An adiabatic type of palladium membrane reactor for coupling endothermic and exothermic reactions

Palladium can play an interesting role as a catalytic membrane, that is, a hydrogen separative and catalytically active wall. Utilizing this function, a palladium membrane reactor capable of working under an adiabatic condition was designed in this study for coupling two conjugated reactions. On one side of the membrane, dehydrogenation of cyclohexane as a model takes place in the catalyst-packed layer, and on the membrane surface of the other side hydrogen permeated reacts in-situ with oxygen. In the adiabatic membrane reactor, a heat compensation between the endothermic dehydrogenation and the exothermic oxidation is expected to be realized. As a result, it became obvious experimentally that the generated heat due to the oxidation refluxed to the dehydrogenation side, heated up the catalyst layer and therefore enhanced the dehydrogenation. A simple mathematical model derived for analyzing the reaction process could simulate the practical reactor performances well.     

The use of high pressure DTA,heat flow and adiabatic calorimetry to study exothermic reactions

Cellulose, a commonly found carbohydrate, exhibits exothermic decomposition, when heated above 180C. These reactions have been detected and quantified by a range of thermal analysis and calorimetric techniques giving complementary information. The presented results are of interest for process safety.

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Zusammenfassung Cellulose, ein weit verbreitetes Kohlenhydrat, zeigt beim ErwÄrmen auf Temperaturen oberhalb 180C exotherme Zersetzung. Die damit verbundenen Reaktionen wurden mittels einer Reihe komplementÄrer thermoanalytischer und kalorimertrischer Methoden erfasst und quantifiziert. Die vorgestellten Resultate sind für die AbschÄtzung von Prozessrisiken bedeutungsvoll.

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The authors gratefully acknowledge many discussions with C. Foetisch (Linor, Orbe, CH), Prof. R. C. Reid (Lexington, Massachusetts, USA) and I. Horman (Nestlé Research Centre, Vers-chez-les-Blanc, CH).     

Small scale adiabatic dewar techniques to determine exothermic onset temperatures

The heightened focus on safety in the chemical process industry has resulted in a growing trend to provide reliable thermochemical information before process scale-up. The exothermic onset temperature of compounds is of key concern. The small scale adiabatic dewar age (SSAD) test, developed at Merck Research laboratories, is used to determine exothermic onset temperatures. These tests are conducted under nearly adiabatic conditions using specially designed dewar sample cells and commercially available programmable calorimeters. The dewar cell method offers a rapid and accurate method to enhance standard test procedures and allows measurement of auto catalytic behavior using small samples.The SSAD technique is applicable to both solids and liquids and has the following advantages: 1) small sample size, 2) ability to handle compounds in which exothermic activity is accompanied by large pressure increases without damaging experimental equipment, 3) accurate determination of exothermic onset temperature, 4) ease of experiment set-up, 5) ease of data interpretation and 6) rapid experimental turn around time.The determination of the exothermic onset temperature using the SSAD technique will be presented and compared to data obtained using standard and isothermal age techniques.     

Characterisation of an exothermic reaction using adiabatic and isothermal calorimetry

A simple esterification reaction is used to demonstrate standard procedures for determining the thermokinetic parameters of an exothermic reaction from adiabatic calorimetric data. The influence of variations in the heat capacity of the sample due to changes in temperature and concentration is explored. Shortcomings in the simple interpretation of adiabatic data are identified and isothermal heatflow calorimetry is used to reveal autocatalytic effects which were not apparent from the adiabatic experiments. A more rigourous interpretation of the adiabatic and isothermal data is outlined and used to predict the conditions which can lead to exothermic runaway in a batch reactor. Mathematical simulation of the conditions in a jacketed reactor is used to demonstrate the importance of developing reliable kinetic expressions before assessing the safety of a batch process.

