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2.1 General properties and uses of carbon dioxide
Carbon dioxide gas is found in small proportions in the atmosphere (about 385 ppmvd); it is assimilated by plants which in turn produce oxygen by photosynthesis. It is produced from the combustion of coal or hydrocarbons, the fermentation of liquids and the breathing of humans and animals. Humans exhale carbon dioxide at approximately 4,4 % v/v. Carbon dioxide is also found beneath the earth’s surface, and emerges during volcanic activity, in hot springs and other places where the earth’s crust is thin. It is found in lakes at depth under the sea1, and comingled with oil and gas deposits.
Carbon dioxide comprises two oxygen atoms covalently bonded to a single carbon atom, with an O-C-O angle of 180°. As such it is very stable, no process other than photosynthesis having been discovered that is able effectively to reduce carbon dioxide to carbon monoxide. Carbon dioxide is not classified by the UN as toxic2.
Carbon dioxide is widely used commercially. It is used in the chemicals processing industries to control reactor temperatures, to neutralise alkaline effluents and used under supercritical conditions for purifying or dying polymer, animal or vegetal fibres.
In the food and beverage industries, carbon dioxide is used for carbonation of fizzing beverages such as soft drinks, mineral water or beer, for packaging of foodstuffs, as a cryogenic fluid in chilling or freezing operations or as dry ice for temperature control during the distribution of foodstuffs. Caffeine is removed from coffee using supercritical carbon dioxide.
In the medical field, carbon dioxide produces close-to-physiologic atmospheres for the operation of artificial organs. Carbon dioxide is used as a component in a mixture of oxygen or air as respiratory stimulant to promote deep breathing. It is also used for the surgical dilation by intra-abdominal insufflations.
Industrially, carbon dioxide is typically used for process control, examples of which include the use of carbon dioxide for red fume suppression during scrap and carbon charging, for nitrogen pick-up reduction during electric arc furnace tapping and for bottom stirring. In non-ferrous metallurgy, carbon dioxide is used for fume suppression during ladle transfer of matte (Cu/Ni production) or bullion (Zn/Pb production). Carbon dioxide is used to enhance the recovery of oil from wells where primary and secondary methods are no longer cost-effective on their own.
Carbon dioxide is also used as a fire extinguishant and as ‘dry ice’ for storage and other effects.
Physical properties of carbon dioxide
Pure carbon dioxide exhibits triple point behaviour dependent on the temperature and pressure, as shown in Figure 1:
Figure 1 CO2 Phase diagram
The triple point (at a pressure 5,11 bar and temperature of −56,7 °C) is defined as the temperature and pressure where three phases (gas, liquid and solid) can exist simultaneously in thermodynamic equilibrium. The solid-gas phase boundary is called the sublimation line, as a solid changing state directly into a gas is called sublimation. Physically, this boundary implies that the gas and solid can co-exist and transform back and forth without the presence of liquid as an intermediate phase.
Above the critical point (73,8 bar and 31,1 °C), the liquid and gas phases cannot exist as separate phases, and liquid phase carbon dioxide develops supercritical properties, where it has some characteristics of a gas and others of a liquid.
In the event of an uncontrolled release of carbon dioxide (e.g. damage to a pipe containing liquid carbon dioxide), a portion of the escaping fluid will quickly expand to carbon dioxide gas. The temperature of the released carbon dioxide gas will fall rapidly due to the pressure drop (Joule-Thompson effect – see later description), causing some of the released carbon dioxide to form carbon dioxide snow. As a result of the low temperature of the carbon dioxide, the surrounding air will also be cooled down, which will cause the water vapour in the air to condense locally, which will look like a thick fog. This will continue (to a greater or lesser extent) as long as there is cold carbon dioxide present (e.g. subliming ‘snow’).
A phase diagram, as shown in Figure 1, is a common way to represent the various phases of a substance and the conditions under which each phase exists. However, it tells us little regarding how the changes of state for carbon dioxide occur during a transient. The carbon dioxide pressure-enthalpy diagram (P-h), shown in Figure 2, or temperature- entropy (T-s) diagrams provide insight to the phase changes.
Figure 2 CO2 pressure-enthalpy diagram
P-h and T-s diagrams can be used to examine phase changes, energy transfers, and density, pressure and temperature changes during depressurisation, e.g. for a leak of carbon dioxide from a vessel or a pipeline. In order to understand and interpret such a diagram some basic thermodynamic theory and terms need to be established.
The adiabatic (no heat exchanged) expansion of a gas may occur in a number of ways. The change in temperature experienced by the gas during expansion depends not only on the initial and final pressures, but also on the manner in which the expansion is carried out.
Isenthalpic expansion is a theoretical expansion which takes place without any change in enthalpy. In a free expansion, the gas does no work and absorbs no heat, so the internal energy is conserved. Expanded in this manner, the temperature of an ideal gas would remain constant, but the temperature of a real gas may either increase or decrease, depending on the initial temperature and pressure. This is called the Joule-Thompson effect. The amount by which the gas cools on expansion (measured in °C/bar) is called the Joule-Thompson coefficient, μJT. Carbon dioxide has a particularly high μJT compared with other gases, as shown in Table 1.
Table 1 Joule-Thompson coefficient for a number of gases
Figure 3 Changes in Joule-Thompson coefficient (μJT) for a number of different CO2 temperatures and pressures
Positive or negative temperature change can occur during the Joule-Thompson process. Each real gas has a Joule-Thompson inversion temperature above which expansion at constant enthalpy causes the temperature to rise, and below which such expansion causes cooling. For carbon dioxide the inversion temperature, at atmospheric pressure, is 1 500K4 (1 226,85 °C), which means that carbon dioxide gas always cools by isenthalpic expansion for all conditions relevant in CCS applications.
