Modeling of CO₂ Decompression across the Triple Point

Abstract

The formation of significant quantities of solid CO 2 as a result of surpassing its triple point during rapid decompression of CO 2 pipelines employed as part of the Carbon Capture and Sequestration (CCS) chain can present serious operational and safety challenges. In this paper, the development, testing and validation of a rigorous Computational Fluid Dynamics (CFD) flow model for predicting solid CO 2 formation during decompression is presented. Multiphase flow is modelled by assuming homogeneous equilibrium, and the pertinent thermodynamic data are computed using real-fluid equations of state. The flow model is validated against pressure and temperature data recorded during the decompression of an extensively instrumented 144 m long, 150 mm i.d. CO 2 pipe initially at 5.25 o C and 153.3 bar. For the conditions tested, the simulated results indicate CO 2 solid mass fractions as high as 35% at the rupture plane, whose magnitude gradually decreases with distance towards the pipe's intact end.

Adsorption Isotherms of Low-Pressure H2O on a Low-Temperature Surface Measured by a Quartz Crystal Microbalance

“…In terms of CO 2 , its triple-point temperature is about 216 K, relatively far lower than the wall temperature in the experiment, and its saturated vapor pressures at every experimental temperature are far higher than the experimental pressure. 55 These characteristics prevented CO 2 from noticeable condensation and formed a stable crystal structure. This is the reason why there is no more capacity for more CO 2 molecules on the outer layer when the adsorption process is reaching a dynamic equilibrium.…”

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

“…To model the outflow at the puncture end, the variation in the flow area from that of the pipe to the area of the puncture hole is prescribed using an explicit function ) (x A . This approach enables application of the outflow condition following our previous study [26], where an integral form of a Riemann invariant is applied to express the discharge flow velocity [35]:…”

“…c = 0. In order to overcome this singularity, the speed of sound at the triple point is calculated using the following expression for the homogeneous frozen mixture [26]:…”

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

“…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].…”