Effect of Magnesium and Ferrous Ions on CaCO3 Scaling in MEG Regeneration Pre-Treatment

Abstract

Monoethylene glycol (MEG) regeneration may include a pre-treatment to reduce the concentration of cations that tend to induce scaling in the downstream process. This work reproduced pre-treatment conditions in a continuous stirred-tank reactor. The experiments were performed in 50 wt% MEG solutions at 80°C. Divalent cations and alkalinity solutions were dosed into the reactor and the mixed solution was pumped out at controlled rates. Steel rods were inserted into the test solution to measure scaling rates. The growth of scale and particles in bulk solution with varying Mg2+, Fe2+ and SO42− ions were studied as function of supersaturation with respect to calcite. The experimental results show that crystallization fouling, rather than particulate fouling, is the dominating mechanism controlling the formation of calcium carbonate scale in MEG pre-treatment. The supersaturation at steady state controlled the amount of scale. The presence of Mg2+ retarded the nucleation rate of calcium carbonate and thereby lowered the surface areas available for consumption of Ca2+ and CO32− in in the bulk solution. It resulted in higher CaCO3 supersaturation which promoted scaling. Addition of Fe2+ had little effect on scale formation. At these conditions, the calcium carbonate scale that formed on steel rods and as solids in the bulk were exclusively the aragonite polymorph. Seeding with aragonite reduced the scaling tendency in the experiments where Mg2+ was present. The result indicates that maintaining a large active surface area for growth in the bulk solution can reduce the scale formation.