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Magnetic circular dichroism analysis of nitrogen contaminants in crude oil
ANALYTICA
CHIMICA ACTA ELSEVIER
Analytica
Chimica Acta 298 (1994) 341-349
Magnetic circular dichroism analysis of nitrogen contaminants in crude oil J.A. Warner, B.R. Hollebone Ottuwa-Carleton
Chemistry instime,
*
Carleton University, X125 Colonel By Drive, Ottawa, Ontario KIS 5B6, Canada Received 23 May 1994
Abstract Nitrogen pollution of air can occur from combustion of fossil fuels contaminated with fossilized nitrogen bearing organic molecules. Upgrading and refining processes have been designed to remove nitrogen but analytical monitoring feedstocks and products in real time is impractical with standard analytical techniques for nitrogen. In this study, magnetic circular dichroism (MCD) detection and quantitation of fossil porphyrins is shown to correlate well with total nitrogen in fractions representing over 80% of crude oil. Non-linear correlation in the heaviest fraction due to non-porphyrin neutral substances can be recognized and corrected. The resulting MCD procedure has the potential to become a fast on-site quality control system for upgrading crude oil. Keywords:
Crude oil; Magnetic
circular
dichroism;
Nitrogen;
Oil
1. Introduction The level of nitrogen found in crude oils is in the range of 1% by weight [l]. It occurs principally as a heteroatomic replacement of carbon in unsaturated, cyclic or aromatic structures [2]. Regardless of its speciation however, nitrogen in all fossil fuels interferes with the sequence of processes used for upgrading and refining to final products [3]. As well, when fuels are used, the trace nitrogen species are converted to mixed nitrogen oxides regardless of the design of the combustion system [4]. In these forms,
* Corresponding
author.
0003.2670/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0003-2670(94)00301-7
it becomes a contribution to urban smog and acid rain. To avoid these problems, the initial upgrading steps have been designed to remove the bulk of nitrogen species during the coking process which precipitates the aromatic materials as amorphous carbon [5]. This succeeds because the nitrogen substituted species are similar thermodynamically to the aromatics and often contain fossilized porphyrins or its degradation fragments [6-g]. The most common of these are listed in Table 1 together with their standard enthalpies of formation. The nitrogen level of crude oils varies widely and controlling the upgrading process to ensure complete removal of nitrogen is difficult with current techniques. Typically the feedstock passes through the
342 Tabk 1 Stabilities
J.A. Warner, B.R. Hollebone / A&pica
of nitrogen contaminants
Name
Pyrrole Indole Carbazole Pyridine Isoquinoline Acridine Phenanthridine 21 H,23H-Porphin Etioporphyrin I Etioporphyrin II Octaethylporphyrin
Molecular formula
C4H5N
C&N G&N C,“,N C,H,N C,,“,N Ci3H9N
C,oH,,N C32H38N4
of crude oils Standard enthalpy formation &I/m00 Hf (298.15 K) 63.1 86.1 125.1 100.2 144.5 200.9 148.9 - 1106.9 -25.2 1.6
C3ZH38N4 C36
“46N4
-
183.2
upgrades in lo-15 min. Attempts to control the process with traditional Kjeldahl analysis for nitrogen become impractical since it typically requires several hours to perform the analysis [lo]. Recent introduction of inductively coupled plasma mass spectrometry (ICP-MS) methods, based on observation of NO [ 111 still requires sample preparation and analysis at a remote laboratory. Because practical, real-time, on-site analysis for nitrogen has not been available upgrading plants tend to be overbuilt to allow for the most extreme conditions [12]. They are also operated to overcompensate the coking. This raises the initial capital cost and wastes some feedstock which could otherwise be recovered as final products. To overcome these problems a chemical analysis for nitrogen which completes the determination routinely within the hold-up time of the process is needed. These requirements essentially preclude all but the simplest and most rapid manipulations and strongly suggest that direct spectroscopic determination on the crude oil itself is necessary. Several types of spectroscopy are available but the chemical and physical constraints of this problem preclude most of them. The major chemical problems arise from the variability of the chemical components and the suspended solids in the crude oil. These make it very difficult to establish a reference material for absorbance spectrometry or to normalize backgrounds for fluorimetry. At the same time, the major physical difficulties arise from the intense, broad band spectra
Chimica Acta 298 (1994) 341-349
of crude oils. Detection of signals from trace contaminants against these intense backgrounds involves detection of small differences between large signals for either absorbance or fluorescence. The most promising spectrometric technique for this application is magnetic circular dichroism (MCD) [13]. In this form of spectroscopy, the signal measures the preferential absorption of left or right circularly polarized light by a sample held in a strong magnetic field. This preferential absorption may be plotted as a positive (LCP) or negative (RCP) signal. The signal arises from the rotation of electrons around the applied field during excitation by the light to an excited state. For practical purposes, this means that molecules capable of magnetic response can become highly visible against a background of magnetically silent substances, even if the background has a high electronic absorption [14]. In the present case, the nitrogen occurring as a porphyrin is easily detected because of the strong magnetic signal of this structure in several bands in the visible and near ultraviolet regions [15]. This overcomes the basic physical difficulties since the MCD spectra of the deeply coloured oil background are very weak. At the same time, MCD also, in effect, overcomes the chemical problem of the reference. Right and left hand light scatter equally from suspensions so MCD spectrometry is not sensitive to sample turbidity. As well, the differential absorbance means that the cpflcentration is estimated, using Beer’s law, as a difference in intensities of the two handed beams of light. In this sense, one beam acts as a reference for the other and MCD appears to provide single-beam analysis. In fact, a baseline correction may be needed [14], but the characteristic porphyrin signal is easily recognized against poorly defined and weak spectra of other oil components. In spite of this ability to discriminate and quantify the signal from porphyrins, MCD can only become a reliable indicator of nitrogen contamination if total nitrogen is correlated strongly to porphyrin content of oil. The processes of fossilization and metamorphosis which lead to oil formation suggest that such a correlation is possible [16,17]. The porphyrins occur in the original life forms and may also serve as the thermodynamically stable end point of anaerobic metamorphosis. To test this hypothesis a synthetic crude oil obtained from oil sand deposits has been
.i.A. Warner, B.R. Hollebone /Analytica
Chimica Acta 298 (1994) 341-349
studied. In particular, fractions separated by solution in homologous alkane solvents were analyzed to determine the reliability of the proposed correlation.
