Thermodynamics and Stoichiometry of Reactions between Hydrogen Sulfide and Concentrated Sulfuric Acid

A thermodynamic analysis of the possible reactions between hydrogen sulfide and concentrated sulfuric acid shows that four reactions are feasible. However, only two of these reactions apparently occur in experiments at 1 atm and 0°C to 150°C. Hydrogen sulfide is first oxidized by molecular sulfuric acid, forming SO₂, sulfur and water, and then the H₂S may react with the dissolved product, SO₂, to generate sulfur and water. The stoichiometry of the consecutive reactions and their dependence on acid concentration were determined experimentally using mass balance measurements. The results of this study suggest a possible alternative method for sulfur removal and recovery that has more advantages.     

Kinetics of sulfur transfer from H2S to Fe1-xS

The rate of sulfur transfer across the gas/solid interface involving H₂S(g) and Fe_1-xS surface has been investigated using resistance relaxation measurements at 600°C. The rate of the oxidation reaction incorporating sulfur into Fe_1-xS has been found to decrease with sulfur activity (a_S) in the sample as (a_S)^-2/3, while the rate of the reduction reaction corresponding to sulfur loss is found to increase with the sulfur activity as (a_S)^1/3. The kinetic finding has been combined with the appropriate defect models for FeS to identify the rate limiting step for the sulfur transfer reaction from H₂S to FeS. Accordingly, the rate limiting step has been identified to be: H₂S(g) + 2e^-\rightleftharpoons S^2-(ad) + H₂(g). This revised version was published online in July 2006 with corrections to the Cover Date.     

Sulfur removal from coal with supercritical fluid treatment

Conversion and sulfur removal of coal in sub- and supercritical water was studied in a micro reactor in the temperature range of 340-400°C and water density 0-0.27 g/cm³ for 0-90 min under N₂ atmosphere. The experiments were conducted to investigate the effect of reaction temperature, pressure, time and density of water on the sulfur removal in gaseous and liquid effluents, respectively. The results show that supercritical condition is more effective than sub-critical condition to remove the sulfur from coal. It is possible to reduce 57.42% of the original sulfur in coal for the reaction time of 90 min at 400°C and 30 MPa. The main gas containing sulfur in the gaseous effluent is not SO₂ but H₂S, irrespective of operating condition. The sulfur removal in liquid effluents is much greater than that in gas effluents. Compared with temperature, the influence of water density and pressure is less significant.     

Measurement and prediction of LDPE/CO2 solution viscosity

When CO₂ is dissolved into a polymer, the viscosity of the polymer is drastically reduced. In this paper, the melt viscosities of low‐density polyethylene (LDPE)/supercritical CO₂ solutions were measured with a capillary rheometer equipped at a foaming extruder, where CO₂ was injected into a middle of its barrel and dissolved into the molten LDPE. The viscosity measurements were performed by varying the content of CO₂ in the range of 0 to 5.0 wt% and temperature in the range of 150°C to 175°C, while monitoring the dissolved CO₂ concentration on‐line by Near Infrared spectroscopy. Pressures in the capillary tube were maintained higher than an equilibrium saturation pressure so as to prevent foaming in the tube and to realize single‐phase polymer/CO₂ solutions. By measuring the pressure drop and flow rate of polymer running through the tube, the melt viscosities were calculated. The experimental results indicated that the viscosity of LDPE/CO₂ solution was reduced to 30% of the neat polymer by dissolving CO₂ up to 5.0 wt% at temperature 150°C. A mathematical model was proposed to predict viscosity reduction owing to CO₂ dissolution. The model was developed by combining the Cross‐Carreau model with Doolittle's equation in terms of the free volume concept. With the Sanchez‐Lacombe equation of state and the solubility data measured by a magnetic suspension balance, the free volume fractions of LDPE/CO₂ solutions were calculated to accommodate the effects of temperature, pressure and CO₂ content. The developed model can successfully predict the viscosity of LDPE/CO₂ solutions from PVT data of the neat polymer and CO₂ solubility data.     

