11 replies to this topic

[Nadia10]

Hi everyone,

 

I would like to get some kind of trend of what happens with the CO₂ separation efficiency (as CO₂ mols at the outlet of the stripper/CO₂ mols inlet to absorber) when we feed to the absorber a stream where the CO₂ concentration is lower than a previous case, in a gas which does not contain H₂S, only CO₂.

 

So we have 2 cases: a case 1 with a given CO₂ %,  and case 2 where we have all the rest of the parameters the same (amine solution circulation rate, MDEA concentration, volume of gas being treated) but the CO₂ %, that is here lower than case 1. I suppose the driving force will be higher in case 2 than in 1, or maybe it could be explained in different terms, due to the fact that there are more molecules of MDEA free to react with CO₂, so we achieve better separation efficiency in case 2.

 

Thank you very much.

[Art Montemayor]

Nadia:

 

Firstly, if you are trying to selectively absorb only CO2 from a gas stream, then you should not be using MDEA.  You should use MEA (Monoethanolamine) instead for efficient and thorough CO2 removal.  MDEA is a tertiary type of amine that is employed in the selective removal of H2S from natural gas streams containing CO2 as well.

 

If you are trying to understand the difference in the  absorption driving force and how it is calculated theoretically, read the chapter on Ethanolamines for Hydrogen Sulfide and Carbon Dioxide Removal found in the book "Gas Purification" by Kohl and Neilsen.   For additional information on the use of MDEA, read the book "Gas and Liquid Sweetening" by Robert N. Maddox.  These two books are the most authoritative and recognized writings to date on the subject of acid gas and sour gas processing.  I believe all your questions and concerns are addressed there.

[Nadia10]

Dear Art,

 

thank you very much for your help and the book recommendations. I am reading them. I will tell you my progress in this issue.

Well be in contact

 

Regards

[thorium90]

Hi Art, I have a difference of opinion. I have a plant using MDEA with activator that reduces the CO2 concentration to less than 10ppm. My feed gas also doesnt have H2S, the unit is just for removing CO2.

"Gas Purification" by Kohl and Neilsen on page 46 also says that "tertiary amines are unable to form carbamates", "CO2 reactions which do not produce carbamate... is very slow", "this problem is overcome by the addition of activator"

And also abit more on pg 54 and 55.

 piperazine.pdf   67.82KB   72 downloads

 

MDEA can be useful for removing CO2 as Nadia's plant might already be doing and is pretty effective too and has less problems than MEA.

Edited by thorium90, 06 February 2013 - 02:42 PM.

[Art Montemayor]

Thorium:

 

You are comparing apples with oranges.  Previous to your mentioning BASF's activated MDEA (I assume you mean to mention aMDEA), Nadia and I have been discussing MDEA.  That means just that - MDEA plain and simple, without any "helpers" added.  Nadia describes her use of MDEA - not aMDEA, which is a totally different animal.

 

I am perfectly aware of the reported successes and accomplishments of aMDEA.  However, the jury is still out on what are the inevitable trade offs - besides the extra royalty rights you must pay BASF and the market price as well as availability.  All experienced engineers know a proven fact: "There are no free rides or free lunches".  This means that there are trade offs to every selected option in life.  Nevertheless, all indications point to the fact that MDEA is a much more "friendly" amine because it has much less corrosion worries, practically no degradation, a lower vapor pressure, a lower reboiler requirement, etc.  We old timers have known that for many years.  What you fail to mention is that, by itself, MDEA is notoriously much less selective towards CO₂.  It can never compete with MEA in lowering CO₂ level down to negligible levels.  No one in industry uses MDEA to remove CO₂ to the levels of MEA - at least not those that know the difference 

 

What BASF has done is simply added a common chemical that we old timers have always known about - Piperzine - and used the inherent CO₂ selectivity that chemical has to convert plain MDEA into a superior absorbent solution.

 

Therefore, if you use your words and understandings correctly you will find that we are not at odds but rather in agreement.  You have to be specific in engineering in order to paint a clear and accurate picture.

[thorium90]

Ah, I assumed that she just neglected to mention the use of activator. I believe her plant must have already been built and running so they would have chosen MEA if they wanted to long ago. So she mentioned just MDEA and not aMDEA (out of ignorance, neglect, I dont know) and its just CO2 absorption so i simply assumed an activator must have been used otherwise it would be a difficult process to be running and her question would have to be quite different. I mean her post sounds calm and composed... And her question is also not of the "complicated" type (no offense Nadia) so I thought its likely just an oversight on her part.

 

and I did mention it was slow..

 

 

..., "CO2 reactions which do not produce carbamate... is very slow", "this problem is overcome by the
addition of activator"

Edited by thorium90, 06 February 2013 - 02:58 PM.