2. Experimental 2.1. Crude oil A sample of synthetic crude oil was obtained from a Clark hot water extractor operated by Syncrude Canada at Mildred Lake, Alberta. This viscous material was diluted with benzene, and filtered through sintered glass to remove the entrained sand and clay particles. The benzene was then removed by rotary evaporation. 2.2. Fractionation
of crude oil
Crude oil is known to contain alkanes, cycloalkanes, aromatics and heteroatomic compounds [2,18]. In general, these are separable by their solubilities in increasingly polarizable alkane solvents [19]. A very convenient series of these solvents is the straight chain hydrocarbons from propane to heptane. In this series, the polarizability increases linearly with the number of carbon atoms, as shown in Fig. 1. Thus, these solvents can be used to extract increasingly polar or polarized components of crude oil [19]. In particular, this discrimination should include the nitrogen-bearing contaminants. The working scheme of the fractionation sequence is shown in Figs. 2 and 3. Because of the specific design, no explicit extraction with butane is incIuded. The corresponding fraction is the maltene resins, isolated here as soluble in pentane but insoluble in propane at - 80°C. After solvent separations, all fractions except the heptane asphaltenes were dissolved in cyclohexane and subjected to cation exchange on Amberlist 15 cation-exchange resin columns (Rohm and Haas). The columns were washed with four volumes of cyclohexane. The retained bases were then eluted successively with benzene, then benzene-methanol azeotrope and finally the benzene-methanol-isopropylamine azeotrope, to separate the weak, medium and strong bases. The non basic faction in cyclohexane was subjected to anion exchange on the base form of Amberlyst 29 anion exchange resin (Rohm
Fig. 1. Relationship between the average electric polarizability volume (1O-24 cm’) of homologous alkane straight chain solvents and the number of carbon atoms.
and Haas). The acids were eluted with 5% acetic acid-95% benzene. Finally, the neutral fraction was recovered from the cyclohexane, redissolved in dichloromethane and loaded onto Davisil, silica gel 150A (Aldrich). The dichloromethane was evaporated and the silica gel eluted with cyclohexane to remove saturates, then with the benzene-methanol azeotrope to remove the aromatics. 2.3. Analyses The recoveries of all of the separable components were analysed first by gas chromatography (GC) then GC-MS. The preliminary data were obtained on a Varian 3400 gas chromatograph with a 1045 injection system and 30-m DB5 column with a thermionic detector. Final data were determined on a Varian Saturn GC/MS system with a 30-m DB5 column programmed with a rapid ramp to 300°C and a 70 V ion trap source. With each fraction and subfraction where possi-
344
J.A. Warner, B.R. Hollebone/Analytica
Chimica Acta 298 (1994) 341-349
porphyrin. From these, two calibration determined, following the relations;
ble, analyses for C, H and N were performed by Canadian Microanalytical Services. The porphyrin analyses were performed by MCD spectrometry on a custom-built system in this laboratory. Data were obtained in the Q band and when possible, the B band. This technique was calibrated with cobalt sulphate [13] and then with vanadyl porphyrin. The latter was chosen since it is the dominant porphyrin found in these oil sand deposits. It was synthesized by well defined procedures [20]. From the known absorbance extinction coefficient [21] and the cobalt sulphate calibration of MCD intensities, the differential extinction coefficients were determined for the Q and B bands of vanadyl
A, - 3.0228 x lo+ [ VOP], =
0.0193
A, - 2.7577 x 1O-7 [ VOP], =
0.01332
These calibrations
are shown in Fig. 4.
3. Results The recoveries of fractions from the crude oil and the analyses of elements and vanadyl porphyrin in
Situmen 40:1 (v/w) pentane soluble
Maltenes
I251
(v/w) propane
insoluble
anion
anlo exchang
exchange
+ Neutrals
Arom d tics
Sat rates
Fig. 2. Experimental
’ Acids
curves were
Acid
scheme for separating
maltene fractions from bitumen.
