3 replies to this topic

[Ankit Sharma]

Hello all,

I am working on simulating a process for a diluent recovery unit (DRU) which involves using a distillation tower.

I am using petrofrac model in aspenplus and I can get my simulation to converge with 20 actual stages. Now, I am starting to size my column, however, I haven't taken into account the tray efficiencies during my simulation..and using several CHEM ENG books (treybal, etc.) I have found that typically the overall tray efficiency value is 0.8 (seive) for hydrocarbon applications. The feed stage to the column is currently at 10 and I would like to now incorportate tray efficiencies within Aspen so I can properly size the column from within Aspen but I have no idea how to get started on that because it uses murphree efficiencies... I would greatly appreciate if someone could provide me with some reference material or help in this matter.

Please let me know if you require more info.

Ankit

[sheiko]

Hello Ankit,

Overall tray efficiency (Fenske) is the number of theoretical stages minus reboiler and condenser divided by number of actual trays (because when you calculate a tray or packing efficiency from the field, you don't account for any separation stages gained in the condenser or reboiler). After calculating number of theoretical stages add a stage for reboiler and another for condenser to include in simulation (20*0.8 + 2 = 18 theoretical stages). This way you can keep tray efficiency (Fenske, point efficiency, Murphree or whatever) equal to 1 in the simulation.

The rules for the various number of stages to be gained from different configurations of reboilers and condensers can be found in one of Kister's books (distillation design or distillation operation). Both books are excellent.

Hope this helps.

[Ankit Sharma]

Hello sheiko,

Thanks for your quick response..the 20 'actual stages' I mentioned earlier is just the total number of stages I set in my petrofrac distillation model (and it includes the reboiler and condenser)..so then I have 18 trays in Aspen currently with efficiencies all set to 1...and basically I want to take into account the efficiencies of the trays as I have not done that yet.

So then based on your reply if i wanted to incorporate the efficiency in Aspen, I should just find the 'actual' number of stages by => (18/0.8 + 2) ~ 25 stages and put this value in petrofrac with the tray efficiencies still set to 1?

Thanks,

Ankit

Hello Ankit,

Overall tray efficiency (Fenske) is the number of theoretical stages minus reboiler and condenser divided by number of actual trays (because when you calculate a tray or packing efficiency from the field, you don't account for any separation stages gained in the condenser or reboiler). After calculating number of theoretical stages add a stage for reboiler and another for condenser to include in simulation (20*0.8 + 2 = 18 theoretical stages). This way you can keep tray efficiency (Fenske, point efficiency, Murphree or whatever) equal to 1 in the simulation.

The rules for the various number of stages to be gained from different configurations of reboilers and condensers can be found in one of Kister's books (distillation design or distillation operation). Both books are excellent.

Hope this helps.

[sheiko]

Hello sheiko,

Thanks for your quick response..the 20 'actual stages' I mentioned earlier is just the total number of stages I set in my petrofrac distillation model (and it includes the reboiler and condenser)..so then I have 18 trays in Aspen currently with efficiencies all set to 1...and basically I want to take into account the efficiencies of the trays as I have not done that yet.

So then based on your reply if i wanted to incorporate the efficiency in Aspen, I should just find the 'actual' number of stages by => (18/0.8 + 2) ~ 25 stages and put this value in petrofrac with the tray efficiencies still set to 1?

Thanks,

Ankit

Sorry i did not read your query carefully...i thought '20' was the number of actual trays...

As i said, overall efficiency (Fenske efficiency) = Number of theoretical stages (excluding condenser and reboiler) / Number of actual trays.

Thus, in your case, 0.8 = (20-2)/X = 18/X => X = 18/0.8 = 22.5 => 23 actual trays with an overall efficiency of 0.8.
Then a few extra trays are normally added for offload conditions, such as a change in feed composition. Experience for a given service is the best guide for extra trays.

Now, assuming that tray efficiencies do not vary from the stripping section to the rectifying section, your actual feed tray would be tray #12 <= (10-1)/0.8 = 11.25 ('1' represents the condenser).

By the way, i think it should be possible to change the type of efficiency from Murphee to Fenske in Aspen Plus. Try the Help menu.

To finish, remember: if you set the Murphee efficiency of top and bottom stages as less than 1, you will see liquid in overhead stream and vapor in reboiler outlet - which may not happen in practice. That is why it is recommended to use theoretical stages (100% efficiency) rather than playing with actual number of trays and correspondent efficiencies.

N.B.: I also think it is a good idea to check the O’Connell correlation to ensure that there is a general agreement with the value of overall efficiency you found in the litterature (0.8).

Hope this helps.