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New pathways for Asphaltene Upgrading Using Oxy-Cracking Process Maryam Ashtari, Lante A Carbognani Ortega, Francisco A LopezLinares, Abdellatif Eldood, and Pedro Rafael Pereira-Almao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00385 • Publication Date (Web): 20 May 2016 Downloaded from http://pubs.acs.org on May 22, 2016
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New pathways for Asphaltenes Upgrading Using Oxy-Cracking Process 5 6 7 8
Maryam Ashtari,a,* Lante Carbognani Ortega,a Francisco Lopez-Linares,a,b Abdelatif Eldood,a 9 10
Pedro Pereira-Almao a 1 12 13
a Department of Chemical & Petroleum Engineering, Schulich School of Engineering University of Calgary,
14
Calgary, Canada, T2N1N4
15 16
b. Present address; Petroleum and Materials Characterization Unit, Chevron Energy Technology Company,
17 18 19
100 Chevron Way, Richmond, California, 94801, United States
20 21 2
Abstract 23 24 25
Solvent deasphalting of the bottom of vacuum distillation columns (vacuum residue, VR) is a 27
26
process practiced worldwide. In Northern Alberta, a solvent deasphalting plant was designed to 28
process up to 4000 tons/day of the asphaltenic pitch. Asphaltenes oxy-cracking in liquid phase 30
29
could be a new approach to asphaltenes upgrading and conversion into valuable chemicals. Oxy32
cracking is a combination of oxidation and cracking in basic aqueous media at moderate 34
3
31
temperatures (170-225 °C) and pressures (300-500 psi). This process could act very selectively 36
35
producing smaller amounts of greenhouse gases like CO2, thus being considered environmental 37
friendly. In this work, a mild oxy-cracking treatment of C5-asphaltenes solid from Athabasca 39
38
vacuum residue was investigated. The reaction kinetics and possible reaction mechanism for C541
asphaltenes oxy-cracking in water under alkali conditions were studied. Products solubilized under 43
42
40
different severities were characterized using Fourier Transform Infra-red and Nuclear Magnetic 4
Resonance Spectroscopies, Simulated Distillation, elemental analysis and Ultraviolet-visible 46
45
spectrophotometry to investigate the structure of solubilized products and changes in asphaltenes 48
47
structures after the reaction. A model based on sequential-parallel reactions from the asphaltenes 50
to water-soluble products and CO2 was found to describe the process successfully. Production of 52
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oxidized functionalities like carboxylic acids, their salts, methyl ethers and esters, sulfur-oxidized 53
forms plus phenolics, were determined as the most significant fractions soluble in water. 54 56
5 *
Corresponding Author. Tel:+14032109590. Email address:
[email protected] 60
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Solubilization of asphaltenes in water could also decrease challenges regarding facilities and 5
4
pipelines plugging. 6 7 9
8
Keywords: 10 12
1
Asphaltenes, Oxy-cracking, Solubilization, Acidic functionalities, CO2 13 14 15 17
16
1. Introduction 19
Asphaltenes comprise substantial fractions from the residue of bitumen and heavy oil 21
20
18
distillation.1,2 They are the most massive (high molecular weight), most complex and most polar 23
2
components of heavy oils and bitumen.3,4 Asphaltenes are insoluble in light alkanes like n-pentane, 24 26
25
n-hexane or n-heptane and soluble in aromatic solvents like benzene and toluene.1 Different 28
27
average structures and molecular weights have been proposed for asphaltenes based on their 29 31
30
elemental analysis and their behaviors on Petroleum Processing.
6,7
Two molecular architectures
3
32
are widely accepted nowadays as asphaltenes structures, i.e., the island structure, and the 35
34
archipelago structure. In the island structure, one polycyclic aromatic hydrocarbon (PAH) 36 37
containing in average seven fused aromatic rings is connected to peripheral alkyl chains.8 40
39
38
However, bulk decomposition studies on asphaltenes showed that there could be several aromatic 41 42
cores per asphaltenes molecules linked by alkyl functionalities. These structures, which are defined 43 45
4
as archipelagos are often accepted for large molecular weight asphaltenes.9,10 46 48
47
Asphaltenes presence in crude oils could be the source of problems in upstream and 49 50
downstream facilities. Asphaltenes precipitation plugs wellbores, production lines, and 51 53
52
transportation pipelines. They also stabilize water-oil emulsions in reservoirs, affecting the oil path 54 5 57
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to the surface. Precipitation of asphaltenes is often caused by changing pressure, temperature and 5
4
chemical composition of crude oil during processing and transportation.1,6,11 6 7 8
Large deposits of extra-heavy oil or bitumen are located in the Athabasca region, northern 9 1
10
Alberta. Precipitated n-C7 asphaltenes comprise between 15-20% of Athabasca vacuum residua, 13
12
which are the products of vacuum distillation. One facility for heavy oil upgrading in this region 14 16
15
can produce up to 4000 tons of asphaltenes per day as a result of solvent de-asphalting. 17 19
18
Increased asphaltenes production in heavy oil facilities using total or partial deasphalting, 21
20
led to explore simple and more environmentally friendly alternatives than coking, hydrocracking 2 23
or gasification to dispose of that material.12 Alberta plans to decrease green-house gas (GHG) 24 26
25
emissions by 17 wt. % by 2020. As oil sands account for more than 7 wt.% of current GHG 28
27
emissions, new technologies complying with environmental future standards are desirable.13 29 30 31
