8 replies to this topic

[cruam]

Dear All

there are two design specs for amine absorber and stripper in the Promax software, Lean approach and Rich approach.
Do anybody know the desiered value of these specs for a CO2recovery plant in which Co2 from a flue gas is absorbed by MEA and desorbed in the stripper?

these specs in my final simulation for absorber column are rich approach=0.97 lean approach=0 . I know for rich approach , how this value is close to 100% is better. but I don't khnow about the lean approach!

in this condition, the mass ratio CO2 in offgas from top of the absorber to CO2 in the feed flue gas is 2%.
(the packed absorber contains 10 theoritical trays and top three trays are for washing section. the pressure drop through the column is 0.25 bar)
I don't know what's the meaning of lean approach=0?

In this situation my absorber is rich end pinch?


Thanks all
cruam

Edited by cruam, 17 April 2010 - 12:11 AM.

[Zauberberg]

Cruam,

What is the definition of terms "lean approach" and "rich approach"? Strangely, but I have never encountered those so far although I have been working on a wide variety of amine units. The terms I have seen are normally "rich amine loading" and "lean amine loading". Basically, these are the most important thermodynamic parameters for design of an amine system.

In general, these two figures depend on the design of specific plant and feed gas/product gas quality. In addition, there are some corrosion/safety concerns with respect to what the maximum RAL and LAL should be.

I'm attaching two interesting papers that can assist you in further calculations.

Attached Files

•  Amine Process Guidelines.pdf   43.24KB   821 downloads
•  Gas treating fundamentals.pdf   216.28KB   646 downloads

[cruam]

Dear Zauberberg

Thank you so much for the attached files. I know the meaning and importance of RAL & LAL, but for an initial design I use promax software to simulate this plant and in this software "Lean approach and rich approach" are defined to help us to know how our plant work as it mentiened in its help!

I attached two files from Promax help in which there is the meaning of them. but my problem is that I couldn't understand them thoroughly!?
If possible, please see them and help me!

thank you again!
Cruam

Attached Files

•  lean rich approach.pdf   45.26KB   515 downloads
•  promax guide.pdf   86.57KB   355 downloads

[Zauberberg]

Cruam,

It is more clear now. Essentially, these are parameters similar to maximum rich loading and maximum lean loading for which you can still ensure required acid gas pickup.

Rich approach = approach to thermodynamic equilibrium on bottom of the Absorber, which is the maximum theoretical amine loading at given partial pressure of acid gas and solution temperature. This parameter changes with Absorber pressure (= acid gas partial pressure). For example, if you run your absorber at 5 barg the maximum thermodynamic loading of the solvent is, let's say 0.40 mol/mol. You decide to go for 75% of this value that is 0.40 x 0.75 = 0.30 mol/mol and set the required amine circulation based on this figure. If, another example, Absorber pressure is 50barg and the maximum thermodynamic loading of solvent is, let's say 0.75 mol/mol then you calculate 0.75 x 0.75 = 0.56mol/mol as maximum practical solvent loading. On some occasions, the solvent loading can be even higher than 1 (water absorbs acid gas as well!) but normally we never consider such cases.

Lean approach = approach to equilibrium on top of the Absorber, which is basically the content of acid gas in the overhead gas that is in equilibrium with the inflowing lean amine. To illustrate this point, think of the following: the more acid gas is contained in the regenerated amine, the more difficult will be to remove it from the feed gas on top of the Absorber. That sounds logical, you agree? In real plants, you can always play and try to regenerate your amine as less as possible and, after a certain point you will see that you are not able to meet the product gas specification regardless of how much you increase the solvent circulation rate. And that is the "lean pinch", meaning that the acid gas content of overhead stream is such that it cannot be reduced further until you reduce the acid gas content in lean amine.

As a conclusion: maximum RAL sets the solvent circulation rate, maximum LAL sets the required level of solvent regeneration (reboiler duty).

Hope this is much more clear now.
Best regards,

[cruam]

Dear Zauberberg

I appreciate you for this complete description and helpful guide.

- you mean: max LAL leads to min reboiler duty and
max RAL leads to min amine circulation rate, is it true?
and also,
the more RAL, the more rich approach and
the more LAL, the more lean approach, isn't it?

It means that when rich approach is near to 100 we have maximum rich loading? is this description true for lean approach and lean load? I mean to find Max LAL and RAL in an amine plant we should meet 100% for both lean and rich approach?

- also you pointed to a 75 percent of maximum loading in your description, is it a typical value recomended for basic design? I mean an amine plant should be worked at howmany percent of the Equilibrim loading?

Excuse me for asking these alot of questions! because now I meet lean approach equal to 12% in my simulation and rich approach is 96%. according to the defenition of lean approach and your description I think [b]lean approach should be more than this value and rich approach should be less??

I should say that in my simulation the value of RAL=0.45 and LAL=0.107 to approach minimum amine crculation rate and minimum reboiler duty in the stripper respectively. these values are in the safe rang for MEA (from GPSA and Promax help)

what should I do to optimize these values in my simulation?

Thank you so much in advance.

best regards,
Cruam

[Zauberberg]

Cruam,

Rich Approach = [Actual Amine Loading] / [Equilibrium Amine Loading] - this figure takes into account solvent temperature and Absorber pressure.

As recommended from ProMax and CCR (also by AmineExperts), this figure should never exceed 75-80% due to operation reliability and corrosion/safety concerns. If you got 96% rich approach, you have to increase the amine circulation rate till the Rich Approach falls to 75%.

Lean Approach = [Equilibrium Composition of Acid Gas in Overheads] / [Actual Composition of Acid Gas in Overheads] - this figure takes into account the actual quality (LAL) of the inflowing solvent including its temperature and Absorber pressure.

A lean approach of 12% indicates that with the actual lean amine loading you have, you are far away from reaching the pinch point on top of the Absorber. In theory, this means that you could reduce the amine regeneration level further and increase the LAL but, on the other hand, by doing so you will definitely affect (increase) RAL, and probably create corrosion problems in the lean amine circuit and the regenerator reboiler. If the lean amine loading is close to the experience-based guidelines, I would say it is good enough for the basic design.

Best regards,

[cruam]

Dear Zauberberg

Thank you so much for your reply. I really appreciate you!

I do as you said and decrease Rich approach till 80% by increasing the amine circulation rate. but the temperature profile of the absorber compeletly changed. one of my freinds who has worked on many amine plants had told me that temperature profile in an amine absorber should have a maximum which is preffered to be at the middle of column rather than the bottom.

I'm attaching both temperature profiles before and after changing amine flow rate .
If possible , please see them and tell me your idea. which one is true in practice?

Thank you again

Best regards

Attached Files

•  temp profile.pdf   45.84KB   250 downloads

[Zauberberg]

I wouldn't be so much interested in the temperature profile. The temperature peak usually moves towards the bottom of the tower along with the front of mass transfer zone. It depends on several parameters: acid gas content in feed gas, treated gas specification, amine circulation rate, amine type and concentration, number of stages etc. You can read more about it in "Gas Purification" by Kohl & Nielsen. There is an excellent chapter dedicated to the temperature profile of amine absorbers.

[cruam]

I wouldn't be so much interested in the temperature profile. The temperature peak usually moves towards the bottom of the tower along with the front of mass transfer zone. It depends on several parameters: acid gas content in feed gas, treated gas specification, amine circulation rate, amine type and concentration, number of stages etc. You can read more about it in "Gas Purification" by Kohl & Nielsen. There is an excellent chapter dedicated to the temperature profile of amine absorbers.

Thanks alot dear zauberberg!!