Density and Refractive Index of Petroleum, Cuts, and Mixtures

Aug 21, 2015 - (b) midrange scale. Figure 3. Measured and ...... Atmospheric Pressure for Distillation Cuts from an Algerian. Crude Oil13. TBP, °C d2...
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Density and Refractive Index of Petroleum, Cuts, and Mixtures Harvey William Yarranton, Jane C. Okafor, Diana P. Ortiz, and Frans G.A. van den Berg Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 21 Aug 2015 Downloaded from http://pubs.acs.org on August 21, 2015

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Density and Refractive Index of Petroleum, Cuts, and Mixtures H.W. Yarranton1, J.C. Okafor1, D.P. Ortiz1, F.G.A. van den Berg2 1. Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada, T2N 1N4. 2. Shell Technology Centre Amsterdam, Shell Global Solutions International BV, PO Box 38000, 1030 BN Amsterdam, The Netherlands

ABSTRACT Densities and refractive indices were measured for native crude oils, distillation cuts, and thermo- and hydrocracked oils at 20°C and for some samples up to 60°C, all at atmospheric pressure. These data as well as pure component and crude oil data from the literature were used to develop correlations between: the refractive index function (FRI) and density; the thermal and FRI expansion coefficients, and; the binary interaction parameters used in mixing rules for the density and FRI of mixtures. The density (or FRI) of the components (or liquids treated as a single component) were both correlated to within 2%. The density (or FRI) of a mixture was fitted also to within 2% using mixing rules with only the known component density (or FRI) at 20°C as the input. The mixture density (or FRI) can be predicted to within 4% using only the correlated component density (or FRI) at 20°C. The correlations have yet to be tested at temperatures above 60°C and pressures above atmospheric.

KEYWORDS: density, refractive index, FRI, hydrocarbons, crude oil, petroleum, distillation cuts, cracked oils, mixtures

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Introduction Density and refractive index are closely related properties and both can be used for petroleum fluid characterization. Density is a defining property for crude oils, is a key parameter for almost all process calculations, and is an input for many property correlations. Refractive index is easily measurable and, since it correlates to density 1, it has been used as a proxy measurement for density in some processes 2. The refractive index has also been used to determine solubility parameters of crude oils for asphaltene stability modeling 3. Measurements for both properties are not always available and it is desirable to find an accurate correlation between the two properties not only for native crude oils but also for the refined products from distillates to cracked residues produced in refineries. Usually, the two properties are determined at a reference temperature, which is easy and convenient to measure, and then corrected to the temperature of interest, assuming no phase change in that temperature interval. Hence, a correlation at a reference temperature and a method to correct for temperature are required.

Many published correlations for oil industry applications relate density to refractive index at 20°C. Raizi

1

presented a number of correlations between refractive index and two other

properties such as specific gravity and boiling point. Other correlations 4,5 relate density linearly to only the refractive index itself or a function of the refractive index, FRI, given by:

FRI =

nD2 − 1 nD2 + 2

(1)

where nD is the refractive index at the sodium D-line. Vargas et al. 6 noted that the FRI of many crude oils was directly proportional to density (1/3 of the density value in g/cm³ units). Vargas et al.

7

noted a deviation from this one-third rule for lighter crude oils and proposed a third order

polynomial fit. It has yet to be established what form of correlation is sufficient to describe a broad range of hydrocarbons.

The density of liquid crude oils and petroleum products decreases linearly with temperature unless near the saturation pressure or critical point 8. FRI appears to remain proportional to

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density at different temperatures 7; that is, FRI is linearly related to temperature as well. There are as yet few data for the magnitude of the FRI temperature dependence of petroleum liquids.

In some applications, the density and FRI must be predicted for mixtures of components; for example, distillates, blended refinery streams, and diluted bitumen. In some cases, the objective is to determine compositions from density or refractive index measurements. For these applications, mixing rules are also required to determine mixture properties from the component properties. For density, the API correlation provides reasonable predictions for binary mixtures as do excess volume mixing rules with correlated binary interaction parameters 9. To the authors’ knowledge, excess FRI of mixing for liquid crude oils and petroleum products and their relationship to excess volumes of mixing have yet to be explored.

The objective of this study is to examine the relationship between density and refractive index (or FRI) and to assess the accuracy of the one-third rule, linear, and higher order correlations between the two properties. Pure components, native oils, and reacted oils are included in the evaluation. The temperature dependence of the properties and the relationship between the excess volume and excess FRI are also examined.

