Subscriber access provided by University of Newcastle, Australia
Article
Molecular Selectivity in Supercritical CO2 Extraction of a Crude Oil Xin Cao, Bo Peng, Sutian Ma, Hongxing Ni, Linzhou Zhang, Weilai Zhang, Mingyuan Li, Chang Samuel Hsu, and Quan Shi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00415 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 12, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 18
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
Energy & Fuels
Molecular Selectivity in Supercritical CO2 Extraction of a Crude Oil Xin Cao†, Bo Peng†*, Sutian Ma‡, Hongxing Ni‡, Linzhou Zhang‡, Weilai Zhang‡, Mingyuan Li†, Chang Samuel Hsu§‡, Quan Shi‡* †
(a) Research Institute of EOR, China University of Petroleum, Beijing 102249, China; (b)Beijing Key
Laboratory of CO2 Storage and EOR, China University of Petroleum, Beijing, 102249, China ‡
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China
§
(a) Petro Bio Oil Consulting, Tallahassee, FL 32312 USA, (b) Department of Chemical and Biomedical
Engineering, Florida A&M University/Florida State University, Tallahassee, FL 32310 USA
Abstract Supercritical CO2 flooding has been considered as a promising enhanced oil recovery (EOR) method because it can effectively improve the oil recovery and promote greenhouse gas sequestration. However, the solubility of different petroleum components in supercritical CO2 (SC-CO2) has not been well investigated. This paper presents the molecular selectivity of SC-CO2 extraction on crude oil under different pressures and temperatures. The crude oils were loaded on the surface of kieselguhr and extracted by SC-CO2. The extracts and the residues from the SC-CO2 extraction were analyzed by gas chromatography-mass spectrometry (GC-MS) and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Our results showed that the operating pressure (20-30 MPa) affected the extraction yields more than the temperature (50-70 ºC). SC-CO2 preferentially extracted small molecules with relatively low aromaticity and polarity. Compound classes containing multiple heteroatoms had lower extraction yields than hydrocarbons. The carbon number distribution ranges of various compound classes in the residues were largely different. Carboxylic acids and phenolic compounds were found to have poor solubility in SC-CO2. The risk of asphaltenes precipitation in CO2 EOR is also discussed.
ACS Paragon Plus Environment
Energy & Fuels
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
1. Introduction Supercritical carbon dioxide (SC-CO2, Tc ≥ 31ºC, Pc≥7.3 MPa) flooding has been used as a commercial process for enhanced oil recovery (EOR) since 1970s.[1] The physical properties and chemical composition of crude oil, such as flow and phase behavior changes of the reservoir fluids, would be changed during CO2 flooding. Extraction of small components by CO2 could lead to precipitation of relatively large components.[2] The precipitated organic solids can reduce porosity and permeability, alter rock wettability, and affect well injectability and productivity.[3, 4] Well and pipeline clogging can lead to significant economic losses and operational delays.[5] The organic solid precipitation inside a reservoir is affected by many factors, such as the nature of the reservoir fluids, the properties of the rock, the pressure and temperature, the nature of injection fluids, the nature of formation water and composition/characteristics of crude oil, etc.[6] Allawzi et al.[7] investigated the extraction yields of shale oil by CO2 at temperature of 450ºC and pressure of 22 MPa, with hexane and acetone as co-solvents. Nuclear magnetic resonance (NMR) analysis showed that most of the components in extracts were saturates, olefins, and some aromatics. Guiliano et al.
