A.1 BASIC THEORY
A material is classified as dielectric if it has the ability to store energy when an external
electric field is applied. For instant, if a DC voltage source is placed across a parallel
plate capacitor as shown in Figure A.1, more charge is stored when a dielectric material
is between the plates than if no material (a vacuum) is between the plates. The
dielectric material increases the storage capacity of the capacitor by neutralizing charges
at the electrodes which ordinarily would contribute to the external fields. The
capacitance with the dielectric material is related to dielectric constant as follows:
κ’ = εr’= C/Co (A.1)
C = capacitance with material = qd/V
Co = capacitance without material (vacuum) = A/t
A = area of the capacitor plates
t = distance between plates
Figure A.1 Parallel plate capacitor DC case
If an AC sinusoidal voltage source (V) is placed across the same capacitor (Figure A.2),
the resulting current will be made up of a charging current (Ic) and a loss current (Il)
Appendix __________________________________________________________________________
- 212 -
which is related to dielectric constant. The losses in the material can be represented as a
conductance (G) in parallel with a capacitor (C) as follows:
I = I charge + I loss (A.2)
I = V(jωC + G) (A.3)
I = V(jωCo κ’ + G) (A.4)
If G = ωCo κ”, then
I = V(jωCo κ’ + ωCo κ”) (A.5)
I = V(jωCo)(κ’ - jκ”) (A.6)
κ* = κ’ - jκ” (A.8)
consists of a real part (κ’) which represents the storage and an imaginary part (κ”) which
represents the loss. The following notations are used for the complex dielectric constant
interchangeably κ = κ* = εr = ε* r
Figure A.2 Parallel plate capacitor AC case
εr * = κ* = ε* / εo = (ε’ / εo) – j(ε” / εo) (A.9)
where:
ε0 = permittivity in free space ≈ 8.854 x 10-12 Farad/m
The real part of permittivity (εr ’) is a measure of how much energy from an external
electric field is stored in a material. εr ’ is > 1 for most solids and liquids. The imaginary
part of permittivity (εr ’’) is called the loss factor. It is a measure of how dissipative or
lossy a material is to an external electric field.
A.2 ORIENTATION (DIPOLAR) POLARIZATION
A molecule is formed when atoms combine to share one or more of theirs electrons.
This arrangement of electrons may cause an imbalance in charge distribution creating a
permanent dipole moment. These moments are oriented in a random manner in the
absence of an electric field so that no polarization exists. The electric field E will
exercise torque T on the electric dipole, and the dipole will rotate to align with the
electric field causing orientation polarization to occur (Figure A.3). If the field changes
the direction, the torque will also change.
The friction accompanying the orientation of the dipole will contribute to the dielectric
losses. The dipole rotation causes a variation in both εr ’ and εr
” at the relaxation
Figure A.3 Dipole rotation in electrical field
Appendix __________________________________________________________________________
- 214 -
A.3 MEASUREMENT
The schematic layout of the PNA system for measurement is shown in Figure A.4. The
dimensions and pictures of dielectric probes used for this study are shown in Figure A.5
for a Slim Probe and Figure A.6 for a New Performance Probe.
Figure A.4. Layout of Measurement system
Figure A.5. Slim Probe
A.4 COMPARISON BETWEEN SLIM PROBE AND NEW PERFORMANCE
PROBE
During initial studies using slim probe, we demonstrated that the dielectric constant
measurement has a potential application to detect hydrate water history. Further
investigation was done during this phase of work to investigate similar application using
a new Performance Probe. The experiment was designed to test water samples after
hydrate formation and dissociation (HDW).
The result indicated that the water memory was a combined effect of changes in water
structure and gas solubility. Water samples saturated with natural gas outside hydrate
phase boundary (SDW) was analyzed to investigate the effect of gas solubility.
Deionised water was used in all experiment as a reference data for determining changes
of dielectric constant with respect to various types of water samples. The measurement
was made for DW, HDW and SDW at atmospheric pressure and 4 oC.
