An experimental study of calcium carbonate precipitation with hydrate inhibitor in MEG recovery unit

Abstract

Monoethylene glycol (MEG) is a thermodynamic inhibitor of gas hydrate formation used in the oil industry. Regeneration of MEG for reinjection in wells is necessary to minimize operating costs due to the large amounts of this additive employed. However, this scenario favors the precipitation of inorganic salts from the produced water, mainly calcium carbonate (CaCO₃). This work is devoted to evaluating the CaCO₃ precipitation in water + MEG mixtures (0–50 vol.% MEG) at different concentrations of reacting salts (0.01–0.1 mol L⁻¹) and temperatures (25–60 °C). The focused beam reflectance measurement (FBRM) technique was used for inline monitoring of chord length and CaCO₃ particles distribution in the suspension for 60 min. Optical microscopy was used to understand the particle precipitation phenomena. FBRM results show that the size distribution and the number of CaCO₃ particles in the aqueous solution vary with time, temperature, reacting salts, and MEG concentrations. The higher the salt concentration, the larger both the size and number of precipitated chords. Temperature expressively affects salt precipitation. For a given concentration of MEG, the enhancement in temperature favors the increase in the amount and size of chords. Specifically, for 10% v/v of MEG solutions, the particle size increases from 8.0 ± 0.5 μm (at 25 °C) to 20.4 ± 2.1 μm (at 60 °C). Additionally, at 30% v/v of MEG in the solution, the particle size increases from 5.4 ± 0.4 μm (at 25 °C) to 15.5 ± 0.2 μm (at 60 °C). These outcomes are related to the reduction in CaCO₃ solubility and the improvement in MEG viscosity with temperature. Optical microscopy measurements corroborate the FBRM data, thus demonstrating the influence of the parameters MEG concentration, ionic concentration, and temperature have on the number and size of precipitated carbonate crystals.