a top product of C2 (and lighter) and a bottom product of C3 (and heavier).
To simulate such a column you need to specify the C3 content of the top product and the C2 content of the bottom product.
Bottom product spec : ethane 0.045%
So you know already the C2 spec to be used for the bottom product. Question is only: what does % mean in this case? Mol or weight or volume?
That value of 0.045 % sounds very low though. Must it really be that low?
Now you only need to figure out what the C3 in the ethane top product is supposed to be. How much % C3 is in the C2 product during present operation? And how much should it really be to make everybody happy? What is this C2 product to be used for?
You also need to specify the condensor temperature, and of course the condensor pressure and column pressure top to bottom.
The condensor T and p determine how much overhead vent you will get and how much C2 you lose with that. It also determines how much lights dissolve in the liquid ethane top product. These dissolved lights plus some C3 reduce the purity of the liquid ethane top product. You say it is now around 72~74% (mol or weight or volume % ?) but you should really have the lab determine how much C3 is in it.
By increasing the condensor temperature you could increase the liquid ethane top product purity by reducing dissolved lights in it, but in that way you would lose more ethane with the vent. Increasing the ethane product purity is then best done by operating the upstream demethanizer such that it leaves less lights in the deethanizer feed. Note that the feed now contains quite a lot of C1.
Let's not worry too much about the ethane product purity right now.
If you don't know the actual C3 spec for the ethane product then just assume something realistic to get the simulation going. Take for example 0.5 mol% to start with and worry about the correct value later.
For the bottom product specify that 0.045 % C2, or a higher value if allowed.
Specify condensor T and P.
Specify pressure profile from condensor to column bottom.
Specify number of stages.
Select a thermodynamic model.
And that's it really.
Maybe it will help to add some estimates of flowrates, tray loadings, duties. As I never use Hysys, but often another process simulator, I don't know what it takes to get it converged in Hysys.
Could you tell us what the T and P of the condensor are, as well as the T and P of the feed?
You mentioned before that number of stages is 34. Is that theoretical stages or actual? And is that including condensor and reboiler, or not?