Inhibition of calcium carbonate precipitation in NaCl brines from 25 to 90°C

The nucleation induction period of CaCO3 in NaCl brines in the absence and presence of scale inhibitors was experimentally measured at temperatures from 25 to 90°C. A semi-empirical mathematical inhibitor model is presented for the CaCO3 scale control in industrial processes based upon nucleation theory and experimental observations. Results show that the minimum inhibitor dosage (Cinh) may be obtained from: Cinh=f(s)/binh log [tinh/t0], where tinh is the inhibition time, e.g., 20 min, t0 is the nucleation induction period in the absence of inhibitors, binh is the inhibitor efficiency, and f(s) is the safety factor, e.g., 2. Important factors for the kinetics of both nucleation and inhibition have been incorporated in this model including the calcite saturation index (SI), temperature (T), and the molar ratio of Ca to HCO3 alkalinity (R). In this paper, model parameters are presented for commonly used inhibitors, including 1-hydroxyethylidene-1,1-diphosphonic-acid (HEDP) and nitrilotri(methylene phosphonic) acid (NTMP). Results show that HEDP and NTMP are the best inhibitors for calcite scaling in the systems examined.