J.A. Warner, B.R. Hollebone/Analytica
these fractions are all reported in Table 2. In several instances, the fractions represent very small recoveries and full analyses were not possible from the quantities obtained. In particular, the hexane fraction was so small that only C, H and N analyses together with porphyrin determinations were possible. The porphyrin analyses rely essentially on the Q band data, both because it is stronger and because the B band is often obscured by other transitions. These data can be used to determine whether the porphyrin content of these fractions is correlated to the total nitrogen levels. In Fig. 5, the total porphyrin content is plotted against the total level of nitrogen in each of the fractions obtained by homologous
345
Chimica Acta 298 (1994) 341-349
alkane solvent separations. The four successive fractions extracted by C, up to C, solvent yield a good correlation of the porphyrin determination to the nitrogen content. In the remainder fraction from heptane, the porphyrin level is much lower than predicted by the trend of the first four data points. The reason for this decreased porphyrin level in the heptane fraction can be examined in the subfractionation data for bases, acids and neutrals, in Table 2. The porphyrin content tends to be concentrated in the basic subfractions, particularly in those subfractions denoted as either weak or medium bases. The total nitrogen however is found distributed over most of the subfractions. From these observations, the
Bitumen
soluble
basic
base base
base
Asphalienes
base
b-
barn
basic
I
t
Ih Ne
tr.r~ gel
Acids
Saturates
Fig. 3. Experimental
Aromatics scheme for separating
I-
Aromatics asphaltene
Safufiires
fractions from bitumen.
Acids
346
J.A. Warner, B.R. Hollebone/Analytica
Chirnica Acta 298 (1994) 341-349
total porphyrin content of a solvent fraction should correlate to the proportion of nitrogen found in these basic subfractions. This is illustrated by a plot of total fraction porphyrin against the nitrogen content of medium base subfractions, corrected for their relative subfraction yield, in Fig. 6. In this plot, the
porphyrin of the extracted heptane fraction (C,) correlates closely with that of the other fractions for which subfraction data were available. This result shows that the distribution of elemental nitrogen amongst various types of molecules is reasonably constant through fractions soluble in C,
Table 2 Analysis of crude oil fractions Bitumen
Recovery as % bitumen
Fraction
Description
wt.% f 0.5
Description
Dephased oil
34 * 0.3 Weak bases Medium bases Strong bases Acids Saturates Aromatics
Resins
C
wt.% + 0.5
wt.% f 0.8
8 9 1.0 7 69 7
35 + 0.3 Weak bases Medium bases Strong bases Acids Saturates Aromatics
Pentane asphaltenes
Recovery as % fraction
3.0 15 7 21 0 54
4.0 fO.l Weak bases Medium bases Strong bases Acids Saturates Aromatics
15 23 4.0 52 <1
H
N
wt.% +0.1
Porphyrin content MCD Q band
MCD B band
wt.% 4 0.03
wt.% * 0.05
wt.% * 0.05
84.8
10.5
0.15
0.17
0.22
_
_
0.14 0.13 0.00 0.00 O.M3 0.00
0.13 0.00 0.00 0.00 0.28 0.00
66.4 _
8.91 _
52.8 85.6 _
7.65 11.5 _
_ 0.88 _ 1.43 _ -
83.1
9.8
0.44
82.4 71.9 81.4 55.8 _ 78.8
8.37 8.49 9.21 7.03 _ 8.07
0.85 0.99 1.49 0.92 _ 1.17
4.2 2.97 1.56 0.39 0.00 0.41
0.00 0.00 0.00 0.00 0.52 0.00
81.9
9.09
0.76
5.5
0.00
79.3 68.2 _
7.94 7.80 _
0.81 1.00 _ _
8.3 6.8 4.7 0.010 0.00 0.00
1.65 3.13 0.00 0.06 0.00 0.00
2.69
Hexane asphaltenes
1.0 fO.l
80.7
8.53
0.96
10.23
2.28
Heptane asphaltenes
19
80.2
7.94
1.2
1.85
0.00
79.3 78.5 78.0 71.3 84.7 79.0
7.53 6.13 8.14 7.72 11.3 9.25
1.35 0.77 1.44 1.02 _ 0.57
0.99 3.9 2.51 1.0 0.00 4.1
0.00 2.90 0.00 0.84 0.53 4.24
fl Weak bases Medium bases Strong bases Acids Saturates Aromatics
32 14 7.0 6 28 12
J.A. Warner, B.R. Hollebone/Analytica Chimica Acra 298 (1994) 341-349
O.uOOO
0.0003 0.0006 0.0009
347
0.0012 0.0015
[oflWL1 Fig. 4. Calibration curves for MCD quantification porphyrin using both Q and B baud correlations.
4
of vanadyl
0.2
0.4
0.6
0.6
0.9
1.0
[NT1
Fig. 5. The dependence of % total fractions on % total nitrogen.
porphyrin
for successive
to C, solvents. Beyond this point however, the distribution appears to change. This is illustrated in Fig. 7, using all the available subfraction data.
up
4. Discussion The correlation in Fig. 5 shows that an MCD assay of total porphyrin is a good indicator of the total nitrogen content of fractions representing more than 80% of the total synthetic crude oil. The correlation in Fig. 6 shows that the MCD assay continues to be valid when applied to the basic subfraction of the heptane-soluble fraction. The excess nitrogen content of this fraction appears to arise in the high aromatics and acids content, as shown in Fig. 7. This distribution of aromatics into the heptane fraction follows the increasing polarizability of the solvent. The least polar&able solvent, propane, dissolves saturates and small amounts of permanently polar acids and bases. With increasing polarizability, the solubility of saturates drops quickly, that of polar bases and acids rises continuously and the aromatics
Fig. 6. The dependence of % total porphyrin for successive fractions on % nitrogen present as medium bases in each fraction.
