(163b) Ultrasupercritical CO2 Transport in Flexible Flowlines

In order to transport supercritical carbon dioxide safely in polymer pipelines, it is necessary to understand several thermodynamic and transport properties of the polymer/gas system. Measurements and modelling were carried out with supercritical CO₂ and a mixture of CO₂ and CH₄ in two polymers - PVDF and XLPE (a highly cross-linked polyethylene). Both of these polymers find service in the offshore industry and the samples were obtained from flexible pipes. The properties measured are solubility of the gas in the polymers, using a Magnetic Suspension Balance, as well as permeability through the polymers using a 2-D pressure permeation cell. Pressures up to 650 bar and temperatures up to 90 °C were studied.

An advanced equation of state (simplified PC-SAFT) which is suitable for modelling polymers was used to model the gas/polymer system. Since pure component parameters for this model are usually fitted to saturated properties remote from the critical region, a refit of the CO₂ pure-component parameters was required for application in the supercritical region. Pure component properties for polymers are also problematic, and here we fitted parameters to polymer density data combined with an extrapolation from the polymer monomer properties. Solubility was successfully modeled and it was found that correct modelling of the fluid phase (ultrasupercritical CO2) was most important for correlating binary phase data.