Robert W. Carr

A continuous chromatographic reactor

Abstract Reaction chromatography has been done on a continuous basis in a reactor configuration consisting of a packed cylindrical annulus with a rotating feed port. Chromatographic action causes spatial separation of product streams emerging from the annular exit. This has been demonstrated by an experimental study of the acid catalysed hydrolysis of aqueous methyl formate. Furthermore, separation of the products, formic acid and methanol, suppresses the reverse reaction causing conversions to be significantly greater than equilibrium conversions. A mathematical model of the reactor has been developed and used for numerical simulation of reactor behavior. Dispersion characteristics of the bed, adsorption isotherms and reaction kinetics were all determined in independent experiments, and used as input data for the model. Comparisons of numerically simulated reactor performance with experimental results showed good agreement. 1. 1 A continuous chromatographic reactor has been designed and constructed for the aqueous, acid catalysed hydrolysis of methylformate. The reaction products, methanol and formic acid, gave reasonably well defined chromatographic peaks. No reactant peak was detected, indicating that larger than equilibrium conversion was obtained. 2. 2 An ideal chromatographic model, modified by simulating small dispersion effects, is capable of giving excellent prediction of the experimental results. 3. 3 In the numerical simulation the Freundlich adsorption isotherm was used. Binary component adsorption isotherm data are capable of giving good agreement between the simulation results and the experimental data. This seems to be due to the quick disappearance of the high concentration of methylformate a short distance from the reactor inlet and almost no competitive adsorption between methanol and formic acid. 4. 4 To reduce the scatter of data points in the rotating sampling scheme it may be necessary to improve or develop a more effective packing technology.

Collision‐Induced Singlet→Triplet Intersystem Crossing of Methylene and Methylene‐d2

The photolysis of ketene or ketene‐d2 in the presence of propane, oxygen, and inert gas was studied at 300°K at each of the four wavelengths 2600, 3130, 3340, and 3500 A. Inert gases used were He, Ar, Xe, N2, and CF4. The inert gases cause the transition of methylene from the first singlet state to the triplet ground state to occur. The observed kinetics of the intersystem crossing process, being first order in methylene and first order in inert gas, are second order. The second‐order rate constants, relative to the rate constant for reaction of singlet methylene with propane, are independent of wavelength, and increase in the order He<Ar<CF4≤Xe. The rate‐constant ratios for methylene‐d2 show the same trend with inert gas but are 2–3 times larger. The results are consistent with a bimolecular intersystem‐crossing mechanism where the role of the collision partner is to cause the transition to occur by sufficiently perturbing the singlet methylene stationary states.

Dynamic sensitivity analysis of chemical reaction systems: A variational method

Abstract An approach to the sensitivity analysis of systems governed by ordinary differential equations is presented. It is based on a variational approach and is developed here in the context of chemically reacting systems, although in principal it is more general. The linear analysis generates sensitivity indexes which are a measure of the sensitivity of an objective function to the j th reaction of a mechanism. Both instantaneous and time-averaged sensitivities can be obtained. It also gives the sensitivity to initial concentrations of reactants, or to concentration perturbations at arbitrary times. The use of an objective function permits one to obtain the sensitivity of several species simultaneously in a single analysis, in contrast to other methods where the sensitivity of one species at a time must be determined. In this work the objective functions used are norms involving an arbitrary number of dependent variables. A ranking of the relative importance of each reaction in governing the concentrations of species appearing in the objective function is obtained. Since the sensitivity indexes are time dependent, the analysis may be said to be a dynamic sensitivity analysis. The sensitivity indexes given by this method are related to other standard sensitivity indexes.

Molecular velocity distribution effects in kinetic studies by time-resolved mass spectrometry

Abstract Mathematical models were developed to describe the sampling of a rapidly reacting gas by time-resolved mass spectrometry. The flow of reaction mixture from a sampling orifice in the reactor wall to the mass spectrometer ion source was treated by application of the Boltzmann equation. The calculations demonstrate that for sufficiently rapid reactions the molecular velocity distribution of the detected species causes a distribution of arrival times at the ion source which complicates the kinetic analysis. The time-dependent signal is composed of two parts; one due to the rate of chemical reaction and the other due to the ion source arrival time distribution. Calculations done on the basis of both first and second order kinetics indicate the upper limit to the reaction rate which can be measured without causing substantial errors in rate constant determinations when conventional types of concentration time plots are used. These limits hold for effusive flow through the orifice, and at least to a first approximation also hold for convective and transition flow.