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Zusammenfassung Anhand einer einfachen Veresterungsreaktion wurden Standardverfahren zur Ermittlung thermokinetischer Parameter exothermen Reaktionen aus adiabatischen kalorimetrischen Daten demonstriert. Dabei wurde der Einfluß von Temperatur und Konzentration auf Änderungen der Wärmekapazität untersucht. Fehler bei der einfachen Interpretation adiabatischer Daten wurden identifiziert und isotherme Wärmeflußkalorimetrie wurde angewendet, um autokatalytische Effekte aufzuzeigen, die sich anhand der adiabatischen Experimente nicht ersehen lassen. Es wurde eine gründlichere Interpretation adiabatischer und isothermer Daten umrissen und verwendet, um die Bedingungen vorherzusagen, die in einem Kesselreaktor zu einem exothermen Davonlaufen der Reaktion führen. Mathematische Simulation der Bedingungen in einem Mantelkessel wurde angewendet, um zu zeigen, von welch großer Bedeutung die Entwicklung zuverlässiger kinetischer Ausdrücke ist, bevor man die Sicherheit einer Reaktion in einem Kesselreaktor beurteilt.

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Statistical theory of exothermic ion-molecule reactions

A statistical theory is presented for ion-molecule reactions of the type A₂+A⁺ ₂A⁺ ₃+A, with allowance for conservation of the total energy and of the angular momentum of the composite system A⁺ ₄. The conservation of the momentum greatly reduces the phase volume of the initial reaction channel; the probability of decomposition in the forward direction is less than one even for exothermic reactions. A reaction of this type is considered for hydrogen, for which the statistical theory gives a qualitative explanation of the observed relation of reaction cross-section to vibrational quantum number for H⁺ ₂.     

Predicted rate constants for the exothermic reactions of ground state oxygen atoms and CH radicals

A potential function has been derived for the ground-state surface of HCO which reproduces the spectroscopic properties of the equilibrium molecule and the results of ab-initio calculations at other stationary points on the surface. The potential has been used for a classical trajectory study of the vibrational excitation of CO on collision with fast H atoms and for a study of the reaction of ground-state oxygen atoms and CH radicals.     

On the determination of the activation energy of solid-state reactions from the maximum reaction rate of isothermal runs

The method of kinetic analysis of isothermal traces recently proposed by Dharwadkar et al. is criticized. It is concluded that this method would be a proper one in the case of reactions following the mechanism of Prout and Tompkins or Avrami-Erofeev, but it would lead to misinterpretation when diffusion or phase-boundarycontrolled reactions are involved.     

Uncertainty analysis of theoretical methods for adiabatic temperature rise determination in calorimetry

The main methods for the determination of the temperature rise in calorimetric experiments corrected of heat losses to surroundings (called adiabatic temperature rise) are described thereafter. This corrected temperature rise is obtained analytically from experimental temperature–time curves. The general scheme reported by Henri Régnault and Leopold Pfaundler for the first time in the 19th century is considered as the basis for all methods elaborated afterwards. A bibliographical study raised five methods including Régnault–Pfaundler’s. These methods have been applied on five experimental temperature–time curves obtained with an isoperibolic reference gas calorimeter at the French national metrology and testing institute (LNE) on combustion of pure methane. This paper deeply details a new analytical method elaborated at LNE exposing best heat transfer phenomena representation occurring in the water bath calorimeter. A comparative study of the five methods of the temperature rise determination and of their associated uncertainties is here presented.     

Fragmentation of organic anions induced by exothermic addition reactions

The addition reactions of a series of carbanions with CO₂, COS and CS₂ have been studied in a flowing afterglow apparatus. Carbon dioxide simply forms the adduct, but the more highly exothermic additions to COS and CS₂ lead both to addition and addition followed by fragmentation. A number of novel fragmentation pathways have been observed for additions to diazo anions and to anions derived by proton abstraction from allenes. In addition to these reactions, highly basic anions are observed to undergo sulfur atom transfer reactions with CS₂.     

