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.     

Simultaneous Absorption of H2S and Co2 Into Aqueous Methyldiethanolamine

Abstract

The tertiary araine methyldiethanolamine (MDEA) is finding increasing application as a chemical solvent for selective absorption of hydrogen sulfide from gases containing hydrogen sulfide and carbon dioxide. Gas streams of this type include some natural gases, synthetic gases from coal and heavy oil gasification and tail gases from sulfur plants. Selectivity for H2S is needed either to enrich Glaus sulfur plant feed in H2S or to remove only H₂S when CO₂ removal is not necessary or economic. For the absorption of hydrogen sulfide into MDEA, the reaction which occurs can be considered to be instantaneous while carbon dioxide undergoes a second-order reaction with MDEA.

In this work, the simultaneous absorption of two gases into a liquid containing a reactant with which both gases react is modelled using the film theory. Physical properties and kinetic rate parameters used in the model have been measured in our laboratory. The model is used to study the effect of process variables on the selectivity of MDEA for H₂S over C0₂. The simultaneous absorption of H₂S and CO₂ gases into aqueous MDEA is studied experimentally using a continuous stirred tank absorber. Experimental absorption rates are compared to predictions based on a multicomponent mass transfer model. The average deviations of the theoretical calculations from the experimental results are 10.2% and 12.9% for C0₂ and H₂S, respectively.     

Development and reactivity tests of Ce-Zr-based Claus catalysts for coal gas cleanup

Claus reaction (2H₂S + SO₂ ↔ 3/nS_n + 2H₂O) was used to clean the gasified coal gas and the reactivity of several metal oxide-based catalysts on Claus reaction was investigated at various operating conditions. In order to convert H₂S contained in the gasified coal gas to elemental sulfur during Claus reaction, the catalysts having the high activity under the highly reducing condition with the moisture should be developed. CeO₂, ZrO₂, and Ce_1−xZr_xO₂ catalysts were prepared for Claus reaction and their reactivity changes due to the existence of the reducing gases and H₂O in the fuel gas was investigated in this study. The Ce-based catalysts shows that their activity was deteriorated by the reduction of the catalyst due to the reducing gases at higher than 220 °C. Meanwhile, the effect of the reducing gases on the catalytic activity was not considerable at low temperature. The activities of all three catalysts were degraded on the condition that the moisture existed in the test gas. Specifically, the Ce-based catalysts were remarkably deactivated by their sulfation. The Ce-Zr-based catalyst had a high catalytic activity when the reducing gases and the moisture co-existed in the simulated fuel gas. The deactivation of the Ce-Zr-based catalyst was not observed in this study. The lattice oxygen of the Ce-based catalyst was used for the oxidation of H₂S and the lattice oxygen vacancy on the catalyst was contributed to the reduction of SO₂. ZrO₂ added to the Ce-Zr-based catalyst improved the redox properties of the catalyst in Claus reaction by increasing the mobility of the lattice oxygen of CeO₂.     

Improvement of the method of obtaining gas sulfur

An improved method of obtaining gas sulfur using the Claus and Sulfren processes, which provides an increase in its yield to 99.6–99.8%, is suggested. To realize this method, new catalysts are developed, namely, the alumina catalyst for the Claus process, the catalyst for the reduction of SO₂, the catalyst for the Sulfren process, and the low-temperature catalyst for the direct oxidation of H₂S, which provided for the utilization of previously not used components of the gas medium H₂ and CO forming at the thermal stage. This method is recommended for introduction at the enterprises of OAO GAZPROM, Orenburg and Astrakhan, gas processing plants. No substantial changes in the hardware implementation of technological lines will be necessary; it will be sufficient to reconstruct the reactors of the Sulfren process.     

