Article pubs.acs.org/EF
Application of Low-Field NMR for the Determination of Physical Properties of Petroleum Fractions Lúcio L. Barbosa,*,† Flávio V. C. Kock,† Renzo C. Silva,† Jair C. C. Freitas,‡ Valdemar Lacerda, Jr.,† and Eustáquio V. R. Castro† †
Department of Chemistry, Federal University of Espírito Santo, Vitoria, Brazil Department of Physics, Federal University of Espírito Santo, 29.075-910, Vitória, Brazil
‡
ABSTRACT: We propose the use of the low-field 1H NMR technique to predict various properties of petroleum fractions with °API ranging from 21.7 to 32.7. The experimental data obtained by standard methodologies (ASTM D-1218, D 445-06, D-66406, D-2892, and D-4052) were correlated with the mean values of the 1H transverse relaxation time in the range between 25 and 675 ms. Results of the present work showed good correlations between the NMR relaxation data with viscosity, total acid number, refractive index, and API gravity. The main advantage of the proposed method is its nondestructiveness, together with its speed and the fact that it does not require solvents/dilution. This allows the assessment of several properties of petroleum fractions simultaneously, based on the output of only one NMR experiment, leading to large economy in terms of energy, time, and costs.
1. INTRODUCTION Among the petroleum products used in the U.S., nearly half are gasoline and almost 40% are other distillate products with boiling points below 345 °C.1 Given this scenario, the analysis of petroleum composition together with the physical and chemical characterization of petroleum fractions is very important to improve refinery operations. With the recent discovery of new oil wells offshore, especially in the Campos and Espirito Santo basins, Brazil stands as the 13th highest producer of petroleum in the world.2 Thus, the characterization of petroleum and its fractions (mixtures of hydrocarbons with a limited boiling point range)3 constitutes a very important step for the operation, primary treatment, and refining processes in Brazil, as well as in other countries. Each type of petroleum has its own distillation curve, which defines its chemical identity. In addition, after the physical process of distillation, different hydrocarbon groups are produced within different temperature ranges, named fractions or cuts. In the Brazilian petroleum industry, the fraction boiling ranges are represented by different temperature intervals: residual gas (510 °C).4,5 Petroleum fractions consist of a mixture of many hydrocarbons whose compositions depend on distillation range. Therefore, a single value for boiling point is not absolute for the complete characterization of the fractions.6 Viscosity is one of the most important properties in the determination of the fluid’s behavior during the production process (flowing in the reservoir pores and through piping), and its assessment is essential for the technical and economic success of petroleum production. Low-field NMR is used in the petroleum industry to predict the viscosity and development models taking into account some parameters, such as the hydrogen index, temperature, and instrumental parameters, as reported in the literature.7−13 © 2012 American Chemical Society
It is relevant to estimate other physical properties and also compositions of fractions.3 The density also constitutes an important physical property of petroleum and its products, since crude oils are commercially classified based on the density value.3,14 In the petroleum industry, the American Petroleum Institute (API) gravity (commonly known as API gravity and expressed in “degrees”) is used to classify crude oils and its products according to their density relative to pure water. In this way, petroleum can be classified as light (°API ≥ 32), medium (26 ≤ °API < 32), heavy (10.5 ≤ °API < 26), or extra heavy (°API ≤ 10.5).3,14 Refractive index (n) is a physical property of interest to characterize petroleum fractions, especially in what concerns the molecular composition. It is important to stress that n values vary from about 1.3 for propane to 1.6 for some aromatic compounds.3,14 Petroleum fractions may contain acidic constituents (phenol, carboxylic acid, alcohol, and others). Generally, the amount of these constituents is determined by KOH titration. The total acid number (TAN) is very important to evaluate the acid and corrosive power of petroleum fractions. In Brazil, the crude oils