Different available technologies for asphaltenes upgrading include delayed coking, 32 34
3
hydrocracking and gasification. Among them, delayed coking is a thermal cracking process usually 36
35
carried out at high temperatures (>400 °C) which converts asphaltenes to distillates; however, the 37 39
38
efficiency is less than 50 % along with the considerable amount of undesirable coke as the 41
40
byproduct. The produced coke contains significant amounts of sulfur and metals, which are highly 42 43
undesirable according to environmental regulations. Therefore, handling coke disposal is one of 4 46
45
the major concerns affecting this process. The distillate value from coking is very little for large 48
47
molecular weight asphaltenes, requiring further expensive hydroprocessing to reach industrial 49 50
standards.14-15 51 52 53 54 5 57
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Hydrocracking processes, which could be one attractive alternative for asphaltenes valorization, 5
4
require high pressures (1200 –2000 psi) and temperatures (380 - 440°C range) making these 6 8
7
processes expensive, leading to catalyst deactivation as result of coke formation over the catalyst 10
9
surface.16-17 1 12 13
Gasification processes, which produce syngas using controlled combustion and water-gas shift 14 16
15
reaction, operate at very high temperatures (800-1800 °C) and pressures (1-10 MPa) with large 18
17
capital investments. Under the current gas prices and recent advances in shale gas production, 19 21
20
these processes would not be attractive at first glance. Additionally, one of the main products of 23
2
the reaction is CO2, and with the new environmental regulations, any new technologies are 25
24
required to produce lower amounts of CO2.18,19 26 28
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A new alternative oriented to creating more economic value from asphaltenes towards light 30
29
products has been investigated in our research group in the last five years, through the combination 31 32
of two reactions; oxidation and cracking (oxy-cracking) processes carried out in aqueous alkaline 3 35
34
media. The present work will focus on the kinetics involved and products nature; previous reports 37
36
from the group addressed detailed composition of selected products 38
20
and production of light
39
distillates via hydro-processing 21. Via oxy-cracking, solid asphaltenes are converted to products 40 42
41
like carboxylic and naphthenic acids and their corresponding salts among others products, through 4
43
a controlled oxidation reaction on basic media; the reaction occurs at moderate temperatures (17045 47
46
225 °C), pressures (300-500 psi), with the pH range from 8-10. The out coming products from 49
48
oxy-cracking can be used as raw material for organic synthesis and petrochemical processes. By 50 51
maximizing solubilization, the selectivity to other products such as CO2 is reduced considerably 52 54
53
and as a result, the process could be considered environmentally friendly. Processed products 56
5
obtained in aqueous phase decrease challenges regarding asphaltenes clogging and transportation. 57
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The oxy-cracking process is inspired on asphaltenes ozonolysis and oxidation studies reported 5
4
in the literature.22-25 As an example, a bubble reactor was utilized for ozonisation of asphaltenes 6 8
7
extracted from coal. The reaction occurred in chloroform at 20 °C for 30 hours, using 2.5% ozone 10
9
in air.23 Characterization of filtered products showed that they contained 22.5% oxalic acid. The 1 12
by-products of the reaction were succinic acid, poly carboxylic acids, and salicylic acid. Cleavage 13 15
14
of carbon-carbon double bonds and aromatics framework of the asphaltenes during ozonolysis 17
16
forms ozonides, followed by radical oxidation and decomposition of ozonides to carboxylic acids 18 19
and ketones was proposed as primary pathway.22 20 2
21
In the same order of ideas, other authors dissolved extra heavy Hamaca crude oil in carbon 23 24
tetrachloride and treated it with ozone for a few minutes. Reduction in molecular weight and 25 27
26
aromaticity with increasing oxygen in the product were the significant findings. The products were 29
28
mainly carboxylic acids, aldehydes or ketones. The products surface tensions were compared with 30 31
commercial surfactants, and it was concluded that the products could be suitable alternatives for 32 34
3
commercial surfactants for oil production.23 36
35
The oxidation of Athabasca bitumen using acidic and alkaline peroxides for 30 hrs at 37 38
ambient temperature and pressure has been reported.24,25 The results showed that despite low 39 41
40
conversions (less than 10 %), phenolic and carboxyl groups were formed on the asphaltenes 43
42
molecules. As plausible reaction pathway, the naphthenic moieties were aromatized, and reactive 4 46
45
methylene groups were oxidized to ketones, which at longer reaction periods were further 48
47
converted to carboxylic acid functions. Interestingly, sulfonation and sulfomethylation reactions 49 50
were not possible for the virgin asphaltenes, however, after oxidation, sulfonation reaction 51 53
52
proceeded and converted asphaltenes to materials applicable as drilling mud thinners, according 5
54
to the researchers findings.24,25 57
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Oxidation of asphaltenes recovered from froth treatment process using oxidizing agents at 5
4
25-95 °C and ambient pressure has been recently reported.26,27 Reduction in molecular weight up 6 8
7
to 50 % and aromatic rings cleavage up to 50 % were the primary findings. The resultant product 10
