5 replies to this topic

[erin512]

Hi, was just wondering if anyone knows where I'm going wrong in my reactive distillation simulation with the following errors: Thanks!

 

*** SEVERE ERROR

MATERIAL AND ENERGY BALANCES FAILED TO CONVERGE: CHECK COL-SPECS

OR SUPPLY BETTER TEMPERATURE AND COMPOSITION ESTIMATES.

 

*** SEVERE ERROR

THE FOLLOWING STAGES DRIED UP:

7 10 11 15

A LIMIT OF 0.10000E-04 * SUM OF FEEDS WAS IMPOSED ON THE FLOW RATES.

 

 

[thorium90]

"THE FOLLOWING STAGES DRIED UP", I've never seen that one, but I suppose it means what it says...

 

Perhaps you can zip it up and post here so we can look at it? Its kinda hard to tell which module created the problem just by looking at the control panel errors. Im guessing some recycle stream problem? But its better to look at the whole file.

[Pilesar]

There MUST be a vapor and liquid on each stage for the calculations to converge. Computer distillation algorithms include overall and componential mass balances on each stage in addition to enthalpy balances. The algorithm was unable to solve those equations while also meeting the requested column specifications. Your column specifications are likely such that a mass balance is impossible. Different initial estimates might help the computation, but the column will never converge if the specs are impossible so check those first. Get out a pencil and paper and make the separation yourself to find out what the expected products will be. If you are over-specified using product purity specs, change them to something simpler like a product flow rate and reflux ratio. The reaction adds additional complexity so you may need to simplify that as well. 

[abhi_agrawa]

Erin,

 

I have seen this error several times with new engineers. The most common cause that I have found is incorrect column specifications. The easiest specifications to ensure column convergence would be to specify reflux ratio and one of the product rate. After a converged run, try to change the specifications to reach the desired point.

 

Also make sure that you do a material balance by hand to make sure that your specifications are realistic.

 

abhishek

[erin512]

Thank you all for your reply. The system is a reactive distillation column to produce sodium methylate:

NaOH + CH3OH → NaOCH3 + H2O

There is about 30 times the amount of methanol added to the amount of NaOH to ensure very high conversion. NaOH solution coming fromtop of column, MeOH vapour coming from bottom of column. 

I ran the simulation without the reaction, just as a methanol/water stripper. I also ran the reaction on its own in a reactor. There were no problems and I got expected results. When I put them together in a radfrac block with reactive distillation reaction, I had the above errors. I noticed in the column profile that I am getting much higher temperatures (200C) in the column than I should be getting. This is "drying up" the stages of the liquid I believe. I don't know why the temperatures are so high, it should be nearer to 75C. 

[abhi_agrawa]

Erin,

 

From you description, it seems that you are trying to model a reactive absorption column. The column has vapor feed at the bottom (Methanol) and liquid is fed at the top (Sodium Hydroxide).  Is this reaction exothermic? At what temperature are you feeding Methanol ad Sodium Hydroxide? Is it possible to lower this temperature and see the effect?

 

What thermodynamic method are you using to model your flowsheet? Are you using the Absorber = Yes option in Radfrac block?

 

Just for trouble shooting, if you further increase Methanol by a factor of 10, what happens?

 

I hope that answers to some of the above question will help in solving you problem.

 

Abhisek