Carbon dioxide (CO2) is a by-product of burning fossil fuels. As we all know, CO2 emissions should be minimised. Carbon capture (utilization) and storage (CCS or CCUS ) is a process where the CO2 is separated, treated and transferred in a storage where it does not return back to the atmosphere. Usually, CO2 is compressed in a supercritical form (SC-CO2) and pumped using carbon steel pipelines in the ground under a dense geological layer.
SC-CO2 flows include often impurities such as water, H2S, SO2, oxygen and NOx. The impurities affect the critical point pressure and temperature, but they also promote corrosion. The individual purities may not accelerate corrosion much, but their combination may increase the corrosion rates a lot. A representative corrosion testing must combine the SC-CO2 and the gaseous and liquid impurities. One must also pay attention to the regeneration of the testing environment to maintain a stable testing environment.
A SC-CO2 instrument has the following major sections:
- dosing and mixing of the gas flows (CO2 and the impurities),
- pressurization and heating of the SC-CO2 + impurities flow to create SC-CO2 mixture,
- testing cell operation + injection of liquids / inhibitors,
- cooling and depressurization of the SC-CO2 + impurities flow.
Various gases and gas mixtures will be mixed using mass flow controllers. There is normally a mixing tank in which the gas mixture is created. The SC-CO2 environment (> 31.1°C temperature and > 73.8 bara pressure) is created using a booster pump and a heater. The testing cell pressure vessel construction material is typically Alloy C-276 to avoid any contamination and corrosion due to H2S, chlorides, etc.
SC-CO2 phase must be maintained in the pipelines and in the testing cell to prevent the separation of phases. Correspondingly, the pipes and the critical components upstream to the testing cell must be trace heated.
The depressurization of the SC-CO2 is somewhat tricky as the CO2 enthalpy increases with the decreasing pressure. Many components and pipeline parts must be trace heated downstream of the autoclave. Additionally, one has to pay attention to the gasket materials that survive in the CO2 environment.
Cormet has built SC-CO2 instruments for various environments such as 650°C / 300 bar and
200°C / 200 bar. Common for all the SC-CO2 instruments is accurate and continuous temperature and pressure management.
