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Distillation Column Convergence For Biodiesel Production

biodiesel distillation column methanol recovery solvent recovery

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#1 sukanta87


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Posted 02 November 2017 - 11:57 PM

I am working on a simulation in Aspen Hysys for the Biodiesel production. Couldn't converge the distillation column. The inputs I  put in the column are:


Feed stream contains > NaOH, Methanol, methyl esters (biodiesel), glycerol etc at 68 deg C and 12 psig

No. of stages > 10

P cond > 5 psig, pressure drop: 2 psi

P reb > 8 psig, pressure drop: 2 psi


In the column specification tab > Reflux ratio: 2, Reboiler temperature: 100 deg c.


From the distillation column, I want to take out the unreacted methanol ( normal boiling point is around 65 deg C) as a distillate from the top of the column. In the distillate stream, it was also mentioned the Std Vol frac as 1.0 for the methanol.


Most of the values I put here was the assumptions. The column was not converged. The above parameters were also varied at different values. Would you please suggest me how the column can be converged for the methanol recovery? How are actually the values should be?



#2 Bobby Strain

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Posted 03 November 2017 - 09:46 AM

Pick some specification other than reboiler temperature.



#3 sukanta87


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Posted 03 November 2017 - 10:58 AM

Hi Bobby, thank you. I specified the distillate rate initially. In the column feed stream there was 4.325 bbl/d methanol (from the compostion tab). So, I mentioed the same flowrate in the distillate rate. But it was not converging, and the Hysys was calculating over 200 bbl/d distillate rate! Even, the distillate rate was almost the same when I specified another parameter, reboiler temperature as 100 deg C.


The bottom product rate should be over 200 bbl/d in my simulation based on the column operating conditions and the normal boiling point of the bottom product( which is above 300 deg C). But at the bottom,I am getting around 80 bbl/d.


For your better understanding the specification and block diagram is attached here.

Attached Files

#4 sukanta87


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Posted 03 November 2017 - 06:48 PM

The distillation column is converged but the reboiler temperature is very high ( ~ 380 deg C with the duty of 2.3e6 btu/hr)! The temperature was calculated by Hysys.

#5 Pilesar


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Posted 06 November 2017 - 11:01 AM

It is difficult to get any useful information from a non-converged column. Once you have a converged simulation of your column, then you adjust the specifications marginally to better understand how the column works. Trial and error is common with simulations. Use the experimental method if you like and perform case studies. There is a reason for the converged answer you are getting. If it is not what you expect, check your inputs. Are your simulation streams actually in bbls/day or are they bbls/hr? Is your composition input on the correct basis? Is the thermo system correct? Simulation is a puzzle and the engineer has to find the clues.

#6 sukanta87


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Posted 06 November 2017 - 06:06 PM

Thank you, pilesar. It's bbl/d. For the methanol compositon in the distillate, I used the basis as liq vol fractions (earlier I mentioed as std vol frac which was not correct actually). I have checked in so many ways to reduce the reboiler duty as well as the column temperature but still it's 383 deg C! The normal boiling point of methanol is ~ 65 dec C. I want to separate only the methanol from the rest of the materials in the distillation column. So I was expecting to get the column temperature around 100 deg C.  The pressure at inlet stream, reboiler, and in condenser are at 4 psig, 3 psig, and 2 psig respectively in the simulation.


Apart from this discussion, how to deal with the flowrate unit for the batch process in Aspen? All the units are ()/time, for example bbl/d, kg/hr, etc. But if I want to feed 3 barrel of a certain liquid in a distillation column considering a batch process, how to deal with the unit in the aspen input stream?

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