9
showed lower boiling point and viscosity, which could be used as fuel oil or as a solvent, according 1 12
to the authors. 13 15
14
Using oxidation for enhanced oil recovery purposes was also reported in another patent. Ozone 17
16
was injected to tar sand; then the material was flushed with water-caustic- ionic surfactant solution 18 20
19
to solubilize bitumen in water and improve the wettability of sand. The inventors claimed that by 2
21
this process, bitumen could separate from the sand and move through the reservoir.28 23 24
Desulfurization using oxidation was investigated in another patent. Hydrocarbon feeds like fuel 25 27
26
oil plus water soluble oxidant and water soluble catalyst were oxidized at 70 °C and ambient 29
28
pressure for a few hours, to achieve the desired desulfurization. However, the inventors did not 30 32
31
explain the achieved desulfurization level, being the process used as pre-upgrading before 34
3
desalting and de-asphalting.29 36
35
Meanwhile, asphaltenes obtained from solvent de-asphalting of tar sands were treated with 37 39
38
superheated water containing sodium carbonate salts plus ferric oxide at the concentration from 1 41
40
to 10 wt. % and temperatures of 300 °C to 425 °C, with residence times between 1-3 hours. The 42 4
43
water/carbonate treated asphaltenes showed lower average molecular weight, heteroatom removal, 46
45
and less aromatic compounds. The resultant product was an oil-like fraction and could be upgraded 48
47
using conventional distillation techniques.30 49 51
50
Even though different attempts have been reported to convert asphaltenes through oxidation reactions, 53
52
there are not published reports on products characteristics, kinetics and mechanistic studies regarding n-C5 5
54
asphaltenes oxidized with pure oxygen in aqueous alkaline media. Previous published works described the 57
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use of ozone or other oxidation reagents applied to whole bitumen or heavy oils feedstocks. Ozonolysis 5
4
proved to be a severe reaction leading to aromaticity decrease, contrary to what will be discussed pertaining 7
6
aqueous oxy-cracking, where increased aromaticity and different products characteristics were found. 8 9
Furthermore, upgrading asphaltenes in liquid phase water at high temperature and pressures (supercritical 10 12
1
conditions) has been reported; however, operation under moderate temperatures and pressures in 14
13
aqueous alkaline conditions has not been reported in the open literature. Process conditions were 16
15
set up to achieve total conversions spanning from 25-100 %. Experiments were carried out at 17 19
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temperatures varying between 180-250 °C and residence times from 0-2 hr range, under constant 21
20
pressure of 500 psi to keep water in liquid phase. The minimum pressures to keep water in liquid 2 23
state are 145 psi and 371 psi respectively at 180 °C and 225 °C according to Antoine equation.31 24 26
25
Gas and liquid products were characterized using gas chromatography, simulated 28
27
distillation analysis, NMR, FTIR and elemental analysis to find the nature of formed compounds 29 31
30
at different reaction severities, particularly those solubilized in the liquid phase. Based on the 3
32
characterization results, a reaction kinetics model has been proposed as well as a potential 34 35
mechanism for asphaltenes oxy-cracking in the liquid phase. 36 37 39
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2. EXPERIMENTAL 41
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2.1. Materials. The asphaltenes were extracted from Athabasca bitumen vacuum residue (ABVR) 42 4
43
following IP.143 standard, using industrial grade pentane as a precipitant solvent. NaOH solution 46
45
(5N), was used as solubilization enhancer. HNO3 (70%, ACS reagent), thioacetamide (ACS 47 48
reagent), ICP cobalt standard solution (993 ppm Co, 1 wt. % HNO3), CS2 (carbon disulfide), 49 51
50
Toluene (ACS reagent > 99.9 %), chloroform (ACS reagent >99.8 %), and MeOH (ACS reagent 53
52
>99.9 %), all of them from Sigma-Aldrich (Oakville, Ontario, Canada) and tetrahydrofuran, (Omni 54 5 57
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Solv, 99.9%) were also used in experiments as received. Deionized water was used as reaction 5
4
medium. Oxygen, 99.993 % purity, (Praxair, Canada) was used as the reactant gas. 6 7 8
2.2 Experimental procedures and apparatus. Preliminary experiments were done using one 9 1
10
batch stainless steel model 4598 Parr reactor (Parr Instrument Company, Moline, Il, USA). The 12 13
reactor depth was 4.6 inches with inside diameter of 1.3 inches, and its volume was 100 ml. In all 14 16
15
the experiments the asphaltenes mass was 0.5 g and the water weight was 20 g. As a result of 18
17
highly acidic media derived from oxidation reactions, 1 ml NaOH solution (5N) was added to 19 21
20
neutralize the final liquid product. Also, a glass liner was used to prevent corrosion on the reactor 23
2
walls. 24 26
25
As oxidizing gas, pure oxygen was used. Excess O2 was used for avoiding this reagent to 28
27
become limiting. Temperature, mixing rate and pressure was controlled using the 4598 Parr reactor 29 30
controller. In each experiment, it took about 40 min for the feed to reach the set point temperature, 31 3
32
and then the reaction was carried out for the selected reaction time. After each reaction test, the 35
34
reactor was cooled down, and gas chromatography was conducted for the gases produced. Liquid 36 38
37
effluents were filtered (Whatman #2 filter paper), and TOC (Total Organic Carbon, see ensuing 40
39
