Safety when liquid becomes gas in CCS

At low temperatures, gaseous CO2 occupies more than 80 times the space of liquid CO2 (at the same pressure). If a pipeline, storage container or equipment has been designed for gas to form, this is likely unproblematic. 

The problems arise when gas forms unexpected. This can happen because of a change of operation conditions, or due to a breach in the structure. Significant amounts of gas bubbles in the fluid can have a range of consequences on flow assurance and safety: 

  • They can change the flow pattern, possibly leading to surges in the flow velocity because gas occupies a much larger volume than liquid. 
  • Pockets of gas (bubbles) can form between liquid slugs and move rapidly through the equipment, leading to vibrations, noise, and potential material fatigue.
  • The pressure drop is likely to increase,  potentially reducing pipeline efficiency if it exceeds the design specifications of the pumps.
  • Operating pumps at too low pressures can cause rapid formation and collapse of vapor cavities, which can erode the inner surfaces of pumps and piping. This phenomenon not only damages the pump but may also affect the connected piping and other associated equipment.
  • Expansion of the gas can lead to the “Joule-Thomson effect”, where the gas cools down. The gas can then cool down surrounding infrastructure, possibly leading to a loss of structural integrity in CCS injection wells.
  • Contact between gas pockets and liquid CO2 can lead to rapid gas condensation, which accelerates the surrounding liquid and triggers a phenomenon known as “condensation-induced water hammer”. This can result in damaging pressure surges that pose a risk to the integrity of the system.

The illustration above shows the regions where liquid turns into gas for various CO2-mixtures. If the CO2 contains only 5% nitrogen, the boundary where gas forms shifts to much higher pressures (upper green dotted line) compared to pure CO2 (solid line). On the contrary, if the CO2-mixture contains 5% SO2, the pressure where gas forms shifts to lower values (upper blue dashed line). 

In CCS systems, CO2 is often accompanied by impurities. When designing CCS infrastructure, it is crucial to precisely determine the composition, temperature and pressure range at which gas formation occurs. The software TPCloud will use the most accurate models and algorithms to identify this range, ensuring that appropriate safety barriers can be effectively maintained.