Low-field nuclear magnetic resonance for petroleum

Fuel Processing Technology 138 (2015) 202–209

Contents lists available at ScienceDirect

Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Low-field nuclear magnetic resonance for petroleum distillate characterization Lúcio L. Barbosa a,⁎, Flávio V.C. Kock a, Vinícius M.D.L. Almeida a, Sônia M.C. Menezes b, Eustáquio V.R. Castro a a b

Department of Chemistry, Federal University of Espírito Santo, Vitória, ES, Brazil Petrobras/Cenpes/QM, Ilha do Fundão, 21941–598 Rio de Janeiro, RJ, Brazil

a r t i c l e

i n f o

Article history: Received 16 January 2015 Received in revised form 25 May 2015 Accepted 27 May 2015 Available online xxxx Keywords: Low-field NMR Petroleum Petroleum fractions Distillates

a b s t r a c t Low field nuclear magnetic resonance (LF-NMR) has several applications in the oilfield industry such as in predicting the viscosity and evaluating porosity, permeability, fluid saturation of reservoir rocks, and the water content in fluids. However, the studies to determine the physical and chemical properties of petroleum distillates are uncommon. So, the aim of this study was to determine the physical and chemical properties of distillates using the transverse relaxation time (T2) in the range from 73.43 to 1810.74 ms. LF-NMR was employed in this research, due to its rapid and non-destructive analytical method. From LF-NMR data, it was possible to estimate the molar mass, correlation index, characterization factor, API gravity, relative hydrogen index, and number of hydrogen in distillates obtained up to 350 °C. T2 and the properties determined by standard methodologies (ASTM D-1218, D-445-06, D-664-06, D-2892, and D-4052) were strongly correlated. So, low field NMR constitutes an interesting alternative to ASTM methods. The results also show that changes in the chemical and physical properties depend on boiling point and molecular mobility. Besides, LF-NMR enabled the classification of the fractions into gasoline, kerosene, and light and heavy gas oil. © 2015 Published by Elsevier B.V.

1. Introduction Knowledge of the physical and chemical properties of petroleum fractions is indispensable for the adequate industrial production of petroleum derivatives. The standard methods frequently used to determine these properties are very laborious; involving the use of solvents and presenting high costs as well as destroying samples during the trials. A possible alternative to traditional methods is the low-field nuclear magnetic resonance (LF-NMR). This technique has numerous applications in the oilfield industry because of its potential uses both in situ and ex situ to estimate fluid and rock properties. Also is used to identify and classify crude oil that is extracted from different reservoirs and fields, as well as crude oil fractions [1]; to estimate viscosity [2–4]; to evaluate the petrophysical characteristics (porosity, permeability, fluid saturation) of reservoir rocks [5–9]; to determine the droplet size distribution of emulsions [9–12]; and to quantify the water content in biphasic mixtures [13]. A recently developed application of LF-NMR involves the use of two-dimensional pulse sequences to measure longitudinal relaxation time (T1) and transverse relaxation time (T2), T1–T2, and the diffusion coefficient and T2 (D-T2), in a unique measurement [14]. The above-mentioned sequences are applied in researches about the dynamics of reservoirs, the properties of fluids, and the size distribution chain of molecular groups [15,16]. NMR logs constitute an ⁎ Corresponding author. E-mail address: [email protected] (L.L. Barbosa).

http://dx.doi.org/10.1016/j.fuproc.2015.05.027 0378-3820/© 2015 Published by Elsevier B.V.

important technique, where hydrogen frequency lower than 2 MHz is applied to evaluate the reservoir profile [17]. Xiao et al. [18] from the analysis of 54 core samples, developed a new calibration technique for the construction of capillary pressure curves with the aim to predict the pore throat radius and reservoir permeability, based on T2 distribution measure. NMR phenomena are observed when the nucleus that contains the nuclear spin magnetic moment (μ) is subjected to a magnetic field (B0), promoting an interaction and aligning the spins in parallel or opposite to B0. The result of net alignment with direction and energetic preference is called resultant magnetization (M0) — a vector quantity characterized by magnitude and direction [19]. Such vector is expressed as the sum of all the magnetic moments of a sample (M0 = ∑iμi). To a nuclei with spherical charge distributions (e.g., 1H, 13C, and 31P; spin number equal to − 1/2), relaxation is caused by the fluctuating magnetic field at spin sites; the fluctuation is caused by the thermal motion of molecules [20,21]. Relaxation is defined as the process by which spins return to thermal equilibrium after radiofrequency irradiation. It occurs primarily through two ways: (1) spin–lattice relaxation, which involves energetic exchange between excited nuclear spins and the lattice, to reduce the total energy of the spin population and recover the equilibrium state and (2) spin–spin relaxation, in which spins precess in distinct orientations, i.e., the spins are oriented toward or deviate from the magnetic field, thereby exhibiting “flip-flop” motions. The first process has an enthalpic character, whereas the second has an entropic nature [22].

