Pascal Diévart

Detection and Identification of the Keto-Hydroperoxide (HOOCH2OCHO) and other Intermediates during Low-Temperature Oxidation of Dimethyl Ether

In this paper we report the detection and identification of the keto-hydroperoxide (hydroperoxymethyl formate, HPMF, HOOCH2OCHO) and other partially oxidized intermediate species arising from the low-temperature (540 K) oxidation of dimethyl ether (DME). These observations were made possible by coupling a jet-stirred reactor with molecular-beam sampling capabilities, operated near atmospheric pressure, to a reflectron time-of-flight mass spectrometer that employs single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation. On the basis of experimentally observed ionization thresholds and fragmentation appearance energies, interpreted with the aid of ab initio calculations, we have identified HPMF and its conceivable decomposition products HC(O)O(O)CH (formic acid anhydride), HC(O)OOH (performic acid), and HOC(O)OH (carbonic acid). Other intermediates that were detected and identified include HC(O)OCH3 (methyl formate), cycl-CH2-O-CH2-O- (1,3-dioxetane), CH3OOH (methyl hydroperoxide), HC(O)OH (formic acid), and H2O2 (hydrogen peroxide). We show that the theoretical characterization of multiple conformeric structures of some intermediates is required when interpreting the experimentally observed ionization thresholds, and a simple method is presented for estimating the importance of multiple conformers at the estimated temperature (∼100 K) of the present molecular beam. We also discuss possible formation pathways of the detected species: for example, supported by potential energy surface calculations, we show that performic acid may be a minor channel of the O2 + ĊH2OCH2OOH reaction, resulting from the decomposition of the HOOCH2OĊHOOH intermediate, which predominantly leads to the HPMF.

Improved optimization of polycyclic aromatic hydrocarbons (PAHs) mixtures resolution in reversed-phase high-performance liquid chromatography by using factorial design and response surface methodology

A new procedure for optimizing PAHs separation in very complex mixtures by reverse phase high performance (RPLC) is proposed. It is based on changing gradually the experimental conditions all along the chromatographic procedure as a function of the physical properties of the compounds eluted. The temperature and speed flow gradients allowed obtaining the optimum resolution in large chromatographic determinations where PAHs with very different medium polarizability have to be separated. Whereas optimization procedures of RPLC methodologies had always been accomplished regardless of the physico-chemical properties of the target analytes, we found that resolution is highly dependent on the physico-chemical properties of the target analytes. Based on resolution criterion, optimization process for a 16 EPA PAHs mixture was performed on three sets of difficult-to-separate PAHs pairs: acenaphthene-fluorene (for the optimization procedure in the first part of the chromatogram where light PAHs elute), benzo[g,h,i]perylene-dibenzo[a,h]anthracene and benzo[g,h,i]perylene-indeno[1,2,3-cd]pyrene (for the optimization procedure of the second part of the chromatogram where the heavier PAHs elute). Two-level full factorial designs were applied to detect interactions among variables to be optimized: speed flow, temperature of column oven and mobile-phase gradient in the two parts of the studied chromatogram. Experimental data were fitted by multivariate nonlinear regression models and optimum values of speed flow and temperature were obtained through mathematical analysis of the constructed models. An HPLC system equipped with a reversed phase 5 microm C18, 250 mm x 4.6mm column (with acetonitrile/water mobile phase), a column oven, a binary pump, a photodiode array detector (PDA), and a fluorimetric detector were used in this work. Optimum resolution was achieved operating at 1.0 mL/min in the first part of the chromatogram (until 45 min) and 0.5 mL/min in the second one (from 45 min to the end) and by applying programmed temperature gradient (15 degrees C until 30 min and progressively increasing temperature until reaching 40 degrees C at 45 min).

Experimental Study of the Oxidation of N-Tetradecane in a Jet-Stirred Reactor (JSR) and Detailed Chemical Kinetic Modeling

The kinetics of oxidation of n-tetradecane was studied experimentally in a jet-stirred reactor (JSR) at high pressure (10 atm), at temperatures ranging from 560 to 1030 K, at a constant residence time (τ) of 1 s, and for three equivalence ratios (φ = 0.5, 1.0, and 2.0). Chemical analyses by Fourier transform infrared spectrometry and gas chromatography yielded mole fractions of reactants, stable intermediates, and final products as a function of temperature. A kinetic reaction mechanism based on previous studies from this laboratory was developed and validated by comparison with the present experimental results. The proposed reaction mechanism consisted of 7,885 reversible reactions involving 1,813 species. Experimental data and simulation results obtained in the current work were found in good agreement. Ignition delays of n-tetradecane/air mixtures were also simulated.

Combustion of a Gas-to-Liquid–Based Alternative Jet Fuel: Experimental and Detailed Kinetic Modeling

The kinetics of oxidation of blends of gas-to-liquid (GtL) jet fuel with 20% vol. of 1-hexanol was studied. New experimental results were obtained using a jet-stirred reactor (p = 10 bar, constant mean residence time of 1 s, over the temperature range 550–1150 K, and for equivalence ratios of 0.5, 1, and 2). Concentration profiles of reactants, stable intermediates, and final products were obtained by probe sampling followed by on-line and off-line gas chromatography analyses and on-line Fourier transformed infra-red spectrometry. A comparison with corresponding results for Jet A-1 and pure GtL showed these fuels have similar combustion properties. The oxidation of the GtL-hexanol fuel under these conditions was modeled using a detailed kinetic reaction mechanism consisting of 8217 reactions and 2185 species and a 4-component surrogate fuel mixture (n-decane, iso-octane, n-propyl cyclohexane, and 1-hexanol). The proposed kinetic model showed good agreement with the presently reported jet-stirred reactor concentration profiles.

