3 replies to this topic

[chegurke]

Hello everybody,

Within my bachelor thesis i'm trying to design a reboiler for stripping CO₂ from an aqueous monoethanolamine (MEA) solution. It shall be used for post-combustion CO₂-capture in coal-fired power plants. Previous works and models i got chose vertical thermosiphon reboiler to bring the MEA-solution to a vapor fraction of 0.3. As i read in Schluender's Heat Exchanger Design Handbook the vapor fraction for aqueous solutions with narrow boiling range (what i expect the solution to be) should be in the range of 0.02 to 0.10.

The main argument for the vertical thermosiphon is, that those capture-plants are used for retrofitting existing power plants where the available space is strongly limited.

Now my question: Is it possible to design the vertical reboiler for higher vapor fractions? If i do so, what problems will occur?


Thanks for any reply

Jan

Edited by chegurke, 24 August 2010 - 09:04 AM.

[Zauberberg]

Why do you need such a high vapor fraction in the reboiler? MEA-based systems can be operated with highest lean amine loadings (when compared to other amines), and I'm surprised to see 30% vaporization in the reboiler. What is the Rich MEA temperature entering the Stripper? Is there a proper heat integration in the unit, particularly between Rich and Lean amine circuits?

I have never seen vertical thermosyphons in Amine service, since these devices are commonly used for smaller duties and very clean services which. I am afraid, MEA certainly isn't. It seems like you want to achieve high duty with a fairly small heat transfer area (like vertical thermosyphons have), which results in requirement for high percentage of feed vaporization.

Yes, for water-based systems, the literature cites 10% vaporization being the practical maximum for vertical thermosyphons. And I wouldn't try to challenge this figure without some extensive experience in this field, if I were at your place.

[chegurke]

Thanks for your answer so far.

The value 30% was given to me within an Aspen Plus model which is the basis of my work. The vertical thermosyphon was chosen because, as you said, high duty should be archieved with as small area as possible (and furthermore little space requirement). Furthermore in Couper et al. "Chemical Process Equipent" there are some values for heat transfer coefficents and for amines and water with steam there this value is for vertical thermosyphons.

Literature also told me vert. thermosyphons would be preferred if there is no clean service especially compared to kettle reboiler (said to be dirt collectors)

The model i've been given uses a lean/rich-exchanger so the MEA entering the reboiler at approx. 125°C at 2.05 bar.

[Zauberberg]

I would still challenge the 30% figure. Kettles are known as reliable devices if the system is operated properly, which includes the MEA reclaimer as well (mandatory). And they can handle very high vaporization rates, up to 70-80% of feed. There is no better way to confirm whether you are on the right track, than to contact some of the heat exchanger vendors and see if they can support your preliminary design. It looks very unlikely.

What is the lean MEA loading at the Regenerator outlet?