Experimental investigation of near-critical CO2 tube-flow and Joule–Thompson throttling for carbon capture and sequestration

• Optically accessible flow of CO2 near its critical point (74 bar, 31 °C).
• Experiments performed in conditions relevant to carbon capture and sequestration.
• Shadowgraphs show transition of CO2 from a supercritical state to a two-phase state.
• Pressure drop in pipe is sensitive to changes in inlet conditions.
• Classical Moody chart is applicable in these conditions.

Flow of CO2 in the vicinity of its critical point was studied experimentally in two different flow configurations. First, a 60 cm long stainless steel pipe with 2.1 mm inner diameter was used to study near-critical CO2 pipe flow. In terms of raw flow data, the results indicated high sensitivity of pressure drop to mass flow rate as well as to inlet conditions; i.e. pressure and temperature. Remarkably though, when friction factor and Reynolds number were defined in terms of the inlet conditions, it was established that the classical Moody chart described the flow with satisfactory accuracy. This was rationalized using shadowgraphs that visualized the process of transition from a supercritical state to a two-phase subcritical state. During the transition the two phases were separated due to density mismatch and an interface was established that traveled in the direction of the flow. This interface separated two regions of essentially single-phase flow, which explained the effective validity of the classical Moody chart. Second, Joule–Thomson throttling was studied using a 0.36-mm-diameter orifice. For conditions relevant to carbon capture and sequestration, the fluid underwent Joule–Thompson cooling of approximately 0.5 °C/bar. The temperature difference during the cooling increased with increasing inlet enthalpy. Discrepancies with previous computed and experimentally measured values of Joule–Thompson throttling were discussed in detail.