section) evaluation was carried out for the liquid product. After rotary evaporation had removed 41 42
the water, elemental analysis was performed for the dried recovered solid. 43 4 46
45
2.3. Analytical methods 47 48 49
Analytical characterization was conducted on the feedstock asphaltenes; water solubilized 50 52
51
asphaltenes (WSA) and the remaining solid asphaltenes after the oxy-solubilizing reaction with 54
53
the ensuing procedures. 5 57
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2.3.1 Fourier Transformed Infrared spectroscopy (FTIR). FTIR spectra in diffuse reflectance 5
4
mode were recorded on a Nicolet 6700 spectrometer (Thermo Scientific, Waltham, MA USA) with 6 7
a spectral resolution of 8 cm-1, over the range of 4000-500 cm-1. The background (pure KBr) was 10
9
8
collected every two hours, being all spectra baseline corrected. Water solubilized asphaltenes were 1 12
brought to dryness using a rotary evaporator and a vacuum oven, to avoid water contribution to IR 13 15
14
spectra. 16 18
17
2.3.2 Elemental Analysis. For analyzing C, H, N contents a combustion method using a Perkin 20
19
Elmer 2400 CHN (Waltham, Massachusetts, USA) analyzer was followed. S and N contents for 21 23
2
organic materials were determined with an Antek 9000 system (Houston, TX, USA) by running 25
24
toluene solutions (10% wt/vol.). Calibration was performed with Accustandard IS-17368 (N) and 26 27
Accustandard SCO-500x (S) standards. Calibration for aqueous solutions was achieved with 28 30
29
thioacetamide. The relative standard deviations for measurements were: 0.7% (carbon), 5% 32
31
(hydrogen, sulfur and nitrogen) 3 34
For metal analysis, Microwave assisted acid digestion experiments were carried out in a 35 37
36
commercial unit model MARS 6 from CEM Corporation (Matthews, NC, USA), provided with 39
38
UltraPrep vessels of 100 ml capacity and MARSXpress DuoTemp controller. The system was 40 42
41
operated at a frequency of 2.45 GHz at 100% of full power (maximum of 1600 W). WSA sample 4
43
(0.1g-0.5 g weighted to the nearest 0.1 mg) was placed in the vessel, and 1 ppm cobalt standard, 46
45
(micro pipetting 25.2 μL, 25.1 mg, from 993 ppm Co in 1 wt. % HNO3) was added. Finally, nitric 47 49
48
acid, 70%, was added with a manual siphon pump (10.5 mL). After finishing digestion, the sample 51
50
was cooled down to around 25 °C, the carousel was transferred to a fume hood, the holder was 52 53
removed, and caps slowly loosen taking care that vessel cap holes face towards inside of fume 54 56
5
hood while releasing the brown NOx gas mixture. Metal concentrations in the samples were 57
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determined by ICP–AES using an IRIS Intrepid II XDL, from Thermo-Instruments Canada, Inc. 5
4
(Ontario, Canada). The relative standard deviation for metal analysis was 10%. 6 8
7
The elemental analysis for Athabasca bitumen vacuum residue nC5 asphaltenes was: C: 81.18 10
9
wt%, H: 8.25 wt%, N: 1.24 wt%, S: 8.00wt%. For the whole vacuum residue (ABVR) was: C: 1 12
82.30 wt%, H: 9.72 wt%, N: 0.62 wt%, S: 5.31 mg/kg, V: 265 mg/kg, Ni: 115 mg/kg. 13 15
14
2.3.3 Total Organic Carbon Analysis. A TOC-Vcp equipment from Shimadzu (Tokio, Japan) 17
16
was used to determine the content of the aqueous soluble organic and inorganic contents of carbon 18 19
present in the WSA samples. All the measurements were performed in triplicate using the average 20 2
21
of measurements for calculations. The relative standard deviation for analysis was 5%. 24
23
2.3.4 Gas Chromatography analysis. Compositional analysis of the reaction gas phase was 25 27
26
carried out by using a gas chromatograph (SRI Model 8610C, Menlo Park, CA, USA) fitted with 29
28
a Thermal Conductivity Detector (TCD) and two packed columns connected in parallel, a 31
30
Molecular sieve 13X and a Hayesep-D from SRI Instruments. 32 3 34
The amount of CO2, O2, CO, CH4, C2H4 and other low molecular weight hydrocarbons (less than 35 37
36
seven carbon atoms) in the gas effluents from the reaction, were measured five times at the end of 39
38
each test run, and the average of measurements was used for calculations. The relative standard 40 42
41
deviation for analysis was 5 %. 43 45
4
2.3.5 1H and 13C Nuclear Magnetic Resonance (NMR) spectroscopy. Liquid state NMR spectra 47
46
were determined for dried WSA samples in a Bruker 600 MHz spectrometer. Spectra were 48 49
acquired with D2O solvent, a 5mm internal diameter probe and experiments ran at 298ºK. 1H-NMR 50 52
51
spectra were taken with a pulse sequence zg30, with a relaxation time of 2 sec and adding 160 53 54 5 57
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scans/ run. 13C-NMR spectra were run with a zgig pulse sequence, the relaxation time of 10 sec 5
4
and addition of 14,100 scans/run. 6 7 8
WSA CP-MAS 9
13
C-NMR spectra were acquired with a Bruker AMX300 instrument 4mm BL4
1
10
solid probe, cross polarization program, spin rate at 8K, NS=2k, D1=4s, and mixing time= 50 ms 12 14
13
2.3.6 Ultraviolet-visible (UV-Vis) spectrophotometry. The water-solubilized asphaltenes were 15 16
analyzed using Evolution 260 Bio UV-Visible model (Thermo Scientific, Ontario, Canada) to find 17 19
18
E4/E6 ratio absorbance (400/600 nm wavelengths). Deionized water for sample dilutions was used 21
20
for spectra acquisition. The relative standard deviation for analysis was 5%. 2 24
23
2.3.7 Simulated distillation. An Agilent 6890N gas chromatograph (Agilent Technologies, Santa 25 27
26
Clara, CA, USA) provided with an autosampler, and the automatic injector was used for HTSD 29
28
analysis. Experimental conditions are described elsewhere.32 Aqueous solutions were analyzed 31
30
following 32
routine
operational
conditions.