348
J-4. Warner, B.R. Holiebone / Analytica Chimica Acta 298 (1994) 341-349
appear to split into two different groups, those soluble at butane and those in heptane. Significantly, the aromatics soluble in butane carry on unusually low nitrogen content, while those soluble in heptane have a nitrogen content ten times higher. Since these substances emerged from the subfractionation process as apparent aromatic neutral species, they are most probably, nitrogen heteroaromatic compounds in which the nitrogen is fully substituted. The nonbasic nitrogen compounds typically found in these synthetic crude oils [22] include the indoles, carbazoles, benzocarbazoles and substituted pyrroles shown in Fig. 8. There may be some complexes of nitrogenous molecules with metals which would also fall into this group but none have been positively identified [23,24]. These conditions suggest that an MCD assay of porphyrins in oils could be used in process control. The scan time for the Q band region is typically 2 to 3 min, depending on the gain required. Thus, the only limitation to real-time, on-line data would arise from sample preparation. Since the results show that porphyrin content correlates linearly to total nitrogen up to the hexane fraction, a rapid, one-step hexane
/\ c) #
indole
carbazole
quinaline (benzo[bIpyridine)
i8OlpiIlOlin0
UJenzoklpyri~e)
acridine
pbenanthridine
Fig. 8. Non-porphyrin crude oils.
C,_ (117)
=I or, ’
A
CIJfW (167)
WW ( 129)
WW
(129)
WW (179)
WW (179)
nitrogen contaminants
commonly
found in
extraction could be performed to prepare a representative sample at a dilution appropriate to good MCD signal to noise. Since this is such a sample operation, it should be readily automated with a robotic extraction apparatus. Presuming this would require only a few minutes, feed stock data could be available to plant operators at 10 min intervals. This would provide the real-time data needed for continued optimization of denitrification operations.
9
References
6 8 POLARIZABILITY
IO 12 14 a OF SOLVENT MOLECULES
Fig. 7. The dependence of % total porphyrin in each fraction on polarizability of each solvent for components of each subfraction.
[l] O.P. Strausz, in D.A. Redford and G.A. Weinstock (Eds.), The Oil Sands of Canada-Venezuela, CIM Special Volume 17, 1977, pp. 146-152. [2] J.G. Speight and S.E. Moschopedis, Adv. Chem. Ser., 195 (1981) 1. [3] CD. Ford, S.A. Holmes, L.F. Thompson and D.R. Latham, Anal. Chem., 53 (1981) 831. [4] Z.R. Ismagilov and M.A. Kerzhentsev, Catal. Rev. Sci. Eng., 32 (1990) 51. [5] M. Dorbon, I. Ignatiadis, J.M. Schmitter, P. Arpino, G. Guiochon, H. Toulhoat and A. Hut, Fuel, 63 (1984) 565.
J-4. Warner, B.R. Hollebone /Analytica [6] J.M. Schmitter, Z. Vajta and P. Arpino, in A.G. Douglas and J.R. Maxwell (Eds.), Investigation of Nitrogen Bases in Petroleum, Advances in Organic Geometry 1979, Pergamon Press, Oxford, 1980, pp. 67-76. [7] Z. Frakman, Y. Ignasiak, D. Montgomery and O.P. Strausz, AOSTRA, J. Res., 3 (1987) 131. [8] J.M. Schmitter, P. Garrigues, I. Ignatradis, R. De Vazelkes, F. Perin, M. Ewald and P. Arpino, Org. Geochim., 6 (1984) 579. [9] J.B. Pedley, R.D. Nayler and S.B. Kirby, Thermodynamic Data of Organic Compounds, Chapman Hall, London, 2nd edn., 1986. [lo] D.A. Skoog and D.M. West, Fundamentals of Analytical Chemistry, Saunders, Philadelphia, PA, 4th edn., 1982, pp. 249-251. [ 1 l] C. Gregoire, personal communication. [12] M. Nagai, T. Masunaga and N. Hanu-oka, Energy and Falls, 2 (1988) 645. [13] B. Holmquist and B.L. Valee, Methods Enzymol., 49 (1978) 149. [14] B.R. Hollebone, Spectrochim. Acta. Rev., 1.5 (1994) 493. [1_5] J.B. Lambert, H.F. Shurovell, L. Verbit, R. Cooks and G.H.
Chimica Acta 298 (1994) 341-349
[16] [17] [18]
[19] [20]
[21] [22] [23] [24]
349
Stout, Organic Structural Analysis, McMillan, New York, 1976. A.H. Whittaker and P. Dyson, Oil Gas J., 28 (1980) 141. I. Ignatiadis, M. Ewald, P. Arpino and G. Guiochon, Org. Geochem., 7 (1984) 111. B. Tissot and D.H. Welte, Petroleum Formation and occurrance, Springer Verlag, Berlin, Part IV, Chap. 2, 2nd edn., 1981. L.W. Corbett and V. Petrozzi, Ind. Eng. Chem. Prod. Rev. Dev., 17 (1978) 342. J.W. Buchler, G. Eikelmann, L. Puppe, K. Rohbock, H.H. Schneehage and D. Week, Justus Liebis Ann. Chem., 745 (1971) 135. M.M. Barbooti, E.Z. Said, E.B. Hassan and SM. AbdulRidah, Fuel, 68 (1989) 84. Z. Frakman, T.M. Ignasiak, D.S. Montgomery and O.P. Strausz, AOSTRA, J. Res., 3 (1987) 131. J.G. Reynolds, W.R. Biggs and S.A. Bezman, ACS Symp. Ser., 344 (1987) 205. Z. Frakman, ‘P.M. Ignasiak, D.S. Montgomery and O.P. Strausz, AOSTRA, J. Res. 4 (1988) 171.
