AQUAlibrium©
Software for Phase Equilibria in Natural Gas-Water Systems
Carbon Dioxide Sequestration
in the Ocean
John J. Carroll
©1999 - Publication Rights Reserved
We encourage feedback on this concept and on
the paper itself.
We'd be pleased to publish reasonable comments
in future versions of this site.
Please restrict your comments to carbon dioxide
sequestration, we do not want a debate on global warming.
New links have been added at the end of this page.
With the current concerns about global warming, the
Kyoto agreement, carbon taxes, and the like (which you
may or may not share), there has been increased concern
about the fate of atmospheric carbon dioxide. It has been
suggested by Yamasaki, et al. (1998), and others [also see
Chopey (1999) for example], that one way to dispose of
anthropogenic (man-made) carbon dioxide is to compress it
and inject it into the ocean. If it is injected a sufficient
depth it will form a stable gas hydrate and should remain
there indefinitely.
Introduction
Global warming via greenhouse gases (such as carbon dioxide) is
a hot topic worldwide. The recent Kyoto meeting on
greenhouse gas emissions, and other environmental topics, is a clear
example of this.
Among engineers (and others), global warming
remains very controversial. Many question whether or not
the earth is warming at all. Others question whether or not
greenhouses gases are to blame for the apparent warming.
Regardless, legislatures the world over are convinced and slowly
reductions in emissions of gases, such as carbon dioxide, are becoming
law. Some European nations have already imposed a "Carbon Tax".
Others are sure to follow.
It is the responsibility of engineers to
discover ways to deal with reductions in emissions whether or not they
believe them to be problematic. Legislation will force them
into making these designs. And like any engineering design, it
must be a combination of technical feasibility and economic prudence.
One scheme proposed to deal with flue gas emissions is to
inject them into the ocean. At the temperature and pressure in the
ocean a hydrate will form and thus the carbon dioxide is
not emitted to the atmosphere.
Presented in this paper are some preliminary engineering
considerations for the design of such a scheme.
What Depth?
The first question that must be answered is "What depth
must the carbon dioxide be injected to in order to form
hydrates?" The depth must be such that a there is
sufficient pressure for a hydrate to form.
If we assume that seawater is a 3.5 wt% solution
of sodium chloride, CSMHYD
(Sloan, 1998) can be used to
estimate the hydrate formation for pure carbon dioxide. The
temperature-pressure for such a hydrate locus is given in Table 1.
Table 1 Hydrate Formation Pressure and Depths
for Sequestering Carbon Dioxide
Temp Press Depth
(deg C) (kPa) (m)
------- ----- ------
-1 1334 121
0 1490 136
1 1667 153
2 1869 173
3 2100 196
4 2366 222
5 2676 252
Note, the inhibiting effect of the salt in the seawater
is fairly small, but it was included in this as subsequent
calculations.
From these pressures the hydrostatic equation can be
used to calculate the depth. The hydrostatic equation is:
P - P(atm) = d g h
where P is the hydrate formation pressure, Pa
P(atm) is atmospheric pressure, Pa (101 325 Pa)
d is the density of seawater, kg/m¦
g is the acceleration due to gravity (9.81 m¦/s)
h is the depth, m
When using this equation, the user is cautioned to use the proper units
(as noted above). This comment is more appropriate when American
Engineering Units are used. It is very easy to make a mistake.
If we further assume that the density of seawater is
1.04 g/cm³, it is an easy matter to calculate the depth required
to achieve the hydrate pressure. The estimated depths are also
given in Table 1. From this table it can be seen that if
the carbon dioxide is injected to a depth of greater than 250 m
a hydrate will form.
Injection Pressure
Next the required injection pressure must be calculated.
This is done in a manner similar to that for acid gas injection.
Using AGIProfile (Carroll and Maddocks, 1999) the injection
pressure is estimated to be 2527 kPa.
(output from AGIProfile
- use BACK on your browser to return).
Temperature Fluctuations
If the carbon dioxide is injected to a depth of 140 m (i.e.,
assuming the water is at 0°C), what happens if the water warms?
If the temperature rises, the pressure required to keep the
carbon dioxide in the hydrate form also increases. Thus if the
carbon dioxide is not injected to a sufficient depth, then the
warming could release the carbon dioxide.
Hydrate Density
The density of the carbon dioxide hydrate is approximately 1.1 g/cm³ -
slightly larger than seawater. This means that the hydrate will sink
to the ocean floor. If the hydrate were less dense than seawater (such
as ice), then the hydrate would float. Once it reached the surface it
would quickly melt releasing the carbon dioxide.
In contrast, the methane hydrate is less dense
than water (density approximately 0.91 g/cm³). Therefore, methane
could not be sequestered in this fashion.
Therefore, this
process works because the carbon dioxide hydrate is denser than seawater.
Impurities
It was assumed that the gas injected was pure carbon dioxide.
What if the gas contain other components? If the gas to be
disposed is from a fossil fuel gas plant, the gas is most
likely to be a mixture of carbon dioxide, nitrogen, oxygen,
and a few other impurities (notably SOx and NOx).
For the next scenario, assume that the gas to be injected
is 90% carbon dioxide and 10% nitrogen. Again, using
CSMHYD,
the hydrate formation pressure can be estimated. Note the presence
of the nitrogen increases the hydrate formation pressures by
between 10 and 20%.
Table 2 Hydrate Formation Pressure and Depths for
Sequestering Carbon Dioxide(90%) and Nitrogen (10%)
Temp Press Depth
(deg C) (kPa) (m)
------- ----- ------
-1 1487 136
0 1663 153
1 1863 173
2 2092 195
3 2355 221
4 2660 251
5 3019 296
Assuming a pressure of 3000 kPa, AGIProfile can be used to
estimate the required surface injection pressure for this
mixture. The estimated injection pressure is 2800 kPa.
Liquefaction
A check of the pressure calculated for both the hydrate
forming conditions and for the phase envelope indicate that
in neither case is it necessary to liquefy the gas.
Other Effects
The question of whether or not this should be done has not yet
been raised. In essence, this process is really dumping human waste
into the ocean. Is it ever a good idea to dispose of human waste
in the ocean?
What is the effect of this hydrate on the ecosystem of the sea floor?
Surely it is not totally benign. If nothing else, the solids will
cover the sea floor. Would this be any different from covering the
sea floor with other solid material?
Reader Feedback
We welcome your comments.
References
- Carroll, J.J. and Maddocks, J.,
"Design Considerations for Acid Gas Injection",
Laurance Reid Gas Conditioning Conference,
Norman, OK, February (1999).
- Chopey, N.P., "Technology to Cool Down Global Warming",
Chemical Engineering, pp. 37-41, January (1999).
- Slaon E.D., Clathrate Hydrates of Natural Gas, 2nd Ed.,
Marcel Dekker, New York, NY, (1998).
- Yamasaki, A., et al. "A Novel Ocean Disposal-Scenario:
Disposal of Anthropogenic Carbon Dioxide in the Ocean
via a Submerged Hydrate Crystallizer",
AIChE Annual Meeting, Miami Beach, FL, Nov. (1998).
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