The result shows that the new performance probe has higher sensitivity to detect
changes in different type of water samples. The improvement in sensitivity was
significant even with the worst case scenario where the sample prepared did not have
continuous mixing as compared the sample preparation with continuous mixing. The
comparison beween Slim probe and New Performance probe results are shown in Figure
Appendix __________________________________________________________________________
- 216 -
A.7 after 0.5 hour and Figure A.8 after 1 hour. As can be seen in Figure A.7, the
difference in dielectric constant with DW for HDW at 4 GHz is increased by 10 times
using New Performance Probe. The difference for HDW and SDW is very significant
in the case of New Performance Probe as opposed to Slim Probe (old) at 1 hour as
shown in Figure A.8.
Figure A.7. Comparison between Slim probe (old) and New Performance probe after
0.5 -0.6 hours.
Figure A.8. Comparison between Slim probe (old) and New Performance probe after 1
hour.
0
5
10
15
20
25
30
35
40
45
50
0 2 4 6 8 10 12 14 16 18 20 frequency / GHz
D iff
. i n
di el
ec tr
ic c
on st
W
HDW (0.5 hrs) SDW (0.5 hrs) HDW (0.6 hrs) - old SDW (0.6 hrs) -old
0
5
10
15
20
25
0 2 4 6 8 10 12 14 16 18 20 frequency / GHz
D iff
. i n
di el
ec tr
ic c
on st
W
HDW (1 hr) SDW (1 hr) HDW (1 hrs) - old SDW (1 hrs) - old
Appendix __________________________________________________________________________
- 217 -
B.1 INTRODUCTION
The objective of this work is to investigate techniques for monitoring changes in the
composition of vapour and liquid phases to detect the hydrate formation and the hydrate
‘memory’ effect for use in early warning systems. Hydrate ‘memory’ has been found to
remain in the aqueous phase for some time after hydrate dissociation. The studies based
on dielectric constant technique shows that the hydrate memory was influenced by both
gas solubility and changes in water structure. A very preliminary analysis on water
samples taken from hydrate dissociation which was used for dielectric constant
indicated that significant traces of i- and n-butane were present in water with a history
of hydrate formation. Therefore, a series of tests have been designed to investigate this
observation for natural gas-water systems. The main idea was to study changes in the
composition of liquid and vapour phases.
The study was focused into three parts. The first part was compositional changes during,
after hydrate formation and dissociation at atmospheric pressure. The second part was
the analysis of changes in liquid samples after hydrate dissociation from all samples
prepared for dielectric constant studies described in Section 3.3 of Chapter 3. The third
part was online analysis of vapour samples for two systems; natural gas – water samples
at low pressure and three components (95% C1, 4.5% C2 and 0.5% i-C4) system at
constant pressure. These results are included in this appendix except the second part
which are included in Section 3.3 of Chapter 3.
B.2. CHANGES DURING, AFTER FORMATION AND DISSOCIATION
B.2.1. Experimental set-up and procedure
Kinetic Rig 1 (shown in Figure B.1) with a total cell volume of 2400 ml was used. The
rig was charged with 1000 ml of distilled water then vacuumed to remove any air.
Natural gas (supplied by Air Products with composition given in Table B.1) was
injected into the rig to a pressure of 103 bar (1500 psia) at 20 oC. Fluids were mixed by
Appendix __________________________________________________________________________
- 218 -
means of a magnetic motor stirrer at 400 - 600 rpm. The fluid was allowed to equilibrate
for 24 hours before sampling vapour and liquid phases for analyses. The system
temperature was then reduced to 8 oC to observe changes in phase composition outside
the hydrate structure-I region (predicted based on methane), before the temperature was
reduced to 4oC to form hydrate. The fluid phases were then sampled again. Hydrates
were then left in the system for several days before a further sample was taken to
monitor any changes in the composition of the vapour phase. Hydrate was subsequently
dissociated to outside the structure-I region (based on the predicted hydrate phase
boundary using in-house HWHYD software given in Figure B.2) and vapour sample
was analysed again before the temperature was increased to 20 ºC to dissociate all
hydrate. The fluid phase was then sampled at several stages to monitor the effect of
hydrate water memory with time. The system was heated to 35oC for 24 hours, and then
the temperature was reduced to 20oC to study the effect of heating on hydrate water
memory.
Table B.1 Natural Gas Composition
Component N2 C1 CO2 C2 C3 iC4 nC4 iC5 nC5+C6 Total
Mole (%) 2.16 89.69 1.69 4.62 1.26 0.17 0.30 0.06 0.05 100
Pressure transducer
Coolant Jacket
Appendix __________________________________________________________________________
- 219 -
Figure B.2. Hydrate phase boundaries for s-I (methane) and s-II hydrates.
