Claus Furnace Research Articles

A Reaction Kinetics Study on Benzene Oxidation in the Claus Process by Sulfur Monoxide

The Claus process is used in natural gas processing plants to treat H2S-rich acid gas to recover sulfur, but the process suffers from catalytic deactivation when aromatic contaminants such as benzene, toluene, ethylbenzene, and xylene isomers (collectively called as BTEX) are present in the acid gas feed. To safeguard the catalytic reactors, it is desired to oxidize aromatic contaminants in the furnace that are present upstream of the catalytic reactors in the process by oxidants present in it. This work develops a reaction mechanism and evaluates the reaction kinetics for the oxidation of phenyl radical by SO using CBS-QBS for reaction energetics and RRKM and transition state theory for reaction kinetics. The mechanism explores the possible products that are formed from the barrierless addition of SO on phenyl through the O atom as well as through the S atom. The exothermicity of the addition reaction is higher when the addition of SO on the aromatic structure takes place through the S atom. The major products formed from phenyl oxidation by SO are cyclopentadienyl, cyclopentadienethiol and thiopyran radicals. A remarkable similarity between the pathways for phenyl radical oxidation by O2 and its oxidation by SO at high temperatures is observed. The proposed reactions and their rate constants are used to conduct reactor simulations to determine the important reactions that contribute to the formation of major products during phenyl-SO reactions and the temperatures suitable for benzene oxidation by SO under process conditions similar to the Claus furnace.

Математическая модель процесса сжигания сероводородных и аммиачных газов в реакционной печи Клауса с использованием вычислительной гидродинамики

Based on the method of computational fluid dynamics using the ANSYS Fluent software package, the combustion process in the Claus furnace of sour gas containing ammonia was simulated. The vortex dissipation model was used to describe the chemical kinetics of combustion (Eddy-dissipation model). The object of study was the sulfur recovery unit of the Heavy Residue Processing Complex. Options for increasing the efficiency of the process of burning ammonia-containing sour gases, depending on the scheme for supplying acid to the furnace, its moisture content, as well as the temperature of air and sour gases at the furnace inlet were considered.

A split-flow sulfur recovery process for the destruction of aromatic hydrocarbon contaminants in acid gas

Sulfur recovery from acid gas containing H2S through Claus process is an efficient process. However, the acid gas contains several impurities such as benzene, toluene, ethylbenzene and xylene (collectively known as BTEX), light aliphatic hydrocarbons, H2O, NH3, CO2, COS, CS2, and N2. Among the hydrocarbons, while the aliphatics get destroyed in the Claus furnace, the aromatics (BTEX) often survive the high-temperature environment to reach the catalytic units to form soot deposits on the catalyst active sites and reduce their activity. This necessitates catalyst replacement and increases operational cost. This paper proposes a split-flow sulfur recovery method with enhanced gas temperature to effectively destroy BTEX in the Claus furnace to reduce the operational cost, where a portion of the acid gas feed is injected at the furnace inlet while the remaining is injected in the furnace in the post-combustion zone. A detailed reaction mechanism, developed for acid gas combustion and validated with experimental data, is utilized to conduct furnace simulations with the split-flow configuration to predict temperature, BTEX destruction, and acid gas conversion to sulfur. The results indicate that the suggested configuration for the Claus furnace can significantly improve BTEX destruction in the furnace with enhance temperature and SO2 concentration in the initial part of the furnace, while the sulfur recovery efficiency remains unaltered. The novel flow configuration for BTEX destruction also evades the need to co-fire natural gas in the furnace that was mandatory earlier to ensure BTEX destruction.

