Depressurization of CO2 Rich Mixtures: Challenges for the Safe Process Design of CCS Facilities and CO2 EOR Systems

The design of systems with very 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 depressurisation operations (“blowdown”). The minimum metal temperature usually sets the material of construction: if metal temperatures below -46oC (-50oF) are possible then the usual requirement is to select materials that exhibit ductile behaviour below this point. Such choices have a huge impact on project costs and order times and ultimately project viability.

The design of the safety depressurization system for CO2 rich mixtures is difficult; CO2 introduces complex thermodynamic behavior, 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, typical depressurization segments consist of multi-vessels and significant amounts of piping with low points where condensate may accumulate. The design of such systems is not handled well using conventional 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 analyzing accurately the depressurization of high pressure gas processing facilities with retrograde condensation. We describe the application of the methodology to the design of a CCS facility for a 500 MWe coal-fired power-plant. The resulting optimized, safe design decreased the stainless steel construction with project savings of hundreds of millions of dollars.