Molecular Simulation Of Gas Permeation In PVDF Under Oil-Gas Environment

In recent years,the reinforced thermoplastic composite pipes with the advantages of corrosion resistance,impact resistance,easy bending,easy laying,light weight and low cost has been widely used in the field of surface oil and gas gathering and transportation.However,with the extension of service time,more and more thermoplastic plastic lined pipes have blistering failure,which leads to a decrease in conveying pressure and brings great hidden dangers to the safe operation of gathering and transportation pipelines.In order to reduce the failure risk of thermoplastic lined pipes in the process of oil and gas transportation,effective measures to control gas permeation are fundamentally proposed.It is necessary to understand the permeability characteristics of gaseous media in thermoplastics in an oil and gas environment and clarify its permeation mechanism.Molecular simulation technology can not only clarify the permeation behavior of gas molecules from the microscopic particle level,but also directly observe the evolution process of gas adsorption and diffusion from a microscopic point of view,and determine its permeation mechanism.Therefore,based on the oil and gas gathering and transportation operating environment,this paper comprehensively considers the effects of gas permeation and liquid petroleum swelling.The molecular simulation method was used to study the permeation behavior and permeation mechanism of single-component gas,mixed gas and oil-gas coupling medium in polyvinylidene fluoride（PVDF）under different temperature and pressure conditions.The main research contents and results are as follows:The molecular simulation method is used to establish the adsorption and diffusion model of CH₄ in PVDF and perform simulation calculations.It is found that the molecular simulation results are consistent with the experimental results,and the error is small.It shows that the established molecular model has high simulation accuracy and can simulate and study the permeation behavior of gas in PVDF well.Based on the established molecular model,the permeation behavior of single-component CH₄ gas in PVDF at different temperatures and pressures was studied.It is found that the solubility coefficient of single-component CH₄ gas in PVDF decreases with the increase of temperature and increases with the increase of pressure.The diffusion coefficient increases with the increase of temperature and pressure.The permeability coefficient decreases with the increase of temperature as a whole,and increases with the pressure.Based on the above-mentioned single-component gas model,molecular models of CH₄/CO₂ binary mixed gas medium and CH₄/CO₂/H₂S ternary mixed gas medium are established.Further explore the permeation behavior of multi-component gases in PVDF and the competition behavior between molecules in the permeation process.The study found that the dissolution coefficient and diffusion coefficient of the mixed gas medium in PVDF at different temperatures and pressures change law basically the same as that of single-component CH₄.However,in a mixed gas medium,the dissolution-diffusion behavior of gas molecules in the polymer will be greatly affected by another gas.There is a competitive relationship between gas molecules during adsorption.The more mixed gas,the smaller the solubility coefficient.However,there is a synergistic effect between gas molecules during diffusion.The more mixed gas,the greater the diffusion coefficient.Among them,the order of solubility and permeability of gas molecules in PVDF is H₂S>CO₂>CH₄,and the order of diffusion capacity is CH₄>CO₂>H₂S.On the basis of the ternary mixed media model,crude oil molecules are added to establish an oil-gas coupling medium model,and the permeability behavior of the oil-gas coupling medium in PVDF and the influence of crude oil components on the permeability behavior of gas molecules are studied and determined.It is found that the gas permeability coefficient under the oil-gas coupling medium has the same law as the gas-only medium.However,the swelling effect of liquid crude oil molecules will reduce the solubility of gas molecules in PVDF and improve the diffusion and permeability of gas molecules in PVDF.Research on the permeation mechanism of gas molecules in various media found that the permeation mechanism of gas molecules has nothing to do with the medium（single component gas,mixed component gas,oil-gas coupling medium）.The gas molecules exhibit selective aggregation and adsorption in PVDF,and are mostly adsorbed in the low-potential energy region in the simulated unit cell.During diffusion,gas molecules are combined with small vibrations and large jumps,which conforms to the"cavity jump diffusion theory".In most cases,gas molecules vibrate at a small amplitude in a cavity in PVDF,and a larger range of transitions between the cavities occur when the temperature or pressure increases.