A reaction kinetics study and model development to predict the formation and destruction of organosulfur species (carbonyl sulfide and mercaptans) in Claus furnace

AbstractCarbonyl sulfide (COS), carbon disulfide (CS2), and mercaptans (e.g., CH3SH) are unwanted sulfurous species that are produced in sulfur recovery units (SRUs). They can cause equipment corrosion, reduce catalyst activity, and are pollutants for the environment. This necessitates process optimization to minimize their formation in the SRUs using an accurate kinetic model. In this paper, new reactions of COS and mercaptans are studied using the CBS‐QB3 level of theory, and their kinetics are evaluated using transition state theory. Such reactions are added to a detailed SRU reaction mechanism, and the updated mechanism is validated using the data on COS and mercaptans from lab‐scale experimental setups and an industrial plant. Compared to the previous models, significant improvements in the predicted concentrations of the important species such as COS, carbon monoxide (CO), and sulfur oxides (SO and SO2) are observed due to the inclusion of the new reactions in the mechanism. The simulation studies in a homogeneous reactor within the temperature range of 800–2000 K revealed that furnace temperatures near 1300 K are required to minimize CO and COS formation, while ensuring the oxidation of methane (CH4), mercaptans, and aromatics. Mercaptan formation mainly took place at low temperatures near 900 K. Claus furnace simulations are conducted using industrial feed and operating conditions to show that the optimized inlet air temperature and fuel gas flow rate can assist in simultaneously reducing the emissions of CO and harmful sulfurous species such as COS, CH3SH, and CS2 from the furnace.

Failure Occurrence Probability Assessment of Claus Furnace Package in Sulfur Recovery Unit Using Fault Tree Analysis Based on Conventional and Fuzzy-Based Approach

Background: If shutdown scenario of burner of the sulfur recovery unit takes place, toxic release, fire, and explosion accident can easily occur. Therefore, it is essential to assess the basic causes of burner shutdown scenario. Fault Tree Analysis (FTA) could be used to assess the occurrence probabilities of burner shutdown scenario and its basic causes/events. The failure occurrence probability of these Basic Events (BE) are often not available in lack of data and uncertain conditions. Objectives: This study was done to provide a comprehensive approach for analyzing and calculating BE failure occurrence probability affecting the shutdown scenario, using combined fuzzy logic, expert judgment, and FTA. Methods: The study was carried out from June to December, 2016. In this study, a fuzzy-based approach based on expert judgment was proposed to calculate the occurrence probability of burner shutdown scenario in lack of data and uncertain conditions. The brainstorming and FMEA, and HAZOP study were first used to identify fault events of the fault tree. Then, based on these methods, the fault tree was constructed. Subsequently, the failure occurrence probabilities of BE and shutdown scenario were calculated using the fuzzy-based approach and a conventional approach. Finally, the Fussell-Vesely importance analysis was used to rank the BE in FTA. Results: Results showed that the occurrence probability of shutdown scenario was 4.76E-04 per year. Since the failure occurrence probabilities of some BEs were not available, using failure probability functions in the conventional approach cannot provide failure occurrence probabilities of those BE. Therefore, the occurrence probability of shutdown scenario based on the conventional approach was not available. Based on the Fussell-Vesely importance measure analysis, it was determined that the blower failure while running, air pre-heater blockage, and shut-off valve fail close were 3 major causes of “burner shutdown scenario”. Conclusions: The fuzzy-based approach could derive a failure occurrence probability of BE based on expert subjective judgments using the failure possibility distributions (FPDs) in lack of data conditions. This study overcame the weaknesses of the conventional approach, to calculate BE failure occurrence probabilities via fuzzy logic.

Reduction in Natural Gas Consumption in Sulfur Recovery Units through Kinetic Simulation Using a Detailed Reaction Mechanism

H2S, a toxic gas present in raw natural gas and in oil fields, is converted to sulfur using the Claus process in sulfur recovery units. The low and fluctuating demand and sale price of sulfur and its increasing production due to the discovery of new sour fields have mandated optimization studies to reduce the production cost. Natural gas (mainly methane) is continuously cofired with acid gas (H2S and CO2) in the Claus furnace to maintain temperatures above 1050 °C, which adds to the operating cost. This study aims at reducing the usage of natural gas in sulfur recovery units using a detailed reaction mechanism for Claus feed combustion, while maintaining the desired furnace temperature. An optimal combination of feed-preheating using natural gas and its cofiring in the furnace is determined through furnace and fired heater simulations. A model for the feed preheater is developed using plant measurements. Through simulation studies by varying natural gas flow rates into the Claus furnace and to the air pre...