The vapour and liquid phases were analysed using a Varian 3800 Gas Chromatograph
(GC). Helium was used as the carrier gas with the flow rate adjusted to obtain a
maximum sensitivity of the detector. Pure grade helium (3 ppm water trace and 0.5 ppm
hydrocarbon) from Air Products was used. The carrier gas was cleaned by a molecular
sieve positioned at the outlet of the helium bottle. Two detectors were used to perform
the analyses: a Thermal Conductivity Detector (TCD) and a Flame Ionization Detector
(FID). The TCD measures continuously the variation of the thermal conductivity of the
carrier gas. The TCD is a non-destructive detector. The FID measures the compound
capabilities to form ions when passing through a flame. The FID is a destructive
detector, thus it is connected in series, at the outlet of the TCD. The analytical column
used was a Hayesep R 100/120 Mesh column (silcosteel tube, length: 1.5 m, diameter:
1/8 inch). The hydrocarbon components were detected by FID while water, nitrogen and
carbon dioxide were detected by TCD. 400µl of vapour sample and 0.5 µl of liquid
sample were injected in the chromatograph via the injector with syringes.
B.2.2 Results and discussions
Two tests were performed for natural gas-water systems based on the procedure
described in previous section. For the first test, hydrate started to form as the
temperature was reduced to 8 ºC based on the increase in torque and pressure drop
(approximately 4 bar) as shown in Figures B.3 and B.4. Significant hydrate formation
0
50
100
150
200
250
pr es
su re
Appendix __________________________________________________________________________
- 220 -
was detected as the system temperature was further reduced to 4oC. At this condition,
the stirrer appeared to stop functioning, most likely due to massive hydrate formation in
the rig at this condition. The stirrer was subsequently turned off to avoid over heating of
the motor. The system was left at this condition to form hydrates for 3 days before
proceeding to the next step. The system temperature was increased to 13oC to dissociate
s-I hydrate and the stirrer was then turned on to observe any reduction in torque
following this partial hydrate dissociation. This step was followed by hydrate
dissociation to outside phase boundary and heating to 35oC. The experimental path
within hydrate phase boundary is shown in Figure B.5.
Figure B.3 Gas hydrate formation and dissociation for Test 1.
Figure B.4 Gas hydrate formation and dissociation for Test 1. The numbers represent
samples taken for compositional analysis.
50
60
70
80
90
100
110
time / hr
pr es
su re
time / hr
pr es
su re
time / hr
pr es
su re
time / hr
pr es
su re
2
3,4
5
6,7,8
Appendix __________________________________________________________________________
- 221 -
Figure B.5 The path followed during experiment with predicted hydrate phase boundary
for Test 1. The numbers represent positions where samples were taken for
compositional analysis.
Table B.2 Samples collected for compositional analysis (Test 1) Sample
no
Description
Temperature
(oC)
Pressure
1 Before formation 20 105 / 1530 -
2 After formation 8 71 / 1030 -
3 After formation 4 67 / 970 -
4 After formation 4 59 / 860 After 3 days, vapour only
5 After dissociation
outside SI region
6 After dissociation 19.5 100 / 1457 -
7 After dissociation 20 100 / 1457 After 1 day
8 After dissociation 20 100 / 1456 After 1.2 days
9 After heating 20 99.8 / 1448
The liquid and vapour phase samples taken at various stages for compositional analysis
are detailed in Table B.2. For the vapour phase, the compositional changes with respect
to methane are presented in the form of percentage ratio of each component as shown in
Figure B.6 and also the percentage ratio of before to that of after hydrate formation
shown in Figure B.7. As seen from Figure B.7, significant reduction of the ratio of
ethane to methane (C2/C1), propane to methane (C3/C1) and iso-butane/methane
(iC4/C1) was observed due to hydrate formation. The ratio of carbon dioxide/methane
50
60
70
80
90
100
110
0 5 10 15 20 25 30 35 temperature / C
pr es
su re
1
2
3,4
5
6,7,8,9
50
60
70
80
90
100
110
0 5 10 15 20 25 30 35 temperature / C
pr es
su re
1
2
3,4
5
6,7,8,9
Heating
Cooling
Appendix __________________________________________________________________________
- 222 -
(CO2/C1) was observed to remain relatively unchanged in the vapour phase. This result
suggests that CO2 did not take part in hydrate formation to the same extent as C2 and
C3. Other components in the system were not significantly affected. After hydrate
dissociation, it was observed that the final liquid composition is very close to the initial
composition before hydrate formation after about 1.5 days. Similarly, the vapour
composition after subjected to heating at 35oC was close to that before hydrate
formation. On the other hand, there is no clear trend of compositional changes in liquid
phase as shown in Figure B.8 and B.9. This could be contributed to limited amount of
liquid withdrawn during sampling. The CO2 component was clearly detected due to its
high solubility in water phase. C1 and some traces of C2, C3 and C4 components were
also detected. During hydrate formation there were no liquid samples available for
analysis possibly due to conversion of most water to hydrate.