[Art Montemayor]

Thorium:

 

As you mentioned, you are running a BASF aMDEA unit.  Can you share your operational experience and results with this unit?  I - and I'm sure a lot of our members - would be very interested in knowing more about how the BASF aMDEA application is really doing.  You can't believe everything you read from BASF; actual field data is the real proof.

 

Stuff that I would be interest in:

• Age of unit;
• Raw gas composition, temperature, & pressure;
• solution concentration & gpm;
• Absorber size;
• Stripper size;
• Stripper pressure and temperature;
• Reboiler capacity;
• Solution make up requirements.

Thanks

[thorium90]

Ok..

 

Its about 12 years old. Its located downstream of an SMR unit.Inlet pressure and temperature is about 16barg and 30C. Inlet gas composition varies slightly depending on the required plant load. Typically ~57%H2, 19%CO, 20%CO2, 2%CH4 with a flow of 3500-4500Nm3/hr. Flowrate of lean solution to the top of the absorber also varies with the plant load but typically maintained at 25-27m3/hr. CO2 concentration out of the top of the absorber is typically <10ppm CO2. Absorber bottom temperature is about 70C. Stripper unit uses LP steam for reboiling. Typical bottom and top temperatures are 117C and 92C. Stripper top pressure is about 0.7barg. Both are packed columns. Amine concentration is about 45-50%. Makeup is done only when CO2 slippage starts to increase, which is not often, like a few liters of additional solvent once or twice a year? Depending on the results of the routine analysis.

Edited by thorium90, 06 February 2013 - 03:49 PM.

[Art Montemayor]

 

Thanks a lot thorium.  I don’t know what an “SMR” is; I presume it’s a methane steam reformer and you are producing syn gas for shift conversion to pure hydrogen.  The aMDEA unit must be used to clean up the associated CO₂.

 

The Rich aMDEA at the absorber bottom is pretty hot at 70 ^oC (158 ^oF).  You don’t mention the Lean aMDEA temperature at the top of the absorber and I presume it is running relatively hot as well – perhaps 65 ^oC?  If so, that also makes the exit gas out of the absorber also relatively much higher than the inlet 30 ^oC.

 

If the unit is 12 years old, was it originally an aMDEA unit or was it later modified?

[Nadia10]

Dear Art, Thorium,

 

First of all, thank you very much for your kind and wise responses. Now I have to say that you both are totally and absolutly right. I did not mention that my plant uses aMDEA and from BASF. Im very sorry Art, that is a big mistake from my part, I assumed that today the addition of the activator was supposed for the MDEA when its for CO₂ only. That won't happen never again.

 

Thorium, based on your experience with the aMDEA plant, could you give me another advice related to my question? I mean, what would happen with the CO₂ separation efficiency when we feed all the same but less CO₂ % in the acid gas? Would it be lower (more CO₂ in the clean gas from the absorber) than previously?

 

My plant is already built and its running for internal research. I have to say that the conditions for the plant that Thorium described are very very similar to the conditions of mine. Where is it, if it could be asked?

If you all guys are interested, I could ask permission and give here the technical and operational data.

 

Thank you both so much again.

 

Regards.

[thorium90]

Yes, SMR is steam methane reforming and syngas is produced. The temperature of the lean aMDEA to the absorber top is about 50C. The temperature of the gas at the absorber top is abit problematic. No transmitter was ever installed and the only temperature gauge is sort of "yellow". To be frank, I cant tell what the temperature is over there... But there is a bfw flush at the absorber top to prevent entrainment and maintain the water balance. The temperature of this bfw is cooled before entering but the temperature is unknown too since there is no temperature gauge, but the piping is touchable so its probably 40+C. There is also only one temperature gauge at the absorber bottom, so I cant be 100% sure if its real, although its still mostly "white". It was designed as an aMDEA unit.

 

From what I observed, feeding gas with a lower % of CO2 to the absorber but maintaining the same flow of lean aMDEA to the absorber top does result in cleaner gas. But I think one has to think about the increased pumping requirements etc to maintain a high flow. These are after all mechanical equipment, so is it necessary to push for such purity? I have a coldbox downstream of the unit and a TSA dryer just before that, therefore its necessary to go so low.

Edited by thorium90, 06 February 2013 - 08:42 PM.

[pathapati]

Hii all,

 

I have just joined a new organization where I'm involved in the CO2 removal unit. Our plant is yet to be commissioned. We will be using aMDEA for CO2 removal. I've been looking for some sort of literature on web and stumbled on to this community. Looking forward to share experiences with all you guys  on this.

 

Regards