Lowest and highest stage refer to the bottom and top stages of each PA section, not between PA sections.
Each PA circulation is over 3 or 4 actual trays, which are to be simulated as 2 theoretical stages.
For example: if Kero PA is drawn from theoretical stage 7 then it is returned to theoretical stage 6.
The corresponding PA duty is then removed from stage 6. At least, in PRO/II you would then specify a DUTY on stage 6, I don't know how a PA is best modelled in HYSYS.
Note that by adding an extra stage for each PA you probably need to increase theoretical stages in each section to account for that, as PA stages do not really contribute to fractionation. So kero draw and kero PA draw will not be from stage 7 anymore, but probably from stage 9, due to the 2 extra stages added for top PA and kero PA.
Edited by PingPong, 17 October 2013 - 06:25 AM.