L.L. Barbosa et al. / Fuel Processing Technology 138 (2015) 202–209

After the nuclear spins align parallel to the external magnetic field (Bo), the magnetization vector in the equilibrium state lies along the B0 direction and the z-direction in the coordinate system. Petroleum or fractions are irradiated at the Larmor frequency (v0) to enable the petroleum/fractions to reach the resonance condition (v0 = γB0, where γ is the gyromagnetic ratio). The relaxation of nuclear spins is described in terms of the magnetization vector rotating at v0 in the coordinate system. Such consideration is represented by Mz ðt Þ ¼ M 0 1−e−ðt−t i Þ=T1 ;

ð1Þ

where Mz is the magnetization along the z-axis at time t, M0 is the magnetization in the equilibrium state, ti denotes the initial time at which a sample is subjected to B0, and T1 represents longitudinal relaxation (recovery of the Z-component of vector magnetization with constant time T1 of a new equilibrium state after system disturbance). Similar to the previously discussed, another relaxation process consists in the exponential magnetization decay in function of time, governed by constant T2. This variable corresponds to the disappearance of signals in the transverse plane, i.e., Mxy = 0: −t

Mxy ðt Þ ¼ M0 eT2 ;

ð2Þ

where T2 is the transverse relaxation time, and Mxy(t) is the transverse magnetization at time t. The phase coherence produced by polarization and transfer to the xy plane via the application of radiofrequency pulses is lost, because the nucleus precesses under the influence of the local magnetic field. Several physical and chemical properties are estimated on the basis of T2, which is an important NMR parameter typically obtained by Carr– Purcell–Meiboom–Gill (CPMG) pulse sequence. Because of molecular diffusion (one of the most important relaxation mechanisms in liquids), the transverse relaxation rate (T−1 2 ) is affected by the molecular weight (MW), molecular mobility, and size of molecules, as described by Coates et al. [17]. An intuitive phenomenon is that no molecular diffusion occurs in solid samples when these are analyzed in a solid-state NMR experiment; thus, dipolar decay and intermolecular motion contribute to T1 to be higher than T2 [23]. For small molecules in liquids, T1 and T2 values have the same order of magnitude (i.e., several seconds), but in solids or large molecules, T2 may be shorter than T1 because of the loss in synchrony due to nuclear precession [23]. Such relaxation times are intrinsic to the molecular mobility of a material. Depending on the material, only one relaxometric component (e.g., water bulk) or multiple relaxometric components (e.g., crude oil in saturated reservoir rocks) are produced [4]. Bryan et al. [3] correlated the relative hydrogen index (RHI) with dynamic viscosity (η) and geometric mean transverse relaxation time (T2gm). Such viscosity model was applied to estimate a viscosity in the range of 1,000,000–3,000,000 mPa·s− 1. Studies have demonstrated the relationship between RHI and the viscosity of crude oil [4]. This parameter is determined by Eq. (3) [3]: RHI ¼ AIðoilÞ =AIðwaterÞ ;

ð3Þ

where AI(Oil) is the amplitude index of the signal of oil, and AI(water) denotes the amplitude index of the signal of water. The amplitude index is the ratio between amplitude and mass. Kantzas [2] determined a correlation between the logarithmic mean of transverse relaxation time (T2LM) and the hydrogen index (HI). At low viscosity, T2LM strongly depends on viscosity, but the HI is insensitive to changes. The HI quantifies hydrogen in a given mass of crude oil vis-à-vis hydrogen content in the same mass of water. After production and primary treatment, crude oil is transported to a refinery, where it is converted into a consumable product. Crude oil refining involves three main steps: separation, conversion, and

203

accomplishment. The most common separation step is distillation, which is initiated in a column where compounds are separated on the basis of differences in boiling point [24–26]. Crude oil and distillates contain a substantial amount of chemical compounds; thus, the best approach to analyzing them is through their physical properties [25]. MW is an indication of molecular size. This important property is normally determined by chromatographic techniques (ASTM-D 5296) and cryoscopic methods (ASTM-D 2503) [24]. In this work, the MW of petroleum fractions was obtained by the Pedersen equation (Eq. (4)). This mathematical expression relates MW to density and mean boiling point [24,26]: MW ¼ 42:965 exp 2:097 10‐4 – 7:78712 : ρ þ 2:08476 10‐3 Tb: ρ Tb 1:26007 ρ4:9803 ;

ð4Þ where Tb is the mean boiling temperature in Kelvin, and ρ represents the specific gravity. The correlation index (CI) is another relevant parameter that can provide information about the chemical nature of fractions [25]. CI values are estimated thus: CI ¼ 473:3:ρ – 456:8 þ 48; 640=Tb ;

ð5Þ

where ρ denotes the specific gravity (g cm−3), and Tb (°C) is the mean boiling point of the petroleum fraction determined by a standard distillation method (ASTM D-86 or ASTM D-1160). According to Speight [25], a CI between 0 and 15 reflects the predominance of paraffinic hydrocarbons, values of 15 to 50 indicate the predominance of either naphthenes or a mixture of compounds (paraffins + naphthenes), and a CI higher than 50 indicates the predominance of aromatic species in a fraction. An equally relevant parameter for characterization is the universal oil product called the characterization factor (K), which is widely used in characterizing petroleum. It is expressed as K ¼ 3 √ Tb =ρ;