[8]
studied the effects of temperature (40∼50ºC),
pressure (13∼30 MPa), co-solvents (DCM and toluene) and extraction time. Gas chromatography-mass spectrometry (GC-MS) and Fourier-transform infrared analysis (FTIR) showed that temperature had little effect on extraction, while pressure and co-solvent were sensitive. The extracts contained saturates, aromatics, and polar compounds. Al-Marzouqi et al.[9] reported the effects of temperature and pressure on the extraction yields of crude oil, in which n-alkane distributions in the extract and the residue were analyzed by gas chromatography (GC). Morselli et al.[10] investigated the effects of SC-CO2 extraction on the yields of saturates and aromatics in crude oil at different temperatures pressures, and concentrations of co-solvent (acetone). Ni et al.[11] compared the SC-CO2 extraction with Soxhlet solvent extraction of petroleum. SC-CO2 showed lower extraction yield than DCM Soxhlet extraction, while the SC-CO2 extracts could reserve lighter components since it did
ACS Paragon Plus Environment
Page 2 of 18
Page 3 of 18
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
Energy & Fuels
not require a de-solvent procedure. Srivastava et al.[12] performed the static asphaltene precipitation experiment on an improved pressure-volume-temperature (PVT) apparatus. The asphaltene deposition of three kinds of crude oils was investigated under different pressures, CO2 concentrations, gas purities, and formation water. Dehghani et al.[6] studied the differences of asphaltene deposition in sandstone and limestone in the process of CO2 flooding, such as deposition amount, permeability change, and wettability alternative. Behbahanid et al.[13] investigated the effects of rock composition on asphaltene deposition by scanning electron microscopy (SEM), X-ray, and elemental analysis. Novosad et al.[14] reported the relationship between CO2 injection and oil recovery/asphaltene precipitation. Takahashi et al.[15] investigated the characteristics and impact of asphaltene precipitation in carbonate and sandstone cores during CO2 injection. It has been known that the molecular structure of crude oil has a serious impact on the asphaltene deposition. Ibrahim, et al.[16] studied the asphaltene deposition of three crude oils in CO2 miscible flooding and found that the asphaltene deposition is greatly affected by asphaltene structures. Zanganeh, et al.[17] studied the asphaltene deposition under different pressures, temperatures, and CO2 concentrations in a set of visual experimental apparatus. Their results showed that molecular structure was critical to the asphaltene deposition. Monger et al.[18] studied the effect of crude oil composition (carbon number, acid content, etc.) on deposition in the presence of CO2. Previous studies[18-22] have shown that the injection of CO2 in the reservoir will not only lead to the deposition of nC7 asphaltenes, but also deposited wax, aromatics, and resins. Priyanto et al.[23] considered that the small aromatics (toluene, methylnaphthalene, etc.) in crude oil keep asphaltenes in micellar form, while paraffins keep asphaltenes in colloidal forms. The SC-CO2 extraction on the small saturates and aromatics broke the original equilibrium state and eventually led to the deposition of large compounds. Ni et al.[11] reported
the
molecular
compositions
in
the
SC-CO2
ACS Paragon Plus Environment
extracts
and
their
Energy & Fuels
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
saturate/aromatic/resin/asphaltene (SARA) fractions. However, the molecular selectivity at different extraction conditions was still unclear. In this paper, SC-CO2 extractions on the crude oil were conducted at different temperatures and pressures. The extracts and residues were characterized by gas chromatography-mass spectrometry (GC-MS) and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to reveal the molecular selectivity of petroleum components in SC-CO2 extraction.
2 Experimental 2.1 Materials The crude oil was obtained from Daqing oil field (the largest oil filed in China), which has an elemental composition of 86.54 wt% C, 12.39 wt% H, 0.30 wt% N, and 0.15 wt% S. Its density is 862.8kg/m3 (20ºC), kinematic viscosity is 20.65mm2/s(50ºC), and condensation point is 31ºC. The crude oil was loaded on kieselguhr (100-200 meshes) and extracted by 99.95% CO2. For each gram of oil, it was mixed with 4 grams of kieselguhr. 2.2 Supercritical CO2 extraction Supercritical fluid extraction (SFE) was performed on a Waters MV-10 ASFE System, which was equipped with a CO2 pump, a co-solvent pump, a 25 mL extraction cell and a collector. The schematic of the SCF experimental apparatus could be found elsewhere.[24] Ten gram oil-loaded kieselguhr was loaded into a 25 mL extraction cell. The extraction can be performed in two modes: (1) dynamic extraction where the supercritical fluid is allowed to pass through the extraction cell to extract components out of the sample continuously and (2) static extraction where the sample being extracted is soaked in the supercritical fluid before the extract is taken out after a period of time. Our extraction sequence was as follows: 20 min dynamic, 40 min static, and 20 min dynamic. The 3-stage extraction sequences were repeated for 5 times. In the dynamic mode, the CO2 flow rate was at 6 mL/min. After the extraction, the pipeline was cleaned by one mL/min dichloromethane (DCM). Three extraction
