12 replies to this topic

[knvrmnd]

Hello everyone,

 

I'm simulating a carbon dioxide capture process using MDEA as solvent on Aspen HYSYS V8.8 using acid gas package. My goal is to achieve 90% recovery of CO2 with following conditions that my professor specified for me:

 

----

<Feed gas> 50 ºC, 1.2 bar, 130,000 kmole/h

composition (mole fraction):

0.05 carbon dioxide

0.75 nitrogen

0.13 oxygen

0.07 water

 

<Lean amine solvent> 50 ºC, 1.2 bar, 49 wt% MDEA

lean loading (CO2 mol / MDEA mol): 0.05

 

<Process conditions>

Fluid package: Acid gas

Heat exchanger minimum delta T: 10 ºC

Pressure drop: 0.2 bar

CO2 product temperature: 40 ºC

Number of trays in columns (absorber / stripper): 15 / 14

Operating pressure of columns (absorber / stripper): 1.2 bar / 2.2 bar

----

 

As far as I understand, once you decide the L/G ratio (lean amine flowrate / feed gas flowrate), degree of freedom in this process is zero. The problem is, however, rich amine liquid from the absorber bottom has very low loading (around 0.075) when using lean amine loaded with 0.05 CO2 as the given condition. And even when L/G is as high as 20, only 50-60% of recovery is achieved.

 

Does anyone know why the rich amine's loading appears to be so low? It just seems like the absorber tower itself does not function well, when most of its important variables are already decided in the given condition.

 

Also, it was possible to close the recycle loop and achieve >90% recovery when I set the lean amine's CO2 loading as low as 0.0007 or something. But I'm supposed to use 0.05 lean loading...

 

Another thing is, when designing the stripper column, what kind of specs do you usually specify to make its degree of freedom zero? I've been using 0.3 reflux ratio and some random value for vent rate, however I don't think it's logical at all. When I tried specifying reboiler temperature instead of vent rate, it never converged.

 

Thanks for reading this lengthy post. Btw this is my bachelor's thesis project!

 

+ Edit: PFD screenshot from HYSYS V8.8 attached: PFD.PNG

Edited by knvrmnd, 31 May 2022 - 04:01 AM.

[Nikolay_]

Hi,

It would be better if you uploaded PFD from simulator.

 

Possible specifications for regenerator are:

- temperature of Condenser;

- component fraction CO2 in liquid of Reboiler (lean amine).

Edited by Nikolai T, 29 May 2022 - 10:30 PM.

[breizh]

Hi,

Consider the literature attached ( operation side) to support your work.

http://www.gasproces...f-acid-gas.aspx

https://www.jmcampbe...mine-contactor/

 

Good luck 

Breizh 

[Bobby Strain]

Commercial operations use MEA, not MDEA.

 

Bobby

[knvrmnd]

Hi,

It would be better if you uploaded PFD from simulator.

 

Possible specifications for regenerator are:

- temperature of Condenser;

- component fraction CO2 in liquid of Reboiler (lean amine).

I added a link to the PFD: PFD.PNG

 

Thank you so much! I tried specifying temperature of Condenser and lean loading like you said. I don't have a screenshot of that version of the simulation file, but it worked and the product temperature was successfully calculated to be 40ºC.

 

However CO2 recovery was very low like around 10-20%. I think the main problem here is poor performance of the absorber column. I'm a little confused here, because once top and bottom pressures and number of stages are specified, the degree of freedom is zero (as long as I don't mess with column diameter, etc.). Maybe it's something to do with extremely slow absorption rate of CO2 to MDEA, but then what do I need to do to achieve 90% recovery?

 

Seonggyun

Edited by knvrmnd, 31 May 2022 - 04:11 AM.

[knvrmnd]

Hi,

Consider the literature attached ( operation side) to support your work.

http://www.gasproces...f-acid-gas.aspx

https://www.jmcampbe...mine-contactor/

 

Good luck 

Breizh 

Thanks Breizh! I'll take a deep look at each of them. I hope the literature you attached help me fill the gap between the real processes and my shallow and narrow knowledge. I do feel overwhelmed to decide all the parameters and conditions when I have no idea how main variables like pressure and temperature are decided in real processes.

Seonggyun

[knvrmnd]

Commercial operations use MEA, not MDEA.

 

Bobby

 

That's exactly my point! I just emailed my professor asking why do I have to use MDEA as solvent to capture CO2 when the biggest benefit of using MDEA is its high selectivity of H2S absorption in presence of both CO2 and H2S...

 

Seonggyun

Edited by knvrmnd, 31 May 2022 - 04:12 AM.

[ChEf]

MDEA is not a good choice for CO2 unless it is activated e.g. with piperazine

Lean loading seems a bit too high

[Nikolay_]

Also look reboiler heat load if it exceeds 800...900 BTU/gal MDEA solution. For MEA solution it's 1000....1200 BTU/gal. Otherwise regenerator and reboiler will be oversized.

[Art Montemayor]

Before you attempt any computer simulations, you should do some hard, basic, and detailed study of the CO2 removal process you have been given.

 

You have a very "weird" looking gas stream - essentially a nitrogen stream with an oxygen content that is 3x that of the CO2 content.  Your problem basically is that you have one of the worst amine "destroyers"  in your feed gas - oxygen.  If you haven't done so already, you have a lot of reading and studying ahead of you in researching the proper chemical to employ.

