Hydrates in liquid pipeline 3

[MortenA]

Not quite sure where to put this - but here goes:

Consider a condensate/water mixture in a pipeline.

Will hydrates form if there is no gas phase (but lots of methane/ethane and water) - and could they potetially dreate the same problems (blockage) as for a multiphase pipeline?

Best regards

Morten

 

[btrueblood]

Morten,

It's not clear to me what pressure/temperature you are talking about? From what I understand about methane clathrates, they form due to the slow production of methane in sediments, in the presence of low temperature (0-2 C) liquid water. The low rate of production of methane is necessary, since otherwise it would nucleate to form bubbles, and rise through the sediment to escape. Not clear that you have the same situation in a high-pressure nat. gas line, and that is your question?

 

[btrueblood]

Ah, wait. From wikipedia:

"Methane clathrates (hydrates) are also commonly formed during natural gas production operations, when liquid water is condensed in the presence of methane at high pressure. It is known that larger hydrocarbon molecules such as ethane and propane can also form hydrates, although as the molecule length increases (butanes, pentanes), they cannot fit into the water cage structure and tend to destabilise the formation of hydrates.

Once formed, hydrates can block pipeline and processing equipment. They are generally then removed by reducing the pressure, heating them, or dissolving them by chemical means (methanol is commonly used). Care must be taken to ensure that the removal of the hydrates is carefully controlled, because of the risk of massive increases in pressure as the methane is released, and the potential for the hydrate to let go with high velocity as it is exposed to a high pressure differential.

It is generally preferable to prevent hydrates from forming or blocking equipment. This is commonly achieved by removing water, or by the addition of ethylene glycol (MEG) or methanol, which act to depress the temperature at which hydrates will form. In recent years, development of other forms of hydrate inhibitors have been developed, such as Kinetic Hydrate Inhibitors (which dramatically slow the rate of hydrate formation) and anti-agglomerates, which do not prevent hydrates forming, but do prevent them sticking together to block equipment."

 

[MortenA]

I know all of that

Even wrote some of it

But im uncertain what would happen in a liquid pipeline with unstabilised condensate - plenty of methane - and water. Would hydrates form in the liquid suspension - or do they require methane/ethane in gaseous form?

The operating pressure is around 100-120 barg and HYSYS (and PVTsim) says that hydrate formation temperature is around 25ºC.

Best regards

Morten

 

[vzeos]

see attached publication "Gas Hydrates and Their Relation to the Operation of Natural-Gas
Pipe Lines

http://www.prci.com/publications/FreeReports/L41020e.pdf

 

[MortenA]

seems to be only for gasses - but still a good overview - a star for bringing this work to attention.

Best regards

Morten

 

[vzeos]

MortenA,
Thanks, see page 17. Reference is also made to liquefied gases. "...in tests with ethane and propane no difficulty was experienced in forming hydrates when the liquefied gases were in contact with water."

 

[Unotec]

Typically, you will not get hydrates as long as you maintain the conditions away from the gas phase. But brace yourself in case of an upset where the condy gasifies.

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[MortenA]

Unotec - Do you have a reference for this? Do you know if its thermodynamic or kinetics (the hydrates could form - but due to low formation speed they dont become a problem)?

Best regards

Morten

 

[Unotec]

I am based on my experience, but I stand corrected. Hydrates can form in a liquid phase, given a bunch of condition that I have never encountered in the field.... yet

"Experiments and theory both demonstrate that crystalline hydrates can form from a single-phase system consisting of liquid water with dissolved hydrate former under appropriate conditions of temperature, pressure, and dissolved hydrate former content. At these conditions, the pressure required for hydrate formation will be equal to or greater than that required when a separate gas or liquid phase of hydrate former is present. It is possible to form hydrates from a single phase when the mole fraction of dissolved hydrate former is greater than the mole fraction which would exist in the water-rich liquid phase at the three-phase (vapor/water-rich liquid/hydrate or VLH) hydrate equilibrium pressure. However, this would represent non-equilibrium or super-saturated conditions with respect to hydrate formation. It is also possible to form hydrates from a single water-rich phase when the mole fraction of the dissolved hydrate former is less than that which would exist in the presence of a gas phase at the three-phase VLH hydrate equilibrium pressure. The pressure requirement for the formation of hydrate increases as the mole fraction of hydrate former decreases under these conditions.

http://www.sciencedirect.com/scienc...erid=10&md5=63017a837dcfe53bd608a8562d36eee5"

Take a look, and I am getting the book. After this post I am sure I have jinxed the hydrates and I will now get them in my lines


<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[MikeClay]

If the system consits of a hydrocarbon condensate with free water, then hydrate formation can take place.

I'm sorry that I cannot give the specific referece, but this issue is addressed in the hydrate myths section of a client's hyrate guidelines manual.

--Mike--

 

[MortenA]

Mike

I think your reference is related to the "age old" question if hydrates will form in only if there is FREE water as a phase in the pipeline.

Thermodynamically you can prove that this is NOT true - hydrate can form even if the gas has a much lower dew point - but they tend not to - or to form very small crystals that dosnt become "sticky".

Best regards

Morten

 

[MikeClay]

Sorry, I from the words "lots of methane/ethane and water" I assumed the question involed condensate and free water?

--Mike--

 

[MortenA]

yes - but only as a liquid - with a free water phase (unstabilised oil) so i may have misunderstood you?

Best regards

Morten

 

[MikeClay]

I think we are talking the same. A mixture of condensate and free water with no vapor phase can form hydrates.

--Mike--

 

[dcasto]

Dr Dendy Sloan at CSM is the defacto expert. Hydrates will form with C4 and lighter and some alkene heavier. They will form in the liquid pipelines, been there and done that.

 

[MortenA]

I usually refer to Mr. Sloans book "Clatherate hydrates of natural gases" - and had i done that right away i could have saved myself the embarresment of displaying my ignorance ;-) The answer is right there on page 213 and some more pratical comments on page 569.

There you have it. And then i also learned (from the same book) that CSM is of course Colorado School of Mines. Not to be confused with CMS - something entirely diffirent i would assume.

Best regards

Morten