ACS Paragon Plus Environment
Page 4 of 18
Page 5 of 18
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
Energy & Fuels
procedures, namely A, B, and C, were conducted with the extraction sequence mentioned above (20 min dynamic, 40 min static, and 20 min dynamic) at 30 MPa and 50°C, 20 MPa and 70°C, and 30 MPa and 70°C, respectively. It should be noted that the pressure was closed to the upper limit of instrument. The residual oil (residue) remained in the kieselguhr matrix was recovered by Soxhlet extraction using 93/7 (V/V) dichloromethane/methanol co-solvent, the solvent was evaporated in a rotary evaporator. 2.3 SARA fractionation The original oil as well as the extracts and residues of the extraction procedures were separated into saturate, aromatic, resin, and asphaltene fractions using a modified ASTM D2007-93 method (equivalent to Chinese industry standard method SH/T 0509-92). Generally, About one gram oil sample was dissolved in 50 mL n-heptane. The mixture was refluxed for 30 min and stored in the dark for 2 h. The n-heptane precipitate was obtained by filtration using a quantitative filter paper. The n-heptane precipitate on the filter paper was rinsed with n-heptane until the solvent is colourless. The precipitate on the filter paper was washed with toluene. The asphaltenes were obtained by evaporating the toluene from the filtrate solution. The n-heptane soluble fraction, namely maltane, were obtained by vacuum rotary evaporation of the n-heptane solution. The maltenes in n-heptane.were concentrated to about 5 mL. A glass column (50 mm i.d. x 700 mm length) was packed with 40 g of neutral alumina adsorbent (100-200 mesh, activated at 450°C for 6 h, 1 wt% water added). The maltene solution was added on the top of the neutral alumina adsorbent in the glass column. Saturates and aromatics were obtained by eluting the packed column with 80 mL n-heptane and 80 mL toluene, respectively. Forty milliliters of 50:50 (v/v) toluene/ethanol mixture, 40 mL toluene, and 40 mL ethanol were added sequentially to elute the resins. The solvent in each effluent was dried by vacuum rotary evaporation and weighed. 2.4 Simulated Distillation analysis The high temperature simulated distillation (SimDis) analysis was performed on a modified Analytical Controls (AC) Agilent 6890 N GC system. The GC column was an AC
ACS Paragon Plus Environment
Energy & Fuels
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
HT-750 high temperature column (5 m × 0.53 mm internal diameter × 0.18 µm film thickness). The oven was held at 40ºC for 1 min, ramped to 430ºC at 10ºC/min, and then held at 430ºC for 5 min. Helium was used as the carrier gas at a flow rate of 19 mL/min. A flame ionization detector (FID) was maintained at 430ºC. The initial temperature of the programmable temperature vaporizer (PTV) was 100ºC, then programed to 430ºC at 15ºC/min, and held at 430ºC for 22 min. The sample injection was at 1 µL. 2.5 GC-MS An Agilent 7890A GC coupled with 5975C MS equipped with an electron-impact ionization (EI) source was used to analyze the saturates and aromatics. The column used in the GC-MS analysis was a HP-5 MS column (60 m × 0.25 mm internal diameter × 0.25 µm film thickness). The oven was held at 50°C for 1 min, ramped from 50°C to 120°C at 20°C /min, 120°C to 250°C at 4°C /min, 250°C to 310°C at 3°C /min, and then held at 310°C for 30 min. Both the injector and transfer line were held at 300ºC. Helium was used as the carrier gas, with a flow rate of 1 mL/min. The ion source temperature of MS was maintained at 250°C with an electron beam ionizing energy at 70 eV. 2.6 FT-ICR MS A Bruker Apex-Ultra FT-ICR mass spectrometer with a 9.4 T magnet (operating at 9.0 T) was used for determining the molecular composition of heavy components. Positive and negative ion electrospray (ESI) were carried out respectively. The oil sample was diluted in 1:3 toluene:methanol co-solvent with a concentration of about 0.2 mg/mL, which was infused into the ionization source at a flow rate of 180 µL/min. Typical (-)ESI conditions were 4 kV spray shield voltage, 4.5 kV capillary column introduce voltage, and -320 V capillary column end voltage; (+)ESI conditions were -3.5 kV spray shield voltage, -4.0 kV capillary column introduce voltage, and 320 V capillary column end voltage. The ion accumulation time was 0.5-1 s. The optimized time-of-flight (ToF) for the ions extracted from the collision pool to the ICR cell was 1.0 ms. The optimized mass for the quadrupole (Q1) was 200 Da. Atmospheric pressure photoionization (APPI) was also conducted for the analysis of