The rate of selenium removal from tellurium by hydrogen purging

The reaction between hydrogen gas and selenium dissolved in liquid tellurium is described as follows H₂(g) + Se ? H₂Se(g) The kinetics of this reaction have been investigated at temperatures ranging between 525 and 625°C and at concentrations of selenium varying between 250 and 3,300 ppm. Under all conditions investigated the reaction was controlled by transport processes in one or other of the two phases depending on the relative values of the mass transfer coefficients. Subject to the assumption that the solution of selenium in tellurium is an ideal one the molecular diffusion coefficient of mixtures of hydrogen selenide in hydrogen, Dg, is approximately 1.6 cm.² sec.^?1 at 525°C, and 2.1 cm.² sec.^?1 at 625°C. In view of complicating factors accurate values of the diffusivity of selenium in liquid tellurium cannot be deducted from the experimental results. However, the value obtained from experimental measurements of 7.59 × 10^?4 cm.² sec.^?1 at 525° is in reasonable agreement with the value of 9.85 × 10^?5 cm.² sec.^?1 reported in the literature.     

Partial Oxidation of H2S to Sulfur Over Mo and/or W-Containing Bronzes with a TTB-Structure

Mo- and W-based oxidic bronzes, with and without Te-atoms in framework positions are active and selective, for the partial oxidation of H₂S to sulfur in the 160?C220 °C temperature range. The catalysts have been prepared by a hydrothermal synthesis and heat-treated in N₂ at 600 °C, and characterized by several physico-chemical techniques, i.e. AAS, S_BET, XRD, SEM?CEDX, DR UV?Cvis and XPS. Te-free catalysts are active, selective and stable for the partial oxidation of H₂S to sulfur. V-atoms in Mo?CO?CV pairs can be proposed as the active sites. Moreover, Te-containing materials show fast catalyst decay. This catalyst deactivation can be related to the presence of Te⁰ and MoS₂, which are observed in used catalyst.     

Vapor pressure and heat of solution of sulfur dioxide in 1-methyl-2-pyrrolidone and its aqueous solution

A new method was developed to measure the vapor pressure of sulfur dioxide above various liquid absorbents. It was applied to pure 1-methyl-2-pyrrolidone and its aqueous solution at 50°C to 91°C for SO₂ loadings of 3.73 × 10 ⁴ kg/kg to 2.16 × 10 ⁻² kg/kg. A chemical model was developed. The heat of solution was calculated from the dependence of the vapor pressure on temperature and was 3.98 × 10⁷ J/kmol (exothermic) for pure 1-methyl-2-pyrrolidone. The vapor pressures disagreed with the only previously published set of results but were confirmed by independent measurements with a gas chromatograph.     

Vanadium oxide catalysts on structured microfiber supports for the selective oxidation of hydrogen sulfide

Vanadium(V) oxide catalysts for the selective oxidation of hydrogen sulfide to sulfur on a nonporous glass-fiber support with a surface layer of a porous secondary support (SiO₂) are studied. The catalysts are obtained by means of pulsed surface thermosynthesis. Such catalysts are shown to have high activity and acceptable selectivity in the industrially important region of temperatures below 200°C. A glass-fiber catalyst containing vanadium oxide (10.3 wt % of vanadium) in particular ensures the complete conversion of H₂S at a temperature of 175°C and a reaction mixture hourly space velocity (RMHSV) of 1 cm³/(g_cat s) with a sulfur yield of 67%; this is at least 1.35 times higher than for the traditional iron oxide catalyst. Using a structured glass-fiber woven support effectively minimizes diffusion resistance and greatly simplifies the scaleup of processes based on such catalysts. Such catalysts can be used for the cleansing of tail gases from Claus units and in other processes based on the selective oxidation of H₂S.     

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.     

