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Aspen Plus Stripping Distillation Column With Ionic Liquids


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#1 Jorge_ChemE

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Posted 30 July 2018 - 07:00 AM

Hi,

 

I am with my final thesis Masters Degree in Chemical Engineering. It consists basically in using a stripping distillation column (a distillation column without condenser) to recover ionic liquids from 3 different streams. The first stream is ionic liquid with aromatic-aliphatic compounds. The stream is at 40 ºC (313 K) and 1 atm. The distillation column model is a radfrac without condenser. My professor's assistant gave me a template with all the ionic liquids since they had researched and pusblished several articles about that, they have a database. He told me to specify a Reboiler Duty, fixing a temperature in the last stage with a Design Specification and with a wide range of variation in Reboiler duty. I did so and this is what I am having in control panel:
 

<< Loading Simulation Engine 13:59:08 Mon Jul 30, 2018>>
 
 
 
->Processing input specifications ...
 
  *   WARNING IN PHYSICAL PROPERTY SYSTEM
      THE FOLLOWING CONVENTIONAL COMPONENTS WERE NOT IN A DATA BANK.
      CHECK DATA BANK NAMES FOR SPELLING.
      CID       DATA BANK NAME
        C105A104  MMIMNTF2
        C105A203  MMIMMECOO
        C105A70B  MMIMMESO4
        C106A101  EMIMBF4
        C106A104  EMIMNTF2
        C106A107  EMIMFEP
        C106A203  EMIMMECOO
 
  Flowsheet Analysis :
 
 COMPUTATION ORDER FOR THE FLOWSHEET:
 T-100
 
->Calculations begin ...
 
 
   Block: T-100    Model: RADFRAC
 
       Convergence iterations:
         OL   ML   IL     Err/Tol
          1    1    7      1191.5
          2    1    5      763.06
  *** SEVERE ERROR
      FORTRAN DIVIDE BY ZERO ENCOUNTERED.
 
          3   21   27      5318.4
          4    1   10      5596.2
          5    1   10      4177.4
          6    1   10      1710.6
          7    1   10     0.98558E+06
          8    1   10      1137.5
          9    1   10      145.43
         10    1   10      66.430
  *** SEVERE ERROR
      THE FOLLOWING STAGES DRIED UP (VAPOR OR LIQUID FLOW APPROACHES 0):
       10
      A LIMIT OF  0.10000E-04 * SUM OF FEEDS WAS IMPOSED ON THE FLOW RATES
      OR A LIMIT OF  0.10000E-04 WAS IMPOSED ON THE STAGE V/L OR L/V RATIO.
 
 
  **  ERROR
      BLOCK T-100 IS NOT IN MASS BALANCE:
      MASS INLET FLOW = 0.47548667E+03, MASS OUTLET FLOW = 0.44608284E+03
      RELATIVE DIFFERENCE = 0.65915620E-01
 
 
->Simulation calculations completed ...
 
 
 
  ***  Warning(s) were issued during Input Translation ***
  ***  Check the Run Status Results for more Information ***
 
 
               ***  Summary of Simulation Errors  ***
 
                   Physical
                   Property         System        Simulation
  Terminal Errors      0               0               0
    Severe Errors      0               0               2
           Errors      0               0               1
         Warnings      0               0               0
 
---------------------------------------------------------------------
 
I have read that this   *** SEVERE ERROR THE FOLLOWING STAGES DRIED UP (VAPOR OR LIQUID FLOW APPROACHES 0) happens with Reboiler Duty specification but I don't have any idea how to solve it and why this is happening. If you have a clue, I would appreciate your help.
 
Thank you in advance.


#2 Pilesar

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Posted 30 July 2018 - 10:22 PM

It seems you are trying to solve a Radfrac distillation unit with two products and a variable reboiler duty. Your first goal should be to get a converged model. Do not worry at this step about the product quality or if it is even close to the 'correct' answer. Without a converged model you will not be able to make further progress. Consider your material balance. Can you calculate approximately what fraction of the feed will be in the overhead product? It does not have to be exact. What is the approximate mass of the overhead stream? Make that the specification and vary the reboiler duty in the model. This should give you a converged run. Then examine the results. Are the components separating in the way you expect? Is your column temperature and pressure profile close? Make adjustments in your spec and review the results again. If you get an unconverged run, then back up to the last working spec and make a different adjustment. This method will require more intervention than your Design Spec method recommended by your prof assistant, but you must have converged models to troubleshoot. 

  So why did your stage dry up? Most likely the software was given impossible specifications. That does not necessarily mean the temperature spec is 'wrong'. It may mean your input streams are incorrect or your thermodynamic system is flawed. Once you have a converged run, you can begin to examine how your model behaves and try to deduce how to fix it. Save your work to a new model before major changes so you can revert to a converged model easier. It is helpful to make a paper log of the changes you make during troubleshooting because it can get confusing. Good luck.