Figure B.6 Compositional analysis of vapour phase based on percentage ratio of
component (mole %) /methane (mole %) for test 1.
0%
1%
2%
3%
4%
5%
6%
7%
1 2 3 4 5
6 7 8 9
0 5 10 15 20 25 30 35 temperature / C
pr es
su re
6,7,8,9
1
1 2 3 4 5
6 7 8 9
0 5 10 15 20 25 30 35 temperature / C
pr es
su re
6,7,8,9
1
5
Appendix __________________________________________________________________________
- 223 -
Figure B.7 Compositional analysis of vapour phase based on percentage ratio of R after /
R before. R represent component (mole %) / methane (mole %) for test 1.
Figure B.8 Compositional analysis of liquid phase for test 1.
0%
20%
40%
60%
80%
100%
120%
140%
160%
180%
C2/C1 C3/C1 CO2/C1 iC4/C1 nC4/C1 iC5/C1 nC5/C1
1: Before Formation (20 C) 2: After Formation (8 C) 3: After Formation (4 C) 4: After Formation (4 C, 3 days) 5: outside SI (13 C) 6: After Dissociation (20 C) 7: After Dissociation (20 C, 1 day) 8: After Dissociation (20 C, 1.2 days) 9: After Dissociation (20 C)
0 10 20 30 40 50 60 70 80 90
100 110 120 130
m ol
e fra
ct io
n (1
0 -6 )
1: Before formation (20 C) 2: After formation (7 C) 6: Dissociation (20 C) 7: Dissociation (20 C, 1 day)
Appendix __________________________________________________________________________
- 224 -
Figure B.9 Compositional analysis of liquid phase based on percentage ratio of
component (mole %) /methane (mole %) for test 1.
A further test was conducted using a similar gas composition and distilled water. The
main difference from the first test was that the system was allowed to equilibrate for
about 90 hours (more than 3 days) prior to reducing temperature to form hydrate. In the
first test the system was left to equilibrate for only about 10 hours.
As shown in Figure B.11 and B.12, hydrates started to form as soon as the temperature
reached 4 ºC based on the increase in torque (Figure B.11) with a pressure drop of
approximately 20 bars within 40 hours. This was followed by a very slow growth of
hydrate for about 4 days before heating the system to dissociate hydrate. The path
followed within the hydrate phase boundary during this experiment is plotted in Figure
B.13.
0%
50%
100%
150%
200%
250%
300%
350%
400%
450%
500%
N2/C1 C2/C1 CO2/C1 C3/C1 C4/C1
1: Before formation (20 C) 2: After formation (7 C) 6: Dissociation (20 C) 7: Dissociation (20 C, 1 day)
Appendix __________________________________________________________________________
- 225 -
Figure B.11 Gas hydrate formation and dissociation for Test 2.
Figure B.12 Gas hydrate formation and dissociation for Test 2. The numbers represent
positions where samples were taken for compositional analysis.
30
40
50
60
70
80
90
100
110
time / hr
pr es
su re
time / hr
pr es
su re
time / hr
pr es
su re
time / hr
pr es
su re
6
7
Appendix __________________________________________________________________________
- 226 -
Figure B.13 The path followed during experiment with predicted hydrate phase
boundary for Test 2. The numbers represent samples taken for compositional analysis.
