Kinetics and reaction mechanism of catalytic oxidation of low concentrations of hydrogen sulfide in natural gas over activated carbon

The kinetics of the hydrogen sulfide oxidation process, producing mostly sulfur and water, was studied using 0.25 to 1.0 g Hydrodarco activated carbon catalyst and varying the O₂/H₂S ratio (molar basis) in the feed gas between 0.5 to 0.6 in the temperature, and pressure ranges from 125 to 200°C and 225 to 780 kPa. SO₂ was obtained as an undesirable by-product during H₂S oxidation reaction or as a product during regeneration of the catalyst. The feed gas contained 0.9 — 1.3 mol% H₂S with approximately 80 mol% CH_4. In this paper, the factors affecting the H₂S conversion and SO₂ formation are presented. The rate expressions for (a) H₂S conversion and (b) SO₂ formation were developed from the Langmuir-Hinshelwood surface control reaction model. The experimental data were well correlated by the rate equations. Also, the rate parameters were evaluated and correlated with temperature. The activation energies for H₂S oxidation and SO₂ production reactions were calculated to be 34.2 and 62.5 kJ/mol, respectively. Partial pressures of oxygen and H₂S were found to influence H₂S conversion whereas, the presence of water in the feed gas up to 10.5 mol% did not affect H₂S conversion significantly. Heats of adsorption for various species on the active sites were calculated. SO₂ production was, as expected, enhanced at higher temperature, and its rate was much smaller than the oxidation rate of H₂S under the reaction conditions used.     

Thermal oxidation of polypropylene in the temperature range of 120–280°c

Thermal oxidation of isotactic polypropylene films at 120–280°C in air was studied. Separation and identification of the volatiles formed was carried out by gas chromatography–mass spectrometry. Sixteen products were tentatively identified for the first time. Altogether, 50 compounds representing hydrocarbons, ethers, alcohols, aldehydes, ketones, and acids are reported. Oxygen deficiency is manifested in diffusion-limited products of olefines, dienes, and aromatic compounds. The relative amounts of acetaldehyde and acetone are almost temperature independent in the range of 120–280°C. This indicates a similarity of oxidative degradation of the polymer in a broad temperature range. Addition of an antioxidant to the polymer depresses the evolution of the main volatiles by 9–10 times at 280°C. The relative amounts of the volatiles formed are, nevertheless, the same as for the polymer without an antioxidant. The mechanism of formation of the degradation products is discussed.     

Kinetics of the pyrolysis of cellulose in the temperature range 250–300°C.

Samples of α-cellulose, containing 0.11–0.14% ash, were isothermally pyrolyzed in a fluidized bath in a nitrogen environment at 250–298°C. Results were reported in terms of volatilization (based on weight-loss measurements) and decomposition (in term of glucosan loss). The findings show three distinct stages of pyrolysis: (1) an initial period of rapid decomposition and weight loss; (2) a range in which both the volatilization and decomposition are of zero order; (3) a region in which the volatilization follows a first-order rate, leaving a char deposit which does not undergo further pyrolysis. The degree of decomposition and volatilization occurring during the zero-order phase increases with increasing temperature. A single activation energy of 42 kcal./mole describes both the decomposition and volatilization rates in the zero-order phase over the entire 250–298°C. range.     

Oxidation of low concentrations of hydrogen sulfide over activated carbon

The oxidation of low concentrations of hydrogen sulfide with air over activated carbon was studied over the temperature range 24-200°C using both fixed and fluid bed reactors. The predominant reaction, H₂S + ½ O_a → H₂O + S, was found to have an order of 0.5 with respect of H₂S concentration. Activity of the catalyst decreased as the amount of sulfur deposited on it increased. Indirect evidence suggests that adsorption of water by the carbon also decreases its activity as a catalyst at lower temperatures. Values of the activation energy and the frequency factor were determined for various sulfur loadings using the fixed bed reaction system. Regeneration of the carbon loaded with sulfur was studied at temperatures between 150 and 500°C using steam as a carrier gas. Bright yellow sulfur was recovered. The regenerated carbon was shown to have its original activity.     

