A 1H NMR method for the estimation of hydrogen content for all petroleum products
© Mondal et al. 2015
Received: 24 February 2015
Accepted: 18 June 2015
Published: 3 July 2015
Hydrogen content is an important parameter for all petroleum products, because the performance of the products for specific application depends on the concentration of hydrogen in it. Further, hydrogen content can be used as a measure for quality control during the production process and assess the quality of the products, which is governed by the catalyst used. Therefore, to get the desired petroleum products like MS and HSD, pilot scale evaluation of different catalysts plays an important role in problem solving during troubleshooting in refineries. During evaluation studies the performance of catalyst depends upon the hydrogen consumption and mass balance in any catalytic process. In order to calculate total hydrogen consumption during production of different petroleum products an effort has been made to develop a universal method based on nuclear magnetic resonance (NMR) technique, that allows estimating hydrogen content in all petroleum fractions, ranging from IBP to 530+ °C.
The method uses hexamethyldisiloxane (HMDSO) for the first time as a quantitative reference standard respect to which the H content of unknown samples has been estimated. The newly developed method can also determine H/C and O/C ratio of ethanol blended fuel in a given sample without any additional experimentation.
Hydrogen content for twenty five model compounds was determined along with nearly hundred petroleum fractions. There has been found to be good correlation between the existing ASTM D5291 and developed NMR spectroscopic based methods. For low boiling petroleum fractions, where ASTM D5291 is not suitable, there is no direct way to correlate the data. However, as the hydrogen content estimated for some model compounds shows a high degree of correlation R 2 = 0.998, between theoretical values and estimated values, indirectly validate the developed method.
A universal NMR based method for the estimation of hydrogen content in all sort of petroleum products irrespective of their origin, composition, boiling range has been developed.
KeywordsNMR Crude oil Petroleum fractions Recycle delay Hydrogen content
Combustion of fossil fuel is what we think of as burning processes basically a reaction with oxygen. Fossil fuels are composed primarily of hydrocarbons. The amount of energy released during combustion is dependent on the oxidation state of the carbons in the hydrocarbon which is related to the hydrogen/carbon (H/C) ratio. The more hydrogen per carbon, the lower the oxidation state and the more energy that will be released during the oxidation reaction. The presence of oxygenates in fuels reduces the amount of carbon monoxide and unburned fuel in the exhaust gas. Oxygenate blending is basically the addition of oxygen-bearing compounds such as methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and ethanol to gasoline/diesel. Throughout the USA, oxygenate blending is mandated by EPA regulations to reduce smog and other airborne pollutants. For example, in Southern California, a fuel must contain 2 % oxygen by weight, resulting in a mixture of 5.6 % ethanol in gasoline (Removal of reformulated gasoline oxygen content requirement (national) and revision of commingling prohibition to address non-oxygenated reformulated gasoline (national), 2006). The resulting fuel is often known as reformulated gasoline (RFG) or oxygenated gasoline. Ethanol-blended fuel has been a great success in Brazil where ethanol is being used as a primary fuel for several decades; E-85 to E-100 are the most common in them. Several countries in the European Union also use ethanol-blended gasoline due to environmental consideration. In India, the government has mandated up to 5 % ethanol for metro cities, recently for all over the country, and has a plan to extend it up to 10 % in the future (Government to take a call on ethanol price soon, 2011). Thus, in the case of a blended fuel (oxy-diesel, reformulated gasoline), in addition to the H/C ratio, the O/C ratio would be a parameter of interest (Sarpal et al. 1997).
The pilot plants of refining technology division at the Research and Development Center of Indian Oil Corporation play an important role in troubleshooting problems in refineries. Pilot plant research is also essential in evaluating catalysts produced by IOC as well as from various vendors prior to the purchase. The quality of the research depends on accurate calculations of mass balance and hydrogen consumption during catalytic processes. Additionally, optimizing the feed severity for the existing process units, e.g., hydrocracker, FCC units is having great significance for the refineries. Identifying little changes in the catalyst performance can only be made possible with well-supported pilot plant studies. The quality of this support can be improved by enhancing the existing analytical methodologies. Even a small difference in the catalyst performance can substantially affect the bottom line of a refinery. The development of a universal nuclear magnetic resonance (NMR) technique which will allow us to calculate H/C (hydrogen content) ratio as well as O/C (oxygen content) ratio for all petroleum fractions, ranging from initial boiling point (IBP) to vacuum residue (VR), and in blended fuels is an effort in this direction.
