2 replies to this topic

[gjt]

I am designing a depropanizer for a HYSYS simulation, and am struggling with the pressures. Background on the problem:

-Column employs reboiler and total condenser
-38 trays, .12 psi loss/tray
-Condenser pressure loss of 10psi
-Overhead pressure= 260 psig
-Reboiler duty = 4Mbtu/hr
-Overhead is 100% Propane
-Feed is 547 bpd propane, 1760 bpd butane, 434 bpd iso-butane, 40 bpd iso-pentane @ 120 Deg. F and 200 psig
-Feed enters Tray 19

Where I run into problems is the feed pressure; HYSYS alternates between complaining that the feed pressure doesn't match that of the stage it enters and that the system is overspecified.

The DOF is 0, so I believe the problem must be with the pressures. I currently have the reboiler pressure at 274.56 psig (Condenser Pressure+Condenser Pressure Loss+38 trays*.12 psi/tray), so obviously the feed pressure is significantly lower than the column's.

However, this is my first time doing computer-aided engineering, so the only way I could think to fix this was to disregard the basis and set the feed to 272.28 psig- which resulted in an overspecification error.

Any recommendations would be greatly appreciated.

[Padmakar Katre]

Dear,
From your input it seems that column feed pressure is low as compared to the prevailing pressure at the feed stage. Normally I haven't seen any issue weather the the pressure is low or high for your feed till you don't switch from steady-state to dynamics.
In steady-state simulation Hysys will just give warning that the feed enters below stage pressure but you can run the column by ignoring the warning. But I will suggest you to give pressure of the feed at least equal to or a slight higher pressure than the prevailing pressure at feed tray. Now your one problem is solved. The another problem which I see in your simulation input is specification which you are giving i.e. Prpane Purity in Ovhd is 100% which you could not specify or column solver will not be able meet this specification. In your case your tightening the Column Solver by giving the purity specification and providing the column reboiler duty. Better you make either propane purity spec or reflux ratio as an acyive spec and another as just an estimate. Give the purity of propane as 99.99% and not 100%.
As you have the total condenser and no side stream you will have 2 active specifications (I am not sure but seems to be as one out of three will exhausted againest the specified reboiler duty keeping 2 active specs to define.)
Now you run the column it should work. Still you need further assistance please let me know I could give you the procedure to configure you simulation here in this forum.

[engprocess]

take a feed pressure as 300 psig et run your simulation , don,t forgot to correct the propane purity to 99.99%.