#3 Jorge_ChemE

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Posted 31 July 2018 - 05:07 AM

Thank you very much for your help, Pilesar. Yes, this is a stripping distillation column without a condenser with a feed stream and two products, a gas* (top) and a liquid (bottom) (*please sorry if I am saying wrong concepts since I am not familiar with this column, I am more familiar with distillation columns with condenser and reboiler) The only mistake in my feed stream was that I specified temperature, pressure, vapour fraction, so I deleted the vapour fraction which is 0. After doing what you suggested I have something that may bring you some light on how is wrong with this. I put as specification a bottom rate which is of 99% of purity of ionic liquid. Something is wrong, trully wrong, simulation converged, meets the bottom rate, ionic liquid is in bottom product as should be, but temperature is 1215K, that is barbarity! My components are: n-hexane, n-heptane, n-octane, benzene, toluene, ethylbenzene, m-xylene, MMIMNTF2 (ionic liquid). The feed stream is at 313 K (40 C) and 1 atm, mass flow is 1,71175e+6 kg/h, composition of MMIMNTF2 is 91% in mass (0,91 mass fraction). I put the aromatics as they are present in database, the heaviest has NBP 412 K, I can only see NBPs of ionic liquids in pseudocomponents, the NBP of MMIMNTF2 is 1060K. My thermodynamic model is COSMOSAC as given in template. It should be really easy to separate those components... I can only think about something is missing in my component definition or the thermodynamic model needs something in addition. The problem may be in properties/thermodinamic model...

 

Edited/Update: I even tried with other thermodinamic model even is not suitable for ionic liquids, Peng Robinson, I know is crazy but just to get some figure what is going on with this... I got a temperature of 992 K. Summarizing, something is wrong with energy balances, the program is telling me that I need a crazy temperature to separate ionic liquids from aromatics... I tried with other ionic liquids, the result is the same. I am lost at this point.


Edited by Jorge_ChemE, 31 July 2018 - 05:25 AM.


#4 Pilesar

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Posted 31 July 2018 - 07:10 AM

Modelling work takes some time. Check your inputs carefully and have someone else check them too. Your simulation program will not give correct answers if you ask it the wrong questions. Your feed mass flow seems unreasonably high, so I suspect you were not accurate in defining the problem. Although computer programs have some error checking, they still function on a 'garbage in, garbage out' basis. Defining a component from scratch in AspenPlus is not easy to get right. Put a stream containing just that component into a flash drum and check that it behaves like you expect.

 To specify 'a bottom rate which is of 99% of purity of ionic liquid' would be to use this equation:

  [Feed Rate] X 0.91 / 0.99 = [Bottoms Rate]  So you specified Bottoms Rate as 1,5734e+6 kg/h? That would result in a decent split and should be a good starting point as your converged model.



#5 Jorge_ChemE

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Posted 31 July 2018 - 10:45 AM

Modelling work takes some time. Check your inputs carefully and have someone else check them too. Your simulation program will not give correct answers if you ask it the wrong questions. Your feed mass flow seems unreasonably high, so I suspect you were not accurate in defining the problem. Although computer programs have some error checking, they still function on a 'garbage in, garbage out' basis. Defining a component from scratch in AspenPlus is not easy to get right. Put a stream containing just that component into a flash drum and check that it behaves like you expect.

 To specify 'a bottom rate which is of 99% of purity of ionic liquid' would be to use this equation:

  [Feed Rate] X 0.91 / 0.99 = [Bottoms Rate]  So you specified Bottoms Rate as 1,5734e+6 kg/h? That would result in a decent split and should be a good starting point as your converged model.

 

Yes, I specified 1,5734e+6 kg/h, I know is a reasonable split according to the nature of the components (boiling points) but I got a reboiler temperature of about 1250 K. The mass flow rate was obtained from one of their articles. I will check the mass flow to see if this is the problem. I also check a flash operation to see its behaviour.



#6 Jorge_ChemE

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Posted 31 July 2018 - 12:26 PM

Pilesar, after studying this with a flash, I noticed that I have to operate below 1 atm, I mean, vacuum pressure. Then, I tried with the stripping distillation column and converged with 353 K and 0,2 atm (just a quick trial and error, I will try better tomorrow). My professor told me that I will evaluate to recover ionic liquids in just a single equipment, a stripping distillation column and that I will compare it against absorber, extraction column and I can't recall at this moment if against separation trains that they researched previously.

 

I have an engineering question: I can make whatever I want with Aspen Plus, but how feasible is to make a vacuum of 0,2 atm?



#7 DHKUSUMA68

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Posted 21 August 2018 - 12:10 AM

Hai there

I am doing simulation for heat exchange between vapor (water contain terephtalic acid [TPA]) and water to generate steam.

 

I got warning as follows and simulation stopped. Anyone can help, what to do?

Thank you.

 

->Generating results ...

      VISCOSITY MODEL MUV2WILK HAS MISSING PARAMETERS:
      STKPAR/1ST ELEMENT (DATA SET 1) MISSING FOR COMPONENT TPA

      STKPAR/2ND ELEMENT (DATA SET 1) MISSING FOR COMPONENT TPA

      TB (DATA SET 1) MISSING FOR COMPONENT TPA; TB NEEDED TO
      CALCULATE STOCKMAYER POTENTIALS WHEN STKPAR PARAMS NOT INPUTTED BY USER

  ****PROPERTY PARAMETER ERROR
      ERRORS ENCOUNTERED IN CALCULATION OF VAPOR MIXTURE PROPS USING
      OPTION SET NRTL-RK FOR MUMX.

 

! Calculations stopped because of missing  property parameters
 






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