NMR spectroscopic technique has long been used for both qualitative (Gillet et al. 1980; Kvalheim et al. 1985) and quantitative (Srivastava 1982; Allen 1985; Abu-Dagga and Ruegger 1988; Smirnov et al. 1992; Sarpal et al. 1998; Bansal et al. 1998; Bansal et al. 2007; Bansal et al. 2014; Mirotchnik et al. 2001; Young and Galya 1984; Poveda and Molina 2012) studies of petroleum products. The main focus of these studies revolve around understanding the qualitative composition and to quantify average structural parameters such as aromatic and aliphatic content, average chain length for aliphatic moieties, and average number of substituents on the aromatic ring. A relatively few number of studies (ASTM D-4808 1992; ASTM D-3701 1992; ASTM D-7171 2011; Gautier and Quignard 1995; Kennedy et al. 1998) have appeared on the determination of carbon and hydrogen contents in crude oil and petroleum fractions using NMR technique. The traditional combustion method ASTM D5291 (ASTM D-5291 1996) has its own limitations for low boiling fractions and affords 0.4 % repeatability and 0.8 % reproducibility for a medium hydrogen content of 10 %. The ASTM methods D4808 (ASTM D-4808 1992) and D3701 (ASTM D-3701 1992) make use of a low-resolution continuous-wave NMR spectrometer and also use dodecane (C12) as an external standard for quantitative determination of hydrogen. The ASTM method D7171 (ASTM D-7171 2011) covers the determination of the hydrogen content of middle distillate petroleum products using a low-resolution pulsed NMR spectrometer. These methods are restricted to a low-resolution NMR equipment that involves large quantities of samples (~20 g), standard, and large volumes of organic solvents such as tetrachloroethylene (~½ l).
So far, only one report has come out in which Khadim et al. (2003) used high-resolution NMR spectroscopy to determine the hydrogen and carbon contents in crude oil and petroleum fractions. In their study, for the first time, a new reference standard, bis(trimethylsilyl)methane (BTMSM), has been used for both proton and carbon-13 NMR to quantify the hydrogen and carbon in petroleum products. BTMSM has two sets of proton coming at δ 0.02 ppm (CH3) and δ −0.27 ppm (CH2) with respect to tetramethylsilane (TMS) making the measurement less predictable.
Herein, we report the development of a simple and reliable spectroscopic method for determining hydrogen, carbon, and oxygen contents in a wide range of petroleum products including gasoline, naphtha, kerosene, diesel, vacuum gas oil (VGO), and residua as well as in composite mixture, reformulated gasoline, and oxy-diesel. The study is based on using hexamethyldisiloxane (HMDSO), as an internal reference standard for quantitative calculation of hydrogen content and oxygenates in petroleum brands. The method uses high-resolution NMR spectroscopy and small amount of sample along with 12 to 15 wt% of the aforesaid internal standard. HMDSO is used in liquid bandages to protect damaged skin from irritation from other bodily fluids. It is being studied for making low-k dielectric materials for the semi-conductor industries by plasma-enhanced chemical vapor deposition (PECVD) (Barni et al. 2012). HMDSO has recently been used in quantitative tissue oximetry (Kodibagkar et al. 2006; Kodibagkar et al. 2008). To our knowledge, HMDSO is highly inert and has not previously been employed as an internal standard or frequency reference for quantitative purposes in conventional NMR spectroscopy.
Twenty five model compounds and about 100 oil fractions were analyzed by the 1H NMR method using HMDSO as the reference standard. In some cases, dioxane was also used as reference standard. Most of the samples were also analyzed by ASTM D5291 combustion method and the results were compared. Model compounds were purchased either from Sigma-Aldrich and Merck (New Delhi). Crude oil fractions, naphtha, gasoline, aviation turbine fuel (ATF), diesel, light cycle oil (LCO), clarified oil (CLO), light gas oil (LGO), vacuum gas oil (VGO), vacuum residue (VR), and several composite samples were collected from different pilot plants (PP) and refineries of Indian oil.
