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Co-Current Absorber In Aspen Plus


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#1 ikha21

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Posted 19 July 2018 - 08:35 AM

Hello everyone,

 

 

I just want to ask you, i have to simulate co-current Absorber in Aspen Plus for flue gas cleaning. I am using currently RadFrac, namely Absorber 2 Model in Aspen Plus. But it seems, as if that all Absorber model in Aspen Plus is only for counter-current flow.

 

What i did, for example, i specified there are 5 stages in the column, with no reboiler and no condenser. I also specified that the feed gas and feed liquid enter the 1st stage, and the product streams for gas leaves the column at 4th stage, meanwhile the liquid at 5th stage. But then, Aspen requires me to Input a stream to the bottom stage, since i introduced all feed streams in the top stage.

 

So, is it true, that we can not model Absorber with co-current flow in Aspen Plus? Has anyone had experience with co-current Absorber in Aspen Plus?

 

 

Thank you for your reply and sharing.

 

 

Best Regards,

Ikha



#2 Pilesar

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Posted 19 July 2018 - 12:28 PM

You must have a vapor and liquid phase on each stage of a RadFrac model. I did not quite follow your specific explanation of your simulation, but one trick to force a vapor phase is to feed a very small stream of Nitrogen to the stage with no vapor. You add an inconsequential amount -- just enough to allow the model to run, but not enough that it makes an appreciable difference in the final converged solution.



#3 Pilesar

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Posted 19 July 2018 - 12:34 PM

If you really want the co-current simulation, you may want to model the five stages as a series of five flash drums. These should converge easily in co-current operation.



#4 Bobby Strain

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Posted 19 July 2018 - 01:14 PM

How do you get 5 stages in a co-current operation?

 

Bobby



#5 Pilesar

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Posted 19 July 2018 - 03:46 PM

How do you get 5 stages in a co-current operation?
  I had in mind sending the vapor and liquid from one flash drum as feed to the next one in series. The flash drums in series would have gradually lower pressures for simulating process pressure drop. Fraction liquid entrainment in the vapor stream could be specified to serve as a pseudo vaporization efficiency. I don't know how flash drums could mimic a rate-based column model. Will five co-current stages in series give a different result than one stage? Probably not! But if the premise of the question is 'how do I model five stages co-current' then a series of five flash drums should reveal the vapor and liquid compositions on each stage and whether or not they are changing. I do not think the original poster got a working simulation with a column block. At least with five flash drums, Ikha should have a converged model and may learn something about the process. But as I said, I don't really understand Ikha's explanation of what the model is supposed to do. Would reactors in series be a better match?


#6 ikha21

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Posted 20 July 2018 - 02:24 AM

Hello Pilesar,

 

 

Thank you for your suggestions.

 

 

So the gas is flue gas containing acid gases mainly HCl and HBr. Absorption process with water in co-current is used for the removal of acid gases. So the flue gas and water enters the column at the top. At the top stage, gas is cooled and saturated with water, and then there is structured packing where the acid removal mainly takes place. The other stages are trays. There is no condenser or reboiler. Sorry, there is something i forgot, so the water is also introduced to each trays. I have tried that, i introduced water into each stages. However, aspen still requires a vapor feed/pumparound to the bottom stage. Attached the snapshots of the Simulation. I have also to put an imaginary stream (S14), since Aspen required vapor distrilate stream if i defined the main vapor product leaves the column at 4th stage. I am quite new to this Software, so maybe there is something wrong with my input.

 

I tried your trick to add small amount of Nitrogen. But it did not work. So i guess using 5 series of Flash drum if the Absorber does not work  is a good idea. Thank you.

Attached Files



#7 ikha21

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Posted 20 July 2018 - 02:36 AM

Hello Bobby,

 

 

Thanks for your respond.

 

 

Actually, i did not calculate the number of stages. It was a Trial value, since the informaton i have is the column has trays and structured packing. And my task is to do the Simulation of this Absorption process with Aspen Plus, so that certain amount of HCl is removed from the flue gas stream. But i could not even figure out how to simulate an Absorption process in co-current flow in Aspen Plus by using Absorber Block.

 

Best Regards,

Ikha



#8 Nikolai T

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Posted 02 August 2018 - 08:03 AM

Hello,
Could you please explain me what is the model for? Is it for Hydraulics calculations data or energy and material balance calculations?

Regards,
Nikolai

#9 ikha21

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Posted 05 August 2018 - 04:12 AM

Hello,
Could you please explain me what is the model for? Is it for Hydraulics calculations data or energy and material balance calculations?

Regards,
Nikolai

 

Hello Nikolai,

 

 

The model is for flue gas cleaning process, only for material balance. It is expected that not only physical absorption occurs but also there are also chemical reactions.

 

Regards,

ikha



#10 Nikolai T

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Posted 07 August 2018 - 10:38 AM

The absorption is exotermic, that is why there are water inputs in different points of column. If heat of reaction, water flow and temeratures are defined you can calculate energy balance and then material balance without equilibrium or rate base model.

Regards,
Nikolai




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