[Show abstract][Hide abstract] ABSTRACT: The use of a rule-based automated kinetic model builder Genesys is illustrated for the thermal decomposition of diethyl sulfide and ethyl methyl sulfide. Rule-based automatic kinetic model generation builds-upon the users’ expert knowledge to define constraints per reaction family to limit the model size and exclude species and reactions that are considered irrelevant. In the case of alkyl sulfide pyrolysis intermolecular hydrogen abstractions, intermolecular additions/β-scissions and intermolecular homolytic substitutions are used to iteratively expand the model not considering species with more than 5 heavy atoms. Furthermore, the formation of biradical species and cyclic structures was avoided. Rate coefficients of elementary reactions and thermochemical properties of molecules were estimated through group additive methods, with parameters solely derived from high level ab initio calculations. 39 reactions were added to the model after the automatic generation out of which 37 got rate constants assigned from ab initio calculations. The generated model, consisting of 444 reactions between 28 molecules and 38 radical species, was validated using experimental data for the thermal decomposition of diethyl sulfide and ethyl methyl sulfide and showed that measured and predicted species mole fractions are in good agreement. However, there is a clear need for more accurate and more detailed experimental data for the pyrolysis of sulfur containing compounds.
[Show abstract][Hide abstract] ABSTRACT: High concentrations of fuel-range hydrocarbons may be recovered from heavier alkyl-aromatic compounds in crude oil after supercritical water (SCW) treatment. Arabian Heavy (AH) crude oil was treated in SCW and analyzed using two-dimensional gas chromatography (GC × GC-FID). Cracking mechanisms were investigated using the model compound hexylbenzene under similar SCW treatment conditions. The results of the model compound experiments were compared to predictions of a kinetic model built by the Reaction Mechanism Generator (RMG). AH crude cracked significantly during SCW treatment. The GC-observable mass fraction increased by 90%. We conducted studies on the distilled samples of crude oil and found that significant changes in the composition of the SCW-treated heavy fraction occurred. Significant formation of aliphatic hydrocarbons and small-chain BTX-type compounds were found in the SCW-processed samples. Hexylbenzene conversions differed between the crude oil studies and the model compound studies. The mechanistic model for the cracking of hydrocarbons during SCW treatment of the model compound hexylbenzene predicted the observed major liquid products toluene, styrene, and ethylbenzene. The selectivity of ethylbenzene and styrene changed over time. The apparent conversion of styrene into ethylbenzene was possibly via a reverse disproportionation reaction. Ultimately, a mechanism was built that serves as a basis for understanding the kinetics of hydrocarbon cracking in SCW.
Energy & Fuels 07/2015; 29(8):150721115614005. DOI:10.1021/acs.energyfuels.5b01040 · 2.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Natural gas has the potential to increase the biofuel production output by combining Gas- and Biomass-to-Liquids (GBTL) processes followed by naphtha and diesel fuel synthesis via Fischer-Tropsch (FT). This study reflects on the use of commercial-ready configurations of GBTL technologies and the environmental impact of enhancing biofuels with natural gas. The autothermal and steam-methane reforming processes for natural gas conversion and the gasification of biomass for FT fuel synthesis are modeled to estimate system well-to-wheel emissions and compare them to limits established by U.S. renewable fuel mandates. We show that natural gas can enhance FT biofuel production by reducing the need for water gas shift (WGS) of biomass-derived syngas to achieve appropriate H2:CO ratios. Specifically, fuel yields are increased from less than 60 gallons per ton to over 100 gallons per ton with increasing natural gas input. However, GBTL facilities would need to limit natural gas use to less than 19.1% on a LHV energy basis (7.83 wt. %) to avoid exceeding the emissions limits established by the Renewable Fuels Standard (RFS2) for clean, advanced biofuels. This effectively constitutes a blending limit that constrains the use of natural gas for enhancing the Biomass-To-Liquids (BTL) process.
