Investigation of Inlet Gas Streams Effect on the Modified Claus Reaction Furnace

The objective of this paper is to model the main reactions that take place in the Claus reactor furnace and compare it with actual data and simulated process. Since the most important point is the selection of suitable reaction conditions to increase the reactor performance, the model is formulated to predict the performance of the Claus plant. To substantiate the theoretical model, we used actual process condition and feed composition in Shahid Hasheminejad Gas Refinery. Model equations have been solved by using MATLAB program. Results from MATLAB are compared with those from SULSIM ® simulator and with actual plant data. The AAD (Average Absolute Deviation) of modeling results with actual data is 2.07% and AAD of simulation results with real data is 4.77%. Error values are very little and show accuracy and precision of modeling and simulation. The predicting curve for different parameters of the reactor furnace according to variable conditions and specifications are given.


Introduction
Hydrogen sulfide (H 2 S) is produced from sulfur compounds in fossil fuels such as natural gas or oil.Sour gases (H 2 S and CO 2 ), are removed from the natural gas or refinery gas by means of one of the gas treating processes.Due to global environmental rules, refineries have to recover sulfur from nature.H 2 S containing acid gas stream is flared, incinerated, or fed to a sulfur recovery unit.The Claus process is commonly used to reduce the emission of sulfur compounds into the atmosphere.Recently recovery of sulfur is done by means of the modified Claus tail gas clean-up processes.In these processes, H 2 S over a catalyst converts to elemental sulfur where the reaction takes place in a high temperature furnace.The recovery process is the reaction between H 2 S and air to form sufur and water.Following reaction is the main reaction in the recovery process: In the original Claus process, control of this reaction was difficult and sulfur recovery efficiencies were low.
In order to overcome these difficulties and also increase the efficiency of the process, several modifications of the Claus process have been developed.In modified process, free flame total oxidation of 1/3 of the H 2 S to SO 2 followed by a reaction over the catalyst of SO 2 with the remaining 2/3 H 2 S. According to Mohamed Sassi and Ashwani K. Gupta modified Claus process for a Sulfur Recovery Plant consists of several stages [1]: 1) Combustion (In the Reactor Furnace) 2) Redox (Catalytic Converter) These are simplified reactions which actually take place in a Claus unit.There are various species of gaseous sulfur S 2 , S 3 , S 4 , S 5 , S 6 , S 7 , and S 8 .Equilibrium concentrations of these sulfur compounds are not known in the entire of process.Additionally, gas stream contains water saturated with 15 -80 mol% H 2 S, 0.5 -1.5 mol% hydrocarbons, and CO 2 which can result in carbonyl sulfide (COS) carbon disulfide (CS 2 ), carbon monoxide (CO), and hydrogen [2].Most Claus plants operate in the multistep process "straight-through" mode as shown in Figure 1.The combustion is carried out in a reducing atmosphere with only enough air 1) to oxidize one-third of the H 2 S to SO 2 , 2) to burn hydrocarbons and mercaptans, and 3) for many refinery Claus units, to oxidize ammonia and cyanides.The process includes the following operations:  Combustion: burn hydrocarbons and other combustibles and oxidize one-third of the H 2 S to provide necessary SO 2 to react with remainder H 2 S for producing S 2 in the furnace. Waste Heat Recovery: Cool combustion products. Sulfur Condensing: Cool outlet streams from waste heat recovery unit and from catalytic converters. Reheating: Reheat process stream, after sulfur condensation and separation, to a temperature high enough to remain sufficiently above the sulfur dew point.
In order to investigate different aspects of the modified Claus process, a number of studies have been performed on main burner and sulfur recovery in this process.Monnery et al. modeled the modified Claus process [3].Kelly Anne Hawboldt has studied mathematical modeling of reactions in the process [4].Recently, S. Asadi et al. used TSWEET simulator to optimize the recovery of sulfur [5].
At first approach, we have used a mathematical model for the key reactions that take place in the reactor furnace.In the second approach, we have simulated the process with a commercial simulator.Finally using the model and simulation, we have compared obtained results and proposed some improvements on the base case.