Experimental Methods Materials In addition to data available in the literature, this study utilizes density data compiled from two projects conducted at the University of Calgary. The first project, referred to in this study as the SARA Fractions Project, focused on generation and property measurement of saturate, aromatic, resin, and asphaltene (SARA) fractions from native and reacted bitumen mostly from Western Canadian (WC) feedstocks. The second project, referred to as the Distilled Cuts Project, focused on the generation and property measurement of distillation cuts from de-asphalted heavy oils from different sources. In each project, the choice of samples was dictated by availability of samples and their fractions but altogether the dataset encompasses a range of sources and reaction history.

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For the SARA Fractions Project, the samples, the whole oil density, and their saturate, aromatic, resin and asphaltene compositions (SARA) are listed in Table 1. The Native samples include conventional oils (CV), heavy oil (HO), a bitumen (B), a diluted bitumen product (DB), and a vacuum residue (VR). The In Situ Thermocracked (B-IS79 and 98) samples were obtained from an in-situ thermal process and were partially cracked. The VISB Thermocracked (VR-TC31) sample was the product after vacuum distillation of a visbreaker pilot plant. The Hydrocracked set of samples includes the bottoms product from a heavy oil stripper in a hydrocracking process (SR-HC77C) and two hydrocracked samples (SR-HC56 and 80) from a pilot plant. Finally, the last sample (XX-CV-A1) in the dataset is a sample of unknown origin and reaction history.

The sample set for the Distilled Cuts Project is listed in Table 2. The WC-B-B1 sample was provided by Shell. The samples denoted as CO-HO-A1, CO-B-B1, (Colombian heavy oil and bitumen) MX-HO-A1 (Mexican heavy oil), US-HO-A1 (Californian heavy oil) and WC-B-A1 (Canadian bitumen) were received from Schlumberger. These oils were de-asphalted prior to distillation and the density of the de-asphalted oils are also provided in Table 2. The procedures used to obtain the distillation cuts and SARA fractions are provided elsewhere 9. Technical grade (EMD) n-heptane, n-pentane, toluene and acetone were purchased from VWR International, LLC. These solvents were used for asphaltene precipitation, solids removal, SARA fractionation, and in some density and refractive index measurements.

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Table 1. SARA assays and densities for the oils from the SARA project. Oil Sample

Sample Density* kg/m³

Saturates

Aromatics

Resins

Asphaltenes

wt%

wt%

wt%

wt%

Native AU-CV-G1** 796 61.2 22.3 13.9 2.6 ME-CV-A1 ** 870 35.3 41.2 15.6 7.8 WC-B-B2 1105 17.1 44.0 19.4 19.4 WC-B-C1 1001 14.7 45.0 21.2 19.1 WC-DB-A1 904 21.1 44.9 23.4 10.6 5.3 37.4 20.1 37.2 WC-VR-B2 In Situ Thermocracked WC-B-IS79** 878 17.6 46.5 20.7 15.2 WC-B-IS98** 870 21.1 52.2 15.2 11.5 VISB Thermocracked WC-VR-TC31 4.0 31.4 13.4 51.2 Hydrocracked WC-SR-HC77C 980 19.7 47.3 17.3 15.7 WC-SR-HC56 954 32.8 49.3 15.1 2.8 WC-SR-HC80 15.3 45.1 14.5 25.1 Unknown Origin XX-CV-A1** 846 46.4 31.5 17.2 4.8 * Density at 293K and atmospheric pressure ** SARA composition for the heavy fraction after atmospheric distillation up to 300°C.

Table 2. Distillation summary and density of the deasphalted oils from the distillation project. Oil Sample

C5 Asphaltene Cumulative Mass Content Distilled† wt% wt% WC-B-B2 17.4 51.7 CO-B-A1 25.8 42.8 MX-HO-A1 21.2 41.4 CO-B-B1 22.7 46.0 US-HO-A1 12.7 53.4 WC-B-D1 16.2 41.9 †Cumulative mass distilled based on the whole oil mass *Density at 293K and atmospheric pressure

Deasphalted Oil Density* kg/m³ 998.4 959.5 958.4 959.6 948.5 972.4

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Density Measurement Densities were measured at 293 K and atmospheric pressure using an Anton Paar DMA 4500M density meter. The instrument precision was ±0.01 kg/m³ with a repeatability of ±0.1 kg/m³ for unmixed components such as pure solvents, distillation cuts, or saturate and aromatic fractions. The repeatability of mixture densities was ±0.3 kg/m³ based on a 95% confidence interval.

