Effect of Methanol on Carbonate Equilibrium and Calcite Solubility in a Gas/Methanol/Water/Salt Mixed System

Methanol is a common industrial solvent and is added to water to enhance hydrocarbon solubility and to prevent solid hydrate from forming, as well as other applications. One of the side effects of methanol addition to water is to greatly reduce the solubility of ionic solids, particularly divalent solids. The effect of methanol on ionic solubility has been reported for only a few isolated conditions. A self-consistent activity model is proposed to describe the effect of methanol on carbonate equilibrium and calcite solubility in gas/methanol/water/salt solutions. The model is semiempirical in nature, which uses the Pitzer theory to model the effect of salt and a Born type equation to model the effect of methanol. The model parameters are derived from experimental studies at 0−3 m ionic strength, 4−25 °C, and 0−0.75 mole fraction methanol. The experimentally determined methanol activity coefficients, with respect to dissolved CO 2, bicarbonate, carbonate, and calcium, are determined as a function of temperature and ionic strength. Excellent agreements between the model predictions and the experimental results are presented. From model predictions, significant scaling problems can be anticipated with as low as 20% (by volume) of methanol.