However,
samples
were
dissolved
in
34
3
water/tetrahydrofuran (THF):80/20: vol/vol and blanks with deionized water between sample 36
35
injections were set in standard conditions in all analysis. 37 39
38
2.3.8 TLC-FID chromatographic separation. One Iatroscan model MK-6 chromatograph 40 42
41
provided with synterized silica chromarods model S-III (Iatron Laboratories, Japan) was used for 4
43
feedstock asphaltenes and water solubilized materials separation. Solution concentrations were 46
45
about 1% wt/vol. The feedstock was dissolved in a mixture toluene/chloroform: 8:2:vol. 1μl 47 49
48
solution was spotted over the rods, left to dry in a gas hood, and then the rods were sequentially 51
50
developed with 1). 10 cm CHCl3; 2). 6 cm CHCl3/MeOH:80/20%: vol./vol.; 3). 3 cm 52 53
CHCl3/MeOH:50/50%:vol./vol. 54 5 57
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3. Results and discussion. 4 6
5
3.1 Reaction kinetics. As it was mentioned before, oxy-cracking is the result of combining 8
7
oxidation and cracking (oxy-cracking) processes in the aqueous phase at mild conditions. Solid 9 1
10
asphaltenes convert to products like carboxylic and naphthenic acids and their corresponding salts 13
12
among others minor products. The process conditions were set up to achieve the maximum yield 14 15
and selectivity towards WSA and less amount of CO2 during the oxy-cracking reaction. Initial 16 18
17
assessment of the oxy-cracking reaction along as reaction kinetics is discussed in detail in the 20
19
present article. A detailed study of the process conditions such as the effect of temperature, 21 2
residence time, pressure, stirring rate and asphaltenes/water ratio over conversion yield and 23 25
24
selectivity will be discussed in an ensuing article. 26 28
27
From the experimental results, it can be seen that consumed oxygen is not only utilized for 30
29
converting the asphaltenes to carbon dioxide, but also a good proportion is converted to water31 3
32
soluble material which contains carboxylic, sulfonic and phenolic functionalities as will be shown 35
34
in ensuing sections. Asphaltene particles initially suspended in water were slowly dissolved with 36 37
little quantities of CO2 produced at early stages of the reaction. Under severe reaction conditions, 38 40
39
i.e., longer residence time and higher temperature, these "soluble asphaltenes” further react with 42
41
oxygen, producing CO2. The schematic presented in Figure 1 describes the reactions taking place 43 4
in the studied oxy-cracking process. 45 47
46
Our results showed that no reaction occurred at temperatures lower than 150 °C. For finding 48 49
reaction orders and kinetic constants, conversion and selectivity to water solubilized asphaltenes 50 52
51
(WSA) and CO2 were calculated using TOC and GC measurements at conversion levels from 25% 54
53
to 100% obtained at temperatures ranging from 180°C- 225 °C with residence times of 0-2 hrs. 5 57
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Considering that asphaltenes have complex structures, conversion, and selectivity to products were 5
4
calculated based on carbon mass in reactants and products of the reaction. The carbon input to the 6 8
7
reactions comes from the elemental analysis from the initial feedstock: 9 10 1 12
Carbon amount before reaction = asphaltene mass × carbon % in feed 13 14 15 17
16
Equations (1-3) show how carbon conversion and selectivity to products could be calculated. 18 19
Keys for identifying fractions are organic components in the liquid phase, which comprise: 20 2
21
product B: WSA 24
23
product C: CO2 in gas phase + CO2 in liquid phase (carbonates) 25 26 27 29
28
𝒕𝒐𝒕𝒂𝒍 𝒄𝒐𝒏𝒗𝒆𝒓𝒔𝒊𝒐𝒏 = 30
(𝑶𝑪 (𝑾𝑺𝑨)+𝑰𝑪(𝑾𝑺𝑨)+𝑪𝑮 )×𝟏𝟎𝟎
(1)
𝑪𝑭
31 32 34
3
𝑺𝒆𝒍𝒆𝒄𝒕𝒊𝒗𝒊𝒕𝒚 𝒕𝒐 𝒑𝒓𝒐𝒅𝒖𝒄𝒕 𝑩 = 35
𝑶𝑪(𝑾𝑺𝑨) 𝑶𝑪(𝑾𝑺𝑨)+𝑰𝑪(𝑾𝑺𝑨)+𝑪𝑮
(2)
36 37
𝑰𝑪(𝑾𝑺𝑨)+𝑪
𝑮 𝑺𝒆𝒍𝒆𝒄𝒕𝒊𝒗𝒊𝒕𝒚 𝒕𝒐 𝒑𝒓𝒐𝒅𝒖𝒄𝒕 𝑪 = 𝑶𝑪(𝑾𝑺𝑨)+𝑰𝑪(𝑾𝑺𝑨)+𝑪
39
38
𝑮
(3)
40 41 42 43 4 45
CO2 48
47
46
𝑃𝑉
𝐺𝑎𝑠
𝐶𝐺 = 𝑅𝑇 × 12
Liquid
TOC
Other gas contents were determined small, thus neglected.
(4)
50
49
Where OC(WSA) is organic carbon in WSA, IC(WSA) is inorganic carbon in WSA, CG is carbon 52
51
content in gas product and CF is carbon amount in feedstock. 53 54 5 57
56
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For conversion calculation parameters such as temperature, residence time, stirring rate, pressure 5
4
and water/ asphaltenes ratio were studied. Both temperature and residence time proved to be those 6 8
7
most relevant. Preliminarily parts of the results are disclosed in the supporting information (see 10
9
S1, S2). However, this will be discussed in detail in an upcoming article, as this is out of the scope 1 12
of the present work. 13 14 16
15
On Figure 2, the carbon selectivity to products B and C is plotted as a function of asphaltenes 18
17
conversion. The selectivity to product B ranged from 29 to 84% while the asphaltenes total 20
19
conversion was higher than 26%. The selectivity to product B dropped while it increased for 21 23
2
product C as a function of conversion, i.e., under higher temperatures and residence times. 24 26
25
These findings corroborate that a continuous supply of oxygen avoids hot spots over the 28
27
particles conducting to a moderate (partial oxidation) of asphaltenes, thus favoring solubilization 29 31
30
over combustion. In the same direction, at low conversion (C20) contain carbon backbones large enough to prevent their solubilization in 19
18
aqueous bases.36 20 2
21
For determining whether or not NaOH exerts additional effects over the oxy-cracking process other 24
23
than acids neutralization, a blank reaction without oxygen was performed at 200ºC using 1 ml 25 27
26
NaOH (5N) in an inert gas (He) for 2 hours. No change in asphaltenes mass and pH of the liquid 29
28
(pH=12.3) was observed. However, the aqueous phase showed a bright yellow color and the TOC 31
30
analysis showed total carbon content of 216 mg/l, which indicates that only small amounts (about 32 34
3
2%) of asphaltenes were able to dissolve in the liquid phase. Therefore, it is possible to conclude 36
35
from the preceding that sodium hydroxide in the absence of oxygen is not solubilizing important 37 38
