Reaction Rates of OH Radicals with CH3OCF2CHFCF3 and CHF2CF2OCH2CF2CHF2: Measurements and Estimation Using Neural Network Method.

The reaction rates of OH radicals with CH3OCF2CHFCF3 (k1) and CHF2CF2OCH2CF2CHF2 (k2) were measured over a temperature range of 250-430 K. Kinetic measurements were performed using the flash and laser photolysis methods combined with a laser-induced fluorescence technique. The Arrhenius rate parameters were determined as k1 = (2.52 ± 0.25) × 10-12·exp[-(1390 ± 30)/T], k2 = (1.83 ± 0.20) × 10-12·exp[-(1420 ± 35)/T] cm3 molecule-1 s-1. The infrared absorption spectra of the two hydrofluoroethers were measured at approximately 298 K in 760 Torr of N2. The atmospheric lifetimes of CH3OCF2CHFCF3 and CHF2CF2OCH2CF2CHF2 have been estimated as 2.5 and 3.8 years, respectively, and their global warming potentials were determined as 245 and 405, respectively. Additionally, a method, using a three-layered feed-forward neural network, for estimating the rates of the reaction of the OH radicals with alkanes, ethers, and alcohols was investigated. The ratios of the calculated reaction rates to the observed ones agreed within a factor of 2. The ability of the neural network method to predict reaction rates was examined by using a leave-one-out test.

Correction to: Rate constants for the reactions of OH radicals with CF3CX=CY2 (X=H, F, CF3, Y=H, F, Cl).

We make a correction to the original paper because there was a mistake in our preceding paper (Tokuhashi et al. 2018) that was cited in the original paper. In our preceding paper, we reported the measured rate constant of OH radicals with (E)-CHF=CHF.

Rate Constants for the Reactions of OH Radicals with the ( E)/( Z) Isomers of CFCl=CFCl and ( E)-CHF=CHF.

The rate constants for the OH radical reactions with halogenated ethenes were investigated experimentally and computationally. The rate constants for the reactions of OH radicals with ( E)-CFCl=CFCl ( k1), ( Z)-CFCl=CFCl ( k2), and ( E)-CHF=CHF ( k3) were measured using flash and laser photolysis methods. The temporal profile of the OH radical was monitored by a laser-induced fluorescence technique. Kinetic measurements were carried out over the temperature range of 250-430 K. Arrhenius rate constants were determined to be k1 = (1.67 ± 0.06) × 10-12·exp[(140 ± 10) K/ T], k2 = (1.75 ± 0.04) × 10-12·exp[(140 ± 10) K/ T], and k3 = (3.99 ± 0.15) × 10-12·exp[(260 ± 10) K/ T] cm3 molecule-1 s-1. The quoted uncertainties are 95% confidence levels and do not include systematic errors. Infrared absorption spectra were measured at room temperature. The atmospheric lifetimes and the global warming potentials of ( E)-CFCl=CFCl, ( Z)-CFCl=CFCl, and ( E)-CHF=CHF were estimated to be 4.3, 4.2, and 1.2 days and 0.035, 0.036, and 0.0056, respectively. The ozone depletion potentials of ( E)-CFCl=CFCl and ( Z)-CFCl=CFCl were determined to be 0.00011 and 0.00010, respectively. The photochemical ozone creation potentials of the halogenated ethenes were less than 1/4 that of ethene. In addition, the ( E)/( Z) differences in the energy and IR spectra of the CFCl=CFCl and CHF=CHF molecules were computationally examined. The reactivities of these halogenated ethenes toward OH radicals were investigated through the combination of DFT and ab initio computations. The rate constants calculated for the OH radical reactions of these halogenated ethenes showed reasonable agreement with the experimentally determined values. Our computational results for the CFCl=CFCl and CHF=CHF ( E)/( Z) isomeric pairs indicated that the rate constants toward OH radicals are larger for the higher-energy geometrical isomers than for the lower-energy counterparts.

Rate Constants for the Reactions of OH Radicals with Fluorinated Ethenes: Kinetic Measurements and Correlation between Structure and Reactivity.