Electrochemical polishing of hydrogen sulfide from coal synthesis gas

An advanced process has been developed for the separation of H₂S from coal gasification product streams through an electrochemical membrane. This technology is developed for use in coal gasification facilities providing fuel for cogeneration coal fired electrical power facilities and molten carbonate fuel cell (MCFC) electrical power facilities. H₂S is removed from the syn-gas by reduction to the sulfide ion and hydrogen gas at the cathode. The sulfide ion migrates to the anode through a molten salt electrolyte suspended in an inert ceramic matrix. Once at the anode it is oxidized to elemental sulfur and swept away for condensation in an inert gas stream. The syn-gas is enriched with the hydrogen. Order of magnitude reductions in H₂S have been repeatedly recorded (100 ppm to 10 ppm H₂S) on a single pass through the cell. This process allows removal of H₂S without cooling the gas stream and with negligible pressure loss through the separator. Since there are no absorbents used, there is no absorption/regeneration step as with conventional technology. Elemental sulfur is produced as a byproduct directly, so there is no need for a Claus process for sulfur recovery. This makes the process economically attractive since it is much less equipment intensive than conventional technology.     

Iron—molybdenum—oxide catalysts for selective oxidation of hydrogen sulfide to sulfur

The selective oxidation of hydrogen sulfide to sulfur was studied over iron-molybdenum oxides with various Fe-Mo ratios. Strong synergistic phenomenon in catalytic activity was observed for the Fe-Mo-O binary oxides. Under identical reaction conditions, the areal rates of the binary oxides were superior to those of the corresponding single oxide catalysts, which suggest that the new compound Fe₂(MO₄)₃ formed in the binary oxide is more active than Fe₂O₃ and MoO₃. The oxidation rates of H₂S were found to exhibit first-order dependence on the hydrogen sulfide concentration, which implies that the activation of H₂S is the rate-limiting step.     

Low-temperature catalytic decomposition of hydrogen sulfide on metal catalysts under layer of solvent

When hydrogen sulfide decomposition {2 H₂S ? 2 H₂?+?S₂^(gas)} is carried out in the flow regime at room temperature on metal catalysts placed in a liquid capable of dissolving H₂S and sulfur, the reaction equilibrium can be significantly (up to 100%) shifted to the right yielding the desired product – hydrogen. The process efficiency was demonstrated using aqueous solutions of monoethanolamine (MEA), sodium carbonate, which is widely used in industry for H₂S absorption from tail gases, and aqueous hydrazine as examples. IR and Raman spectroscopy data demonstrated that sulfur obtained in the solutions is in the form of diatomic molecules. DFT calculations showed that diatomic sulfur forms weakly bound coordinative complexes with solvent molecules. Some problems related to sulfur accumulation and recovery from the solvents are discussed.     

Selective oxidation of H<Subscript>2</Subscript>S to sulfur over CeO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> catalyst

The catalytic oxidation of hydrogen sulfide (H₂S) to elemental sulfur was studied over CeO₂-TiO₂ catalysts. The synthesized catalysts were characterized by various techniques such as X-ray diffraction, BET, X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of ammonia, and scanning electron microscopy (SEM). Catalytic performance studies of the CeO₂-TiO₂ catalysts showed that H₂S was successfully converted to elemental sulfur without considerable emission of sulfur dioxide. CeO₂-TiO₂ catalysts with Ce/Ti=1/5 and 1/3 exhibited the highest H₂S conversion, possibly due to the uniform dispersion of metal oxides, high surface area, and high amount of acid sites.     

Poisoning and regeneration of claus alumina catalysts

Samples of poisoned alumina catalysts obtained from Claus sulfur recovery plants in Alberta have been regenerated. Poisoned catalysts had SO₄⁼, NO₃^?, sulfur and carbon deposits, discoloration, and low surface areas. Whereas the Claus catalytic activity of the poisoned catalysts varied widely, regeneration uniformly enhanced the catalytic activity to the same level as fresh alumina. The activity comparisons were made under conditions similar to those of a third Claus converter of low partial pressures of H₂S and SO₂ and high partial pressure of H₂O, where maximum catalytic activity is required to promote Claus reaction.     

