Available online at www.sciencedirect.com
Energy Procedia 00 (2013) 000–000
www.elsevier.com/locate/procedia
GHGT-13
Transient phase behavior of CO2 leakage from a high-pressure
pipeline to the atmosphere with visualization of internal phase
change and near-field shock wave structures
Peixue Jianga, Xiaolu Lia, Feng Liua, Xue Chena, Ruina Xua,*
a
Beijing Key Laboratory for CO2 Utilization and Reduction Technology
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education
Department of Thermal Engineering, Tsinghua University, Beijing 100084, China
Abstract
As the development of Carbon Capture and Storage technology as a promising solution for greenhouse gas control, large
quantities of CO2 will be transported over a large distance in the near future. High pressure transportation of CO2 via pipeline is
the most economically attractive for the high volume transports. Once the pipeline is ruptured, there will be a large number of
high-concentration CO2 leaking into the atmosphere, thus threatening the environment and lives. Therefore, it is necessary to
conduct a detailed understanding of the mechanism of CO2 leakage so as to reduce the risk. In this paper, an experimental study
was conducted on the flow and phase change of CO2 in different initial state during release from the pipelines. The temperature,
pressure and concentration of CO2 during and after the release were measured, as well as the visualization of internal phase
change and near-field shock wave structures. It was found that during the rupture, CO2 underwent 3 stages: liquid phase stage,
saturation stage and gas phase stage. Inside the pipe, it was dramatically thermodynamic unequilibrium. Outside the pipe, dry ice
was generated in the beginning, following with a formed underexpanded jet.
© 2013 The Authors. Published by Elsevier Ltd.
Selection and peer-review under responsibility of GHGT.
Keywords: CO2 transport pipe; transient phase behavior; leakage; experimental investigation
1. Introduction
Carbon Capture and Storage (CCS) is an effective technology to reduce CO2 emissions. The aim of CCS is to
capture CO2 from industry or other sources, transport CO2 to a suitable storage site and inject it into the underground
storage area in order to achieve long-term isolation from the atmosphere. Currently there are at least 74 large-scale
1876-6102 © 2013 The Authors. Published by Elsevier Ltd.
Selection and peer-review under responsibility of GHGT.
Peixue Jiang et al. / Energy Procedia 00 (2013) 000–000
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integrated CCS projects in operation or under construction. As the development of Carbon Capture and Storage
technology as a promising solution for greenhouse gas control, large quantities of CO2 will be transported over a
large distance in the near future. High pressure transportation of CO2 via pipeline is the most economically attractive
for the high volume transports [1,2]. Once the pipeline leakage occurs, there will be a large number of highconcentration CO2 leaking into the atmosphere, thus threatening the environment and lives. Therefore, the
quantitative risk assessment (QRA) of CO2 pipeline leakage is essential for the limited experience on CO2 pipelines.
A lot of investigations on the modelling of QRA were performed all over the world. Witlox et al.[3] used Phast
software to simulate the CO2 leakage and diffusion process from the pipeline. After considering the effect of solid
phase, they stated that the concentration of CO2 would rise with the increase of working pressure in the pipe, and at
saturation point there was no continuous phase transition. Jennifer et al.[4] theoretically analyzed the diffusion
process of CO2 in the atmosphere, especially considering the influence of wind, and combined with the experiment
to obtain the gas plume concentration in different locations; Antonin et al.[5] obtained physical properties and phase
transformation of CO2 in the case of containing specific impurities from the experimental and theoretical analysis.
Maciej et al.[6] analyzed CO2 pipeline hydrodynamic characteristics with disturbances and obtained appropriate
mathematical model. Mazzoldi et al.[7] found from the experiment that, for vertically downward leakage of a
horizontal pipe, there is ice bank on the ground, and they calculated the sublimation rate of ice bank over time.
It can be concluded that none of the models have a complete system, and the existing correlations have
discrepancy. The results of the safe distance are quite various in different models due to the different assumptions
and standards in every model including the physical state of the released CO2, the jet release to be free or impinging
turbulent flow, with or without formation and fall out of solid CO2, etc. It can be indicated that the small-scale and
middle-scale experiments of CO2 pipeline release are urgently needed to valid the different models [16].
In this context, this paper aimed to study the leakage process of CO2 on the characteristics of fluid mechanics,
thermodynamics and heat transfer, and will be helpful for a better understanding of leakage and diffusion behavior
of high-pressure CO2 pipeline. A laboratory-scale experimental system was setup in this study, in which the pipe
pressure, temperature and leakage flow rate were measured and the underexpanded jet structure outside the pipe
near the orifice was recorded by the schieren device. Moreover, a high-pressure visualization unit was designed to
record the phase change within the pipe. A sensor array was distributed in the atmosphere to measure CO2
concentration and temperature in large-field.
2. Experimental system and procedure
2.1. System setup
An experimental system was built up for investigating the leakage of supercritical pressure CO2, including dense
phase, gas phase, liquid phase and supercritical phase, from high-pressure pipeline to the atmosphere and the
subsequent CO2 jet diffusion in the atmosphere. With help of supercritical CO2 pump and the circulating water bath,
CO2 was adjusted to the set point and then be pumped into the test pipe. CO2 in the pipe was pressurized to about 69 MPa, and was suddenly released from an orifice with a diameter of 0.7mm, 0.9 mm, 1.1 mm or 1.3 mm along the
radial direction to the atmosphere by opening a leakage valve with a response time of 0.04s. After the test period of
2 min, the leakage valve was closed remotely to stop the release process. A visualization unit was connected to the
pipeline and the phase behavior of CO2 could be recorded using a CMOS camera and a high-speed CCD camera. A
Schlieren apparatus was installed to record the shock wave images and the whole CO2 plume could be recorded by a
DV. A detector matrix was installed in front of the orifice to record CO2 concentration and temperature in the
atmosphere. All these data were transmitted to the datalogger apparatuses (Agilent 34972A).
