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Estimation Of Number Of Trays - Mea-Co2 Absorber/stripper System

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#1 DanBR


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Posted 01 November 2016 - 05:52 AM

Hi all,


I am trying to design a CO2 capture plant. The inlet stream of the absorption process is the output from a 50 MW power plant: either from Gas Turbine (typical value CO2 = 3% molar) or from Direct Fired Steam Turbine (typical value CO2 = 8%), with no H2S. Any thoughts on the values presented is valid, since I don't have practical experience and took the values from hand calculation + numerous papers presenting such values (for example, Gas Turbine with 100% conversion of 93% methane and 40:1 partial pressure of air to methane). We are at first going for the higher CO2 content, which will supposedly make the towers smaller. Please, correct me if I'm wrong.


Now, on the design, I have read many topics here with responses from Art Montemayor, which are extremely valuable. Reading some 4 books of gas sweetening (Ken Arnold Vol2, Kohl, Maddox Vol4, and Perry ChemE Handbook 8th), they all mention the "previous experience" into calculating height or number of stages.

Perry provides the Solubility curve of CO2 in MEA for dilute (up to 1% of CO2) at typical temperatures (40C, 30% MEA, but I am using 20 for simulation), but it is highly difficult to get the values. Mr Art has stated that for low pressures, this curve isn't necessary, which is my case, I want to do absorption at atmospheric. Besides, the fact that it is chemical absorption, makes the use of the curve not simple (although "Perry" also states that for VERY FAST chemical reactions, like MEA/CO2, the problem can be treated just like the physical absorption).


So, the problem statement is: Using Hysys or any other book method, how can I estimate the number of trays?


Via equilibrium curve from Perry 8th ed (14-8) , after struggle some hours to get the values from graph by eye+computer zoom, I got maximum 2 ideal plates, which using 33% efficiency for CO2 would turn to 6 plates and using 50% safety margin would go to 9. Is that reasonable? Any other suggestion on how to calculate this number of trays? The spacing I'm convinced is 30 inches, since diameter is > 6 m (see further).

Another way I calculated was with Colburn equation: Np = LN[(1-1/A)(y1 - y20)/(y2-y20) + 1/A]/LN(A). The data was: y1 = 8%, y2 = 0.8%, y20 = 0 (pure solvent), (Lm/Gm = 1,2 [dy/dx = (0.0792-0.00792)/(0.0598 - 0)], the 0.0598 taken from equilibrium curve of MEA 30% at 40C from Perry, so the Lm/mGm = 1.78 (1.5 Lm/Gm) 

Np = 2.75 = 4, 33% CO2 Eff = 8.35 trays = 9 trays.



- Is this algebraic calculation of number of trays correct (colburn equation)?

- Is the use of the equilibrium curve for this system reliable?

- Is 33% plate efficiency reasonable?


More data:

Mass treated: 107 kg/s inlet

Temperature of inlet gas: 40 C

y1 CO2: 8%

y2 CO2: 0.8% (90% capture);

Amine Solution: MEA 20% (weigh);

Calculated mass of Lean Amine (Ken Arnold: Surface Production Operations, Volume 2, pg 186): 287 kg/s -> 15% excess: 330 kg/s. A very similar result comes from hysys simulation, but it ends up with 5% of CO2 (%w) in the Lean Amine.

Calculated diameter of tower: now, this is tricky. I used Ken Arnold Volume 1 method: d^2 = 5040*T*Z*Qg/P * (rhog/(rhol-rhog)*Cd/dm)^1/2, as an iterative method (assume Cd, calculate Re calculate terminal velocity, calculate Cd again, repeat til converge), using Cd initial as 0.34 (recommended), dm = 150 mm. I arrive in the end at Cd = 6.18, Re = 5.28, Vt = 1.64 and that gives me a diameter of 15.4 meters. With 15% safety: 17 m.

Hysys Equipment design gives me, for valve tray and some other specs, from 10 to 11.5 meters. If I don't iterate Ken's method, just assume Cd = 0.69 (for no reason), it gives me around 11.4 m.

A third method, presented by Maddox on page 43/44 is using the equation: W = C((rhoL - rhoG)/rhoG)^0.5 and C = 350 or 395 for amine absorbers (24 and 30 inches spacing respectively), where W is the gas mass velocity (from which I'm able to calculate diameter by setting W = Q/d and checking which d gives me the closest Qg to my original Qg). That gave me a value of 10.5 m, which is 12m with 15% safety. Maddox doesn't present a way to estimate number of trays.


Thoughts on that?


I appreciate any comments. Any additional info can be provided.

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#2 Pilesar


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Posted 01 November 2016 - 12:07 PM   Best Answer

I will answer assuming this an academic exercise that might have gotten more responses if it had been posted in the student forum. Since all the expert books you read say to use 'previous experience' to calculate the number of stages, I suggest you do not insist on using theoretical calculations. Try to find an example of a real column for a go-by and claim this is the result of previous industry experience. There are good reasons the books do not explain how to derive actual number of trays from a theoretical basis for this service. If you cannot find any useful reference, I would use 20 or more real trays and ignore the 'expected tray efficiency'. Design tray spacing and tower diameter are primarily influenced by hydraulics heavily dependent on vapor flow. Higher operating pressure reduces required column diameter. Your computer simulation can help with these calculations to stay away from flooding problems in the column. MEA/CO2 solutions tend to be corrosive to metal and these problems increase with concentration. 20% MEA is probably the highest concentration you should use. Real-world amine solutions tend to use lower MEA concentrations or blend the MEA with MDEA or avoid MEA altogether. This is especially applicable to your large column that will be expensive and you don't want it corroding away in a short time. Different amine solutions also affect the amount of energy required for operation which can significantly influence the economic analysis. Even if you continue to use 20% MEA as the basis of your material balance, you may want to consider the column size requirement if the solution were changed to another amine or a weaker concentration such as 12% MEA so that your column design is somewhat flexible.

#3 DanBR


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Posted 02 November 2016 - 02:45 AM

Dear Pilesar,


It is a real problem I am trying to solve, not a student exercise. But indeed, I posted a few days ago in the Student Forum and got some 2 or 3 replies that gave me some clues, but didn't drive me to a conclusive answer. Therefore I tried here, although I think I should have posted on the "Professional Forum".


Your answer is extremely valuable, I do appreciate it. It goes in agreement with Art Montemayor's reported experiences.


I will need to give an answer to this problem at some point and the real world problem is highly dependent on the size estimation. But yes, I believe I will state it as "between 15 and 20", because it sounds plausible based on everything I have read, especially from Art.


Anyway, thank you very much for your time and thoughts. Feel free to add any comments.


Kind regards,


#4 Pilesar


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Posted 02 November 2016 - 10:26 AM

There are specialty companies that focus exclusively on designing and supplying amine systems for CO2 removal. For your real-world problem, they would probably be eager to offer advice.

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