The Carbon Capture and Storage (CCS) is the technology that has been proposed to reduce high concentrations of CO2 in the atmosphere. This method involves transporting the CO2 to the storage site, usually in pipelines. In order to ensure safe transport, the required pipe toughness must be accurately estimated. This, in turn, requires an accurate estimate of the decompression wave speed in the fluid. In this paper, a multi-phase Computational Fluid Dynamics (CFD) model is presented to simulate the decompression of a CO2-N2 mixture in a pipe. A real gas EOS (GERG-2008) was incorporated into the CFD code. The non-equilibrium liquid/vapour transition was modelled by introducing source terms for mass transfer and latent heat. The model is validated through simulation of a ‘shock tube’ test. The effects of the non-equilibrium phase transition on the decompression wave speed are discussed.