Abstract

Large quantities of hydrate inhibitors, such as methanol, MEG (ethane 1, 2 diol), and TEG (triethylene glycol) are used to prevent gas hydrate formation during production from a gas/oil well. Due to environmental and economic concerns, MEG is often regenerated via a thermal vacuum distillation process. The hydrate inhibitors adversely enhance scale formation. The impact of temperature and concentration on mineral scale formation during MEG regeneration is largely unknown. The solubility of barite and halite in methanol/brine and MEG/brine solution was measured up to 473.15 K (200 °C ) and 6.6 MPa (1000 psia) to evaluate the impact of high temperature on mineral scale formation in hydrate inhibitor/brine solutions. A model is developed to assist the process design to prevent scaling risk during MEG regeneration. The "unified theory" of electrolytes developed by one of the authors (EDJ), for prediction of the standard state thermodynamics properties of electrolytes to extreme temperatures and pressures, has been adapted to model the effect of temperature and pressure on mineral salt solubility in methanol, ethanol and MEG/brine solutions. The model requires knowing the Gibbs free energy of hydration at some temperature, usually 298.15 K (25 °C), and only two previously determined constants for each electrolyte. Once these constants are fixed, the model can be used to predict the standard state partial molar Gibbs free energies of electrolytes up to supercritical temperatures. The temperature and pressure behavior of electrolytes can now be accurately predicted from existing low temperature data alone. A modification of Pitzer's model for activity coefficients is also used to account for the effect of mixed electrolytes on solubility in cosolvent/brine systems. The combined model will be applied to typical reboiler systems in use on off-shore platforms and on shore.

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