Estudo dos interferentes na determinação do número de acidez total (NAT) em petróleo

Abstract

The standard method for determination of the total acid number (TAN) crude oil is the ASTM D664, which uses potentiometric titration in non-aqueous solvents. In this paper, the interference of the salts NaCl, CaCl 2 , MgCl 2 , and SrCl 2 , in the TAN determination method, was evaluated and quantified. The interference of the salts in the titration solvent (test blank), and in two samples of crude oil, classified as light and extra-heavy oil, was evaluated. Salt was added in the titration solvent (test blank) of the ASTM D664 method individually and mixed. For the crude oil samples, it was evaluated the mixed effect of the salts. Salt was added to the titration solvent and crude oil samples in proportions simulating brine compositions of the pre-salt (76% NaCl, 14% CaCl MgCl 2 and 3% SrCl 2 ) and post-salt (70% % NaCl, 10% CaCl 2 , 7% 2 and 20% MgCl ) oil reservoirs. When added individually to the titration solvent, it was observed that the presence of NaCl in up to 8000 mg.kg 2-1 the presence of 50 mg.kg-1 does not affect TAN. For the remaining salts, of salt in the titration solvent was enough to overestimate the value of TAN. Among the evaluated salts, MgCl 2 and CaCl 2 are responsible for the largest interference in TAN. The increase in TAN due to the presence of SrCl is less pronounced, approximately 1.3 times. The interference of salts in TAN is ranked in the following order MgCl 2 > CaCl 2 >> SrCl 2 2 ; the presence of these salts can increase the TAN value by up to 9.59 mg KOH/g, 5.77 mg of KOH/g, and 0.22 mg KOH/g, respectively. Additionally, it was observed that the presence of salts in the titration solvent (test blank) in the pre-salt and post-salt proportions overestimates the TAN value by up to 1.68 mg KOH/g and 2.7 mg KOH/g, respectively. Similar behavior was observed in crude oil, the presence of the mixture of salts CaCl 2 , MgCl 2 , in both light and extra-heavy oil samples, overestimates the NAT value by up to 1.62 mg KOH/g, and 2.19 mg KOH/g for the pre-salt and post-salt proportions, respectively. , and SrCl 2 solvent, it was noted the formation of a precipitate when the salts CaCl 2 , MgCl 2 , and SrCl 2 were present, which suggests an interaction mechanism between the titrant and alkaline earth metals. To better understand the influence of the alkaline earth metals in the potentiometric titration, the precipitates formed during the were characterized using gravimetry, X-ray powder diffraction, thermogravimetry, Scanning Electron Microscopy with Energy Dispersive Spectroscopy, Fourier transform infrared spectroscopy, chromatography, and potentiometry. Using the techniques cited above, the precipitate was characterized; the compounds KCl, magnesium, and calcium hydroxide were identified. It was not possible to identify strontium hydroxide; it was identified that the precipitate, formed when SrCl 2 was present, was composed mainly of the SrCl titration started, which explains why Sr(OH) 2 2 deposited before the was not identified. The identification of KCl and hydroxides in the precipitates suggests that the reaction mechanism responsible for the increase in TAN is the reaction between alcoholic KOH and the salts CaCl 2 , MgCl hydroxides. 2 and SrCl 2 salts forming calcium, magnesium and strontium.