Comparison of activation methods for mercury determination

A comparative study is made on different activation methods for mercury analysis. Mercury concentrations down to 0.1 ppm were determined instrumentally via the isotopes¹⁹⁷Hg (T=65 h) and²⁰³Hg (T=47 d). A high-resolution Ge(Li) detector was used in measuring the activity and a small computer for data reduction. Up to 500 samples were measured daily. Chemical separations were performed on samples with low mercury concentrations. Sensitivity of 0.01 ppm was attained by precipitating HgS from basic solutions and counting¹⁹⁷Hg on NaI(T1) detector. A new rapid instrumental method was also developed based on^199mHg (T=43 min). This short-lived isotope was activated with resonance neutrons. The sensitivity of the method is 0.5 μg and it requires only 1 hr.     

Gas chromatographic determination of rate constants in bimolecular gaseous reactions

Summary Rate constants for bimolecular reactions in the gas phase, under diffusion controlled conditions, can easily be determined by the reversed-flow gas chromatography (RF-GC) technique. The analysis of the diffusion band by means of a simple PC programme gives directly an apparent, second-order rate constant for gaseous reactions. By varying the amounts of the reactants, one can calculate the true order of the reaction and the true non-first-order rate constant of gaseous reactions. The calibration problem of the analytical techniques in non-first-order reaction kinetics is absent as are other disadvantages connected with carrier gas flow, peak shape and their instrumental spreading. The method can be used for atmospheric reactions and was applied in the gaseous reaction systems: SO₂+NO₂, SO₂+Br₂, C₆H₆+NO₂, C₆H₅CH₃+NO₂ and C₃H₆+NO₂ with various concentrations of reactants in nitrogen. The effect of the NO₂ concentration on the apparent second-order rate constant of C₂H₄+NO₂ at 333.2 K was also studied. Finally, the effect of sun light pre-irradiation of C₂H₂+NO₂ in nitrogen was investigated.     

Comparison of macro-gravimetric and micro-colorimetric lipid determination methods

In order to validate a method for lipid analysis of small tissue samples, the standard macro-gravimetric method of Bligh-Dyer (1959) [E.G. Bligh, W.J. Dyer, Can. J. Biochem. Physiol. 37 (1959) 911] and a modification of the micro-colorimetric assay developed by Van Handel (1985) [E. Van Handel, J. Am. Mosq. Control Assoc. 1 (1985) 302] were compared. No significant differences were observed for wet tissues of two species of fish. However, limited analysis of wet tissue of the amphipod, Leptocheirusplumulosus, indicated that the Bligh-Dyer gravimetric method generated higher lipid values, most likely due to the inclusion of non-lipid materials. Additionally, significant differences between the methods were observed with dry tissues, with the micro-colorimetric method consistently reporting calculated lipid values greater than as reported by the gravimetric method. This was most likely due to poor extraction of dry tissue in the standard Bligh-Dyer method, as no significant differences were found when analyzing a single composite extract. The data presented supports the conclusion that the micro-colorimetric method described in this paper is accurate, rapid, and minimizes time and solvent use.     

Semiquantal analysis of adiabatic hydrogen transfer rate

The reaction rate of adiabatic proton/hydrogen/hydride (H) transfers in condensed phase is examined by combining the semiquantal time-dependent Hartree theory and the multidimensional transition state theory, which takes into account the zero-point effect and the dynamical modulation of the wavepacket width in the adiabatic transfer regime. By applying the theory to a model potential consisting of a quartic double well coupled linearly and quadratically (symmetrically) to external degrees of freedom, a set of compact analytical formulas was derived for the adiabatic H transfer rate. The analysis suggests that the kinetic isotope effect on the H transfer rate may exhibit a maximum as a function of the coupling strength to the external degrees of freedom measured by the reorganization energy.     

