Hydrate Formation for Gas with (-15degC) Water Dew Point 2

[EmmanuelTop]

Good day,

Can somebody help in determining Hydrate formation temperatures for a partially dehydrated gas stream, at 30 barg and 45 barg?
Water dew point of gas is (-15)degC at 45 barg.

Thank you
Best regards

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[PaoloPemi]

you can obtain a estimate of hydrate formation conditions with methods discussed in GPSA,
for a good review of the different methods available in literature I suggest Sloan's book, Clathrate hydrates of natural gas, the book includes also a software which allows to estimate hydrate equilibria conditions,
you may also consider a process software, I do these calc's with PRODE PROPERTIES (see prode.com) which includes Excel pages to print hydrate formation temperatures, pressures etc.
the std. methods are based on EOS (PRODE adopts CPA which should give good accuracy) for gas & liquid fugacities plus Van der Waals and Plateeuw for hydrates.

 

[EmmanuelTop]

Thanks for the answer.

What confused me is the fact that Aspen predicts hydrate formation below the water dew point temperature, for given pressure (-11degC @ 45 barg). Is that possible?

Regards

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[EmmanuelTop]

Apologize - it should have been written above the water dew point temperature, at given pressure.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[PaoloPemi]

hydrates can form above water dew point,
and below water dew point temperature (with free water present),
in fact the presence of free water is not required,
however consider that water is present (with different amounts) in almost all natural gas streams,
the std. procedures available in process software (see previous post) do not estimate the time required for hydrate formation but only the conditions,
for the details you may consider the above book or equivalent literature.

 

[MortenA]

@Paolo
I agree that hydrate can form even in the absence of free water, and that both thery and experimetal data confirms this. However, in practise its my firm belief that this has never been a problem in actual operation of pipelines. I belive this to be caused by kinectics of the hydrate formations at very low temperature, perhaps in combination with the nature of the crystals that forms (small and without tendency to aglomerate). So at the end o the day - while i also agree that this is a complex subjet that cannot be "designed" here i would consider this a very low risk (in the risk matrix: "Never observed").

 

[zdas04]

Morton,
I don't think you have that one quite right. Just like the presence of liquid water will poison the reaction in a glycol dehydrator, liquid water will poison hydrate formation in my experience. Clathrates can only form in gaseous species.

With a -15C dew point at 45 barg, the water content is 0.054 mg/SCM. That amount of water vapor available is too small for the hydrates that do form at around 7C to be more than just ice crystals in the flow and lack the volume to cause flow line problems.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[MortenA]

@David

Im not certain i get that - if you had a free water phase in a gas pipeline, you say that this will actaully _prevent_ growth of hydrates? Your description of what happens when hydrate form in gas with a very low water dew point mayches my understanding

PS: Its Morten (not Morton).

 

[zdas04]

Sorry about the spelling, my eyes are so bad that I can't make out most letters. When I get home from this trip I'm going to have the cataracts cut out.

I've seen quite a bit of hydrate problems over the years and I can't ever remember seeing water ice in conjunction with the hydrate. A common sequence is hydrates form at some temperature above the freezing point of water. Virtually all of the water vapor participates in the hydrate formation so there is minimal to no condensation. While the line is plugged, ambient temperature drops further and any water vapor in the non-flowing line would either condense and freeze as water ice or form hydrates. Looking at freed plugs I don't see any water ice (not fluffy) in with the hydrate (fluffy). I've never done a chemical analysis to confirm this, but that is my impression. I just don't see hydrates and water ice in the same place.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[dcasto]

I agree with Morten. in real life the hydrates are formed and sublime continually but rarely stay as solid when the gas is under saturated with water. If that were not true every pipeline dehydrated with regarding would eventually freeze up always

 

[PaoloPemi]

gas hydrates can form when gas molecules are trapped in a lattice of water molecules,
this happens everyday in oceans,
the total quantity of methane hydrates in the ocean sediment is estimated to be around 10,000 GtC...

 

[rotw]

MortenA,

Quoted
I agree that hydrate can form even in the absence of free water, and that both theory and experimental data confirms this.
Unquoted

I have limited knowledge and education on the phenomenon of hydrate formation. But that is often an issue to verify on seals in machinery for instance.
The teaching I have received on this matter is that when there is NO water in the gas, there is no need to check as hydrates cannot form. But here I read the contrary.

Could you please elaborate and explain more in details ? Is that a recent scientific finding?
What can be a typical gas content where there is NO water and presence of hydrate?

 

[PaoloPemi]

you are correct,
"hydrate structures have repetitive crystal units, composed of asymmetric, spherical cages of hydrogen-bonded water molecules" (from Sloan's book)
the point in discussion here is that water can be present in both vapor and/or liquid phase (see the original question),
we do not discuss the dynamics of formation (see my above notes) only the conditions.

 

[zdas04]

Hydrates in the ocean floor are really not a mystery. Where they form (which is a couple of percent of the gas seeps in the ocean floor, but staggeringly large masses at those points), the flowing gas displaces the liquid water and you have continuous-phase saturated-gas flow displacing the surrounding water at the point of entry. Over geologic time, many many tonnes of clathrate hydrates form and cap the seep down to a velocity where the gas can only bubble instead of flow. Hydrates still form within the bubbles, but much slower than in the earlier days. In any case the hydrates form from water vapor and a hydrocarbon gas, not from liquid water.

The presence of liquid water does not "prevent" hydrate formation across a system, but it does stop it at the point of coherent gas/liquid interface. Generally within a pipe, that interface is a tiny percentage of the total volume of gas.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[PaoloPemi]

actually the exact mechanism of formation is not clear,
there are light formers as C1 which are incondensable at those conditions but for heavy formers there are different points of view.
My note about the formation of hydrates in oceans (which has been a field of research since half past century) should suggest that hydrates can form in presence of liquid water, even at high pressures,
one could speculate about the exact mechanism but we can assume that it happens.
So, I suppose, there is sufficient evidence that hydrates can form with presence of water in vapor or liquid phase.

 

[MortenA]

David,

I think we agree on the ice-hydrate issue

At temperatures below freezing of water (normal) ice will form not hydrates.

Free water is not a "must" to form hydrate from a theoretical point of view although i have not heard of a pipeline where hydrates were a problem but no free water phase existed.

The formation occurs at the interface and this is often "small" eg at a dip - but in a multiphase pipeline this may actually be large. This - of course -assumes that the mp pipeline is operated below HFT. In most cases of concern its the shut-in case thats of concern and here the interface is small.

Looking for good litterature references? Include "Dendy Sloan" in your search - hes one of the best on this subject (if not the no. 1 man)

 

[PaoloPemi]

I do not think that once ice has formed it may change structure,
however ice doesn't trap all water present, ice has a vapor pressure and that water (in equilibrium with solid) can form hydrates.
As conclusion the software which I have tested (GEM, PRODE PROPERTIES etc.) do predict possible formation of hydrates below 0 C (water freezing point) when free water forms ice,
you may get a more accurate estimate with the multiphase (including ice) option available in both GEM and PRODE.
Of course with free water below freezing point one has additional problems to manage :-(

 

[MarcTroy]

thanks all for sharing knowledge on these topics.