[Show abstract][Hide abstract] ABSTRACT: This work presents shock tube experiments and kinetic modeling efforts on the pyrolysis and combustion of JP-10. The experiments were performed at 6–8 atm using 2000 ppm of JP-10 over a temperature range of 1000–1600 K for pyrolysis and oxidation equivalence ratios from 0.14 to 1.0. This work distinguishes itself from previous studies as GC/MS was used to identify and quantify the products within the shocked samples, enabling the tracking of product yield dependence on equivalence ratio as well as identifying several new intermediates that form during JP-10’s decomposition. A detailed, comprehensive model of JP-10’s combustion and pyrolysis kinetics was constructed with the help of RMG, an open-source reaction mechanism generation software package. The resulting model, which includes 691 species reacting in 15,518 reactions, was extensively validated against the shock tube experimental dataset as well as newly published flow tube pyrolysis data from Ghent. Most of the important rate coefficients were computed using quantum chemistry. The model succeeds in identifying all major pyrolysis and combustion products and captures key trends in the product distribution. Simulated ignition delays agree within a factor of 4 with most experimental ignition delay data gathered from literature. The presented experimental work and modeling efforts yield new insights on JP-10’s complex decomposition and oxidation chemistry and identify key pathways towards aromatics formation.
Combustion and Flame 05/2015; DOI:10.1016/j.combustflame.2015.02.010 · 3.08 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a simple protocol which allows fully automated discovery of
elementary chemical reaction steps using in cooperation double- and
single-ended transition-state optimization algorithms - the freezing string and
Berny optimization methods, respectively. To demonstrate the utility of the
proposed approach, the reactivity of several systems of combustion and
atmospheric chemistry importance is investigated. The proposed algorithm
allowed us to detect without any human intervention not only "known" reaction
pathways, manually detected in the previous studies, but also new, previously
"unknown", reaction pathways which involve significant atom rearrangements. We
believe that applying such a systematic approach to elementary reaction path
finding will greatly accelerate the possibility of discovery of new chemistry
and will lead to more accurate computer simulations of various chemical
Journal of Chemical Theory and Computation 05/2015; DOI:10.1021/acs.jctc.5b00407 · 5.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Despite the beneficial impact of biofuels on most regulated pollutants and carbon dioxide emission, their combustion results in an increased production of aldehydes which are highly toxic. The oxidation of ac-etaldehyde by oxygen behind reflected shock waves has been investigated by simultaneously recording the emission profiles of excited OH*, CO 2 * and CH*. Experiments were performed at T 5 =1295-1580 K and P 5 =306-404 kPa. Equivalence ratios were Φ=0.5, 1 and 1.5. The argon dilution was held constant at 97%. Five detailed reaction mechanisms were tested with respect to the presently obtained data and those from the literature. The JetSurf model and one obtained using the RMG software overestimate the present ignition delay-times but reproduce fairly well the data from Yasunaga et al., Wang et al. and Kern et al.. The models from Konnov and Dagaut are in good agreement with the present measurements and reproduce some of the results of Yasunaga et al., Wang et al., Kern et al. and Bentz et al.. Overall, the model of Mével predicts a too high reactivity. Analyses have demonstrated the importance of the following reactions: R 1 : CH 3 CHO=CH 3 + HCO; R 2 : CH 3 CHO+CH 3 =CH 3 CO+CH 4 ; R 3 : CH 3 CHO+H=CH 3 CO+H 2 ; and R 4 : CH 3 CHO+CH 3 =CH 2 HCO+CH 4 .
US National Combustion Meeting, Cincinnati; 05/2015
[Show abstract][Hide abstract] ABSTRACT: The reaction between vinyl radical, C_2 H_3, and 1,3-butadiene, 1,3-C_4 H_6, has long been recognized as a potential route to benzene, particularly in 1,3-butadiene flames, but the lack of reliable rate coefficients has hindered assessments of its true contribution. Using laser flash photolysis and visible laser absorbance (λ=423.2 nm) we measured the overall rate coefficient for C_2 H_3+1,3-C_4 H_6, k_1, at 297 K≤T≤494 K and 4≤P≤100 Torr. k_1 is well-described in this range by the high-pressure-limit modified Arrhenius expression below. k_1=6.5×〖10〗^(-20) cm^3 molecule^(-1) s^(-1)×T^2.40 exp(-(1.76 kJ mol^(-1))/RT) Using photoionization time-of-flight mass spectrometry (PI TOF-MS) we also investigated the products formed. At T≤494 K and P=25 Torr we found only C_6 H_9 adduct species, while at 494 K≤T≤700 K and P=4 Torr, we observed ≤~10% branching to cyclohexadiene in addition to C_6 H_9. Quantum chemistry master-equation calculations indicate that n-C_6 H_9 is the dominant product at low T, consistent with our experimental results, and predict the rate and branching ratios at higher T where chemically activated channels become important.