For the SARA Fractions Project, density data was collected for the saturate and aromatic fractions from all of the oil samples listed in Table 1. Mixture densities of saturates and aromatics in heptane or toluene were also measured. For the Distillated Cuts Project, densities of the individual distillation cuts and mixtures of the distillation cuts with toluene were measured for each oil in the dataset.

The density of low viscosity fractions (saturates, aromatic and light distillation fractions) and all mixture densities were measured directly at standard conditions of 293K and atmospheric pressure. The densities at 293 K of the more viscous fractions (samples with densities higher than 990 kg/m³) were obtained by linearly extrapolating the density of the sample at higher temperatures down to 293 K. To validate the linear extrapolation, the same procedure was used on some light fractions. The extrapolated densities at 293 K were within the experimental error of the densities measured directly at 293K.

Resin and asphaltene densities were determined indirectly from the measured densities of solutions of resins in toluene at different concentrations (from 2 to 160 g/L for resins and from 2 to 6 g/L for asphaltenes). Densities were then calculated using a regular solution mixing rule. The validity of the assumption of regular solution behavior for asphaltene-toluene mixtures is a potential source of error. The low solubility of asphaltenes limits the measurements to low concentrations. At these concentrations, the densities of irregular and regular solutions are indistinguishable, but the ultimate value of density calculated by each assumption can be noticeably different. However, the fitted asphaltene densities determined in solutions with toluene were the same as the densities determined with dichlorobenzene 10. Such good agreement

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is only likely when a regular solution is formed in both cases. The repeatability for the indirect determined densities of resins and asphaltenes were ±6 and ±23 kg/m³, respectively.

Refractive Index Measurement Refractive indexes were measured with an Anton Paar Abbemat HP refractometer with a sodium D lamp at a reference temperature of 20°C and a wavelength of 589.3 nm. The apparatus includes a measuring prism made of a glass of a high refractive index (YAG -YttriumAluminum-Garnet). The refractometer is designed to be used with samples with lower refractive index than the prism. A liquid sample in contact with the prism is illuminated by an LED and the critical angle of the total reflection at 589.3 nm sodium D wavelength is measured with a highresolution sensor array. The refractive index (nD) is calculated from this value. The instrument has a precision of 0.00002.

To measure the refractive index of a liquid sample, it was placed on the measuring (refractive) prism and covered with a lid. Up to a minute was required for the sample to attain the prism temperature and to obtain a constant reading. Constant readings were taken 5 seconds after temperature equilibration (typically 30 seconds for light/volatile samples and 60 seconds for viscous samples). The measuring prism was cleaned with solvent (toluene or acetone) and dried before another sample was applied.

As with density, the FRI were determined from direct measurements of the refractive index where possible and otherwise were calculated indirectly from the FRI of mixtures of the given fraction (2 to 20 g/L) in toluene. Very viscous samples were warmed up to 60oC in a water bath prior to measurements to facilitate sample placement. The repeatability of the FRI from direct measurements was ±0.0002. The FRI calculated indirectly from solutions have additional uncertainties in the measurement from concentration errors. The repeatability of the indirect FRI (calculated assuming regular solution behavior) was found to be ±0.003.

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Datasets Ambient Condition Dataset The densities and refractive indices of pure hydrocarbons were obtained from Riazi NIST database

11

8

and the

and are summarized in the appended Tables A1 to A6 for paraffins,

naphthenes, alkylaromatics, olefins, heteroatomic species, and polynuclear aromatics, respectively. The nomenclatures is as follows: d20 is the density at 20°C, nD20 is the sodium line refractive index at 20°C, and FRI20 is the FRI at 20°C. The measured densities and refractive indices of a) crude oils, maltenes, and distillation cuts, b) deep vacuum fractionation cuts, c) saturates and aromatics, and d) resins and asphaltenes from the combined SARA Fractions and Distillation Cuts Projects are reported in the appended Tables A7 to A10, respectively. Note, the resin and asphaltene density and refractive index were determined indirectly from the density and refractive index of solutions of these components in toluene as explained earlier. The properties of all other samples were measured directly. This dataset and the pure hydrocarbon dataset were used to develop a correlation between density and FRI.

An additional dataset was collated from densities and refractive indices reported for petroleum fractions in the literature. The dataset included distillation products, liquid-liquid extraction cuts, vacuum residue from cracking processes, and coal derived liquids, as reported in Tables B1 to B7. This dataset was used to evaluate the proposed correlation between density and FRI.

Elevated Temperature Dataset The density and refractive index were measured for many of the saturate and aromatic fractions and bitumens from the ambient condition dataset. The densities and refractive indices of the saturates and aromatics are reported in the appended Tables A11 to A12 at 40 and 60°C, respectively. The densities of several bitumens at temperatures between 20 and 175°C are provided in Tables A13 to A15.