ACS Paragon Plus Environment
Page 6 of 18
Page 7 of 18
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
Energy & Fuels
petroleum fractions. The operating conditions were as follows: capillary column end voltage, 320 V; skimmer voltage, 30 V. Ions were accumulated for 0.001 s in a hexapole with 2.4 V DC voltage and 400 Vp-p RF amplitude. The optimized mass for the Q1 was 200 Da. Ions accumulation time was 0.5 s. The extraction period for ions from the hexapole to the ICR cell was set to 1.0 ms. The RF excitation was attenuated at 11 dB. The mass ranges for all FT-ICR MS analysis were m/z 200-1000. A total of 128 time domain signals with 4M word size were co-added to enhance the signal-to-noise ratio (S/N). The data processing procedures could be found elsewhere.[25]
3 Results and Discussion 3.1 Bulk properties of the extracts and residues. The yields of extracts from the three extractions are listed in Table 1. No more than three quarters of the crude oil can be extracted by SC-CO2 with the selected operating conditions. High pressure and high temperature correspond to high extraction yields. However, the pressure seems to be more sensitive in the extraction yield than the temperature, this is in agreement with the previous studies.[8, 9] The least amount of extract with the most amount of residue was found in B, the procedure at the lowest pressure. The difference between A and C (3.8%), both at same high pressure with only difference in temperature, is smaller than that between B and C (12.4%) at the same temperature but different pressures. However, the highest yield in extract is obtained at the highest pressure and temperature.
Table 1 Extract yields of three extractions
A B C
Pressure/MPa
Temperature/°C
Yield (wt%)
30 20 30
50 70 70
69.9 61.3 73.7
The SARA compositions of the crude oil, extracts, and residues are listed in Table 2.
ACS Paragon Plus Environment
Energy & Fuels
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
Page 8 of 18
Compared with the original oil and extracts, the content of saturates in the residues decreased significantly, while the relative contents of the other three fractions increased, especially for resins and asphaltenes.
Table 2. SARA composition of the crude oil, extracts, and DCM extractable residual oils. Samples
Conditions (°C/MPa)
Saturates
Aromatics
Resins
Asphaltenes
Crude oil A- Extract B- Extract C- Extract A- Residue B- Residue C- Residue
50/30 70/20 70/30 50/30 70/20 70/30
64.55 82.55 85.78 85.62 18.87 22.65 15.27
20.73
11.87
2.85
13.55
3.60
0.30
12.21
1.81
0.21
11.86
2.27
0.25
40.96
29.16
11.01
34.95
29.92
12.47
40.82
33.24
10.67
The simulated distillation curves of the crude oil and its three pairs of extracts/residues are
shown in Figure 1, with the initial and final boiling points, 30%- and 70%-yield distillation temperatures tabulated in Table 3. Note that the components with boiling points less than 195°C were lost upon the SC-CO2 extraction. The yields of the simulated distillation experiment are also listed in parentheses in Table 3. The extracts contain mostly volatile components which could be eluted through the SimDis GC column, lead to higher distillation yields than that of the original crude oil. The extracts contain mostly volatile components with almost full recoveries except C-Extract that contains about 10% of nonvolatiles not recovered. All of the residues contain almost 20% of nonvolatiles,
ACS Paragon Plus Environment
Page 9 of 18
100
A-Ext (30MPa/50^C) B-Ext (20MPa/70^C) C-Ext (30MPa/70^C) A-Res B-Res C-Res Crude Oil
90 80 70 Yield wt%
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
Energy & Fuels
60 50 40 30 20 10 0 0
100
200
300
400
500
600
700
800
BP(°C)
Figure 1. Simulated distillation curves of the extracts and residues. Table 3 Simulated distillation temperatures (°C). Samples Crude oil A-Ext B-Ext C-Ext A-Res B-Res C-Res
IBP 59 195 215 194 440 465 475
30 wt% yield 350 360 350 370 660 635 665
70 wt% yield 595 510 465 535 730 725 735
FBP (yield, wt%) 750（88.2） 750（96.0） 612（99.5） 750（89.2） 750（79.6） 750（80.1） 750（78.3）
These results showed that the major differences between the extracts and residues were the contents of high boiling heavy components. The heavy components can be extracted by SC-CO2 at higher temperatures and/or pressures, especially at the higher pressures. Therefore, for CO2 flooding, the pressure would significantly affect the extraction efficiency of heavy crude oils. On the other hand, if the pressure is not high enough, heavy components would remain or precipitate, causing deposit problems in the reservoir.
