2.3 Other Properties of CO2

2.3.1 Density of CO2

The density of liquid CO2 and liquid CO2 admixed with other gases in water has already been discussed in 2.1.2. From Figure 2.11 it can be seen that for the pressure ranges that could be expected for offshore pipelines (80 – 200 bar), the CO2 may, or may not, be denser than water at the point that it emerges, dependent on the gaseous impurities.

The density of seawater increases with depth down to about 1 km, as shown in Figure 2.2219. Offshore UK CO2 pipelines are unlikely to be below 300 m, thus the seawater density will be in the range 1,0250 – 1,0264 g/cm3. Superimposing this range on to Figure 2.11 shows how the density of liquid CO2 compares to that of seawater (see Figure 2.23). In theory, this shows the potential that pure CO2, and with the inclusion of SO2 and H2S as impurities, will make the liquid CO2 denser than seawater. In practice, where there is a loss of containment, the liquid CO2 will absorb heat from the surrounding water, which will precipitate a phase change. Depending on the depth of the release and the size of the rising liquid, some of the CO2 may reach the surface in vapour form.

Figure 2.22 Changes in the density of sea water with depth

Figure 2.23 Density of CO2 and that of seawater

Because the pressure of seawater at a depth of 300 m is only 30,2 bar, it may be concluded that any liquid CO2 released at depth will rapidly depressurise, become less dense, and rise towards the surface.

2.3.2 Speed of sound in liquid CO2

Two-phase flow in CO2 pumping and pipeline systems has the potential to cause water hammer. Water hammer can also result from valves closing too quickly. Modelling this (see 3.3.9) requires the speed of sound in the CO2 as a source term. The speed of sound in CO2 at 4 °C is shown in Figure 2.24 which shows the CO2 in both the liquid and vapour phases20.

Figure 2.24 Speed of sound in CO2 at 4 °C

(source: National Institute of Science and Technology)

In a CCS application at a pressure of 200 bar, the speed of sound is 717 47 m/s. For comparison, the speed of sound in water under the same physical conditions is 1 454 m/s.

2.3.3 CO2 BLEVE

Boiling Liquid Expanding Vapour Explosion (BLEVE) is an explosion resulting from the failure of a liquid container at a temperature above its boiling point at atmospheric pressure. A further explanation of CO2 BLEVE is included as Annex F. The term was first applied to steam explosions, but often and incorrectly, it is thought to relate only to flammable materials. BLEVE involving non-flammable liquids produce only two effects: a blast due to the expansion of the vapour in the container combined with the flashing of the liquid, and fragmentation of the container (whether that container is a vessel or pipe).

A BLEVE can be caused by a number of events:

- an external fire;

- external impact;

- corrosion of the pressure containment;

- excessive internal pressure, and

- metallurgical failure of the pressure containment.

19 Density of sea water, Windows to the Universe, National Earth Science Teachers Association

20 National Institute of Science and Technology