Apparent Reaction Order Research Articles

Char Gasification with O2, CO2, and H2O: Effects of Pressure on Intrinsic Reaction Kinetics

Measurements of the intrinsic reaction rates of two Australian coal chars (made under laboratory conditions) with O2, CO2, and H2O at increased pressures (up to 30 atm) have been made using a pressurized thermogravimetric analyzer (TGA). It was found that the reaction order in CO2 and H2O was not constant over the pressure range investigatedvarying from 0.5 to 0.8 at atmospheric pressure and decreasing at pressures above approximately 10 atm. The apparent reaction order in oxygen was less affected by pressure over the range 1 to 16 atm. Char surface area after reaction at higher pressures was generally greater than that after reaction at lower pressures. This resulted in a reduced effect of pressure on the intrinsic rates at 10% conversion. Activation energies for all three reactions were not significantly affected by an increase in reaction pressure. The intrinsic rate data obtained in this work were used to estimate the high-temperature reactivity of the chars using a basic knowledge of the pore structure of the samples. This provided a framework within which coal behavior under high-temperature gasification conditions can be predicted using intrinsic rate data and a knowledge of the structure of the char.

CATALYTIC TREATMENT OF PETROLEUM ASPHALTS

ABSTRACT Catalytic Sulphurization of residual asphalts obtained from Nigeria;i crudes has been investigated at 210 - 250 °C for a period of 1 hour. The overall effect of the reaction was the manifold increase in the asphaltene content of the sulphurized asphalt. The growth of asphaltenes however decreases with temperature which is in sharp contrast to the non-catalytic reaction reported in the literature. Kinetic analysis shows that the apparent reaction order varied from 1.75 to 3.60 while the rate constant follows a decreasing trend with temperature. These results indicate that the role of the catalyst in the sulphurization reaction is to lower the temperature of the reaction.

Experimental Study on Low-Temperature Oxidation of an Australian Coal

Low-temperature oxidation of an Australian coal is examined in an isothermal flow reactor, operated at atmospheric pressure and 50 °C. Strong dependence of the rates of consumption of oxygen and the formation of carbon dioxide are observed on coal particle size and oxygen concentration in the gas stream. Following an induction period, oxygen consumption proceeds at a quasi steady state rate indicating that this process is limited by oxygen diffusion in the pores of coal particles. The rate of carbon dioxide production decreases with time, due to the progressive reduction in the number of active sites at the internal surfaces of pores in coal particles. This points to the kinetic control of CO2 production. The apparent reaction order for oxygen consumption at quasi steady state varies between 0.63 for large and 0.41 for small particles, with the latter number providing the best estimate of the intrinsic order. Fitting the rate of CO2 production to the expression developed by Kam et al. for higher temperatures suggests that, under the present experimental conditions, CO2 is formed via parallel pathways of (i) direct oxidation of coal to CO2 and (ii) formation and subsequent decomposition of solid coal−oxygen complexes, with carbon dioxide produced mainly via the first pathway at long times. The results presented in this paper have applications to self-heating and spontaneous combustion of coal.

Growth and structure of aggregates of heat‐denatured β‐Lactoglobulin

SummaryThe aggregation of the globular protein β‐lactoglobulin after heat‐denaturation was studied in aqueous solution at pH 7 using static and dynamic light scattering. The structure of the aggregates is self‐similar with fractal dimension 2.0. Size exclusion chromatography and dynamic light scattering measurements show that the aggregates have a broad size distribution. Initially clusters of about 85 proteins and 15 nm radius are formed which are the elementary units of the larger fractal aggregates. At low ionic strength the formation of the larger aggregates is impeded by electrostatic interactions.The structure of the aggregates is independent of the concentration and the temperature. The rate of aggregation has an Arrhenius temperature dependence with an activation energy of about 350 kJ/mol weakly dependent on the concentration. The apparent reaction order of the aggregation is 1.5. In the mixture both variants A and B have the same aggregation rate. The gel time increases with decreasing concentration and diverges at about 0.7g L−1. At lower concentration the aggregate growth stagnates when all protein has aggregated.