Kinetic Studies
Claus process has been investigated via different aspects, experimental and theoretical perspectives.Paskal et al. gives a summary of the main reactions thought to occur within the Claus furnace [6,7].Clark et al. discussed the mechanisms behind the formation of key sulfur containing species found within the furnace, and in a subsequent study outline primary reaction pathways for the principal components in the furnace [8,9].While there have been numerous attempts to model the Claus process based on simplified kinetic expressions, the complexity of the chemistry and the number of involved reactions has precluded the accurate prediction of outlet compositions.As it mentioned before, gas stream contains different compounds such as H 2 S, CO 2 and hydrocarbons.Most important compound is H 2 S and several groups have studied it's decomposition under different condition.As result, it suggested that there are numerous reactions on the catalytic decomposition of H 2 S in the clause process.According to the studies, gaseous H 2 S exists in chemical equilibrium with elemental hydrogen and sulfur by the following equation: Oxidation includes two staged reaction, first oxidation of H 2 S and followed by reaction between H 2 S and SO 2 that limiting stage of the Claus reaction is the second part [10].
During the reaction in the furnace and according to the existence of ammonia in the gas stream, oxidation of NH 3 will take place.Recently, Clark has mentioned that under 1100˚C ammonia oxidation is negligible.Additionally, he noted in competitive oxidation, first of all H 2 S, and then methane, finally NH 3 react.On the other hand, Goar et al. found rate of hydrocarbon combustion is more than ammonia and NH 3 is more than H 2 S [11].Formation of COS and CS 2 in the Claus reaction furnace are also very important in the modeling.Field studies have revealed that concentrations of COS and CS 2 at the exit of the reaction furnace/ waste heat boiler typically lie between 100 ppm and 2 mol% [12].However, these seemingly small concentrations in the furnace product stream can represent nearly half the sulfur emissions from a tail gas clean-up unit [13].It is possible to hydrolyze COS and CS 2 back into H 2 S in the Claus catalytic converters according to the following stoichiometry: As it mentioned, there many reactions which may take place in the furnace according to the conditions such as temperature and pressure.Full list of reactions that occur in the furnace is not obvious and for the known reactions, reaction rate expressions are not available.In current work we assumed that gas stream consists of CH 4 , CO 2 , H 2 S, H 2 O, O 2 , N 2 , CO, CS 2 , COS, S 2 , SO 2 , H 2 .Regarding gas stream composition, important reactions which take place in furnace and we use in the modeling are listed below.

Mathematical Model
The basic structure of the model consist of the equations of mole and energy conservative rule the furnace, which are related to each other and are function of molar conversion of H 2 S in equilibrium reaction and temperature.In order to model the reactor, a steady-state simulation has been used for mole and energy balance.Sames and Paskal presented empirical correlations to predict the fraction of CO, H 2 , COS, CS 2 and sulfur (as S) in the effluent of the Claus furnace.The correlations are obtained from more than 300 tests on 100 different sulfur trains; with different flow configurations processing acid gas feed streams [12].These empirical correlations are presented in Appendix.We use these equations to model the furnace and mole balance.In this work, furnace pressure is 130 kPa (absolute) and pressure drop (ΔP) is 10 kPa.Using empirical equations and applying in the mole balance for the compounds, we get the mole balance equations, for each compound.
For gas components in the outlet gas stream: H 2 S: Total moles: X: Flow rate of H 2 S conversion at the equilibrium Claus reaction  In order to establish a reference point, calculations are carried out for a "base case" ed are given in Table 3.Our base case is Shahid Hasheminejad Gas Refinery.Shahid Hasheminejad (Khangiran) gas refinery is in Sarakhs, Khorasan province.