Density,viscosity and surface tension of high-silicate CaO–SiO2 and CaO–SiO2–Fe2O3 slags derived by aerodynamic levitation. The behavior of Fe3+ in high-silicate melts

Thermophysical data for liquid slag are required for the optimization and control of metallurgical processes. The density, surface tension and viscosity were measured by employing aerodynamic levitation under contactless conditions. The high-silicate slag (44 and 63 mass-% of SiO₂) of the CaO–SiO₂ and CaO–SiO₂–Fe₂O₃ systems (with 5 and 10 mass-% Fe₂O₃) was investigated under (80% Ar + 20% O₂) gas atmosphere. The temperature ranges were between 800 °C and 2000 °C for the density and 1500 °C–2000 °C for surface tension and viscosity measurements. The influence of the CaO/SiO₂ ratio on the investigated properties and the behavior of Fe³⁺ ions in high-silicate melts were examined. The density of the CaO–SiO₂ melt was lower than that of the CaO–SiO₂–Fe₂O₃ systems. The surface tension of all compositions tested decreased with temperature and showed compositional dependence. The viscosity measured was higher in the Fe₂O₃-containing slag. The Raman spectroscopy analyses confirmed the increase in the degree of polymerization with the addition of Fe₂O₃ for the silicate-rich slag. The formation of a complex anion of a ferric ion and contribution to the silicate network were assumed. The trends observed were related to the structural properties and different interionic bonding. Urbain's viscosity model and FactSage? 7.3 were applied for the assessment of the experimental data.     

Characteristics of the mercury vapor removal from coal combustion flue gas by activated carbon using H2S

Removal of Hg⁰ vapor from the simulated coal combustion flue gases with a commercial activated carbon was investigated using H₂S. This method is based on the reaction of H₂S and Hg over the adsorbents. The Hg⁰ removal experiments were carried out in a conventional flow type packed bed reactor system in the temperature range of 80–150 °C using simulated flue gases having the composition of Hg⁰ (4.9 ppb), H₂S (0–20 ppm), SO₂ (0–487 ppm), CO₂ (10%), H₂O (0–15%), O₂ (0–5%), N₂ (balance gas). The following results were obtained: in the presence of both H₂S and SO₂, Hg removal was favored at lower temperatures (80–100 °C). At 150 °C, presence of O₂ was indispensable for Hg⁰ removal from H₂S–SO₂ flue gas system. It is suggested that the partial oxidation of H₂S with O₂ to elemental sulfur (H₂S+1/2O₂=S_ad+H₂O) and the Clause reaction (SO₂+2H₂S=3S_ad+2H₂O) may contribute to the Hg⁰ removal over activated carbon by the following reaction: S_ad+Hg=HgS. The formation of elemental sulfur on the activated carbon was confirmed by a visual observation.     

A study on selective oxidation of hydrogen sulfide over zeolite-NaX and-KX catalysts

Selective oxidation of hydrogen sulfide (H₂S) was studied on zeolite-NaX and zeolite-KX. Elemental sulfur yield over zeolite-NaX was achieved about 90% at 225 °C for the first 4 hours, but it gradually decreased to 55% at 40 hours after the reaction started. However, yield of elemental sulfur on zeolite-KX was obtained within the range of 86% at 250 °C after 40 hours. The deactivation of the zeolite-NaX and -KX catalysts was caused by the coverage of a sulfur compound, produced by the selective oxidation of H₂S over the catalysts. The coverage of a sulfur compound over the zeolite-NaX and -KX was confirmed by the TPD (temperature-programmed desorption) tests utilizing thermogravimetric analysis and FT-IR analysis. Even though high temperature was required to prevent the deactivation of zeolite-NaX, the temperature cannot be raised to 250 °C or above due to the SO₂ production and the decrease of thermodynamic equilibrium constant. Zeolite-KX was superior to the zeolite-NaX for both its selectivity to elemental sulfur and its resistance to deactivation in the selective oxidation of H₂S.     