SULFUR RECOVERY WITH REDUCED EMISSIONS, LOW CAPITAL INVESTMENT AND HYDROGEN CO-PRODUCTION

The main disadvantage of the Claus process is that by introducing air as oxidant a large volume of tail gas is produced. This must be treated to reduce atmospheric emissions of sulfur-containing gases. The costs of the tail-gas unit are a significant fraction of the total capital and operating costs for sulfur recovery. A new process uses thermal decomposition of hydrogen sulfide in the presence of carbon dioxide instead of air oxidation. The products of this reaction are hydrogen, carbon monoxide, elemental sulfur, water vapor and carbonyl sulfide. Carbonyl sulfide is easily converted to H₂S and C0₂ by liquid- or vapor-phase hydrolysis. Unreacted H₂S and C0₂ are recovered by absorption and recycled to the reactor. Since no air is introduced, there is no tail gas and the tail-gas unit is eliminated, giving a substantial reduction in capital investment. The concentrations of sulfur-containing gases in the product streams depend only on the operation of the absorber and stripper units and can be controlled to very low levels by increasing stripper boil-up. Process operating costs depend on the level of sulfur recovery required and can also be much lower than those of the modified Claus Process.

The process chemistry depends on a shift in the equilibrium of H₂S decomposition caused by reaction of hydrogen with C0₂ by the reverse of the water-gas-shift reaction. Catalysts for this chemistry have been identified. Reactor conversion is further improved by rapid cooling of the reactor effluent gas. Other aspects of process design and operation confer further advantages with respect to the Claus process; however, the process equipment used is similar to that used in a Claus plant. Retrofit of existing plant to the new technology can therefore be considered.     

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.     

Activity monitoring of Claus catalysts in sulfur recovery units

Methodical principles of catalyst activity monitoring in Claus reactors based on the determination of the rate constant of the reaction of hydrogen sulfide conversion at catalyst temperatures lower than 280°C are discussed. The procedure is justified by data from laboratory experiments (in the range of concentrations [H₂S]₀ = 1.5–7 vol %), pilot tests ([H₂S]₀ = 0.8–37.4 vol %) of an alumina-based catalyst AO-NKZ-2 produced by ZAO Novomichurinsk Catalyst Plant, and by the results of its test in the Claus reactor of the department for coke oven gas purification of by-product-coke plant at the OAO Magnitogorsk Integrated Iron-and-Steel Works. The procedure is recommended for reliable monitoring of the current activity and estimation of the residual life of catalysts in the Claus industrial reactors operating under conditions of substantial variations in the composition of the process gas, as well as for comparative estimates of catalyst activity in the Claus process.     

Modifying effects of hydrogen sulfide on the rheometric properties of liquid elemental sulfur

Handling molten sulfur is inherently difficult due to liquid sulfur's extreme rheological behavior. Upon melting at 115°C, sulfur's viscosity remains low until reaching 160°C, the λ-transition region, where the viscosity increases to a maximum of 93,000 × 10⁻³ Pa s at 187°C. Within this study, our previous viscosity measurements for pure liquid elemental sulfur have been discussed along with new measurements on sulfur containing physically and chemically dissolved hydrogen sulfide (H₂S). H₂S is always incorporated into industrial sulfur which has been recovered through the modified Claus process in gas plants and oil refineries. Using the experimental data from this study, a semi-empirical correlation model was reported based on the reptation model of Cates to estimate the impact of H₂S on liquid sulfur's viscosity as a function of temperature. The equation can be applied to commercial sources of sulfur with 0–500 ppm of total dissolved H₂S.     

