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Topic: Design of an MDEA unit: some questions  (Read 30451 times)

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Offline apex

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Re: Design of an MDEA unit: some questions
« Reply #15 on: May 04, 2008, 05:23:43 AM »
Hi eugenedakin,

yes i meant corrosion inhibition. So in what form do you add corrosion inhibitors, it seems they can just form heat stables salts of any kind and don't decrease the corrosion rate? Could you give me some more examples of HSS? And why do HSS cause foaming, is this only when insoluble HSS are formed? What kind of passivation can occur?

Also R3NH+H2PO4- should be soluble, or at least at concentrations of 2wt%. Is that possible?

Best regards,

apex
« Last Edit: May 05, 2008, 06:09:13 AM by apex »

Offline eugenedakin

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Re: Design of an MDEA unit: some questions
« Reply #16 on: May 05, 2008, 08:50:21 PM »
Hello apex,

I am a specialist on corrision inhibitors. There are many ways (and methods) in which corrosion inhibitors are added. Some form heat stable salts, and some don't. My concern with the phophate inhibitor that you are proposing is that it will form insoluble precipitates. Yes, the initial amine-phosphate complex may be initially soluble, and this will change as soon as any type of contamination occurs.

Heat stable salts... there are many. Most are organic acids that are good foamers (emulsifiers). The list is quite long.

Passivation is actually a type of corrosion prevention. Is this what you mean?

Sincerely,

Eugene

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Offline apex

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Re: Design of an MDEA unit: some questions
« Reply #17 on: May 06, 2008, 02:46:00 AM »
Hi eugenedakin,

Yes I mean passivation as a prevention for corrosion. For example in
H2SO4 plants, the circulating concentration must be kept above 93%, below this value corrosion will increase greatly. Above 93% a solid layer will be formed, ironsulphate, that will prevent further corrosion.

What kind of passivation can occur in MDEA units?

Many thanks,

apex

Offline eugenedakin

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Re: Design of an MDEA unit: some questions
« Reply #18 on: May 06, 2008, 08:07:19 PM »
Hello apex,

Here are some estimated corrosion rates. If all things remain the same (which often it does't) here are some corrosion rates for various metals in amine sweetening plants:

Carbon Steel ~ 45 Microns per year
304 SS ~ 10 Microns per year
316 SS ~ 5 Microns per year
Monel ~ 2 Microns per year

http://www.gastreating.com/pdf/Hc%20Processing%20Apr-May%201993.pdf

There is most likely iron sulphides as the passivation chemical present to lower corrosion rates. The reason behind this is that corrosion rates severely increase in turbulent flow. This is most likely not limited to FeS, but various forms of iron sulphides.

Please be careful with dissimilar metals, as galvanic corrosion can easily occur in this conductive environment.

I hope this helps.

Sincerely,

Eugene


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Offline apex

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Re: Design of an MDEA unit: some questions
« Reply #19 on: May 08, 2008, 03:14:40 PM »
Hi eugenedakin,

What's the best way to design the carbon filter? We have no real hydrocarbon data here on the plant. I guess I need a Freundlich curve to design an accurate carbon filter? The engineer in the plant says I just need to look at the pressure drop, but that's a bit vague though. Do you know of any data I can use?

Best regards,

apex

Offline eugenedakin

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Re: Design of an MDEA unit: some questions
« Reply #20 on: May 08, 2008, 08:03:42 PM »
Hello apex,

If you are measuring a carbon filters 'end-life' by its pressure differential, then you don't need a carbon filter.  A carbon filter only removes organic compounds, it should not be used as a regular particle filter.

I hope this helps,

Sincerely,

Eugene
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Offline apex

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Re: Design of an MDEA unit: some questions
« Reply #21 on: May 09, 2008, 02:41:03 AM »
Hi eugenedakin

There already is a particle filter. What I ment was that I just decide the dimensions of the carbon filter so the pressure drop won't exceed a given value.