CHIMICA ACTA ELSEVIER
Analytica
Chimica Acta 298 (1994) 341-349
Magnetic circular dichroism analysis of nitrogen contaminants in crude oil J.A. Warner, B.R. Hollebone Ottuwa-Carleton
Chemistry instime,
*
Carleton University, X125 Colonel By Drive, Ottawa, Ontario KIS 5B6, Canada Received 23 May 1994
Abstract Nitrogen pollution of air can occur from combustion of fossil fuels contaminated with fossilized nitrogen bearing organic molecules. Upgrading and refining processes have been designed to remove nitrogen but analytical monitoring feedstocks and products in real time is impractical with standard analytical techniques for nitrogen. In this study, magnetic circular dichroism (MCD) detection and quantitation of fossil porphyrins is shown to correlate well with total nitrogen in fractions representing over 80% of crude oil. Non-linear correlation in the heaviest fraction due to non-porphyrin neutral substances can be recognized and corrected. The resulting MCD procedure has the potential to become a fast on-site quality control system for upgrading crude oil. Keywords:
Crude oil; Magnetic
circular
dichroism;
Nitrogen;
Oil
1. Introduction The level of nitrogen found in crude oils is in the range of 1% by weight [l]. It occurs principally as a heteroatomic replacement of carbon in unsaturated, cyclic or aromatic structures [2]. Regardless of its speciation however, nitrogen in all fossil fuels interferes with the sequence of processes used for upgrading and refining to final products [3]. As well, when fuels are used, the trace nitrogen species are converted to mixed nitrogen oxides regardless of the design of the combustion system [4]. In these forms,
* Corresponding
author.
0003.2670/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0003-2670(94)00301-7
it becomes a contribution to urban smog and acid rain. To avoid these problems, the initial upgrading steps have been designed to remove the bulk of nitrogen species during the coking process which precipitates the aromatic materials as amorphous carbon [5]. This succeeds because the nitrogen substituted species are similar thermodynamically to the aromatics and often contain fossilized porphyrins or its degradation fragments [6-g]. The most common of these are listed in Table 1 together with their standard enthalpies of formation. The nitrogen level of crude oils varies widely and controlling the upgrading process to ensure complete removal of nitrogen is difficult with current techniques. Typically the feedstock passes through the
342 Tabk 1 Stabilities
J.A. Warner, B.R. Hollebone / A&pica
of nitrogen contaminants
Name
Pyrrole Indole Carbazole Pyridine Isoquinoline Acridine Phenanthridine 21 H,23H-Porphin Etioporphyrin I Etioporphyrin II Octaethylporphyrin
Molecular formula
C4H5N
C&N G&N C,“,N C,H,N C,,“,N Ci3H9N
C,oH,,N C32H38N4
of crude oils Standard enthalpy formation &I/m00 Hf (298.15 K) 63.1 86.1 125.1 100.2 144.5 200.9 148.9 - 1106.9 -25.2 1.6
C3ZH38N4 C36
“46N4
-
183.2
upgrades in lo-15 min. Attempts to control the process with traditional Kjeldahl analysis for nitrogen become impractical since it typically requires several hours to perform the analysis [lo]. Recent introduction of inductively coupled plasma mass spectrometry (ICP-MS) methods, based on observation of NO [ 111 still requires sample preparation and analysis at a remote laboratory. Because practical, real-time, on-site analysis for nitrogen has not been available upgrading plants tend to be overbuilt to allow for the most extreme conditions [12]. They are also operated to overcompensate the coking. This raises the initial capital cost and wastes some feedstock which could otherwise be recovered as final products. To overcome these problems a chemical analysis for nitrogen which completes the determination routinely within the hold-up time of the process is needed. These requirements essentially preclude all but the simplest and most rapid manipulations and strongly suggest that direct spectroscopic determination on the crude oil itself is necessary. Several types of spectroscopy are available but the chemical and physical constraints of this problem preclude most of them. The major chemical problems arise from the variability of the chemical components and the suspended solids in the crude oil. These make it very difficult to establish a reference material for absorbance spectrometry or to normalize backgrounds for fluorimetry. At the same time, the major physical difficulties arise from the intense, broad band spectra
Chimica Acta 298 (1994) 341-349
of crude oils. Detection of signals from trace contaminants against these intense backgrounds involves detection of small differences between large signals for either absorbance or fluorescence. The most promising spectrometric technique for this application is magnetic circular dichroism (MCD) [13]. In this form of spectroscopy, the signal measures the preferential absorption of left or right circularly polarized light by a sample held in a strong magnetic field. This preferential absorption may be plotted as a positive (LCP) or negative (RCP) signal. The signal arises from the rotation of electrons around the applied field during excitation by the light to an excited state. For practical purposes, this means that molecules capable of magnetic response can become highly visible against a background of magnetically silent substances, even if the background has a high electronic absorption [14]. In the present case, the nitrogen occurring as a porphyrin is easily detected because of the strong magnetic signal of this structure in several bands in the visible and near ultraviolet regions [15]. This overcomes the basic physical difficulties since the MCD spectra of the deeply coloured oil background are very weak. At the same time, MCD also, in effect, overcomes the chemical problem of the reference. Right and left hand light scatter equally from suspensions so MCD spectrometry is not sensitive to sample turbidity. As well, the differential absorbance means that the cpflcentration is estimated, using Beer’s law, as a difference in intensities of the two handed beams of light. In this sense, one beam acts as a reference for the other and MCD appears to provide single-beam analysis. In fact, a baseline correction may be needed [14], but the characteristic porphyrin signal is easily recognized against poorly defined and weak spectra of other oil components. In spite of this ability to discriminate and quantify the signal from porphyrins, MCD can only become a reliable indicator of nitrogen contamination if total nitrogen is correlated strongly to porphyrin content of oil. The processes of fossilization and metamorphosis which lead to oil formation suggest that such a correlation is possible [16,17]. The porphyrins occur in the original life forms and may also serve as the thermodynamically stable end point of anaerobic metamorphosis. To test this hypothesis a synthetic crude oil obtained from oil sand deposits has been
.i.A. Warner, B.R. Hollebone /Analytica
Chimica Acta 298 (1994) 341-349
studied. In particular, fractions separated by solution in homologous alkane solvents were analyzed to determine the reliability of the proposed correlation.
