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

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

Simultaneous Determination of Boiling Range Distribution of Hydrocarbon, Sulfur, and Nitrogen in Petroleum Crude Oil by Gas Chromatography with Flame Ionization and Chemiluminescence Detections

We present a quick and efficient gas chromatographic method to simultaneously determine the boiling range distribution of hydrocarbon (C), sulfur (S), and nitrogen (N) in crude oils by a high temperature–CNS simulated distillation (HT-CNS SimDis) analyzer. The analyzer is a gas chromatograph equipped with flame ionization (FID) and sulfur and nitrogen chemiluminescence (SCD and NCD) detectors with simulated distillation features. The hydrocarbon yield profile of crude oil obtained by FID response was applied to calculate S and N content in various isolated fractions such as naphtha, kerosene, diesel, and vacuum gas oil. This method was used to analyze 10 different crude oils of variable composition. A fraction of crude oil that boils above the atmospheric equivalent temperature (AET) of 700 °C does not elute fully and forms a coke inside the chromatographic column. As a result, it is not possible to quantify total sulfur and total nitrogen content in the high-boiling vacuum residue (VR) fraction (565 °C and above) of crude oil by this method. However, we have addressed this issue by calculating sulfur in the VR fraction as a difference between total sulfur in crude oil (using X-ray fluorescence or combustion methods) and sulfur in the rest of the fractions (using HT-CNS SimDis). A similar technique was employed to determine nitrogen in the VR fraction of crude oil. The <i>gas oil reference standard</i> with known boiling range distribution was used to check the system suitability and generate the response factor for the calculation of hydrocarbon yield, and <i>VGO NS Reference</i> (internal nitrogen/sulfur QC standard) was used as a calibration standard for sulfur and nitrogen quantification. Currently, there is no single method available for the simultaneous determination of C, S, and N present in crude oil. This method produces detailed temperature distribution of S and N in a crude oil sample that cannot be obtained by either total sulfur and total nitrogen analysis or analysis of sulfur and nitrogen in discrete distillation cuts. As a result, this technique is extremely valuable to the refining industry for the valuation of crude oil, plant troubleshooting, and optimization of refinery processes

Property Prediction of Diesel Fuel Based on the Composition Analysis Data by two-Dimensional Gas Chromatography

The objective of the present study is to develop robust statistical models for the prediction of critical diesel properties such as cloud point, pour point, and cetane index with composition inputs such as <i>n</i>-Paraffins, Iso-paraffins, Naphthenes, and Aromatics (PINA) obtained by flow modulated two-dimensional gas chromatography with flame ionization detection (GC×GC-FID). A single gas chromatographic measurement coupled with models to predict the key physical properties is attractive for refiners to make quick decisions in optimizing diesel blending. We present a partial least-squares (PLS) linear regression statistical model that has been developed using 41 data sets of diesel samples with different compositions, out of which 33 samples were used for the calibration and eight samples for validation of the model. The <i>R</i>² values obtained for cloud point, pour point, and cetane index were 0.92, 0.93, and 0.92 with standard deviations of 1.20, 1.50, and 0.40, respectively. The average relative errors for predicted values of cloud point, pour point, and cetane index are found to be 0.86, 1.02, and 0.25, respectively. The PINA analyses of diesel and kerosene samples were carried out using flow modulated GC×GC with flame ionization detection (FID). The technique adapts reverse phase gas chromatography with two capillary chromatographic columns; the columns differ in length, diameter, stationary phase, and film thickness to get maximum peak resolution. The gravimetric blends of high purity reference standards of paraffins, naphthenes, and aromatic compounds (PINA) with variable carbon numbers were used for identification and to draw the boundaries for group types. Monoaromatic and polyaromatic content obtained for diesel and kerosene samples by the flow modulated GC×GC method were comparable to the results obtained by the High Performance Liquid Chromatographic (HPLC) method as per IP 391 or ASTM D 6591. Repeatability and reproducibility of the GC×GC analysis were performed for several samples to validate the method. It has been found that the HPLC method for the determination of aromatics content using a single calibration standard for each type, such as mono-, di-, and polyaromatics, causes a small error in the quantification in some of the samples as the refractive indices of all the aromatic species present in the diesel and kerosene samples vary depending on the addition of alkyl side chains; the presence of heteroatoms such as sulfur, nitrogen, and oxygen; etc