The fate of methane in a claus plant reaction furnace

Experimental kinetic data are reported for key side reactions occurring in the front end [i. e. the reaction furnace (RF) and the waste heat boiler (WHB)] of modified Claus plants used for sulfur recovery from the sour gases evolved in the treatment of natural gas. An extensive experimental study was conducted in a high temperature tubular reactor system for two important homogenous gas‐phase reactions. Firstly, experiments were carried out to study the oxidation of hydrogen sulfide and methane mixtures in the presence of oxygen. Secondly, the reaction between methane and sulfur dioxide was investigated experimentally. These results showed that methane was much less competitive for oxygen than hydrogen sulfide. Hence, in a partially oxidizing environment of a RF, data showed that methane reacted significantly with other major sulfur containing species, as secondary reactions, to form COS and especially CS². This is highly problematic from an environmental point of view.     

Catalytic performance of vanadia-doped alumina-pillared clay for selective oxidation of H2S

The catalytic oxidation of hydrogen sulfide over V₂O₅ supported on Al-pillared clay (V/Al-PILCs) was studied. The synthesized catalysts were examined using a variety of characterization techniques such as XRD, BET, XPS, ⁵¹V NMR, H₂-TPR and NH₃-TPD. A catalytic activity study performed by using V/Al-PILC catalysts showed that H₂S was successfully converted to elemental sulfur without considerable emission of sulfur dioxide. The H₂S conversion over V/Al-PILCs increased with the vanadia content up to 6 wt.%. This superior catalytic performance might be related to the uniform dispersion of vanadia species on the Al-PILC support.     

The reaction thermodynamics of hydrogen sulfide decomposition into hydrogen and diatomic sulfur

To verify a hypothesis about the electronic state of diatomic gaseous sulfur formed during the low-temperature catalytic decomposition of hydrogen sulfide, we carried out some experiments to examine elemental sulfur dissociation. As shown, after heating at ～1000?K, elemental sulfur sealed in quartz ampoules with metal catalysts followed by quenching at room temperature did not produce any visible changes on solid sulfur. However, conversion of solid sulfur into gaseous diatomic sulfur can be realized via intermediate interaction of melted sulfur with hydrogen in the presence of Pt followed by decomposition of H₂S formed on the surface of the metal catalyst at room temperature. It is suggested that the conversion of the singlet sulfur atoms into the ground triplet state becomes feasible only on the surface of metal catalysts resulted from the dissociation of hydrogen sulfide into adsorbed atomic species.     

A study on the reactivity of Ce-based Claus catalysts and the mechanism of its catalysis for removal of H2S contained in coal gas

In this study, Claus reaction was applied for the selective removal of H₂S contained in the gasified coal gas, and the characteristics of Claus reaction over the Ce-based catalysts were investigated to propose the reaction mechanism. The Ce-based catalysts showed a high activity on Claus reaction. Specially, Ce_0.8Zr_0.2O₂ catalyst had a higher activity than CeO. On the basis of our experimental results, it was proposed that the selective oxidation of H₂S was carried out by the lattice oxygen in the Ce-based catalysts and that the reduction of SO₂ was performed by the lattice oxygen vacancy in the reduced catalyst. Since the mobility of the lattice oxygen in Ce_0.8Zr_0.2O₂ composite catalyst was better than the one in CeO₂, Ce_0.8Zr_0.2O₂ provided more lattice oxygen for the selective oxidation of H₂S. It was presumed that the reaction mechanism to convert H₂S and SO₂ into elemental sulphur over our prepared catalysts was different from the mechanism over the solid-acid catalysts. It is believed that Claus reaction over the Ce-based catalysts was carried out by the redox mechanism. Since the moisture was contained in the major components, CO and H, of the gasified fuel gas, the effects of CO and H₂O on the catalytic reaction were investigated over a Ce-based catalyst. The conversion of H₂S and SO₂ was decreased in Claus reaction over the Ce-based catalysts as the concentration of either H₂O or CO in the gasified coal gas was increased. Under the circumstances of the coexistence of both moisture and CO, however, the conversion was increased as the concentration of CO was increased. The reactivity of Claus reaction was varied in terms of the concentration ratio of CO to H₂O. The maximum conversion of H₂S and SO₂ was achieved in the condition of that the concentration of CO contained in the reacting gas was higher than the one of H₂O. The conversions of H₂S and SO₂ did not match to the stoichiometric ratios of Claus reaction. The higher conversion of H₂S was obtained in the higher concentration of H₂O, while the higher conversion of SO₂ was achieved in the higher concentration of CO. It was another evidence to indicate that the Claus reaction over the Ce-based catalysts was carried out by the redox mechanism.     

