5 replies to this topic

[ChemistryDude]

Hello,

I'm currently undertaking a design project at university and have been tasked with designing a CO2 absorber. I have found thermodynamic solubility data for CO2 in 30 mass % aqueous MEA solution, trouble is the data is presented as CO2 partial pressure Vs Loading (mol CO2/mol MEA).

I have been able to convert the CO2 partial pressure to gas mole fraction using yco2=Pco2/Ptotal (this was relatively straight forward). I'm a little stuck as to how I should convert the loading to liquid mole fraction. I tried a few combinations, but each time the y Vs x graph I generate looks quite strange (I have attached the speadsheet of the data im using/generating for clarification). Any help with this would be really awesome.

Regards,

CD

Attached Files

•  Book1.xlsx   28.81KB   65 downloads

Edited by ChemistryDude, 11 February 2015 - 12:09 AM.

[Zauberberg]

It is very hard to grasp the concept of your calculations, because I fail to see what is the given information and what needs to be calculated. Can you upload a document clearly showing what is the input data, and what needs to be developed further? Please upload all the data from your task.

[ChemistryDude]

It is very hard to grasp the concept of your calculations, because I fail to see what is the given information and what needs to be calculated. Can you upload a document clearly showing what is the input data, and what needs to be developed further? Please upload all the data from your task.

Hey,

Thanks for the feedback - to be honest my calculations might not seem very clear since I'm a little confused as to what needs to be calculated myself. Essentially, I am trying to determine the equilibrium line for my CO2-MEA absorber system. I have managed to find solubility data for CO2 in (aqueous) MEA solution. The data is for a MEA solution that is 30% mass solution of MEA. The total pressure for the system is given, the partial pressure of CO2 is given and the solution loading is given (in mol CO2/ mol MEA). I am trying to convert this data into gas mole fraction and liquid mole fraction.

I converted the partial pressure of CO2 into gas mole fraction using Dalton's Law: Yco2 = Pco2/Ptotal
 

The main problem I'm having is trying to convert loading (the mol CO2/ mol MEA value) into liquid mole fraction. The calculation I need to perform is:

nCO2/(nCO2 + nMEA + nH20) (expect I'm a little stuck as to how I determine each of these values from the data I have)

I hope this explanation is a little clearer (if Im still missing information please let me know) - thanks again for your time

CD
 

[Zauberberg]

Again, I don't see any specific data but I will try to help with the following:

 

1) It is highly recommended to design absorbers at a certain Rich Amine Loading (RAL) which is always less than the thermodynamic equilibrium at given operating conditions (a function of partial pressure of acid gas, operating temperature, and number of stages in the absorber) - especially at lower pressures. See attached article for more guidance. There is also an experience driven rule of thumb for maximum RAL for every type of commercial amine.

 

2) I don't understand what do you mean by "liquid mole fraction of CO₂", but that is not important for absorber design. What you need to do is to establish the design point (for clarity, let's say it is at 80% of the thermodynamic equilibrium at given operating conditions of the absorber), so this would give you the quantity of amine required to absorb the entire CO₂ from feed gas (let's say the result would be 0.32 mol MEA per mol CO₂). For your task this means: you will obtain the result how many moles of pure MEA you need to absorb 1 mole of CO₂, at given partial pressure of CO₂ and operating temperature. Now looking at MEA which is normally designed for maximum solution concentration of 25% wt MEA and the residual amine loading after regeneration in the range of 0.12 mol/mol, the solution circulation rate is easily calculated.

Attached Files

•  Understanding Gas Treatment Fundamentals.pdf   216.28KB   85 downloads
•  GPSA Reference.pdf   126.64KB   71 downloads

[ChemistryDude]

Again, I don't see any specific data but I will try to help with the following:

 

1) It is highly recommended to design absorbers at a certain Rich Amine Loading (RAL) which is always less than the thermodynamic equilibrium at given operating conditions (a function of partial pressure of acid gas, operating temperature, and number of stages in the absorber) - especially at lower pressures. See attached article for more guidance. There is also an experience driven rule of thumb for maximum RAL for every type of commercial amine.

 

2) I don't understand what do you mean by "liquid mole fraction of CO₂", but that is not important for absorber design. What you need to do is to establish the design point (for clarity, let's say it is at 80% of the thermodynamic equilibrium at given operating conditions of the absorber), so this would give you the quantity of amine required to absorb the entire CO₂ from feed gas (let's say the result would be 0.32 mol MEA per mol CO₂). For your task this means: you will obtain the result how many moles of pure MEA you need to absorb 1 mole of CO₂, at given partial pressure of CO₂ and operating temperature. Now looking at MEA which is normally designed for maximum solution concentration of 25% wt MEA and the residual amine loading after regeneration in the range of 0.12 mol/mol, the solution circulation rate is easily calculated.

Thank you again for your time Zauberberg, I will have a thorough read through the attachments to get a better understanding of the design process as a whole. 

The precise trouble I'm having is, I think, quite similar to the second post on this thread (though Im not sure if this will clarify the matter any further):
http://www.cheresour...0-mea-solution/



 

[Zauberberg]

You have got there some really insightful reply from Art Montemayor. The key point in designing an amine absorber is to define how many moles of acid gas you can absorb with 1 mole of circulating amine (calculated on pure component). The rest is just arithmetic.