How do you measure the end-life the carbon filter in your plant? You actually measure the hydrocarbon concentration in the amine solution? Or you look at foaming, the level indicator in the absorber or something?

Best regards,

apex
« Last Edit: May 09, 2008, 09:36:18 AM by apex »

Offline eugenedakin

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Re: Design of an MDEA unit: some questions
« Reply #22 on: May 09, 2008, 08:55:17 PM »
Hello apex,

I see, you already have a particle filter.  :) A carbon filter works by diffusion.  The more contact time that a fluid has with the activated carbon, the more hydrocarbon is removed from the fluid.

Most carbon filters are assembed in bypass-flows.  Usually, a sidestream flow (at a very low flow rate) is passed through the carbon filter.  This provides sufficient time for the hydrocarbons to diffuse into the activated carbon.

Quote
How do you measure the end-life the carbon filter in your plant?

Great question!  Usually, a constant increase in hydrocarbon concentration typically indicates that the carbon filter is saturated and needs to be replaced.  The best way to maximize the life of the filter is to prevent the hydrocarbons from entering the amine in the first place  ;)

Yes, foaming is another indicator of excess hydrocarbons.  Usually, the hydrocarbons which are causing foaming are large (asphaltenic or paraffinic).  Light hydrocarbons are typically removed with the acid gasses because they have a low boiling point (similiar to acid gases).

Great questions!

Sincerely,

Eugene
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Offline apex

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Re: Design of an MDEA unit: some questions
« Reply #23 on: May 10, 2008, 09:28:15 AM »
Hi eugenakin,

I see, you just put a small stream throught the carbon filter. In the current MDEA-unit in the plant where am i doing my internship, they send the whole flow over the carbon filter, and when the pressure drop becomes to high, they just regenerate it with steam.

I was wondering though, will the pressure drop really increase when the filter becomes more and more saturated? As you said, if there is a particle filter in place before the carbon filter, the pressure drop over the carbon filter should not increase much.

Best regards,

apex

Offline eugenedakin

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Re: Design of an MDEA unit: some questions
« Reply #24 on: May 10, 2008, 03:17:56 PM »
Hi apex,

Yes, when you put the entire stream through the carbon filter, the carbon filter is quite inefficient (diffusion doesn't work as well)   ;)

If you are using a carbon filter the way in which it is designed, there should be almost no pressure drop. If there is a differential across the carbon filter, and you have a particle filter upstream, then the particle filter should be exchanged with a higher mesh or lower particle size.

I hope this helps.

Sincerely,

Eugene
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Offline apex

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Re: Design of an MDEA unit: some questions
« Reply #25 on: May 15, 2008, 12:19:06 PM »
Hi eugenedakin,

I wrote this about stripper pressure in my endreport:

The regeneration process is not always better at lower pressures. The stripperpressure sets the reboiler temperature, how higher the pressure, how higher the bubble point in the reboiler. The reaction which sets the H2S free out of the amine is faster at higher temperatures, seeing the reaction constant is higher at higher temperatures according to Arrhenius. The masstransport of H2S however is slower at higher pressure. The best conditions in the stripper are a combination of these effects and therefore differ from situation to situation. Setting the lowest possible pressure is not always the best solution.

Do you agree with this?

Best regards,

apex

Offline eugenedakin

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Re: Design of an MDEA unit: some questions
« Reply #26 on: May 15, 2008, 08:00:51 PM »
Hi apex,

It looks like you have put alot of thought into this.  Well done.

The regeneration process is usually better at a higher temperature and a lower temperature.

But you are correct in saying that the lowest possible pressure is not always the best solution...the only time this is possible is when you have a very high amount of foaming - which means that you have a high level of contaminants.

I suggest that you mention that the regeneration process is better with both lower pressures and higher temperatures.

I hope you get a good mark on your report.  :)

Sincerely,

Eugene
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