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[ChemStudent99]

Hi all,

 

I'm designing a CO₂ absorption column using MEA solvent as part of a university assignment.

My feed consists primarily of hydrogen, carbon dioxide and methane, in that order. There are traces of carbon monoxide (mol fraction =0.03), oxygen (mol frac=0.002), nitrogen and extremely low quantities of longer-chain hydrocarbons.

I don't have any H₂S in my feed, nor any NH₃ or HCN contaminants and am using 15% wt MEA solvent. I am operating at a slightly higher pressure than atmospheric.

I am doing the mechanical design and was wondering if I can use carbon steel for the absorber bottom? Is SOHIC/HIC something to be concerned about under these conditions? From what I understand this only affects carbon steel if atomic hydrogen is introduced into the steel, which is caused by hydrogen sulphide. There is no possibility for hydrogen sulphide to be introduced into the plant anywhere, either. I will be using an immersed gas disperser to prevent wet CO₂ corrossion in the bottom of the absorber.  Will the quantity of hydrogen in my feed cause cracking? 

 

Edited by ChemStudent99, 22 February 2015 - 09:28 AM.

[Art Montemayor]

ChemStudent99:

 

I’ve had extensive experience in designing and building amine - especially MEA - systems for the removal of CO₂.  I started with the use of engineering contractor’s designs based on 20% and quickly abandoned that to an MEA system using 12-15% MEA (wt. %) that succeeded in eliminating corrosion and operating problems.  Your 15% wt MEA (as max.) is the correct solution strength.  If you are a ChE student, you should be aware that the MEA is in a solution state that absorbs CO₂; it is NOT a solvent.

 

I’ve been a professional ChE for 55 years.  I’ve never heard or read of “SOHIC/HIC”.  Please: always identify your choice of acronyms.  I can’t read your mind.

 

Carbon steel - specifically A-516 - is perfectly suited for MEA absorber fabrication.  It will last a very very long time.  Of the carbon steel absorbers I designed and built, all lasted well over 25 years.  The only reason they were demolished was to replace or remove them for bigger or other processes.  We never detected any corrosion in the absorbers.  I have even fabricated MEA absorbers using carbon steel scrap plates from older absorbers.  Heat treatment is something I would definitely apply to the fabrication for stress removal - especially if the absorber is under relatively high pressure - over 100 psig.

 

I see no need for an “immersed gas disperser” to prevent wet CO₂ corrosion in the bottom of the absorber.  All you need to have is a sufficiently designed absorber sump to contain a suitable rich MEA solution inventory as well as a suitable feed gas injection space and volume to allow the gas to ascend up into the first stage of absorber contact with a conservative superficial gas velocity and no descending MEA solution channeling.  Presumably you have been issued a process design of the absorber specifying the correct diameter and height with a complete detailing of the contact internals.  You have also failed to explain this requirement.

 

You should be well aware of the fact that experience has shown and proven that MEA solution systems should always incorporate a "reclaimer" solution system.  This is fully detailed and explained in such texts as Kohl and Neilsen's "Gas Purification" (which you should have already read and studied).  This part of the process - as well as the use of in-line activated carbon filters - removes any tendency of the solution to degenerate and turn corrosive. 

 

I don’t have any idea of the effect of the Hydrogen content.  You fail to tell us the total composition and pressure of the raw gas feed.  That is simply bad specifications for a ChE.  You need to work on your skill in defining your engineering task in an engineering manner.  You need to be specific and accurate in your scope descriptions.