Isotherm, Thermodynamic and Kinetic Studies of Elemental Sulfur Removal from Mineral Insulating Oils Using Highly Selective Adsorbent

Abstract

Elemental sulfur (S8) is a corrosive sulfur compound which was found to be extremely reactive to silver, causing intensive silver sulfide (Ag2S) deposition on on-load tap changer (OLTC) contacts in power transformers. A highly selective adsorbent (HSA), called Tesla’Ssorb, for the removal of S8 from mineral insulating oils was prepared from raw material (RM) using the novel procedure. In this study, the adsorption properties of HSA for the removal of S8 from the oil were determined. RM and HSA were characterized using various techniques, such as field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD). The performance of HSA was determined by adsorption equilibrium, thermodynamic, and kinetic study through batch experiments, at various temperatures and initial concentrations of S8. The obtained results were analyzed by Langmuir and Freundlich adsorption isotherms and it was found that equilibrium data were fitted better with the Langmuir isotherm model. The maximum adsorption capacity was 4.84 mg of S8/g of HSA at 353 K. Thermodynamic parameters, such as enthalpy (ΔH°), Gibbs free energy (ΔG°), and entropy (ΔS°), were calculated and it was found that the sorption process was spontaneous (ΔG° < 0) and endothermic in nature (ΔH° > 0). It was found that the adsorption of S8 follows pseudo-second-order kinetic model, and the activation energy indicated the activated chemisorption process.

Kinetic, Isothermal, and Thermodynamic Analyses of Adsorption of Humic Acid on Quaternized Porous Cellulose Beads

“…As shown in Figure 7, the activation energy decreased with an increase in the content of quaternary ammonium groups, and their values were much higher than 4.2 kJ/mol. If an adsorbate molecule is held on the adsorbent surface mainly by van der Waals type of interaction, the activation energy is usually no more than 4.2 kJ/mol [50][51][52]. Therefore, the results shown in Figure 7 indicate that HA adsorption on the quaternized cellulose beads is controlled by chemical reaction.…”

“…where A is a term that includes factors like the frequency of collisions (g/mol•s) and R is the universal gas constant (8.314 J/K•mol) [50][51][52]. Since a good linear relationship held between 1/T and ln k 2 , the activation energy was calculated from the slope.…”