The hydrogen content of all high boiling range (diesel and above) samples were analyzed by a CHNS analyzer Vario EL-III from M/s Elementar, Germany, as per ASTM D5291 method. The samples are weighed in tin or aluminum tray/capsule. The tray/capsule after folding properly is injected into a high-temperature furnace and combusted in pure oxygen under static conditions.
All proton NMR spectra were recorded either on a Agilent DD2 NMR spectrometer operating at a proton frequency of 500 MHz, spectral width of 8012.8 Hz (−2.0 to 14.0 ppm), 90° pulse = 11.8 μs, relaxation delay = 20 s, and digital resolution of 0.49 Hz/point or on a JEOL ECA-500 spectrometer operating at a proton frequency of 500 MHz and the same specified parameters as above (90° pulse = 10.7 μs). Sixteen repetitions were averaged with 32K data points and 6.24- and 6.38-min experimental time, respectively, for DD2 and ECA machines. All the NMR spectra were integrated after baseline correction, and a mean of minimum three integration values has been taken for each calculation. It has been found that increasing the relaxation delay from 5 to 20 s does significantly influences the integral value and so most of the samples were recorded with 20-s relaxation delay.
Measurement of T1 by inversion recovery method
The spin-lattice relaxation times for proton, T1s (H), of HMDSO and T1s (H) for some model compounds and fractions have been measured following weighted linear inversion recovery method using 32 points. Best line fitting was ensured after phase and drift correction along with complete inversion during processing of T1 data in a JEOL ECA-500 spectrometer. The T1 (H) of HMDSO has thus been measured in CDCl3 at different concentrations ~0.14, ~0.19, ~0.24, and ~0.25 M giving rise to corresponding T1s 4.15, 3.70, 3.26, and 3.17 s, respectively, at an ambient temperature (~25 °C) (see Additional file 1).
Results and discussion
Selection of a reference compound
HMDSO is chemically inert, and it does not react with the usual NMR solvents or the solutes generally used in petroleum industry.
The boiling point of HMDSO (101 °C) is suitable for quantitative measurement and handling purpose.
HMDSO is highly hydrophobic and have strong nuclear magnetic resonance spin lattice relaxation rate, hence provides quantitative hydrogen in NMR (ASTM D-7171 2011). Molecular symmetry provides a single NMR signal.
The hydrogen and carbon contents of HMDSO are 11.17 and 44.38 %, respectively. An H content of 11.17 % lies almost at the focal point of the H content of hydrocarbons ranging from 6 to 16 % making it suitable for quantification through comparing.
Purity of HMDSO
Mutual purity check of HMDSO and dioxane by proton NMR (d1 = 20 s, scan = 32) in CDCl3
No. of entry
Dioxane (Fluka, 99.99 %)
HMDSO (Aldrich, >99.5 %)
C, 54.53; H, 9.15; O, 36.32 (wt%)
C, 44.38; H, 11.17; O, 9.85; Si, 34.59 (wt%)
Weight taken (mg)
Weight (mg) by NMRa
H, by NMRa (wt%)
Weight taken (mg)
Weight (mg) by NMRb
H, by NMRb (wt%)
Effect of recycle delay (d1) on hydrogen content
It has been noticed that the variation of the number of scan (16 to 64) does not have any significant effect on the integral values in the different region of a 1H NMR spectrum for a fairly concentrated solution and hence on the estimated H content thereof. A rather variation of the relaxation/recycle delay (d1) has dramatic effect on the outcome of H content. Quantitative representation of a NMR active nuclei to the corresponding integral value depends primarily on the spin-lattice (or longitudinal) relaxation time (T1) for that nuclei and the d1 given during data acquisition. Though the T1 (H) of HMDSO diverge a little in a mixture of various samples (model compounds, gasoline, diesel, VGO, naphtha, etc.) depending on the concentration of the sample and HMDSO in the solution, it has always been found varying from 3.0 to 4.4 s at various samples examined (see Additional file 1). So to get a representative H content using HMDSO as internal standard, the experiment was run at d1 = 20 s (minimum) and scan = 16.