2. Experimental 2.1. Crude oil A sample of synthetic crude oil was obtained from a Clark hot water extractor operated by Syncrude Canada at Mildred Lake, Alberta. This viscous material was diluted with benzene, and filtered through sintered glass to remove the entrained sand and clay particles. The benzene was then removed by rotary evaporation. 2.2. Fractionation
of crude oil
Crude oil is known to contain alkanes, cycloalkanes, aromatics and heteroatomic compounds [2,18]. In general, these are separable by their solubilities in increasingly polarizable alkane solvents [19]. A very convenient series of these solvents is the straight chain hydrocarbons from propane to heptane. In this series, the polarizability increases linearly with the number of carbon atoms, as shown in Fig. 1. Thus, these solvents can be used to extract increasingly polar or polarized components of crude oil [19]. In particular, this discrimination should include the nitrogen-bearing contaminants. The working scheme of the fractionation sequence is shown in Figs. 2 and 3. Because of the specific design, no explicit extraction with butane is incIuded. The corresponding fraction is the maltene resins, isolated here as soluble in pentane but insoluble in propane at - 80°C. After solvent separations, all fractions except the heptane asphaltenes were dissolved in cyclohexane and subjected to cation exchange on Amberlist 15 cation-exchange resin columns (Rohm and Haas). The columns were washed with four volumes of cyclohexane. The retained bases were then eluted successively with benzene, then benzene-methanol azeotrope and finally the benzene-methanol-isopropylamine azeotrope, to separate the weak, medium and strong bases. The non basic faction in cyclohexane was subjected to anion exchange on the base form of Amberlyst 29 anion exchange resin (Rohm
Fig. 1. Relationship between the average electric polarizability volume (1O-24 cm’) of homologous alkane straight chain solvents and the number of carbon atoms.
and Haas). The acids were eluted with 5% acetic acid-95% benzene. Finally, the neutral fraction was recovered from the cyclohexane, redissolved in dichloromethane and loaded onto Davisil, silica gel 150A (Aldrich). The dichloromethane was evaporated and the silica gel eluted with cyclohexane to remove saturates, then with the benzene-methanol azeotrope to remove the aromatics. 2.3. Analyses The recoveries of all of the separable components were analysed first by gas chromatography (GC) then GC-MS. The preliminary data were obtained on a Varian 3400 gas chromatograph with a 1045 injection system and 30-m DB5 column with a thermionic detector. Final data were determined on a Varian Saturn GC/MS system with a 30-m DB5 column programmed with a rapid ramp to 300°C and a 70 V ion trap source. With each fraction and subfraction where possi-
344
J.A. Warner, B.R. Hollebone/Analytica
Chimica Acta 298 (1994) 341-349
porphyrin. From these, two calibration determined, following the relations;
ble, analyses for C, H and N were performed by Canadian Microanalytical Services. The porphyrin analyses were performed by MCD spectrometry on a custom-built system in this laboratory. Data were obtained in the Q band and when possible, the B band. This technique was calibrated with cobalt sulphate [13] and then with vanadyl porphyrin. The latter was chosen since it is the dominant porphyrin found in these oil sand deposits. It was synthesized by well defined procedures [20]. From the known absorbance extinction coefficient [21] and the cobalt sulphate calibration of MCD intensities, the differential extinction coefficients were determined for the Q and B bands of vanadyl
A, - 3.0228 x lo+ [ VOP], =
0.0193
A, - 2.7577 x 1O-7 [ VOP], =
0.01332
These calibrations
are shown in Fig. 4.
3. Results The recoveries of fractions from the crude oil and the analyses of elements and vanadyl porphyrin in
Situmen 40:1 (v/w) pentane soluble
Maltenes
I251
(v/w) propane
insoluble
anion
anlo exchang
exchange
+ Neutrals
Arom d tics
Sat rates
Fig. 2. Experimental
’ Acids
curves were
Acid
scheme for separating
maltene fractions from bitumen.