3.2 Molecular composition of the extracts and residues
ACS Paragon Plus Environment
Energy & Fuels
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
The composition and structure of the organic molecules in complex mixtures determine the properties, behavior and reactivity of the mixtures.[26] Difference in mixtures can be determined by their molecular composition even though their bulk physical and/or chemical properties appear to be similar.
3.2.1 Paraffin carbon number distribution characterized by GC-MS We used the m/z 85 mass chromatogram of the GC-MS analysis, shown in Figure 2, to monitor the distribution of normal paraffins in saturates of the crude oil and its SC-CO2 fractions. Due to the temperature limitation of the GC column, the peak measurement can be up to only 40 carbons. The normal paraffin distributions of SC-CO2 extracts are all similar to that of the crude oil. The difference in extraction efficiency is much more apparent in the residues. The n-alkanes with a carbon number less than 23 are not present substantially in the C-residue. The decrease in pressure of 10 MPa in Condition B at the same temperature as Condition C increases the SC-CO2 extraction of n-alkanes with carbon number less than 27, accompanying by the decrease in higher carbon numbers. The decrease in temperature of 20º C in Condition A at the same pressure as Condition C increases the SC-CO2 extraction of n-alkanes with carbon number between 15 and 23. In other words, with increasing temperature and pressure in SC-CO2 extraction, more high boiling n-paraffins would remain in the residues. Therefore, the following conclusions can be drawn: (1) The effect of pressure on SC-CO2 extraction is more significant than temperature; (2) Under more severe flooding conditions, large wax molecules in the reservoir are not easy to be dissolved by SC-CO2 and form deposits. From Figure 2, we could see that increasing temperature mainly influences the extraction on light components, i.e., nC < 25 in this study.
ACS Paragon Plus Environment
Page 10 of 18
Page 11 of 18
16
Relative Concentration%
Normal alkanes
Crude Oil A-Ext (30MPa/50^C) A-Res B-Ext (20MPa/70^C) B-Res C-Ext (20MPa/70^C) C-Res
14 12 10 8 6 4 2 0 10
15
20
25
30
35
40
Carbon Number
Figure 2. Relative concentrations of n-alkanes in the crude oil, extracts and residues. 3.2.2 FT-ICR MS characterization. 80
80 Negative Ions 60
C-Extracts C-Residues
40
10
0
C-Extracts C-Residues
Positive Ions
70
Relative Abundance%
Relative Abundance%
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
Energy & Fuels
60
10
0 N1 N1O1 N1O2 N1O3 N1O4 N1S1
O1 O1S1
O2 O2S1 O3 O3S1
N1
N1O1
N1O2
N1O3
N1O4
N 1S 1
N2
O1S1
O3S1
Figure 3. Compound classes assigned from negative and positive ESI FT-ICR mass spectra for extracts and residues
Figure 3 shows the relative abundances of heteroatom classes in the 70°C/30 MPa SC-CO2 extract and residue determined by the negative and positive ion ESI FT-ICR MS analysis. A total of 12 and 9 classes were detected in the negative and positive ion ESI analyses, respectively. The SC-CO2 extracted oil showed higher abundance of N1 species in
ACS Paragon Plus Environment
Energy & Fuels
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
both ionization modes. The residues had relatively higher abundances in classes containing multiple heteroatoms, such as Ox (x = 1-3), NOx (x = 1-4), NS, Nx (x = 1-2) and SOx (x=1-3). The results showed that classes containing multiple heteroatoms are more difficult to be extracted by SC-CO2. According to the previous studies,[27, 28] the N1 classes detected in negative and positive ion ESI are neutral (non-basic) and basic nitrogen compounds, respectively; O1 and O2 classes detected in negative ion ESI are mainly phenols and carboxylic acids, respectively. Figure 4 compares the abundance plots of double bond equivalent (DBE) as a function