Isothermal diffusion—reaction in a slab catalyst with bimolecular Langmuir-Hinshelwood kinetics: Connections with negative first-order kinetics

The problem of isothermal diffusion-reaction in a slab catalyst with bimolecular Langmuir—Hinshelwood kinetics is studied. Analytic solutions for the concentration profile and the catalytic effectiveness factor are reported for the full range of Thiele modulus values, in the limit of large adsorption inhibition constant. In this limit, connections with negative first-order kinetics are shown, and rather good explicit estimates of the region of multiple steady states are provided. Comparisons with numerical calculations are also made.     

The hysteresis of steady states in a gradientless liquid-phase reaction system

The dynamic behaviour of a reaction system in a continuous-flow stirred tank reactor (CSTR) is described by a model comprising the unsteady mass and heat balances and the kinetics of the reaction system concerned.This model predicts that under certain conditions a hysteresis effect will occur, which manifests itself when the steady-state temperature of the reactIn the present paper a number of experiments concerning the hysteresis effect will be reported. The measurements were carried out with a well-mixed andIn a wide range of conditions with regard to acidity and water content of the reaction system, the reaction rate is found to be first-order in the MO cIn view of the low boiling point of MO, the temperature of the reaction system had to be kept well below 300 K.Since the total change of the temperature of the reaction system was estimated to exceed 40 K, feed-stream temperatures of about 250 K were required toAs a consequence of these low feed-stream temperatures, methanol was added to the feed streams to prevent crystallizing. The methanol did not take partWith respect to the hysteresis of steady states very good agreement between experimental results and calculated values was obtained.     

Multiple steady states in reaction controlled surface catalysed reactions

Even when gas composition and pressure above an isothermal catalyst surface are constant and rate parameters are independent of coverage, the reaction rate may not be unique. Necessary and sufficient conditions for reaction rate multiplicity are obtained here for uni- and bimolecular reactions when mo than one vacant site is required for reaction. The work of Eigenberger is extended and generalized for this situation. Bimolecular reaction with compet or noncompetitive chemisorption of at least two species may lead to multiple reaction rates.The presence of a precursor (noncompetitively adsorbed intermediate state) may also lead to multiple steady states, even when less than two vacant site are required for reaction. Conditions for steady state multiplicity are also considered for the Eley-Rideal reaction mechanism.     

Production of zinc ferrite in a mechano-chemical reaction by grinding in a ball mill

Zinc ferrite was obtained by prolonged dry grinding of stoichiometric powder mixtures of ZnO + -Fe₂O₃ or ZnCO₃ + -Fe₂O₃. The kinetics of transformation was studied by X-ray diffraction, Mössbauer spectroscopy and DTA. Crystallite size reduction followed the process, and defect structures resulted. The carbonate decomposed as a necessary step in the process. In the mixture of NiO + -Fe₂O₃, nickel ferrite was not obtainable, although considerable grain size reduction was observed. This was attributed to the inverse spinel configuration of the nickel ferrite, differing from the normal spinel configuration of the zinc ferrite. In the -Fe₂O₃ and the zinc ferrite, iron ions occupy octahedral sites, while in ZnO and the ferrite, zinc ions occupy tetrahedral sites, and metal ion—oxygen distance is almost identical. These facilitate the transformation. In the nickel ferrite, iron ions occupy both sites, which calls for high strains of -Fe₂O₃ before it can undergo transformation. Further implications of the processes are discussed.     

Application of the reaction kinetics and dispersion model to gas-solid reactors for removal of sulfur dioxide from flue gas

The conversions of a reactive, micro-grained limestone were studied in terms of exposure time and concentration of sulfur dioxide in the flue gas. The rates of the sulfation reaction were correlated as a function of the conversion of calcium oxide to sulfate and concentration of sulfur dioxide in the gas phase. In comparison with active sodium carbonate the sulfation of limestone particles proceeds at the rate which is substantially lower than the sulfation rate of soda particles.The empirical kinetic equation developed in this study is further applied in a two-phase dispersion model of simplified non-ideal flow behavior of the gas and solid in the reactor. The model can serve a rational basis for the conceptual design of a suitable contacting apparatus. Numerical solutions of the model equations outline possibilities and limitations of the dry lime process for the removal of sulfur dioxide from hot flue gas.     

