Annex F CO2 BLEVE
BLEVE can occur when a vessel containing a pressurised gas is ruptured or being blown down in an inappropriate manner. BLEVEs occur with a number of gases, notably with propane at higher temperatures, usually as a result of fire impingement.
BLEVEs are very unusual, but catastrophic events can occur with CO2 under pressure. An example of a severe explosion with CO2 occurred at Worms, Germany.
There is a range of conditions of pressure and temperature where BLEVEs can theoretically occur, and even under these conditions a BLEVE is unlikely. However, because of the potential size of BLEVE explosions, a CO2 plant should be operated in a manner that would avoid these conditions arising.
Figure F.1 Theoretical pressure/volume graph for CO2, showing spinodal curve
Imagine liquid CO2 in a large pipe, such that the volume is significant. The pipe is being depressurised, for example, blown down for maintenance. The blue line A-B-C-D shows the behaviour of the CO2 at a constant temperature and at thermodynamic equilibrium. Along the line A-B, the CO2 is a liquid, and as the volume it occupies is expanded the pressure falls rapidly. Eventually the pressure falls to the vapour pressure of the liquid at the particular temperature at B. The liquid CO2 then starts to evaporate to become a liquid-gas mixture, and the pressure stays constant at the vapour pressure. Eventually it reaches C, where the liquid has been completely converted to gas. The pressure then drops as it is expanded further as a gas (to D).
However, if, having reached the vapour pressure line (B-C), the CO2 pressure falls suddenly, (for example, due to a failure in the containment, or a valve being rapidly opened), the CO2 can become an unstable liquid along the path B-X, the solid red line. Along this line the CO2 is metastable, and could at any time boil to return to the equilibrium horizontal line B-C. Were this to happen, a sudden and a violent disturbance would take place, although it would not become a BLEVE until it reached the point X.
X is called the spinodal point: here the slope of the solid red line is zero. The dashed line (spinodal curve) indicates the loci of spinodal points with changing temperature. Along the spinodal curve large, because the slope of the line approaching it is zero, density changes can take place because pressure can increase without any volume increase.
Once the spinodal curve is reached, the CO2 (which is an equilibrium of liquid and gas) gas will separate into gas and liquid states. This occurs homogeneously throughout the whole of the mass of the CO2. The rise in pressure to (X') on the vapour pressure line B-C may not be not large, but takes place very quickly145, and the pressure shock to the vessel in which the CO2 is contained is significant, and failure (BLEVE) is likely to occur.
142 The rapid depressurization of hot, high pressure liquids or supercritical fluids Kim and Reid, Chemical engineering at supercritical fluid conditions, Paulaitis, Anne Arbor Science,1983
143 BLEVE and its importance in Enhanced Gas Recovery and Carbon Capture and Storage, AA Clifford, Leeds University Department of Chemistry.
144 CO2 BLEVE, W.Ke, Telemark University College, Norway, 2009.
145 On a time scale of molecular motions, ref. Calculation of BLEVE conditions for carbon dioxide, Critical Processes Ltd