The effect of carbonyl complexation on highly exothermic vanadium oxidation reactions

The effects of CO complexation on highly exothermic vanadium oxidation reactions is evaluated. We study the chemiluminescent (CL) reaction products formed when vanadium vapor entrained in Ar or CO is oxidized by O₃ or NO₂. The multiple collision V+Ar+O₃→VO^*(C ⁴Σ⁻, ⁴Φ, ²X)+Ar+O₂ reactive encounter yields two previously unreported VO excited states, whereas the V+Ar+NO₂→VO^*+Ar+NO reactive encounter populates states up to and including VO^* C ⁴Σ⁻. The multiple collision V+nCO+O₃ reactive encounter would appear to form a VOCO excited state complex, emitting in the region 420–560 nm, via the formation and oxidation of V(CO)₂ viz. V(CO)₂+O₃→VOCO^*+CO+O₂ and a relaxed VO excited state emitter via V+nCO+O₃→VO^*+nCO+O₂ where the VO excited state excitation is mediated by V–CO complexation. In complement, the much less exothermic V–NO₂ encounter displays an emission which, in concert with previous studies of CO complexation, suggests the formation of a VO(CO)₂ excited state complex viz. V(CO)₂+NO₂→VO(CO)₂^*+NO. The experiments characterizing CL are complemented by comparative laser-induced fluorescence studies of the VO X ⁴Σ⁻–CO and Ar interactions and their influence on the VO C ⁴Σ⁻–X ⁴Σ⁻ laser-induced excitation spectrum. These studies, in conjunction with further attempts to excite LIF in the 420–560 nm region, suggest that the observed complex emissions result primarily from VO excited state interactions. Complementary time-of-flight mass spectroscopy of vanadium and vanadium-oxide–carbonyl complex formation demonstrates the formation of V(CO), V(CO)₂, V₂(CO), and VOCO, the latter three of which demonstrate clear metastable-ion dissociation peaks for the processes VOCO⁺→V⁺+CO₂, V(CO)₂⁺→V⁺+2CO, and V₂(CO)⁺→V₂⁺+CO, suggesting that these vanadium complexes when formed in a reaction-based environment may be photodissociated with light in the visible and ultraviolet regions.     

Dynamics of competitive reactions: endothermic proton transfer and exothermic substitution

Dynamics of an endothermic proton-transfer reaction, F(-) with dimethyl sulfoxide, and an endothermic proton-transfer reaction with a competing exothermic substitution (S(N)2) channel, F(-) with borane-methyl sulfide complex, were investigated using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) and kinetic modeling. The two proton-transfer reactions have slightly positive and a small negative overall free energy changes, respectively. Energy-dependent rate constants were measured as a function of F(-) ion translational energy, and the resulting kinetics were modeled with the RRKM (Rice-Ramsperger-Kassel-Marcus) theory. The observed rate constants for the proton-transfer reactions of F(-) with dimethyl sulfoxide and with borane-methyl sulfide complex are identical, with a value of 0.17 x 10(-9) cm(3) molecule(-1) s(-1); for the S(N)2 reaction, k = 0.90 x 10(-9) cm(3) molecule(-1) s(-1) at 350 K. Both proton-transfer reactions have positive entropy changes in the forward direction and show positive energy dependences. The competing S(N)2 reaction exhibits negative energy dependence and becomes less important at higher energies. The changes of the observed rate constants agree with RRKM theory predictions for a few kcal/mol of additional kinetic energy. The dynamic change of the branching ratio for the competing proton transfer and the substitution reactions results from the competition between the microscopic rate constants associated with each channel.     

Properties of combustion waves in a model with competitive exothermic reactions

We study the properties of travelling combustion waves in a diffusional thermal model with a two-step competitive exothermic-exothermic reaction mechanism. This investigation considers the system in one spatial dimension under adiabatic conditions. Based on the notion of the crossover temperature, the model is examined analytically using the activation energy asymptotic method to predict travelling combustion wave behaviour in the limit of large activation energies. The model is then studied numerically using a shooting-relaxation method over a wide range of parameter values, such as those describing the ratios of enthalpies, pre-exponential factors and activation energies. It is demonstrated that the flame speed as a function of these parameters is a single-valued monotonic function and there are two flame regimes identified—each region representing parameter values when one reaction dominates the other.