amounts of asphaltenes, because the content of oxygen functionalities in naturally occurring 39 41
40
asphaltenes is very low for compensating their large carbon backbones. Combined presence of O2 43
42
and NaOH generated the oxygen functions and their Na-salts that can provide the right balance 4 45
(carbon functions/ polar functions) able to produce asphaltenes solubilization in water. 46 48
47
Recently, mass spectrometry analysis of high converted materials derived from asphaltenes oxy50
49
cracking using KOH showed that solubilized materials contained in average 8 oxygen / 20 carbon 51 52
atoms per molecule.20 The feedstock asphaltenes contained about 0.5 oxygen/40 carbon atoms per 53 54 5 57
56
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average molecule, as calculated from their elemental analysis. The preceding findings suggest that 5
4
asphaltenes oxygen incorporation could have a dramatic effect over their aqueous solubilization. 6 8
7
3.3. FTIR analysis of WSA products and parent asphaltenes 9 1
10
FTIR analysis was carried out for the original asphaltenes, insoluble remaining solids after 12 13
reaction and water solubilized products obtained from experiments ran at different severities to 14 16
15
unravel the chemical functionalities present in these materials. Figure 6 illustrates the results that 18
17
were pooled into two sets for better appraisal. Panel A shows the trend occurring from the original 19 20
feedstock towards the unconverted non-soluble solid and the soluble portion existing after 21 23
2
processing under high severity conditions (94% carbon conversion). 24 26
25
The presence of hydrocarbon functions, i.e., alkyls (~3,000-2,800 cm-1) and aromatics (~3,050 27 28
and 950-750 cm-1) is observed for the non-converted solid, which resembles the original feedstock. 31
30
29
The water soluble material was found dramatically different, having high absorption in regions 3
32
corresponding to oxygenates like the –OH area (~3,700 to 2,700 cm-1), C=O (2,550 cm-1, possible 34 36
35
aldehyde functions, and 1,700 cm-1, typical of carboxylic acids) and O=S=O (~1,150 cm-1). 38
37
Hydrocarbon functions are difficult to observe for water solubilized asphaltenes (WSA), being the 40
39
aromatic out of plane bands non-existing anymore (950-750 cm-1) and the alkyls barely visible 41 43
42
(~3,000-2,800 cm-1). Aromatics can be present (C=C stretching at 1,600 cm-1). However, the 45
4
signals corresponding to these species overlap with the strong doublet appearing in 1,400/1,600 46 47
cm-1, assigned to carboxylate anions. Presence of esters (~1,750cm-1)35,37 is possible also. 48 50
49
Formation of carboxylic salts and esters has been reported by others studying the reaction of 52
51
asphaltene with diazotized ethyl-p-aminobenzoate.38 53 54 5 57
56
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Figure 6, Panel B presents a comparison of FTIR bands for the original feedstock and WSA 5
4
products obtained under medium and high severity conditions (42% and 90% carbon conversion). 6 8
7
The progress of formation of carbon-oxygen (fig. 6 signals 1, 4, 5, 6,7) and sulfur-oxygen (fig.6 10
9
signals 8, 9, 10) functionalities as a function of oxidation severity is clearly put into evidence with 1 12
these results. The data suggests the existence of both protonated carboxylic acids (signals 1, 6) 13 15
14
plus their neutralized carboxylate derivatives (signal 7). To confirm these findings, the pH of the 17
16
aqueous WSA solution was brought to ~1, thus being able to separate the protonated free acids, 18 19
species not soluble in water from the absence of the carboxyl salts that facilitate their dissolution 20 2
21
process. 23 25
24
These aspects are illustrated with the FTIR spectra plotted in Figure 7. The presence of 26 27
carboxylic salts suggests first an important leading factor for solubilization in water, second that 28 30
29
the solubilized carboxylate salts possibly can display potential surface active properties. The 32
31
presence of the C=C stretching band in 1,600 cm-1 for both, the free acids and their salts, indicates 3 34
that aromatic moieties probably do not react during the oxy-cracking process in comparison to the 35 37
36
alkyl moieties, findings similarly published in others studies related to coal oxidation.35,39,40 These 39
38
aspects will be further discussed later in connection with NMR spectroscopy data. 40 41 42
3.4 Elemental analysis. C, H, N contents were determined for the feedstock and several non43 45
4
soluble unconverted solids and WSA materials. Figure 8 presents the results. The three elements 47
46
are observed to decrease for all the processed samples compared to the original feedstock. The 48 50
49
unconverted solids were found to have more of the three elements, compared to WSA materials. 52
51
The findings indicate that the decreasing of C, H, N content for WSA shows a continuous trend; 54
53
however, the unconverted (non-water soluble) solids show a sudden decrease occurring under high 5 57
56
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severity conversion, aligned with the massive production of CO2, indicating that at such conditions 5
4
combustion is the primary reaction rather than oxygen incorporation. 6 7 8
One highly converted sample (97% carbon conversion) was analyzed, being oxygen determined 9 1
10
as the difference of 100%. Figure 9 presents the results indicating that about 62 wt% of this WSA 13
12
sample is composed of oxygen+sodium. These findings suggest that the decreasing trends 14 16
15
observed before (Figure 8), can be in part ascribed to a dilution effect from the presence of these 18
17
two new elements, inexistent in significant quantities in the feedstock. Oxygen content comes from 20
19
oxidation reactions occurring during asphaltenes oxy-cracking processing while sodium was 21 23
2
introduced at the beginning of the reaction (about 16 wt% of Na as NaOH) to neutralize the 25
24
produced acids and avoid damaging the reactor walls and enhance solubilization in the aqueous 26 27
phase. 28 29 31
30