J.A. Warner, B.R. Hollebone/Analytica
these fractions are all reported in Table 2. In several instances, the fractions represent very small recoveries and full analyses were not possible from the quantities obtained. In particular, the hexane fraction was so small that only C, H and N analyses together with porphyrin determinations were possible. The porphyrin analyses rely essentially on the Q band data, both because it is stronger and because the B band is often obscured by other transitions. These data can be used to determine whether the porphyrin content of these fractions is correlated to the total nitrogen levels. In Fig. 5, the total porphyrin content is plotted against the total level of nitrogen in each of the fractions obtained by homologous
345
Chimica Acta 298 (1994) 341-349
alkane solvent separations. The four successive fractions extracted by C, up to C, solvent yield a good correlation of the porphyrin determination to the nitrogen content. In the remainder fraction from heptane, the porphyrin level is much lower than predicted by the trend of the first four data points. The reason for this decreased porphyrin level in the heptane fraction can be examined in the subfractionation data for bases, acids and neutrals, in Table 2. The porphyrin content tends to be concentrated in the basic subfractions, particularly in those subfractions denoted as either weak or medium bases. The total nitrogen however is found distributed over most of the subfractions. From these observations, the
Bitumen
soluble
basic
base base
base
Asphalienes
base
b-
barn
basic
I
t
Ih Ne
tr.r~ gel
Acids
Saturates
Fig. 3. Experimental
Aromatics scheme for separating
I-
Aromatics asphaltene
Safufiires
fractions from bitumen.
Acids
346
J.A. Warner, B.R. Hollebone/Analytica
Chirnica Acta 298 (1994) 341-349
total porphyrin content of a solvent fraction should correlate to the proportion of nitrogen found in these basic subfractions. This is illustrated by a plot of total fraction porphyrin against the nitrogen content of medium base subfractions, corrected for their relative subfraction yield, in Fig. 6. In this plot, the
porphyrin of the extracted heptane fraction (C,) correlates closely with that of the other fractions for which subfraction data were available. This result shows that the distribution of elemental nitrogen amongst various types of molecules is reasonably constant through fractions soluble in C,
Table 2 Analysis of crude oil fractions Bitumen
Recovery as % bitumen
Fraction
Description
wt.% f 0.5
Description
Dephased oil
34 * 0.3 Weak bases Medium bases Strong bases Acids Saturates Aromatics
Resins
C
wt.% + 0.5
wt.% f 0.8
8 9 1.0 7 69 7
35 + 0.3 Weak bases Medium bases Strong bases Acids Saturates Aromatics
Pentane asphaltenes
Recovery as % fraction
3.0 15 7 21 0 54
4.0 fO.l Weak bases Medium bases Strong bases Acids Saturates Aromatics
15 23 4.0 52 <1
H
N
wt.% +0.1
Porphyrin content MCD Q band
MCD B band
wt.% 4 0.03
wt.% * 0.05
wt.% * 0.05
84.8
10.5
0.15
0.17
0.22
_
_
0.14 0.13 0.00 0.00 O.M3 0.00
0.13 0.00 0.00 0.00 0.28 0.00
66.4 _
8.91 _
52.8 85.6 _
7.65 11.5 _
_ 0.88 _ 1.43 _ -
83.1
9.8
0.44
82.4 71.9 81.4 55.8 _ 78.8
8.37 8.49 9.21 7.03 _ 8.07
0.85 0.99 1.49 0.92 _ 1.17
4.2 2.97 1.56 0.39 0.00 0.41
0.00 0.00 0.00 0.00 0.52 0.00
81.9
9.09
0.76
5.5
0.00
79.3 68.2 _
7.94 7.80 _
0.81 1.00 _ _
8.3 6.8 4.7 0.010 0.00 0.00
1.65 3.13 0.00 0.06 0.00 0.00
2.69
Hexane asphaltenes
1.0 fO.l
80.7
8.53
0.96
10.23
2.28
Heptane asphaltenes
19
80.2
7.94
1.2
1.85
0.00
79.3 78.5 78.0 71.3 84.7 79.0
7.53 6.13 8.14 7.72 11.3 9.25
1.35 0.77 1.44 1.02 _ 0.57
0.99 3.9 2.51 1.0 0.00 4.1
0.00 2.90 0.00 0.84 0.53 4.24
fl Weak bases Medium bases Strong bases Acids Saturates Aromatics
32 14 7.0 6 28 12
J.A. Warner, B.R. Hollebone/Analytica Chimica Acra 298 (1994) 341-349
O.uOOO
0.0003 0.0006 0.0009
347
0.0012 0.0015
[oflWL1 Fig. 4. Calibration curves for MCD quantification porphyrin using both Q and B baud correlations.
4
of vanadyl
0.2
0.4
0.6
0.6
0.9
1.0
[NT1
Fig. 5. The dependence of % total fractions on % total nitrogen.
porphyrin
for successive
to C, solvents. Beyond this point however, the distribution appears to change. This is illustrated in Fig. 7, using all the available subfraction data.
up
4. Discussion The correlation in Fig. 5 shows that an MCD assay of total porphyrin is a good indicator of the total nitrogen content of fractions representing more than 80% of the total synthetic crude oil. The correlation in Fig. 6 shows that the MCD assay continues to be valid when applied to the basic subfraction of the heptane-soluble fraction. The excess nitrogen content of this fraction appears to arise in the high aromatics and acids content, as shown in Fig. 7. This distribution of aromatics into the heptane fraction follows the increasing polarizability of the solvent. The least polar&able solvent, propane, dissolves saturates and small amounts of permanently polar acids and bases. With increasing polarizability, the solubility of saturates drops quickly, that of polar bases and acids rises continuously and the aromatics
Fig. 6. The dependence of % total porphyrin for successive fractions on % nitrogen present as medium bases in each fraction.