 

You don't give us the basic facts of your assignment:

• Are you obligated to strictly follow what your prof dictates?
• Can you challenge his choice of an amine?
• Have you studied the various components required in a CO2 contactor+CO2 stripper process?
• Are you aware that no reflux is required or justified on a CO2 stripper?
• Are you aware of the Benfield Process?
• Have you read and studied "Gas Purification" by Kohl & Nielsen?
• Is your assignment subjected to economics?  

Try to use a solvent that won't be attacked by the O2 content.

Removing only 90% of the CO2 is not a tough job for a solvent.

 

 

 Effect of Oxygen and Heat Stable Salts on the Corrosion of Carbon Steel in MDEA-Based CO2 Capture Process_Choi_2010.pdf   7.36MB   19 downloads

 Post-combustion CO2 Capture with Amino-Acid Salts.pdf   454.72KB   16 downloads

 

[breizh]

Hi,

To add to the list of books suggested by Art , Let you consider 

Gas treating (absorption theory and practice) by Dag A Eimer , wiley 

GPSA  (gas Processors Suppliers Association) .

 

Good luck 

Breizh 

[knvrmnd]

To: ChEf

Thanks for your comment. I agree that activating MDEA with PZ would make it more reasonable and that the given lean loading is too high.

 

 

To: Nikolai T

I will definitely check that tomorrow when I go to the computer lab where I have access to the software. I've been struggling with finding reasonable range of values of heat duties of the process components. Like I see those numbers calculated by the softwares but I don't have enough knowledge to tell if something is unrealistically too high or low..

 

 

To: Art Montemayor

I'm in the process of finding out whether I have to strictly follow the given condition including the choice of solvent. I have studied components involved in the process and am aware that reflux is not essential. And economics is not really in the scope of this assignment.

 

I will read about Benfield Process and the textbook you mentioned as well as the literature you and Breizh attached below. Great great thanks for all the help!

 

 

To: Breizh

Thank you so much! I will definitely check them out.

 

 

My sincere thanks to each one of you! I have been studying about carbon capture process since last year but to be fair I was having difficulties setting direction and finding resources until I found this forum. This community is truly amazing and inspiring.

Edited by Art Montemayor, 02 June 2022 - 07:56 PM.
Simplify; consolidate

[Art Montemayor]

Seonggyun:

 

Thank you for your quick response, which is a positive indication of the potential help this Forum can give you in starting your project on the “right foot”.  As you have noted, our expert Forum members are willing to give you their expert advice and experience in dealing with and confronting the CO₂ removal project you are embarking on.

 

I had many years in CO₂ removal design, construction, and operations.  I have handled CO₂ as a compressed gas, saturated liquid, and a solid (Dry Ice).  I have a patent on producing solid pellets of Dry Ice for refrigeration purposes.  My point here is not my credentials, but my willingness – as most of my Forum colleagues – to help you get started on a project that will ensure you success and a potential future in engineering.  The more details and basic data you furnish our Forum, the more results and positive recommendations and information you will receive.

 

You have started on the right track by being honest with us and giving some of the details – albeit not all of them – of your assignment.  Please believe me when I tell you that you cannot successfully justify using an amine solvent solution to remove the CO₂ from the gas stream that you describe.  It is not only not practical, it is embarking on a path that is unreasonable, un-engineering, and totally uneconomical in the reality of projects.  In the mass hysteria spread by today’s “scientific” so-called gurus, countless amount of money and effort has been done at the academic levels to copy, duplicate, and try to “perfect” the amine process that we, experienced and proven engineers have known for decades.  Universities have issued subsidized “studies” on how to “capture” CO₂ with amines – principally MEA.  I’ve read most of the published ones and have filed them in the trash can because they fail to indicate any new or novel knowledge or experience in designing or operating an ethanolamine process in the field.  Ethanolamine processes have been used since the late 1950s – nearly 70 years ago.  There is little – or nothing at all – left to “study”.  The issue is not that we need to further study how the amine contactor and stripper work.  The issue is how to extract more subsidy money from politicians who are looking for the magical solution of removing all CO₂ from the atmosphere – regardless of all that is produced by volcanoes, cow farts, forest fires, and what-not.  It’s all in the cause of justifying a carbon tax - for more political revenue.

 

My point above is that I can’t blame a prof that hasn’t the experience with amines that I’ve had in the field.  He/she have been studying and teaching – not producing chemicals.  Therefore, they don’t have an insight of the un-practical and difficult problem that has been assigned: amines can’t be expected to treat or tolerate oxygen gases.  They will react and degenerate to corrosive waste products producing an intolerable and very expensive operating condition.  That is a fact, not fiction or my opinion.  My sincere and best recommendation for you is to have the choice of solvent be left up to you – and not to your prof.  If pride, prejudice, or discipline does not allow this, then a change in the fictitious feed gas stream is called for.  In a free, and democratic society I believe you should have a voice in deciding what is a just and appropriate description of your assignment.  Of course, I’m assuming you live in South Korea – and not in North Korea.  I hope your prof listens to reason and issues reasonable project specifications and data.

 

In the meantime, I enclose a workbook to entertain you with a CO₂ removal process flow sheet that I have used in various projects in the past.  Don’t forget to do the basic reading research on the basic process before starting any simulation.

 

 Amine Process for CO2 Removal.xlsx   102.93KB   45 downloads