×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

liquid water carryover in natural gas

liquid water carryover in natural gas

liquid water carryover in natural gas

(OP)
Hi,

Can someone indicate a formula to calculate the quantity of liquid water carry over in a natural gas stream (hydrocarbon gas), knowing the gas composition, actual pressure and temperature ?
It can be also a formula which is function of the relative humidity ?

any indication please ?

Thank you
regards

RE: liquid water carryover in natural gas

Is this free water entrained in the gas stream or molecular water absorbed or adsorbed by the gas molecules in the gas stream?

RE: liquid water carryover in natural gas

(OP)
this is just free water entrained in the gas stream.

RE: liquid water carryover in natural gas

Then continue to dream.

This month's SPE Oil & Gas Facilities Engineering magazine has an excellent article by Mark Bothamley (Chief Engineer at JM Campbell) that goes into the exotic and esoteric math and the way I read his conclusion is that you gotta guess.

If you want water vapor (which SNORGY was correct in assuming when you said "it can also be a formula that is a function of relative humidity") then there is no formula. There are a family of curves called the McKenna Chart that you can look stuff up, but there isn't an equation to go with the curves. Several have been presented at eng-tips.com but none of them do a very good job across the possible range. You can generate your own relationship based on the ASTM tabular functions (which can be purchased from anyone who sells codes and standards). I did that and spent just over 2,000 hours developing three empirical equations that match every point on the McKenna Chart. I use the equations in programs that I write, but I don't give them away (I figure at my hourly rate I spent nearly $400k developing them). You could do the same. I found that a solve block in MathCad with 7 unknowns gave me the best fit, and it improved when I put temperature in an exponential term, but the content of that term was kind of difficult to determine. It is a different equation less than 3 bara, more than 10 bara, and between 3-10 bara.

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"

RE: liquid water carryover in natural gas

(OP)
Thank you David for your insight.
I was not aware it was that difficult.
I looked into the well known formula used for moist air then I only guessed that it would be a different story for other gas like Hydrocarbons.
But then how this is being calculated then in Hysys when you use a mixer component?
Do they have some complex built in algorithms for that?

RE: liquid water carryover in natural gas

Pretty much the standard answer to anything about HySys is "they have complex built in algorithms (and tables) for that".

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"

RE: liquid water carryover in natural gas

as said by zdas04 there are no simple formulas capable to provide high accuracy,

however you may consider ISO 18453 standard,
it's more accurate than charts,
another good option is GERG 2008 and, of course,
a EOS with complex mixing rules.

All these methods are available in tools which you can utilize
in union with Excel, Marlab or Mathcad,,
or embed in your code,
see for example

http://www.prode.com/en/properties.htm

RE: liquid water carryover in natural gas

(OP)
Thanks for the link. I am currently looking at the ISO 18453.

By the way with regard to the pressure of saturation of the water (in order to calculate the relative humidity for instance) and dew point calculation, Is it correct to used the IAWPS steam correlation when the water is mixed to Hydrocarbon ?
Is it a kind of good approximation?

I have heard that in presence of acid gas especially at high pressure, the water dew point prediction using the IAWPS correlation becomes less reliable. is it the case especially for acid gas or are you aware about other gases than acid gas which compromise the use of the correlation above ?

RE: liquid water carryover in natural gas

it depends from the model which you decide to adopt,

ISO 18453 has modified Peng Robinson with specific alpha functions and temperature dependent Kij
this because water is a polar fluid and std. VDW mixing rules don't work well.
Note that ISO 18453 has a relatively limited range of application but it is accurate.

With water vapor pressure model (based on IAPWS or another vapor pressure correlation) you treat water as separate component, you cannot not get accurate values with this assumption (although it can work in some cases).

A better approach (especially if you have H2S, CO2 Glycols etc. in the mixture) is to use a EOS with complex mixing rules (Huron Vidal, Wong Sandler etc.) or association term (as Cubic Plus association) or others,

all these models are not easy to code,
as said I prefer to use a process library (PRODE PROPERTIES) which does all the calc's for me.

RE: liquid water carryover in natural gas

(OP)
thanks Paolo.

quote
With water vapor pressure model (based on IAPWS or another vapor pressure correlation) you treat water as separate component, you cannot not get accurate values with this assumption (although it can work in some cases).
unquote

What are the cases where it can work ?

RE: liquid water carryover in natural gas

it may work well when you can model the components in the mixture as near ideal fluids,
for example at relatively low pressures and with limited interactions with water (not the case of CO2, H2s etc.)

RE: liquid water carryover in natural gas

(OP)
understand. But what about low CO2 content (say below 5-10% mol weight) and pressure ranges say below 100-150 bar.
are you confident to treat the water as separate component in these ranges or should be more restrictive ?

RE: liquid water carryover in natural gas

you can compare your model with a std. as ISO 18453
or some reliable procedure (see above posts) or experimental data (tables, charts) and see how it works,
this is the best procedure for all cases as I do not know a
simple rule to validate a model apart to compare results against reliable data...

