Density functional theory studies on the formation of CaCO₃ depositions on cristobalite, diamond, and titanium carbide surfaces

Fouling caused by inversely soluble salts, like CaCO₃, is a general problem on heat transfer surfaces. Carbonate depositions are typically cleanable with acids, but costs of energy losses, operation, and maintenance are significant. In this study, formation of CaCO₃ depositions was investigated on cristobalite, diamond, and titanium carbide surfaces. The aim of the study was to clarify the detailed mechanisms of crystallization fouling during the initiation on crystalline phases existing in materials used as coatings (SiO_x, TiCN, diamond-like carbon [DLC]), and to compare the results to the fouling mechanism of stainless steel (Cr₂O₃). In experimental studies of fouling, detailed mechanisms and description of sterical and electrostatic factors of surfaces are often very much simplified. In this work, molecular modeling was used to describe surface structures and to investigate the effect of process fluid (water) on the structures. The adsorption of water can be molecular or dissociative. During the dissociative adsorption, hydroxylated surface structures are formed. The existence of hydroxyl groups on the surfaces has an effect on the fouling mechanism. First, the dissociation probability of water on different surfaces was determined according to the adsorption mechanism and energies, and then the attachment of CaCO₃ onto optimized and hydroxylated surfaces was investigated. As a result, the formation mechanism with detailed intermediate steps of CaCO₃ deposition was obtained. The fouling takes place via hydrogen carbonate intermediates, but the final deposition structure was found to vary between surfaces.