Phase behavior of natural gas production !

[quangkhoa90]

In natural gas production, the wet gas coming to the processing plant is in the retrograde region ( in our plant, the natural gas is coming at 109 bar ). I would like to understand why 109 bar but not 130 bar or 100 bar.

Here is my thought: The presence of liquid phase may impair the velocity of the gas in the pipeline, hence, in order to avoid liquid formation, the wet gas is compressed above the critical pressure on offshore platform, so that the wet gas is considered as dense fluid. Through the pipeline from offshore to processing plant, due to the pressure drop, the wet gas coming into the Slug Catcher may be at the point between critical point and Cricondentherm point of the phase diagram.

Thanks in advance

 

[apetri]

you should ask the engineers who designed the plant...
there are many variables to consider,
you may evaluate P&I / PFD etc. , phase diagram & fluid properties etc.

 

[EmmanuelTop]

What is the nature of the reservoir fluid? Is it gas only, or gas condensate? You need to provide us with more information - particularly in view of the data you provided in your earlier thread where you say there is liquid phase collected in the plant slug catcher. So are you getting condensation in the pipeline or not?

Operation in dense or super-critical phase is not uncommon, particularly for long distance onshore and offshore pipelines. The key reasoning behind this concept is to prevent from liquid condensation along the pipeline and avoid all problems associated with two-phase flow (slugging, erosion, high pressure drop, corrosion in stagnant areas, etc.). There is a good deal of information in Campbell's Volume 1 (see attached extract on phase envelope) and various petroleum engineering handbooks. Also see

http://www.slb.com/~/media/Files/resources/oilfield_review/ors89/jan89/1_pvt.pdf

Dejan IVANOVIC
Process Engineer, MSChE

 

[LittleInch]

quang,

A bit difficult to say without more details, but dense phase pipelines normally try and operate like that al the way into the plant to prevent the issues you are talking about. However things ma have changed since deign in terms of the fluid or maybe the balance between high pressure and line size / wall thickness went the way of lower pressure at the plant.

The fact you have a slug catcher there implies that the design was always going to accept arrival below the critical pressure

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[quangkhoa90]

Dear all,

Thanks for your help, it is gas condensate and here is the composition of gas coming from offshore (%mol) : N2 (0.21), CO2 (0.06), C1 (70.85), C2 (13.41), C3 (7.5), iC4 (1.65), nC4 (2.37), iC5 ( 0.6), at 109b (gas pressure coming from offshore) , 26°C.

I am using Hysys V7.3 and looking for a good book for training myself, do you have any recommendation ?

Thanks

 

[georgeverghese]

To get an accurate locus of the dewpoint and bubble point map ( and other V/L locus lines) for a multicomponent non associated natural gas stream, it is essential to get good characterisation of the C6plus components - we see here there is nothing stated about C6+, so observations from the phase map with this component composition will be far from what is happening in reality.

 

[apetri]

in addition to the Campbell's books suggested by EmmanuelTop I would recommend the GPA Engineering Data Book, on a short list I would include also Phase Equilibria (by Valas) Perry (to cover many topics) and some specific for compression (for example Hanlon), transport etc.

as noted by georgeverghese natural gas compositions in most cases do include heavy components (above the C5 you mentioned) and these have a large influence on phase equilibria (the same for water and other polar fluids in the mixture),
see GPA Engineering Data Book for the procedures to characterize heavy components,
also, with water or polar fluids you may need to select specific thermodynamic models and the resulting phase diagram may show some irregularities, for some examples see

"

http://www.prode.com/en/phaseenvelope.htm"

 

[srfish]

I ran this gas sample through our in-house condensing curve program and got a dewpoint of 11 C.

 

[quangkhoa90]

Hi,

Here is the feed composition. Could you please generate the phase diagram ? What is the fluid characteristic at 109 barg, 26°C (feed condition) ?

N2: 2.09998E-3
CO2: 5.9994E-04
C1: 0.7085
C2: 0.1341
C3: 0.075
iC4: 0.0165
nC4: 0.0237
iC5: 6.2994E03
nC5: 7.2993E-03
C6: 5.0995E-03
C7: 2.5997E-03
C8: 1.7998E-03
C9: 7.9992E-04
C10: 2.9997E-04
CyclC5: 4.9995E-04
MCyclC5: 4.9995E-04
CyclC6: 3.9996E-0.4
MCyclC6: 4.9995E-04
Benzene: 3.9996E-04
Water: 0.013

 

[apetri]

in that mixture the water 0.013 fraction could create problems with std. EOSs such as PR,
the above mentioned Prode Properties includes several models capable to handle hydrocarbons + water,
in order to evaluate the number of phases and properties of each phase at 109 Bar.g 26 C simply solve a isothermal multiphase flash (with your software or Prode Properties or another sofware).

 

[srfish]

I would run this again but our software doesn't handle the new components listed below C10.