Table B.3 Samples collected for compositional analysis (Test 2). Sample
no
4 After formation 4 41 / 590 After 1 days
5 After formation 4 39 / 570 After 5 days
6 After dissociation
outside SI region
12 60 / 866 -
8 After dissociation 20 85 / 1230 After 1 day
(limited liquid phase)
The samples collected during this test are detailed in the Table B.3. Vapour phase
compositional data are presented in the form of percentage ratio, as described in the first
experiment, and are plotted in Figures B.14 and B.15. Similar trends in the vapour
compositional changes were observed in this experiment as compared to the first
experiment. Significant reduction of ethane/methane (C2/C1), propane/methane
(C3/C1) and iso-butane/methane (iC4/C1) was observed due to hydrate formation, while
the ratio of carbon dioxide to methane (CO2/C1) remained unchanged.
30
40
50
60
70
80
90
100
110
pr es
su re
1 2
pr es
su re
1 2
3,4
5
6
7
Cooling
Heating
Appendix __________________________________________________________________________
- 227 -
For the liquid phase, again there is no clear trend of compositional changes as shown in
Figure B.9 and B.10. As expected, CO2 component was clearly detected due to its high
solubility in water phase. C1, N2 and some traces of C2 and C4 components were also
detected. It is interesting to point out that small traces of iC4 and nC4 were present in
the sample after dissociation. It is important to point out that the analysis of liquid
samples may not very representative of the liquid present in the system due to the
current limitation on the configuration of the liquid sampling point at the rig. Further
modifications of sampling point will be required for future tests to improve the
sampling method.
Figure B.14 Compositional analysis of vapour phase based on percentage ratio of
component (mole %) /methane (mole %) for test 2.
0%
1%
2%
3%
4%
5%
6%
7%
8%
1 2 3 4
5 6 7 8
pr es
su re
6
12
1 2 3 4
5 6 7 8
pr es
su re
6
12
pr es
su re
6
12
9
Appendix __________________________________________________________________________
- 228 -
Figure B.15 Compositional analysis of vapour phase based on percentage ratio of R after
/ R before. R represent component (mole %) / methane (mole %) for test 2.
Figure B.16 Compositional analysis of liquid phase for test 2.
0%
20%
40%
60%
80%
100%
120%
140%
160%
180%
200%
C2/C1 C3/C1 CO2/C1 iC4/C1 nC4/C1 iC5/C1 nC5/C1
1: Before formation (20 C) 2: Before formation (14 C) 3: After formation (4 C) 4: After formation (4 C, 1 day) 5: After formation (4 C, 5 days) 6: Outside SI (12 C) 7: Dissociation (20 C) 8: Dissociation (20 C, 1 day)
0
10
20
30
40
50
60
m ol
e fra
ct io
n (1
0 -6 )
1: Before formation (20 C) 2: Before formation (14 C) 3: After formation (4 C) 4: After formation (4 C, 1 day) 5: After formation 4 C, 5 days) 6: Outside S1 (12 C) 7: Dissociation (20 C)
Appendix __________________________________________________________________________
- 229 -
Figure B.17 Compositional analysis of liquid phase based on percentage ratio of
component A (mole %) /methane (mole %) for test 2.
B.2.3. Summary
Preliminary studies on fluid compositional analysis, particularly of the vapour phase,
suggest that this technique has a potential to be used for monitoring the changes in
composition due to hydrate formation. Significant reduction of the ratio of ethane to
methane (C2/C1), propane to methane (C3/C1) and iso-butane/methane (iC4/C1) was
observed due to hydrate formation. The ratio of carbon dioxide/methane (CO2/C1) was
observed to remain relatively unchanged in the vapour phase. This result suggests that
CO2 did not take part in hydrate formation to the same extent as C2 and C3. Other
components in the system were not significantly affected. However, monitoring water
memory based on soluble gas present in the liquid phase required improvement on
sampling technique.