Effects of H2O in the Feed of Sulfur Recovery Unit on Sulfur Production and Aromatics Emission from Claus Furnace

An increase in the exploitation of sour reservoirs with a high content of acid gas (mainly H2S and CO2) and the enforcement of stringent emission standards have triggered demand for high sulfur recovery efficiency in sulfur recovery units (SRUs). The high amount of H2O that is present in feed gases to the SRU can act as a diluent in the thermal section of the SRU and can also pose operational difficulties and have substantial chemical effect on the equilibrium-limited reactions of sulfur production and the destruction of impurities. This paper explores the role of H2O (0–12% in the furnace feed) in sulfur recovery and in the fate of undesirable aromatics, COS, and CS2 in the thermal section using a detailed reaction mechanism and industrial SRU operating parameters. An increasing H2O concentration in the feed decreased the sulfur recovery efficiency in the thermal section significantly, as the equilibrium conversion of H2S to sulfur was affected. The higher H2O concentrations increased the concentrations ...

Roles of hydrogen sulfide concentration and fuel gas injection on aromatics emission from Claus furnace

Acid gas (H2S and CO2) recovered from sour feedstocks is treated in Sulfur recovery units (SRU) to obtain sulfur. The frequent changes in acid gas composition often result in lean acid gas, containing higher volume of CO2 than H2S. Lean acid gas causes flame instability that result in inefficient destruction of feed impurities in Claus furnace. To mitigate this problem, fuel gas (with >90% CH4) is co-fired with acid gas to increase furnace temperature and prevent flame extinction. The addition of fuel gas in Claus furnace may aggravate the production of polycyclic aromatics hydrocarbons (that cause frequent catalyst deactivation), and CS2 and COS (that reduce sulfur yield). Clearly, the amount of fuel gas injected into Claus furnace needs to be optimized. The previous optimization studies have given less importance to the roles of H2S concentrations in acid gas and the injected fuel gas on the production of aromatics and organosulfur species. This paper explores the fate of undesired aromatics, COS and CS2 in the Claus furnace over a wide range of H2S concentrations in acid gas using a detailed reaction mechanism. Experimental data on species concentration from a SRU is presented, and is used for mechanism validation. The effect of fuel gas co-firing with typical Claus feed containing 30–50% H2S is presented alongside detailed analysis of species compositions in the thermal section. An increase in fuel gas flow rate (from 0 to 200kmol/h) increased aromatics production in the furnace for lean acid gases. For rich acid gas, a decrease in aromatics production occurred as fuel gas flow rates was increased. The divergent trends were found to be occurring due to the reduced reactivity of methane in comparison to H2S at low flame temperatures, the dilution effect of unburnt CH4, CO2, and N2 in the furnace, and the endothermic hydrocarbon pyrolysis reactions leading to aromatics formation and growth. Fuel gas addition also led to COS and CS2 formation to reduce sulfur yield. The results indicate that fuel gas may not be the best solution for aromatics destruction in the Claus process for lean acid gas feeds.

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.

Modeling and simulation of an improved ammonia-based desulfurization process for Claus tail gas treatment

An improved ammonia-based desulfurization technology was proposed, in which the produced excess NH4HSO3 was concentrated by using the heat from the Claus tail gas and then SO2 was recovered by decomposing the concentrated NH4HSO3 in the Claus furnace. Furthermore, the process was modeled and simulated using the commercial software Aspen Plus. The characteristics of the ammonia-based desulfurization model were validated and compared with experimental results. Moreover, the water balance in the system was maintained, therefore, no extra water was required. Under the water balance condition, the SO2 removal performance in the system was predicted with various operating conditions. The model exhibited a reliable prediction of the desulfurization performance of the innovative process, which will be favorable for actual industrial application.