A Comparison of 0°–0° and 45°–45° bridge‐Seeded,YBCO single grains

A variety of multiseeding techniques have been investigated over the past 20 yr in an attempt to enlarge bulk (RE)BCO superconducting samples fabricated by the top‐seeded melt growth (TSMG) process for practical applications. Unfortunately, these studies have failed to establish whether technically useful values of trapped field can be achieved in multiseeded bulk samples. In this work specially designed, 0°–0° and 45°–45° bridge seeds of different lengths have been employed to produce improved alignment of the seeds during the TSMG process. The ability of these bridge‐seeded samples to trap magnetic field, which is the key superconducting property for practical applications of bulk (RE)BCO, is compared for the samples seeded using 0°–0° and 45°–45° bridge seeds of different lengths. The grain boundaries produced by these bridge seeds are analyzed in detail, and the similarities and differences between the two bridge‐seeding processes are discussed.     

Fracture toughness of some adhesive cement systems measured in the temperature range 20–700°C

The adhesive fracture energy (G_IC) of several adhesive cement systems has been measured at temperatures extending to 700°C using alumina adherends in the double torsion test geometry. Two commercially available low-temperature curing systems were evaluated together with three laboratory-formulated cements, two of which were cured at 1000°C. The room temperature values of G_IC rangee from 0.6 to 4.9 J m⁻² for the former and from 6.7 to 11.4 J m⁻² for the latter. Despite the adoption of a linear compliance test geometry it was essential to precrack the specimens in order to obtain ‘valid’ G_IC data. Generally the fracture energy of the adhesive cement joints increased with temperature up to a peak between 400 and 600°C, and then decreased. The maximum G_IC value recorded was 40.2 J m⁻². The increase in toughness can be associated with viscous effects in the glassy phases present in the adhesive cements.     

Effect of long-term oxidation at 200–300°C on six types of aromatic amide and imide resins

Six types of aromatic amide and imide resins in the form of wire enamels, paper, and film were subjected to thermal and oxidative deterioration at temperatures ranging from 200 to 300°C and for aging periods of 3 and 6 months. No HCN was evolved at any temperature and aging time. The main degradation product was, in all cases, carbon dioxide, in quantities increasing as functions of time and temperature. Other materials evolved were, in approximate order of magnitude, carbon monoxide, water, and nitrogen. Traces of acetonitrile were recovered from a film and a paper sample, while benzene, also in trace quantities, was evolved from the wire enamel samples at temperatures ranging from 250 to 300°C.     

Selective oxidation of hydrogen sulfide containing excess water and ammonia over vanadia–titania aerogel catalysts

A series of V₂O₅–TiO₂ aerogel catalysts were prepared by sol–gel method with subsequent supercritical drying with CO₂. The aerogel catalysts showed much higher surface areas and total pore volumes than V₂O₅–TiO₂ xerogel and impregnated V₂O₅–TiO₂ catalysts. Two species of surface vanadium in the aerogel catalysts were identified by Raman measurements: monomeric vanadyl and polymeric vanadates. The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was studied over these catalysts. Aerogel catalysts showed very high conversion of H₂S without harmful emission of SO₂. Temperature programmed reduction (TPR), XRD and Raman analyses revealed that the high catalytic performance of the aerogel catalysts originated from their highly dispersed VO_x species and high reducibility.     

Catalytic oxidation of hydrogen sulphide by air over an activated carbon fibre

Activated carbon fiber (ACF), FN-200CF-15, made from coal tar, oxidized hydrogen sulphide effectively to sulphuric acid at an inlet concentration of 200 ppm and space velocity (SV = flow rate/packing volume) of 100 h⁻¹ (sulphur load 11 g-S/day kg-fiber) for one and a half months only by keeping the moisture content of ACF more than 50%. This phenomenon is different from granular activated carbon (GAC) reported previously.     