are considerably more acid in comparison to oils from other parts of the world.4 The TAN average for Brazilian oils is 0.64 mg KOH/g; however, some oils from Espirito Santo Basin have an acidity equal to 2.93 mg KOH/g.4 TAN values above 1 are undesirable for refining due to corrosion issues, which requires the refineries to increase investments to reformulate the production plant.4,15 In the past decade, there have been many successful applications of nuclear magnetic resonance in the petroleum sector, such as the determination of total hydrogen content,10,16 reservoir simulations, laboratory plant development, and estimative reserve;10,17 estimates of viscosity;10,16,17 determiReceived: September 27, 2012 Revised: December 12, 2012 Published: December 19, 2012 673
dx.doi.org/10.1021/ef301588r | Energy Fuels 2013, 27, 673−679
Energy & Fuels
Article
below 4 °C to reduce the natural degradation process and the loss of light components. It is important to stress that the fractions presented colors from light yellow (fraction 1) to dark brown (fraction 8). The color observed for each fraction is associated with the chemical composition, reflecting the predominance of hydrocarbon compounds, such as naphthenic acids and aromatics.3,14 2.2. NMR Analysis. The low-field 1H NMR experiments were performed using a Maran 2 Ultra NMR spectrometer manufactured by Oxford Instruments, operating at 2.2 MHz for 1H with a 51 mm diameter probe. The transverse relaxation time (T2) was measured using the Carr−Purcell−Meiboom−Gill (CPMG) pulse sequence.25−27 Before each CPMG experiment, the samples were stabilized at 27.5 °C, with the sample temperature being monitored using an optical thermometer (FOT Lab Kit Fluoroptic Thermometer, manufactured by LUXTRON). The samples were placed into a glass tube for the NMR analysis. The CPMG pulse sequence was applied employing π/2 and π pulses with durations of 8.15 and 16 μs, respectively, and 8192 echoes were recorded for each transient, with one point per echo and an echo spacing (τ) of 0.2 ms. In each CPMG experiment, 32 transients were recorded, with a recycle time of 3 s. The echo decay signal from each measurement was inverted using an inverse Laplace transform (ILT) to obtain the distribution of T2 relaxation times. The ILT was performed using the WINDXP 7.0 software package. The log-mean transverse relaxation times (T2LM) were computed from the T2 distributions. All correlations investigated in this work used this single parameter, T2LM, as representative of the whole T2 distribution curves. It is important to know the values of longitudinal relaxation times (T1) in order to determine the minimum values of recycle delays used in the CPMG experiments and thus to avoid saturation problems. For this purpose, inversion−recovery experiments were performed in selected samples, allowing the recycle delay to be set at 3 s. Taking into account that experiments to determine T2 are significantly faster than those used for measurements of T1 and considering that many well-known physical properties of oils (such as viscosity) are correlated in a similar way to either T1 or T2,31,32 in this work, only T2 distributions were used to investigate correlations with the physical properties of the petroleum fractions. 2.3. Specific Gravity. The specific gravity (ρ) was determined at 20 °C by using an Anton Paar DMA 4500 digital density meter, according to the ASTM-D4052 standard.33 Approximately 0.7 mL of the petroleum fraction was introduced using a dry and clean syringe into an oscillating sample tube, and the change in oscillation frequency caused by the change in the mass of the tube was used in conjunction with calibration data to determine the specific gravity of the sample. 2.4. Total Acid Number (TAN). All the petroleum fraction samples were dissolved in a toluene and propan-2-ol alcohol mixture containing a small amount of water. After that, they were titrated potentiometrically with alcoholic potassium hydroxide using a glass indicating electrode and a reference electrode or a combination electrode (ASTM D664-06).34 2.5. Refractive Index. The refractive index (n) of samples was measured using a refractometer in accordance with the standard methodology (ASTM D-1218).35 2.6. Kinematic Viscosity. The test method for determination of the kinematic viscosity, ν, of liquid petroleum products consists of measuring the time a volume of liquid takes to flow under gravity through a calibrated glass capillary viscometer. The viscosity was measured following the ASTM D-445-06 standard.36 2.7. Distillation and Boiling Point. The ASTM D-2892 test method is a distillation procedure used for production of liquefied substances, such as naphthas and fractions with initial boiling points above 400 °C. Using this method, it is possible to obtain fractions and estimate yields based on both mass and volume for various boiling point ranges. The results are presented as a graph of temperature versus distilled mass percentage, known as the distillation curve.37
