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.     

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.     

Catalytic processes and catalysts for production of elemental sulfur from sulfur-containing gases

The modern technologies for production of elemental sulfur are considered. It is demonstrated that along with the further wide application of the conventional Claus process with conventional alumina catalyst in the observable future some new trends which may significantly influence the technological picture of recovered sulfur manufacturing may be formulated: active development of Claus tail gas cleanup processes with the stress on replacement of subdewpoint Sulfreen-type processes by processes of hydrogen sulfide selective oxidation by oxygen; development of novel highly-efficient technologies for hydrogen sulfide decomposition to sulfur and hydrogen; application of new catalysts forms, first of all — at microfiber supports for Claus and H₂S oxidation processes; wider application of titania and vanadia catalysts at the newly constructed Claus units; development of technologies and catalysts for direct purification of H₂S-containing gases and for catalytic reduction of SO₂ for sulfur recovery from smelter gases. All these prospective routes are actively developed by Russian science and some of them are completely based on domestic developments in this area.     

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.     

Elemental sulfur production through regeneration of a SO2-adsorbed V2O5-CoO/AC in H2

H₂ regeneration of an activated carbon supported vanadium and cobalt oxides (V₂O₅-CoO/AC) catalyst–sorbent used for flue gas SO₂ removal is studied in this paper. Elemental sulfur is produced during the H₂-regeneration when effluent gas of the regeneration is recycled back to the reactor. The regeneration conditions affect the regeneration efficiency and the elemental sulfur yield. The regeneration efficiency is the highest at 330 °C, with SO₂ as the product. The production of elemental sulfur occurs at 350 °C and higher with the highest elemental sulfur yield of 9.8 mg-S/g-Cat. at 380 °C. A lower effluent gas recycle rate is beneficial to elemental sulfur production. Intermittent H₂ feeding strategy can be used to control H₂S concentration in the gas phase and increase the elemental sulfur yield. Two types of reactions occur in the regeneration, reduction of sulfuric acid to SO₂ by AC and reduction of SO₂ to elemental sulfur through Claus reaction. H₂S is an intermediate, which is important for elemental sulfur formation and for conversion of CoO to CoS that catalyzes the Claus reaction. The catalyst–sorbent exhibits good stability in SO₂ removal capacity and in elemental sulfur yield.     

Fluidized bed Claus reactor studies

Fluidized bed studies have been performed on the Claus reaction to determine whether the conversion efficiency of the Claus process could be improved by replacing conventional fixed bed reactors with fluidized bed reactors. Various idealized Claus plants, incorporating fluidized bed technology, were simulated using the equilibrium constant method. The results of the simulation indicated that, for feed gases consisting of pure H₂S, sulphur conversions in excess of 99% are attainable by using a Claus furnace and two fluidized bed reactors in series. To substantiate the theoretical predictions, experimental studies were performed using a single fluidized bed reactor (0.1 m I.D.) containing Kaiser alumina S-501 catalyst. The effects of temperature (150–300°C), flow rates (15–30 l min⁻¹), feed composition (0.06 < H₂S < 18%, 0.03 < SO₂ < 9%, 73 < N₂ < 99.91%) and bed height (0.12, 0.25 m) on the sulphur conversion were examined. The experimental results showed the same general trends as the theoretical predictions but the measured sulphur conversions exceed the theoretical values by up to 8%.     

Catalytic activity of faujasite-type zeolites towards the claus reaction

Commercial NaY and NaX zeolites showed significant catalytic activity towards the Claus reaction. Bronsted acidity retarded the catalytic activity to a much greater extent than Lewis acidity. Electron Spin Resonance (ESR) spectroscopic studies indicated the formation of SO₂^? anion radicals on the catalyst surface upon SO₂ adsorption. The reactivity of SO₂^? towards H₂S depended upon many factors. When the electron donating property of NaX was increased by impregnation with a small amount of NaOH, the Claus activity was enhanced and the activity of NaX containing 2.0 wt% NaOH was similar to that of a commercial Claus alumina catalyst. Infrared (IR) spectroscopic studies indicated physical and dissociative adsorption of H₂S; SO₂ was found to chemisorb on the zeolites.     

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.     

Oxidation of low concentrations of hydrogen sulphide: Process optimization and kinetic studies

