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.     

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.     

Kinetics of glucose decomposition in formic acid

Cellulose is abundantly available in the form of forestry and agricultural lignocellulosic residues. These residues offer the most potential source for the production of cellulosic glucose, which is a prerequisite for the sustainable production of glucose-based fuels and chemicals. Acid catalysis is one path to lignocellulosic glucose and further to its dehydration end products. Furthermore, many studied lignocellulose pretreatment methods for enzymatic hydrolysis are carried out in acidic conditions, in which the unwanted release of hemicellulose-based glucose and its further reactions to harmful end products are possible. Thus, in order to maximize glucose production from non-food cellulosic raw materials, data on the kinetics of cellulose decomposition and formation rates of end products are required. Glucose decomposition is a complex reaction system that has often been modelled with empirical, simplified models. In this study, a kinetic model was developed for glucose decomposition in formic acid solution. The experimentation was carried out in batch reactors at 180-220 °C in 5-20% (w/w) formic acid. The model developed relies on a mechanistic step through an unknown substance and gives excellent correspondence to the experimental data despite the pseudo-elementary nature of the model structure.     

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.     

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.     

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.     

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.     

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.     

Analysis of the deactivation of Claus alumina catalyst during its industrial exploitation

Change in the activity of AO-NKZ-2 (AO-MK-2) alumina catalyst in the Claus reaction and transformations of carbonyl sulfide during operation over four years in the Claus reactor at the Magnitogorsk Metallurgical Combine’s coke-oven gas purification shop were studied at an average temperature of 245–260°C and a volume velocity of ∼2000 h⁻¹. The rate constants of the Claus reactions and COS transformation were determined, and the changes in the active surface area of the catalyst were investigated. Fundamental discrepancies in the rate and deactivation mechanism of the Claus catalysts were revealed with respect to the reactions of the conversion of hydrogen sulfide and carbonyl sulfide.     

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.     

Sulphur recovery from sour gas by using a modified low-temperature Claus process on sepiolite

In order to introduce an alternative catalyst for the reduction at low temperature, 343–423 K, of the Claus process streams tail gas, the authors studied the performance of sepiolite, a natural, very abundant and cheap hydrated magnesium phylosilicate whose unique textural and structural characteristics confer the solid with the possibility of use as an adsorbent and catalyst. Apart from the effectiveness of the solid in catalysing the sulphur dioxide reduction by hydrogen sulphide, an expression for the reaction rate has been deduced from the analytical data. The kinetic expression tells about a mechanism in which the chemical reaction between adsorbed species is the controlling step of the process. Water present in the Claus stream as a reaction by-product is adsorbed on the substrate and plays an important role in the reaction kinetics. From deactivation experiments under mass control conditions a better performance of sepiolite with respect to γ-alumina is made apparent.     

石灰石和氧化钙脱除H_2S和COS动力学研究

向煤气化炉中喷入石灰石或氧化钙粉是脱除煤气中含硫化合物最简便的方法之一。文中采用热重分析仪在常压(绝压为0.1 MPa)下对石灰石和氧化钙分别脱除H2S和COS的反应进行了动力学研究。实验条件为:温度1 025—1 450 K、H2S和COS的分压范围0.000 3—0.001 5 MPa、颗粒粒径为0.84—1.0 mm。结果表明,石灰石和氧化钙与H2S和COS的反应均表现为一级反应;与CaCO脱除H2S和COS的反应相比,CaO3脱除H2S和COS反应的初速率几乎快10倍。在0.105—1.0 mm粒径范围内的实验表明,CaO脱除H2S和COS的反应与颗粒粒度成反比关系。     

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.     

超声波对磷矿酸解过程的强化

本文主要研究了超声波对硫酸分解磷矿过程的影响.讨论了超声的声强、时间、温度对磷矿酸解的影响.实验结果表明,磷矿分解率随着超声功率、反应时间和水浴温度的增加而提高.超声波对分解率的影响很大,特别是在酸解过程后期,可以提高16%左右.反应速率常数与温度的关系符合阿累尼乌斯方程.利用固体粒度不变的缩芯模型回归实验数据,得到宏观动力学模型.     