A A T B T C T D T X B AT B T C T D T
the Claus process in the refinery.Next step, application of the feed gas condition in the equations resulted into the parameters of mole and energy balance equations.Calculated parameters are presented in  This software has widely accepted thermodynamic data and propriety thermodynamic properties for all of the gas components and sulfur species found in sulfur recovery processes.Figure 2 shows the flow diagram of the simulated Claus unit of Shahid Hasheminejad Gas Refinery.

Results and Discussion
As described previously, we implemented real data from gas refinery in the model.In this section to verify the model, we compare the output result from reaction furthree set of results.It is obvious that furnace temperature obtained using model (1098 K) is lower than actual temperature of reaction furnace (1113 K), as the 15˚C temperature difference is negligible and it results into 1.35% error.Simulated results shows that predicted temperature using SULSIM ® software is 1121 K and higher than Claus furnace temperature.Error occurred using software is lower (0.72%).In this case simulation is more reliable.Additionally, sulfur conversion obtained from model results (56.635%) is in good agreement with conversion of sulfur in Claus plant (54%) in gas refinery.On the other hand, results oduct is higher, conversion.It is obvious that presented model in these conditions, is effective even more than simulated process.As it is presented in the Table 5, in the furnace effluent there is sulfur vapor.It's due to the high temperature in the furnace; this high temperature converts sulfur to S 2 vapor.According to Table 5, inlet air ratio to acid gas feed in Claus plant is 0.86 and in our model this ratio is 0.83.As we used this obtained ratio in our simulation, simulation and modeling resulted in the same ratio.Therefore modeling and simulation errors are low and about 3.5%.Since sulfur production is high in comparing with actual plant; air consumption is low, CO 2 concentration difference is about 0.05 (mole %), it means error is 0.34%; we can conclude that in all cases, simulated and modeling are more efficient.
Predicted N 2 concentration using equilibrium model is 36.067(mole %) whereas N 2 content in the plant outlet gas stream is 39 (mole %).Since In empirical equations of model, it is assumed air consumption is low, results are logic and difference between real state and model results is S acceptable.ince air consumption is low, its acid gas capacity is more than actual plant and can predict better results.Simulation has the same manner in the prediction of N 2 concentration.H 2 O concentration in the outlet stream from model is less than plant outlet water content.The model performance was not good in water case and error value is about 7.8%.While H 2 O content in simulation is closer to the actual data and lower error has occurred.Simulation performance is better in this case.
O 2 component in Claus plant damages the equipments (catalyst exchanger) and must be minimized, in Claus plants O 2 content is zero.Predicted concentration is    Predicted S 2 content in both methods is greater than plant data.There is a big difference between real and predicted values for S 2 ; it is due to the formation of liquid sulfur in WHB.According to Table 5 and comparison between results and plant data, and also neglecting the error in CO and H 2 predicted concentrations, average error is about 3.5% and 5.36% for model and SULSIM ® simulation; also AAD (Average Absolute Deviation) in comparing actual data with modeling and simulation results are 2.07% and 4.92%, respectively.We can conclude that our model is more efficient and applicable for other Claus plants with different inlet composition.The results indicate that in lower concentrations, furnace temperature is low and increase in the H 2 S (to 30%) content would increase the slope of H 2 S conversion line that results to decrease in H 2 S content in the outlet stream of furnace.
In the higher concentrations, since furnace temperature is more than previous, H 2 S cracking and conversion in e rted hydrogen sul ration in feed Claus r action increases and unconve  in the outlet decreases.As can be seen in the figure, the reduction of H 2 S in the effluent is agreement with increase in H 2 S content in acid feed gas.For this case, model prediction is more reliable than simulation.