Survey of model compounds
H content in model compounds by proton NMR
Name of the compound
%H, by NMRa
H at d1 = 5 s/10 s/40 s/60 s
Scan = 16
1,2,4-Trichlorobenzene, Spectrochem, 99.5 %
1,2-Dichlorobenzene, Spectrochem, 99.5 %
Furfuraldehyde, freshly distilled
2-Methylnaphthalene, Aldrich, 98 %
Toluene, Merck, 99.8 %
4-(Decyloxy)benzoic acid, Aldrich, 98 %
Tetralin, Merck, 98 %
Dioxane, Fluka, 99.99 %
Ethylbenzene, Aldrich, 99 %
Mesitylene, Aldrich, >99 %
Propylbenzene, Merck, >98 %
2-(2-Ethoxyethoxy)ethanol, 98 %
Octylphenol, Aldrich, 98 %
Tricyclo[126.96.36.199]decane, Aldrich, 98 %
2,4-Dimethyl-1,3-pentadiene, Aldrich, 98 %
2,5-Dimethyl-2,4-hexadiene, Aldrich, >96 %
2-Ethylhexanol, Aldrich, 99 %
Pentylcyclohexane, Koch-Light Lab Ltd., >99 %
2,4,4-Trimethyl-1-pentene, Aldrich, 99 %
Pentacosane, Aldrich, 99 %
Octadecane, Aldrich, 99 %
Dodecane, Aldrich, >99 %
Decane, Aldrich, >99 %
Nonane, Aldrich, >99 %
2,2,4-Trimethylpentane, SDH, >99 %
Crude oil fractions
H in petroleum fractions by 1H NMR (d1 = 20 s, scan = 16) and ASTM D5291 methods
H, by NMR (HMDSO)a
H, by combustion
H, by NMR (dioxane)b
Crude 1 C5-95d
Crude 1 95-149d
Crude 1 149-250d
Crude 1 250-369d
Crude 2 C5-95d
Crude 2 95-149d
Crude 2 149-250d
Crude 2 250-369d
Crude 3 C5-95d
Crude 3 95-149d
Crude 3 149-250d
Crude 3 250-369d
Crude 4 C5-95d
Crude 4 95-149d
Crude 4 149-250d
Crude 4 250-369d
Crude 5 C5-95d
Crude 5 95-149d
Crude 5 149-250d
Crude 5 250-369d
Crude 6 C5-95d
Crude 6 95-149d
Crude 6 149-250d
Crude 6 250-369d
Crude 7 C5-95d
Crude 7 95-149d
Crude 7 149-250d
Crude 7 250-369d
Crude 8 C5-95d
Crude 8 95-149d
Crude 8 149-250d
Crude 8 250-369d
Mangala crude 1d
HR OHCU mixed feedf
HR OHCU test feedf
MRU2 Pdt VGOe
PR OHCU test feedf
MRU2 feed VGOe
HR OHCU furfural extractf
MRU10 LCO feede
ATF biojet blend (UOP)e
ATF biojet blend (Jat)e
High diesel feede
Composite samples: new NMR method vs combustion method
H, by NMR
H, by combustion
Feed crude 9
Feed crude 9 (after 2 months)
Pdt at rxn 380 °C (from crude 9)
Pdt at rxn 400 °C (from crude 9)
Pdt at rxn 420 °C (from crude 9)
Feed crude 10
Pdt at rxn 380 °C (from crude 10)
Pdt at rxn 400 °C (from crude 10)
Feed crude 11
Pdt at rxn 400 °C (from crude 11)
Pdt cut IBP-70 °C (1)
Pdt cut IBP-70 °C (2)
Cut C5-140 (1)
Cut C5-140 (2)
Base case Pdt
Diesel case Pdt
Pdt cut 70–200 °C (1)
Pdt cut 140–330 °C
Pdt cut 70–200 °C (2)
Pdt cut 330–400 °C
Pdt cut 200+ °C
Pdt cut 400–480 °C
Pdt cut 480+ °C
VGO feed + 10 %LGO
Pdt at MRU
Mangala crude 2
Distribution of the H content calculated by NMR and ASTD D5291 combustion methods
Repeatability and reproducibility in H content for crude oil samples
Repeatability in 1H NMR method
No. of Exp
H content (wt%)
Crude 1 C5-95
Crude 2 C5-95
Crude 3 C5-95
Crude 4 C5-95
Crude 1 149-250
Crude 5 149-250
Crude 7 149-250
Crude 4 149-250
The reproducibility of the developed proton NMR method has been established and found to be satisfactory when some of the samples were recorded by two different operators following the same experimental conditions, sometimes in two different spectrometers as well.
