Deethaniser simulation 3

[fifi121]

Hello,

I am trying to simulate a Deethaniser to give 95 mol % ethane in the OVHD and minimum Propane. It is not converging with recovery spec or vapor fraction spec in the OVHD. Can anyone give any idea how to converge the column and which specs to be used. The Feed stream contains 9.5% Ethane, 0.5% Methane, 13.5% Propane, 18% Butane, and the rest higher HCs i.e. Pentane etc.

Thanks

 

[ngb23]

What simulation software are you using?

Besides the feed stream and OH stream, what else is specified within the system? I would suggest doing a DOF analysis on the system after you determine all the parameters you have specified (depending on the software, this may be done for you).

NGB

 

[fifi121]

To start with, only the feed stream is completely defined and the degree of freedom is 2 therefore I add two specs to make degree of freedom 0. I have tried with adding various specs as a)Ethane mol fraction in OVHD and Propane recovery in the bottoms b) Ethane recovery in the OVHD and Propane recovery in the bottoms. etc. I have tried with various specs, I get zero degree of freedom but it does not converge. Therefore I think there may be some recommended combination of specs to meet the desired product. If anyone has done it before would know it.

 

[ngb23]

Try specifying OH temp instead of 95 mol% ethane in the OH. Then toggle the OH temp to manually target the 95 mol% ethane. Also, increasing the reflux rate may help to increase the mol% ethane in the OH.

 

[fifi121]

speciying OVHD temp. does not work. Actually by specifying Ethane recovery and Reflux ratio specs, the specs are converged but the column flowsheet is not converged. Dono why is that...

 

[mfelzien]

fifi121,

Not sure if this will help, but I'll give it a shot. For me it was insightful to understand that HYSYS models REAL things (not only that it is retardedly/brilliantly dedicated to that idea). That means if you want a model to actually work in reality you need to know some reasonable variables to "define" it. Say, feed rate of stuff going in, condenser duty, re-boiler duty, tray efficiency. Given those physical variables a real column is defined. In other-words it could be constructed in the real world. I believe mathematically you could define several linearly independent equations but they don't specify a physically real case.

Perhaps thinking in terms of a real physical situation will give you insights into why the column is not converging. Hope this helps a bit.

Yours,

Mike F

 

[dcasto]

it will never solve. Look at you feed stream and think about what you asked for.

 

[ngb23]

Can't believe I didn't see this at first

9.5% ethane feed + 0.5% methane + 13.5% propane + remaining C4+s feed

Since the cut between ethane and propane cannot be made perfectly, even in a simulation, the overhead mol% ethane can never get to 95%. (Think if it were ONLY ethane and methane, ethane would be 95% and methane 5% based on the feed).

Wow, I feel dumb.

 

[UmeshMathur]

fifi121:

Since you cannot condense methane, you must have an overhead vapor leaving the accumulator. The overhead liquid distillate would need to be a draw from the accumulator.

I ran a simple simulation and have attached the report. This simulation contains many assumptions, such as feed rate (1000 lbmole/h), and feed thermal condition (saturated liquid). The tower overhead pressure was 300 psia, with a profile supplied as usual for condenser and tower DP.

Then, I specified the overhead distillate liquid impurities at 5%, and a reasonable bottoms C2 leakage (2%), all on a mole basis.

The calculated reflux ratio was 14.3 and the condenser duty was 5.49 MMBTU/h.

Hope that suffices.

Umesh Mathur

 

[UmeshMathur]

Sorry, the report did not get uploaded properly. Here it is:

 

[UmeshMathur]

Well, this time I uploaded it to Engineering.com

Hope it shows up for you.

Umesh Mathur

 

[dcasto]

umesh, yes you can and you do condense the methane in the overhead. As I see it, you will take the vapours from a partial condensor. Chill them until you condense a mixture of liquids with only 5% methane in it. When you do this, you will discard to the flare 8% of your overhead product, it works, but is that what you want to do.

Meanwhile your 300 psia column has an overhead temp of 16 F, if you raise the pressure to 450, its a 45 F chiller.

Since we still can not see your runs I'm guessing on the trays and like.

 

[MortenA]

Actually this looks a lot like homework That feed spec and the requirement for excactly 95% ethane i OVHD....

Best regardcs

Morten