The rate constants for the reaction of OH radicals with four fluorinated ethenes (CF2═CHF, ( E)-CHF═CHF, CF2═CH2, and CHF═CH2) have been measured over the temperature range of 250-430 K. Kinetic measurements have been carried out using flash photolysis and laser photolysis methods combined with a laser-induced fluorescence technique. The Arrhenius expressions for the rate constant have been determined as k(CF2═CHF) = (3.12 ± 0.11) × 10-12 exp[(270 ± 10)/ T], k(( E)-CHF═CHF) = (3.75 ± 0.08) × 10-12 exp[(230 ± 10)/ T], k(CF2═CH2) = (1.15 ± 0.07) × 10-12 exp[(230 ± 20)/ T], and k(CHF═CH2) = (1.16 ± 0.09) × 10-12 exp[(390 ± 20)/ T] cm3 molecule-1 s-1. Infrared absorption spectra of the fluorinated ethenes have been measured at room temperature. The atmospheric lifetimes and global warming potentials of the fluorinated ethenes have been estimated. The correlation between the reactivity and the structure of the halogenated ethenes has been investigated by considering the structure containing the atoms attached to the carbons on both sides of the double bond. The calculated rate constants of 14 halogenated ethenes showed agreement with the measured rate constants within a factor of 2, except for that of one compound.

Rate constants for the reactions of OH radicals with CF3CX=CY2 (X = H, F, CF3, Y = H, F, Cl).

The rate constants of OH radicals with CF3CF=CCl2, CF3CH=CF2, CF3CF=CH2, CF3CH=CH2, and (CF3)2C=CH2 have been measured over the temperature range 250-430 K. Kinetic measurements have been carried out using flash photolysis and laser photolysis methods combined respectively with laser-induced fluorescence technique. The Arrhenius rate parameters have been determined as k(CF3CF=CCl2) = (6.50 ± 0.22) × 10-13∙exp[(200 ± 10)/T], k(CF3CH=CF2) = (4.85 ± 0.14) × 10-13∙exp[(120 ± 10)/T], k(CF3CF=CH2) = (1.54 ± 0.03) × 10-12∙exp[- (100 ± 10)/T], k(CF3CH=CH2) = (1.06 ± 0.02) × 10-12∙exp[(80 ± 10)/T], and k((CF3)2C=CH2) = (8.75 ± 0.23) × 10-13∙exp[- (20 ± 10)/T] cm3 molecule-1 s-1. Infrared absorption spectra of the halogenated alkenes have been measured at room temperature. The atmospheric lifetime, global warming potential, ozone depleting potential, and photochemical ozone creation potential have been estimated. The change in the reactivity of halogenated alkenes by the substitution has been examined by considering the structure containing the atoms or atomic groups attached to the carbons on both sides of the double bond.

Rate Constants for the Reactions of OH Radical with the ( E)/( Z) Isomers of CF3CF═CHCl and CHF2CF═CHCl.

The rate constants for the reactions of OH radical with ( E)- and ( Z)-isomers of CF3CF═CHCl and CHF2CF═CHCl have been measured over the temperature range of 250-430 K. Kinetic measurements have been performed using flash and laser photolysis methods combined with laser-induced fluorescence. Arrhenius rate constants have been determined as k(( E)-CF3CF═CHCl) = (1.09 ± 0.03) × 10-12 · exp[(50 ± 10)K/ T], k(( Z)-CF3CF═CHCl) = (8.02 ± 0.19) × 10-13 · exp[-(100 ± 10)K/ T], k(( E)-CHF2CF═CHCl) = (1.50 ± 0.03) × 10-12 · exp[(160 ± 10)K/ T], and k(( Z)-CHF2CF═CHCl) = (1.36 ± 0.03) × 10-12 · exp[(360 ± 10)K/ T] cm3 molecule-1 s-1. Infrared absorption spectra have also been measured at room temperature. The atmospheric lifetimes of ( E)-CF3CF═CHCl, ( Z)-CF3CF═CHCl, ( E)-CHF2CF═CHCl, and ( Z)-CHF2CF═CHCl have been estimated as 8.9, 20, 4.6, and 2.6 days, respectively, and their global warming potentials and ozone depletion potentials were determined as 0.23, 0.88, 0.060, and 0.016 and 0.00010, 0.00023, 0.000057, and 0.000030, respectively. Additionally, the rate constants for OH radical addition and IR spectra of these compounds were determined computationally. Consistent with experiment, our calculations indicate that the reactivity toward OH radical addition is reduced as ( Z)-CHF2CF═CHCl > ( E)-CHF2CF═CHCl > ( E)-CF3CF═CHCl > ( Z)-CF3CF═CHCl, where the ( E)/( Z) reactivity is reversed for CF3CF═CHCl and CHF2CF═CHCl. The calculations reproduced the observed temperature dependencies of the rate constants for the OH radical reactions, which is slightly positive for ( Z)-CF3CF═CHCl but negative for the other compounds.

