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OXIDATION OF N-ALKANE (n-C8H18) UNDER RESERVOIR CONDITIONS, IN RESPONSE TO GAS MIXTURE INJECTION (CO2/ O2): Understanding of oxygenated compounds distribution Claire Pacini-Petitjean, Pranay Morajkar, Valerie Anne Burkle-Vitzthum, Aurélien Randi, Catherine Lorgeoux, Jacques Pironon, and Pierre Faure Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01323 • Publication Date (Web): 17 Aug 2016 Downloaded from http://pubs.acs.org on August 18, 2016
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Energy & Fuels
OXIDATION OF N-ALKANE (n-C8H18) UNDER RESERVOIR CONDITIONS, IN RESPONSE TO GAS MIXTURE INJECTION (CO2/O2): Understanding of oxygenated compounds distribution
C. PACINI-PETITJEAN a,b, P. MORAJKAR c, V. BURKLE-VITZTHUM c, A. RANDI a, C. LORGEOUXa, J. PIRONON a and P. FAURE b
a
Université de Lorraine, CNRS, CREGU, GeoRessources lab, BP 70239, Vandœuvre-lès-Nancy, F-54506, France
b
Université de Lorraine, CNRS, LIEC, Vandœuvre-lès-Nancy, F-54506, France
c
, Université de Lorraine, CNRS, LRGP, Nancy, F-54001, France
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Highlights: -
n-octane oxidation was carried out to study oxygenated compounds formation
-
Oxygenated compounds, CO2 and water were quantified at molecular scale
-
Major oxygenated products are ketones and carboxylic acids
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Initial O2 content influences oxygenated products yield but not CO2 and water content
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Preservation of the aliphatic length chain and breakdown processes are discussed
KEYWORDS: CCS; EOR; hydrocarbon oxidation; quantitative analysis; oxygenated compounds; closed isochoric and isothermal reactor; high pressure; low temperature;
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Abstract
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CO2 geo-sequestration (CCS) and enhanced oil recovery (EOR) by CO2 injection in
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hydrocarbon-depleted reservoirs could limit the CO2 atmospheric accumulation. In the case of
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CO2 capture by oxy-combustion, the main annex gas associated with CO2 is O2. The O2 that
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remains in the flue gas for injection can induce the oxidation of the hydrocarbons contained in
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the reservoirs. The effect of O2 must be studied in terms of benefit and/or risk for CCS or
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EOR.
8
To investigate the mechanism of hydrocarbons oxidation, it is essential to analyze the
9
distributions of the formed oxygenated compounds. That’s why experiments have been
10
performed with a model compound (n-octane) in a closed reactor under high pressure at
11
different temperatures and with different oxygen concentrations.
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The product distribution suggests two pathways of n-alkane oxidation, with (i) the
13
preservation of the aliphatic chain length of the initial n-alkane, which generates oxygenated
14
products with the same number of carbon, and (ii) the breakdown processes of the initial
15
n-alkane, which generates low molecular weight oxygenated products.
16
The new understanding of the mechanism of n-alkane oxidation could be incorporated into
17
the detailed kinetic model of our previous study which is specific to the reservoir conditions.
18
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1. Introduction
2 3
The combustion of fossil fuels is the largest emission source of carbon dioxide (CO2),
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which is the main greenhouse gas emitted in the atmosphere 1. Several options are considered
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to prevent the release of large quantities of CO2 into the atmosphere, and hence mitigating the
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contribution of industrial source emission to global warming via (i) Enhanced Oil Recovery
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(EOR), and (ii) Carbon Capture and Storage (CCS).
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EOR processes are applied in depleted oil fields and can produce additional oil. The main
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objective of these operations is to increase the oil production, by decreasing the oil viscosity 2.
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One of such techniques is EOR promoted by CO2 injection. The EOR using CO2 injection has
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2 objectives: improvement of oil recovery and geo-sequestration of more than a half of the
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injected CO2 into the petroleum reservoir (approximately 60 % of CO2 injected) 3.
