Precipitation of Calcium Carbonate (CaCO 3 ) in Water− Monoethylene Glycol Solutions P. D. Natsi, S. G. Rokidi, and P. G. Koutsoukos* Department of Chemical Engineering, University of Patras and FORTH ICEHT, Patras, GR 26500, Greece *S Supporting Information ABSTRACT: The influence of monoethylene glycol (MEG) on the precipitation process of calcium carbonate at constant supersaturation, 25 °C, and pH 8.5 was investigated. MEG aqueous solutions supersaturated with respect to all calcium carbonate polymorphs were used since they are often encountered in oil production processes where they are used as a hydrate inhibitor. Both seeded and unseeded experiments were done in aqueous supersaturated calcium carbonate solutions containing MEG over the concentration range 0−80% v/v. The presence of MEG affected the induction times and the subsequent rates of precipitation in unseeded, spontaneous precipitation of calcium carbonate. At low values of the supersaturation ratio (SR calcite = 77.62−15 135.61 or σ calcite = 7.81−122) and for MEG concentrations in the range 30−80% v/v, the solutions were stable for 4 days. At lower than 30% v/v MEG concentrations, calcium carbonate precipitation took place past induction periods between 160 and 500 min. The morphological examination of the precipitates showed that in the presence of low MEG concentrations (10−20% v/v) the precursor vaterite was stabilized. The homogeneous nucleation and growth of calcium carbonate in the absence and in the presence of MEG was also investigated by the inoculation of stable supersaturated solutions with quartz and calcite seed crystals. In both cases nucleation and growth of calcium carbonate was significantly reduced with increasing MEG concentration in the solutions. ■ INTRODUCTION Calcium carbonate (CaCO 3 ) is a sparingly soluble salt, encountered in three distinct polymorphs, calcite, aragonite, and vaterite. Two more hydrated forms of calcium carbonate the hexahydrate (ikaite) and monohydrate are also known to form in addition to amorphous calcium carbonate (ACC). 1,2 The first mineral phase forming is the least stable, transforming to a more stable phase possibly through a dissolution−precipitation process. 3 The polymorphic phases and the hydrates of calcium carbonate, are encountered in biological mineralization, in geological and industrial processes and have been the subject of numerous investigations over the past decades. 4,5 A large number of studies have been concerned with the mechanism of calcium carbonate nucleation and growth in aqueous super- saturated solutions. 5−8 Of particular importance is scale formation in the production processes of fossil fuels and natural gas. In this case oil and gas is produced from subsurface reservoirs the walls of which have been permeated with water, gas, and oil or combinations of these fluids. It is believed that the rocks of oil and gas reservoirs were saturated with water before the intrusion of oil and gas. As a result, there is always a portion of water left, known as connate water. 9 The composition of this and other water used in the oil production process is enriched in hardness metal anions and carbonic species providing aqueous environment favorable for the formation of a number of sparingly soluble salts including carbonates. 10−12 The formation of these salts in drilling and production equipment increases operational costs as treatment processes are necessary to minimize losses. Another significant scaling problem however is the corresponding to the formation of gas hydrates because of the presence of hydrocarbons C1−C4, CO 2 , and H 2 S. 13 For this reason MEG is most often used in oil and gas production as an efficient thermodynamic inhibitor at various concentrations Received: August 29, 2018 Revised: November 21, 2018 Accepted: March 5, 2019 Published: March 5, 2019 Article pubs.acs.org/IECR Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.iecr.8b04180 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX Downloaded via IOWA STATE UNIV on March 26, 2019 at 12:58:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.