nation of petrophysical properties of rock cores;18 studies of water/oil emulsions;19,20 and characterization of fluids.21,22 The instruments used for low-field nuclear magnetic resonance (LF-NMR) experiments are commonly composed of electromagnets or permanent magnets, producing nonhomogeneous static magnetic fields with strengths in the range between 0.05 and 2.2 T. Characterization techniques, such as the potentiometric titration and viscosimetry, are complicated, laborious, and involve the destruction of samples and the application of poisonous solvents. On other hand, LF-NMR presents several advantages, such as its nondestructiveness, low cost for analysis (about U$30), and short analysis time around 30 s for petroleum and 1 min for fractions. Until the present and to the best of our knowledge, this method has not been applied yet as an alternative technique for fast, nondestructive, and simultaneous analyses of physicalchemical properties of petroleum fractions. Sophisticated techniques, such as gas chromatography (GC), infrared (IR),23−25 and mass spectrometry,26 have been applied in the study of petroleum fractions. High-resolution NMR spectroscopy was also used to determine structural parameters and assess functional groups of aromatics and naphthenic and paraffinic compounds from petroleum fractions.1 Previous studies3,6 showed many correlations allowing the characterization of properties of petroleum fractions, for example, vapor pressure, surface tension, aniline point, and aromatics content. One model has been used by Albahri6 for predicting the properties of light petroleum fractions. The method applied a molecularly explicit characterization model to define the molecular composition of petroleum. Qian et al.26 used ion electrospray ionization mass spectrometry (ESI-MS) for the determination of TAN and molecular weight distribution of petroleum products. According to the authors, the ions are subjected to a series of collisions with gas molecules, thus causing adverse effects on the ionization of low-molecular-weight (MW) species and fragmentation of small acids. In this work, we present the simultaneous characterization of several properties of petroleum fractions (refractive index, viscosity, density, and TAN) based on the outcome of just one NMR experiment. As stated above, LF-NMR presents the advantages of being fast and giving results that are easy to interpret,27−30 besides being nondestructive (it does not cause fragmentation or molecular dissociation, as previously described for other techniques) and presenting low cost. Hence, it is worth making an effort to correlate the LF-NMR results with important physical and chemical properties of petroleum fractions.
2. EXPERIMENTAL SECTION 2.1. Samples. The Brazilian crude oil used (petroleum 1) to obtain the fraction presented the following characteristics: TAN = 1.15 mg KOH/g, density of 0.9749 g cm−3 (13.1 °API), and kinematic viscosity of 5800 mm2 s−1 at 40 °C. A set of eight samples of petroleum fractions with masses ranging from 13 to 15 g were obtained by atmospheric distillation using a homemade manual distillation system. The residue of the distillation process presented a density of 0.9968 g cm−3 (9.9 °API). Another crude oil (petroleum 2) with TAN = 1.42 mg KOH/g, a density of 0.9164 g cm−3 (22.3 °API), and a kinematic viscosity of 58.115 mm2 s−1 at 40 °C also was distillated. The samples of Brazilian crudes and fractions (produced in a range from 240 to 390 °C) were sealed and stored in a polyethylene flask 674
dx.doi.org/10.1021/ef301588r | Energy Fuels 2013, 27, 673−679
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Article
Table 1. Physical Properties and NMR Data of Petroleum Fractions cut
BP (°C)
1 2 3 4 5 6 7 8
240 279 308 332 339 360 373 390
v (mm2 s−1)
n 1.4707 1.4811 1.4901 1.4992 1.5042 1.5085 1.5119 1.5149
± ± ± ± ± ± ± ±