ð6Þ

where Tb is the mean boiling point in Rankin degrees (°F + 460), and ρ is the specific gravity at 60 °F. Numeric values of K can be used to determine the chemical nature of petroleum and distillates. For example, highly paraffinic oils have a K between 12.5 and 13.0, naphthenic oils present K ≈ 10.5–12.5, and aromatics exhibit a K value lower than 10.5 [25]. CI and K are critical to the oilfield industry because they can be used to acquire relevant qualitative information about the chemical properties of petroleum or fractions. Scientists and engineers can use the values as reference for deciding on the best production and refining strategies. On the basis of such knowledge, as well, a refinery can evaluate which refining processes (alkylation, isomerization, catalytic reforming, catalytic cracking, thermal cracking, hydrocracking, coking) excellently convert distillates into desired consumable products, such as kerosene, gasoline, diesel, and lubricants. As previously stated, several studies have used LF-NMR to analyze crude oil, but the use of such technique to investigate distillates is rare. Barbosa et al. [27] conducted a pioneering study that involved the determination of the physical properties of petroleum distillates by LF-NMR. The authors used a transverse relaxation time in the range of 25–675 ms to predict the viscosity, total acid number, refractive index, and API gravity of heavy oil distillates. The main advantages highlighted in the study are that the technique enables rapid analysis and presents the possibility of simultaneously determining the aforementioned properties. The aim of the present study was to estimate the physical and chemical properties of petroleum fractions, as well as the parameters that are important to the petroleum industry, on the basis of transverse relaxation time (T2). The parameters estimated included API gravity, correlation index, characterization factor, RHI, molar weight, molecular

204


Table 1 Physical properties and NMR parameters of the petroleum fractions obtained from crude oils 1 and 2.

Crude oil 2

Tb (°C)

T2 (ms)

API gravity

RHI

1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

110 140 170 210 240 270 290 320 350 140 170 190 210 240 270 290 320 350

1810.74 ± 46.76 1553.01 ± 77.56 1360.46 ± 56.94 1113.96 ± 46.62 780.20 ± 23.64 555.78 ± 20.49 394.01 ± 22.50 255.56 ± 15.80 146.24 ± 9.02 1577.04 ± 31–54 1391.85 ± 27.83 1252.04 ± 25.04 1088.76 ± 21.77 849.87 ± 16.99 833.83 ± 16.67 457.03 ± 9.24 332.67 ± 6.65 179.28 ± 3.38

61.6 54.4 48.7 41.9 35.8 31.6 29.5 27.7 25.3 56.7 50.7 45.4 41.5 38.4 34.8 31 30.7 26.6

1.33 1.30 1.26 1.23 1.18 1.15 1.14 1.13 1.11 1.30 1.26 1.22 1.20 1.18 1.17 1.13 1.13 1.10

Note: The signal intensity of the twenty first points obtained by FID from a given mass of fraction and water employing Eq. (3). Tb is the final boiling point for each range of temperature used to produce a fraction by distillation using the ATM D-2892 method [31].

formula, and boiling point. Given that characterizing and classifying petroleum fractions rapidly provide information and save on time and costs for oilfield analyses, we determined certain correlations by LF-NMR and ASTM methods.

Signal magnitude

Crude oil 1

Fraction

Fraction

1,0

1 2 3 4 5 6 7 8 9

0,8 0,6 0,4 0,2 0,0 -2

0

2

4

6

8

10

12

14

Time (s) Fig. 2. Decay curves of the CPMG of the nine fractions from crude oil 1 obtained by atmospheric distillation. Fractions obtained at high boiling points exhibited lower molecular mobility and transverse relaxation time (T2). The same behavior was observed for crude oil 2 (data not shown).

at 110–350 °C for crude oil 1. Nine fractions were also produced at 140–350 °C for crude oil 2. The fractions of crude oil 3 were used to validate some of the results. The distillates in the liquid state were analyzed by LF-NMR. The produced distillates were sealed in a polyethylene flask and stored below 4 °C to reduce natural degradation and loss of light components.

2. Experimental 2.2. NMR analysis 2.1. Samples To obtain the fractions, we used three types of dehydrated Brazilian crude oil with the following characteristics: crude oil 1 had a specific gravity of 0.9164 g cm−3, a total acidic number of 1.42 mg KOH/g, API of 22.3°, and a kinematic viscosity of 182 mm2·s−1 at 20 °C (crude oil 1). Crude oil 2 had a specific gravity equal to 0.9268 g cm−3, a total acidic number equal to 0.4848 mg KOH/g, API equal to 20.6°, and a kinematic viscosity equal to 391.6 mm2·s−1 at 20 °C. Crude oil 3 had a specific gravity of 0.9749 g cm−3, a total acidic number equal to 1.15 mg KOH/g, API equal to 13.1°, and a kinematic viscosity of 5800 mm2·s−1 at 40 °C. The distiller used in this work was manufactured by LabPetro (Research and Methodology Development Laboratory for Crude Oil Analysis). The atmospheric distillation process produced nine fractions