THE pH DEPENDENCE OF HORNBLENDE DISSOLUTION

Hornblende minerals are important in proton neutralization and base cation release processes in many soils. This study was conducted to determine the pH dependence of hornblende dissolution rates and the rate of proton consumption during weathering under laboratory conditions. Hornblende was crushed mechanically and purified by magnetic and density separation. The samples were initially weathered (22 days) in pH 4.0, 0.01mol/L HOAc-LiOAc buffer to minimize the effect of grinding on dissolution rates. The pH range investigated was from 3.4 to 4.6 at approximately 0.2 pH unit intervals. The hornblende dissolution experiment was conducted in a 0.01 mol/L HOAc-LiOAc buffer in batch-type reactors for 30 days. The rate of proton consumption was determined for both the initially weathered samples and for samples not initially weathered in 0.01 mol/L LiCl at pH near 4.0. The dissolution was nonstoichiometric during the study period, with preferential release of Al, Fe, and Mg relative to Si. Nearly linear kinetics were observed for the release of Si, but the release rates of Al, Fe, and Mg decreased with time. Hornblende dissolution rates in the pH range studied decreased as pH increased. The overall apparent linear reaction orders, with respect to solution pH, were found to be 0.71, 0.39, 0.42, and 0.79 for Al, Fe, Mg, and Si release, respectively. The reaction orders at pH values between 3.4 and 4.2 were lower than those between pH 4.2 and 4.6. The quantity of proton consumed was equivalent to the total charges of the cations released. The proton consumption rate of samples not initially weathered was five times higher than that of the initially weathered samples. Dissolution rates obtained in acetate buffer were 3 to 4 times higher than those obtained in unbuffered LiCl solution at comparable pH and ionic strengths. The rate of hornblende dissolution is a function of solution pH and concentration of organic ligand.

Carbochlorination kinetics of tantalum and niobium pentoxides

The carbochlorination kinetics of pure Nb 2 O 5 and Ta 2 O 5 by gas mixture (C1 2 + CO + N 2 ) between 380 and 1,000 °C is studied. A calculation of the standard free energy of the carbochlorination reactions is made. A diagram of the phases stability is drawn. The influence of the gas flow, temperature and the pardal pressure of Cl 2 and CO at temperatures below 650 °C on the reaction rate is studied. The apparent activation energy is approximately 75 and 110 kJ/mol for Nb 2 Os and Ta 2 O 5 , respectively. At temperatures above 650°C the Arrhenius diagram presents an anomaly which may be attributed to the decomposition of the COCl 2 formed in situ. The apparent reaction order of the carbochlorination of these oxides against Cl 2 +CO is approximately 2. The carbochlorination rates of these oxides are much greater than those of chlorination by Cl 2 + N 2 . The carbochlorination kinetics of tin furnace slag leaching concentrates containing tantalum and niobium compounds are also studied and compared with the carbochlorination kinetics of the pure oxides.

Selective catalytic reduction of NO with C 3H 6 over Rh/alumina in the presence and absence of SO 2 in the feed

The selective catalytic reduction of NO in excess O 2 with C 3H 6 as reductant was studied over a Rh/alumina catalyst. The effect of SO 2 on the extent of NO reduction and product formation was extensively examined. TPD and FT-IR studies using different feeds aimed at the identification of species adsorbed on alumina and Rh/alumina samples. Experiments were also carried out in a differential fixed-bed reactor to estimate apparent orders of reaction and activation energies. The rate-controlling step of the proposed reaction mechanism for the NO reduction is the reaction of Rh–NCO and Rh–NO + for an SO 2-free feed. In the presence of SO 2 in the feed the NO and C 3H 6 oxidation over the catalyst were inhibited and a less complicated reaction mechanism is proposed, based on the reaction of Rh–NO + and partially oxidized C 3H 6.

Kinetics of the Selective Reduction of NO with NH3 over a V2O5(WO3)/TiO2 Commercial SCR Catalyst

In order to clarify the mechanism of the selective catalytic reduction of nitric oxide with ammonia over a V2O5(WO3)/TiO2 commercial SCR catalyst, measurements were made on the reaction rate, rNO, as a function of partial pressure of nitric oxide, PNO, partial pressure of ammonia, PNH3, and partial pressure of oxygen, PO2, from 513 to 553 K under steady-state conditions. The adsorption of NO and NH3 on the catalyst was also observed by infrared spectroscopy (DRIFT). The apparent reaction orders with respect to NO were observed to be less than unity, 0.6–0.8. The reaction rate was nearly independent on PNH3 at lower temperatures. As temperature increased, rNO became slightly increased with increasing PNH3 at lower partial pressures of ammonia and tended to be saturated with further increases of PNH3. The dependence of rNO on PO2 was similar to that of PNH3: rNO increased with increasing PO2 at lower partial pressures of oxygen and was saturated with further increase of PO2. The spectroscopic study showed that NO does not adsorb significantly on the oxidized nor on the NH3 preadsorbed surface of catalysts above at least 473 K. The SCR reaction was considered to proceed as follows. NH3 adsorbed on the Brønsted acid sites as ammonium ions. Ammonium ions were activated with the terminal oxygen groups, V5+=O, prior to the reaction with gaseous NO. Subsequent reaction with NO produced N2, H2O, and the hydroxyl groups bonded to the reduced vanadium, V4+−OH, which would be reoxidized by oxygen to the V5+=O species. The Brønsted acid sites where NH3 adsorbed were then recreated. The results obtained in this study suggested that the Brønsted acid sites and/or the V5+=O species were equilibrated with the other species on the surface, implying that the number of each site changed with the experimental conditions such as PO2. The relative amount of the V5+=O species would vary from ∼0.1 to ∼0.4 with increasing PO2.