Efficiency of a Claus furnace in the coke-oven gas desulphurization circuit of MMK

The poor efficiency of the Claus furnace in sulfur removal at the coke plant of OAO Magnitogorskii Metallurgicheskii Kombinat (MMK) is analyzed on the basis of monitoring data for the sulfur recovery unit and thermodynamic calculations. The low level of sulfur removal from coke-oven gas (~60%) at OAO MMK is mainly due to thermodynamic limitations on the thermal stage of hydrogen-sulfide oxidation with the specific composition of the sour gas at coke plants: low hydrogen-sulfide concentration and combustion of the additional fuel (natural gas). With the kinetic constraints in the Claus furnace, the high CO₂ content in the sour gas (up to 37 vol %) results in increased yield of carbonyl sulfide (~14%). That calls for particular care in optimizing the temperature during the catalytic stages and selecting the best catalysts.     

An investigation on the most important influencing parameters regarding the selection of the proper catalysts for Claus SRU converters

In the processing of sour natural gas, the conversion of hydrogen sulfide into elemental sulfur is often carried out using the modified Claus sulfur recovery process.In this work, the most important influencing parameters regarding the selection of the proper catalysts for Claus SRU (sulfur recovery unit) converters are technically evaluated. It is shown that the pore size and pore size distribution, size, shape, bulk density, mechanical strength and attrition resistance of the catalyst particles are the main parameters which should be considered in selection of Claus catalysts. The general classification of Claus catalysts and the most important specifications required for proper selection of converter catalysts are also presented.     

Analysis of sulfur poisoning on a PEM fuel cell electrode

The extent of irreversible deactivation of Pt towards hydrogen oxidation reaction (HOR) due to sulfur adsorption and subsequent electrochemical oxidation is quantified in a functional polymer electrolyte membrane (PEM) fuel cell. At 70 °C, sequential hydrogen sulfide (H₂S) exposure and electrochemical oxidation experiments indicate that as much as 6% of total Pt sites are deactivated per monolayer sulfur adsorption at open-circuit potential of a PEM fuel cell followed by its removal. The extent of such deactivation is much higher when the electrode is exposed to H₂S while the fuel cell is operating at a finite load, and is dependent on the local overpotential as well as the duration of exposure. Regardless of this deactivation, the H₂/O₂ polarization curves obtained on post-recovery electrodes do not show performance losses suggesting that such performance curves alone cannot be used to assess the extent of recovery due to sulfur poisoning. A concise mechanism for the adsorption and electro-oxidation of H₂S on Pt anode is presented. H₂S dissociatively adsorbs onto Pt as two different sulfur species and at intermediate oxidation potentials, undergoes electro-oxidation to sulfur and then to sulfur dioxide. This mechanism is validated by charge balances between hydrogen desorption and sulfur electro-oxidation on Pt. The ignition potential for sulfur oxidation decreases with increase in temperature, which coupled with faster electro-oxidation kinetics result in the easier removal of adsorbed sulfur at higher temperatures. Furthermore, the adsorption potential is found to influence sulfur coverage of an electrode exposed to H₂S. As an implication, the local potential of a PEM fuel cell anode exposed to H₂S contaminated fuel should be kept below the equilibrium potential for sulfur oxidation to prevent irreversible loss of Pt sites.