Interpretation of Adsorption Thermodynamics and Kinetics

A complete study of adsorption processes will be less complete if the structure and dynamics of its different elements and how they interact is not well captured. Therefore, the extensive study of adsorption thermodynamics in conjunction with adsorption kinetics is inevitable. Measurable thermodynamic properties such as temperature equilibrium constant and their non-measurable counterparts such as Gibbs free energy change, enthalpy, entropy etc. are very important design variables usually deployed for the evaluation and prediction of the mechanism of adsorption processes.

Investigation of adsorption behaviors of paraffin waxes on iron, iron oxide, and iron Ⅲ oxide pipeline's internal surfaces using adsorption locator model

Wax deposition in pipelines leads to pressure drop,reduced effective cross-sectional area,and blockages.Although mathematical models and experimental loops were used to model wax precipitation on pipeline surfaces,its prediction at molecular levels is not fully recognized.Molecular dynamics is another powerful approach that can predict wax precipitation at the molecular level.This paper uses molecular dynamics simulations with the adsorption locator model found in Material Studio Software to investigate the adsorption behaviors of Icosane-C20H42,Docosane-C22H46,and Tetracosane-C24H50 paraffin waxes on the Fe,FeO,and Fe2O3 pipeline internal surfaces.Modeling is performed by varying temperature values and validated with experimental data.It was found that as the temperature altered,the adsorption energies,probability energy distribution and adsorption density field on the surfaces also changed;on the other hand,the energetic analysis results showed adsorption energies increase with carbon numbers increase due to its larger surface contacting areas and lower aspect ratio,which resulted in stronger interaction with the surfaces.Further,paraffin waxes showed to adsorb easily on Fe surfaces than oxide surfaces.At temperatures below Wax Appearance Temperature(WAT)on both simulations and experiments showed wax deposition.The lower adsorption energy capacity observed on the Fe2O3 pipeline surface confirms it's vitality and suitability for crude oil transportation pipelines surface lining material.

A combined DFT and molecular dynamics study of U(Ⅵ)/calcite interaction in aqueous solution

Here we present a combined DFF and molecular dynamics study of uranyl （U（VI）） interaction mecha- nisms with the calcite （104） surface in aqueous solution. The roles of three anion ligands （CO2 , HCO3, OH ） and solvation effect in U（VI） interaction with calcite have been evaluated. According to our calculations, water adsorbed on the calcite （104） surface prefers to exist in molecular state rather than dis- sociative state. Energy analysis indicate that the positively charged uranyl species prefers to form surface complexes on the surface, while neutral uranyl species may bind with the surface via both surface complexing and ion exchange reactions of U（VI） → Ca（II）. In contrast, the negatively charged uranyl species prefer to interact with the surface via ion exchange reactions of U（VI）→ Ca（II）, and the one with UO2（CO3）2（H2O）^2- as the reactant becomes the most favorable one in energy. We also found that uranyl adsorption increases the hydrophilicability of the （104） surface to different extents, where the UO2（CO3）3Ca2 species contributes to the largest degree of energy changes （ 53 kcal/mol）. Our calcula- tions proved that the （104） surface also has the ability to immobilize U（VI） via either surface complexing or ion exchange mechanisms under different pH values.