Thermal Decomposition Of Carbonyl Sulfide At Temperatures Encountered In The Front End Of Modified Claus Plants

Understanding the kinetics of the formation and consumption of COS and CS₂ in the front end of the modified Claus process will be a significant step towards reducing the environmental impact of these plants. Specifically, homogeneous intrinsic rate expressions are needed for engineering design and simulation, which will lead to new, optimized ways of operating these plants. Hence, a high-temperature kinetic study of the COS decomposition reaction was carried out. Experiments were performed with inlet COS compositions in the range of 0.20-2.33 mol%, with pressures at 101-150 kPa and temperatures at 800-1100°C; these conditions cover the conditions typically encountered in the front end of the modified Claus process. The experimental results showed that COS conversion is dependent on the inlet concentration of COS, which contrasts with previously reported higher temperature studies. Finally, the COS decomposition kinetics were modeled as the sum of two reactions, which provided a satisfactory representation of experimental data.     

Use of new reaction kinetics for COS formation to achieve reduced sulfur emissions from claus plants

A simulation study of COS formation in the tubes of the waste heat boiler (WHB) located just after the reaction furnace of a Claus plant is reported. First, the kinetics of the COS forming reaction were obtained from a recently completed experimental program in our laboratory. The predictions for COS formation from the newly developed kinetic model were compared with the data from an actual industrial waste heat boiler and found to be in good agreement. The simulation results showed that up to a 50% reduction in the COS production may be achieved by operating the WHB at the maximum allowable gas mass velocity in the WHB tubes coupled with the use of a smaller diameter tube. These reductions have major implications on the overall sulfur recovery from Claus plants.     

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.     

Drying Kinetics of Oil Palm Frond Particles in an Agitated Fluidized Bed Dryer

The drying kinetics of oil palm frond particles in a laboratory-scale agitated fluidized bed dryer were investigated under various operating conditions: inlet air temperature (50–80°C), superficial air velocity (0.6–1.0 m/s), bed load (200–300 g), and agitation speed (300–500 rpm). To study the effects of these variables on the drying time and drying rate, an experimental design using Taguchi orthogonal array was employed. Based on analysis of variance (ANOVA), the results indicated that inlet air temperature greatly affected the drying rate, followed by superficial air velocity and bed load. The effect of agitation speed on the drying rate was found to be small. The experimental drying kinetics data were compared with the values obtained from three different models, namely, the Page model, modified quasi-stationary method (MQSM), and a new composite model. It was found that the proposed new model could satisfactorily predict the complete drying rate curve for the drying of oil palm fronds.     

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%.     

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.     

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.     

Appraisal of catalysts for the hydrolysis of carbon disulfide

Industrial alumina- and titania-based catalysts were tested for CS₂ hydrolysis in a Berty reactor. Activated bauxite is the most active catalyst below 290°C and titanium oxide above 290°C. TGA experiments examining water desorption from 50°C to 550°C showed complete desorption from titania by 300°C and continuous desorption even beyond 550°C for alumina and bauxite. Using the Kelvin equation to predict capillary condensation over the range of pore sizes of these catalysts, condensation of water vapour should be negligible but for sulfur vapour considerable blocking of pores is predicted for alumina. Much of the internal area of aluminas may thus become inaccessible to vapor molecules. These observations suggest why alumina-based catalysts exhibit lower activity for CS₂ and COS hydrolysis in modified Claus plant reactors.     

Modelling the modified claus process reaction furnace and the implications on plant design and recovery

The calculation of product composition, flow rate and temperature of the modified Claus process reaction furnace is typically done by assuming either thermodynamic equilibrium or by empirical methods fitted to plant data. This paper extensively reviews the literature on the Claus reaction furnace and compares equilibrium and empirical results of the predicted concentrations of the key components: hydrogen (H₂), carbon monoxide (CO), carbonyl sulphide (COS) and carbon disulphide (CS₂). The implication of the reaction furnace model on the overall plant design and sulphur recovery is subsequently presented. It is well known that results of equilibrium calculations do not match plant data taken both before and after the waste heat boiler (WHB). Moreover, even though results of empirical methods do not match plant data taken before the WHB, one empirical method provides the best fit of highly scattered data taken after the WHB and provides a conservative plant design and estimates of sulphur recovery and emissions.     

Influence of the glass–calcium carbonate mixture's characteristics on the foaming process and the properties of the foam glass

We prepared foam glasses from cathode-ray-tube panel glass and CaCO₃ as a foaming agent. We investigated the influences of powder preparation, CaCO₃ concentration and foaming temperature and time on the density, porosity and homogeneity of the foam glasses. The results show that the decomposition kinetics of CaCO₃ has a strong influence on the foaming process. The decomposition temperature can be modified by varying the milling time of the glass–CaCO₃ mixture and thus for a specific CaCO₃ concentration an optimum milling time exists, at which a minimum in density and a homogeneous closed porosity are obtained. Under the optimum preparation conditions the samples exhibit a density of 260 kg/m³. The thermal conductivity of the foam glass was measured to be 50–53 mW/(m K). The observed dependence of the foaming process on the decomposition kinetics of the foaming agent can be applied as a universal rule for foaming processes based on thermal decomposition.     

Influence of calcination temperature on the hydrolysis of carbonyl sulfide over hydrotalcite-derived Zn–Ni–Al catalyst

A novel catalyst for low temperature hydrolysis of carbonyl sulfide (COS) was prepared by thermal decomposition of Zn–Ni–Al hydrotalcite-like compounds (HTLCs). As the key factors of catalyst activity, effects of calcination temperature have been studied. The samples were carefully characterized by XRD, FTIR, SEM, CO₂-TPD and N₂ adsorption/desorption. Results showed that HTLCs calcined at 350 °C exhibited excellent activity due to the production of more M–O pairs which are active sites of the hydrolysis of COS. However, calcination at 500 °C led to the destruction of pore structure and reduction of active sites, ultimately led to a lower COS conversion.     

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.     

Synthesis of LaFeO3 perovskite-type oxide via solid-state combustion of a cyano complex precursor: The effect of oxygen diffusion

The effect of oxygen diffusion on the thermal decomposition kinetics of La[Fe(CN)₆]·5H₂O has been explored. In particular, the critical conditions under which LaFeO₃ can be synthesized via solid-state combustion of this cyano complex precursor were analytically and numerically investigated. Thermal analysis experiments as well as simulations showed that the oxygen diffusion enhancement facilitates the formation of a self-propagating combustion front during the decomposition of La[Fe(CN)₆]·5H₂O. As a consequence, the sample undergoes local overheating that raises its temperature by several hundreds of degrees. This enables the production of LaFeO₃ perovskite-type oxide with a minimum contribution of external heat resources. Although the self-propagating high-temperature method has already proven to be successful for the synthesis of perovskite-type oxides from cyano complex precursors under oxygen atmosphere, we will show that under the appropriate settings, air can be used instead. Moreover, the temperature of the front is related to the ease of oxygen diffusion. Therefore, the surface area and the crystal size of LaFeO₃ perovskite-type powders obtained via solid state combustion have been controlled by controlling the gas flow rate at which the sample has been exposed during the treatment.     

Removal of carbonyl sulfide at low temperature: Experiment and modeling

A mathematic model of carbonyl sulfide (COS) removal at low temperature with fouling of catalyst has been developed based on experimental results. Kinetic studies were conducted in a fixed bed reactor under atmospheric pressure and at low temperature (40-70 °C). Experimental results of breakthrough curves were used to obtain kinetic parameters accounting for axial dispersion, external and internal mass-transfer resistances as well as the sulphur deposition on inner-face of catalyst. Initial bulk porosity of particle (?_P0), deactivation coefficient (α), sulfide deposition coefficient (β) were used to quantify the behavior of COS removal at different operating conditions. Adsorption heat of H₂O and activation energy of COS removal was 21.5 and 62.4 kJ/mol respectively. The effects of flow rate, COS inlet concentration, temperature and relative humidity(RH) were analyzed, and it was found that relative humidity carried a heavier weight than temperature on ε_P0, α, β within our experimental conditions. The model agreed well with the experimental breakthrough curves and satisfactorily predicted the fixed-bed reactor performance, and this model can be used as a reliable tool for process design and scaling-up of similar system.     

Molecular beam mass spectrometry and modelling of CH4–CO2 plasmas in relation with polycrystalline and nanocrystalline diamond deposition

CH₄–CO₂ microwave plasmas have been studied by optical emission spectroscopy, microwave interferometry, Langmuir probing and molecular beam mass spectrometry. The variations of plasma parameters and the concentration variations of both stable species and radicals in the plasma had been reported previously as a function of the power density; the influence of the total inlet flow rate is reported here. While the power density influences directly the plasma kinetics, the flow rate changes the residence time in the plasma and then the degree of conversion of the chemical system that is the extent to which the gas composition moves toward its steady-state composition. This is studied by modelling of plasma kinetics taking into account the coupled fluid dynamics of the gaseous species and the gas-phase chemistry including electron dissociation and surface recombination at the reactor wall. The experimental and modelling studies are used for correlating: – the relative concentration of important hydrocarbon radicals and etching radicals in the plasma and the gradients of all these species in front of the surface; – to the deposition domain, the structure (polycrystalline or nanocrystalline) and the quality of diamond films, which is the ratio of sp³ to (sp³ + sp²)-hybridized carbon in the film. All results evidence the plasma kinetic effect on the diamond deposition domain and the diamond deposition quality and structure, due to different degrees of conversion of the chemical system. The deposition of diamond coating from CH₄–CO₂ is shown to be a versatile process that permits deposition of a great variety of diamond films. However it requires particular attention because of the variation of the deposition conditions and then diamond quality and structure of the deposits depending on the extent of conversion of the inlet species to various intermediate and finally stable species formed in the plasma chemical system.     