Low concentrations (e.g. < 3) of H₂ S in natural gas can be selectively oxidized over an “granular Hydrodarco” activated carbon catalyst to elemental sulphur, water and a small fraction of by-product sulphur dioxide, SO₂. To optimize the H₂ S catalytic oxidation process, the process was conducted in the temperature range 125—200 °C, at pressures 230—3200 kPa, with the O/H₂ S ratio being varied from 1.05 to 1.20 and using different types of sour and acid gases as feed. The optimum temperature was determined to be approximately 175 °C for high H₂ S conversion and low SO₂ production with an O/H₂ S ratio 1.05 times the stoichiometric ratio. The life of the activated carbon catalyst has been extended by removing heavy hydrocarbons from the feed gas. The process has been performed at elevated pressures to increase H₂ S conversion, to maintain it for a longer period and to minimize SO₂ production. The process is not impeded by water vapour up to 10 mol% in the feed gas containing low concentrations of CO₂ (< 1.0). A decrease in H₂ S conversion and an increase in SO₂ production were obtained with an increase in water vapour in the feed gas containing a high percentage of CO₂. The process works well with “sour natural gas” containing approximately 1% H₂ S and with “acid gas” containing both H₂ S and CO₂. It gives somewhat higher H₂ S conversion and low SO₂ production with feed gas containing low concentrations of CO₂. A kinetics study to determine the rate-controlling step for the H₂ S catalytic oxidation reaction over “granular Hydrodarco” activated carbon has been conducted. It was concluded that either adsorption of O₂ or H₂ S from the bulk phase onto the catalyst surface is the rate-controlling step of the H₂ S catalytic oxidation reaction.     

Effect of the Porous Structure of a Catalyst on the Rate of Its Deactivation in the Claus Reaction

The effect produced by the content of different size pores and the diameter of a catalyst grain on the deactivation rate has been estimated based on the experimental data on the conversion of H₂S in the Claus reaction. Catalysts with different pore size distributions have been used for analysis. Deactivation has been shown to occur in different fashions for a fine catalyst fraction and catalyst grains. A mathematical model that describes the process of deactivation has been proposed.     

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.     

Catalysts for the desulfurization of coke-oven gas

Two promising catalysts are tested in the desulfurization of coke-oven gas at OAO Magnitogorskii Metallurgicheskii Kombinat in a pilot plant (with a gas flow rate of up to 0.78 m³/h): AOK-78-57 titaniumoxide catalyst (produced by OOO Katalizator, Novosibirsk); and VA-2 vanadium catalyst on an aluminumoxide substrate (developed at the Institute of Catalysis, Siberian Branch, Russian Academy of Sciences and produced by OOO Novomichurinskii Katalizatornyi Zavod). The VA-2 catalyst shows some benefits in the conversion of hydrogen sulfide, with practically the same activity in the conversion of organosulfur components (CS₂ and COS). With a final temperature of 450°C, CS₂ conversion at VA-2 catalyst exceeds 99.5%, on average. Accordingly, VA-2 catalyst is promising for use in the first stages of catalytic conversion in sulfurremoval units of Claus type, including those for the desulfurization of coke-oven gas.     

Catalysis of gasification of coal-derived cokes and chars

Calcium is the most important in-situ catalyst for gasification of US coal chars in O₂, CO₂ and H₂O. It is a poor catalyst for gasification of chars by H₂. Potassium and sodium added to low-rank coals by ion exchange and high-rank coals by impregnation are excellent catalysts for char gasification in O₂, CO₂ and H₂O. Carbon monoxide inhibits catalysis of the CH₂O reaction by calcium, potassium and sodium; H₂ inhibits catalysis by calcium. Thus injection of synthesis gas into the gasifier will inhibit the CH₂O reaction. Iron is not an important catalyst for the gasification of chars in O₂, CO₂ and H₂O, because it is invariably in the oxidized state. Carbon monoxide disproportionates to deposit carbon from a dry synthesis gas mixture (3 vol H₂ + 1 vol CO) over potassium-, sodium- and iron-loaded lignite char and a raw bituminous coal char, high in pyrite, at 1123 K and 0.1 MPa pressure. The carbon is highly reactive, with the injection of 2.7 kPa H₂O to the synthesis gas resulting in net carbon gasification. The effect of traces of sulphur in the gas stream on catalysis of gasification or carbon-forming reactions by calcium, potassium, or sodium is not well understood at present. Traces of sulphur do, however, inhibit catalysis by iron.     

Reaction intermediate over mordenite-type zeolite catalysts for no reduction by hydrocarbons

An infrared spectroscopic study has been made over mordenite-type zeolite catalysts prepared by the ion exchanging method to observe a surface species during the selective reduction of NO by hydrocarbons with and without H₂O. The strong absorptions at 2,274 and 2,325 cm_-1 were observed over HM and CuHM as well as over CuNZA catalysts, respectively after the reaction without H₂O, regardless of the types of reductant employed. It may be attributed to the isocyanate (-NCO) species formed on the catalyst surface which may be one of the most probable reaction intermediates for this reaction system. When H₂O was added to the feed gas stream, its formation on the synthetic mordenite catalysts such as HM and CuHM was significantly suppressed, but not for CuNZA catalyst It agrees well with the fact that CuNZA catalyst exhibits a strong water tolerance for this reaction system. It also reveals that the formation of the -NCO species on the catalyst surface plays a crucial role for the maintenance of NO removal activity when H₂O exists in the feed gas stream.     