0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0002
2.4458 3.9074 6.2517 12.0388 17.7122 26.1688 41.1277 64.7977
± ± ± ± ± ± ± ±
ρ (g cm−3)
0.0010 0.0015 0.0037 0.0003 0.0061 0.0043 0.026 0.153
0.8548 0.8707 0.8842 0.8956 0.9033 0.9088 0.9131 0.9167
3. RESULTS AND DISCUSSION Table 1 shows the main physical and chemical properties obtained by ASTM methods for the set of eight petroleum fractions (crude oil 1) investigated. The initial boiling point for the crude oil is 199 °C. It is possible to observe that the refractive index, kinematic viscosity, TAN, and the density increase with the boiling point. This is explained on the basis of the molecular properties of the fractions, considering especially the growth in size of the carbon chains with increasing boiling points.3 As will be demonstrated, results show that the transverse relaxation associated with each petroleum fraction can be used to provide an indication of the properties of the fractions. Figure 1 shows the linear variation of density with the increase in boiling point (BP) of the petroleum fractions in the
± ± ± ± ± ±
0.0001 0.0001 0.0001 0.0001 0.0001 0.0001
± 0.0001
TAN (mg KOH/g) 0.0654 0.1806 0.4716 0.8566 1.1456 1.3384 1.3384 1.4945
± ± ± ± ± ± ± ±
0.03021 0.03021 0.03021 0.03021 0.03021 0.03021 0.03021 0.03021
T2 (ms) 675.75 390.85 229.01 110.82 73.43 48.55 30.74 25.53
± ± ± ± ± ± ± ±
13.15 19.54 11.45 5.54 3.67 2.42 1.53 1.27
It is possible to observe in Figure 3 that crude oil 1 has a transversal relaxation time of 3.8 ms. This value is considerably lower than T2LM of crude oil 2 because its viscosity is approximately 100 higher. Comparing the relaxation time of crude oil 1 (solid curve) of Figure 3 with the T2 of fractions (Table 1), it is possible conclude that crude oil has a higher density and viscosity than the fractions; thus, the T2 is smaller. Figure 4a shows the CPMG decay curves for some of the analyzed petroleum fractions obtained from crude oil 1. It can be observed that these curves have different decay time constants that can be attributed to distinct physical characteristics (e.g., viscosity). The transverse relaxation time values for each fraction reflect the differences in oil properties, such as the increase in chain length (carbon number), the saturation degree, and the presence of heteroatoms in the oil.38 It can also be observed that fraction 1 has the longest decay (T2LM = 675 ms) and the lowest viscosity (v = 2.44 mm2 s−1). Sample 3 presents intermediate values, and petroleum fraction 8 has the shortest T2 (T2LM = 25 ms) and the highest kinematic viscosity (v = 64.80 mm2 s−1). It is verified in Figure 4b that, as expected, pure water relaxes more slowly than all the oil fractions, with a single exponential behavior and a value of T2 = 2.74 s. Figure 5 presents the T2 distribution curves corresponding to the CPMG decay curves of Figure 4a. It can be verified that the distribution curves exhibit a displacement of the peaks toward lower T2 values with the increase in viscosity. These changes in T2 are provoked by reduction of molecular mobility of each fraction. The appearance of one a minor peak in the distribution curve of sample 3 is attributed to an artifact of the ILT process. 3.1. Prediction of API Gravity. Density is a function of temperature and pressure and is one of the most important physical properties of fluid and petroleum derivates.14 An exponential-like relation of the T2LM values from 25 to 675 ms with the °API of the studied petroleum fractions can be seen in Figure 6a. The results of Figure 6a were linearized, leading to a linear correlation between T2LM and °API in the range from 22 to 33 °API (Figure 6b). Therefore, T2 measurements by LFNMR can be used as a “probe” to monitor density changes of petroleum fractions and to determine indirectly the °API, thus allowing the classification of the fractions in the usual ranges as heavy, medium, or light oils.14 From Figure 6b, it is possible to obtain the following mathematical expression:
Figure 1. Relation between density and boiling point for the petroleum fractions obtained from crude oil 1. The dotted line represents the boundary between the temperature regions for two categories of petroleum fractions.
range from 240 to 390 °C. This increase in density is explained by the increase in carbon content and average size of the carbon chains. From this result, the petroleum fractions were classified in two categories: light gas oil (