The test section was a 2-metre long stainless steel pipe with a 6 mm outer diameter and 4 mm inner diameter. In
this experiment, four T-type thermocouples were installed equidistantly inside the pipe and two pressure transmitters
were used both in the beginning and in the end of the pipe. Additionally, there were thermocouples arranged
uniformly along the pipe wall. Thermocouples were all OMEGA T-type thermocouple (copper-constantan), with the
socket diameter of 1/16 inch, an accuracy of 1.5% and a response time of less than 1s. For the wall temperatures,
79μm T-type thermocouple wires were used. Pressure transmitters were from Yokogawa with a maximum range of
16MPa (which can be adjusted within the allowable range in the actual operation), an accuracy of 0.065% and the
Peixue Jiang et al. / Energy Procedia 00 (2013) 000–000
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response time of less than 0.2s. Mass flowmeter was located before the test section to measure the flow, with a
precision of ±1% and measuring ranges from 0 to 350kg/h.
2.2. Visualization method
At the end of the test section there was a high-pressure visualization unit for fluid visualizing which is made of
sapphire glass. Within the visualization unit, the pressure and temperature were measured. Out of the orifice there
was a 200 mm concave-mirror Schlieren to record the highly underexpanded jet structure, which used the difference
in the density field to characterize the complex physical phenomena.
2.3. Concentration measurement
Outside the orifice there was a detector matrix in the atmosphere to measure CO2 concentration and temperature.
The matrix was in the axis direction of leakage, in the distance of orifice from 0.4m to 6m there were 10 planes to
make three-dimensional arrangement of 26 CO2 concentration sensors respectively. The Xensor Integration XENTCG3880-P2-R-W micro thermal conductivity gauge was chosen to measure the temperature and concentration at
the same time. Before the experiment the sensors were calibrated by standard gases with CO2 concentrations of 0,
5.17%, 10.18% and 20.17%.
The experimental procedures are as follows. After evacuating the system, CO2 parameters were adjusted to the
set operating conditions by the pump and circulation water bath. When the temperature and the pressure of CO2 in
the reservoir reached the pre-set point, the water bath circulation pump was kept working continuously for some
time until the state of CO2 in the reservoir was stable. Later on, the solenoid valve was opened by remote control to
start the leakage and all the image capture devices was started to operate. After 2 min, the solenoid valve was closed
to stop the release. 10 min was allowed after the release to record the CO2 diffusion process inside the laboratory.
3. Experimental results and discussion
3.1. CO2 behavior transformation inside the tube
With the initial state of supercritical pressure CO2 (7.8 MPa, 26 oC), the pressure and temperature of CO2 finally
decreased along the liquid/gas saturation line of CO2 (Fig. 1), having bubbles within the liquid CO2 along the whole
leakage process. Initially after the leakage process, there were small bubbles inside the pipe, and the bubbles grew
and increasing rapidly over time.
Fig. 1. Pressure and temperature variation of CO2 in the visualization unit with the initial state of supercritical pressure state (7.8 MPa, 26 oC)
along the whole leakage process. It was found that pressure and temperature decreased along the saturation line.
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3.2. Jet flow outside the leakage orifice
High pressure jet flow structures in the near-field ahead of the leakage orifice were recorded as shown in Fig. 2.
Jet structure can be divided into two types depending on whether the solid phase was generated outside the tube. If
the isotropic theory was established，the extremum enthalpy can be calculated. When the initial enthalpy was
greater than the extremum enthalpy, outside the pipe there was no solid phase, otherwise it would generate dry ice.
Without the formation of dry ice in the near-field, the barrel shock, reflected shock and Mach disk were clearly
seen (Fig. 2). The distance between the Mach disk and the leakage orifice was measured to be 4.5 mm with the
initial state of 7.8 MPa and 42 oC. At the distance of 4.5 mm ahead of the leakage point, measured CO2 temperature
dropped to as low as -77 oC which was the sublimation temperature of CO2 at 1 atm.
Fig. 2. Shock wave structure in the near-field ahead of the leakage orifice (Before leakage: 7.8 MPa, 42 oC).
4. Conclusion
From the experiment it can be concluded that, when the initial state of the leakage is in liquid or dense phase,
phase change within the pipe can be divided into three stages: the liquid phase, the saturation state and gas phase.
Theoretically, CO2 wouldn’t change into dry ice until its pressure in the pipe drops along the saturation line to
5.18bar, and the experimental results also suggest that the leakage did not produce ice in the pipeline. In addition, in
the actual projects, pipeline in the leakage process may be significantly endothermic, which would suppress the ice
production in the pipe.
In this work, the simplest diffusion conditions with horizontal leakage and no obstacles were studied. However,
in the actual pipeline transport processes, there are complex terrain and surrounding environment, which require the
consideration of the impact of ground slope, wind direction and other factors. All of them are subject to further
studies.
5. Acknowledgements
This project was supported by the Key Project Fund from the National Natural Science Foundation of China.
(No. 51536004), National Natural Science Foundation of China (No. 51376104), the Foundation for Innovative
Research Groups of the National Natural Science Foundation of China (No. 51321002).
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