The Journal of Physical Chemistry A 04/2015; 119(28). DOI:10.1021/jp512705r · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Potential energy surfaces and reaction kinetics were calculated for 40 reactions involving sulfur and oxygen. This includes 11 H2O addition, 8 H2S addition, 11 hydrogen abstraction, 7 beta scission, and 3 elementary tautomerization reactions, which are potentially relevant in the combustion and desulfurization of sulfur compounds found in various fuel sources. Geometry optimizations and frequencies were calculated for reactants and transition states using B3LYP/CBSB7, and potential energies were calculated using CBS-QB3 and CCSD(T)-F12a/VTZ-F12. Rate coefficients were calculated using conventional transition state theory, with corrections for internal rotations and tunneling. Additionally, thermochemical parameters were calculated for each of the compounds involved in these reactions. With few exceptions, rate parameters calculated using the two potential energy methods agreed reasonably, with calculated activation energies differing by less than 5 kJ/mol. The computed rate coefficients and thermochemical parameters are expected to be useful for kinetic modeling
[Show abstract][Hide abstract] ABSTRACT: Supercritical water upgrading (SCWU) of heavy oils reduces sulfur content and decreases average molecular weight, without rejecting carbon as coke products. Despite many years of industrial and academic scrutiny, many fundamental questions remain in the field: intrinsic reaction rates and mechanisms; the role of water; the need for catalysts; the importance of phase behavior and mixing. In 2009, MIT initiated a SCWU research program aimed at improving the understanding of the relevant physical, chemical, and catalytic phenomena. This overview summarizes the work performed at MIT within the historical context of SCWU with a particular focus on new kinetic rate measurements and modeling, reaction mechanism analysis, catalyst investigation, and combined mass/heat transport modeling of hydrocarbon/water mixtures. Kinetic rate measurements showed that sulfide decomposition during SCWU is consistent with a radical chain reaction pathway. Mechanistic studies and product distribution analysis identified that sulfide decomposition likely occurs via thioaldehyde and aldehyde intermediates and that water plays important roles in thioaldehyde hydrolysis (as a reactant) and aldehyde decarbonylation (as a catalyst). Catalytic investigation found that ZnO has potential to improve sulfur removal during SCWU, without addition of molecular hydrogen. Mixing studies revealed the complex dynamic processes that occur when hydrocarbons are injected into near or supercritical water. The article concludes with a summary of research needs and thoughts on the future of SCWU.
Journal of Supercritical Fluids The 01/2015; 96:114-123. DOI:10.1016/j.supflu.2014.09.015 · 2.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A detailed kinetic model for the thermal decomposition of the advanced fuel Jet-Propellant 10 (JP-10) was constructed using a combination of automated mechanism generation techniques and ab initio calculations. Rate coefficients for important unimolecular initiation routes of exo-TCD were calculated using the multireference method CAS-PT2, while rate coefficients for the various primary decompositions of the exo-TCD-derived monoradicals were obtained using CBS-QB3. Rate-of-production analysis showed the importance of four dominating JP-10 decomposition channels. The model predictions agree well with five independent experimental data sets for JP-10 pyrolysis that cover a wide range of operating conditions (T = 300-1500 K, P = 300 Pa-1.7 × 105 Pa, dilution = 0.7-100 mol% JP-10, conversion = 0-100%) without any adjustment of the model parameters. A significant part of the model comprises secondary conversion routes to aromatic and polyaromatic hydrocarbons and could thus be used to assess the tendency for deposit formation in fuel-rich zones of endothermic fuel applications.
Energy & Fuels 01/2015; 29(1):413-427. DOI:10.1021/ef502274r · 2.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Following our previous study of prototypical insertion reactions of energetically asymmetric type with the RPMD (Ring-Polymer Molecular Dynamics) method [Y. Li, Y. Suleimanov, and H. Guo, J. Phys. Chem. Lett. 5, 700 (2014)], we extend it to two other prototypical insertion reactions with much less exothermicity (near thermoneutral), namely, X + H2 → HX + H where X = C(1
D), in order to assess the accuracy of this method for calculating thermal rate coefficients for this class of reactions. For both chemical reactions, RPMD displays remarkable accuracy and agreement with the previous quantum dynamic results that make it encouraging for the future application of the RPMD to other barrier-less, complex-forming reactions involving polyatomic reactants with any exothermicity.