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The CCS process consists of (i) capturing waste CO2 from large point sources such as fossil
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fuel power plant, (ii) transporting it to a geological storage site and (iii) long-term isolation
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from the atmosphere in deep geological formation 4. Several geo-sequestration options are
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conceivable for CCS like unmineable coal seams, deep saline formations, or depleted oil and
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gas fields. Among these storage options, oil and gas fields present several advantages. These
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natural fields have proved their potential containment by retention of hydrocarbons during
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millions years. Moreover, these sites being already exploited, they present the advantage of
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being amply studied in context of storage capacity, structural and stratigraphic patterns etc. 4.
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In both cases, EOR and CCS, the first step of CO2 injection involves CO2 capture from
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fossil organic matter combustion (pre-combustion, post-combustion, oxy-combustion). This
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paper focuses on oxy-combustion (combustion with elevated oxygen concentration = 90-95
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%) capture because it has major advantages over conventional air combustion, such as (i) a
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better combustion efficiency, (ii) a decrease in nitrogen oxide emissions 5. Oxy-combustion
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flue gas contains a relatively high level of impurities, like water, Ar (0-5 %), N2 (0-15 %), O2
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(0-7 %), SO2 (0-1.5 %) 6. But a complete separation of CO2 is not planned because of (i) the
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high cost of purification stages, (ii) the increase in the energy penalty and by consequence (iii)
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the increase in the amount of CO2 produced 6. Therefore it is envisioned to compress the flue
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gas without additional treatments and inject it directly into a suitable geologic formation.
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Impurities could greatly affect the thermodynamic properties of the gas stream, and in turn
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would affect the different steps of CO2 injection process 6. Moreover, the oxygen can induce
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oxidation of the residual hydrocarbons in the reservoir, and modify the composition of oil
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with the production of several oxygenated compounds 7. It is important to study the oxidation
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of oil (i) for geo-sequestration, to estimate the chemical stability of the storage system, (ii) for
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EOR process, to assess the chemical and physical modification of oil (viscosity, acidity etc.),
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in order to estimate the potential of oil recovery and the oil quality.
13 14
However, most oxidation studies do not mimic the reservoir conditions or, they do not take into account the analysis of all products of the hydrocarbon oxidation.
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Some authors studied the oxidation of oils at low temperature with different compositions
16
and densities. As an example, Dechelette et al. (2006) performed oxidation of oil by gradually
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increasing the reaction temperature. A global oxidation reaction was proposed, based on the
18
molecular analysis of final oxidation products such as CO, CO2, residual coke and water.
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However, no identification of the intermediate and stable oxygenated compounds were
20
performed 8.
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More recently, Khansari et al. (2014) investigated the oxidation of a heavy oil
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(Lloydminster, Canada) in a TGA (Thermo-Gravimetric Analysis) study from 323 to 623 K
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and at atmospheric pressure. This study does not mimic any known reservoir conditions.
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Moreover, elementary analyses (C, H and O) were performed to determine the major
25
products, by mere comparison of the functional group ratios. A reaction scheme was
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proposed, but no product detection and identification were performed
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estimation methods (comparison of the functional group ratios) are based on an old analysis
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method of coal established by D.W. Krevelen in 1961
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determine precisely the product distribution and the chemical mechanisms.
11
9, 10
. These indirect
, but they are no sufficient to
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On the other hand, some studies are based on the oxidation of a mixture of pure
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compounds. As an example, Faure et al. (2003) investigated the oxidation at low temperature
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and atmospheric pressure of a mixture of pure aliphatic compounds. Molecular analysis were
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performed to identify the oxygenated compounds and they proposed a reaction scheme based
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on free radical chemistry 12.
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However, to understand the oxidation of oil in response to CO2/O2 injection, it is important
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to work in reservoir conditions (low temperature and high pressure, LT – HP). Furthermore,
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since the oil consists of a complex mixture of numerous organic compounds, it is difficult to
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identify the oxidation reaction pathways and so to describe the detailed scheme of
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hydrocarbon oxidation. Therefore, in this work we have chosen to study the oxidation of a
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representative single hydrocarbon that is n-octane, to enable the determination and
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understanding of the product distribution. Moreover, we have chosen experimental conditions
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similar to reservoir conditions, LT – HP.
