TOPIC 1: Gas Hydrates
53
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
The condition for hydrate formation in a non-aqueous liquid is then simply
given by
0
.
1
1
,
∑
=
=
n
i
i
i
LS
x
K
……………………………
.
………
..(1.17)
K
VL
values for the hydrocarbon species can be obtained from the literature or
calculated in the usual way (see notes from B49CE).
These are quite basic methods but have some reliability
and accuracy for
preliminary results:
•
The gas gravity method is usually accurate to within 20% for sweet
gases.
•
The K-factor method is usually accurate to between 10% and 15%,
plus the error associated with reading the chart.
Neither method can predict which hydrate structure is formed. However, there
are some rules of thumb that provide an indication:
•
Type II predominates.
•
Whenever only a small amount of type II hydrate
former is present in
the gas, the resultant hydrate is type II.
•
Type I hydrate formers will occupy the smaller cavities in the Type II
structure.
A small change in gas composition can result
in a big change in hydrate
formation pressure (this is because a different hydrate structure is formed). For
example, pure methane forms type I hydrate at 288 K and 12.8 MPa but the
presence of 1 % propane leads to the formation of type II hydrate at 7.7 MPa
at the same temperature.
These methods assume that there is a large quantity of water present and
therefore should not be used to determine hydrate
forming conditions in a
dehydrated gas.
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