6 replies to this topic

Hi all,

I have to design a reboiler for a full scale CO2 capture plant (~300 tco2/h) using amines (30wt-% MEA in this case). See attached figure for plant configuration.
The main problem is that the plant is operated under variable conditions:
• The feed stream (rich solution) to the reboiler is not constant as
o the flue gas volume flow to the absorber is in the range of 50%-100%
o the CO2 removal rate is in the range of 50-90%
o the L/G ratio is optimized
Therefore I need to have a variable control of the reboilerd duty:
The stripper is equipped with structured packings operates at 2bara (29psi), to avoid thermal degradation at Temperatures > 130°C (266°F). I did not planned to use a reflux at the top of the desorber as the water can be used somewhere else in the process. The heating medium will be saturated steam from a power plant.
Heating steam at 135°C (275°F), 2.13bar (30.89 psi).
The boiling temperature of the 30wt-% Mea solution at 2 bara(29psi) is about 124°C(255,2°F)

My consideration is to use a vertical thermosyphon and control the condensate level. But I am not realy sure if such a configuration is possible at this size.
It would be kind if could get some expertise on this case.

regards,
Sebastian

Attached Files

•  captureprocess_eng_091113.jpg   85.67KB   95 downloads

[Zauberberg]

Having a common kettle reboiler will do the work. For evaluating range of duties that have to be delivered by the reboiler, use the following logic.
For each operating case, calculate:

1. Duty required to heat the rich solvent from tower feed T to tower bottoms T
2. Duty required to desorb given quantity of acid gas from the solvent
3. Duty required to operate the tower with required reflux ration (I forgot the figures for MEA systems, Google around)

If the range of duties is within the 40-100% limit, kettle is your choice. And don't forget the reclaimer for MEA.

Attached Files

•  Amines.pdf   20.26KB   187 downloads
•  Gas Treating Fundamentals.pdf   126.77KB   191 downloads
•  Huntsman Complete Technical Guide.pdf   224.84KB   197 downloads

Thx for your answer..


A kettle reboiler was also my first choice. But in my last 1 year working on this topic as a phd student most articles, presentations and people I spoke to were talking about thermosyphons. For example Fluor (see attached presentation p.25). I guess the problem of a kettle might be the higher recidence time and therefore the high dgradation rates due to the high wt-% of MEA. And/Or the size of the reboiler. I could imagine that it will be easier to build a thermosyphon in this this size (100kgco2 * 3,6 GJ/tco2).

What do you think?

Sebastian

Attached Files

•  Reddy08 - Econamine FG PlusSM Technology for Post-Combustion CO2 Capture.pdf   417.21KB   121 downloads

[Zauberberg]

Hello Sebastian,

How things are going in Munich? It looks like all beer festivals are finished long time ago, so you finally got yourself focused on chemical engineering, am I right?

If you envisage significant fouling and degradation of MEA inside the reboiler, thermosyphon would be a better choice. Kettle is known as "dirt collector". However, if rest of the plant (filtering on rich and lean amine side) works properly, and if thermal design of reboiler is done in such way it prevents high thermal gradients and excessive amine film temperatures, everything should be OK. I have seen and read about successful kettle reboiler applications in MEA systems. If you decide to go for thermosyphon, there is a tough decision to be made with regards to circulating vs. once-through systems. The economics will definitely drive your choice towards once-through (higher vapor flow from circulating thermosyphons and therefore probably larger diameter tower), while there are plenty of operational difficulties and design challenges for once-through system.

Here's one chapter about reboilers so you can enjoy while drinking beer and laying on the greenfield, during the next beer festival.
Good luck,

Attached Files

•  Reboiler Details.pdf   942.38KB   252 downloads

[Zauberberg]

And remember the MEA Reclaimer. And to stress-relieve all piping and vessels in MEA service. There's another interesting story about these - stress corrosion cracking incident that took several people's lives.

[SSWBoy]

30 wt% looks high for MEA, what is your basis for this number? I'm accustomed to seeing lower values in the range of 15-20%

[Art Montemayor]

Sebastian:

It appears that you have reading a lot of theoretical papers and dissertations by CO2 capture professors and wanna bes. 30% MEA is outlandish and I wish your design a lot of luck; but my many years of dealing with the design, fabrication, and operation of CO2 - amine plants tells me it will never be practical and much less economic. But since it is only a thesis, I guess it is worth the exercise.

You have answered your own question when you state: "The main problem is that the plant is operated under variable conditions". Under no circumstances should a thermosyphon reboiler be considered for the variations you are outlining. Both theoretical and empirical evidence show us that a thermosyphon is not designed for varying loads. It is likened to a centrifugal compressor: it is a great solution for a steady, consistant flow rate. It starts to come apart economically and practically when you start to vary the flow rate (or turn down). A kettle is a big, ugly, hulk of steel and liquid inventory - but it has the inherent ability to handle any reasonable size of turn down capacity without any problem. Just because you are handling a big volume doesn't mean you will have more MEA degradation. That has nothing to do with degradation. The Heat flux you employ on the reboiler tube bundle has a large part to play in degradation, but not the volume in the reboiler.

You are going to suffer degradation whether you like it or not and, as Zauber has said, it would be very foolish not to employ a redistillation still for the MEA - as well as full-flow activated carbon filters. Additives are not going to save you. If anything, they will make your process (& your life) more difficult.

It is apparent to me that you have a lot to learn about the MEA-CO2 system by the parameters you are setting for your design. I would never opt for having the reboiler operate at 2 barA. I consider this totally illogical for MEA, especially so at 30%wt solution. It makes for good common sense to keep your reboiler temperature as low as you can justify - keep the top of the stripper at about 2 psig, barely enough to keep the suction of the CO2 compressor downstream of the CO2 cooler at a positive pressure.

Although I don't require to see your flow diagram to understand exactly what you have to do, it would have been nice if you had uploaded it.

Good Luck.