Substrate-inhibited enzyme reaction in a tubular reactor with axial dispersion

For a fixed set of physico-chemical parameters, an isothermal substrate-inhibited enzyme reaction in a tubular reactor with axial dispersion can give rise to multiple steady states. Criteria previously developed for non-isothermal reactions are applied to this case to develop analytic conditions which ensure uniqueness of steady states. The criteria compare well with values obtained from exact numerical calculations.     

Simultaneous diffusion and chemical reaction in two-phase systems

An approximation is given for the case of simultaneous steady state diffusion and chemical reaction in a liquid—liquid system under such conditions that diffusion and chemical reaction are of the same order of magnitude in the two phases. The two-film model was adopted to represent the system, and results were plotted as variations of the enhancement factor with the diverse parameters affecting it. Comparison was made with an exact numerical solution, and the results show that the proposed approximation leads to small errors for low reaction rates, and gives a first approximation for the mass transfer rate for very rapid chemical reactions.     

Bubble dissolution with chemical reaction

The dissolution of an isolated stationary gas bubble, enhanced by a chemical reaction, in a large body of liquid is investigated. A theoretical model which properly accounts for the movement of the interface due to dissolution is formulated and solved using the method of finite differences. The finite difference results are used as a standard of comparison for the testing of various approximate solutions. These include a perturbation expansion, a quasi-stationary approximation and a quasi-steady approximation. The results of this work indicate that the quasi-steady approximation, which provides very simple analytical solutions, is an excellent one in the presence of a sufficiently rapid chemical reaction.     

Study of interfacial kinetics for liquid-liquid systems—II: Intrinsic kinetics of the copper chelation reaction with an oxime-based reagent

The interfacial chelation of copper from copper sulfate-sulfuric acid solutions by the commerical oxime extractant LIX65N in toluene is studied using the liquid jet recycle reactor. The intrinsic initial rate of copper chelation by LIX65N is found to be represented in mol. cm^?2-sec^?1 by the expression R = $\text{1.22 × 10}{}^{\mathrm{?8}}\text{{Cu}{}^{\mathrm{2+}}\text{}[LIX65N]}\text{{H}{}^{\mathrm{+}}\text{} + 0.0288[LIX65N]}$ where {Cu²⁺} and {H⁺} are the cupric and hydrogen activities. The interfacial rate expression above is determined as free from diffusional contributions by using a numerical solution to model mass transfer with interfacial reaction in the LJRR. The above expression is selected on statistical and physical grounds from a set of models systematically generated from the most probable reaction pathways. The liquid jet technique has been shown to be an effective and accurate (±6% experimental error) technique for the study of liquid-liquid interfacial reactions.     

Different behavior between Pt and Pd catalysts in the hydrogenolysis of cyclohexanediones and hydroxycyclohexanones

Hydrogenolysis reactions of cyclohexanediones, hydroxycyclohexanones, and some related alicyclic ketones were studied over Pt, Pd, Ir, and Rh catalysts at atmospheric hydrogen pressure in t-butyl alcohol as a solvent. Pt and Pd had high catalytic activities for the hydrogenolysis of carbon-oxygen bonds. However, Ir and Rh scarcely had any activity unless 1,3-cyclohexanedione and 3-hydroxycyclohexanone were involved. The mechanisms of the hydrogenolysis differed with Pt and Pd. In the hydrogenation of 4-methoxycyclohexanone, Pt afforded cyclohexyl methyl ether as the hydrogenolysis product; while Pd afforded cyclohexanone, which was then hydrogenated to cyclohexanol. Thus Pt cleaved the carbon-oxygen double bond, and Pd cleaved the carbon-oxygen single bond. Deuterolysis of cyclohexanone and 4-methoxycyclohexanone on Pt gave mainly d₂ species of cyclohexane and cyclohexyl methyl ether as the hydrogenolysis products. This indicated that the carbon-oxygen double bonds were directly cleaved to yield methylene groups on Pt. Almost of all 3-hydroxycyclohexanone was hydrogenolyzed to cyclohexanone on Pd; whereas cyclohexanone as well as cyclohexanol was not hydrogenolyzed at all. In the case of Pd, the carbon-oxygen single bond was cleaved when it was activated by formation of π-oxoallyl adsorbed species on the catalyst at the carbon-oxygen double bond.     