B.3.1. Experimental set-up and procedure
The online compositional analysis set-up is shown in Figure B.18. Gas hydrate
formation and dissociation was prepared using a high-pressure kinetic rig. This rig is
equipped with 525 ml high-pressure cell (4500 psia), a magnetic bar for stirring, cooling
jacket and temperature/pressure recording equipment controlled by computer. It has an
operating temperature range of –30 to 75 oC with temperature being controlled within
+/-0.05 oC. Temperature is measured by PRTs and pressure by Quartzdyne pressure
0%
100%
200%
300%
400%
500%
600%
N2/C1 C2/C1 CO2/C1 iC4/C1 nC4/C1
1: Before formation (20 C) 2: Before formation (14 C) 3: After formation (4 C) 4: After formation (4 C, 1 day) 5: After formation 4 C, 5 days) 6: Outside S1 (12 C) 7: Dissociation (20 C)
Appendix __________________________________________________________________________
- 230 -
transducers. Natural gas supplied by Air Products with the composition listed in Table
B.1 was used in this study.
Experimental was setup by charging approximately 200 ml of deionized water into the
kinetic rig at 20 oC. The kinetic rig was pressurised to 350 psia by injecting natural gas.
The water-natural gas system was step by step cooled to 4 oC (inside the hydrate phase
boundary) to form hydrates, while stirring at 240 rpm. The hydrate formation is
detected based on the sudden temperature rise due to the release of latent heat of hydrate
formation and the pressure reduction. After gas hydrate formation the system
temperature was increased step by step to dissociate the hydrates. The composition of
vapour phase was measure by directly sampled into gas chromatography to observe the
changes in SII composition during the process.
The sampling is carried out using a capillary sampler injector, which is connected to the
top of the cell through 0.1mm internal diameter capillary tube. The withdrawn samples
are swept to a Varian 3800 gas chromatograph for analysis.
Figure B.18. Flow diagram of apparatus for on-line compositional analysis
A three-component gas mixture was used for this study which consists of 95 mole% of
methane, 4.5 mole% of ethane and 0.5 mole% of iso-butane. Similar procedure and set-
up describe above was followed. The main difference was that this analysis was
conducted at constant pressure of 1500 psia and a HPLC pump was attached to the
system to maintain the pressure via make-gas gas. The vapour samples was analysed at
Appendix __________________________________________________________________________
- 231 -
several equilibrium temperatures which are 17 oC, 3.2 oC during hydrate formation,
12.3 oC, 13.6 oC, 17 oC, and 21 oC during hydrate dissociation.
B.3.2. Results and Discussions
On-line compositional changes in vapour phase at low pressure
The online analysis of vapour phase was made during hydrate formation and
dissociation. The main purpose was to analyse if this set-up is workable for simulating
on-line measurement. The analysis was done at low pressure (350 psia). Based on
preliminary results shown in Figure B.19, it was observed that there is significant
reduction (from 2.2 mole % to 0.2 mole %) in the concentrations of SII hydrate forming
compounds during hydrate formation. The SII composition is the sum of all the
hydrocarbon components that are known to form structure II hydrate, which are C3, i-
C4, and n-C4. A gradual increase in the concentrations of SII hydrate forming
components was observed during hydrate dissociation as more gas was released from
the aqueous phase. Similar trend was observed in the case of only monitoring i-C4
concentration as shown in Figure B.20. This concentration of i-C4 was reduced from
0.2 mole% to 0.02 mole %.
Figure B.19 Online compositional analysis of vapour phase for SII hydrate forming
components concentrations
200
220
240
260
280
300
320
340
360
380
400
-2 0 2 4 6 8 10 12 14 16 18 20 22 Temperature /oC
Pr es
su re
/ p sig
200
220
240
260
280
300
320
340
360
380
400
-2 0 2 4 6 8 10 12 14 16 18 20 22 Temperature /oC
Pr es
su re
/ p sig
Figure B.20. Online compositional analysis of vapour phase for i-C4 concentration
On-line compositional changes in vapour phase at constant pressure
To simulate the pipeline condition the compositional changes of vapour phase for
simple system were investigated at constant pressure. As can be seen from Figure B.21,
the composition of iso-butane in the vapour phase started to reduce at about 10 oC. This
observation may indicate that hydrate started to form in the system. Further reduction in
temperature leads to significant reduction in iso-butane concentration. At equilibrium
temperature of about 3 oC, the composition reduced down to 60 % of its original feed
value. During hydrate dissociation, the composition remains almost constant until
reaching closer to the hydrate dissociation temperature predicted by in-house HWHYD
software. The composition increased significantly as hydrate dissociated. This
preliminary result may suggest that there is potential of monitoring compositional
changes in vapour phase at constant pressure for hydrate early warning system.