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Oxidation mechanisms of hydrocarbons and especially alkanes are well described in
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homogeneous phase via free-radical chemistry, but concern mainly high temperature – low
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pressure conditions, for engine optimization. During the hydrocarbon oxidation, peroxy
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radicals and various intermediate oxygenated compounds are formed, until transformation
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into the final products, i.e. water and CO2. Knox et al. (1967), followed by Fish (1968),
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carried out the first low temperature oxidation of alkanes
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have undergone major improvement
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reservoir conditions was expected to be in accordance with the low temperature kinetic model
15, 16
13, 14
. The detailed kinetic models
. The oxidation chemistry of hydrocarbons in the
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(600-900 K), but the model needed to be upgraded to take the high pressure (1 MPa to
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100 MPa) into account and the particularly low temperature (< 500 K) of a petroleum
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reservoir. In a previous study, these modifications were considered, i.e. some new reactions
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were added to the literature kinetic model and some kinetic parameters were optimized, in the
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primary mechanism of the oxidation of n-octane 17. This improved kinetic model perfectly
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enables the simulation of the n-alkane conversion.
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This new study aims to better understand the byproduct distribution. Experiments have been
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performed in a closed reactor at 3 different temperatures and with 2 different concentrations
9
of O2, in order to highlight the influence of temperature and O2 concentration on the
10
byproduct distribution. The main reaction pathways of the oxygenated products have also
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been proposed.
12 13
2. Materials and methods
14 15
2.1 Reactants
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Oxidation experiments were carried out with a model compound, i.e. n-C8H18 used as
17
received (Sigma, purity 99+ %), because n-alkanes are (i) representative of crude oil
18
constituents, and (ii) are simple and linear compounds. To simulate the gas injection, artificial
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air (Table 1), and a mixture of oxygen (3 %) (Table 2) and nitrogen (97 %) (Table 3) were
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injected into the reactor.
21
The gas solvent was tested in a previous study
17
; we showed that neither nitrogen nor
22
carbon dioxide have any influence on the oxidation kinetics and reactivity. Therefore,
23
artificial air was selected rather than a mixture of CO2 + O2, to be able to quantify CO2
24
production during the oxidation.
25
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2.2 Experimental device
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2.2.1 Experimentation
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To be close to the reservoir conditions, oxidation experiments were carried out in a
4
high-pressure/low-temperature reactor, already described
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(volume: 500 ml) and isothermal. An electromagnetic propeller stirrer enabled the
6
homogenization of the reactor content. The reactor was placed in an oven. The pressure vessel
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was built in titanium and the other components (stirrer, valves, tubes…) were in hastelloy to
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avoid corrosion problems.
17
. The system was isochoric
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The influence of wall reactions potentially catalyzed by titanium was tested by varying the
10
surface to volume ratio (factor of 2). Similar oxidation kinetics were observed in the test
11
conditions as those in a regular experiment. Hence, it was concluded that the effect of the
12
titanium surface is insignificant under the experimental conditions of this study.
13
The temperature and the pressure of the vessel were recorded every 10 seconds. The entire
14
experimental system was maintained at the same temperature to avoid cold spots, and so
15
compound condensation.
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The reactor was connected to a Rolsi valve (Rapid On-Line Sampler Injector), which
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performed an on-line micro sampling of the gas phase in the system. The valve was connected
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to a transfer line, which was heated to the reactor temperature, under 0.05 MPa of argon. It
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enabled performing a continuous gas monitoring of the system during the experiment.
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2.2.2 Product analysis
22
The transfer line was connected to a gas micro-chromatograph SRA R-3000 (µ-GC),
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already described 17. The µ-GC was coupled with three modules. Each module contained a
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Thermal Conductivity Detector (TCD).
25
The µ-GC enabled the analysis of the following compounds:
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1st module: He, N2, O2, CO, CH4
2
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2nd module: CO2
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3rd module: H2O, n-C8H18
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For each analysis, ten samples with the Rolsi valve were analyzed to check the repeatability
5
of the results (O2: 4.2%, CO2: 3.7%, H2O: 2.3%, n-C8H18: 10.5%).
