Salt scales on process equipment: Measurement of the induction time for calcium carbonate nucleation

Calcium carbonate and calcium sulphate scales can lead to an increase in running costs, can decline or stop the flow of fluids, and can cause early equipment fouling and failure in several industrial sectors, such as oil industries and desalted water production plants. A way to delay or hinder the formation of these salts is to use a proper additive, able to modify the nucleation kinetic increasing the induction times for salt nucleation. However, in order to properly understand the overall phenomenon it is worth defining the scenario of calcium sulphate and carbonate salts nucleation kinetics in the absence of any additive. The scope of the work is to measure, with a high resolution and in-line laser technique previously devised, the induction times for calcium carbonate nucleation on a lab scale plant. The experiments have been carried out at temperatures of 25 °C and 35 °C, with a supersaturation ratio S varying in the interval 2–210, and show that as expected the measured induction times for calcium carbonate nucleation strongly decrease while increasing supersaturation for each fixed temperature, outlining a wide metastable region. The obtained values have been used to evaluate the interfacial tension between salt crystals and mother solution (109.3 mJ/m2 for T = 25 °C and γsl = 95.7 mJ/m2 for T = 35 °C) that are in good agreement with other available literature values.