Effect of Inlet Temperature
In this section, temperature effects have been investigated.We can predict the effect of preheating on the furnace temperature and conversion in furnace.Figure 6(a) shows the variation of furnace temperature vs. inlet temperature of acid gas.According to the figure, furnace temperature increases 4.4˚C by 10˚C increase in inlet temperature.Therefore, if we can increase the design temperature (52˚C) to 252˚C, furnace reactions will take place in 914˚C.conversion increases by 0.156% when inlet temperature increases 10˚C.Also this figure demonstrates that H 2 S conversion in furnace would increases from 72.66% to 75.74% in preheated feed (252˚C).Also 10˚C increase in temperature of inlet air results into 2.52˚C increases in furnace temperature, 0.103% increase in H 2 S conversion and 0.153% increase in sulfur conversion.If both acid gas feed and inlet air preheated separately and equally 10˚C, reaction furnace temperature increases 7.1˚C, H 2 S conversion 0.21% and sulfur conversion 0.31%.
From a theoretical point of view, there is an optimal temperature in the furnace reactor to get the more efficient performance, maximizes sulfur production and H 2 S conversion as reported in the previous sections for Claus process.A solution for this problem is fuel gas injection to the furnace in order to incr perature.By one percent increase fuel cont in the inlet gas, furnace temperature would increase 30˚C -50˚C.Calculation showed that 2000 Sm 3 /hr fuel injections in acid gas feed (50000 Sm 3 /hr) for Shahid Hasheminejad Refinery, furnace temperature would increase 130˚C.More hydrocarbon content in the feed will produces more CS 2 and COS in the furnace.Increasing flow rate causes decrease in furnace capacity.It was observed the positive effect of fuel injection by increasing the temperature, led to reduction in plant capacity.

Conclusion
The reactor furnace for an industrial three-stage straightthrough sulfur plant with identical feed gas composition and operating conditions was molded and compared.The results of the modeling and steady state simulation have been presented in Table 5.The results showed that H 2 S conversion could be promoted by an increase in hydrogen sulfide content in the feed gas.Therefore if we coul enhance the H 2 S concent n, sulfur conversion and order to increase furnace temperature, fuel pacity in the Claus pl A. K. Gupta, "Sulfur Recovery from Acid ease the tem ent d ratio overall efficiency of the furnace would improve.This also could lead to decomposition of aromatic compounds such as BTEX, additionally furnace temperature would increase.In injection is possible but, it must be optimized to prevent plant capacity reduction.On the other hand, reduction in CO 2 and N 2 inlet flow helps to reduce the volume of effluent and increases the furnace ca ant.Also results demonstrated that by utilization of heat input (preheated feed and air) in a furnace of a plant, the performance of the reactor would improve.

1
35) A 0 , A 4 , A 5 , B 0 , B 4 , B 5 , C 4 , C 5 , D 4 , D 5 are cons ters that would calculated numerically according to the and the operating conditions us Operating conditions is composition of inlet gas stream to 4. Process Simulation lications of the reaction fur-

Figure 4 (
Figure 4(a).It is due to errors occurred in the simulation.Figure5shows the effect of H S concent str

Figure 5
Figure 4(a).It is due to errors occurred in the simulation.Figure5shows the effect of H S concent str case.Obtained results both are matched.It should be noted that sulfur conversion in Figure4(b) shows model and simulation have similar trend.Therefore there is a negligible difference between the model and simulation prediction in 18% H 2 S in increases 0.12%.Simulation has sim 2 eam on the effluent H 2 S content.As H 2 S content increase in the feed, model predicts a trend for H 2 S content in outlet which decreases and then with a lower slope increases and finally decreases.

Figure 3 .Figure 4 .
Figure 3. Simulation and model estimation for Modified Claus plant reaction furnace temperature vs. H 2 S content.

Figure 5 .
Figure 5. Estimation for H 2 S content in outlet stream vs. H 2 S mole fraction in feed stream.

Figure 6 ( 6 ,Figure 6 .
Figure 6.(a) Modified Claus furnace temperature prediction for Khangiran Gas refinery vs. inlet acid gas temperature; (b) H 2 S and sulfur conversion vs. inlet acid gas temperature.

Table 4 .
In order to compare the imp