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Mixture Dataset Mixture densities were presented elsewhere 9. FRI data were collected in this study for pseudobinary mixtures of saturates, aromatics, and bitumen with either toluene or n-heptane. FRI data for hydrocarbon mixtures were obtained from Riazi 8 and NIST 11. The excess properties of the mixtures from both sources are presented later.

Results and Discussion We first examine the relationship between density and FRI at 20°C at atmospheric pressure. Then, the effect of temperature is considered, and finally, the relationship between excess volume and excess FRI of mixtures. The investigation is limited to hydrocarbons and atmospheric pressure (the amount of FRI data reported at higher pressures is limited).

Density and FRI at 20°C The pure hydrocarbon and petroleum fraction densities are plotted versus FRI in Figure 1. Simple hydrocarbons (paraffins, single ring cyclics and aromatics) and petroleum fractions appear to follow a common trend, Figure 1a. Heteroatomic species and multiple ring aromatics deviate upwards from this trend, Figure 1b. It is interesting that the resins and asphaltenes follow the simple hydrocarbon trend or even deviate downwards from extrapolated simple hydrocarbon properties. This observation suggests that the heteroatoms and polynuclear contributions to the petroleum cut properties are relative small compared with the overall “simple” hydrocarbon contributions. For example, there may be relative strong interactions between polar groups or ring structures in the pure components that are weaker in the more complex and varied structures found in the petroleum cuts. We have only used the simple hydrocarbons in the subsequent analysis.

The density and FRI appear to be linearly related over much of the range but there is a significant deviation from linearity at the lower and upper extremes (density below 0.7 and higher than 1.1). The deviation at lower densities was also noted by Vargas et al. 7. The deviation at the higher 9

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densities occurs in the resins and asphaltenes. The latter deviation may be an artifact because these densities and refractive indices were determined indirectly from solution data. However, the identical results when diluted with toluene or dichlorobenzene suggest otherwise. A more likely possibility is that the deviation is caused by resin and asphaltene self-association. A similar deviation is expected for any fraction with a significant resin and asphaltene content. The reason for the deviation at low densities is not known but may be related to the proximity of those fluids to their saturation pressure and/or critical point. The refractive index and density are related through the Lorentz-Lorenz equation given by: FRI =

4παNa 3MW

(2)

where α is polarizability, Na is Avogadro’s number, and MW is molecular weight. The implicit assumption for a proportionality between density and FRI is that α/MW is constant for hydrocarbons; the assumption may break down near a saturation condition or the critical point. 1.3

1.3

(a)

(b)

1.2 1.1

1.0

Density, g/cm³

1.1

Density, g/cm³

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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saturates aromatics resins asphaltenes paraffins naphthenes alkylaromatics olefins oils dist cuts DVF cuts quadratic fit

0.9 0.8 0.7 0.6

saturates aromatics resins asphaltenes paraffins naphthenes alkylaromatics olefins oils dist cuts DVF cuts quadratic fit heteroatoms polynuclear

0.9

0.7

0.5

0.5 0.2

0.3

0.4

0.5

0.2

FRI

0.3

0.4

0.5

FRI

Figure 1. Measured and quadratic fit of density versus FRI at 20°C of pure hydrocarbons 8,11 and petroleum fraction data (this work): with (a) and without (b) heteroatomic species and polynuclear hydrocarbons. The one-third rule and the following correlations from the literature were tested against these data: Khan 4

 = −2.08 + 1.977

(3) 10

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Angle et al. 5

 = −1.6235 + 1.6631

(4)

Angle et al. 5

 = −0.0647 + 3.24

(5)

The average and maximum absolute and relative deviation in the fitted densities are reported in Table 3. The one-third rule and the linear correlation to FRI outperform the correlations to refractive index. As expected, the main source of error is at densities above 1.1 g/cm³, Figure 2a. However, even in the 0.75 to 1.05 g/cm³ range, the majority of the data do not follow an exact linear trend, Figure 2b.