348
J-4. Warner, B.R. Holiebone / Analytica Chimica Acta 298 (1994) 341-349
appear to split into two different groups, those soluble at butane and those in heptane. Significantly, the aromatics soluble in butane carry on unusually low nitrogen content, while those soluble in heptane have a nitrogen content ten times higher. Since these substances emerged from the subfractionation process as apparent aromatic neutral species, they are most probably, nitrogen heteroaromatic compounds in which the nitrogen is fully substituted. The nonbasic nitrogen compounds typically found in these synthetic crude oils [22] include the indoles, carbazoles, benzocarbazoles and substituted pyrroles shown in Fig. 8. There may be some complexes of nitrogenous molecules with metals which would also fall into this group but none have been positively identified [23,24]. These conditions suggest that an MCD assay of porphyrins in oils could be used in process control. The scan time for the Q band region is typically 2 to 3 min, depending on the gain required. Thus, the only limitation to real-time, on-line data would arise from sample preparation. Since the results show that porphyrin content correlates linearly to total nitrogen up to the hexane fraction, a rapid, one-step hexane
/\ c) #
indole
carbazole
quinaline (benzo[bIpyridine)
i8OlpiIlOlin0
UJenzoklpyri~e)
acridine
pbenanthridine
Fig. 8. Non-porphyrin crude oils.
C,_ (117)
=I or, ’
A
CIJfW (167)
WW ( 129)
WW
(129)
WW (179)
WW (179)
nitrogen contaminants
commonly
found in
extraction could be performed to prepare a representative sample at a dilution appropriate to good MCD signal to noise. Since this is such a sample operation, it should be readily automated with a robotic extraction apparatus. Presuming this would require only a few minutes, feed stock data could be available to plant operators at 10 min intervals. This would provide the real-time data needed for continued optimization of denitrification operations.
9
References
6 8 POLARIZABILITY
IO 12 14 a OF SOLVENT MOLECULES
Fig. 7. The dependence of % total porphyrin in each fraction on polarizability of each solvent for components of each subfraction.
[l] O.P. Strausz, in D.A. Redford and G.A. Weinstock (Eds.), The Oil Sands of Canada-Venezuela, CIM Special Volume 17, 1977, pp. 146-152. [2] J.G. Speight and S.E. Moschopedis, Adv. Chem. Ser., 195 (1981) 1. [3] CD. Ford, S.A. Holmes, L.F. Thompson and D.R. Latham, Anal. Chem., 53 (1981) 831. [4] Z.R. Ismagilov and M.A. Kerzhentsev, Catal. Rev. Sci. Eng., 32 (1990) 51. [5] M. Dorbon, I. Ignatiadis, J.M. Schmitter, P. Arpino, G. Guiochon, H. Toulhoat and A. Hut, Fuel, 63 (1984) 565.
J-4. Warner, B.R. Hollebone /Analytica [6] J.M. Schmitter, Z. Vajta and P. Arpino, in A.G. Douglas and J.R. Maxwell (Eds.), Investigation of Nitrogen Bases in Petroleum, Advances in Organic Geometry 1979, Pergamon Press, Oxford, 1980, pp. 67-76. [7] Z. Frakman, Y. Ignasiak, D. Montgomery and O.P. Strausz, AOSTRA, J. Res., 3 (1987) 131. [8] J.M. Schmitter, P. Garrigues, I. Ignatradis, R. De Vazelkes, F. Perin, M. Ewald and P. Arpino, Org. Geochim., 6 (1984) 579. [9] J.B. Pedley, R.D. Nayler and S.B. Kirby, Thermodynamic Data of Organic Compounds, Chapman Hall, London, 2nd edn., 1986. [lo] D.A. Skoog and D.M. West, Fundamentals of Analytical Chemistry, Saunders, Philadelphia, PA, 4th edn., 1982, pp. 249-251. [ 1 l] C. Gregoire, personal communication. [12] M. Nagai, T. Masunaga and N. Hanu-oka, Energy and Falls, 2 (1988) 645. [13] B. Holmquist and B.L. Valee, Methods Enzymol., 49 (1978) 149. [14] B.R. Hollebone, Spectrochim. Acta. Rev., 1.5 (1994) 493. [1_5] J.B. Lambert, H.F. Shurovell, L. Verbit, R. Cooks and G.H.
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[16] [17] [18]
[19] [20]
[21] [22] [23] [24]
349
Stout, Organic Structural Analysis, McMillan, New York, 1976. A.H. Whittaker and P. Dyson, Oil Gas J., 28 (1980) 141. I. Ignatiadis, M. Ewald, P. Arpino and G. Guiochon, Org. Geochem., 7 (1984) 111. B. Tissot and D.H. Welte, Petroleum Formation and occurrance, Springer Verlag, Berlin, Part IV, Chap. 2, 2nd edn., 1981. L.W. Corbett and V. Petrozzi, Ind. Eng. Chem. Prod. Rev. Dev., 17 (1978) 342. J.W. Buchler, G. Eikelmann, L. Puppe, K. Rohbock, H.H. Schneehage and D. Week, Justus Liebis Ann. Chem., 745 (1971) 135. M.M. Barbooti, E.Z. Said, E.B. Hassan and SM. AbdulRidah, Fuel, 68 (1989) 84. Z. Frakman, T.M. Ignasiak, D.S. Montgomery and O.P. Strausz, AOSTRA, J. Res., 3 (1987) 131. J.G. Reynolds, W.R. Biggs and S.A. Bezman, ACS Symp. Ser., 344 (1987) 205. Z. Frakman, ‘P.M. Ignasiak, D.S. Montgomery and O.P. Strausz, AOSTRA, J. Res. 4 (1988) 171.