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Azeotropic Column Not Converging When Changing Feed Stage


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#1 Chemengbabe

Chemengbabe

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Posted 18 May 2017 - 02:57 PM

I am simulating a a 1,2, Ethylene Dichloride Drying (distillation column) which forms an EDC- Water Azeotrope. I got the system to conversion but to optimise it I need to find the feed stage but everytime I change the feed stage it stops working. I have tried changing the tolerence, damping factor and the solving method. This all worked when trying to converge the original column but not now. The specs have to be fixed at 32 degrees celsius at the top and 4ppm of water at the bottom. I am sure these specs work since the same column was simulated on Chemcad. Some times it says fatal error, zero solutions and other times it just runs for ages and doesn't converge. Help!

 

 



#2 Bobby Strain

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Posted 18 May 2017 - 04:10 PM

Relax the specifications until it converges. Then gradually change them, one at a time. This sometimes works. Also try a spec different from temperature at the top.

 

Bobby



#3 Saml

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Posted 19 May 2017 - 10:19 AM

I've seen this when you formally have a degree of freedom but the margin is very low because you are close to a limit .

 

Take for example a dearator: water liquid + water vapor + dissolved oxygen.

Components: 2

Phases: 2

 

F = C-P+2 = 2. For a given composition, you should be able to set pressure and temperature, but at ppm oxygen concentrations, the system behaves almost as pure water. So once you set one variable (P or T), the margin f on the other is very low. If you set the pressure, the margin on temperature (between the gas phase disappearing and the liquid phase disappearing) is very small. The simulator  will let you set an arbitrary set of pressure and temperature because the degree of freedom checks, but the solution in the L-V equilibrium zone is not feasible.

 

Same happens when you are close to an azeotrope. The exact azeotropic point reduces a degree of freedom (you cannot specify pressure, temperature, and being at the azeotrope simultanously) , but when you are close to it, while formally you have the degree of freedom,  the margin is low and you may get into an unfeasible condition for L-V equilibrium with just a few tenths of a degree.

 

I your case you are basically trying to set pressure and temperature at the top of the column in a region near the azeotrope.

 

So, try with actual variables like distillate rate and reflux ratio. Then use internal specs or external vary to get to the desired ouput (ppm water in the bottom, máximum EDC at the top).






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