6 replies to this topic

[Rainbow21]

Hi all,

 

I am currently designing an absorber unit to remove CO2 from flue gas.

 

I am currently calculating the column height and well be using  the height transfer units (HTU) and Over gas coffincets, KGa method. For this I need to construct an equilibrium curve from data obtained from literature and also the operating line (this i am yet to attempt).

 

I have obtained data from literature, but the information is in partial pressure rather than, CO2 in gas (x) and co2 mol in mea mol (y).

 

I know to use Raoult's law, y=P(MEA-CO2 sol.)/pco2. Where P(MEA-CO2 sol.) is total pressure exerted by the co2-mea, and pco2 is the partial pressure of co2 in mea.however my graph does not seem correct. Can some light be shed on if I am using the data correctly or if i am doing this entirely wrong.I have attached the literature and my calculations below

 

Many thanks,

Rainbow

Attached Files

•  Finalonlinepublication.pdf   1.33MB   32 downloads
•  VLE CO2.xlsx   15.88KB   27 downloads

Edited by Rainbow21, 20 August 2018 - 09:24 AM.

[breizh]

Hello Rainbow1,

You should consider to review your statement about Raoult's law . Any thermo text book will help you.

 

good luck

 

Breizh

[Rainbow21]

Breizh:

 

Thank you, I have edited my question

[MrShorty]

A few observations on your spreadsheet:

 

1) It appears that you are you using the data from Table 4 at 40 C from the publication. I observe that the quantity you are labeling X (usually used to denote overall mole fraction in the liquid) in your spreadsheet is labeled alpha (ratio of moles of CO2 to moles of MEA in the liquid) in the publication. Perhaps the first thing you need to look at is the relationship between this alpha (loading) quantity and overall mole fraction.

 

2) I note that your spreadsheet is assuming ~1 atm total pressure, and I see nothing in your spreadsheet that appears to be using Raoult's law. If I assume that all of the total pressure is due to water and assume that water obeys Raoult's law (as briezh suggests -- check those assumptions. Maybe look at figure 3 of the publication which describes some VLE data for the aqueous MEA binary system), I get something closer to 6 kPa for the solvent contribution to the total pressure. Where did the 1 atm pressure come from?

 

At this point, I think you are misunderstanding what alpha means in the publication, and you are misunderstanding what the other components of the vapor phase are so that you understand how to get the total system pressure. Perhaps correcting those two things will give you a y vs x chart that makes more sense.

[Nikolay_]

Dear participants,

 

Please, look at modified file. Are there suggestions about the solution?

 

Regards

Nikolai

 

 

Attached Files

•  VLE CO2 m.xlsx   19.87KB   25 downloads

[MrShorty]

@Nikolai T: That's the kind of thing I was thinking of. I do note that I got different results, but I think we read the description differently. It appears that you assumed that, when the paper spoke of "solubility of CO2 in 30 mass% MEA" that the mass fraction of MEA was constant at 30 mass%. I read "30 mass% MEA" to mean that the solvent, before CO2 was added, was 30 mass% MEA, so my calculations assumed a constant ratio of MEA to H2O to be 0.3:0.7. There could be some value in reviewing the paper to make sure the OP understands exactly what the authors mean by 30 mass% MEA. In either case, the idea is to convert the "loading" alpha to liquid mole fraction CO2.

[Nikolay_]

Hello Rainbow 21,

 

By now, I have no thoughts how to account changes in MEA concentration. But in my experience difference between in/out MEA concentrations can take about 2 %. It is not much as you see. Take assumption about concentration MEA.

 

Regards,

Nikolai