Page 243 - Handbook Of Multiphase Flow Assurance

242 10. Research methods in flow assurance

Apparatus used for xenon hydrate formation
A xenon gas hydrate may form from ice as well as from liquid water. In order to study
thermodynamic properties of gas hydrate below ice point the low temperature apparatus has
been used. A schematic diagram of the apparatus was previously shown in Fig. 10.12.
A spherical stainless steel reactor of spherical shape rated to 1500 psia was used to form
hydrate. One hundred and fifty stainless steel balls of 1/8 in. diameter were placed inside
the reactor. A reactor with stainless steel balls was shaken by the Thermolyne orbital shaker
in order to mix the contents and renew the hydrate formation surface. Reactor maintenance
consisted of regular cleaning with acetone, alcohol, and, sometimes, nitric acid to remove the
fouling from the walls of the reactor and steel balls.
The reactor was immersed in the constant temperature methanol bath. The temperature
of the bath was maintained within ±0.3 K at temperatures below 260 K, and within ±0.1 K at
higher temperatures by the Neslab on/off temperature controller. Cooling was provided by a
Neslab CC-100 II low temperature cooler.
The pressure of the reactor was sensed by two Heise gauges and by a Barocel manometer.
The Heise gauges were Bourdon tubes with ranges 0–2.07 MPa ±1.38 kPa and 0–13.79 MPa
±13.79 kPa. The Barocel manometer was a differential electronic manometer with the range
0–2000 Torr ± 0.001 Torr (0–0.267 MPa ± 0.133 Pa).
Water and liquid hydrocarbon were supplied to the reactor by vacuum distillation.
Evacuation of the apparatus was provided by the Trivac vacuum pump generating a vacuum
to 4 Pa. The cold trap was placed between the apparatus and the vacuum pump in order to
prevent pollution of the apparatus tubing with vacuum oil and to collect some of the waste
liquid after the experiment. The cold trap was cooled by the Flexi-cool cooler to 263 K.

Experimental procedure
A typical experiment started with evacuation of all tubing and the reactor. Stabilization
of the pressure with the vacuum pump valved off indicated complete removal of water and
hydrocarbon liquid.
A weighed sample of degassed water was vacuum distilled into the reactor immersed
into the cold bath by evaporation from the inlet flask and condensation on the cold walls
of reactor. Vacuum distillation transported 98–99% of liquid into the reactor which was de-
termined by weighing the reactor before and after distillation. Non-condensed vapor was
evacuated. Liquid hydrocarbon was then, if required, distilled into the reactor, which was
immersed into liquid nitrogen. All lines were evacuated after the distillation was complete.
Xenon of 99.999% purity purchased from Matheson Gas Products, Inc. was used in this
set of experiments. The gas pressure was set and gas was allowed to cool in the stationary
reactor for 10 min. The pressure was then adjusted to the starting value and the shaker was
started. Pressure was monitored with intervals of 10–30 min and recorded into the notebook
for further analysis. A typical curve of pressure versus time is shown in Fig. 10.15, similar to
that of Fig. 10.13. Gas was partially vented after hydrate formation rate approached zero and
hydrate decomposed. Pressure was then increased to the lowest point in the previous forma-
tion cycle and hydrates formed.
This process was repeated until the differential between the lower formation and upper
decomposition pressures narrowed to 7–14 kPa. The arithmetic average of the two values of