Experimental investigation of sodium surfactin as potential methane-water hydrate formation inhibitor
The leading technologies employed in gas processing operations, including gas pipeline transport, are generally chemical based technologies. Chemical based substances are widely used as surfactants in these operations, including hydrate formation prevention and control. Surfactants usage in oil and gas processes stem from their tensioactive capabilities. However, dosage and environmental compatibility continue to be an issue arising from the use of these chemical surfactants. More so, chemical surfactants are being used as hydrate formation inhibitors. This is because, the effect of pipe plugging (due to hydrate formation) could be devastating as it may lead to the eventual shut down of operation. Besides, economics of unplugging and maintenance is a huge burden to the industry. Biosurfactants, on the other hand, are low-dosage and environmentally friendly surfactants. Their application was tested and therefore has been established to serve as alternative to conventional chemical surfactants in enhanced oil recovery (EOR). However, the application of biosurfactants in multiphase methane/water system under field operating conditions has not been duly investigated and reported. Thus, this provided the primary motivation for this study.In view of the above, this novel research study experimentally investigated the potential of sodium surfactin as methane-water hydrate formation inhibitor. The investigation was conducted in three (3) phases using sodium surfactin. Evaluation was made considering surfactant’s macro size (dosage) and its interaction in the solution, temperature stability and salinity tolerance. Parameters that were employed for the experimental investigation were: temperature in the range of 1 to 50 oC, surfactant dosages of 0.025 to 1 % and 0.1 to 1.5 M saline (NaCl) concentration.Phase I of the study investigated the physicochemical (density, pH, electrical conductivity, functional group) and thermal characteristics of surfactin. The results revealed that surfactin was effectively soluble and foams in aqueous medium. This confirmed the surfactant’s adsorptivity, a critical property that enhance performance at reducing surface tension. However, the characteristics were significantly affected by salinity at 1.0 Molarity (M) and above. Dispersion and adsorptivity potential were further confirmed by the lowest density value of 996.641 kgm-3 at 0.075 and 0.5 % surfactin dosages. More so, temperature increase resulted in density decrease to a minimum value of 988.262 kgm-3 at 50 oC. Nonetheless, density was significantly affected by changes in salinity. Aqueous surfactin will pose no threat of corrosion in pipeline, with pH value of 6.68 and 7.31 respectively at 0.025 and 1.0 % surfactin dosage. Functional group analysis indicated that surfactin is cyclic and contains aliphatic chains (–CH3; –CH2–), carbonyl group (C=O), CO–N bond and aromatic C—H group. The cyclic nature of the molecule confirms its ability for surface adhering properties. Also, thermal stability test showed that surfactin is thermally stable within the temperature range of -20 to 170 oC. Flow behaviour of surfactin was investigated in the phase II of the study. This was performed using rheology as the basis for the investigation, at variable temperature and salinity. Findings revealed that surfactin exhibited pseudoplastic flow behaviour characteristic based on viscosity-shear rate interactions. More so, solution of surfactin satisfied the shear rates for situations involving mixing and stirring and pipe flow which are respectively in the range of 101 to 103 s-1 and 100 to 103 s-1. This finding is an indication of ease of flow of surfactin solution during fluid transport in pipes.Phase III investigated the effect of surfactin on methane-water surface tension (ST) using rising bubble technique at different operating conditions. The finding showed that surfactin effectively reduced methane-water surface tension from 72 to 34.13 mN/m at 32.5 oC and pressure of 7.58 MPa. As a key indicator of surfactant’s ability to alter fluids interfacial interaction, the ST result further demonstrated that surfactin can potentially confer on multiphase gas/water system. Hence the potential of sodium surfactin as hydrate formation inhibitor.Hydrate formation is a surface activity and therefore surfactants must effectively adhere at fluids surface to efficiently be able to reduce surface tension. Dispersion and adsorptivity characteristics are key indicators of surface activities which were confirmed by density and surface tension measurements. Furthermore, surfactin will flow to delivery point in pipeline without failure (considering both laminar and turbulent flow regime). More so, utilization of surfactin is expected to be safe without risk of corrosion based on the pH and electrical conductivity results. Therefore, sodium surfactin can be said to be a potential hydrate-formation inhibitor. These new experimental data will provide a basis for further investigation into mechanism of biosurfactant’s effect on methane-water hydrate formation inhibition.
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