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Thread: FCC Main Frac Simulation

  1. #1

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    FCC Main Frac Simulation

    Dear all
    I am trying to simulate FCC main fractionator using aspen Plus. I am trying to model with actual trays and efficiency. I have configured all the PA with the specs of PA duty and flows. I am trying to run the columns with specs of all the product withdrawal flow rates are fixed. Cuurenty I am generating FCC rx effluent by using product backblending. (feed is generated using mixer of FG, LPG,gasoline, LCO and Slurry oil/decant oil)
    Currently I am not able to match the Column internal traffic with actual observed one. Simulation is showing much higher Reflux, LCO IR, HCO IR flows than actual one. (almost 1.8-2 times than actual one) I have already rechecked all the input values.

    Has anybody observed the same? Can you please suggest something (some thermodynamic option/ may be pseudo component property estimation option) to minimize this mismatch.. Currently I a m using Grayson Street as thermodynamic option, and aspen defaults methods for pseudo component property estimation.

    Thanks in advance
    Suhas

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  3. #2

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    I also face the same problem in simulating complex problems like crude column simulation ,Coker main fractionator and FCC main fractionator......If some has the solution to any of these plz reply.......

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  5. #3

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    Do not give actual trays and tray efficiency. It is better to give number of stages based on some typical efficiency and converge the simulation. Then try to vary the number of stages and try to match the actual flows / product specs.

    All the best

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  6. #4

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    hi
    try to increase duty of PA and decrease flowrate of PA
    ckeck top column, temperature,condenser duty and reflux ratio.
    check liiquid free water on tray/activate two liquid phase on tray

    Quote Originally Posted by suhas.nehete View Post
    Dear all
    I am trying to simulate FCC main fractionator using aspen Plus. I am trying to model with actual trays and efficiency. I have configured all the PA with the specs of PA duty and flows. I am trying to run the columns with specs of all the product withdrawal flow rates are fixed. Cuurenty I am generating FCC rx effluent by using product backblending. (feed is generated using mixer of FG, LPG,gasoline, LCO and Slurry oil/decant oil)
    Currently I am not able to match the Column internal traffic with actual observed one. Simulation is showing much higher Reflux, LCO IR, HCO IR flows than actual one. (almost 1.8-2 times than actual one) I have already rechecked all the input values.

    Has anybody observed the same? Can you please suggest something (some thermodynamic option/ may be pseudo component property estimation option) to minimize this mismatch.. Currently I a m using Grayson Street as thermodynamic option, and aspen defaults methods for pseudo component property estimation.

    Thanks in advance
    Suhas

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    Thanks Kermitel

    I am able to match the reflux and internal traffic by increasing the Bottoms PA duty. But then the problem is there will be huge offset between Bottom PA duty in simulation vs actual observed one in plant. (offset is about 23% in bottom duty) which is unaccetable.

    I have tried to change the duty in different PA and it is obsrved that internal traffic gets reasonably match even I change only the bottom PA duty. But the mismatch is quiet huge... We dont expect that calculated duty from unit will have this much error.

    Any way thanks for your advice.

    Suhas

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  8. #6

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    Suhas,

    could you upload your created file with the input data here so every one can have a look and commend?

    Thanks.

  9. #7

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    Answers & suggestions:
    1. Selection of Greyson Streed is correct
    2. I have no experience in simulation of FCC MF in Aspen Plus, but have successfully done it in PRO/II. PRO/II gives a better match for refinery fractionators like FCC MF, coker MF, CDU, VDU etc.
    3. Use theoretical trays with appropriate efficiencies. Efficiency will vary from zone to zone.
    4. In PRO/II, there is an option of using different thermo models in a single column. Use this option and select SRK for top tray to bottom most tray in lean oil/heavy naphtha zone. LCO, HCO and slurry zone should be modelled with Greyson Streed package. This may lead to slightly off temperature on the tray where thermo changes in column, but gives very good match overall.
    5. Have you estimated the heat loss from the column to atmosphere? This is a significant amount (whatever may be the insulation) with respect to the tolerances in simulation. Estimate this by measuring skin temperature of the column at various zones and then calculate the heat loss. This heat loss should be entered in column simulation.
    6. You are saying that simulation is not matching the plant figures. But are you sure that plant instruments are showing correct figures? Slurry flow meters are notorious and flow should be cross verified by checking rundown tank dip or by checking duty in slurry exchangers.
    7. Try to do a material balance across FCC feed and all the product flows using plant data. If your material balance closes within 100+/-2% then only you can say instruments are correct and can use them for validation of your simulation.

    Any further queries? You can mail me to shubhajit05@yahoo.com

    Good luck

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