Table 3: Average and maximum deviations in the predicted density using proportional, linear, and quadratic correlations to refractive index and FRI. Source Function AD ARD MAD MARD g/cm³ % g/cm³ % One-third rule Proportional (FRI) 0.021 2.2 0.14 11 Khan 4 Linear (nD) 0.039 3.9 0.42 15 5 Angle et al. Linear (nD) 0.032 3.2 0.28 23 Angle et al. 5 Linear (FRI) 0.020 2.0 0.19 15 This work Quadratic (FRI) 0.010 1.1 0.04 4.2

1.05

1.3

(a)

(b)

1.2

Density, g/cm³

1.1

Density, g/cm³

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1.0 0.9 data

0.8

1/3 rule

0.7 0.6

0.95

0.85

Khan

data

Angle (FRI)

1/3 rule

Angle (RI)

Angle (FRI)

polynomial

polynomial

0.5 0.2

0.3

0.4

0.5

0.75 0.25

FRI

0.3 FRI

Figure 2. Comparison of density versus FRI correlations at 20°C for pure hydrocarbons petroleum fraction data (this work): a) full scale; b) mid-range scale.

0.35 8,11

and

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The following quadratic expression was found to provide a better fit to the data:  = −0.6656 + 7.375 − 6.984 

(6a)

where ρ20 and FRI20 are the density and refractive index, respectively, at 20°C. The average and maximum absolute deviation in the fitted densities were 0.010 g/cm³ (1.1%) and 0.039 g/cm³ (4.2%). The deviations are significantly lower than the other correlations. Much of the difference is in the improved fit at densities above 1.1 g/cm³, Figure 2a. However, some improved is realized at lower densities as well, Figure 2b. Note, the equivalent expression for the refractive index as a function of density is given by:  = 0.5280 − 0.37841.2813 −  .

(6b)

The average and maximum absolute deviation in the fitted FRI were 0.0034 (1.1%) and 0.026 (6.1%). Equations 6a and 6b are valid for densities from 0.55 to 1.25 g/cm³ and FRI from 0.2 to 0.5.

All of the correlations were tested on the ambient condition dataset obtained from the literature. The dataset and the FRI based correlations are shown in Figure 3. The average and maximum deviations are summarized in Table 4. All of the correlations perform similarly over the reported ranges of densities (0.7 to 1.2 g/cm³). In this case, the scatter in the datasets is more significant than the differences between the correlations. Overall, it appears that the one-third rule, linear, and quadratic correlations can predict density from FRI with an average error of approximately 0.015 g/cm³ (2%) for most petroleum liquids.

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1.2 Mazumdar El Hadi Shishavan Tzekhanovich Goossens Coto Riazi quadratic one third rule Angle (FRI)

1.1

Density, g/cm³

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0

0.9

0.8

0.7 0.2

0.25

0.3 FRI

0.35

0.4

Figure 3. Measured and quadratic fit of density versus FRI at 20°C of petroleum fraction data from the literature 5,8,12-17. Table 4: Average and maximum deviations in the predicted density at 20°C using proportional, linear, and quadratic correlations to refractive index and FRI. Source

Function

One-third rule Khan 4 Angle et al. 5 Angle et al. 5 This work

Proportional (FRI) Linear (nD) Linear (nD) Linear (FRI) Quadratic (FRI)

AD g/cm³ 0.017 0.021 0.015 0.016 0.015

ARD % 2.0 2.2 1.7 1.8 1.8

MAD g/cm³ 0.084 0.108 0.088 0.072 0.068

MARD % 8.0 9.1 9.3 7.6 6.3

Effect of Temperature on Density and FRI As Vargas et al.

7

noted, the density and FRI at any pair of temperatures are related through a

simple ratio:

ρ (T ) FRI(T ) = ρ (Tref ) FRI(Tref )

(7)

The applicability of the ratio was assessed on the data for saturates, aromatics, and bitumens at 60°C. The densities at 60°C were determined from the densities measured at the reference temperature of 20°C and FRI measured at 20 and 60°C. The average and maximum deviation of 13

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the predicted densities were 0.0025 g/cm³ (0.27%) and 0.0036 (0.36%), respectively. Similarly, the FRI determined from the densities measured at 60°C were predicted with an average and maximum deviation of 0.0008 (0.27%) and 0.0010 (0.36%), respectively. As with any ratio method, the error increases as temperature increases for non-linear functions; hence the accuracy of this approach beyond a 40°C temperature difference is unknown.

When neither density nor FRI data are available, the effect of temperature must be predicted. To do so, the effect of temperature on saturate, aromatic, and bitumen density and refractive index was determined from measurements at 40°C and 60°C. Both specific volume and FRI were found to be linearly related to temperature for all samples, Figure 4. For density, the relationship to temperature can be expressed in terms of coefficient of thermal expansion, αV, defined as:

1  dv   v  dT 

αV = 

(8)

where v is the specific volume. Eq. 8 can be rearranged to obtain: v = v 20 exp(α V (T − 20))

(9)

For αVT