3.4. Nuclear Magnetic Resonance Spectroscopy (NMR). NMR has been helpful for the study of 3
32
–OH and –COOH groups in Athabasca asphaltenes after these groups were methylated, 41 as well 34 35
as, solution or solid state NMR have been used for the characterization of lignin, soils and humic 36 38
37
substances.42-45 Combined results from high-resolution MS and advanced NMR techniques have 40
39
proven successful for identification of possible isomeric forms of oxidized aromatics.46-47 With 41 42
that information in mind, spectra acquired with a high-resolution spectrometer (600 MHz) would 43 45
4
allow the separation of many resolved signals observed in both H and C modes. Additionally, 47
46
using available commercial NMR simulator software,48 helped to assign most of the structure types 48 50
49
occurring at different frequencies. Solution proton and carbon NMR spectra were taken for WSA 52
51
samples. Panels A, B from Figure 10 respectively present 1H, 13C- NMR results for a high severity 54
53
produced WSA (97% carbon conversion). 5 57
56
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Typical functional chemical shifts in 1H-NMR such as terminal alkyl methyl (0.9 ppm), alkyl 5
4
methylene groups (2.0 ppm), methylene bonded to the aromatic groups (2.7 ppm) as well as 6 7
aromatic molecules (7-8 ppm) has been observed in asphaltene49 and/or dissolved aquatic organic 10
9
8
materials.50 The assignation of the major oxygenated functional groups was possible through 12
1
previous reports;50-51 methyl group from aromatic ester (2.3 ppm), methoxy group bonded to 13 15
14
mono-aromatic (4.0 ppm). The oxygenated aromatic compounds were located from 6-9 ppm as 17
16
follows: methoxy-aromatic carboxylic acid type molecules (6.0 ppm), methoxy-phenol type 18 19
molecules (6.4 ppm) as well as diaromatic carboxylic acid sodium salt type molecules with a strong 20 2
21
signal around 8.2 ppm.48 23 25
24
Meanwhile, the 13C NMR spectrum (Figure 10B) shows multiple signals, characteristics of methyl 27
26
and methylene groups bonded to aliphatic chains (10- 30 ppm), methylene group bonded to the 28 30
29
aromatic molecules (30-40 ppm) as well as alkyl substituted aromatic carbons/internal carbons of 32
31
condensed aromatic compounds (130-145 ppm).52 The oxygenate groups such as methyl ester (45 3 34
ppm), methoxy aromatic (60 ppm), mono aromatic methoxy acids and mono aromatic methoxy 35 37
36
groups (150-160 ppm), diaromatic carboxylic acids and corresponding sodium salts (170-190 ppm 39
38
) and finally monoaromatic carboxylic acids around 220 ppm.48, 52, 53 40 42
41
These signals are typical oxidation products from hydrocarbons; the presence of acids and 43 4
their salts discussed before based on FTIR spectroscopy was confirmed by these NMR results. 45 47
46
Solution NMR shows the presence of methyl esters of the carboxylic acids and the methyl ethers 49
48
derived from phenols, functionalities not easy to identify with other techniques. From the 50 52
51
preceding, production of carboxylic acids and phenols during oxy-cracking is again confirmed, as 54
53
it was formerly evidenced by FTIR spectroscopy. The methyl groups attached to the acidic 5 57
56
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functionalities is speculated to come from the scission of alkyl appendages from the original 5
4
asphaltenes; functionalities observed to disappear as a function of oxy-cracking conversion as 6 8
7
monitored via FTIR spectroscopy. 9 1
10
Solid state 13
12
13
C-NMR analysis was carried out for dried WSA samples. Figure 11 shows the
spectrum taken for a highly converted WSA sample. Signals were assigned according to Siskin et. 15
14
al.54 Signals distribution following the methodology is presented in Table 3 for WSA samples 16 18
17
produced under different severity. The most important feature highlighted with the results shown 20
19
in Table 3 is increased aromaticity (fa' parameter) at the expense of aliphaticity (fal), results 21 2
determined as a function of higher carbon conversions set up during oxy-cracking. These findings 23 25
24
agree with the general information derived from FTIR spectroscopy, i.e., aromatics were observed 27
26
to survive the oxy-cracking process (FTIR), resulting in increased relative amounts within WSA 28 30
29
products as shown with the data derived from the NMR spectra here discussed. 31 3
32
3.5 Relative size of WSA samples determined via UV-Visible spectrophotometry. For having 35
34
insights into average molecular weights of WSA products, samples produced under different 36 37
severities were analyzed using UV-Visible spectrophotometry (UV-Vis). This technique is a 38 40
39
common method used in soil science for evaluating molecular weights of humic substances. 42
41
Molecular weights are correlated with the E4/E6 ratio, determined as the ratio of sample absorption 43 4
at 400 and 600 nm.55 According to Chen et. al., the E4/E6 ratio of humic materials is inversely 45 47
46
proportional to their molecular weight. They also found that E4/E6 ratios were not concentration 49
48
dependent, however, were found depending on the carbon and oxygen contents, pH and amount 50 51
of -COOH groups.56 E4/E6 ratios for humic acids have been determined to be in the 3-5 range and 52 54
53
for fulvic acids in the 5-8 range.55 5 57
56
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Figure 12 shows the E4/E6 ratios for WSA products obtained under increasing carbon 5
4
conversion conditions. The results revealed that for samples obtained with carbon conversions 6 8
7
lower than 92%, the E4/E6 ratios felt below 5, which is characteristic of humic acids or, put into 10
9
other terms, High Molecular Weight (HMW) components. For carbon conversions >92%, Low 1 12
Molecular Weight (LMW) components possibly analogs of fulvic materials were observed, 13 15
14
spanning E4/E6 ratios from 6-12. The higher E4/E6 ratios obtained beyond typical values for 17
16
humic substances are deemed reflecting the fact that the origin of the present WSAs is not the same 18 19
as the organic matter deposited and oxidized in soils. Figure 12 also shows the visual aspects of 20 2
21