Effects of fuel gas addition to Claus furnace on the formation of soot precursors

Claus process is used in oil and gas industry to recover sulfur from H2S rich gas streams. The aromatic contaminants in its feed form soot that clog catalytic beds and deactivate them. Another common problem in this process is flame extinction in the furnace due to low H2S concentration. Therefore, fuel gas (mainly, methane) is added to Claus feed to sustain combustion. However, given that hydrocarbons can form polycyclic aromatic hydrocarbons (PAHs) and soot at high-temperatures in the furnace, fuel gas addition may aggravate the problem of catalyst deactivation. This makes it important to understand the role of fuel gas in forming PAHs and soot in the furnace. In this work, a detailed reaction mechanism consisting of 261 species and 1726 reactions is developed for Claus process including reactions for the interaction of hydrocarbons with S-containing species, and is validated using experimental data from flames and flow reactors. The extent of the conversion of hydrocarbons present in the feed gas to PAHs is reported by simulating the thermal section of Claus process. The variation in PAH profiles with varying fuel gas flow rate is also computed and discussed. The pathways for the formation and destruction of aromatic hydrocarbons are presented.

Reaction Mechanism for m-Xylene Oxidation in the Claus Process by Sulfur Dioxide.

In the Claus process, the presence of aromatic contaminants such benzene, toluene, and xylenes (BTX), in the H2S feed stream has a detrimental effect on catalytic reactors, where BTX form soot particles and clog and deactivate the catalysts. Among BTX, xylenes are proven to be most damaging contaminant for catalysts. BTX oxidation in the Claus furnace, before they enter catalyst beds, provides a solution to this problem. A reaction kinetics study on m-xylene oxidation by SO2, an oxidant present in Claus furnace, is presented. The density functional theory is used to study the formation of m-xylene radicals (3-methylbenzyl, 2,6-dimethylphenyl, 2,4-dimethylphenyl, and 3,5-dimethylphenyl) through H-abstraction and their oxidation by SO2. The mechanism begins with SO2 addition on the radicals through an O-atom rather than the S-atom with the release of 180.0-183.1 kJ/mol of reaction energies. This exothermic reaction involves energy barriers in the range 3.9-5.2 kJ/mol for several m-xylene radicals. Thereafter, O-S bond scission takes place to release SO, and the O-atom remaining on aromatics leads to CO formation. Among four m-xylene radicals, the resonantly stabilized 3-methylbenzyl exhibited the lowest SO2 addition and SO elimination rates. The reaction rate constants are provided to facilitate Claus process simulations to find conditions suitable for BTX oxidation.

Formation of polycyclic aromatic hydrocarbons in Claus process from contaminants in H2S feed gas

Claus process is used in oil and gas industry to produce sulfur from H2S. The presence of aromatic contaminants in its feed is believed to have detrimental effect on the process, as they are able to survive the high-temperature environment of Claus furnace and reach the catalytic units, where they form soot, clog catalyst pores, and deactivate them. Along with aromatics, small hydrocarbons (CH4, C2H6, and C3H8) are also present in feed that are capable of forming polycyclic aromatic hydrocarbons (PAHs) and soot in anoxic, high-temperature environment of the furnace. In this work, a detailed reaction mechanism with 258 species and 1695 reactions for Claus feed containing H2S, aliphatic and aromatic hydrocarbons, CO, CO2 and H2O is developed. The mechanism includes reactions for PAH formation (up to coronene) from hydrocarbons, H2S oxidation, and the interaction of hydrocarbons with S-containing species. The mechanism is validated using experimental species and aromatics profiles in flames. Through furnace simulations, the fate of aromatic contaminants, the amount of PAHs formed from hydrocarbon contaminants and the temperature effects on PAH profiles are determined. The PAH formation pathways are found, and the role of resonantly stabilized radicals is highlighted.

Reaction Mechanism for the Oxidation of Aromatic Contaminants Present in Feed Gas to Claus Process

Abstract Claus process, consisting of a furnace and catalytic reactors, is used to recover sulfur from H 2 S found in raw natural gas and gases from refineries. H 2 S is accompanied by contaminants such as benzene, toluene, xylenes (collectively called BTX), other hydrocarbons, NH 3 , CO 2 , N 2 and sulfur compounds. Among these, BTX have attracted several research activities as they form soot and sulfur-hydrocarbons in catalytic units that clog and deactivate the catalysts. This work focusses on BTX oxidation by SO 2 as a potential solution that can be carried out in a BTX destruction unit placed between Claus furnace and catalytic units. To determine the extent of BTX destruction by SO 2 , reaction mechanisms are developed using density function theory. The rates of elementary reactions are evaluated using transition state theory. The pathways leading to the formation of CO and SO are obtained.

