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.
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.
Dostları ilə paylaş: