Early Reaction Times Research Articles

PAH growth and soot formation in the pyrolysis of acetylene and benzene at high temperatures and pressures: Modeling and experiment

The growth of high molecular polycyclic aromatic hydrocarbons (PAHs) before soot inception is modeled during C 2 H 2 and C 6 H 6 pyrolysis at temperatures between 1600 and 2400 K, p =60 bar, and C-atom concentrations of (3.8–4.0)×10 −6 mol/cm 3 . The formation of high molecular PAH is mainly computed by two different reaction pathways: (1) successive H-abstraction C 2 H 2 additions and (2) a combinative ring-ring condensation of aromatics. The calculations are compared with measurements obtained from C 2 H 2 and C 6 H 6 pyrolysis by the shock wave method. From reaction flux analysis, it is deduced that the combinative route plays a major role in forming PAHs, especially at early reaction times. The calculated induction times of higher PAHs reasonably reflect the experimental trends of soot inception. The slope of the computed and measured induction times in dependence on the temperature resulted in a similar activation energy for high molecular PAH and soot formation. It is concluded that soot mass growth rates during the pyrolysis of C 2 H 2 and C 6 H 6 are strongly related to PAH formation. Thus, the decrease of the soot mass growth rates during C 2 H 2 pyrolysis for T >2000 K seems to be substantially influenced by a change in the nucleation process of soot precursors.

Enhanced SO2 abatement with water‐hydrated dolomitic particles

AbstractA dolomitic calcine was hydrated with liquid water and then recalcined as an effective sorbent toward SO2 removal. The hydrated/recalcined sorbent was tested for SO2 removal in a thermogravimetric analyzer under conditions applicable to an atmospheric fluidized‐bed combustor: 1,123 K, 20 vol. % excess air, and 3,000 ppmv SO2. Specific pore surface areas and volumes of hydrated/recalcined dolomites, hydrated at 80°C for 1 h, increased by four and five times, respectively, compared to unhydrated dolomitic calcines. Due to these improved pore characteristics, the SO2 capture achieved by the hydrated/recalcined calcine, was 1.3–1.6 times higher than that of the unhydrated one after a 1‐h exposure to SO2 concentrations of 0.11–0.31 vol. % at 1,073–1,173 K. In the hydrated/recalcined calcine CaO utilization was enhanced due to the absence of significant pore diffusional limitations, at least during the early reaction times, evidenced by the high apparent activation energy of 198 kJ/mol. On the contrary, the lower CaO utilization in the unhydrated calcine was presumably due to severe pore diffusional limitations, which dropped the apparent activation energy from 149 to 85 kJ/mol at > 1,123 K.

Measurement of O( $^1$ D) formation during thermal decomposition of CO $_2$ behind shock waves

Atomic Resonance Absorption Spectroscopy (ARAS) was applied to measure the time dependent concentration of electronically excited O(\(^1\)D)-atoms during the thermal decomposition of CO\(_2\) behind reflected shock waves. The experiments were performed in the temperature range 4102 K \( \le \) T \( \le \) 6375 K at pressures 0.2 to 1.9 bar with initial gas mixtures of 100 to 1000 ppm CO\(_2\) diluted in Ar. The measured O(\(^1\)D)-formation rate at early reaction times divided by the initial reactant concentrations was found to obey the Arrhenius law: \(\) The assumption of a fast thermalisation between the O(\(^3\)P) and O(\(^1\)D) states is in agreement with previous measurements of the O(\(^3\)P) formation during the thermal decomposition of CO\(_2\), see Burmeister and Roth (1990).

Measurement of O(1D) formation during thermal decomposition of CO2 behind shock waves

Atomic Resonance Absorption Spectroscopy (ARAS) was applied to measure the time dependent concentration of electronically excited O(1D)-atoms during the thermal decomposition of CO2 behind reflected shock waves. The experiments were performed in the temperature range 4102 K≤T≤6375 K at pressures 0.2 to 1.9 bar with initial gas mixtures of 100 to 1000 ppm CO2 diluted in Ar. The measured O(1D)-formation rate at early reaction times divided by the initial reactant concentrations was found to obey the Arrhenius law: $$\begin{gathered} \left. {\frac{{d{{\left[ {O\left( {^1 D} \right)} \right]} \mathord{\left/ {\vphantom {{\left[ {O\left( {^1 D} \right)} \right]} {dt}}} \right. \kern-\nulldelimiterspace} {dt}}}}{{\left[ {CO_2 } \right]_0 \left[ {Ar} \right]}}} \right|_{t \approx 0} \hfill \\ = 1.23 \times 10^{14} \exp \left( { - 74810{K \mathord{\left/ {\vphantom {K T}} \right. \kern-\nulldelimiterspace} T}} \right)cm^3 mol^{ - 1} s^{ - 1} \hfill \\ \end{gathered} $$

Covert orienting to non-informative cues: reaction time studies

Lateralized, non-informative visual cues lengthen reaction time (RT) to successive targets flashed in the same hemified. Early ipsilateral RT facilitation is limited to the co-occurrence of cues and targets. Inhibition from visual cues has sensory components which do not depend on orienting, as well as attentional components which are limited to one side of the vertical meridian. An inhibition of RT to targets ipsilateral to the cues has been found with somatic or auditory cues and targets, and also when somatic targets follow visual cues or visual targets follow somatic cues. The results reviewed in this paper (1) are best accounted for by directional constraints in motor readiness which are induced by the voluntary suppression of an overt orienting toward the location of the cue; (2) indicate that similar mechanisms of covert orienting operate in the whole peripersonal and near extrapersonal space; and (3) point to a common neural substrate mediating both intramodal and cross-modal effects.

Nucleosome structure modulates benzo[a]pyrenediol epoxide adduct formation.

We have studied the binding of a chemical carcinogen to DNA reconstituted with histone octamers to determine the effect that nucleosome structure has on covalent adduct formation. Reconstitution of a plasmid containing the somatic 5S rRNA gene from Xenopus borealis resulted in characteristic nucleosome structure, as determined by micrococcal nuclease digestion, shifted migration in agarose gels, and hydroxyl radical footprinting. Formation of covalent adducts by benzo[a]pyrenediol epoxide (BPDE) occurred initially at a slower rate in reconstituted DNA than in naked plasmid, but after 2 h the total adduction levels (adducts/plasmid) were equal in both samples. Analysis of adduction at the sequence level by primer extension indicated that, after a 2-h BPDE reaction, the degree of adduction within the 5S rRNA nucleosome was suppressed by approximately 50% compared to naked DNA. The rotational setting of the guanines on the helix did not explain the level of adduction observed, since guanines in close proximity to the histone core were equally susceptible to adduction as guanines on the outer nucleosome surface. At early reaction times with BPDE, however, sequences near the 5S nucleosome dyad, where known modulations in the minor groove width occur, were the least susceptible to adduction. These results indicate that the structural features of DNA assembled into nucleosomes contribute to the susceptibility of the DNA to modification by BPDE.

Thermal decomposition of CH2 verified by product concentration measurements of C, H, and CH

At temperature T>-2200 K, ketene (CH2CO) is known as a convenient, reproducible and well-defined CH2 source. In experiments performed behind reflected shock waves, the subsequent reactions of CH2 were monitored by concentration measurements of various product species, including C, H, and CH, using atomic resonance absorption and ring dye laser absorption spectroscopy. The time histories of these species were investigated in various initial ketene (5–75ppm)/argon mixtures in the temperature range 2200–3400 K at pressures 0.7≤p≤2.4 bar. From the concentration slopes at t≈0, apparent rate coefficients for C, H, and CH formation were determined, but only the C concentration profiles at early reaction times have direct kinetic significance. From these data, the rate coefficient of the thermal decomposition reaction C H 2 + M ⇋ k 2 C + H 2 + M k 2 = 1.60 × 10 14 exp ⁡ ( − 32 , 200 K / T ) c m 3 m o l − 1 s − 1 (R2)

A Shock Tube Study of Chlorobenzene Pyrolysis

Abstract The pyrolysis of a 2.1% chlorobenzene-Ne mixture was studied behind reflected shock waves over the temperature range 1580-2000 K and total pressures of 0.30-0.50 atm. Time dependent reactant and major product profiles were obtained using time-of-flight mass spectrometry. Acetylene, hydrogen chloride, and diacetylene are the major products. Minor products include C6,H2, benzene, and C8H2. The only radical detected is phenyl which appears at early reaction times. HCl is the only chlorine containing product recorded; Cl atoms, Cl2, and species with m/e > 114 were not detected and were assumed to be below the TOF detectability limit which is approximately 10−2mol/cm3. The total carbon balance depends upon temperature. The maximum imbalance occurs around 1800 K. C2H2 and C4H2 represent only 25-40% of the carbon resulting from the decomposition of chlorobenzene. C6H2, C6H6 and C8H2 add another 10%; the remaining 50-65% of carbon atoms from C6H5Cl decomposition are lost to soot formation by the end of t...

High-temperature kinetics of the reaction CN + CO 2

The high-temperature reaction of the CN radicals with CO 2 was studied behind reflected shock waves in the temperature range 2510 K ≤ T ≤ 3510 K at pressures of about 1.6 bar. Mixtures of C 2N 2 and CO 2 highly diluted in argon were used as initial reactants, in which C 2N 2 served as a thermal CN source. Its reaction with CO 2 was followed by N- and O-atom concentration measurements, monitored in the postshock reaction zone by using atomic resonance absorption spectroscopy (ARAS). Because of the very fast decomposition of the primary reaction product NCO into N and CO, the rate coefficient of the reaction CN + CO 2 is very sensitive to the measured N-atom concentrations. A rate coefficient for the reaction CN + CO 2 → NCO + CO, k 3 = 4.0 × 10 14 exp(-19,200 K/T) cm 3 mol -1 s -1 could be determined by fitting computed N-atom concentrations to measured N-atom profiles at early reaction times. For further detailed fitting procedures of measured O- and N-atom concentrations at later reaction times, high-temperature values of rate coefficients for the reactions CN + O → CO + N, CN + N → C + N 2, and CO 2 + N → CO + NO were also obtained.

Interplay between U2 snRNP and 3′ splice factor(s) for branch point selection on human β -globin pre-mRNA

We investigated the interaction of U2 snRNP with the branch-3' splice site region of three human beta-globin pre-mRNAs carrying nearly complete (BamHI RNA), 24 nt (Avall RNA) and 14 nt (Accl RNA) of exon 2. All supported splicing, but mRNAs yields were respectively 2 and 10 times lower for Avall and Accl RNAs than for BamHI. Analysis of RNase T1-resistant fragments immunoprecipitated by an anti-(U2)RNP antibody at early times of the splicing reaction showed that the protection encompasses both the branch point region and the end of the intron in BamHI and Avall, but essentially only the branch point in Accl RNAs. Later on, this protection becomes less detectable in BamHI, is reinforced in Avall and remains poorly detectable in Accl RNAs. Similar experiments performed at late times with an anti-Sm antibody recognizing all snRNPs showed that the end of the intron is protected in all but BamHI RNAs. These results support the conclusion that U2 snRNP binds to a fully efficient precursor (BamHI RNA) through another factor(s) recognizing the 3' splice site (U5 snRNP and the so-called U2AF protein are likely candidates). Either the absence of an initial contact between U2 snRNP and the factor(s) recognizing the end of the intron (Accl RNA) or the unability of this ternary complex to undergo a conformational change (Avall RNA) could render these severely truncated precursors poor substrates. These different situations have consequences on the branch point selection itself. BamHI and Avall RNAs use three functional branch points at early times, the usual A residue at -37 and two U residues at -17 and -22. Accl RNA uses only one branch point at -37. Later on, all three branch points are used at the same rate in Avall, while the usual one prevails in BamHI RNAs.

The β Subunit Modulates Bypass and Termination at UV Lesions During in Vitro Replication with DNA Polymerase III Holoenzyme of Escherichia coli

The cycling time of DNA polymerase III holoenzyme during replication of UV-irradiated single-stranded (ss) DNA was longer than with unirradiated DNA (8 versus 3 min, respectively), most likely due to slow dissociation from lesion-terminated nascent DNA strands. Initiation of elongation on primed ssDNA was not significantly inhibited by the presence of UV lesions as indicated by the identical distribution of replication products synthesized at early and late reaction times and by the identical duration of the initial synthesis bursts on both unirradiated and UV-irradiated DNA templates. When replication was performed with DNA polymerase III* supplemented with increasing quantities of purified beta 2 subunit, the cycling time on UV-irradiated DNA decreased from 14.8 min at 1.7 nM beta 2 down to 6 min at 170 nM beta 2, a concentration in which beta 2 was in large excess over the polymerase. In parallel to the reduction in cycling time, also the bypass frequency of cyclobutane-photodimers decreased with increasing beta 2 concentration, and at 170 nM beta 2, bypass of photodimers was essentially eliminated. It has been shown that polymerase complexes with more than one beta 2 per polymerase molecule were formed at high beta 2 concentrations (Lasken, R. S., and Kornberg, A. (1987) J. Biol. Chem. 262, 1720-1724). It is plausible that polymerase complexes obtained under high beta 2 concentration dissociate from lesion-terminated primers faster than polymerase complexes formed at a low beta 2 concentration. This is expected to favor termination over bypass at pyrimidine photodimers and thus decrease their bypass frequency. These results suggest that the beta 2 subunit might act as a sensor for obstacles to replication caused by DNA damage, and that it terminates elongation at these sites by promoting dissociation. The intracellular concentration of beta 2 was estimated to be 250 nM (Kwon-Shin, O., Bodner, J. B., McHenry, C. S., and Bambara, R. A. (1987) J. Biol. Chem. 262, 2121-2130) and is 15-fold higher than the estimated intracellular concentration of DNA polymerase III holoenzyme (15 nM). This high concentration of beta 2 may be responsible for the observation that very little (if any) bypass of pyrimidine photodimers occurred in vivo when the SOS system was not induced. Moreover, it predicts that bypass synthesis under SOS conditions might be associated with an altered form of the beta subunit.

In Vitro Assessment of Nystose as a Sugar Substitute

Nystose represents a fructooligosaccharide with two fructose molecules linked via β(1→2) bonds to the fructosyl moiety of sucrose. This tetrasaccharide was subjected to an array of in vitro tests designed for the assessment of potential sugar substitutes before animal or human studies. β-Fructosidase from yeast cleaved nystose at about 5% of the initial rate observed with sucrose. The terminal fructose was released first. Glycosyltransferase from Streptococcus mutans #620 did not utilize nystose for the formation of a glucan-type polysaccharide. Anaerobic fermentation of nystose by a suspension of mixed dental plaque microorganisms and by S. mutans NCTC 10449 was about half as fast as with sucrose. Thin-layer chromatography at various reaction times with S. mutans NCTC 10449 indicated the terminal fructose as the site of first attack. Analyses for free monosaccharides confirmed these data because free fructose exceeded free glucose at early reaction times far more than would follow from the 3:1 ratio of fructose to glucose in the nystose molecule. High pressure liquid chromatography assays demonstrated lactic and acetic acids as the main fermentation products. Carbohydrases from human jejunal mucosa did not attack nystose. However, cecal anaerobic microorganisms of the rat fermented nystose rapidly into acids.

Poliovirus RNA-dependent RNA polymerase and host cell protein synthesize product RNA twice the size of poliovirion RNA in vitro.

The poliovirus RNA-dependent RNA polymerase required an oligouridylate primer or a HeLa cell protein (host factor) to initiate RNA synthesis on poliovirion RNA in vitro. The polymerase synthesized template-sized product RNA in the oligouridylate-primed reaction. In the host factor-dependent reaction, the largest product RNA synthesized by the polymerase was twice the size of the template RNA. About half of the product RNA recovered from this reaction was shown to exist in the form of a snapback sequence. Time-course reactions and pulse-chase experiments showed that the product RNA was only slightly larger than the template RNA at early reaction times and that with time it increased in size to form the dimer-sized product RNA. Inhibition of the elongation reaction by adding only [alpha-32P]UTP and ATP resulted in the formation of template-sized product RNA. The dimer-sized product RNA was unaffected by phenol extraction or proteinase K treatment but was converted to template-sized molecules by S1 nuclease. Dimer-sized poliovirus RNA that was sensitive to S1 nuclease was also isolated from poliovirus-infected cells. The results from this study indicate that the labeled negative-strand product RNA synthesized in vitro was covalently linked to the positive-strand template RNA. Thus, in vitro, the primer-dependent poliovirus RNA polymerase may initiate RNA synthesis in the presence of the host factor by using the 3' end of the template RNA as a primer.

DNA synthesis catalyzed in vitro by yeast extracts using A 2 ?m DNA containing plasmid as template for enzymatic DNA synthesis

Partially purified cell-free extracts prepared from growing cells of the yeast Saccharomyces cerevisiae catalyze DNA synthesis directed by the chimeric plasmid BTYP-2 containing the whole 2 μm DNA sequence. The biochemical properties of the reaction have been examined and inactivation studies indicate that DNA synthesis is only due to protein factors. Analysis of the products of the DNA synthesis reaction demonstrates that part of the in vitro synthesized DNA is not covalently linked to the template, thus suggesting initiation of new DNA chains. The analysis of the distribution of the labelled products at early times of reaction showed a preferential synthesis on the 2 μm portion of the BTYP-2 DNA template.

二次処理水のオゾン処理による反応生成物の同定と定量

Some batch ozonation experiments on the secondary effluents. The identification of intermediates and end products resulting from the ozonation was studied, setting the reaction time and the ozone concentrations applied as reaction parameters. The feed gas to the reactor had ozone concentration of 9.6-59.5 mg/l. Trace amounts of formaldehyde, acetaldehyde, proponic aldehyde, methyglyoxal, glyoxylic acid and pyruvic acid were detected by gas chromatography and colorimetry for 2, 4-dinitrophenylhydrazones. Formaldehyde was a common product resulting from the ozonation of many organic compounds in the early reaction time. But, this was finally oxidized to carbon dioxide by the prolonged reaction time at a faster rate. Methylglyoxal and pyruvic acid were also oxidized to pyruvic acid and acetic acid, respectively. But, the decomposition rate of pyruvic acid is not so fast as formaldehyde's. Acetic acid, formic acid and propionic acid were also detected by gas chromatography for their benzyl esters. The latter two were present in trace amounts. The quantity of acetic acid increased with the reaction time increment. During the course of the reaction, 1/6-1/5 of total COD can be accounted for by the summations of each TOD on formaldehyde, methylglyoxal, pyruvic acid and acetic acid produced by the ozonation of secondary effluents.