60

(a) Fractions of crude oil 1

60

55

55

50

50

API gravity

API gravity

65

The results were obtained using a Maran 2 Ultra NMR spectrometer (Oxford Instruments) equipped with a 30-cm bore, a 51-mm (diameter) probe, and a frequency of 2.2 MHz for 1H. The petroleum fractions were placed in a glass tube for NMR analysis at a stabilized temperature of 27.5 °C for approximately 10 min. Subsequently, a CPMG pulse sequence was applied to measure the transverse relaxation time (T2) of each sample, from monoexponential fitting of decay curves. The NMR measurement presented in the work had a standard deviation of less than 3.5%. The CPMG pulse sequence was applied employing π/2 and π pulses at durations of 8.3 and 16.3 μs, respectively. Recording was initiated from 8192 to 32,768 echoes for each transient, with an interval of 0.2 ms between two pulses. A recycle time of 15 s and 4 scans were

45 40 35

45 40 35 30

30

25

25 20 100

(b) Fractions of crude oil 2

150

200

250

300 o

Boiling point ( C)

350

20 100

150

200

250

300

350

400

o

Boiling point ( C)

Fig. 1. Dependence of API gravity on boiling point for the fractions produced from the distillation of crude oils 1 (a) and 2 (b). API gravity changed exponentially for the distillates obtained by atmospheric distillation (boiling point up to 350 °C).


2000 Fractions of Crude Oil 1 Fractions of Crude Oil 2

2000

(a)

1600

1600

1400

1400

1200

1200

1000 800

800 600

400

400

200

200

0 150

200

250

300

(b)

1000

600

100

Fractions of the crude Oil 1 Fractions of the crude Oil 2

1800

T2 (ms)

T2 (ms)

1800

205

0

350

0.72

0.75

0.78

o

Boiling point ( C)

0.81

0.84

0.87

0.90

(g.cm-3)

Fig. 3. Correlation of transverse relaxation time (T2) with boiling temperature (a) and specific gravity (b) for the fractions from crude oils 1 and 2.

employed. The experimental time varied from 2.05 to 3.74 min. The information provided by the application of CPMG sequences is T 2, which is correlated with the properties of reservoir fluids and distillates [1–4]. 2.3. ASTM standard Specific gravity (ρ) was determined at 20 °C using an Anton Paar DMA 4500 digital density meter, according to the ASTM-D4052 standard [28]. To measure the ρ value, 0.7 mL of the sample was placed in an oscillating sample tube by using a dry and clean syringe; the syringe was then placed in the digital density meter. Variations in oscillation frequency caused by changes in tube mass were used in conjunction with calibration data to determine the specific gravity of the sample. The API gravity of either the crude oil or the distillates was determined according to the ISO 12185 and ASTM D1250 methods, in which the specific gravity of the samples were measured at 50 °C and then converted into equivalent values at 20 °C [29,30]. The conversion of specific gravity (ρ) to API gravity at 15.56 °C (60 °F) is calculated as follows [24,25]: API ¼

141:5 −131:5: ρ

ð7Þ

In oilfields, API values are an important criteria for evaluating the quality and price of a barrel of oil. In Brazil, light crude oil has an API ≥ 31.1 and heavy crude oil has an API ≤ 12, respectively [31]. The first category of crude oil has more market value due to a higher amount of light compounds produced (for example, fuels). Distillation is one of the processes involved in the initial fractionation of crude oil. Such process indicates the potential of oil to produce light, medium, and heavy fractions. Knowledge of the boiling point distribution of the products is important for a refinery to ensure quality control of final products. The ASTM D-2892 standard [32] was used in this work to produce fractions with final boiling points below 400 °C. This method enables the estimation of yield on the basis of both mass and volume for various boiling point ranges. The results are plotted in a graph that shows the relationship between temperature and distilled mass percentage. This relationship is known as the distillation curve. Table 1 presents the mass and physical properties as the final boiling point (Tb), API gravity, RHI, and transverse relaxation time of distillates produced by atmospheric distillation from crude oils 1 and 2. The mass of the fractions used to determine the RHI changed from 22.54 to 31.40 g; API gravity changed from 25.3 to 61.6 (this variation corresponds to a specific gravity of 0.6598–0.8850 g cm−3). Specific gravity increased with boiling point because of the growth in size of the carbon chains.

450

2000

(a)

1800

Fractions of the crude oil 1 Fractions of the crude oil 2

Fractions of the crude oil 1 Fractions of the crude oil 2

400

(b) 2 R =0.96

1400

light

T2 (ms)

1200

heavy

1000 800 600 400

2 R =0.96

Boiling point (oC)

1600

350

Heavy gas oil

300

Light gas oil

250 200

200

150

0

100

Kerosene Gasoline

-200 100 120 140 160 180 200 220 240 260 280 300

MW (g mol-1)

100

150

200

250

300

350

400

-1

MW (g mol )

Fig. 4. Relationship between transverse relaxation time (T2) and molecular weight (a) and changes in boiling point with molecular weight for crude oils 1 and 2 (b). The vertical line at 220 g mol−1 on the left side of the figure indicates the boundary between light and heavy fractions. The horizontal lines on the right side represent the range of final boiling point given in Table 2 for different categories of distillates.

