Summary
Carbon dioxide (CO2) produced by carbon-capture processes is
generally not pure and can contain impurities such as N2,
H2, CO, H2S, and water. The presence of these impurities
could lead to challenging flow-assurance issues. The presence of water may
result in ice or gas-hydrate formation and cause blockage. Reducing the water
content is commonly required to reduce the potential for corrosion, but, for an
offshore pipeline system, it is also used as a means of preventing gas-hydrate
problems; however, there is little information on the dehydration requirements.
Furthermore, the gaseous CO2-rich stream is generally compressed to
be transported as liquid or dense-phase in order to avoid two-phase flow and
increase in the density of the system. The presence of impurities will also
change the system's bubblepoint pressure, hence affecting the compression
requirement.
The aim of this study is to evaluate the risk of hydrate formation in a
CO2-rich stream and to study the phase behavior of CO2 in
the presence of common impurities. An experimental methodology was developed
for measuring water content in a CO2-rich phase in equilibrium with
hydrates. The water content in equilibrium with hydrates at simulated pipeline
conditions (e.g., 4°C and up to 190 bar) as well as after simulated choke
conditions (e.g., at -2°C and approximately 50 bar) was measured for pure
CO2 and a mixture of 2 mol% H2 and 98 mol%
CO2. Bubblepoint measurements were also taken for this binary
mixture for temperatures ranging from -20 to 25°C.
A thermodynamic approach was employed to model the phase equilibria. The
experimental data available in the literature on gas solubility in water in
binary systems were used in tuning the binary interaction parameters (BIPs).
The thermodynamic model was used to predict the phase behavior and the
hydrate-dissociation conditions of various CO2-rich streams in the
presence of free water and various levels of dehydration (250 and 500 ppm). The
results are in good agreement with the available experimental data. The
developed experimental methodology and thermodynamic model could provide the
necessary data in determining the required dehydration level for
CO2-rich systems, as well as minimum pipeline pressure required to
avoid two-phase flow, hydrates, and water condensation.
© 2011. Society of Petroleum Engineers
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History
- Original manuscript received:
5 June 2009
- Meeting paper published:
8 September 2009
- Revised manuscript received:
23 July 2010
- Manuscript approved:
18 August 2010
- Published online:
29 March 2011
- Version of record:
23 December 2011
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