Towards modelling the evaporation of CO₂ during depressurization using physics-based mass-transfer terms

Accurate models of evaporating flows are needed in a range of engineering applications. An important application is to simulate the depressurization and subsequent phase transition of CO₂ in pipes. This is relevant for the safe design and operation of CO₂ pipelines for large-scale CO₂ capture and storage. Experiments of flows with rapid evaporation show significant departure from equilibrium and relaxation models must be applied to describe the process. Lately, a hierarchy of different relaxation models have been developed and studied, and relaxation techniques have been devised for infinite, finite and arbitrary-rate relaxation terms. The relaxation terms applied in these models often require tuning. We explore the potential of applying physics-based relaxation terms in the relaxation models to improve accuracy and avoid tuning. As a first step, this is tested for a model with chemical potential relaxation. The mass relaxation term is modeled using classical nucleation theory (CNT) and a bubble growth term based on non-equilibrium thermodynamics. As CO₂ flow in pipelines will often be near the critical point where two-step relaxation methods can reach thermodynamically invalid states, we suggest an implicit relaxation technique in the solution procedure. The full model holds for a general equation of state (EOS), and we apply the Peng-Robinson EOS for the simulation of CO₂. We compare the results of this model to experimental data of CO₂ depressurizations.