Reduction in the transport of sour gases and hydrocarbons to underlying PE-RT through thin films of PVDF

Abstract

Raised temperature polyethylene (PE-RT) is being studied for use in the conveyancing of sour hydrocarbon fluids in spoolable composite pipe at temperatures above 90 °C. This grade of polyethylene swells on exposure to all hydrocarbons but specifically aromatic hydrocarbons and so there is a requirement to reduce the flux of hydrocarbons and gases into the base PE-RT. A multi-material pipe wall is one solution to reduce these fluxes but new experimental methods are required for design and validation while saturated in sour hydrocarbon fluids.In this study the simultaneous transport of species such as CO2, H2S, CH4, water vapour and aromatic hydrocarbon vapours such as toluene, xylene, IRM902 and cyclohexane were measured in real time through single films of PVDF and PE-RT and multilayer walls of PVDF/tie layer(s)/PE-RT extracted from a built (co-extruded) pipe section. The permeation test facility contained specialized gas chromatographs modified for continuous quantification of all species. The experimental methods were designed to run at 93 °C with gas pressures of 104 barg or 250 barg. Experiments were run with gas only mixtures (dry tests) and also with gas cap above a mixture of liquid hydrocarbons and water (wet tests). When possible, transport properties such as diffusion and solubility coefficients were derived from the permeation traces. The volume flow rate of all species was higher through PE-RT compared to PVDF and the volume flow rate for CO2, H2S, and CH4, increased when the gas pressure was increased from 104 to 250 barg. The gas permeability was calculated for the base polymers and also the combined multilayer structure. The flow rates estimated for a multilayer specimen using the values measured on individual films were confirmed experimentally. A simple mathematical model was proposed to derive the apparent permeability of the multilayer system and the steady-state concentration profiles across the specimen wall. Specimens were inspected for damage after a final depressurization at 70 bargmin−1. Post exposure assessment of the laminates showed that the adhesion levels were improved and the failure mode was altered from adhesive to cohesive on ageing.