Simultaneous precipitation of Calcium Sulfate and Calcium Carbonate during MEG regeneration
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Simultaneous precipitation of calcium carbonate and calcium sulfate in supersaturated solutions containing a MEG-water concentration containing 0, 50 or 90 wt% MEG, was investigated in a bath setup at constant pressure and two different temperatures, T = 30 and T = 65 degree Celsius. The pH and temperature was monitored and logged automatically during the experiments, samples were taken out to titrate to find the concentration of calcium in the bulk solution, and samples were filtered out, first after precipitation had occured, and then at the end of the experiment, to determine the content of the crystals by XRD and SEM. Experiments to determine whether calcium sulfate precipitation was dependent on pH decrease was also done. The pH was found to only decrease with precipitation of calcium carbonate, and not for calcium sulfate, making it a good indicator for which salt precipitates at a given time. It was further found that 0 and 50 wt % MEG concentrations favors precipitation of gypsum at both temperatures. Solvents with 90 wt % MEG favored precipitation of anhydrite for both temperatures. The presence of MEG in the solvent was found to inhibit nucleation and growth of both calcium sulfate and calcium carbonate crystals. The size of the crystals found with increasing concentration of MEG were found to decrease. The amount of crystals found in the bulk solution at the end of an experiment was also decreased with increasing MEG concentration. The amount and size of the precipitated crystals increased with increasing temperature. The transition between gypsum and anhydrite was found to favor anhydrite at 90 wt % MEG, and gypsum for lower MEG concentrations. For T = 65 degree Celsius and T = 30 degree Celsius the transition occurred between 90 and 50 wt % MEG. Anhydrite was only found at 90 wt % MEG for both temperatures, and not at lower MEG concentrations. High saturation ratios gave large crystals of calcium sulfate, and only small ammounts of calcium carbonate. Low saturation ratios, SR_CaSO_4 = 1, only resulted in calcium carbonate precipitation. Both calcium and sulfate concentrations effect the expected amount of precipitation of both calcium sulfate and calcium carbonate. Further, both calcium and sulfate concentrations must be sufficiently high to give a desired yield of precipitate.