On the temperature dependence of flammability limits of gases.

Flammability limits of several combustible gases were measured at temperatures from 5 to 100 °C in a 12-l spherical flask basically following ASHRAE method. The measurements were done for methane, propane, isobutane, ethylene, propylene, dimethyl ether, methyl formate, 1,1-difluoroethane, ammonia, and carbon monoxide. As the temperature rises, the lower flammability limits are gradually shifted down and the upper limits are shifted up. Both the limits shift almost linearly to temperature within the range examined. The linear temperature dependence of the lower flammability limits is explained well using a limiting flame temperature concept at the lower concentration limit (LFL)--'White's rule'. The geometric mean of the flammability limits has been found to be relatively constant for many compounds over the temperature range studied (5-100 °C). Based on this fact, the temperature dependence of the upper flammability limit (UFL) can be predicted reasonably using the temperature coefficient calculated for the LFL. However, some compounds such as ethylene and dimethyl ether, in particular, have a more complex temperature dependence.

Flammability assessment of CH2CFCF3: comparison with fluoroalkenes and fluoroalkanes.

The burning velocity, flammability limits, and heat of combustion of CH(2)CF=CF(3) (1234yf) have been studied to elucidate the fundamental flammability properties of this new alternative refrigerant with low global-warming potential. The burning velocity of 1234yf was measured independently by schlieren photography and the spherical vessel method. In the spherical vessel method, the burning velocities of 1234yf and its analogues CH(2)=CFCHF(2) (1243yf) and CH(2)=CHCF(3) (1243zf) as well as those of typical fluoroalkanes CH(2)F(2) (HFC-32) and CH(3)=CHF(2) (HFC-152a) were measured in mixtures of air at various O(2)/(N(2)+O(2)) ratios. The maximum burning velocity of 1234yf was found to be 1.2+/-0.3 cm s(-1), which was approximately one-fifth that of HFC-32 (6.7 cm s(-1)) and one order of magnitude less than those of 1243yf (19.8 cm s(-1)) and 1243zf (14.1 cm s(-1)). The flame propagation of 1234yf was highly sensitive to flame temperature compared to that of the other compounds. The measured flammability limits and calculated heat of combustion of 1234yf were also determined.

Flammability limits of olefinic and saturated fluoro-compounds.

Flammability limits were measured for a number of olefinic and saturated fluoro-compounds in a 12l spherical glass vessel. The obtained data together with the ones of previous studies have been analyzed based on the F-number scheme of flammability limits. The flammability limits of these compounds have been found to be explained very well by the present scheme of interpretation. The flammability limits are dependent upon distribution of F atoms in a molecule as well as upon F-substitution rate itself. It has been found that -O-CF(3) group in a molecule conspicuously decreases the flammability of the compound, while -C-CF(3) group does not much. For olefinic compounds, distribution of F atoms around double bonds markedly diminishes the flammability of the molecule.

A study on flammability limits of fuel mixtures.

Flammability limit measurements were made for various binary and ternary mixtures prepared from nine different compounds. The compounds treated are methane, propane, ethylene, propylene, methyl ether, methyl formate, 1,1-difluoroethane, ammonia, and carbon monoxide. The observed values of lower flammability limits of mixtures were found to be in good agreement to the calculated values by Le Chatelier's formula. As for the upper limits, however, some are close to the calculated values but some are not. It has been found that the deviations of the observed values of upper flammability limits from the calculated ones are mostly to lower concentrations. Modification of Le Chatelier's formula was made to better fit to the observed values of upper flammability limits. This procedure reduced the average difference between the observed and calculated values of upper flammability limits to one-third of the initial value.

Burning velocity measurements of nitrogen-containing compounds.

Burning velocity measurements of nitrogen-containing compounds, i.e., ammonia (NH3), methylamine (CH3NH2), ethylamine (C2H5NH2), and propylamine (C3H7NH2), were carried out to assess the flammability of potential natural refrigerants. The spherical-vessel (SV) method was used to measure the burning velocity over a wide range of sample and air concentrations. In addition, flame propagation was directly observed by the schlieren photography method, which showed that the spherical flame model was applicable to flames with a burning velocity higher than approximately 5 cm s(-1). For CH3NH2, the nozzle burner method was also used to confirm the validity of the results obtained by closed vessel methods. We obtained maximum burning velocities (Su0,max) of 7.2, 24.7, 26.9, and 28.3 cm s(-1) for NH3, CH3NH2, C2H5NH2, and C3H7NH2, respectively. It was noted that the burning velocities of NH3 and CH3NH2 were as high as those of the typical hydrofluorocarbon refrigerants difluoromethane (HFC-32, Su0,max=6.7 cm s(-1)) and 1,1-difluoroethane (HFC-152a, Su0,max=23.6 cm s(-1)), respectively. The burning velocities were compared with those of the parent alkanes, and it was found that introducing an NH2 group into hydrocarbon molecules decreases their burning velocity.

