Best Amine Scrubber Temp 2

[sshep]

I have a few easy questions for someone with experience scrubbing H2S (design was 11% H2S) from an off-gas (basically CH4) using MDEA. First the background:

We have a small (18" dia) packed amine scrubber tower. Our amine comes from and returns to a refinery across the street. Originally the system was designed for MEA, but we are now supplied with MDEA. Originally there was no feed heater on the lean amine, but later (still >20 yrs ago) an exchanger was added using 335psi steam trapped to grade (i.e. no feedback controls). There is no documentation for that change, or any operating conditions. Per the operators a 10 gpm amine flow has always been used (no basis or any lab analysis). The heater is currently run only in the winter basically because it is on their winterization checklist, but the operators aren't otherwise sure what function it is supposed to serve. Various weak theories include: better mass transfer effi using hot lean amine, to keep condensibles from dropping out in the amine, and freeze protection of the return stream.

My situation is that the exchanger has been declared no longer fit for service by our inspection department and I am tasked with specifying a replacement. This is a non-code double pipe (or multitube) hairpin unit without any relief protection. I would like some help establishing a basis for a replacement unit and any needed ancillaries. Here are some questions I have before checking into anything:

What temp (I intend to have controls) will give the best scrubbing effi, and how important is the temp?
335psi steam seems a ridiculous choice- is there any hazard to "cooking" these amines?
What other purpose does a feed heater serve?
What sort of feed to amine ratio is typical?
What MOC should be used?
What amine concentration is typical for MDEA, and where can I get some properties for "as used" amine (including freeze temp)?
Any other useful info?

Thanks in advance for any help. If I don't respond between 26-30July it is because I am on holiday, but I will gladly recieve your advice on return. -sshep

 

[Montemayor]

sshep:

Excellent presentation and description of a problem. Before you go holiday, go to Bryan Research and Engineering's website at:

http://www.bre.com/software-TSWEET.asp

Where you will be directed to a library of free Amine Literature on design and operations. There, you will find a treasure trove of experienced Amine information - specifically dealing with what you are facing. I recommend you download and read all their free literature, but specifically I would direct you to read the following during your holiday in your leisure time:

1) Selecting Amines for Sweetening Units
2) Plant Conversion to MDEA
3) Design and Operation of MDEA Unit
4) Decreasing Contactor Temperature Could Increase Performance

Answers to all your concerns and queries are in these papers and the quickest way to transmit them to you is to have you read the material.

You are right about the ridiculous use of 335 psig steam to reboil MDEA. Amines will cook and degenerate into corrosives products. For physical properties on MDEA (like freezing temperature of solutions) go to your supplier - like DOW, Union Carbide, or any other you buy from. This information should be available on the internet as well.

Let us know if you need help after you soak up all the answers in the recommended literature. Good Luck and happy holiday.

Art Montemayor
Spring, TX