Redox reaction of tri(2-deoxy-2-L-ascorbyl)amine. Formation of a persisting free radical species in water

Electrochemical studies showed that tri(2-deoxy-2-L-ascorbyl)amine ($\text{1}$), a new compound isolated as one of the products of reaction of dehydro-L-ascorbic acid with phenylalanine (with added L-ascorbic acid) in ethanol, is oxidized in aqueous solution in two reversible one-electron transfer steps, on mercury or platinum electrode. The first step occurs through the dianion, and its product is an unusually stable blue anion radical giving a characteristic ESR signal. The product of the second step of oxidation is labile and is slowly converted into the oxidized form of di(2-deoxy-2-L-ascorbyl)amine, presumably by hydrolysis with splitting of L-ascorbic acid.     

Verification of a model for estimating the void fraction in a three-component randomly packed bed

The void fraction in a three-component randomly packed bed was calculated from the authors' model, and the calculated values were compared with published experimental data for spherical and irregularly shaped particles and with results from computer simulations. Results from the model were in good agreement with simulated and published experimental data.     

The hydrogen electrode reaction characteristics of thin film electrodes of Ni-based hydrogen storage alloys

Film-type electrodes of hydrogen absorbing intermetallic compound and alloys, LaNi₅, Ni_0.11 Ti_0.89, Ni_0.50 Ti_0.50 and Ni_0.76 Ti_0.24 were prepared by a flash evaporation method. The hydrogen electrode reaction characteristics of the LaNi₅ and NiTi alloy films in 1 M NaOH are very similar to each other. The reaction proceeds via the Volmer-Tafel reaction route with mixed rate-determining characteristics. The exchange current densities of the constituent steps, as well as the overall reaction, are in the range of 10^?6 A cm^?2 (true). Surface analysis by an XPS technique has shown that La or Ti on the electrode surface exists as an electrocatalytically innert oxide of La₂O₃ or TiO₂. Close similarities of these electrodes with pure Ni electrodes indicate that Ni is responsible for the electrocatalytic activity. No synergistic effect is thus noticeable.     

Development of a new system for circulating fluidized particles within a single vessel

A new system for circulating fluidized particles within a single vessel has been developed which has the same advantages as the dual-bed systems widely used for the FCC process and the Fluid Coker process. The interior of the vessel was divided into four sections by intersecting two flat vertical plates at right angles. Two sections were used for the upflowing bubbling fluidized beds and the other two sections were used for the downflowing bubble-free fluidized beds. Fluidized particles were circulated between the two upflowing beds, which were to be used as reactors, through the two downflowing beds, which were to be used as downcomers. Solid particles were fluidized by several streams of gas injected at several stages of the bed. The effects of these gas injection rates on the circulation rate of solids were investigated. The circulation rate of solids was measured using both a measuring box, which collected the overflowing particles, and the downward motion of a gauze net induced by the descending particles. The static pressure distribution within the vessel and the residence time distribution for coarse foreign solids in the system were measured. The present research demonstrated that the proposed system potentially had the same advantages as the conventional dual-bed system, and that the system can be applied to the simple gasification of biomass and solid wastes.     

Effect of anions and pH on the ethanol electro-oxidation at a platinum electrode

The electro-oxidation of ethanol and its inhibition by adsorbed chloride ions have been studied by cyclic voltammetry in 0.64 M HNO₃, 0.64 M HClO₄, 0.64 M NaNO₃, 0.64 M NaClO₄, 0.43 M NaNO₃/0.072 M Na₂SO₄ and 0.43 M NaClO₄/0.072 M Na₂SO₄ solutions at 25°C. The results show that these anions and the pH influence the peak current and potential for all three anodic waves. The anion effect is more pronounced in acidic solutions than in neutral solutions. The magnitude of the effect in the absence of chloride indicates that the surface coverage of these anions increases in the order perchlorate, sulfate, nitrate. The chloride inhibition is dependent on the anions and is smallest in sulfate solutions. Mechanisms proposed in an earlier study are found to be consistent with these results. It is also suggested that the ethanol system may be useful as a probe in studies of anion adsorption properties and platinum surface states.