Transporting carbon dioxide
Physical properties of CO2Carbon dioxide can be transported as a gas, a liquid, a solid, or in a fourth phase, as a super critical gas. Gas has the lowest viscosity but the lowest density. A liquid has a higher density but also a higher viscosity. A super critical gas has a few advantages. It has the viscosity of gas, but the density of liquid. If it is above the critical temperature of 31oC it cannot, by the laws of physics, become a liquid, and cause problems in the pumping. In a pipeline, the pumps are selected to pump a gas, or a liquid, they cannot be optimised to pump both, or a mixture of both. |
Critical point CO2
Triple point
(Critical temperature is the temperature above which, the gas can never be liquefied by pressure alone) |
Problems in pumpingThe CO2 must be dry, or two things can happen. 1) it forms carbonic acid, which though a weak acid, can corrode steel at 10 mm/y. If the CO2 is wet the pipe needs to be of stainless steel which is expensive. 2) At low temperature water can freeze and form a clathrate, a mixture of ice and CO2 that can block pipes, valves etc. Below 31oC the gas can condense to form a liquid. |
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At present there are thousands of KM of pipeline carrying CO2 to oil fields, most of them in the USA. The CO2 is used for Enhanced Oil Recovery - EOR. Next page The thinking is that the pipelines will be developed for EOR first, then extended to inject CO2 into deep saline aquifers which have for more potential. |
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