6 7
At the end of the experiment, the products were analyzed by gas chromatography-mass
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spectrometry (GC-MS). The GC used was an Agilent Technologies 6890 coupling with a
9
split-splitless injector and equipped with a capillary column in silica glass VF-WAXms
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(60 m, 0.25 mm i.d., 25 µm e.f.) (polar column specific to oxygenated compounds), coupled
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to a MS Agilent Technologies 5973 inert detector on the fullscan mode. The temperature
12
program was as following: from 313 K to 513 K at 3 K/min. Helium was used as the carrier
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gas at 1.4 mL.min-1 constant flows. 1µL of the solution was directly injected into the gas
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chromatograph in split mode.
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A solution of standard compounds (alcohols: C8H16O, aldehydes: C4H8O to C8H16O,
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ketones: C4H8O to C8H16O, carboxylic acids: C2H4O2 to C8H16O2 and lactones: C5H8O2,
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C7H12O2, C8H14O2) (Annex) was used as reference to calibrate the GC-MS. To calibrated the
18
different position-isomers an extrapolation was done. 2-undecanone was added in the product
19
solution as an internal standard.
20 21
2.3 Protocol 17
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Experiments were carried out with n-C8H18 at three different temperatures at 20 MPa, and in
23
the presence of artificial air (20 % O2 and 80 % N2) or in the presence of 3 % O2 and 97 % N2
24
(Table 4). Experiments were performed in order to investigate (i) the effect of the initial
25
temperature and (ii) the effect of O2, on the oxidation kinetics.
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The reactant (n-C8H18) and the gas mixture (O2 + N2) were placed in the reactor at ambient
2
temperature, with 0.2 MPa of helium (internal standard for the µ-GC). After heating to the set
3
temperature, the pressure attained 20 MPa.
4
The temperature was measured by a calibrated thermocouple and the pressure by a pressure
5
sensor during the experiment. At the end of the experiment (6 to 96 hours), the heating was
6
stopped, and the reactor was cooled to 258 K by an immersion cooler. When the reactor
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attained 258 K, the pressure was reduced slowly to the atmospheric pressure, in order to
8
minimize the loss of the volatile compounds before opening the reactor. Once the reactor
9
opened, the internal standard (2-undecanone) was added in the liquid sample, which was
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recovered without solvent addition and injected in GC-MS, in split injection.
11 12
3. Results
13
During all experiments, temperature and pressure remained constant.
14 15
3.1 Effect of temperature
16
-
17
Very low production of CO2 was observed while there was no water production. The yield
18
of CO2 was 0.5 % Cini (initial carbon). The n-octane conversion corresponds to
19
25 % (± 5 %) Cini
20
compounds. The chromatogram was dominated by carboxylic acids from C2 to C8 whose
21
global yield was 0.03 % Cini, ketones from C4 to C8 (yield 0.013 % Cini), aldehydes from C4 to
22
C8 (yield 0.005 % Cini), alcohols (only octanols - yield 0.003 % Cini), and γ-lactones from C5 to
23
C8 (yield 0.001 % Cini). The short length chains were dominant (C4 and C5).
24
-
398 K, 20 MPa, n-octane + artificial air, 48 hours (Figure 1)
17
. The GC-MS analysis showed a very low production of oxygenated
423 K, 20 MPa, n-octane + artificial air, 48 hours (Figures 1)
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n-Octane conversion increases to 70 % (± 5 %) Cini 17. The yield of water and CO2 increased
2
to attain respectively 15 % Hini and 8.5 % Cini. The oxygenated compounds are dominated by
3
ketones and carboxylic acids (respectively 16.0 % and 32.2 % of Cini) whereas aldehydes
4
become negligible. Other oxygenated compounds (γ-lactones: 0.7 % Cini, alcohols: 0.5 % Cini)
5
exhibit their maximum yield compared to other temperatures. The carbon number of each
6
oxygenated compounds is similar to the 398 K experiment.
7
-
8
The n-octane conversion attains 96 % (± 5 %) Cini 17. The yield of water and CO2 increased
9
with temperature, to attain respectively, 30 % Hini (initial hydrogen) and 18 % Cini at 448 K.
