Determination of Naphthenic Acid Number in Petroleum Crude Oils and Their Fractions by Mid-Fourier Transform Infrared Spectroscopy

Abstract

Naphthenic acid is a generic name used for all the organic acids present in crude oils. The quantitative determination of naphthenic acid number (NAN) is an essential parameter for petroleum refineries to evaluate corrosive properties of crude oils prior to their processing. Currently, most of the refineries are using total acid number (TAN) as a measure of corrosivity of crudes during their selection, valuation, and processing. Some of the organic molecules are being used as corrosion inhibitors to reduce corrosion in refinery process units, and the dosage of the same depends on the total acid number as it has been understood from the studies that acid inhibitors form a protective layer on the surface of the pipes and thus reduces the corrosion due to acids present in crude oil. TAN measurement by titration overestimates the acid number as each and every molecule like thiols and phenols etc. that are titratable by alkali are also included in the calculation and that causes the improper estimation of the addition of corrosion inhibitors. To get a better refinery margin in the present economic scenario, optimization of the addition of corrosion inhibitors is very much essential and thus accurate measurement of NAN is a primary concern. Hence, we present a quick and efficient mid-Fourier transform infrared (FTIR) spectroscopic method for the determination of NAN using a variable path length liquid cell with calcium fluoride windows. Two distinct photon absorption bands in the region of 1680 to 1800 cm–1 were observed during the spectral measurement and are due to the formation of monomeric and dimeric forms of carbonyl (CO) group of carboxylic acids, and hence both are considered for the quantification. The method is applicable even to highly volatile crude oils that are not measurable by the normal attenuated total reflectance (ATR)-FTIR technique. This article also presents the effect of solvents, hydrogen bonding, formation of monomer and dimer, etc. Currently, this method is being applied for the determination of NAN for crude oils and straight run vacuum gas oil (VGO) samples as they contain either negligible or no carbonyl compounds other than carboxylic acids that interfere in the region of interest.

Metal Ion Interactions with Crude Oil Components: Specificity of Ca2+ Binding to Naphthenic Acid at an Oil/Water Interface

“…Using the average NA molecular mass of 243 calculated above, the product has a composition consistent with the expected 1:2 Ca:NA stoichiometry. Figure 1 shows a comparison of the infrared spectra (Bruker Alpha spectrometer with Diamond ATR (Bruker UK Ltd., Coventry, UK), each sample applied in neat form) for the original naphthenic acid and its calcium salt, from which the shift of the carbonyl band from 1700 to 1543 cm −1 is characteristic of neutralized carboxylic acids [39,40]. The small band at 1709 cm −1 in the calcium naphthenate spectrum indicates a trace amount of unreacted naphthenic acid remains in the product.…”

Adsorption of Polar Species at Crude Oil–Water Interfaces: the Chemoelastic Behavior

“…We interpret this as a result of the heterogeneous chemical environment surrounding the C=O bond, whose absorption peak shifts toward smaller wavenumbers when the carbonyl group is either in conjugation with a double bond or an aromatic group or H-bonded with another molecule. 50 Therefore, we classify the contribution of the carbonyl group as bound, or conjugated, for peak positions below 1725 cm –1 , and free, or nonconjugated ones, for positions above it, 50 shown as diagonally and horizontally striped bar plots in Figure 3 c, respectively. The results highlight that free carbonyl groups are particularly more abundant in films formed against solutions rich in sulfate salts, suggesting that in these films, a larger fraction of molecules have the carbonyl group connected to a bulkier tail which could prevent H bonding due to steric hindrance.…”

Oilfield metal naphthenate formation and mitigation measures: a review

“…Crude oil acid content expressed as TAN (mgKOH/g) can be quantified by the traditional titrimetric method or by FTIR spectroscopy (Chakravarthy et al 2016). A UV-based method for quantification of tetraacids derivatized by chemically engrafting chromophores such as naphthacyl and 2-bromo acetonaphthone is also available (Passade-Boupat et al 2012;Simon et al 2008).…”