However, further investigation is required to further refine the technique and to find the
suitable component for detecting the changes. Possibility of incorporating hydrate
former as a tracer to detect hydrate formation in the system will also be considered.
200
220
240
260
280
300
320
340
360
380
400
-2 0 2 4 6 8 10 12 14 16 18 20 22 Temperature /oC
Pr es
su re
/ p sig
200
220
240
260
280
300
320
340
360
380
400
-2 0 2 4 6 8 10 12 14 16 18 20 22 Temperature /oC
Pr es
su re
/ p sig
Appendix __________________________________________________________________________
- 233 -
Figure B.21 Online compositional analysis of vapour phase for i-butane with make-up
gas at 1500 psia
B.3.3. Summary
The compositional changes of vapour and liquid phase for various systems have been
investigated. This preliminary result suggests that there is potential of monitoring
compositional changes in vapour phase at constant pressure for hydrate early warning
system. However, further investigation is required to further refine the technique and to
find the suitable component for detecting the changes. Possibility of incorporating
hydrate former as a tracer to detect hydrate formation in the system should also be
considered.
SDW
(%)
41.7
60%
65%
70%
75%
80%
85%
90%
95%
100%
105%
110%
115%
0 2 4 6 8 10 12 14 16 18 20 22
Temperature / oC
i-B uta
ne / M
ole Pe
rce nt
(estimated by HWHYD)
60%
65%
70%
75%
80%
85%
90%
95%
100%
105%
110%
115%
0 2 4 6 8 10 12 14 16 18 20 22
Temperature / oC
i-B uta
ne / M
ole Pe
rce nt
(estimated by HWHYD)
2.55E-09 6.01E-09 2.46E-10 2.38E-11 9.7
HDW (outside) 1.45E-09 5.95E-08 4.01E-08 6.52E-09 16.3
17.6
SDW (outside) 4.95E-08 7.18E-08 9.32E-08 9.62E-09 10.3 10.3
* * 3.72E-09 3.71E-10 10.0
* * 3.8E-09 4.04E-10 10.6
2E-09 2E-09 1.86E-09 5.41E-10 29.2
* * 2.94E-10 7.87E-11 26.8
* * 9.56E-11 2.65E-11 27.7
Appendix __________________________________________________________________________
- 235 -
ULTRASONIC RIGS
HP VISUAL CELL
The experimental set-up is comprised of a high pressure cylinder fitted with sapphire
windows at both ends. The cell can be used at pressures up to 517 bar and at temperatures
between –30 and +50°C. The cell temperature is controlled by circulating water from a
heater/chiller through an inbuilt jacket. A platinum resistance temperature probe monitors
the cell temperature. The pressure of the cell is monitored by a strain gauge pressure
transducer connected to the cell via a high pressure line. The accuracy of the temperature
measurement is ±0.1°C and the pressure ±0.3 bar. A computer is used to log the cell
pressure and temperature. A schematic of the experimental rig is shown in Figure C1. The
cell is mounted on a rocking mechanism in order to give mixing when required.
Figure C1. Schematic of high pressure (517 bar) visual cell.
ULTRASONIC RIG
The schematic of the ultrasonic set-up is shown in Figure D2. Its key part is the test cell.
Two ultrasonic transducers are fitted at the two ends of the cell, respectively. One works as
a transmitting transducer that generates acoustic signals, driven by the Pulser/Receiver
(P/R), another one as a receiver that receives the signals through the fluids inside the cell.
Appendix __________________________________________________________________________
- 236 -
The received signals are pre-amplified by the Pulser/Receiver and pre-processed by the
Digital Storage Oscilloscope (DSO) and then sent to a Personal Computer (PC) to display
the waveforms and save the data for further analysis. One of the two ends like a piston is
movable, which is useful to adjust the volume of the test cell. A dial indicator is fitted to the
tail bar of the piston for determination of the piston’s position. A hand pump and a piston
vessel are used to inject test fluids. The test temperature is controlled by means of the
cooling system that consists of a cooling bath and a jacket. In the tests the system
temperature and pressure, and the waveforms can be automatically recorded by logging on
the computer.
-237-
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13 - APPENDICES
APPENDIX CSCHEMATIC OF HIGH PRESSURE VISUAL CELL AND ULTRASONIC RIGS
14 - REFERENCES