Photolysis of ketene in the presence of propylene: Multistep collisional deactivation of methylcyclopropane and the excess energy of singlet methylene

The photolysis of ketene in the presence of propylene‐5% oxygen mixtures was investigated at average wavelengths of 3550, 3340, 3130, and 2680 A, and ketene photolysis in the presence of propylene‐n‐pentane‐5% O2 mixtures was investigated at average wavelengths of 3130 and 2680 A. Reaction pressures ranged approximately between 10 and 760 torr. Chemically activated methylcyclopropane, arising from singlet methylene addition to propylene, isomerized to the four butene isomers at each wavelength. The rate of isomerization increased monotonically with decreasing wavelength, demonstrating that the methylcyclopropane average energy increased with increasing photon energy. Of the chemically activated butenes formed either by methylcyclopropane isomerization or by insertion of singlet methylene into the carbon‐hydrogen bonds of propylene, butene‐1 decomposed significantly at the lowest pressures. A multistep process was required to describe the collisional deactivation of chemically activated methylcyclopropane....

The photo-oxidation of 1,3-dichlorotetrafluoroacetone: mechanism of the reaction of CF2Cl with oxygen

Abstract The 313 nm photolysis of 1,3-dichlorotetrafluoroacetone in the presence of oxygen was investigated at 298 K. CF2O and CO2 were identified as the reaction products, and a mechanism is proposed to explain their formation. Evidence is obtained for the formation of CO2 by the reaction of CF2ClO radicals with CO.

Collisional deactivation of highly vibrationally excited molecules: The effect of activated molecule complexity

Abstract Energy removed from highly vibrationally excited polyatomic molecules by polyatomic collision partners decreases with increasing number of atoms in the hot molecule. The trend is qualitatively predicted by two quasi-statistical models of energy transfer, and calculations of energy removal by the transitional modes model gives agreement with experiment.

Phosphorescence quenching of biacetyl vapor by alcohols and iodides

Abstract The quenching of phosphorescence emission from biacetyl, excited at 404.7 nm, has been investigated in the gas phase for several alcohols and iodides. The data gave linear Stern—Volmer plots, from which quenching rate constants for collisions of 3 B u biacetyl were determined. The rate constant values in l/mol s were: methyl alcohol (7.6 × 10 5 ), ethyl alcohol (6.2 × 10 5 ), n-propyl alcohol (7.2 × 10 5 ), methyl iodide (1.0 × 10 6 ), ethyl iodide (1.1 × 10 6 ), n-propyl iodide (6.7 × 10 5 ), and methylene iodide (2.9 × 10 6 ). Using simple hard sphere collision theory, the collisional efficiencies of quenching, per gas kinetic collision, were all in the range 2 to 5 × 10 −6 , except for methylene iodide, which was 1.4 × 10 −5 . The quenching mechanism for the alcohols is most probably H-atom transfer from the alcohol to 3 B u biacetyl. For the iodides, the data cannot be used to distinguish unambiguously between an external heavy atom effect and chemical quenching.

Intersystem crossing in methylene

Abstract The rate of singlet-triplet intersystem crossing in methylene is a second-order process requiring collisional perturbation to couple with the triplet manifold so that the transition can occur. The efficiency of the process increases with increasing mass of the collision partner, but nevertheless is relatively slow, requiring in excess of 200 gas-kinetic collisions on the average.

Atmospheric diffusion and chemical reaction of the chlorofluoromethanes CHFCl2 and CHF2Cl

Abstract The behavior of the chlorofluoromethanes CHFCl 2 and CHF 2 Cl in the atmosphere is investigated. The rates of removal by rainout and hydrolysis are estimated to be negligible. Hydrogen abstraction by hydroxyl radicals appears to be an important mechanism for the removal of these compounds. Their photolysis can occur in the stratosphere only, where reactions with singlet oxygen atoms are also of interest. Calculations of reaction and diffusion in the atmosphere, up to 60km, are done, using two different sets of chemical and atmospheric data, as well as two different calculational procedures. The model calculations predict fluorocarbon fluxes at the tropopause ranging from 1 to 3% of the ground level input flux for CHFCl 2 , and from 4 to 12% of the ground level input flux for CHF 2 Cl.