A modified brice-phoenix light scattering photometer for the angular range 30° −150°

A simple modification of a BRICE-PHOENIX 2000 DM light scattering photometer which provides for the measurement of the intensity of scattered light at eleven discrete angles in the range 30° ?150° is described. Angular resolution is improved and reflection effects are virtually eliminated for scattering systems with a refractive index close to 1.5. Using this modification, good values for the depolarisation ratio of pure liquids are obtainable. Characterisation data for the Dow Chemical S 114 polystyrene sample are reported.     

Kinetics of carbon monoxide oxidation over chromium sesquioxide with two reaction paths

Kinetics of CO oxidation over chromium sesquioxide with two reaction paths I and II was studied in detail in the temperature range of 378–445 K (105–172°C). The rate constants in each elementary step were estimated as a function of temperature by using transient response and steady-state data without the help of a parameter optimisation technique. A comparison of their values showed that the rate of path I is limited by surface reaction, and the rate of path II is controlled by three steps: surface reaction; desorption of CO₂; and adsorption of O₂, which is the slowest step over the entire temperature range studied. Good agreement of the simulated response with the experimental response due to the linear temperature rise proved the validity of the mechanism and the kinetic analysis. The selectivity of reaction path I was also discussed.     

Hydrothermal synthesis (200 °C) of Co–kaolinite and Al–Co–serpentine

In the systems CoO–Al₂O₃–SiO₂–H₂O and CoO–Al₂O₃–SiO₂–HCl–H₂O, at initial pH between 5.5 and 8.1 and temperature of 200 °C, kaolinite is unstable and the following phases form through a dissolution-precipitation process: a) kaolinite and Co-bearing kaolinite; b) Al–Co–serpentine; and c) poorly crystalline phases. Identification of the several phases was carried out from a combination of X-ray diffraction and transmission/analytical electron microscopy.Co–kaolinite shows variable morphologies: a) Platy lath-shaped particles with very low Co content; b) Spherical particles, with relatively constant Co contents (in the order of 0.10 apfu); c) Kaolinite stacks with very variable Co contents (up to 0.25 apfu). Analytical data indicate that the presence of Co(OH)₂ in the system favors the dissolution process as well as serpentine formation but it leads to the parallel formation of abundant poorly crystalline phases. The Co-content in kaolinite increased as a function of the Co(OH)₂/CoCl₂ ratio in the initial systems, and it is reflected by a parallel increase of the b-cell parameter of kaolinite. The average composition of the coexisting Al–Co–serpentine is: (Al_1.20Fe_0.11Co_1.27)(Si_1.64Al_0.36)O₅(OH,Cl)₂, with Cl contents in the order of 0.14 apfu.The assemblage Co–kaolinite + Al–Co–serpentine, which appears to be stable at 200 °C, has not been described in natural environments, probably because it requires unusual Al- and Co-rich chemical systems.     

Kinetics and mechanism of the alkali-catalyzed p-cresol–formaldehyde reaction

The kinetics of the reaction of p-cresol with formaldehyde in relation to the functionality of p-cresol using NaOH as catalyst has been studied at temperatures of (65 ± 0.05)°C, (70 ± 0.05)°C, (75 ± 0.05)°C, and (80 ± 0.05)°C. The pH maintained was 7.0, 8.0, 9.0, 9.4, and 10.0. The reaction follows a second-order rate law. The rate was found to increase with increase in pH. The stepwise rate constants (k₁ and k₂) for the formation of monomethylol-p-cresol and dimethylol-p-cresol, respectively, were calculated from the overall rate constant k. The values of Arrhenius parameters and the entropy of activation for the overall as well as the stepwise reactions were calculated. The experimental and calculated values of k at pH 10.0 and temperatures 65, 70, 75, and 80°C were found to agree well within experimental errors. A mechanism conforming to the energies and entropies of activation of the reaction is suggested.     