206


2000

Table 2 Range of T2, range of final boiling temperature, mean molecular weight, mean molecular formula, and classification of distilled fractions by LF-NMR to the crude oil 1. Distilled

Range of Tb (°C)

Mean Tb (°C)

MW (g mol−1)

Molecular formula

1810.74–1360.46 1360.46–780.20 780.20–394.01 b394.01

Gasoline Kerosene Light gas oil Heavy gas oil

110–170 170–240 240–290 N290

140 205 265 345

122.38 161.29 202.77 274.96

C8.77H17.71 C12.61H23.37 C15.2H26.23 C19.37H32.91

Note: The molecular mass was calculated by Eq. (4), whereas the molecular formula was determined using Eqs. (8) and (9).

The RHI values were calculated using Eq. (3) and the arithmetic average of the decay signal to free induction decay (FID) from the twenty first points for a given mass of fraction and water. Table 1 indicates that the T2 values of the fractions from crude oil 2 were higher than those of the distillates from crude oil 1. This difference suggests that the distillates from crude oil 2 have a higher amount of light components.

3. Results and discussion Fig. 1(a) and (b) shows the relationship between the API gravity and boiling point for the fractions obtained by atmospheric distillation. Some of the compounds obtained by atmospheric distillation are gas, naphtha, gasoline, diesel, and light and heavy gas oil [25,26]. Fig. 1(a) shows that the API gravity of the distillates decreased exponentially from 61.6 to 25.2, for the crude oil 1. Fig. 1(b) presents the variation in API from 56.7 to 26.6 with boiling temperature in the entire distillation range, for the crude oil 2. The increase in boiling point produced dense distillates (aromatics, resins, etc.); that is, a low API gravity produced such distillates. The exponential behavior observed in the curves of the API gravity for the two sets of fractions can be explained on based in the increase of density with boiling point. Finally, the distillates with a higher amount of carbon contain more restricted protons with lower T2. Gasoline, kerosene, and light gas oil are produced at boiling points (Tb) lower than 305 °C. At Tb values higher than 305 °C, the main fraction produced is heavy gas oil [26]. Light fractions are considered more interesting and more attractive by refineries because of the products that can be obtained from these fractions (such as, kerosene, gasoline, and diesel) [26]. The CPMG decay curves in Fig. 2 show that the amplitude of the signal decayed exponentially with time for the nine atmospheric distilled

R² =0.97

1.35 1.30

RHI

1.25 1.20

1600 1400 1200

T2(ms)

T2 range (ms)

Fraction of oil 1 Fraction of oil 3

1800

1000 800 600 400 200 0 1,10

1,15

1,20

1,25

1,30

1,35

RHI Fig. 6. Correlation between of transverse relaxation time (T2) and RHI for the petroleum fractions of crude oils 1 and 3 (validation fractions). Transverse relaxation time depended on the amount of hydrogen in the fractions. T2 values were obtained via CPMG sequence and RHI using FID sequence.

fractions obtained at a Tb lower than or equal to 350 °C. Fraction 1 showed the highest transverse relaxation time (1810.74 ms), whereas fraction 9 exhibited the lowest T2 (146.24 ms). This difference is attributed to the distinct physical properties of the fractions. For example, the fractions obtained at a higher boiling point presented a larger number of carbon and hydrogen molecules. Thus, their molecular chain length and density were also larger, thereby producing shorter transverse relaxation times. It is important to stress that only one transverse relaxation time constant was obtained from the fitting of each curve that represents the distillates. Fig. 3(a) confirms that transverse relaxation time linearly decreased from 1810.74 to 146.24 ms with increasing boiling point. As previously discussed, T2 is related to molecular mobility. Heavy fractions, such as heavy gas oil, have shorter relaxation times than do light gas oil. Fig. 3(b) shows that the increase in specific gravity (ρ) from 0.73278 to 0.90242 g cm− 3 promotes the decrease in T2 values. This result is expected because dense fractions contain protons (1H) that are bonded to chemical compounds of diverse chain lengths (e.g., naphthenes, asphaltene). These compounds exhibit highly restricted relaxometric proton components, thereby reducing the relaxation time. With the aim of use LF-NMR as a new method for determining MW, we calculated the value of this property using the Pedersen equation, described at Eq. (4) and correlated it with the T2 of the distillates from crude oils 1 and 2. Fig. 4(a) indicates that T2 presented a linear dependence on MW in the range from 120.58 to 262.55 g mol− 1 for all the fractions. The vertical line in the figure indicates the boundary between two categories of fractions: light and heavy fractions. Light fractions that find more interesting by refineries, exhibited an MW below 220 g mol−1 and a T2 higher than 420 ms.

Table 3 Main equations obtained from the correlation between NMR data and characterization parameters.

1.15 1.10 100

150

200

250

300

350

o

Boiling point ( C) Fig. 5. Variation between RHI and boiling point for the fractions obtained by atmospheric distillation from crude oil 1.