Chlorination and carbochlorination of magnesium oxide

Thermogravimetric technique and boat experiments were used to study the chlorination of MgO and its reactivity with respect to Cl2 + air, Cl2 + N2, and Cl2 + CO gas mixtures at temperatures lower than 1000°C. Oxychlorination of MgO occurs at temperatures higher than that of its carbochlorination. Effects of experimental parameters such as gas flow rate, temperature, and partial pressure of the carbochlorinating gas mixture on the reaction rate were examined. At 550 °C, the apparent reaction orders with respect to Cl2 + CO, Cl2, and CO were 2.37, 1.47, and 0.89, respectively. At this temperature, the maximum reaction rate was obtained using a Cl2 + CO gas mixture having a Cl2/CO molar ratio equal to about 0.6. The apparent activation energy of carbochlorination of MgO was calculated as 49 kJ/mol between 425 °C and 600 °C.

Reaction of Hydrogen/Water Mixtures on Nickel‐Zirconia Cermet Electrodes: I. DC Polarization Characteristics

The reaction of hydrogen/water mixtures on nickel‐zirconia cermet electrodes has been investigated as a function of potential and temperature at three different partial pressures of the reactants. Apparent kinetic parameters, namely, reactions orders, activation enthalpies, and pre‐exponential factors, were determined as a function of potential in low‐temperature (725–845°C) and high‐temperature (845–950°C) regions for both the hydrogen oxidation and the hydrogen evolution reactions. In the low‐temperature region, the charge‐transfer coefficient, , calculated according to absolute rate theory was 0.7, independent of temperature. The apparent reaction order of the hydrogen oxidation reactions is in the region of 0.5 at 725°C, from which we conclude that at low temperatures, charge transfer involving atomically adsorbed hydrogen species determines the rate of this reaction. The results for the hydrogen oxidation reaction at temperatures above 845°C as well as for the hydrogen evolution reaction are not consistent with a charge‐transfer‐controlled electrode. Adsorption and chemical reaction between adsorbed species are considered to be dominant for the hydrogen oxidation reaction at temperatures above 845°C and the hydrogen evolution reaction over the entire temperature range studied. © 1999 The Electrochemical Society. All rights reserved.

MECHANISMS AND RATES OF IRON DISSOLUTION FROM BRAZILIAN PLINTHUSTALFS BY DILUTE HYDROCHLORIC AND ASCORBIC ACIDS

Although the dissolution of Fe oxides has been studied extensively, information regarding the dissolution of Fe from tropical plinthic soils is limited. This study examined the influence of acidity, temperature, and chemical reduction on extraction from Fe-rich water-stable aggregates and associated matrices. The two soils studied were Plinthustalfs, developed from the clayey sandstone sediments of the Inferior Cretaceous Itapecuru Formation in Maranhao state, Brazil, and represent an immature Groundwater Laterite soil. Pedon 2 contains more free Fe oxides than Pedon 1. All extractions performed dissolved less Fe than acid ammonium oxalate. Larger apparent reaction orders for solution (H + ) were determined for A and E than for B horizons and are attributed to desorption of organic compounds. Smaller activation energies and larger reactive surface areas (frequency factors) in the A and E horizons could be related to larger organic carbon concentrations. Larger activation energies and smaller reactive surface areas in the water-stable aggregates corresponded with apparently smaller Al substitution in the Fe oxides. The influence of chemical reduction was also stronger in fractions containing larger amounts of organic carbon and greater (apparent) isomorphic substitution of Fe 3+ by Al 3+ . With progressive dissolution, there is a decrease in apparent reaction order and an increase in activation energy as well as in reactive surface area.

