Depressurization of CO2 in pipes: Analysis of experiments and non-equilibrium flashing flow models

Abstract

CO2 capture and storage is expected to play a vital role in reaching net zero emissions by 2050. In this context, large-scale CO2 pipeline transportation networks must be deployed. In order to perform safety analyses and ensure efficient operation of large-scale CO2 transportation systems it is key to predict the flow, in particular during depressurization events. This includes intentional depressurizations, e.g., the opening of a pressure relief valve, and accidental depressurizations, e.g., caused by a pipe fracture. High-capacity CO2 pipelines will be operated in the densephase region, meaning that the CO2 will boil during depressurization. This kind of boiling is denoted as flashing, and flashing often occurs delayed, i.e., at a lower pressure than the saturation pressure, out of equilibrium. The resulting pressure evolution and mass flow is highly dependent on the flashing process. Therefore, the focus of the present thesis is to gain more knowledge of this phenomenon through experiments and coupled thermo and fluid dynamics modeling. In this work, a series of full-bore pipe depressurization experiments were conducted and analyzed, physics-based models for the mass-transfer during flashing have been investigated, and novel numerical methods have been developed for the simulation of non-equilibrium two-phase flows. The effect of homogeneous and heterogeneous bubble nucleation on the maximum attained superheat in the experiments has been investigated. It is found that homogeneous nucleation determines the superheat at warm conditions, i.e., near the critical point, and heterogeneous nucleation dominates otherwise. Homogeneous nucleation can be modeled by classical nucleation theory. This theory is applied herein to account for delayed boiling for flow through orifices and nozzles, and to improve the fluid curve in the Battelle two-curve method for the assessment of pipeline designs with respect to running ductile fracture. It is found that the combination of the crevice model and bubble growth for heterogeneous nucleation can explain the attained superheat at colder temperatures. A main result from this work is a homogeneous flashing model (HFM) for flashing flows. The mass-transfer model in the HFM accounts for bubble nucleation, coalescence, breakup and growth. The key finding from the analysis of this model is that both homogeneous and heterogeneous nucleation must be included in flashing flow models to capture the flow behavior during depressurization at warm conditions, including the relevant operating region of CO2 transportation pipelines. The present findings for flashing flows are general and relevant for other industrial applications including refrigeration systems and water cooling for pressurized water (nuclear) reactors.