RE: liquid water carryover in natural gas

And the 300 kg gorilla in that story is "reliable data". Calibration and repeatability of field instruments is not great and transporting a sample to a reliable instrument will always change it. Also, 5% CO2 is not "low". Take a look at the CO2 saturation curves in chapter 21 of the GPSA Engineering Field Data Book. They are really messy, and include an awful lot of water vapor per volume of gas. If you treat each component in a CH4, CO2, H2S system very carefully, then you can get a result that would lead you to a reasonable decision by combining the water vapor based on a mole percent of the constituents. With more than about 1% of the mixture being any heavier hydrocarbons you have to consider interactions.

Pressures below 100 bara is where the simplifying assumptions start deviating from reality. By the time you get to 4 bara mostly they don't reflect reality at all.

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"

RE: liquid water carryover in natural gas

"Take a look at the CO2 saturation curves in chapter 21 of the GPSA"

I can easily agree that you must decide some criteria to select "reliable data",
this is a common problem in process simulation where one needs good data sources to validate the models.

RE: liquid water carryover in natural gas

Sorry if my word choice is (was) poor...

When I said "...free water entrained in the gas stream...", I was envisioning an otherwise dry - even saturated for that matter - gas stream carrying water droplets at some slip velocity at or higher than the droplet terminal velocity. In such case, an EOS is not of particular importance and GPSA Chapter 7 ought to be able to deal with it. To me, that is what is meant by "...liquid water carryover in a natural gas stream...", i.e., a natural gas stream comes along, picks up liquid water droplets, and carries them over to some other place. That is a transport issue, not an EOS issue.

RE: liquid water carryover in natural gas

(OP)
Snorgy,

A more complete title would have been : "...Amount of liquid water carryover in a natural gas stream".
Trying to keep this issue focused to the EOS problem / correlation story that I do confirm was the intended topic

RE: liquid water carryover in natural gas

ISO 18453 has the advantage to give a "certified" accuracy within the allowable range of application (both usual and extended) which is good.
As noted by zdas04 a chart may result difficult to read and real accuracy in some cases may be questionable.

RE: liquid water carryover in natural gas

In every flow I've ever had to deal with, you cannot isolate this problem to just one thing. Water vapor will condense. Liquid water will evaporate. Both things happen in response to the environment they find themselves in at that nano-second. Both the slip/drag and the EOS are important to understanding your ability to transport a volume of fluid from point "A" to point "B".

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"

RE: liquid water carryover in natural gas

(OP)
The original subject of this post was the amount of water carry over. However this brought another question to mind.

In the case of a hydrocarbon gas comes close to its dew point and finally reach a point (temperature, pressure) where some amount of the gas is condensing and is carried over in the gas stream.
I deem the difficulty to estimate the amount of gas condensate is also a complex task, (or even more complex) and same questions as with water ?
Do we then have to deal also with tricky correlations not necessarily always valid and uncertainties issues, similar of what we have seen with water?

RE: liquid water carryover in natural gas

Water is the most prevalent fluid on the planet. It has been studied more than any other fluid. Hydrocarbon phase change is just as complex as water phase change. Yeah, you have to deal with at least as much complexity and even more uncertainty.

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"

RE: liquid water carryover in natural gas

(OP)
ok so all in all, if you add up all these combinations of water condensation, (eventually hydrates), gas condensate... then it becomes quite a mess going on there.
I was used to use simplistic approach in the past, this is just an example that things are learned to be unlearned, as I discover how complex the problem are in reality, especially with regard to what Zdas mentionned about 4 bar and 1% CO2/H2S in a mixture being sort of threshold.

RE: liquid water carryover in natural gas

As Engineers we often lose track of the concept of "good enough". I can design equipment and systems with enough excess capacity (whatever that means, it is different for every component) to be able to work effectively over most of the required operating range and to fail gracefully for the rest. I design compressors with the design point less than 70% of max available horsepower. I design gas/liquid separators with velocities about 1/2 of max velocity using traditional sizing. I never approach a boundary condition with my design. I have more successes than failures. If I insisted on less "brute force" and more "elegant solutions" then I would be very disappointed when Mother Nature hit me with the systems equivalent of a "100 year flood". I do this by never using code to do what I can do with local-sensing controls, avoiding delicate instruments, minimizing things that use control gas, etc. My projects are not elegant, but they are robust because I admit that multi-phase flow is too fraught with uncertainty and has too much innate randomness for elegant solutions that operate close to a performance envelope.

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"

RE: liquid water carryover in natural gas

(OP)
Zdas,

I really like and adhere to your approach.

This also remind a course I have attended long time ago on numerical methods. The Teacher was a master of computational methods.
But because he was a real engineer instead of a mathematician, although mastering the subject of numerical methods, he always told us to avoid using numerical methods when possible. What to use instead ? try to use basic equations, to define order of magnitude first.
Have a physical understanding of what is going there by defining the amplitude of the variables (velocities, pressures, concentrations etc.) by use of simple (analytical) methods. When you build that physical screening then "maybe" move ahead with numerical methods but with great care (avoid if possible to deal fluid dynamics problems). He also used to tell us that validation of numerical results is not via comparison with experiments, but it has to be mathematical, indeed by bounding the results with upper/lower limits for instance using the order of magnitude philosophy mentioned before.

I really liked this teacher and the caution and the serious in dealing with problems of mother nature.
That is why I say back to the liquid carry over topic, it was good to know about how things deviate from theoretical models.
It is also good to know which application requires accuracy and good confidence and on which one it suffices to have some sort of estimates.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources