Global Rate Constants Research Articles

Theoretical investigation of the OH-initiated atmospheric degradation mechanism of CX2CHX (X = H, F, Cl) by advanced quantum chemical and transition state theory methods.

Halogenated olefins are anthropogenic compounds with many industrial applications but at the same time raising many environmental and health concerns. Gas-phase electrophilic addition of the OH radical to the olefinic CC bond represents the primary sink for these chemicals in the atmosphere, with the degree and type of halogenation playing a significant role in their overall reactivity. In this work, we present a theoretical investigation of the reaction mechanisms and kinetics for the reactions between the OH radical and CH2CH2 (ethylene, ETH), CF2CHF (trifluoroethylene, TFE) and CCl2CHCl (trichloroethylene, TCE), simulated by state-of-the-art protocols and methods, with the aim of providing a detailed interpretation of the available experimental results, as well as new data of relevance to tropospheric chemistry. Specifically, potential energy surfaces (PESs) are obtained using the jun-Cheap (jChS) composite scheme, whereas temperature and pressure dependent rate coefficients and product distributions in the 100-600 K temperature range are calculated within the Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) framework. The rates for barrierless channels are obtained from variable reaction coordinate-variational transition state theory (VRC-VTST) combined with the two transition state model. While the reactions with ETH and TFE proceed mainly via the formation of addition adducts at P = 1 atm and T = 298 K, the dominant channel for TCE is the Cl-elimination reaction. Global rate constants for the two halogenated olefins, TFE and TCE, are found to be pressure-independent, contrary to the case of ETH. The computed rate constants, as well as their temperature and pressure dependence, are in remarkable agreement with the available experimental data, and they are used to derive atmospheric lifetimes (τ) for both TFE and TCE as a function of altitude (h) in the atmosphere, by taking into account variations in the rate coefficients (k (T, P)) and [OH] concentration.

Kinetic model-based group contribution method for derived cetane number prediction of oxygenated fuel components and blends

A four-step autoignition model was used to derive an expression for the ignition delay (ID) measured in ignition quality testers (IQT) with the derived cetane number (DCN) determined from this ID using the ASTM D6890 standard correlation. The model predicts DCN for individual compounds and blends as a function of each compound's global initiation and net chain branching rate constants. Expressions for these values were determined assuming they could be related to the functional groups present in each compound. Measurement data for 125 compounds and 94 binary and ternary blends (including oxygenates: alcohols, aldehydes, esters, ethers, and ketones), from literature and from measurements performed in our lab, were used to obtain the dependence of the measured ignition delay on each functional group. The new kinetic model-based group contribution method was able to predict the ignition delay of both pure compounds and blends, with an average DCN error of 4.4 (19%) and 2.8 (11%), respectively. The blend model was also used to develop an ID mixing rule by incorporating existing IQT ignition delay data for each compound. Use of the mixing rule gave an average DCN error of 3.6 (16%) for blends. Both the blend model and mixing rule were found to be superior compared to standard linear by volume fraction or mole fraction mixing rules commonly used to estimate the DCN of mixtures.

Quantitative assessment of translocator protein (TSPO) in the non-human primate brain and clinical translation of [18F]LW223 as a TSPO-targeted PET radioligand

ObjectivesTranslocator protein 18 kDa (TSPO) positron emission tomography (PET) can be harnessed for the non-invasive detection of macrophage-driven inflammation. [18F]LW223, a newly reported TSPO PET tracer which was insensitive to rs6971 polymorphism, showed favorable performance characteristics in a recent imaging study involving a rat myocardial infarction model. To enable quantitative neuroimaging with [18F]LW223, we conducted kinetic analysis in the non-human primate (NHP) brain. Further, we sought to assess the utility of [18F]LW223-based TSPO imaging in a first-in-human study. MethodsRadiosynthesis of [18F]LW223 was accomplished on an automated module, whereas molar activities, stability in formulation, lipophilicity and unbound free fraction (fu) of the probe were measured. Brain penetration and target specificity of [18F]LW223 in NHPs were corroborated by PET-MR imaging under baseline and pre-blocking conditions using the validated TSPO inhibitor, (R)-PK11195, at doses ranging from 5 to 10 mg/kg. Kinetic modeling was performed using one-tissue compartment model (1TCM), two-tissue compartment model (2TCM) and Logan graphical analyses, using dynamic PET data acquisition, arterial blood collection and metabolic stability testing. Clinical PET scans were performed in two healthy volunteers (HVs). Regional brain standard uptake value ratio (SUVr) was assessed for different time intervals. Results[18F]LW223 was synthesized in non-decay corrected radiochemical yields (n.d.c. RCYs) of 33.3 ± 6.5% with molar activities ranging from 1.8 ± 0.7 Ci/µmol (n = 11). [18F]LW223 was stable in formulation for up to 4 h and LogD7.4 of 2.31 ± 0.13 (n = 6) and fu of 5.80 ± 1.42% (n = 6) were determined. [18F]LW223 exhibited good brain penetration in NHPs, with a peak SUV value of ca. 1.79 in the whole brain. Pre-treatment with (R)-PK11195 substantially accelerated the washout and attenuated the area under the time-activity curve, indicating in vivo specificity of [18F]LW223 towards TSPO. Kinetic modeling demonstrated that 2TCM was the most suitable model for [18F]LW223-based neuroimaging. Global transfer rate constants (K1) and total volumes of distribution (VT) were found to be 0.10 ± 0.01 mL/cm3/min and 2.30 ± 0.17 mL/cm3, respectively. Dynamic PET data analyses across distinct time windows revealed that the VT values were relatively stable after 60 min post-injection. In a preliminary clinical study with two healthy volunteers, [18F]LW223 exhibited good brain uptake and considerable tracer retention across all analyzed brain regions. Of note, an excellent correlation between SUVr with VT was obtained when assessing the time interval from 20 to 40 min post tracer injection (SUVr(20–40 min), R2 = 0.94, p < 0.0001), suggesting this time window may be suitable to estimate specific binding to TSPO in human brain. ConclusionOur findings indicate that [18F]LW223 is suitable for quantitative TSPO-targeted PET imaging in higher species. Employing state-of-the-art kinetic modeling, we found that [18F]LW223 was effective in mapping TSPO throughout the NHP brain, with best model fits obtained from 2TCM and Logan graphical analyses. Overall, our results indicate that [18F]LW223 exhibits favorable tracer performance characteristics in higher species, and this novel imaging tool may hold promise to provide effective neuroinflammation imaging in patients with neurological disease.

Optimisation method for automatic selection of rate constants of global reaction mechanisms

Optimisation approach to automate selection of global reaction mechanisms rate constants is proposed and studied. The objective of optimisation is to find reaction rate constants minimising deviation of some flame characteristics (e.g. laminar burning velocity, ignition delay time, etc.) calculated by global mechanism from their reference values known from experiments or computed by detailed mechanisms. Examples of one, two and four step mechanisms optimisation with respect to laminar burning velocity and concentration distributions in counterflow diffusion flame are given. Computer codes implementing optimisation algorithm for these examples are also afforded and can be modified and used for reaction constants selection in various applications. Uniqueness of singlecriteria and multicriteria optimisation solutions is studied numerically by computations with different initial guesses and by direct evaluation of the objective functions. Particularly, it is found that for considered global mechanisms the minimum value of objective function is reached in some subdomain of the parametric space. This means that any values of rate parameters from this subdomain results in almost the same deviations of chosen flame characteristic from its reference value.

A functional-group-based approach to modeling real-fuel combustion chemistry – III: Application to biodiesels

Real biodiesel fuels are mixtures comprising many high molecular weight components, making it a challenge to predict their combustion chemistry with detailed kinetic models. Our group previously proposed a functional-group approach (FGMech) to model the combustion chemistry of real gasoline and jet fuels; this approach has now been extended to model real biodiesel combustion and mixtures with petroleum fuels. As in our previous work, a decoupling philosophy is adopted for construction of the model. A lumped reaction mechanism describes the (oxidative) pyrolysis of fuels, while a detailed base chemistry model represents the oxidation of key pyrolysis intermediates. However, due to the presence of the ester group, several oxygenated species are identified as additional primary products and incorporated into the lumped reaction steps. In addition to the lumped reactions initiated by unimolecular decomposition and H-atom abstraction reactions, a lumped H-atom addition-elimination reaction is also incorporated as a new reaction class to account for the presence of double bonds. Stoichiometric parameters are obtained based on a multiple linear regression (MLR) model, which establishes relationships between the fuel's functional group distributions and the stoichiometric parameters of the lumped reactions. Global rate constants are developed from consistent rate rules obtained from pure fuels. New pyrolysis experimental data for methyl pentanoate/methyl nonanoate and methyl heptanoate/n-heptane mixtures (50%/50% in mol) are obtained in a jet-stirred reactor at atmospheric pressure. In general, kinetic models developed using the FGMech approach can reasonably reproduce all the validation targets obtained in this work, as well as those in the literature, confirming that functional-group-modeling is a promising approach to simulate combustion behavior of diesel/biodiesel surrogate fuels and real biodiesels.

Charge-Separation and Charge-Recombination Rate Constants in a Donor-Acceptor Buckybowl-Based Supramolecular Complex: Multistate and Solvent Effects.

The kinetics of the nonradiative photoinduced processes (charge-separation and charge-recombination) experimented in solution by a supramolecular complex formed by an electron-donating bowl-shaped truxene-tetrathiafulvalene (truxTTF) derivative and an electron-accepting fullerene fragment (hemifullerene, C30H12) has been theoretically investigated. The truxTTF·C30H12 heterodimer shows a complex decay mechanism after photoexcitation with the participation of several low-lying excited states of different nature (local and charge-transfer excitations) all close in energy. In this scenario, the absolute rate constants for all of the plausible charge-separation (CS) and charge-recombination (CR) channels have been successfully estimated using the Marcus–Levich–Jortner (MLJ) rate expression, electronic structure calculations, and a multistate diabatization method. The outcomes suggest that for a reasonable estimate of the CS and CR rate constants, it is necessary to include the following: (i) optimally tuned long-range (LC) corrected density functionals, to predict a correct energy ordering of the low-lying excited states; (ii) multistate effects, to account for the electronic couplings; and (iii) environmental solvent effects, to provide a proper stabilization of the charge-transfer excited states and accurate external reorganization energies. The predicted rate constants have been incorporated in a simple but insightful kinetic model that allows estimating global CS and CR rate constants in line with the most generalized three-state model used for the CS and CR processes. The values computed for the global CS and CR rates of the donor–acceptor truxTTF·C30H12 supramolecular complex are found to be in good agreement with the experimental values.

Systematic Approach to Find the Global Minimum of Relaxation Dispersion Data for Protein-Induced B-Z Transition of DNA.

Carr–Purcell–Meiboom–Gill (CPMG) relaxation dispersion spectroscopy is commonly used for quantifying conformational changes of protein in μs-to-ms timescale transitions. To elucidate the dynamics and mechanism of protein binding, parameters implementing CPMG relaxation dispersion results must be appropriately determined. Building an analytical model for multi-state transitions is particularly complex. In this study, we developed a new global search algorithm that incorporates a random search approach combined with a field-dependent global parameterization method. The robust inter-dependence of the parameters carrying out the global search for individual residues (GSIR) or the global search for total residues (GSTR) provides information on the global minimum of the conformational transition process of the Zα domain of human ADAR1 (hZαADAR1)–DNA complex. The global search results indicated that a α-helical segment of hZαADAR1 provided the main contribution to the three-state conformational changes of a hZαADAR1—DNA complex with a slow B–Z exchange process. The two global exchange rate constants, kex and kZB, were found to be 844 and 9.8 s−1, respectively, in agreement with two regimes of residue-dependent chemical shift differences—the “dominant oscillatory regime” and “semi-oscillatory regime”. We anticipate that our global search approach will lead to the development of quantification methods for conformational changes not only in Z-DNA binding protein (ZBP) binding interactions but also in various protein binding processes.

Kinetic Study and Numerical Validation of the Cyanide Neutralization Process Using Alkaline Chlorination

In the recovery of precious metals from the mining industry, cyanide is the most used leaching agent due to its high efficiency. However, in some cases cyanide mining waste is dumped into bodies of water without a proper treatment, causing irreversible damage to the environment. Therefore, it is essential to have sustainable, efficient, and economic processes for the adequate treatment of cyanide toxic waste. This work presents a kinetic study of cyanide neutralization using an alkaline chlorination method on waste samples from silver leaching. Results presented demonstrate removal efficiencies of 99.7% at volumetric CN- with loading rates of about 2047 mg/l in a period of 75 h. Additionally, numerical simulation was validated considering the experimental results. For the simulation, fundamental parameters, such as mass balance, temperature and concentration, were considered to find kinetic global rate constants. The results obtained allowed to propose the design of a cyanide neutralization reactor.

Reaction of hydrogen peroxide vapor on platinum on alumina spheres

An experimental and numerical study of the rates of decomposition of hydrogen peroxide vapor on catalytic surfaces was conducted. Hydrogen peroxide vapor and helium carrier gas flowed over a catalyst (platinum on alumina spheres of varying diameter) at a total pressure of <3 Torr, simulating the conditions of a flash evaporation propulsion system for space applications. After reacting on the catalyst, the product gas composition was measured using near-infrared laser absorption on a known hydrogen peroxide line in a multi-pass flow cell. Global reaction rate constants were inferred for varying catalyst surface areas, residence times, and temperatures. The combination of these experimental data with numerical simulations led to the conclusion that under the conditions employed in this study, namely low peroxide partial pressures and low Reynolds numbers, decomposition rates are transport-limited rather than reaction-limited. These results may be applicable to the development of catalyst bed designs for small satellite propulsion systems.

Ab initio kinetic mechanism of OH-initiated atmospheric oxidation of pyrrole

The comprehensive kinetic mechanism of the OH-initiated gas-phase oxidation of pyrrole is first theoretically reported in a broad range of conditions (T = 200–2000 K &P = 1–7600 Torr). On the potential energy surface constructed at the M06–2X/aug-cc-pVTZ level, the temperature- and pressure-dependent behaviors of the title reaction were characterized using the stochastic Rice–Ramsperger–Kassel–Marcus based Master Equation (RRKM-ME) rate model. The corrections of the hindered internal rotation and quantum tunneling treatments were included. The calculated results reveal the competition between the two distinct pathways: OH-addition and direct H-abstraction. The former channels are found favorable at low-temperature and high-pressure range (e.g., T < 900 K and P = 760 Torr) where non-Arrhenius and positive pressure-dependent behaviors of the rate constants are noticeably observed, while the latter predominate at temperatures higher than 900 K at atmospheric pressure and no pressure dependence on the rate constant is found. The predicted global rate constants are in excellent agreement with laboratory values; thus, the derived kinetic parameters are recommended for modeling/simulation of N-heterocycle-related applications in atmospheric and even in combustion conditions. Besides, pyrrole should not be considered as a persistent organic pollutant owing to its short atmospheric lifetime (∼1 h) towards OH radicals. The secondary mechanisms of the subsequent reactions of two OH-pyrrole adducts (namely, I1 and I2) with two abundant species, O2/NO, which are relevant to the atmospheric degradation process, were also investigated. It is also revealed by TD-DFT calculations that two OH-pyrrole adducts (I1 &I2), nine intermediates, Ii (i = 3–11) and four products (P1, P2, P3 and P6) can undergo photodissociation under the sunlight.

Theoretical Studies of the Gas-Phase Reactions of S-Methyl Methanesulfinothioate (Dimethyl Thiosulfinate) with OH and Cl Radicals: Reaction Mechanisms, Energetics, and Kinetics.