The specific volume of poly(tetrafluoroethylene) as a function of temperature (30°–372°C) and pressure (0–2000 kg/cm2)

Experimental data on the specific volume of a 50% crystalline sample of poly(tetrafluorethylene) are presented and discussed. Data points were taken along 22 isotherms spaced 5 to 30°C apart (up to 372°C) in pressure increments of 100 kg/cm² up to 2000 kg/cm². In addition to the melting transition and the first-order solid–solid transition near room temperature, a previously reported second-order transition near 140°C at P = 0 is observed. This transition shifts to higher temperatures with increasing pressure by about 0.015°C per kg/cm². The melt data are discussed in detail. They can be fitted to both the empirical Tait equation, with the usual exponential temperature dependence of the Tait parameter, and to the Simha-Somcynsky hole theory (with the reducing parameters V^* = 0.424 cm³/g, T^* = 7906°K, and P^* = 7100 kg/cm² = 6960 bars), thus providing a test for this theory at high reduced temperatures near T? = 0.08.     

Use of modified activated carbon for the oxidation of aqueous sulfide

The oxidation of aqueous sulfide promoted by activated carbon (AC) was investigated. Raman, infrared, X-ray diffraction, ultraviolet–visible spectroscopy and electron paramagnetic resonance analyses show that AC oxidizes aqueous sulfide to form a complex mixture of polysulfides and sulfur oxide species. This oxidation is strongly promoted by the presence of oxygen surface groups introduced by oxidation of the AC as shown by kinetic measurements. These results are discussed in terms of the similarities the AC have with enzymatic systems, i.e. redox surface groups (e.g. quinone) combined with an efficient electron conduction system.     

油井采出液硫化氢空气氧化反应动力学研究

高含硫化氢油井采出液面临一次处理达不到排放要求的问题,而硫化氢的空气氧化法操作简单,成本低廉,对现有生产流程改动和影响较小,是井口硫化氢预处理的有效方法之一。空气氧化硫化氢是一个伴有化学反应的传质过程,采出液相界面处,硫化氢解离为HS^-离子,在液膜(或液相主体)内HS-离子,在液膜(或液相主体)内HS^-与溶解氧发生氧化还原反应。研究了压力、温度、含水率、硫化氢浓度、氧气浓度等反应条件对表面反应速率的影响,并建立得到反应动力学方程为:■,为井口硫化氢的空气高效处理提供理论依据。并根据反应速率方程和阶梯模拟计算的方法,计算出一定实验条件下,距离空气进气口不同位置的硫化氢转化率,为工业上空气氧化脱硫的设计提供理论依据。     

The three successive stages of creep of PMMA between 55°c and 90°c

After cooling from 140°C, the physical aging of poly(methyl methacrylate), PMMA, has been studied at 55, 70, 84, and 90°C by means of temporary microcreep tests. The systematic analysis of recoveries reveals that at 90°C, creep follows successively three different stages. After stress application, creep starts by a logarithmic function of time, while recovery follows exactly the same kinetics. An Andrade creep begins after the logarithmic creep: It is in fact the beginning of the Kohlrausch-Williams-Watts creep with an exponent of 0.36. This creep is fully recoverable, but the duration of this recovery is ～3 times the creep duration. The third stage of creep is superimposed on the Andrade creep and follows a power law in t^0.8, but it is not recoverable in the usual duration of experiments. By comparison with the recoverable creep of PMMA studied by Plazek above T_g, which can be fitted by a KWW law with an exponent 0.8, this third creep is interpreted as the beginning of the rubbery deformation.     

Peculiar sandwich-like π–π interaction regulating the nonplanarity of the model heme crystals

X-ray crystallographic analysis of bis(pyridine N-oxide) complexes of iron(III) porphyrinates has revealed that the two pyridine rings of the axial ligands correctly sandwich the porphyrin ring to induce the deformation of commonly observed S₄ saddled porphyrin ring.