Variation in H content with integration
The NMR method is sensitive to the values of integration taken for the different region of the spectrum. Though the mean of the three integration values was taken for each spectrum, to determine the relative standard deviation of different integral values, several (8) integrations have been taken after baseline and drift correction for few model compounds as well as for some fractions. RSD in integration values found for model compounds is 0.025 and for fractions is 0.139.
Hydrogen for composite samples
Hydrotreating crude oil is associated with catalytic hydro-desulfurization, hydro-deoxygenation, hydro-denitrigenation, hydrogenation of aromatics, saturation of olefins, etc. In conventional hydrocracking and hydrotreating processes, the hydrogenation of aromatic compounds play a crucial role. As heavy residual compounds are normally aromatic in nature, therefore, the complete or partial saturation of these compounds, by hydrogen addition, is an important step in their cracking into smaller, more valuable compounds. In order to achieve good hydrogenation efficiency, reactions need to take place at a favorable lower temperature range for which expensive noble metal catalysts are usually used. Products of hydrotreating expected to have more hydrogen than the feed. Moreover, products collected from micro reactor units (MRU) of PP operating at different temperatures should have different H contents. Conventionally, the products collected from MRU operating at a higher temperature gives product with a higher H content. Here, we present, in Table 4, a case of differential hydrogen content of the products at different temperatures by NMR method and the corresponding hydrogen content calculated by ASTM D5291 combustion method (entry 1-10).
The hydrogen content of several other composite cut samples have also been estimated by NMR method, and the results are compared with conventional ASTM D5291 method (entry 11-27). It can be concluded from Table 4 that some of these composite samples gave hydrogen contents which varied ±5 % while estimated by NMR and combustion methods. In few cases, the variation is large. Given the fact that the products are a mixture of different boiling range fractions, the variation is reasonable.
Estimation of ethanol and H/C and O/C ratio for ethanol-blended gasoline and oxy-diesel
For reformulated gasoline and oxy-diesel, the percentage of ethanol and thus the H/C and O/C ratio could easily be determined by the described method without any additional experimentation (see Additional file 1).
The experimental details are discussed in Additional file 1.
A simple and highly reliable spectroscopic method has been developed for the estimation of hydrogen content in all petroleum products irrespective of their origin, composition, boiling range, etc. The method is applicable to a wide range of petroleum products such as naphtha, kerosene, diesel, gas oils, vacuum gas oils, and the residua as well as for reformulated gasoline and oxy-diesel. HMDSO, as an internal reference standard, has been introduced for quantitative calculation of hydrogen content and oxygenates in petroleum brands. The internal standard alone serves as a frequency and quantitative reference for proton NMR. The 1H NMR-based method is also applicable to low boiling fractions and compounds containing very low hydrogen content. In most of the model compounds, the NMR method delivers hydrogen close to its theoretical values. In addition to hydrogen content, the developed method can directly be used to estimate the ethanol content/oxygen content in blended gasoline or oxy-diesel.
carbon hydrogen nitrogen sulfur
fluidized catalytic cracking
high speed diesel
heavy vacuum gas oil
initial boiling point
light vacuum gas oil
micro flow unit
micro reactor unit
relative standard deviation
We acknowledge the IOCL R&D Centre for granting permission to publish this paper. Also, we gratefully acknowledge Ms. Suman Mukherjee of Analytical Division for her support in recording the CHNS combustion data.
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