The Journal of Chemical Physics 12/2014; 141(24):244103. DOI:10.1063/1.4904080 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Quantum effects play a crucial role in chemical reactions involving light atoms at low temperatures, especially when a light particle is exchanged between two heavier partners. Different theoretical methodologies have been developed in the last decades attempting to describe zero-point energy and tunneling effects without abandoning a classical or semiclassical framework. In this work, we have chosen the D + HMu -> DMu + H reaction as a stress test system for three well-established methods: two representative versions of transition state theory (TST), canonical variational theory and semiclassical instanton, and ring polymer molecular dynamics (RPMD). These calculations will be compared with accurate quantum mechanical results. Despite its apparent simplicity, the exchange of the extremely light muonium atom (0.114 u) becomes a most challenging reaction for conventional methods. The main result of this work is that RPMD provides an overall better performance than TST-based methods for such a demanding reaction. RPMD might well turn out to be a useful tool beyond TST applicability.
[Show abstract][Hide abstract] ABSTRACT: Chirped-pulse (CP) Fourier transform rotational spectroscopy is uniquely suited for near-universal quantitative detection and structural characterization of mixtures that contain multiple molecular and radical species. In this work, we employ CP spectroscopy to measure product branching and extract information about the reaction mechanism, guided by kinetic modeling. Pyrolysis of ethyl nitrite, CH3CH2ONO, is studied in a Chen type flash pyrolysis reactor at temperatures of 1000-1800 K. The branching between HNO, CH2O, and CH3CHO products is measured and compared to the kinetic models generated by the Reaction Mechanism Generator software. We find that roaming CH3CH2ONO CH3CHO + HNO plays an important role in the thermal decomposition of ethyl nitrite, with its rate, at 1000 K, comparable to that of the radical elimination channel CH3CH2ONO CH3CH2O + NO. HNO is a signature of roaming in this system. [GRAPHICS]
[Show abstract][Hide abstract] ABSTRACT: We present RPMDrate, a computer program for the calculation of gas phase bimolecular reaction rate coefficients using the ring polymer molecular dynamics (RPMD) method. The RPMD rate coefficient is calculated using the Bennett–Chandler method as a product of a static (centroid density quantum transition state theory (QTST) rate) and a dynamic (ring polymer transmission coefficient) factor. The computational procedure is general and can be used to treat bimolecular polyatomic reactions of any complexity in their full dimensionality. The program has been tested for the H+H2, H+CH4, OH+CH4 and H+C2H6 reactions.
[Show abstract][Hide abstract] ABSTRACT: Two-dimensional gas chromatography with sulfur chemiluminescence detection (GC x GC-SCD) is applied to understand the changes in alkylated thiophenes, benzothiophenes (BTs), and dibenzothiophenes (DBTs) during supercritical water (SCW) upgrading of Arabian Heavy crude oil. It is shown that SCW treatment of heavy crude oil has several important effects: (1) The amount of BTs and DBTs in the distillate range increase, primarily due to cracking of heavier compounds. (2) Most of the long side chains on the thiophenes, BTs, and DBTs crack to form the corresponding thiophenic compounds with shorter side chains. (3) A small amount of the alkylated thiophenes undergo ring closure to form BTs during SCW treatment, and a small amount of the alkylated BTs appear to form DBTs in a similar way. As reported earlier, SCW treatment removes some of the sulfur from the oil phase, presumably as hydrogen sulfide (H2S). Distilling the heavy crude oil into light and heavy fractions and treating these fractions individually with SCW showed these effects more clearly. Model compound studies on hexylthiophenes confirm that SCW cleaves alkyl chains bound to thiophenes.