The mechanisms, energetics, and kinetics of the gas-phase reactions of terrestrial plant-derived dimethyl thiosulfinate (DMTS) with the atmospheric oxidants OH and Cl radicals were investigated using high-level ab initio calculations. The results show that the addition of OH and Cl radicals to the sulfinyl [-S(═O)] of DMTS, followed by S(═O)-S single bond cleavage to form methanesulfinic acid + CH3S• and methanesulfinyl chloride + CH3S•, respectively, are the more dominant reactions. The barrier heights for the reactions with OH and Cl radicals were found to be -5.6 and -12.7 kcal/mol relative to the energies of the starting reactants, respectively, when computed at the CCSD(T)/aug-cc-pVTZ//M06-2X/6-311++G(3df,3pd) level of theory. The rate constants for all possible pathways of DMTS + •OH/•Cl reactions were investigated using the MESMER kinetics code over the temperature range between 200 and 300 K. The calculated global rate constants for the DMTS + •OH and DMTS + •Cl reactions at 300 K were found to be 1.42 × 10-11 and 3.72 × 10-11 cm3 molecule-1 s-1, respectively. In addition, the thermochemistry of all possible paths and branching ratios was determined. The atmospheric chemistry implications of the DMTS + •OH/•Cl reactions are discussed.

Binding studies based on ultrafast affinity extraction and single- or two-column systems: Interactions of second- and third-generation sulfonylurea drugs with normal or glycated human serum albumin

Ultrafast affinity extraction was evaluated and used with microcolumns containing human serum albumin (HSA) to measure the global affinity constants and dissociation rate constants for several second- and third-generation sulfonylurea drugs with solution-phase normal HSA or glycated HSA. Glibenclamide, glimepiride and glipizide were used as model drugs for this work. Both single- and two-column systems were considered for the analysis of global affinities for the model drugs. These methods were optimized with respect to the flow rates, column sizes and sample residence times that were employed with each drug for ultrafast affinity extraction. Data acquired with single-column systems were further utilized to estimate the dissociation rate constants for normal HSA and glycated HSA with the given drugs. The binding constants obtained by the single- and two-column systems showed good agreement with each other and with values obtained from the literature. Use of a single-column system indicated that levels of glycation found in controlled or advanced diabetes resulted in a 18–44% decrease in the overall binding strength of the model drugs with HSA. Although the two-column system allowed work with smaller free drug fractions and clinically-relevant drug/protein concentrations, the single-column system required less protein, provided better precision, and was easier to use in binding studies.

Crystallization kinetics of palm oil of different geographic origins and blends thereof by the application of the Avrami model

There is still ambiguity surrounding the application of the Avrami model to estimate kinetic parameters and growth dimension of crystallizing fats. In order to overcome this ambiguity, adjustments to the Avrami model are proposed. This research shows that isothermal palm oil (PO) crystallization is a stepwise process characterized by different consecutive kinetic segments, when dividing the Avrami equation into sub-domains following the study by Narine, Humphrey, and Bouzidi (2006) for an extended Avrami model. Thus, the Avrami model needs to be applied to each step separately. These sub-domains were merged to derive an average global rate constant that demonstrates the crystallization rates of fats follow bell-shaped curves depending on temperature, corroborating the fundamental crystallization literature. Applying this extended Avrami model to three different geographic origins of PO (Ghana, Colombia and Ecuador), the progression and maximum of these bell-shaped curves were found to be highly correlated with the tripalmitin to triolein (PPP/OOO) ratio of PO. Thus, we were able to advance the findings of Narine et al. (2006) and show the applicability of the Avrami model to describe the crystallization process of POs of different geographic origins and its blends with palm stearin.

Examination of the Mechanism of the Yield of N2O from Nitroxyl (HNO) in the Solution Phase by Theoretical Calculations.

The dimerization of HNO and subsequent yield of N2O in aqueous solution are studied based on the theoretical calculations and kinetic simulations. The initial dimerization reactions were computed at various levels of theory, and large divergence was observed in the predictions of the gas-phase free energies. The T1 diagnostics at CCSD(T)/aug-cc-pVTZ suggests multireference characteristics of the HNO dimers and the transition states. The solution-phase free energies were obtained using the wB97XD method and the SMD solvation model. The pKa values of the (HNO)2 tautomers and their first protonated and deprotonated products were estimated using the cluster-continuum approach. The theoretical results confirmed the original conclusion that the favored cis-pathway is comprised of several rapid proton transfer steps leading to either cis-HONNOH or cis-HONNO¯ before decomposition. Several new water-catalyzed and H3O+/water catalyzed reactions are presented to explain the fast kinetics observed in the experiments. To validate the proposed mechanism, kinetic simulations with the consideration of diffusion-limited kinetics were implemented on several related systems, based on which the previously reported global rate constant was explained as the kinetics of the initial dimerization step and the global kinetics in very dilute HNO solutions.

