Transporting CO2

Transporting carbon dioxide

Physical properties of CO2

Carbon 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
  • Critical temperature  : 31 °C
  • Critical pressure  : 73.825 bar
  • Critical density  : 464 kg/m3
Triple point
  • Triple point temperature  : -56.6 °C
  • Triple point pressure  : 5.185 bar

(Critical temperature is the temperature above which, the gas can never be liquefied by pressure alone)

 

Problems in pumping

The 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.

 

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.