10
The chromatogram showed some oxygenated compounds: 11 % Cini of carboxylic acids (C2 to
11
C8), 3 % Cini of ketones (from C4 to C8), 0.3 % Cini of γ-lactones (C4 to C8), 0.008 % Cini of
12
alcohols (only octanols). The short compounds were predominant, and carboxylic acids were
13
the major compounds.
448 K, 20 MPa, n-octane + artificial air, 48 hours (Figure 1)
14 15
3.2 Effect of initial concentration of oxygen and experiment duration
16
To evaluate the effects of the oxygen content and duration of oxidation, series of
17
experiment were carried out at 423 K and 20 MPa. Two oxygen proportions were selected,
18
3 % O2 + 97 % N2 and 20 % O2 + 80 % N2 with a duration of experiment from 6 to 48 hours
19
and 6 to 96 hours respectively (figures 2 and 3).
20
The n-octane conversion increases and attains 75.5 % and 72.6 % Cini after 48 hours of
21
experiment for 3 % and 20 % of O2 respectively. After 96 hours (20 % O2 experiment), the
22
n-octane conversion is equal to 86.6 % Cini. The yield of water and CO2 increased with time,
23
to attain respectively, 13.5 % Hini and 10 % Cini for the 3% O2 experiment, and respectively,
24
15 % Hini and 8.5 % Cini for the 20% O2 experiment, after 48 hours. After 96 hours (20 % O2
25
experiment), the H2O and CO2 yields were respectively 23 % Hini and 14 % Cini (Figure 2).
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The oxygenated compounds were analyzed after 6 hours and every 24 hours until 96 hours
2
(Figure 3). Aldehydes were observed only at 6 hours, with a yield of 0.008 % Cini (3 % O2
3
experiment) and 0.5 % Cini (20 % O2 experiment). Alcohols yield at 6 hours were in the same
4
range than that of aldehydes, and alcohols were mostly observed at 24 hours with a yield of
5
12 % Cini (3 % O2 experiment), and 26 % Cini (20 % O2 experiment). Only alcohols with the
6
same number of carbon than the initial n-alkane were detected, but all the isomer positions
7
were observed.
8
Ketones and carboxylic acids were predominant and increased with time until 24 or
9
48 hours, and then decreased. The maximum of ketones was observed at 24 hours with a yield
10
of 5 % Cini (3 % O2 experiment), and 6 % Cini (20 % O2 experiment); after 96 hours the ketone
11
yields were 4 % Cini for the 20 % O2 experiment.
12
Carboxylic acids increased with time until 24 hours, with a maximum yield of 16 % Cini
13
(3 % O2 experiment) and until 48 hours, with a maximum yield of 32 % Cini (20 % O2
14
experiment). Then, the yield of carboxylic acids decreased to 15 % Cini (20 % O2 experiment)
15
after 96 hours.
16
γ-lactones increased with time until 24 hours, with a maximum at 1 % Cini (3 % O2
17
experiment) and 2 % Cini (20 % O2 experiment), and decreased to 0.6 % Cini (20 % O2
18
experiment) after 96 hours.
19 20
4. Discussion
21 22
4.1 Effect of temperature
23
The oxidation of n-octane at 448 K during 48 hours led to higher yields of water and CO2
24
than the experiments at 423 K and 398 K. The production of CO2 and water continuously
25
increased with temperature between 398 and 448 K.
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Moreover, the composition of the oxygenated residue was different for each temperature.
2
At 398 K after 48 hours, the residue presented a very low concentration of oxygenated
3
compounds; at 423 K after 48 hours, ketones and carboxylic acids were predominant; and
4
finally at 448 K after 48 hours, the liquid residue was mainly composed of carboxylic acids.
5
The oxidation temperature greatly affects the distribution of the intermediate products and
6
the yield of the final products of oxidation. In fact, the oxidation kinetics increases with the
7
temperature of the experiment. Aldehydes and alcohols appear to be primary products of the
8
reaction.
9 10
4.2 Effect of initial concentration of oxygen
11
The experiment with 3 % of O2 showed approximately the same yields of water and CO2
12
(generally slightly lower) than the experiment with 20 % of O2. Hence, the initial
13
concentration of O2 has not a strong influence on the production of the final oxidation
14
products.