CLEPS 1.0: A new protocol for cloud aqueous phase oxidation of VOC mechanisms

Abstract. A new detailed aqueous phase mechanism named the Cloud Explicit Physico-chemical Scheme (CLEPS 1.0) is proposed to describe the oxidation of water soluble organic compounds resulting from isoprene oxidation. It is based on structure activity relationships (SARs) which provide global rate constants together with branching ratios for HO⋅ abstraction and addition on atmospheric organic compounds. The GROMHE SAR allows the evaluation of Henry's law constants for undocumented organic compounds. This new aqueous phase mechanism is coupled with the MCM v3.3.1 gas phase mechanism through a mass transfer scheme between gas phase and aqueous phase. The resulting multiphase mechanism has then been implemented in a model based on the Dynamically Simple Model for Atmospheric Chemical Complexity (DSMACC) using the Kinetic PreProcessor (KPP) that can serve to analyze data from cloud chamber experiments and field campaigns. The simulation of permanent cloud under low-NOx conditions describes the formation of oxidized monoacids and diacids in the aqueous phase as well as a significant influence on the gas phase chemistry and composition and shows that the aqueous phase reactivity leads to an efficient fragmentation and functionalization of organic compounds.

Adsorption of Pharmaceuticals and Personal Care Products on Granular Activated Carbon

We investigated the adsorption to granular activated carbon of two pharmaceuticals (carbamazepine and sildenafil citrate) and a personal-care product (methylparaben) in aqueous solution, characterized the carbon, and evaluated its influence on the kinetics and adsorption equilibrium of the compounds under study. We adjusted data for the analysis of equilibrium to Langmuir and Freundlich models of adsorption isotherms and described adsorption rate using pseudo first- and second-order models; that same analysis was made on the basis of the behavior of the initial rate. In addition, we analyzed the potentiality of a nonlinear adjustment for studying kinetics and equilibrium of adsorption, an approach requiring neither knowledge of equilibrium conditions nor a-priori hypothetical suppositions regarding the order of reaction. The results indicated that the nonlinear model was capable of describing adsorption kinetic behavior, in order to determine concentrations adsorbed at equilibrium, adsorption rates of the system, maximum adsorption capacity, and global rate constant. Granular carbon exhibited an adsorption capacity for carbamazepine and methylparaben of ca. 323 mg/g and for sildenafil citrate of ca. 142 mg/g, though with slow adsorption kinetics characterized by average adsorption times of at least 168 h.

Analysis of Hormone-Protein Binding in Solution by Ultrafast Affinity Extraction: Interactions of Testosterone with Human Serum Albumin and Sex Hormone Binding Globulin.

Ultrafast affinity extraction was used to study hormone-protein interactions in solution, using testosterone and its transport proteins human serum albumin (HSA) and sex hormone binding globulin (SHBG) as models. Both single column and two-dimensional systems based on HSA microcolumns were utilized to measure the free fraction of testosterone in hormone/protein mixtures at equilibrium or that were allowed to dissociate for various lengths of time. These data were used to determine the association equilibrium constants (Ka) or global affinities (nKa') and dissociation rate constants (kd) for testosterone with soluble HSA and SHBG. This method was also used to measure simultaneously the free fraction of testosterone and its equilibrium constants with both these proteins in physiological mixtures of these agents. The kd and Ka values obtained for HSA were 2.1-2.2 s(-1) and 3.2-3.5 × 10(4) M(-1) at pH 7.4 and 37 °C. The corresponding constants for SHBG were 0.053-0.058 s(-1) and 0.7-1.2 × 10(9) M(-1). All of these results gave good agreement with literature values, indicating that this approach could provide information on a wide range of rate constants and binding strengths for hormone-protein interactions in solution and at clinically relevant concentrations. The same method could be extended to alternative hormone-protein systems or other solutes and binding agents.

