TOPIC 1: Gas Hydrates
58
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
The main shortcoming of the graphical technique is that it may face some
problems in systems without a vapour phase, in
particular if the pressure
transducer is not very accurate and/or fluctuations occur in bath temperature.
For these cases the isochoric operation should be coupled with the visual
observations. However, the graphical method seems to be the only reliable
method for measuring the hydrate dissociation
point for oil systems, due to
poor visibility and possible emulsion formation in such cases.
To determine the hydrate dissociation point, the cell is charged with water and
the test fluid:
•
The temperature is lowered until hydrates are formed.
•
The temperature is then raised stepwise allowing sufficient time (at
least 4 hours) at each step for equilibrium to be achieved.
•
At each temperatures step below the dissociation point, some hydrates
are decomposed and the released gas raises the pressure markedly.
•
Once the temperature is above the hydrate dissociation point, with no
more hydrates to decompose, the pressure rise is quite small.
•
A plot of the pressure versus temperature will therefore give two
distinct lines of different slopes, with the break over at the dissociation
point. Figure 31 shows typical hydrate dissociation point measurement
using graphical techniques.
Figure 31: Graphical Hydrate Dissociation Point
Determination
With visual techniques, at each temperature or pressure step the system is
checked for the presence or the absence of solid gas hydrates. According to
some researchers who
have used visual techniques, if the pressure and
temperature of the system remain constant for 3-4 hours with a negligibly
small
quantity of hydrates present, the measured pressure and temperature
are taken as the dissociation conditions.
TOPIC 1: Gas Hydrates
59
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
Dostları ilə paylaş: