Effect of Magnesium and Iron on Precipitation of Calcium Carbonate in Rich Monoethylene Glycol MEG

Abstract

Abstract Monoethylene glycol (MEG) regeneration systems often have a pre-treatment unit for controlled precipitation and subsequent removal of carbonates and hydroxides. The purpose is to avoid scaling in downstream process system. CaCO3 is often the main solid removed in the pre-treatment. This study is on the effect of magnesium and iron(II) ions on the CaCO3 polymorph that forms, its particle size and morphology and the residual calcium concentration in solution. Experiments were performed in a continuous reactor at 80 °C. The residence time was ca. 20 minutes. The feed to the reactor was 50 wt% MEG (representative rich MEG) containing divalent cations and NaCl. Concentrated alkali (NaHCO3, Na2CO3 and/or NaOH) was added to precipitate the divalent cations. The slurry in the reactor was sampled and filtered. The filtrate was analysed for alkalinity and residual divalent cations, and the dried solids were characterised to determine crystal morphology in addition to elemental and phase composition. The experiments were run as a continuous process for ca. 4 hours. Steady state with respect to precipitation was established after ca. 1 hour and from that point in time, the concentration of Ca2+ and CO32- was nearly constant. Aragonite was the dominating CaCO3 polymorph and the main precipitate. At steady state, the saturation ratio (SR, activity product divided by the solubility constant) of aragonite was in the range 1-5. Magnesium ions in solution seemed to maintain the steady state SR of aragonite above 1, while the aragonite crystals became significantly smaller when Mg2+ precipitated as an amorphous solid and the residual concentration in solution was low. Siderite is less soluble than aragonite, and in 50 wt% MEG at 80 °C, its precipitation rate was sufficiently high to give low residual Fe2+ concentration when CaCO3 precipitated to low Ca2+ concentration. Siderite co-precipitating with aragonite at mol ratio siderite:aragonite~1:5 had little effect on the morphology and particle size of aragonite.