the studied samples, clearly showing that less converted materials have the stronger absorbance in 24
23
the visible range of the spectrum compared to high converted samples. Mass spectrometry analysis 25 27
26
of high converted materials derived from asphaltenes oxy-cracking using KOH instead of NaOH, 29
28 20
31
30
apparently agreed with the present results, i.e., "acid soluble WSAs" average molecular weights
were found smaller than the parent asphaltenes. The authors indicated that up to about 115 32 34
3
oxygenated families not initially present in the feedstock were observed to be the main components 36
35
of the water-solubilized materials found soluble under acidic pH.20 The number of oxygen atoms 37 38
per average molecule for these studied materials was found to cover up to 15 oxygens. 39 40 41
3.6 Gas chromatography for molecular weight (MW) and conversion estimation. High 42 4
43
temperature simulated distillation (HTSD) is routinely run in oil laboratories, fundamentally for 46
45
providing samples distillation curves. Organic solutions of hydrocarbon samples are routinely 47 49
48
analyzed for the purpose. Aqueous solutions are not described in the open literature for HTSD 51
50
routine analysis, because continuous injection of water hydrolyzes the polysiloxane phases of 53
52
HTSD columns, thus damaging these. Despite the preceding, HTSD was used in the present work 54 56
5
for the analysis of WSA samples. 57
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The routine output from HTSD, i.e., the distillation curve, was not used for the present 5
4
discussion. Instead, sample chromatograms and their areas were utilized for the study (see Figure 6 8
7
13). Carbon number ranges shown for the chromatograms on Figure 13 were derived from 10
9
comparison with the standard n-paraffin mixture routinely employed for HTSD analysis. Two 1 12
findings are readily observed with the results from Figure 13: I. Areas are found to decrease as a 13 15
14
function of conversion, II. Chromatograms Carbon# maxima were shifted to lower carbon numbers 17
16
as a function of conversion. 18 20
19
Area decreases in HTSD have been observed to depend primarily on the samples thermal 21 2
maturity,32,57 in other words, samples naturally matured (reservoir burial) or man-aged 23 25
24
(processing) display lower areas in the region depicted spanning from about 10-35 minutes in the 27
26
chromatograms (see Figure 13). For WSA samples studied in the present work, area decreases can 28 30
29
be ascribed to two factors: I). Lower crackability potential derived from reduced alkyl functionality 32
31
contents, as indicated before with FTIR spectroscopy, II. The presence of oxygen functionalities 3 34
in WSA, i.e., it is well known that oxygenates provide lower signals in flame ionization detectors 35 37
36
compared to (H, C containing) hydrocarbons. 38 40
39
Regarding the shifts in signals maxima presented in Figure 13, these findings are the further 42
41
indication of oxy-cracking reactions as discussed before (section 3.5 on UV-Vis 43 4
spectrophotometry). Molecular sizes estimated from the spanned carbon numbers suggest that the 45 47
46
original asphaltenes has average molecular masses around 1,275 Dalton (C91) while the high 49
48
converted WSA materials fall in the vicinity of 1,020 Daltons (C73). Discussion on absolute sizes 50 52
51
regarding asphaltenes is often found in the open literature; however, deemed not worth pursuing 54
53
in the present work because aggregation phenomena of polar oxygenate groups makes difficult 5 57
56
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understanding the extent of these over the determined values. What is important to consider is 5
4
what the preceding findings highlight (both UV-Vis and HTSD), i.e., decreased average sizes 6 8
7
determined for WSA samples compared to the original asphaltenes feedstock. The preceding 10
9
support the occurrence of molecular scission phenomena affecting the feedstock during the oxy1 12
cracking reaction. 13 14 15
Figure 14 provides further information based on HTSD chromatographic areas, correlating 16 18
17
these to important parameters monitored during oxy-cracking of the studied C5-asphaltene. HTSD 20
19
areas were correlated with carbon conversion (WSA+CO2 in products), versus total pressure (the 21 2
rough indication of O2 consumption) and produced CO2. Monotonous decreasing correlations are 23 25
24
observed in the three cases, comprising a rapid decrease for HTSD areas in oxidized materials 27
26
produced under low severities (20-30% conversion) and a slow decrease, seemingly reaching a 28 30
29
plateau at higher severities (>30% conversion). Unconverted organics presence was observed 32
31
under low severity oxy-cracking conditions ( 60%. 35 37
36
The former premises were combined so as to provide a general formation pathway presented in 38 39
Figure 16, grossly depicting low and high carbon conversion WSA average mixtures. The 40 42
41
produced materials were determined to have a similar viscosity to water and contain naphthenic, 4
43
carboxylic and sulfonic functions. The aqueous solutions could be easily pumped through pipelines 45 47
46
without clogging and mass transfer limitations. 48 50
49
4. Conclusions 51 53
52
A new feasible pathway for asphaltenes conversion into other chemicals was explored in this work, 5
54
relying on aqueous-alkaline oxidation termed as the oxy-cracking reaction. A model based on 57
56
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sequential-parallel reactions from the asphaltenes to water soluble products and CO2 was found to 5
4
describe the process successfully. Production of oxidized functionalities like carboxylic acids, their 6 8
7
salts, methyl ethers and esters, sulfur-oxidized forms plus phenolics were determined as the most 10
9
significant fractions soluble in water. Scission reactions (cracking) were evidenced to occur 1 12
leading to WSA products, which showed to have higher carbon aromaticities compared to the 13 15
14
original feedstock. The yield of undesirable CO2 was found possible to control to levels
