Catalytic decarboxylation of naphthenic acids in crude oils

Abstract

Naphthenic acids are recognized as the main corrosive species in acidic crude oils, although they represent less than 3 wt%. Alternative methods have been developed in an attempt to remove naphthenic acids from the oil, however, their implementation in petrochemical industry still represents a challenge. Herein, a sub-product of the steel industry, steel slag, is evaluated as an economic alternative catalyst and environmentally feasible to remove naphthenic acids present in crude oils. A high-acidity crude oil (TAN = 4.79 mg KOH g−1 and S = 1.022 wt%) was submitted to thermo-catalytic process at 300 and 350 °C during 2, 4 and 6 h and its degradation products were monitored by negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron mass spectrometry (FT-ICR MS), total acid number (TAN) and total sulfur. The main crystalline phases detected by X-ray diffractometry in the catalyst were calcite (CaCO3), silica (SiO2) and magnesia (MgO). Among them, the MgO contributes effectively to promote the thermo-catalytic decarboxylation of naphthenic acid species, with a TAN reduction of 43.50% (4.79 → 1.89 mg KOH g−1) from the original oil to the degraded oil obtained after treatment at 350 °C for 4 h. Acid species with lower pKa values were selectively removed with the ESI(−)FT-ICR MS data confirming an increase in the DBE values from 1–5 to 5–18 for O2 class. Therefore, the catalyst selectively promoted the removal of naphthenic acids via (i) neutralization reaction; (ii) cracking reaction and (iii) MgCO3 formation from CO2 molecules produced by a previous thermal decarboxylation reaction.