7 replies to this topic

[B-2 Spirit]

Friends,

 

Please refer the following information:

 

Amine absorber operating at 25 bar abs. design pressure 37.5 bar abs.

Rich AMine in column sump and column height below fire limit.

 

I am doing a relief calculation for the PSV on this column. Fire case is the only applicable case as the equipment is designed for blocked outlet.

 

My understanding says that I will have to evaluate two scenarios:

(1) amine vaporization due to heat input from fire

(2) Vapor expansion due to heat input from fire (but this only for surface area between liquid height and fire limit of 7.6 m,

 

For (1) I am not able to find out the latent heat of vaporization of mdea at 37.5*1.21 barg as Hysys gives an error that temperature above range (this is because I am using Amine property package) but online also I am not able to get a latent heat of vaporaization for mdea at relief pressure.

 

For (2) I guess my relieve load will not be higher. Can I safely ignore this?

 

Is my understanding correct for sizing this PSV?

 

Thanks for your help.

 

 

Attached Files

•  Doc1.doc   271KB   64 downloads

[ColinR33]

That is a tough one as MDEA does not readily vapourize at elevated pressures.  You will likely end up boiling off the water portion of the solution while the MDEA decomposes thermally, although some MDEA will certainly be entrained with the steam being generated.  A conservative sizing can be obtained by assuming the liquid in the vessel is water so the relief is essentially steam.

 

Cheers,

[Propacket]

Amine package is good for simulating sweetening process. However, it does not have capabilities as other packages do. It may not be able to provide several properties including critical properties, latent heat etc. I have often observed it giving wrong results when there is a hydrocarbon phase condensation involved for example in Inlet Separator. 

 

Anyways, you can use Peng Robinson EOS and add amine in it. It will give you a warning which you can override. Peng Robinson will give you everything you need. By the way, as Colin suggested, water is the lighter component. So you can base your calculations on it in order to get the worst case sizing.  

[fallah]

B-2 Spirit,

 

1) Appears the PSVs (two) installed aren't for only fire case, because for such case normally one PSV is installed and the spare kept in ware house

 

2) Using ProII, you can calculate the average latent heat of vaporization of MDEA solution, within a specified range of weight percent.

 

3) To calculate relief load in vapor expansion case (dry vessel) normally total surface of the dry vessel is to be considered.

[B-2 Spirit]

Naser,

 

(1) The philosophy in our project is to provide spare for PSVs with Fire Case.

(3) I failed to find this in API 521, Its not specified there that in case of vapor filled vessel, we have to consider the total surface of the vessel.

 

Propacket & ColinR3,

 

yes, one can consider taking the latent heat of water at the set pressure, but I have 2 questions:

(1) if we assume that water is boiled and leaves the PSV, what is the dryness of this steam, I ask this question because the latent heat value will depend on the dryness, 100% dryness that is saturated steam will have the highest latent heat and 0% will have the least. I assume 50% dryness can be a safe assumption.

(2) At elevated pressure the latent heat of vaporization of amine can be lower than steam, I cannot get the latent heat of mdea at that pressure from literature or from simulation, but I guess amine will follow normal behaviour.

 

regards,
B2 Spirit

[ColinR33]

B2,

 

As long as you are below the critical point, the vapour being generated is in equilibrium with the liquid at the relieving pressure.  As the rate of relief is defined by the rate of heat absorption divided by the latent heat of vapourization, this typically means the vapour generated is saturated.  For a multicomponent system this means the temperature will change as the liquid is boiled off, but in this case we are assuming water only (imperfect I know, but adequate for this), so the temperature should stay relatively constant.  With the steam being in equilibrium with the liquid, this means the steam is essentially saturated with minimal superheat.  After the water has boiled off the steam will start to superheat, but the relief rate will drop off dramatically so this will not be the sizing case.

 

The latent heat of vaporization of the amine is irrelevant as it will not boil at the typical relief pressure of an amine contactor (note that amine reclaimers are typically operated at atmospheric or slight vacuum conditions in order to be able to boil the amine off and leave the sludge behind).  The water will boil off first and once it is gone the amine thermally degrades at temperatures above 500°F, breaking down into solids, ethylene oxide, HC's and other components.  Vapour generation rate at this point is much lower than the steam generation rate from the MDEA/water solution, which will be your PSV sizing case.

 

Cheers,

[B-2 Spirit]

_{Thanks Colin,}

 

_{thats was a nice explaination. I will consider this in my calcs. I think I can safely assume that I am below the critical point. whats your view on this?}

 

_regards,

[ColinR33]

Yes, you should be, critical point for water is around 22MPa I believe.