A model of acid gas absorption/stripping using methyldiethanolamine with added acid

A nonequilibrium stage model was developed for the absorption and stripping of H₂S and CO₂ using aqueous methyldiethanolamine (MDEA). Heat and mass transfer are calculated for each stage assuming the liquid is well mixed and the gas moves in plug flow. The vapour-liquid equilibrium is represented by an empirical expression that was fit to experimental data. The mass transfer enhancement factor for CO₂ is based on the surface renewal theory with approximations made to the reaction term by the method of DeCoursey. Calculation of H₂S absorption assumes an instantaneous reaction rate at the gas/liquid interface and accounts for enhancement by equilibrium chemical reactions. Results were generated at Claus tail gas conditions using available equilibrium and rate data for 50 wt% MDEA. The amount of H₂S in the absorber outlet gas, or H₂S leak, was used to measure system performance. The base case resulted in a H₂S leak of 98 ppm with 20 absorber stages, 25 stripper stages, and a steam rate of 1.7 lb/gal solvent. Adding 0.05 equivalents of acid per mole of MDEA to the aqueous solution reduced the H₂S leak to 6 ppm and the steam rate to 1.2 lb/gal. Reducing the base case stripper pressure of 2.0 atm to 1.0 atm reduced the H₂S leak to 22 ppm. Analysis of McCabe-Thiele plots generated by the model showed that system performance improved after adding acid or reducing the stripper pressure because the H₂S equilibrium in the stripper was linearized.     

Partial oxidation reforming of biomass fuel gas over nickel-based monolithic catalyst with naphthalene as model compound

With naphthalene as biomass tar model compound, partial oxidation reforming (with addition of O₂) and dry reforming of biomass fuel gas were investigated over nickel-based monoliths at the same conditions. The results showed that both processes had excellent performance in upgrading biomass raw fuel gas. Above 99% of naphthalene was converted into synthesis gases (H₂+CO). About 2.8 wt% of coke deposition was detected on the catalyst surface for dry reforming process at 750 °C during 108 h lifetime test. However, no coke deposition was detected for partial oxidation reforming process, which indicated that addition of O₂ can effectively prohibit the coke formation. O₂ can also increase the CH₄ conversion and H₂/CO ratio of the producer gas. The average conversion of CH₄ in dry and partial oxidation reforming process was 92% and 95%, respectively. The average H₂/CO ratio increased from 0.95 to 1.1 with the addition of O₂, which was suitable to be used as synthesis gas for dimethyl ether (DME) synthesis.     

Fe2O3/β-SiC: A new high efficient catalyst for the selective oxidation of H2S into elemental sulfur

Over the last decades, sulfur recovery from the H₂S-containing acid gases (issued from oil refineries or natural gas plants) has become more and more important due to the ever increasing standards of efficiency required by environmental protection pressures. The H₂S-tail gas was directly oxidized by oxygen to yield elemental sulfur. A significant improvement of the H₂S conversion and selectivity has been developed, however, the support which is the core of the process still needs to be improved. Recently, β-SiC has been reported to be an efficient and selective catalyst support for the H₂S-to-S reaction. One expected reason for this superior yield should be due to the high thermal conductivity of the support. The high thermal conductivity of the silicon carbide plays an important role in the maintenance of the high selectivity by avoiding the formation of hot spots on the catalyst surface which could favor secondary reactions. On the other hand, insulator supports such as alumina exhibit a poor selectivity due to catalyst surface temperature runaway.     

An integrated catalytic approach for the production of hydrogen by glycerol reforming coupled with water-gas shift

Reaction kinetics measurements of glycerol conversion on carbon-supported Pt-based bimetallic catalysts at temperatures from 548 to 623 K show that the addition of Ru, Re and Os to platinum significantly increases the catalyst activity for the production of synthesis gas (H₂/CO mixtures) at low temperatures (548–573 K). Based on this finding, we demonstrate a gas phase catalytic process for glycerol reforming, based on the use of two catalyst beds that can be tuned to yield hydrogen (and CO₂) or synthesis gas at 573 K and a pressure of 1 atm. The first bed consists of a carbon-supported bimetallic platinum-based catalyst to achieve conversion of glycerol to a H₂/CO gas mixture, followed by a second bed comprised of a catalyst that is effective for water-gas shift, such as 1.0% Pt/CeO₂/ZrO₂. This integrated catalytic system displayed 100% carbon conversion of concentrated glycerol solutions (30–80 wt.%) into CO₂ and CO, with a hydrogen yield equal to 80% of the amount that would ideally be obtained from the stoichiometric conversion of glycerol to H₂ and CO followed by equilibrated water-gas shift with the water present in the feed.     

Effects of Pretreatments of Cu/ZnO-Based Catalysts on Their Activities for the Water–Gas Shift Reaction

The effects of the pretreatments of Cu/ZnO-based catalysts prepared by a coprecipitation method on their activities for the water–gas shift reaction at 523K were investigated. The activity of a Cu/ZnO/ZrO₂/Al₂O₃ catalyst for the water–gas shift reaction was less affected by calcination at temperatures ranging from 673-973K and by H₂ treatment at 573 or 723K than that of a Cu/ZnO/Al₂O₃ catalyst. The catalyst activity could be correlated mainly to the Cu surface area of the catalyst.     

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.