Depressurization of CO₂ rich mixtures: Challenges for the safe process design of CCS facilities and CO₂ EOR systems

The design of systems with high content of CO₂ in the process mixture is of increasing importance. This is particularly true for emerging technologies such as Carbon Capture and Storage (CCS); with over twenty CCS installations worldwide (built or under-construction) and many more now progressing through front-end engineering & design and then to final investment decision. The design of the safety depressurization system for both CCS facilities and CO2 Enhanced Oil Recovery (EOR) installations is of particular importance, due to its impact on project costs. As with Oil & Gas processing facilities, the minimum metal temperatures in process equipment and piping are observed during highly transient depressurization operations ("blowdown"). The minimum metal temperature usually limits the material of construction: if metal temperatures below -46°C (-50°F) are possible then the usual requirement is to select materials that exhibit ductile behaviour below this point, typically stainless steel. Such choices have a huge impact on project costs, vessel order times and ultimately project viability. The design of the safety depressurization system for CO₂ rich mixtures is difficult; CO₂ introduces complex thermodynamic behaviour, for example: physical properties that are not accurately predicted by standard equation of state methods, a narrow phase envelope and the potential formation of solid phases during depressurization. Furthermore, physical plant configurations which are sectionalized for depressurization consist of multiple interconnected vessels and significant quantities of piping low points where condensate may accumulate. These locations are shown to be significant to depressuring temperatures. The design of such systems is not handled well using conventional depressurization methodologies; which rely on the representation of an actual plant segment as a single pseudo-vessel volume. In this paper, we present a validated methodology for analysing accurately the depressurization of high pressure gas processing facilities with rich CO₂ mixtures. We describe the application of the methodology to the design of a CO₂ EOR process.