Energetic Surface Properties in Crystallization Fouling on Modified Surfaces

A modern approach to mitigating crystallization (CaSO4) fouling effectively is to modify heat exchanger surfaces with regard to their surface free energy (SFE) properties. Applying DLC (diamond-like carbon) coatings to the surface provides the possibility to design the SFE parameters. Until now the mitigating effect is not fully understood, although the surface modifications proved their functionality in various fouling experiments. It is discussed in literature that a low total SFE and especially high polar interactions are key factors for mitigating fouling propensities.

From literature, it is found that altered SFE of DLC coatings can result from various cleaning treatments. Therefore, it was investigated how the SFE of diverse solid substrates reacts to consecutive cleaning acid, base and thermal treatments. The thermal and base treatments resulted in significant increases of SFE, especially for polar components. In most cases three repetitions of the respective treatment resulted in a constant property state, withstanding further treatment. Another focus was put on conditioning procedures to test the extent of altering SFE properties and reproduce the constant SFE state. Two appropriate conditioning procedures could be identified for two DLC modifications.

A new investigation procedure is introduced integrating conditioning for fouling determination. In it, surface characterizations were performed, such as analysis for roughness parameters, SFE properties and elemental composition. Hereby, several parameter changes were detected before and after the initial fouling experiment, the conditioning procedure and the consecutive fouling experiments. One type of coating provided excellent fouling mitigation properties after conditioning, which increased the total SFE and polar components significantly. . Further, it could be shown that after conditioning, consecutive fouling experiments altered the SFE parameters, especially the Lewis base polar and acid components and therewith the total SFE. However, fouling propensities could not be correlated to a definite set of SFE parameter, since the SFE altered to no recognizable pattern over the performed investigation procedure. A model needed to be addressed to qualify favorable surface parameters for fouling mitigation.

Several models were considered to study the contribution of molecular interactions to adhesion propensities. From classical nucleation theory, it could be determined that the critical nucleus size for stable nucleation forms at nano-scale. The plate-sphere model was considered to assess adhesion propensities resulting from molecular interactions between calcium sulfate particles and solid substrates immersed in water. The evaluation clearly identified Lewis acid-base polar interactions to be the determining factor to achieve repulsive particle-wall interactions. Lifshitz-van der Waals interactions led to attraction in all examined setups. The contribution to the total energy of interaction resulting from electrostatic double layer gained importance at mid-range. Thus, general rules for repulsion could be concluded. Moreover, the plate-sphere model provides the most promising possibility for fouling assessment when SFE properties are considered.