Synthetic Jet Fuel Combustion: Experimental and Kinetic Modeling Study

Fischer-Tropsch liquid fuels synthesized from syngas, also called synthetic paraffinic jet fuel (SPK), can be used to replace conventional petroleum-derived fuels in jet engines. Whereas currently syngas is mostly produced from coal of natural gas, its production from biomass has been reported. These synthetic liquid fuels contain a very high fraction of iso-alkanes, while conventional jet fuels contain large fractions of n-alkanes, cycloalkanes (naphtenes), and aromatics. In that contest, a jet-stirred reactor (JSR) was used to study the kinetics of oxidation of a 100% SPK and a 50/50 SPK/Jet A-1mixture over a broad range of experimental conditions (10 atm, 560 to 1030K, equivalence ratios of 0.5 to 2, 1000 ppm of fuel). The temperature was varied step-wise, keeping the mean residence time in the JSR constant and equal to 1s. Three combustion regimes were observed over this temperature range: the cool-flame oxidation regime (560–740K), the negative temperature coefficient (NTC) regime (660–740K), and the high-temperature oxidation regime (>740K). More than 15 species were identified and measured by Fourier transform infrared spectrometry (FTIR), gas chromatography/mass spectrometry (CG/MS), flame ionization detection (FID), and thermal conductivity detection (TCD). The results consisting of concentration profiles of reactants, stable intermediates and products as a function of temperature showed similar kinetics of oxidation for the fuels considered, although the 100% SPK was more reactive. A surrogate detailed chemical kinetic reaction mechanism was used to model these experiments and ignition experiments taken from the literature. The kinetic modeling showed reasonable agreement between the data and the computations whereas model improvements could be achieved using more appropriate surrogate model fuels. Kinetic computations involving reaction paths analyses and sensitivity analyses were used to interpret the results.Copyright

Experimental and Modeling Study of the Combustion of Synthetic Jet Fuels: Naphtenic Cut and Blend With a GtL Jet Fuel

Research on the production and combustion of synthetic jet fuels has recently gained importance because of their potential for addressing security of supply and sustainable air transportation challenges. The combustion of a 100% naphtenic cut that fits with typical chemical composition of products coming from biomass or coal liquefaction (C12.64H23.64; M=175.32 g.mol-1; H/C=1.87; DCN=39; density=863.1 g.L-1) and a 50% vol. mixture with Gas to Liquid from Shell (mixture: C11.54H23.35; M=161.83 g.mol-1; H/C=2.02; DCN=46; density=800.3 g.L-1) were studied in a jet-stirred reactor under the same conditions (temperature, 550-1150 K; pressure, 10 bar; equivalence ratio, 0.5, 1, and 2; initial fuel concentration, 1000 ppm). Surrogate model-fuels were designed based on fuel composition and properties for simulating the kinetics of oxidation of these fuels. We used new model-fuels consisting of mixtures of n-decane, decalin, tetralin, 2-methylheptane, 3-methylheptane, n-propyl cyclohexane, and n-propylbenzene. The detailed chemical kinetic reaction mechanism proposed was validated using the entire experimental database obtained in the present work and for the oxidation of pure GtL, we used previous results. Kinetic computations involving reaction paths analyses and sensitivity analyses were used to interpret the results.

Kinetics of Oxidation of a 100% Gas-to-Liquid Synthetic Jet Fuel and a Mixture GTL/1-Hexanol in a Jet-Stirred Reactor: Experimental and Modeling Study

Research activities on the combustion of synthetic jet fuels and bio-derived jet fuels have increased notably over the last 10 years in order to solve the challenging reduction of dependence of air transportation on petroleum. Within the European Community’s Seventh Framework Programme, the combustion of a 100% GtL from Shell and a 80/20% vol. GtL/1-hexanol blend were studied in a jet-stirred reactor (JSR). This synthetic GtL fuel mainly contains n-alkanes, iso-alkanes, and cyclo-alkanes. We studied the oxidation of these alternatives jet fuels under the same conditions (temperature, 550–1150 K; pressure, 10 bar; equivalence ratio, 0.5–2; initial fuel concentration, 1000 ppm). For simulating the oxidation kinetics of these fuels we used a new surrogate mixture consisting of n-dodecane, 3-methylheptane, n-propylcyclohexane, and 1-hexanol. A detailed chemical kinetic reaction mechanism was developed and validated by comparison with the experimental results obtained in a jet-stirred reactor. The current model was also tested for the autoignition of the GtL fuel under shock tubes conditions (φ = 1 and P = 20 atm) using data from the literature. Kinetic computations involving reaction paths analyses and sensitivity analyses were used to interpret the results. The general findings are that the GTL and GTL/hexanol blend have very similar reactivity to Jet A-1, which is important since GTL is a drop-in fuel that should have similar performance to the Jet A-1 baseline and 1-hexanol should not significantly affect the reactivity if it is to be used as an additive.Copyright