Combined use of rare earth (RE)-doped nanostructures and photosensitizers (PS): Possible application in photodynamic therapy (PDT)

The partial oxidation of He-diluted CH4 with O2 has been studied in a laboratory flow reactor packed with fragmented pellets of zeolite-encaged Ru phthalocyanine catalyst. Gas compositions (CH4, O2, CO, CO2, and H2) were determined by on-line GC/TCD. Absolute system pressure was maintained at 445 kPa. Most experiments were performed at a constant gas space velocity=0.717/s. Data were obtained over wide ranges of temperature (523–648 K) and CH4/O2 feed molar ratio (0.5–6.5). Observed conversion of CH4 increased with temperature. Product selectivities of H2 and CO increased, while that of CO2 decreased, with increasing temperature and feed CH4/O2. Based on data analysis with an integral plug flow packed bed reactor model, the power law kinetic rate for CH4 conversion was observed to be first order in each of O2 and CH4. A non-linear Arrhenius plot of the global rate constants, together with a Mears correlation analysis, suggests that an external mass transfer resistance becomes active above 573 K in this system.

The Effects of Temperature on the Yields of the Major Aliphatic and Aromatic Products of Supercritical n-Decane Pyrolysis

To investigate the effects of pyrolysis temperature on the production of polycyclic aromatic hydrocarbons (PAH), precursors to solids, in the precombustion environment of fuels for future high-speed aircraft, we have conducted supercritical pyrolysis experiments with the model fuel n-decane, an alkane component of jet fuels. The experiments are performed in an isothermal, isobaric, silica-lined stainless-steel flow reactor at 94.6 atm, 133 s, and at six temperatures in the range of 530–570 °C, over which n-decane conversion rises from 57% to 92%. The products from the experiments have been analyzed by gas chromatography with flame-ionization and mass-spectrometric detection as well as by high-pressure liquid chromatography with diode-array ultraviolet–visible absorption and mass-spectrometric detection, and the experimentally measured product yields are reported, as functions of pyrolysis temperature, for 20 major aliphatic products (mostly C1–C9 n-alkanes and C2–C9 1-alkenes), 27 one- and two-ring aromatics, and 117 PAH of three to nine aromatic rings. The yield-temperature data of 11 of the one- and two-ring aromatics and all 117 of the PAH products are each found to conform well to the first-order global-kinetic rate expression dY/dt = kapp(1 – X), where Y is the measured aromatic-product yield, X is n-decane conversion, and kapp is the global first-order rate constant, kapp = Aappexp[−Eapp/(RT)]. The derived values of Eapp generally increase with increasing aromatic product size from 68.26 kcal/mol for benzene, to 188.96 kcal/mol for the four-ring PAH pyrene, to 351.94 kcal/mol for the eight-ring PAH benzo[pqr]naphtho[8,1,2-bcd]perylene. In addition to all of the values of Eapp and Aapp determined for the individual product PAH, linear relationships for Eapp and ln Aapp as functions of carbon number have been determined, providing a means of modeling the production of PAH of a given carbon number in a generic way.

Kinetics and Modeling Study on Etherification of Glycerol Using Isobutylene by in Situ Production from tert-Butyl Alcohol

Experimental results on the etherification of glycerol by isobutylene using Indion-130 catalyst are presented. Experiments were carried out at five different temperatures between 45 and 85 °C in an autoclave with mechanical stirring. Isobutylene was produced in situ from tert-butyl alcohol in another autoclave connected in series. Experiments were conducted for a duration of 24 h with sample collection (for analysis) every 2 h. Samples were analyzed by gas chromatography for monitoring mono-, di-, and tri-tert-butyl glycerol ethers (MTBG, DTBG, and TTBG). The studies reveal that the optimal temperature for maximum production of DTBG is 55 °C. A multilump kinetic model has been developed, and global rate constants have been evaluated.