Isentropic expansion takes place if the expansion process is reversible, (meaning that the gas is in thermodynamic equilibrium at all times). In this scenario, the gas does positive work during the expansion, and its temperature decreases. Here, the temperature drop will be greater than for isenthalpic expansion.
Figure 4 Changes in Joule-Thompson coefficient (μJT) for pure CO2 at higher pressures at 10°C
Figure 55 shows the pressure-density behaviour of pure carbon dioxide during rapid decompression under ideal conditions from 130 bar (13,1 MPa) and 5 °C, and it can be seen that solid, liquid and gas phases are all present.
Figure 5 Pressure-density behaviour during rapid CO2 decompression
Figure 6 Effect of impurities on the critical point
Gaseous phase carbon dioxide
Carbon dioxide gas is colourless, heavier than air (1 521 times as heavy, with a density of about 1,98 g/litre), has a slightly irritating odour, and freezes at −78,5 °C to form carbon dioxide snow.
The effects of inhaling carbon dioxide and the limit values for working when there is carbon dioxide in the atmosphere are described in 2.4.1.
An escape of carbon dioxide gas, because it is heavier than air, will tend to accumulate in depressions in the ground and in basements or sumps. This accumulation can occur in low pressure carbon dioxide releases or where there is sufficient impingement on a high pressure release to remove the momentum7. The release will disperse as a result of air movements. Natural releases of carbon dioxide are often at low momentum (e.g. in Italy, Poland and USA). There have also been natural events leading to more hazardous accumulations of carbon dioxide, such as the incident at Lake Nyos in 1986, which is one of only three lakes in the world known to be saturated with carbon dioxide.
Models that assist in the prediction of this are described in EI document Technical guidance on hazard analysis for onshore carbon capture installations and onshore pipelines.
Carbon dioxide dissolves in water to form carbonic acid (H2CO3):
|CO2 + H2O → H2CO3 → H+ + HCO32− pKa = 6,35|
|HCO3− → H+ + CO32− pKa = 10,25|
The solubility of carbon dioxide in water is 1,45 g/litre at 25 °C and 1 bar. Carbonic acid is relatively weak, and it is impossible to obtain pure carbonic acid at room temperatures. Whilst carbonic acid is described as weak, it still carries the potential to corrode carbon steel pipes:
2Fe + H2CO3 → Fe2CO3 + H2
This leaves the designer with the choice of either using a more costly pipe material, lining the inside of the pipe with a corrosion-resistant coating, or reducing the water to a level where significant corrosion will not take place.
There is also a hypothetical acid, orthocarbonic acid, H4CO4.
Carbon dioxide cannot exist as a liquid at atmospheric pressure. At a pressure of anything above 5,11 bar(a) and at a temperature between −56,6 °C and 31,1 °C it becomes liquid (see Figure 1), and its density rises with temperature to 1 180 kg/m3. The liquid/gas equivalent (1,013 bar and 15 °C (per kg of solid)) is 845 vol/vol.
If the temperature of liquid carbon dioxide drops below 56,6 °C it becomes solid (see Figure 1). Solid carbon dioxide usually has a snow-like appearance, and can be compressed into blocks to form ‘dry ice’. Solid CO2 will form in vessels/pipelines when conditions fall below the triple point and this may not be snow-like.
Dry ice manufacturing starts with liquid carbon dioxide held under pressure (about 21 bar) in bulk storage vessels. To begin making dry ice, the liquid CO2, is expanded through a Joule-Thompson valve into an empty chamber where, under normal atmospheric pressure, it flashes into CO2 gas. This change from liquid to gas causes the temperature to drop quickly. About 46 % of the gas will become carbon dioxide snow. The rest of the carbon dioxide gas is either released into the atmosphere or recovered to be used again. The carbon dioxide snow is then collected in a chamber where it is compressed into block, pellet or rice size pieces to meet customer’s requirements. The denser the dry ice is, the longer it will last, the easier it is to handle, and the better it will perform when blast cleaning, if it is to be used for this purpose.
Carbon dioxide above the critical point (in terms of both pressure and temperature) is described as being in the supercritical phase. The properties of supercritical fluids lie between those of gases and liquids; a supercritical fluid has densities similar to those of liquids, while the viscosities and diffusivities are closer to those of gases. A supercritical fluid can diffuse in a solid matrix faster than a liquid, yet possess a solvent strength to extract the solute from the solid matrix.
1 ‘Lakes of CO2 in the deep sea’, Kenneth Nealson, Department of Earth Sciences, University of Southern California, 19 September 2006.
2 Globally Harmonised System of Classification and Labelling of Chemicals, 2007 part 3: Health Hazards.
3 Data from ‘Basic Principles of Membrane Technology’ 2nd edition, May 1996, Marcel Mulder, ISBN 78-0-7923-4248-9 NB. Reference temperature and pressure for these figures is not provided.
4 Reference Perry’s Chemical Engineering Handbook.
5 From ‘Piping Design Handbook’, John McKetta, CRC Press, 1992.
6 Reference Presentation by Julia Race, Newcastle University to Energy Institute, 24 July 2006.
7 Impingement will change the velocity of a jet and hence the source terms for the release. There are a few examples where it is possible to envisage a low momentum outcome. For example, a small high pressure leak from an underground pipeline, where the leak is insufficient to blow away the topsoil material. In this case, the soil will remove the momentum from a leak.