Equation

Graph

T2 (ms) = 2612.48–7.3428 Tb (°C) T2 (ms) = 2728.74–9.99 MW (g mol−1) RHI = 1431.11–993.27 Tb (°C) T2 (ms) = −8015.31 + 7391.30RHI Tb (°C) = −85.21 + 13.14 NH MW (g·mol−1) = −44.73 + 9.47 NH K = 4.79. RHI + 5.00 ρ (g.cm−3) = 1.72–0.74 RHI

Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.

3a 4(a) 5 6 7(a) 7(b) 9 10

L.L. Barbosa et al. / Fuel Processing Technology 138 (2015) 202–209 2

300

R =0.96

(a)

350

(b)

207

2

R =0.99

Molar weight (g mol-1)

o

Boiling point ( C)

270

300 250 200 150

240 210 180 150 120

100

90

14 16 18 20 22 24 26 28 30 32 34 36

14 16 18 20 22 24 26 28 30 32 34 36

Number of hydrogen (NH)

Number of hydrogen (NH)

Fig. 7. Correlation between hydrogen number with boiling point (a) and relationship of MW with hydrogen number (b) for the nine fractions of crude oil 1.

The fractions obtained at high boiling temperatures presented large carbon chains and MW, as reflected in the linear increase in MW up to 350 °C (Fig. 4(b)). The three horizontal lines in the same graph correspond to the final boiling point (see also Table 2) for gasoline, kerosene, and light and heavy gas oil [25,33]. Table 2 shows the boiling point, T2 range, and means MW of the investigated distillates. The results of LF-NMR enabled the identification and classification of the fractions into four categories. Besides, the molecular formula estimate from LF-NMR data also is shown. With the RHI (data in Fig. 5) and the mean MW calculated by Pedersen equation, then the number of carbon and hydrogen in the molecules was calculated. The obtained mean molecular formula is shown in Table 2. Heavy fractions have elevated boiling points and high C/H ratios [23, 24]. An intuitive assumption therefore is that light fractions have a high amount of hydrogen molecules. Fig. 5 shows that RHI decreased from 1.34 to 1.10 with increasing boiling point. Such variation corresponds to the reduction in hydrogen content from 14.88% (gasoline) to 12.22% (heavy gas oil). Given that RHI is correlated with boiling point (Fig. 5) and ρ (Fig. 10), the index can therefore reveal not only hydrogen content, but also the type of fraction (i.e., gasoline, kerosene, or gas oil) produced in a given range of boiling temperature. According to the literature [25,34], the RHI value of crude oil is lower than 1; that is, crude oil contains less hydrogen than water. By contrast, fractions have a higher hydrogen content than water and petroleum, thereby leading to RHI values higher than 1 (Fig. 5). As shown in Fig. 6, the T2 values of the fractions were directly associated with the RHI. Thus, on the basis of the transverse relaxation time measured for the fractions of crude oil 3 (validation fractions)

R =0.96

Nc ¼ ðMW – NH Þ=12:

ð9Þ

From the NH and NC calculations, the average molecular formula of the fractions was determined, as can be verified in Table 2. Fig. 7(a) and (b) shows that NH provokes linear change in boiling point and MW, respectively. Therefore, NH values between 16 and 34 enable the prediction of the MW and boiling point of distillates.

(b)

1500 R² = 0.98

1250 250

T2 (ms)

o

ð8Þ

1750

300

Boiling Point ( C)

NH ¼ MW ðRHI=100Þ 11:11%;

2000

2

(a)

350

and the application of the equation in graph 6 (see Table 3), it was possible to predict the RHI of the fractions produced from crude oil 3 (denoted by a square; Fig. 6). Finally, these results corroborate the findings shown in Figs. 3(a) and (5), indicating that the hydrogen content and boiling point of distillates can be deduced from only one unique CPMG experiment. The conversion of RHI to hydrogen percentages of the samples enabled the correlation with MW and the estimation of the number of hydrogen molecules in each fraction. From the MW calculated by the Pederson equation (Eq. (4)) and with consideration for the fact that each fraction is basically made of hydrocarbons (HxCy), the signal amplitude obtained by FID and expressed by RHI enabled the determination of hydrogen percentage using the formula (RHI / 100) × 11.11% multiplied by MW for a given hydrogen number. With the MW, hydrogen number (NH), and molar mass of carbon (12 g mol−1), the carbon number was also calculated. We propose the following equations to calculate the hydrogen and carbon numbers, NH and NC, as follows:

200

1000 750 500

150

250 0

100 10

15

20

25

30

35

40

Correlation index (CI)

45

50

55

10

15

20

25

30

35

40

45

Correlation index (CI)

Fig. 8. Correlation between boiling point and correlation index (a) and T2 with CI for the fractions of crude oil 1 (b).

50

55


4. Conclusion The results demonstrate that low field nuclear magnetic resonance (LF-NMR) constitutes an alternative to ASTM methods to petroleum distillates analysis produced by atmospheric distillation from different

2

11.6

R =0.97

11.4 11.2

K

11.0 10.8 10.6 10.4 10.2 10.0 1.05

1.10

1.15

1.20

1.25

1.30

1.35

RHI Fig. 9. Correlation between characterization factor (K) and relative hydrogen index (RHI) for crude oil 1.

