A common problem that faces the oil and gas industry is the formation of iron sulphide scale in various stages of production. Recently an effective chemical formulation was proposed to remove all types of iron-sulfide scales (including pyrite), consisting of a chelating agent Di-ethylene-Tri-amine-Penta-Acetic acid (DTPA) at high pH using potassium carbonate (K2CO3). The aim of this Molecular Modelling study is to develop insight into the thermodynamics and kinetics of the chemical reactions during scale removal. A cluster approach was chosen to mimic the overall system. Standard density functional theory (B3LYP/6-31G∗) was used for all calculations. Low spin K4Fe(II)4(S2H)12 and K3Fe(II)(S2H)5 clusters were derived from the crystal structure of pyrite, and used as mimics for surface scale FeS2. In addition, K5DTPA was used as a starting material too. High spin K3Fe(II)DTPA, and K2S2 were considered as products. A series of Km(Fe(II)(S2H)n complexes (m= 3-1, n= 5-0) with various carboxylate and glycinate ligands was used to establish the most plausible reaction pathway. Some ligand exchange reactions were investigated on even simpler Fe(II) complexes in various spin states. It was found that the dissolution of iron sulphide scale with DTPA under basic conditions is thermodynamically favoured and not limited by ligand exchange kinetics as the activation barriers for these reactions are very low. Singlet-quintet spin crossover and aqueous solvation of the products almost equally contribute to the overall reaction energy. Furthermore seven-coordination to Fe(II) was observed both in high spin K3Fe(II)DTPA and K2Fe(II)(EDTA)(H2O) albeit in a slightly different manner.
ASJC Scopus subject areas
- Chemical Engineering(all)
- Industrial and Manufacturing Engineering