9 replies to this topic

[Afshin1355]

Hi,

We have designed an MEG injection package to be installed on an offoshore platfrom in order to inject to the departing pipeline. The MEG tank volume is only 3.5 m3. Depends on operating condition and season, MEG may or may not be injected. In other words, MEG injection will be stopeed for few months. I would like to know whether it is necessary to have Nitrogen Blanketing for MEG tank. MEG is not as volatile as Methanol and there would be no safety concern. However I am not sure if degradation is the case for MEG. Please share your experience and if you know any refernce/code/standard to refer to, please do advise me.

Afshin

[Art Montemayor]

You are storing MEG (Methyl Ethyl Glycol), a very hygroscopic compound, right?  If so, why would you not nitrogen blanket this chemical?  The alternative is to have an atmospheric tank vented to the saturated atmosphere surrounding the ocean platform.  The expected results under such conditions are a contaminated MEG.

 

I would certainly put a nitrogen blanket on this tank - and I would apply a pressure of approx. 1-2 psig.

[fallah]

Afshin,

 

It is necessary the MEG storage tank to be equipped with the nitrogen blanketing system. Then the normal operating pressure for blanketing is around 20 mbarg.

[Afshin1355]

Thanks Naser and Art,

 

We are going to use MEG 80%wt as hydrate inhibitor to inject to Gas Injection Pipeline. This will flow into reservoir and obviously it is not recoverable. I could not find any reference project with the similar purpose. The ones I checked, MEG is re-used after regeneration. For instance, the regenerated MEG is transported through a piggy back line from onshore to offshore for re-injection. In this case degradation can happen and consequently has a big impact on OPEX.

 

In general blanketing shall be provided if:

A- flammable or explosive vapours may accumulate in a tank

B- the chemical may decompose upon contact with air

C- dissolved oxygen within chemical products needs to be minimised

 

I referred to MEG vendors documents to find out whether blanketing for my case is neccessary or not. Here are the findings:

1- MEG has a high boiling point, and the vapors in the tanks are relatively non-flammable (item A above is not applicable)

2- blanketing is reuired to exclude atmospheric moisture and to protect MEG against UV quality degradation. Our MEG is 80%wt i.e. no matter of absorbing moisture. Degaration in contact with air was not highlighted by vendor. (item B and C above are not applicable)

 

I concluded the N2 blanketing can be dismissed in my case. If you think it is really mandatory, please provide me standard/code number, reference or a reason. I know engineers always refer to other go-by projects and usually nobody checks whether the design can be deviated or not.

 

The reason why I am not adding N2 blanketing is another story that we are facing in our contract.

 

Thanks

Edited by Afshin1355, 02 October 2013 - 11:12 PM.

[fallah]

Dear Afshin,

 

 

Please refer to a sample MSDS of the MEG as attached, especially page 2 about the storage.

Attached Files

•  MSDS-MEG.pdf   26.68KB   112 downloads

[Afshin1355]

Dear Naser,

 

The MSDS you attached is for MEG 99.9% and it shall be padded by inert gas. But our case is MEG 80%wt. I have attached another vendor product guide which is not strongly recommend for inert gas blanketing.

 

I also contacted a vendor about the subject. Here is their reply:

 

The purpose of N2 blanketing is to prevent the UV damage and moisture from surrounding. This helps to maintain the MEG to be the polyester grade. If the aqueous MEG is used in their system, I believe both quality parameters are not the inquirer looking at so this is at their option to use N2 padding. The inquirer should have responsibility to determine based on their technical knowledge of their operation system.

 

 

Let me know what you think. 

Attached Files

•  N2 Blanketing for MEG Tank.pdf   122.23KB   129 downloads

[curious_cat]

 

 

Our MEG is 80%wt i.e. no matter of absorbing moisture.

The MSDS you attached is for MEG 99.9% and it shall be padded by inert gas. But our case is MEG 80%wt. 

 

Even if it is 80% the question is if you keep it in contact with humid air you will absorb in more moisture till it gets saturated (in theory at least). 

 

Mass transfer limitations apart, do you really want this to happen? It may not stay 80% it may become 70% or whatever it is that the equilibrium water content is when in contact with your ambient air. 

 

I don't think that's desirable. In the limit you lost all your water uptake capacity that you need when you will inject it into your pipeline. Although not strictly degradation, dilution is your enemy here.  

 

An option is to use dry air to pad instead of N2 but I doubt that's going to be much of an advantage. Probably a hassle. 

[ColinR33]

A little off topic, but on the gas injection units (acid gas disposal, CO2 for EOR) I have worked on we always dehydrated the gas prior to injection.  Have you looked at the economics of a TEG dehydration unit vs MEG consumption?

[fallah]

Dear Afshin,

 

I don't think the concenteration wouldn't affect so much on MEG storage requirements as mentioned in typical MSDS i have attached. In fact, padding is required in MEG storage mostly due to its potential health effects and probable degradation in contact with the air.

[Afshin1355]

Colin,

 

Our project is brownfield offshore platform. There is no room for TEG Dehydration. You are right, always the Injection Gas is dehydrated before flowing to GI pipeline. However, in our project, client decided to use wet gas for this purpose. Actually we had a serious technical challange for hydrate formation in the pipeline and probably downstream of choke valve. Wet Gas at dense phase condition to be used as Injection Gas! We proposed the solution to use MEG in case it is required to prevent hydrate formation. Hope it works!