Study Of Claus Process Simulation And Detailed Mechanism Of Partial Oxidation Of Acid Gas

As one of the most poisonous and corrosive gases,H2S is extensively present in raw natural gas pool.The so-called acid gas(AG)is mostly the mixture of H2S and CO2 separated from raw natural gas.Nowadays the Claus process is the mainstay for treating such acid gas for recovering elemental sulfur.It can be divided into two sections.In the high-temperature section,acid gas is partially oxidized by air in the reaction furnace.And in the catalytic section,the unburned H2S reacts with SO2 to form elemental sulfur on the catalyst bed.Reaction furnace is the most significant unit of the Claus process,and the acid gas combustion has the decisive effect on the primary oxidation of H2S,sulfur production and formation of organic sulfur pollutants(COS and CS2).Moreover,these elements will evidently affect the total sulfur recovery efficiency.Therefore,it is of great importance to study the acid gas combustion process,oxidation mechanism and formation mechanism of pollutants for optimizing and developing the Claus technology,improving the conversion rate of H2S and reducing the discharge of SOx.In this research the software AspenPlus was applied to build up the 3-stage Claus process model and the systematic study of major influencing factors(gas composition,?,O2 enrichment level and air preheating)on furnace performance and sulfur recovery was performed.The acid gas combustion facility was set up to experimentally study the acid gas diffusion flame under atmospheric condition.By measuring the distributions of temperature and major components concentrations,the flame structure and principal reactions distributions were analyzed.The detailed mechanism of acid gas oxidation was developed and validated versus tubular furnace experiment results.With the aid of CHEMKIN software,acid gas premixed flame structure and ignition delay time under different temperatures,gas compositions,equivalence ratio and residence time.Moreover,the conversion paths of CO2 and CH4,and formation of COS and CS2 during the acid gas oxidation were discussed based on the detailed mechanism.(1)The 3-stage Claus process model was established using AspenPlus software.This equilibrium model could give matched results for temperatures of each unit,H2S oxidation rate of the thermal stage and total sulfur recovery efficiency,yet it was inadequacy in predicting the formation of COS and CS2.The impacts of acid gas composition,?,O2 enrichment level and air temperature on furnace reactions and total sulfur recovery were studied.?=3 could ensure the H2S/SO2 molar ratio of the process gas in the catalytic section coincided with stoichiometric ratio.O2 enrichment and air preheated combustion could improve the furnace temperature with? kept unchanged.Obvious temperature increase could be observed by applying low level O2 enrichment while air preheating temperature is in linear relationship with flame temperature.Under the equilibrium model,the total sulfur recovery was significantly depended on ? but insensitive to O2 enrichment or air preheated combustion modes.(2)The diffusion flames of two kinds of acid gases under different conditions were experimentally investigated.The H2S-CH4 fuel-rich diffusion flame could be divided into primary oxidation,intensive combustion and post-flame zone.In the first zone the oxidation of H2S was much faster than that of CH4 and the major products were S2 and H2O.The conversion of CH4 here was realized by reacting with sulfur-containing species to slightly form COS and CS2.The complete oxidation of H2S and CH4 was proceeded in the intense combustion zone located in the outer flame wherein the fonned combustibles such as H2 and C2H2 could also be consumed.The flame structure of H2S-CO2 was similar to that of H2S-CH4.O2 enrichment combustion showed superiority in increasing flame temperature and energy efficiency.However,the higher temperature increased decomposition rate of H2S,radicals concentrations and reactivity of CO2,and also promoted formation of H2,CO,COS and CS2.(3)The acid gas oxidation mechanism covering oxidation of H2S and CH4 and conversions of CO2,COS and CS2 was developed.By CHEMKIN software,the influencing factors of acid gas premixed flame structure and ignition delay time were studied.The premixed flame could be divided into two parts,namely H2S oxidation and CO2 conversion.The presence of O2 would restrict the reactivity of CO2.CO2 was converted to CO and COS via reacting with reducing radicals after O2 was depleted.The ignition delay time could be affected by gas composition,O2 enrichment level and initial pressure.According to the H2S oxidation modes,the ignition of acid gas could be divided into two stages,the representative reaction is H2S+HO2=H2O2+SH in the first stage while at the second stage H2S was mainly oxidized by reactions H2S+OH=SH+H20 and H2S+O=SH+OH.(4)Acid gas decomposition and partial oxidation were studied by changing gas composition,temperature,residence time and ?.The decomposition of H2S was the dominant reaction during acid gas pyrolysis while H2S+CO2=COS+H2O proceeded prior to H2S cracking.H2S reacted with H and S radicals to realize the decomposition and this just supported the self-accelerating phenomenon of H2S decomposition.COS was mainly formed via H2S+CO2=COS+H2O,which was favored at higher temperature(?1473K).H2 could be plentifully formed via partial oxidation of H2S under low temperature,however,H element intended to form H2O but not H2 under higher temperature.The restrict effect of O2 on reactivity of CO2 could be explained by three reasons that O2 competes with CO2 for oxidizing H,the formed OH favors the reverse direction of CO2+H=CO+OH and O2 could oxidize CO into CO2.(5)The thermodynamic results of acid gas partial oxidation with presence of CH4 and CO2 were calculated and its potential in H2 production was studied.Based on the proposed detailed mechanism,pyrolysis and partial oxidation of H2S-CH4 mixture were investigated and conversion mechanisms of CH4 and CS2 were presented.During the pyrolysis reaction,CH4 reactivity required temperature to reach the threshold value of about 1273K.The primary decomposition of CH4 was realized via CH4+S=SH+CH3,subsequently CH3 was converted to CS by reacting with H and SH radicals,and then produced CS2 from reaction between CS and SH.During the oxidation process,O2 accelerated the decomposition and oxidation rate of H2S from two perspectives.The H2S-O2 reaction produced H and S2 and these two components yielded plentiful SH,which was favored for H2S decomposition.And the OH and SO chain branching radicals from H2-O and S2-O reactions promoted H2S oxidation.SO2 functioned as the oxidizing medium after depletion of O2,it could be transformed into OH and SO via reactions with S,SH and H2 species.These oxidizing radicals helped primary decomposition of CH4 and formation of CS2.