Hydrotreating of straight run gas oil–light cycle oil blends

A pilot plant study was conducted to evaluate the effect of up to 50 vol% light cycle oil (LCO) on product quality when it is used together with straight run gas oil (SRGO) as a hydrotreating feedstock. Experiments were carried out at reaction pressures of 54, 70 and 90 kg/cm 2; reaction temperatures of 350, 360, 370 and 380°C, liquid hourly space velocity (LHSV) of 1.0, 1.5 and 2.0 h −1, and constant hydrogen-to-oil ratio of 2000 ft 3/bbl, over a commercial Co–Mo/γ-Al 2O 3 catalyst. The experimental results were used to determine apparent reaction orders and activation energies, which agree with those reported in the literature.

Reduction of NO with NH 3 over carbon catalysts – the influence of carbon surface structures and the global kinetics

The reduction of NO by NH 3 over carbon catalysts was studied at temperatures in the range 140–320°C, and at NO and NH 3 concentrations varying within 290–1160 ppm. The carbon catalysts of different surface areas were prepared by gasifying bituminous and sub-bituminous coal chars in CO 2 to different extents of burn-off. The NO reduction rate was found to increase with the carbon surface area, which is an increasing function of the burn-off level. However, the activity per unit surface area of the carbons decreases with the extent of burn-off. By adding oxygen in the reacting system, the NO conversion can be significantly enhanced. The apparent activation energy was found to be around 20 kJ/mol for the reactions with or without oxygen addition. The apparent reaction orders with respect to NO and NH 3 are functions of the reaction temperature and are affected by the presence of oxygen. It has been suggested on the basis of the kinetic study that the catalytic reduction of NO might be governed by two consecutive steps: the adsorption of NH 3 on active sites, followed by the addition of NO or NO 2 in the neighborhood of the C(NH 3) complexes.

Kinetic Analysis of Au deposition from Aqueous HF onto Si(111) by Surface Second Harmonic Generation

Deposition of Au from aqueous HF onto Si(111) has been studied by Rutherford backscattering (RBS) and surface second harmonic generation (SHG), which at 532 nm is nearly resonant with the surface plasmon of Au nanoclusters. The RBS measurements indicate that Au deposition is rate‐limited by diffusion, while the SHG measurements indicate that Au cluster growth is rate‐limited by either a surface reaction involving a fluoride‐containing species or electron transfer. This apparent contradiction can be reconciled by proposing that initial deposition in the form of AuCN is followed by a slow electroless reduction of Au(I) accompanied by Si oxidation. By addition of HCl and KF, the solution phase equilibria can be separately manipulated, motivating further SHG experiments which indicate that HF and not is the kinetically active fluoride‐containing species. The apparent reaction order for Au cluster growth with respect to HF is approximately 1/2, and the reaction order for Au cluster growth with respect to is near zero in the concentration range to . © 1999 The Electrochemical Society. All rights reserved. ©2001 American Geophysical Union

Etherification rates of 2-methyl-2-butene and 2-methyl-1-butene with ethanol for environmentally clean gasoline production

Etherification of C5 reactive olefins available in light fluidized catalytic cracking (FCC) gasoline is an attractive way to decrease the olefins and to increase the octane number. The reactivities of 2-methyl-1-butene (2M1B) and 2-methyl-2-butene (2M2B) in the etherification reaction with ethanol catalysed by a strongly acidic macroreticular resin catalyst were investigated in a temperature range of 333–360 K using a liquid phase differential flow reactor. In the presence of excess alcohol, the apparent reaction orders of etherification reactions of isoamylenes were found to be 0.93 and 0.69 with respect to 2M1B and 2M2B concentrations, respectively. 2M1B was shown to be more reactive than 2M2B and its activation energy is also lower in the etherification reaction. It was also shown that diffusion resistances, especially in the macropores of the catalyst, may play an important role on the observed rates. © 1999 Society of Chemical Industry

Kinetic study on the thermal degradation of polypropylene and polyethylene

In this work, a kinetic study on the thermal degradation of polyethylene and polypropylene is presented. The thermal degradation of these polymers is investigated under isothermal conditions using a gradient free reactor with on-line mass spectrometry. Apparent kinetic parameters for the overall degradation are determined. In the case of polyethylene degradation, a change of the apparent reaction order with temperature is observed, whereas in the case of polypropylene degradation, a constant apparent order of reaction within the investigated temperature range was found. Fractional pyrolysis under dynamic conditions of polypropylene performed at temperatures below the isothermal measurements reveal a decreasing amount of alkenes and alkanes and an increasing amount of dienes with temperature. On the basis of these data and corresponding literature, a detailed discussion of the reaction mechanisms is possible. Rate equations are formulated and kinetic models for polyethylene and polypropylene degradation are discussed, which are consistent with the measured rate coefficients.