Benzene Destruction in Claus Process by Sulfur Dioxide: A Reaction Kinetics Study

Benzene, toluene and xylene (BTX) are present as contaminants in the H2S gas stream entering a Claus furnace. The exhaust gases from the furnace enter catalytic units, where BTX form soot particles. These particles clog and deactivate the catalysts. A solution to this problem is BTX oxidation before the gases enter catalyst beds. This work presents a theoretical investigation on benzene oxidation by SO2. Density functional theory is used to develop a detailed mechanism for phenyl radical −SO2 interactions. The mechanism begins with SO2 addition to phenyl radical after overcoming an energy barrier of 6.4 kJ/mol. This addition reaction is highly exothermic, where a reaction energy of 182 kJ/mol is released. The most favorable pathway involves O–S bond breakage, leading to the release of SO. A remarkable similarity between the pathways for phenyl radical oxidation by O2 and its oxidation by SO2 is observed. The reaction rate constants are also evaluated to facilitate process simulations.

Sulfur Recovery from Acid Gas Using the Claus Process and High Temperature Air Combustion (HiTAC) Technology

Sulfur-bearing compounds are very detrimental to the environment and to industrial process equipment. They are often obtained or formed as a by-product of separation and thermal processing of fuels containing sulfur, such as coal, crude oil and natural gas. The two sulfur compounds, which need special attention, are: hydrogen sulfide (H2S) and sulfur dioxide (SO2). H2S is a highly corrosive gas with a foul smell. SO2 is a toxic gas responsible for acid rain formation and equipment corrosion. Various methods of reducing pollutants containing sulfur are described in this paper, with a focus on the modified Claus process, enhanced by the use of High Temperature Air Combustion (HiTAC) technology in the Claus furnace. The Claus process has been known and used in the industry for over 100 years. It involves thermal oxidation of hydrogen sulfide and its reaction with sulfur dioxide to form sulfur and water vapor. This process is equilibrium-limited and usually achieves efficiencies in the range of 94-97%, which have been regarded as acceptable in the past years. Nowadays strict air pollution regulations regarding hydrogen sulfide and sulfur dioxide emissions call for nearly 100% efficiency, which can only be achieved with process modifications. High temperature air combustion technology or otherwise called flameless (or colorless) combustion is proposed here for application in Claus furnaces, especially those employing lean acid gas streams, which cannot be burned without the use of auxiliary fuel or oxygen enrichment under standard conditions. With the use of HiTAC it has been shown, however, that fuel-lean, Low Calorific Value (LCV) fuels can be burned with very uniform thermal fields without the need for fuel enrichment or oxygen addition. The uniform temperature distribution favors clean and efficient burning with an additional advantage of significant reduction of NOx, CO and hydrocarbon emission.