Energy & Fuels 10/2014; 28(10):6589-6595. DOI:10.1021/ef5015956 · 2.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Jet Propellant-10 (JP-10) pyrolysis is performed in a continuous flow tubular reactor near atmospheric pressure in the temperature range of 930-1080 K, a conversion range of 4-94%, and two dilution levels of 7 and 10 mol % JP-10 in nitrogen. Identification and quantification of the pyrolysis products of JP-10 are based on online two-dimensional gas chromatography with a time-of-flight mass spectrometer and a flame ionization detector. JP-10 starts to react at 930 K and is fully converted at 1080 K Among the more than 70 species up to C14H10 that were identified and quantified, tricyclo[184.108.40.206(2,6)]dec-4-ene was identified for the first time, indicating the importance of bimolecular H-abstraction reactions in the consumption of JP-10. Critical assessment of the experimental data with the JP-10 combustion model by Magoon et al. [Magoon, G. R.; Aguilera-Iparraguirre, J.; Green, W. H.; Lutz, J. J.; Piecuch, P.; Wong, H. W.; Oluwole, O. O. Detailed chemical kinetic modeling of JP-10 (exo-tetrahydrodicydopentadiene) high-temperature oxidation: Exploring the role of biradical species in initial decomposition steps. Int. J. Chem. Kinet. 2012, 44 (3), 179-193] showed that the model predictions of JP-10 agree reasonably well. The newly acquired and highly detailed experimental data help in understanding the thermal decomposition chemistry of JP-10 and can be used to validate future kinetic models of JP-10 pyrolysis.
Energy & Fuels 08/2014; 28(8):4976-4985. DOI:10.1021/ef500936m · 2.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Integrated gasification combined cycle with CO2 capture and sequestration (IGCC-CCS) emerges as a promising method for reducing emission of greenhouse gases from coal without reducing efficiency significantly. However, the high capital costs of these plants have limited their deployment. The current solvent-based low-temperature CO2 capture process is energy and capital intensive contributing to the problem. Warm CO2 capture has been predicted to be a key enabling technology for IGCC-CCS. Here, we assessed the applicability of CO2 removal technology to IGCC via a warm pressure swing adsorption (PSA) process based on our newly invented sorbent, which has good cyclic sorption-desorption performance at an elevated temperature. A 16-step warm PSA process was simulated using Aspen Adsorption based on the real sorbent properties. We used the model to fully explore the intercorrelation between hydrogen recovery, CO2 capture percentage, regeneration pressure of sorbent, and steam requirement. Their trade-off effects on IGCC efficiency were investigated by integrating the PSA process into the plant-wide IGCC simulation using Aspen Plus. On the basis of our analysis, IGCC-warm PSA using our new sorbent can produce similar thermal efficiencies to IGCC-cold Selexol. In order to achieve this, warm PSA needs a narrow range of process parameters to have a good balance between the hydrogen loss, steam consumption and work requirement for CO2 compression. This paper provides a rigorous analysis framework for assessing the feasibility of warm CO2 capture by sorbents in an IGCC system.
[Show abstract][Hide abstract] ABSTRACT: High pressure, single pulse shock tube oxidation experiments were conducted in order to probe the chemical kinetic effects of the double bond position in long alkenes. All oxidation experiments were carried out with approximately 100 ppm of 1-decene, cis-2-decene, cis-5-decene, and trans-5-decene, in argon, at stoichiometric conditions. The experimental conditions covered the pressure range of 40–66 bar and temperature range of 850–1500 K, with an average reaction time of 2 ms. Gas chromatographic measurements of the stable intermediates indicated increased reactivity for the isomers with more centrally located double bonds, with no influence from the cis–trans configuration observed at these conditions. Significantly different yields in most of the intermediate species measured were observed. Chemical kinetic models were assembled with the aid of Reaction Mechanism Generator where these are able to adequately predict the major product species of all isomers investigated. Simulation of the experiments indicates significantly different reaction pathways that each decene isomers undergoes, controlled entirely by the position of the double bond. The implication for fuels with such molecular structure is that reactivity, as well as pollutant formation characteristics can be significantly different depending on the position of the double bond in very similar molecules.
Proceedings of the Combustion Institute 07/2014; 35(1). DOI:10.1016/j.proci.2014.06.020 · 2.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The rate of self-reaction of the simplest Criegee intermediate, CH2OO, is of importance in many current laboratory experiments where CH2OO concentrations are high, such as flash photolysis and alkene ozonolysis. Using laser flash photolysis while simultaneously probing both CH2OO and I atom by direct absorption, we can accurately determine absolute CH2OO concentrations as well as the UV absorption cross section of CH2OO at our probe wavelength (λ = 375 nm), which is in agreement with a recently published value. Knowing absolute concentrations we can accurately measure kself = 6.0 ± 2.1 × 10–11cm3 molecule–1 s–1 at 297 K. We are also able to put an upper bound on the rate coefficient for CH2OO + I of 1.0 × 10–11 cm3 molecule–1 s–1. Both of these rate coefficients are at least a factor of 5 smaller than other recent measurements of the same reactions.