B41oa oil and Gas Processing Section a flow Assurance Heriot-Watt University

Worked Example 2: Question 
Example: Find the hydrate stability zone for the following gas mixture 
(composition in mole%) at 283 K using specific gravity and distribution 
coefficient methods. 
Gas composition: C
1
=89.9, C
2
=6.0, C
3
=3.6, iC
4
=0.5 

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TOPIC 1: Gas Hydrates 
 
 
 
51 
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3 
Worked Example 2: Solution 
Dissociation pressure = 2.07 + (2.76 - 2.07)/(1.319 - 0.945) x (1 - 0.945)
= 2.17 MPa 
From Figure-22 the natural gas dissociation pressure for the natural gas with 
specific gravity of 0.624 at 283K is around 2.7 MPa. 
………………………………………
..
Using a temperature and pressure combination within the hydrate-stable 
region of the phase diagram, together with the total gas composition (on a 
water-free basis), we can write 
(
)
(
)
0
1
1
1
1
,
,
=
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
−
+
−
∑
=
n
i
i
VS
i
VS
i
K
V
K
z
…………………………
.....(1.10)
In which 
i
z
is the original gas composition (i.e. before hydrate is formed) and 
V
is the vapour phase fraction – this expression is solved iteratively to find the 
vapour fraction 
V
.
When the appropriate vapour phase fraction has been 
obtained, the equilibrium vapour phase composition 
i
y
can be determined 
from 
(
)
1
1
,
,
−
+
=
i
VS
i
VS
i
i
K
V
K
z
y
…………
..
………………
...
…
..(1.11)
Where 
i
y
the composition of the gas that remains after the hydrate is is 
formed.
The composition of the equilibrium solid 
i
s
is calculated from 
i
VS
i
i
K
y
s
,
=
…………………………………………
.
…
(1.12)
Where 
i
s
is the composition of the gas mixture trapped in the solid hydrate 
phase. 

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TOPIC 1: Gas Hydrates 
 
 
 
52 
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3 
These calculations are analogous to vapour-liquid flash calculations. The 
above K-value method was originally designed specifically for hydrate 
formation from gaseous species – that is, gas-solid phase equilibrium.