Flammability limits of isobutane and its mixtures with various gases.

Flammability limits of isobutane and five kinds of binary mixtures of isobutane were measured by the ASHRAE method. Propane, nitrogen, carbon dioxide, chloroform, and HFC-125 (1,1,1,2,2-pentafluoroethane) were used as the counter part gases in the mixtures. The observed data were analyzed using the equations based on Le Chatelier's formula. The flammability limits of mixtures with propane were well explained by the original Le Chatelier's formula. The flammability limits of mixtures with nitrogen and the ones with carbon dioxide were adequately analyzed by the extended Le Chatelier's formula. It was found that the extended Le Chatelier's formula is also applicable to the flammability limits of mixtures with chloroform and HFC-125.

Extended Le Chatelier's formula for carbon dioxide dilution effect on flammability limits.

Carbon dioxide dilution effect on the flammability limits was measured for various flammable gases. The obtained values were analyzed using the extended Le Chatelier's formula developed in a previous study. As a result, it has been found that the flammability limits of methane, propane, propylene, methyl formate, and 1,1-difluoroethane are adequately explained by the extended Le Chatelier's formula using a common set of parameter values. Ethylene, dimethyl ether, and ammonia behave differently from these compounds. The present result is very consistent with what was obtained in the case of nitrogen dilution.

Measurement and numerical analysis of flammability limits of halogenated hydrocarbons.

Flammability limits measurement was made for a number of halogenated compounds by the ASHRAE method. Most of compounds measured are the ones for which discrepancy was noted between the literature values and predicted values of flammability limits. As a result, it has been found that most of the newly obtained values of flammability limits are not in accordance with the literature values. Numerical analysis was carried out for a set of flammability limits data including the newly obtained ones using a modified analytical method based on F-number scheme. In this method, fitting procedure was done directly to flammability limits themselves instead of fitting to F-number. After the fitting process, the average relative deviation between the observed and calculated values is 9.3% for the lower limits and 14.6% for the upper limits.

Effect of vessel size and shape on experimental flammability limits of gases.

The flammability limits of methane and propane have been measured using cylindrical vessels of various sizes and one spherical vessel. An ac discharge ignition method has been employed. For a cylindrical vessel of small diameter with a large height, the flammability limits are primarily determined by the quenching effect of the wall. For cylindrical vessels of smaller heights, the experimental flammability limits are affected by hot gas accumulation at the vessel ceiling, unburned gas heating, self heating of the incipient flame by the reflection both from walls and ceiling, and the quenching effect of the walls. If the vessel size is large enough so that all these effects become negligible, the experimental values of flammability limits may approach to the values that would be obtained in free space. In order to approach this condition for a cylindrical vessel, it is desirable to use a container at least 30 cm in diameter and 60 cm in height. For comparison purpose, the measurement has also been done using ASHRAE type 12l spherical flask.

Calculation of minimum ignition energy of premixed gases.

The minimum ignition energy of premixed gases has been calculated by using two theoretical expressions and compared with the experimental data. One expression considers the amount of energy that the minimal flame should have, and the other the heat loss from the surface of the minimal flamelet. The former is a cubic function of the quenching distance while the latter is a quadratic function of quenching distance. It has been found that the latter expression gives a better fit to the experimental data than the former, though the discrepancy is considerable even for the latter expression. The calculated widths of the fronts of the minimal flame for various fuels were about one-order of magnitude smaller than the corresponding experimentally determined quenching distances, although no clear correlation relationship between the two quantities was found.

Experimental exploration of discrepancies in F-number correlation of flammability limits.

Flammability limits measurement has been made by ASHRAE method for some 20 kinds of combustible gases and vapors. These compounds have been selected mainly because the literature values of flammability limits are not consistent with the F-number calculated ones [J. Hazard. Mater. A 82 (2001) 113]. As a result, it has been found that the newly obtained values of flammable range are classified into three groups. For the first group of compounds, the present values agree well to the literature values. For the second group, the present values do not agree to the literature values but agree with the calculated ones. For the third group ones, the present values neither agree to the literature values nor to the calculated ones. There are 4, 13, and 6 compounds in the respective groups.