Decomposition Kinetics of CaYb2O4

The kinetics of decomposition of CaYb₂O₄ were studied using quantitative analysis by X-ray diffraction methods and a modified method of "standard binary mixtures." This type of method was required as total composition remained constant with only the phase assemblage changing. The experimental factors considered in obtaining meaningful results from such a procedure are covered. The linearity of the fraction decomposed or concentration unreacted versus time curves at various temperatures revealed that the decomposition proceeded according to "zero-order" kinetics. The activation energy for this process was 100 kcal/mole.     

The Kinetics of the Thermal Decomposition of Thermoplastic Polyurethane Elastomers under Thermoplastic Processing Conditions

A theoretical approach to the kinetics of the thermal decomposition of molten thermoplastic polyurethane elastomers, under conditions of thermoplastic processing, is described. On the basis of these considerations, the thermal decomposition in different instruments (melt index analyser and measuring extruder) can be described quantitatively and the various results can be compared. As a result, identical conditions of decomposition of the melt can be defined accurately, thus opening up the possibility of combining experimental values from different instruments. The fundamental kinetic equation obtained for the kinetics of the thermal decomposition of thermoplastic polyurethanes describes the decomposition reaction and the reverse reaction (formation reaction) – which is dependent on the system of measurement and processing – as a function of the molar mass (end‐group concentration) of the original product, determined from the velocity constants for the decomposition reaction and back reaction. The consideration of the limiting value for t → ∞ is in agreement with the equilibrium constant. Consequently, the development of physical characteristic functions of thermoplastic polyurethane elastomers – independent of the system of measurement – is possible.

Experimental values and calculated curves for the thermal decomposition of PUR‐Et in a melt index analyser.     

Influence of phase evolution and thermal decomposition kinetics on the properties of zircon ceramic

In this study, the relationship between the decomposition process and properties of zircon ceramic was analysed. The decomposition process of zircon was investigated by kinetics model, thermodynamics and experimental phenomena. The Coats-Redfern and Kissinger models were used as basic kinetics models, and thermodynamics, thermogravimetry (TG) and differential scanning calorimetry (DSC) were used to characterize the decomposition process and calculate the kinetics parameters. The microstructure, pores characteristics, densification and physical properties were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Archimedes’ principle. The results showed that the decomposition process of zircon could be divided into two stages, with the first and the second stages controlled by the phase boundary reaction and diffusion control reaction, respectively. The decomposition rate of the first decomposition stage was slow and the mechanism function was g(x) = 1-(1-α)^1/3 while the decomposition rate of the second stage was fast and the mechanism function was g(x)=(1–2/3α)-(1-α)^2/3. The large decomposition amount of zircon increased the amount of liquid phase, which would reduce the densification of zircon.     

SURFACE AREA FORMATION IN DRY FGD SORBENTS

Dry flue gas desulfurization (DFGD) technologies are being developed which require sorbents with high specific surface areas capable of large sulfur dioxide uptake. Specifically, novel methods of controlling S0₂ in a fiuidized bed absorber are being studied which will have the capability of isothermal operation at optimum temperature for S0₂ uptake with alkaline sorbents. This paper reports on a study of surface area development with respect to time for three sorbent materials (pure calcium carbonate, dolomitic limestone and pressure hydrated dolomitic limestone) undergoing thermal decomposition in the temperature range of 600-850°C (1100-1550°F). Two of these sorbents (calcium carbonate and dolomitic limestone) confirm observations made by others of a lag time between recrystalization of the product from the reactant and the development of the maximum surface area. A model is presented which predicts the surface area development by accounting for the surface area generated by thermal decomposition and the surface area lost due to sintering. The parameters necessary for the model were obtained from the experimental data. The ability of the model to predict the surface area change with respect to time is shown to be good for the conditions tested.