Catalytic Ammonia Decomposition Over Fe/Fe4N

The catalytic ammonia decomposition over iron and iron nitride, Fe₄N, under the atmosphere of ammonia–hydrogen mixtures of different amounts of ammonia in the temperature range of 400–550 °C by means of thermogravimetry has been studied. A differential tubular reactor with mixing has been used. The ammonia concentration in the gas phase during all the process was analysed. The balance between the inlet and outlet ammonia quantity has been used to determine a degree of ammonia conversion and the values of decomposition reaction rate. The activation energy of ammonia decomposition reaction over Fe and Fe₄N was found to be 68 and 143 kJ/mol, respectively.     

The kinetics modeling and reactor simulation of propylene chlorination reaction process

The kinetics experiments of fast reaction process of propylene chlorination at low temperature (30–90°C) and high temperature (420–480°C) are respectively conducted, and the corresponding reaction mechanisms and kinetics models are proposed. The radical mechanism at high temperature and the molecular mechanism at low temperature are found to be most likely with the experimental results. Specifically, the kinetics model, firstly considering the reversible reaction step of forming C₃H₆Cl · and direct hydrogen abstraction of forming C₃H₅ · , shows better agreement with the experimental data. Furthermore, the critical reaction temperature T_critical is firstly proposed to determine the dominant reaction mechanism in different conditions, and correspondingly the combination method of the high-temperature and low-temperature kinetics models has been adopted for tubular reactor simulation, which can reasonably reflect the influence of wide variation range of temperature in the reactor and guide the industrial reactor design in the further work.     

Formic acid decomposition on Au catalysts: DFT,microkinetic modeling,and reaction kinetics experiments

A combined theoretical and experimental approach is presented that uses a comprehensive mean‐field microkinetic model, reaction kinetics experiments, and scanning transmission electron microscopy imaging to unravel the reaction mechanism and provide insights into the nature of active sites for formic acid (HCOOH) decomposition on Au/SiC catalysts. All input parameters for the microkinetic model are derived from periodic, self‐consistent, generalized gradient approximation (GGA‐PW91) density functional theory calculations on the Au(111), Au(100), and Au(211) surfaces and are subsequently adjusted to describe the experimental HCOOH decomposition rate and selectivity data. It is shown that the HCOOH decomposition follows the formate (HCOO) mediated path, with 100% selectivity toward the dehydrogenation products (CO₂ + H₂) under all reaction conditions. An analysis of the kinetic parameters suggests that an Au surface in which the coordination number of surface Au atoms is ≤4 may provide a better model for the active site of HCOOH decomposition on these specific supported Au catalysts. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1303–1319, 2014     

The effects of operating conditions on COS-induced degradation of aqueous diethanolamine solutions

The COS-induced degradation of diethanolamine (DEA) was examined experimentally under the following conditions: temperature, 120–190°C; DEA concentration, 20–60 wt%; and COS partial pressure, 0.34–1.17 MPa. One insoluble and seven major soluble degradation compounds were detected and their variation in concentration with time is presented. It was found that H₂S, which is formed by COS hydrolysis, can contribute to DEA degradation in the presence of CO₂. The overall DEA degradation is well represented by a first-order mechanism. The study shows that, to minimize degradation and plant operating problems, the solution temperatures should be kept low (preferably under 120°C), the solutions should be filtered and COS hydrolysis should be maximized.     

Claus Catalysis and H2S Selective Oxidation

This review article deals with the development of sulfur recovery from the Claus process to H₂S selective oxidation. Governments are constantly tightening regulations to limit the emission of sulfur compounds into the air. This makes it necessary to constantly enhance the level of sulfur recovery from natural, refinery, or coal gasification geses, and many improvements in the Claus process have been introduced to this end. In this review, emphasis has been put on the mechanism of reactions occurring in most of the sulfur recovery units, reactions between H₂S and SO₂ or O₂ and side reactions such as hydrolysis of COS and CS₂ or sulfation of the catalyst.