Physical solubility of hydrogen sulfide in several aqueous solvents

The solubility of hydrogen sulfide in several aqueous solutions was measured over the temperature range 25°C to 60°C. The solvents investigated in this work include 0 to 50% aqueous solutions of polyethylene glycol, ethylene glycol, methyldiethanolamine and diethanolamine. The amine solutions used in this work were neutralized by the addition of hydrochloric acid in order to suppress the hydrogen sulfide reaction (H₂S → H⁺ + HS^?) so that only the physical solubility of hydrogen sulfide would be measured. The solubility data determined in this work are expressed in terms of Henry's law. The Henry's law constants found in this work were correlated well by a particularly simple empirical formula based on the molecular weight of the solvent.     

Aggregate formation in poly(ethylene oxide) solutions

Static light scattering and viscosity measurements were performed on different molecular weight poly (ethylene oxide) to see the formation of aggregates in its dilute solutions. Viscosity measurements were carried out for PEO samples in water and methanol at 20–45°C and in chloroform at 20–30°C. Using Huggin's equation, the viscosity plots showed distinct upward curvature indicating the presence of aggregates in both PEO/H₂O and PEO/CH₃OH solutions The [η] values for PEO/H₂O and PEO/CH₃OH system were 2–4 times as large as observed for other linear flexible polymers in good solvents thus showing extensive coil swelling/aggregation. This is also apparent from the exponent a values of the Mark–Houwink–Sakurada equation. Light Scattering results using Zimm method showed that aggregation occurred in low molecular weight samples; however, in higher molecular weight samples there was a little evidence for aggregation both in water and methanol. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2578–2583, 2006     

Comparison of hydrodynamic and rheological properties of dilute solutions of a styrene-hydrogenated butadiene copolymer in aliphatic solvents by light scattering and viscometric techniques

Dilute solution of a styrene-hydrogenated butadiene copolymer, a viscosity index improver, were studied by static and dynamic light-scattering techniques. In n-hexane (a model solvent for paraffinic oils) and a mineral base oil, aggregation is observed below 30°C. In cyclohexane (a model solvent for napthenic oils) only isolated polymers are present in the whole temperature range. Kinematic viscometric measurements from 20 to 60°C in the mineral oil show a continuous increase of the intrinsic viscosity together with a decrease of k_H, the Huggins coefficient, from 2.5 to 0.5. At shear rates between 5000 and 40000_s^?1, a large shear thinning is observed at room temperature for the polymer in the mineral base oil. This effect progressively disappears as the temperature increases and the suspension becomes Newtonian near 100°C. All results can be interpreted in terms of micelle formation. © 1994 John Wiley & Sons, Inc.     

Electrochemical Analysis of a System Based on W and Ni Combined with CeO2 as Potential Sulfur‐tolerant SOFC Anode

A formulation of tungsten and nickel combined with CeO₂ (WNi‐Ce) was prepared and evaluated as sulfur‐tolerant anode for SOFC at intermediate temperature. Structural and morphological changes that take place in the system upon interactions with hydrogen sulfide were analyzed. The electrochemical performance was tested in a single cell, WNi‐Ce/LDC/LSGM/LSFC, varying H₂S concentration (0–500 ppm) at 750 °C using I–V curves, impedance spectroscopy and load demands. The highest cell performance was reached in H₂ and decrease with H₂S content increase in the fuel from 226 mW cm⁻² in pure H₂ to 108 mW cm⁻² in 500 ppm H₂S/H₂. Essentially, no decay in the cell performance was observed in the several short‐term load tests studied under several H₂S concentration (0–500 ppm) during 1h, and even in 500 ppm H₂S/H₂ during 70 h, indicating that this material could be a potential sulfur‐tolerant anode.     