Hydrogen Sulfide Combustion: Relevant Issues under Claus Furnace Conditions

The major chemical paths for the combustion of hydrogen sulfide under conditions typical of the Claus furnace (i.e., fuel-rich conditions) are presented. The manuscript begins with a brief survey of recently published research that involves sulfur chemistry in high-temperature environments, including the results of sensitivity analysis for some of the systems involved. Recommended values for the heats of formation of sulfur species are included. The reaction mechanism that is presented consists of more than 150 reactions. Issues such as the formation and destruction of COS and CS2 are presented: new chemical paths for the formation of COS and CS2 (not involving elementary carbon) are illustrated, on the basis of sound thermochemical and kinetic considerations. The formation of COS and CS2 is of great importance in the design of sulfur plants in industry. Possible reactions of COS and CS2 with SO2, and CO2 with H2S and sulfur species, also are discussed, prompted by experimental observations in flow reactors. The mechanism can explain the formation of hydrogen, which also is an important issue in sulfur plant design and associated tail gas units. Species such as H2S2 seem to have an important role during the combustion of hydrogen sulfide. Higher-molecular-weight linear H2Sx species are also considered, and it is concluded that their role is possibly minor. The chemical steps leading to the formation of Sx species by molecular growth are presented. The ring structure of some of the Sx species is discussed, as well as intramolecular ring conversions for S8, S7, S6, and S5. The possibility of H,OH radical recombination catalyzed by oxygenated sulfur species may explain the delayed oxidation of hydrocarbon species in the Claus furnace that has been observed in previous experiments by other authors. This could be an important design consideration for Claus plants to minimize the coking of catalyst beds in the process. The most likely chemical paths for the radical quench are presented and based on past observations. Controversy persists in regard to the actual mechanism and the rate constants of the reactions involved in the radical recombination, as well as the thermochemistry of some of the oxygenated sulfur species involved. More studies are needed to resolve the issues. The study also reveals the lack of high-temperature data for the kinetic coefficients of some of the reactions. Much rate data are based on atmospheric studies, rather than high-temperature oxidation. Similarly, better thermodynamic data are lacking for some important oxygenated sulfur species in the mechanism. This is most important for temperature- and pressure-dependent reactions, such as unimolecular reactions and chemically activated reactions. Studies that involve hydrogen sulfide flames under fuel-rich conditions are lacking. Most of the studies have been limited to the impact of sulfur species on the formation of other species, such as CO and NOx, in flames or reactors.

Kinetic Study of the Pyrolysis of H2S

The pyrolysis of hydrogen sulfide has been studied at residence times between 0.4 and 1.6 s and in the temperature range of 800−1100 °C. A continuous perfectly mixed quartz reactor was used to acquire kinetic data on the thermal dissociation of hydrogen sulfide and elemental sulfur mixtures diluted in argon (95 vol %). The kinetic auto-acceleration effect of sulfur is demonstrated. A detailed radical mechanism is written to account for the experimental results, in particular for the auto-acceleration effect, and validated against experimental results. This pyrolysis kinetic scheme is the first step and core of a complete detailed mechanism capable of modeling the various oxidation reactions encountered in an industrial Claus furnace.

Mechanisms of CO and COS Formation in the Claus Furnace

The aim of this study was to determine the major pathways leading to COS and CO formation and consumption during the processing of H2S and CO2 in the partially oxidizing conditions of the Claus furnace. Both species were found to be produced by a multitude of pathways, which include the direct reaction of H2S with CO2 to form COS and H2O and the reaction of CO2 with S2, one of the major primary products in a Claus furnace. This last reaction produced SO2 and CO as the major products, with COS being formed in lesser quantities. The dissociation of H2S to H2 and S2 at high temperatures (>1000 °C) was shown to promote a further cascade of reactions stemming from the reduction of COS and CO2, both of which lead to CO. Because of the known formation of CS2 from hydrocarbon carry-over into the furnace, the reactions of CS2 with CO2, H2O, and SO2 were also studied as potential CO- and COS-forming reactions. Reaction with CO2 was slow at <1200 °C, but reaction with either H2O or SO2 was fast above 900 °C. Convers...

Ammonia Pyrolysis and Oxidation in the Claus Furnace

The modified Claus process is commonly used in oil refining and gas processing to recover sulfur and destroy contaminants formed in upstream processing. In oil refining, in addition to the typical modified Claus plant feed of H2S and CO2, NH3 is also often present. NH3 is a process contaminant and must be destroyed in the front-end furnace of the modified Claus plant, otherwise, it poses a risk of poisoning the catalyst beds and plugging off downstream equipment because of the formation of ammonium salts. In this paper, the pyrolysis and oxidation of NH3 was studied under Claus furnace temperatures and residence times. Experimental data was taken, and new reaction rate expressions were developed for NH3 pyrolysis and oxidation. The derived rate expressions are outlined as follows: the NH3 pyrolysis rate expression r = A exp(−Ea/RT)PNH31.25, where A is 0.00421 mol s-1 atm-1.25 cm-3 and Ea is 16.5 kcal mol-1, and the NH3 oxidation rate expression r = A exp(−Ea/RT)PNH3PO20.75, where A is 4430 mol s-1 atm-1....