Computational and Experimental Study of Solid-Phase Formation during the Decompression of High-Pressure CO₂ Pipelines

Abstract

Decompression of CO2 pipelines is studied both experimentally and numerically to provide a validated model as the basis for the prediction of the hazards associated with CO2 solid formation. The pipeline decompression experiments, performed using a fully instrumented 36.7 m long and 50 mm internal diameter test pipe up to a maximum pressure of 45 bar, incorporating discharge orifice diameters of 4 and 6 mm, reveal the stabilisation of pressure and temperature near the CO2 triple point. Also, video recordings of the decompression flow in the reinforced transparent section of the steel pipe show that initial stratification of the constituent liquid and vapour phases is followed by rapid CO2 solid formation and accumulation in the pipe. To aid the prediction of hazards associated with solids formation in pipelines, a homogeneous equilibrium pipeline decompression model is developed accounting for the pertinent physical properties of CO2 in the liquid, vapour and solid states. The model is validated against the experimental data, showing ability to accurately predict the measured pressure and temperature variations with time along the pipe, as well as the time and amount of the solid CO2 formed upon decompression across the triple point.

Numerical study of the effect of heat transfer on solid phase formation during decompression of CO2 in pipelines

“…As such, the model indicates that decompression of a compressed CO2 gas state doesn't lead to CO2 solids forming in the pipe. Figure 3 shows the effect of d/D ratio on the mass of solid CO2 formed in the pipe upon its decompression to the triple point, obtained based on the HEM flow model predictions, in comparison with estimates using the thermodynamic method assuming isentropic decompression [12]. As can be seen in Figure 3, predictions by the thermodynamic method are insensitive to the orifice diameter.…”

“…Also, accumulation of solid CO2 in pipelines and vessels may increase the risk of blockage and overfilling of units at later stages of operation [7][8][9][10][11]. In particular, our recent studies showed that solid CO2 may form during decompression of pipelines initially filled with CO2 at 60-80 bar pressures [12,13]. Given that solid CO2 accumulation in safety valves and vented sections of pipelines presents a risk for the system integrity, designing venting equipment and procedures that minimize the amounts of solid phase formed becomes critically important.…”

“…Recent studies showed that Homogeneous Equilibrium Mixture (HEM) model predicts well the pressure and temperature measured in the pipeline Full Bore Rupture test [13] and amounts of solid CO2 formed in the pipeline in the orifice discharge tests [12]. Using this model it was demonstrated that duration of the pipeline decompression to the triple point where CO2 solids can form in the pipe, scales with the pipeline length [14], while the amount of solid CO2 formed depends on the history of decompression flow [12]. The latter depends on a number of factors, including the initial thermodynamic state of the fluid, the discharge hole diameter, and the rate of heat transfer at the pipe wall.…”

Thermophysical evolution during different decompression of N2‐containing S‐CO2 pipelines

“…12 This results in the generation of dry ice in the release valve, which blocks the decompression. According to the results of Martynov et al, and Zheng et al's study on the decompression process of CO 2 across the triple-phase point, 38,39 if dry ice is generated in the pipe during the decompression process, a pressure plateau will appear in the pressure drop curve when the triple-phase point pressure is reached. This is consistent with the experimental results, therefore dry ice is also generated in the pipe.…”

Experimental Campaign of Massive CO2 Atmospheric Releases in an Urban Area

“…Several experimental discharge configurations with large storage conditions [10][11][12][13] have allowed a better understanding and evaluation of the dispersion of a CO 2 cloud. The blowdown of a highly instrumented pipeline has also been deeply studied considering the potential presence of impurities mixed in the CO 2 and their influence on the thermodynamic properties and formation of solids [14], which could lead to clogging and would be a cause of leakage. Ineris has a good knowledge of the source term calculation and dedicated tools which allowed us to focus on dispersion.…”