The Effect of Sulfur on ZrO<Subscript>2</Subscript>-Based Biomass Gasification Gas Clean-Up Catalysts

The effect of sulfur on biomass gasification gas clean-up over ZrO₂, Y₂O₃–ZrO₂ and SiO₂–ZrO₂ catalysts was examined. Experiments were carried out at the temperature range of 600–900 °C with sulfur free and 100 ppm H₂S containing simulated gasification gas feeds. A mixture of toluene and naphthalene was used as a tar model compound. Results revealed that the sulfur addition affected positively on the catalyst properties mainly at 600 and 700 °C: over Y₂O₃–ZrO₂ and ZrO₂ sulfur addition improved naphthalene and ammonia conversion. However, over SiO₂–ZrO₂ no clear effect with H₂S addition was observed. The effect of sulfur addition on the catalyst properties was connected to the formation of SO₂ from H₂S when oxygen was available. The intensity of the sulfur effect increased with the Lewis basicity strength of the catalysts. This indicates that the sulfur adsorption has a role in generating new type of active sites and/or plays role in changing the redox properties of the zirconia. Since the biomass gasification gas contains usually significant amounts of H₂S the sulfur tolerance of the zirconia based catalysts is a remarkable benefit.     

Catalytic Reduction of Nitric Oxide by Hydrogen Sulfide Over γ-alumina

The ability of H₂S to reduce NO in a fixed bed reactor using a γ-alumina catalyst was studied with the objective of generating new methods for conversion of NO to N₂. Compared to the homogenous reaction of NO with H₂S, the catalyzed reaction showed improved conversions of NO to N₂. Using a gas space velocity of 1000 h⁻¹ and a feed of 1% NO and 1% H₂S in argon, it was found that the conversion of NO to N₂ was complete at 800 °C. This result compared to a 38% conversion of NO to N₂ for the homogeneous gas phase reaction at 800 °C. At temperatures below 800 °C, a short fall in the nitrogen balance was discovered when the γ-alumina was employed as a catalyst. This discrepancy was explained by conversion of NO to NH₃ and subsequent reaction of the NH₃ with any SO₂ in the system to form ammonium sulfur oxy-anion salts. This suggestion is supported by the finding that when larger amounts of H₂S were used relative to NO, more NH₃ was formed together in tandem with lower N₂ mass balances. Several reaction pathways have been proposed for the catalytic reduction of NO by H₂S.     

Vapour liquid equilibrium calculations for dilute aqueous solutions of CO2, H2S,NH3 and NaOH to 300°C

Henry's law constants for aqueous CO₂, H₂S and NH₃ up to 300°C have been recalculated from literature vapour pressure, enthalpy and heat capacity data. The high vapour pressure of water above 150°C causes significant solute-water interactions in the gas phase, which were calculated using the Peng-Robinson cubic equation of state. The results were combined with selected ionization constant data to derive a vapour-liquid equilibrium model for dilute solutions. The model reproduces experimental data for binary systems at solute molalities of up to 0.5 m at 100°C, 1.0 m above 250°C and ionic strengths below about 0.1 m.     

Pd-YSZ cermet membranes with self-repairing capability in extreme H2S conditions

A Pd-YSZ cermet membrane that performs coupled operations of hydrogen separation from a mixed-gas stream and simultaneous hydrogen production by non-galvanic water-splitting, and have high sulfur tolerance is fabricated. It is proved that in H₂S containing atmosphere the Pd-YSZ membrane has self-repairing capability, originating mainly from the conversion of Pd₄S back to metallic Pd and SO₂ by ambipolar-diffused oxygen obtained from water-splitting. The performance of membrane was analyzed at different temperatures in high H₂S containing (0–4000 ppm H₂S) mixed gas feed during the operation as a hydrogen separation membrane as well as during the coupled operation of hydrogen separation and hydrogen production. At 900 °C with the feed-stream having ≥2000 ppm H₂S, the hydrogen flux was severely affected due to the formation of some liquid phase of Pd₄S, resulting in the segregation of hydrogen permeating Pd-phase at the membrane surface. But at 800 °C, though the membrane was affected by the Pd₄S formation in high H₂S environment (up to 1200 ppm H₂S), its self-repairing capability and additional hydrogen production by water-splitting is capable of maintaining the hydrogen flux around ~1.24 cm³ (STP)/min.cm², a value expected by the same membrane while performing only the hydrogen separation function in H₂S-free environment.