A molecular beam study of nonlinearity in the CO-induced surface restructuring of Ir{100}

The kinetics of CO chemisorption on both the (1×5) and (1×1) surfaces of Ir{100}, including the CO-induced surface restructuring process, have been studied by measuring the sticking probability as a function of the surface temperature and beam flux. Due to competition between desorption from the (1×5) phase and growth of (1×1) islands, the sticking probability on the initial (1×5) surface is strongly flux-dependent at surface temperatures Ts in the range 480⩽Ts⩽510 K. It is shown that this is due to a strongly nonlinear dependence of the (1×1) growth rate on the local CO coverage on the (1×5) substrate, with an apparent reaction order of around 5. Desorption energies and pre-exponentials of desorption for CO from both the (1×1) and (1×5) surfaces have been determined by means of a modified lifetime measurement technique. Equilibrium coverages as well as isothermal desorption rates of CO were determined for both surface phases. The zero coverage desorption energy of CO from the (1×1) substrate is 196±5 kJ/mol and from the (1×5) surface it is around 150 kJ/mol. This difference in adsorption energies is the driving force for the CO-induced (1×5) to (1×1) phase transition. TEAS data show that the local CO coverage on the growing (1×1) islands during the phase transformation is 0.5 ML.

Methanol decomposition over supported palladium and platinum

Methanol decomposition over supported Pt and Pd catalysts was investigated in the temperature range from 453 to 573 K with the partial pressure of methanol up to 0.8 arm. The specific activity of Pd was higher than that of Pt, and alumina-supported Pd was more active than silicasupported Pd, although aluminasupported catalysts produced dimethyl ether as a by-product. A silica-supported Pd catalyst prepared from a tetraamine salt solution with a proper pH exhibited higher activity than the other silica-supported Pd catalysts. Dependence of the decomposition rate on the partial pressure of methanol was similar regardless of the metal, the support or the preparation method. The apparent reaction orders were near 0.3 at low pressures below about 0.4 atm and became near zero or slightly negative at higher pressures. The apparent activation energies were about the same for most of the catalysts and were in the range from 66 to 77 kJ/g-mol.

Thermal Denaturation of Ovine β-Lactoglobulin

Ovine β-lactoglobulin (β-LG) genetic variants A and B were purified by molecular sieving of acidic whey from homozygous ewes. Heat denaturation of both variants was studied by differential scanning calorimetry in phosphate buffers (0.05 M and 0.5 M, pH 6.6) at different heating rates. Ovine β-LG is more thermostable than its bovine homologue, and variant A is more heat-resistant than variant B. The denaturation temperatures ranged from 71.4°C (variant B in 0.05 M phosphate buffer at a heating rate of 1°C/min) to 89.9°C (variant A in 0.5 M buffer at a heating rate of 20°C/min). Values that were extrapolated to zero heating rate ranged from 70.5°C (variant B, 0.05 M buffer) to 81°C (variant A, 0.5 M buffer). There were no significant differences between variants in enthalpy change or cooperativity in denaturation, but activation energy was higher for variant B. Denaturation of ovine β-LG follows an apparent reaction order that depends on heating rate and buffer molarity, but was always >1. Van’t Hoff enthalpies and the ratio of calorimetric enthalpy to Van’t Hoff enthalpy were dependent on heating rate, revealing that, at low heating rates, important secondary reactions could take place while the protein was being unfolded.

Control of the product distribution in the hydrogenation of vegetable oils over nickel on silica catalysts

AbstractSeveral nickel on silica catalysts, prepared by impregnation or precipitation/deposition, and a commercial catalyst were tested for activity and selectivity in the sunflower seed oil hydrogenation. An average turn‐over frequency of 2.57 s−1 was found for the catalysts, assuming inaccessibility of nickel in pores smaller than 2 nm and a constant nickel surface concentration poisoned by the reaction mixture. After studying the mass‐transfer steps, the effect of temperature (373‐453 K) and pressure (101‐608 kPa) on reaction rates in the kinetic regime was analyzed, and the corresponding apparent activation energies and reaction orders were obtained. Conclusions on the effect of temperature and pressure on the selectivity to the preferential hydrogenation of polyunsaturates (So) and to the formation of trans‐isomers ((Strans)0) in the kinetic regime were derived from the results. Finally, a similar analysis was carried out when diffusion limitations were known to be present.