0.92 R² = 0.99

0.90 0.88 0.86

0.82 0.80 0.78 0.76

Light gas oil

0.84 Heavy gas oil

-3

According the literature [26] MW and boiling point are correlated with the amount of hydrogen. CI values were also calculated for the distillates. The results showed that CI values linearly changed from 14 to 49 in function of Tb (Fig. 8(a)). The increase in index is expected given its dependence with Tb. The considerable relevance of this correlation is depicted in Fig. 8(b), from which the composition of distillates can be inferred as follows: paraffin (T2 N 1810 ms) and naphthenes + paraffins (73 b T2 b 1810 ms). It is possible to observe that CI depends on Tb, so this index linearly decreased with CI, Fig. 8(b). As indicated by Eq. (6), the characterization factor (K) value depends on both boiling point and specific gravity. The results of this study verify that K linearly changes with RHI. Fig. 9 illustrates that K values increased from 10.43 to 11.5 with RHI. For RHI ≤ 1.15 and K close to 10.4, aromatic fraction is predominant. Conversely, the increase in the RHI of light fractions indicates that paraffinic compounds are predominant. Moreover, it is possible to verify that K values are almost constant at RHI b 1.15. For such fractions, the ρ value is minimally changed (from 0.8644 to 0.8992 g cm−3), whereas the fractions with RHI higher than 1.15 presented ρ values that significantly changed from 0.7282 to 0.842 g·cm−3 (see also Fig. 10) in the temperature range from 110 to 240 °C, at which gasoline and kerosene are predominant. Therefore, the specific gravity explains why the K values remained practically constant for distillates with low RHI but linearly changed at a high RHI value. Fig. 10 confirms that the hydrogen content expressed by the RHI increased with decreasing of density. Gasoline presented RHI values higher than 1.26, whereas light gas oil, kerosene, and heavy gas oil exhibited an RHI between 1.1 and 1.25. Therefore, the RHI can also be used to determine density and classify fractions as heavy gas oil, light gas oil, kerosene, and gasoline (Fig. 10). The main equations for the correlations applied in this study were obtained by LF-NMR. The characterization parameters of the distillates are summarized in Table 3. Such correlations were developed using the fractions produced from Brazilian crude oil; thus, these correlations are not universal. The physical and chemical properties of oil considerably vary, depending on location (i.e., different countries) and reservoir [1]. Even so, we believe that the physical and chemical properties can be measured for many distillates produced in other countries.

(g cm )

208

0.74

Kerosene

0.72

Gasoline 1.10

1.15

1.20

1.25

1.30

1.35

RHI Fig. 10. Relation between density and relative hydrogen index for crude oil 1. The vertical bar indicates the type of fraction for each RHI range.

crude oils. Transverse relaxation time (T2) changing from 73.43 to 1810.74 ms were used to determine relative hydrogen index, number of hydrogen per molecule, molecular weight, molecular formula, correlation index (CI), characterization factor (K), and API gravity. Also was possible to infer about the chemical nature of petroleum distillates, i.e., paraffin and naphthenes + paraffins mixture. LF-NMR enabled the classification of distillates into four categories: gasoline, kerosene, light and heavy gas oil. The method saves time and costs, and preserves the sample. So, it has the potential of practical applications in the refinery and petrochemical. Acknowledgments The authors would like to thank the Brazilian agencies PETROBRAS S/A (Proc. 2012/00250-4) for their financial support and also LabPetro of the Federal University of Espírito Santo for their technical support. References [1] L.L. Barbosa, C.M.S. Sad, V.G. Morgan, M.F.P. Santos, E.V.R. Castro, Time domain 1H NMR and chemometrics for identification and classification of Brazilian petroleum, Energy Fuel 27 (2013) 673–679. [2] A. Kantzas, Advances in magnetic resonance relaxometry for heavy oil and bitumen characterization, J. Can. Pet. Technol. 48 (2009) 1–9. [3] J. Bryan, A. Kantzas, C. Bellehumeur, Oil viscosity predictions from low-field NMR measurements, SPE Reserv. Eval. Eng. 8 (2005) 44–52. [4] G.A. LaTorraca, K.J. Dum, K.R. Webber, R.M.N. Carlson, Low field NMR determinations of the properties of heavy oils and water-in-oils emulsions, Magn. Reson. Imaging 16 (1998) 659–662. [5] H. Westphal, I. Surholt, C. Kiels, H.F. Thern, T. Kruspe, NMR measurements in carbonate rocks: problems and an approach to a solution, Pure Appl. Geophys. 162 (2005) 549–570. [6] H. Guan, D. Broughan, K.S. Sorbie, K.J. Packer, Wettability effects in a sandstone reservoir and outcrop cores from NMR relaxation time distributions, Pet. Sci. Eng. 34 (2002) 35–54. [7] J.J. Howard, Quantitative estimates of porous media wettability from proton NMR measurements, Magn. Reson. Imaging 16 (1998) 529–533. [8] P. Fantazzini, Magnetic resonance for fluids in porous media at the University of Bologna, Magn. Reson. Imaging 23 (2005) 125–131. [9] M. Flaum, G.J. Hirasaki, C. Flaum, C. Straley, Measuring pore connectivity by pulsed field gradient diffusion editing with hydrocarbons gases, Magn. Reson. Imaging 23 (2005) 337–339. [10] A.A. Peña, G.J. Hirasaki, Enhanced characterization of oilfield emulsions via NMR diffusion and transverse relaxation experiments, Adv. Colloid Interf. Sci. 105 (2003) 103–150. [11] C.P. Aichele, M. Flaum, T.M. Jiang, G.J. Hirasaki, W.G. Chapman, Water in oil emulsion droplet size characterization using a pulsed field gradient with diffusion editing (PFG-DE) NMR technique, J. Colloid Interface Sci. 315 (2007) 607–619. [12] N.V.D.T. Opedal, G. Sørland, J. Sjöblom, Emulsion stability studied by nuclear magnetic resonance (NMR), Energy Fuel 24 (2010) 3628–3633. [13] R.C. Silva, G.F. Carneiro, L.L. Barbosa, V.L. Junior, J.C.C. Freitas, E.V.R. Castro, Studies on crude oil–water biphasic mixtures by low-field NMR, Magn. Reson. Chem. 50 (2012) 85–88.