15
On the other hand, concerning the intermediate compounds, the rough tendencies were the
16
same but their yields were very different: in almost all experiments, the yields with 3% of O2
17
were lower by a factor between 1.5 and 3, in comparison to the experiments with 20% of O2.
18
Hence, the concentration of initial O2 has a strong influence on the concentration of the
19
intermediate compounds.
20
These observations could suggest that there are reaction pathways for CO2 and H2O
21
production that are independent from the intermediate products and which may be
22
heterogeneous 18, although these pathways were not identified yet. Nevertheless, we showed
23
that the surface to volume ratio of the reactor does not modify the n-octane conversion 17.
24 25
4.3 Effect of time
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During the first 6 hours, aldehydes, alcohols, ketones and carboxylic acids were present in
2
similar amounts. The yields of these families were very low. After 24 hours the analysis did
3
not show aldehydes anymore; alcohols, ketones, and carboxylic acids were the predominant
4
species. Small amounts of γ-lactones were also detected at 24 hours; after 24 hours, all
5
compound families decreased in the oxygenated residue.
6
These intermediate compounds (aldehydes, ketones, alcohols, carboxylic acids and
7
lactones), and the description of their evolution over the time, enable to highlight the
8
pathways of n-alkane oxidation process.
9 10
4.4 Oxidation reaction scheme
11
The identification and quantification of the oxygenated compounds by GC-MS and µ-GC
12
analysis enabled to highlight the reaction pathways, which could explain the production and
13
consumption of the intermediate oxygenated compounds. The evolution of the molecular
14
distribution suggested two pathways: (i) formation of oxygenated compounds with the same
15
carbon number as the initial n-alkane, (ii) production of oxygenated compounds characterized
16
by a shorter aliphatic chain length than the initial n-alkane.
17 18
-
19
At low temperature (< 700 K), the formation of the first radicals is caused by the
20
bimolecular reactions with oxygen. Hydroperoxy (HO2•) and alkyl (R•) radicals are formed;
21
the alkyl radicals contain the same carbon number than the initial n-alkane RH (R1 and R1’).
22
Activation energy of this reaction is 238.5 kJ mol-1. R• radicals react principally by addition
23
to oxygen, to form peroxyalkyl radicals (ROO•) (R2 and R2’). Activation energy is
24
considered likely to be zero.
Initiation reactions of oxidation (Figure 4)
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Energy & Fuels
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Hydroperoxydes (ROOH) can be produced by metathesis between the peroxyalkyl and the
2
initial n-alkane (R3 and R3’). The unstable peroxides are subjected to decomposition by the
3
homolytic cleavage of the O-O bond (159 kJ.mol-1). Alkoxyl (RO•) and hydroxyl (HO•)
4
radicals are formed (R4 and R4’). The radical RO• contains the same carbon number as the
5
initial n-alkane, and depending on the position of the free-radical center, the positional
6
isomers 1-RO•, 2-RO•, 3-RO• and 4-RO• are obtained.
7 8
Radicals RO• are also obtained by the peroxyalkyl radicals (R5 and R5’) and peroxides (R6 and R6’) self-reactions whose importance was demonstrated in our previous study 17.
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-
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Octanols (Figure 5):
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The different chromatograms showed only alcohols with the same number of carbon than
13
the initial n-alkane, but all the positions of isomers were observed. The alkoxy radical RO•
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can induce the formation of alcohols, with the same carbon number of the initial n-alkane.
Preservation of the aliphatic chain
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They are produced by metathesis with the alkoxyl radical RO• and the initial alkane RH
16
(R7 and R7’). Hence, 1-octanol, 2-octanol, 3-octanol, and 4-octanol are formed as a function
17
of the initial free radical position in RO•.
18 19
Octanals (Figure 6):
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Octanals are produced very quickly by oxidation of 1-RO• (R8) 19. This reaction is favored
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at temperature lower than 500 K, because of its very low activation energy (4.574 kJ/mol),
22
and in high concentration of oxygen, as in our study. There are two other pathways with
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peroxyalkyl radicals. The first one (R9) is more difficult (lower rate constant: k9