of carbon number for aromatics and four heteroatom classes (basic nitrogen compounds, neutral nitrogen compounds, phenols, and carboxylic acids) of the SC-CO2 extracts and residues at different extraction conditions. The composite distributions of these classes were obtained by various ionization techniques that yield unique characteristic ions for each class. The sizes of different color dots in Figure 4 denote the relative abundances of different class species determined by FT-ICR MS. The DBE versus carbon number distribution of the crude oil (not shown) is similar to that of extracts. Almost all the carboxylic acids are aliphatic acids of DBE = 1, with carbon numbers ranging from 14 to 30 in the extracts and from 24 to 35 in the residues. Most of the phenolic components have DBE = 4 and 5, with carbon numbers ranging from 20 to 37 in the extracts and 26 to 45 in the residues. Extractable aromatic hydrocarbons and basic nitrogen compounds have similar DBE distributions ranging from 4 to 16, i.e., 1- to 5-ring aromatics, with a normal distribution. The composition of neutral nitrogen compounds in the extracts is relatively simple: three dominant series with DBE values of 9, 12, and 15 corresponding to carbazoles, benzocarbazoles, and dibenzocarbazoles, respectively. In Figure 4, the crossed red dashed lines at DBE=13 and carbon number=45 separates each extract and residue into four quadrants for the convenience of comparison. The components in the extracts concentrate in the lower left quadrant (low DBE’s and small carbon numbers), while the components in residues concentrate in the upper right quadrant
ACS Paragon Plus Environment
Page 12 of 18
Page 13 of 18
(high DBE’s and large carbon numbers). Clearly, the SC-CO2 extraction was preferred to compounds with small molecule weight (carbon number) and low aromaticity (DBE).
30MPa/70^C 35
20MPa/70^C
NN-N1
Aromatics
BN-N1
40
Phenolics-O1
35 30
25
25
DBE
30
20
NN-N1
Aromatics
BN-N1
35
Phenolics-O1
Aromatics
BN-N1
Phenolics-O1
30 25
20
20
15
15
10
10
10
5
5
5
0
0
0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95
Carbon Number 40
Carbon number 40
C-Residues NN-N1
Aromatics
BN-N1
Phenolics-O1
35
Carbonxylics-O2
Carbon Number B-Residues
NN-N1
Aromatics
BN-N1
40
Phenolics-O1 35
Carbonxylics-O2
30
25
25
20
DBE
30
25
DBE
30
20
C-Residues NN-N1
Aromatics
BN-N1
Phenolics-O1
Carbonxylics-O2
20
15
15
15
10
10
10
5
5
5
0
NN-N1
Carbonxylics-O2
Carbonxylics-O2
15
35
A-Extracts
B-Extracts
Carbonxylics-O2
DBE
30MPa/50^C
40
C-Extracts
DBE
40
DBE
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
Energy & Fuels
0 0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95
Carbon Number
Carbon Number
Carbon Number
Figure 4. Consolidated double bond equivalent (DBE) vs carbon number distributions of different compound classes in extracts and residues. The dot size corresponds to the relative abundance of the component, normalized by the total abundance of each class. Neutral Nitrogen (NN-N1), Phenolic-O1 and Carboxylic acids-O2 were detected by (-)ESI; Basic Nitrogen (BN-N1) by (+)ESI; and Aromatics by (+)APPI. The effects of pressure being more significant than temperature are evident by comparing the three pairs of extracts (upper figures) and corresponding residues (lower figures) at different extraction conditions. The overall distributions of left and right pairs at the same pressure of 30 MPa but different temperatures (70°C and 50°C) are quite similar. On the other hand, the overall distributions are quite different between the left and middle pairs at the same temperature of 70°C, but different pressures at 30 and 20 MPa. The upper and lower limits in carbon number and DBE distributions of various compound classes in the extracts and residues were largely different. For example, in the C-Extract (left in Figure 4), the most abundant basic nitrogen species are in the 45