L.L. Barbosa et al. / Fuel Processing Technology 138 (2015) 202–209 [14] Y.Q. Song, L. Venkataramanan, M.D. Hurlimann, M. Flaum, P. Frulla, C. Straley, Correlation spectra obtained using a fast two-dimensional Laplace inversion, J. Magn. Reson. 154 (2002) 261–268. [15] A.R. Mutina, M.D. Hürlimann, Correlation of transverse and rotational diffusion coefficient: a probe of chemical composition in hydrocarbon oils, J. Phys. Chem. 112 (2008) 3291–3301. [16] D.R. Freed, Dependence on chain length of NMR relaxation times in mixtures of alkanes, J. Chem. Phys. 126 (2007) 1–10. [17] G.R. Coates, L. Xiao, M.G. Prammer, NMR Logging Principles and Applications, 1st ed. Halliburton Energy Services, Houston, 1999. [18] L. Xiao, Z.Q. Mao, Z.N. Wang, Y. Jin, Application of NMR logs in tight gas reservoirs for formation evaluation: a case study of Sichuan basin in China, J. Pet. Sci. Eng. 81 (2012) 182–195. [19] J. Keeler, Understanding NMR Spectroscopy, 2nd ed. John Wiley & Sons, Cambridge, 2010. [20] M.H. Levitt, Spin Dynamics: Basics of Nuclear Magnetic Resonance, 2nd ed. John Wiley & Sons, Chichester, 2008. [21] V.M.S. Gil, C.F.G.C. Geraldes, Ressonância Magnética Nuclear: Fundamentos, Metodos e Aplicações, 2nd ed. Fundação Calouste Gulbenkian, Lisboa, 2002. [22] V.I. Bakhmutov, Practical NMR Relaxation for Chemists, John Wiley & Sons, Chichester, 2008. [23] B. Cowan, Nuclear Magnetic Resonance and Relaxation, 1st ed. Cambridge University Press, Cambridge, 2005. [24] M.R. Riazi, Characterization and Properties of Petroleum Fraction, 1st ed. American Society for Testing and Materials, West Conshohocken, 2005.

209

[25] G.J. Speight, Handbook of Petroleum Product Analysis1st ed., Wiley-Interscience, New Jersey, 2002. [26] A.M. Fahim, T.A. Al-Sahhaf, A.S. Elkilani, Introdução ao Refino de Petróleo, Elsevier, Rio de Janeiro, 2012. [27] L.L. Barbosa, F.V.C. Kock, R.C. Silva, J.C.C. Freitas, V.L. Junior, E.V.R. Castro, Application of low-field NMR for the determination of physical properties of petroleum fractions, Energy Fuel 27 (2013) 673–679. [28] American Society for Testing Materials (ASTM), ASTM D4052, Standard Test for Density and Relative Density of Liquids by Digital Meter, 2002. [29] International Organization for Standardization (ISO), ISO 12185, Crude Petroleum and Petroleum Products – Determination of Density – Oscillating U-tube Method, 1996. [30] American Society for Testing Materials (ASTM), ASTM D1250, Standard Guide for Use of the Petroleum Measurement Tables, 2013. [31] Agência Nacional do Petróleo, Gás Natural e Bicombustíveis, http://www.anp.gov.br (Accessed on 02/15/2014). [32] American Society for Testing Materials (ASTM), ASTM D2892-11, Standard Test Method for Distillation of Crude Petroleum, 2011. [33] V. Simanzhenkov, R. Idem, Crude Oil Chemistry, 1st ed. Marcel Dekker, New York, 2003. [34] V.G. Morgan, L.L. Barbosa, V. Lacerda Jr., E.V.R. de Castro, Evaluation of physicochemical properties of the post-salt crude oil for low-field NMR, Ind. Eng. Chem. Res. 53 (2014) 8881–8889.