Claus sulfur recovery process selection

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Selection of Claus Sulfur Recovery Process Hu Wenbin Zhang Yiling Qilu Petrochemical Company Research Institute (Zibo 255400) Abstract The technology of recovering elemental sulfur from sour gas using the Claus process was industrialized more than 100 years ago. It explains how to choose the appropriate process technology for treatment according to the different compositions of acid gas. At the same time, some process technologies that can solve certain operating problems of sulfur recovery units and improve sulfur recovery rates are introduced. Keywords Sulfur recovery process technology, acid gas catalyst 1 Introduction The Claus process has become the standard process flow in the sulfur recovery industry since its invention. However, due to the complex changes in the composition of acid gas, which limits the effectiveness of the Claus process, it is necessary to develop different treatment processes according to the different compositions of acid gas and develop optimized systems for certain specific process conditions. The acid gas entering the Claus unit must contain at least 50% (mol) H2S before sulfur can be recovered using the traditional Claus process. However, sometimes the H2S content in sour gas is very low (5% to 50%) and cannot be burned in a traditional combustion furnace. ； Sometimes sour gas also contains other components such as ammonia and hydrocarbons that can cause many plant operating problems. Therefore, the traditional Claus process must be improved to handle sour gases of various compositions. According to different compositions, acid gases are divided into three categories:: a. Rich acid gas with H2S content greater than 50% (mol) ； b. Lean acid gas with H2S content less than 50% (mol) ； c. Acid-containing gas. How to process acid gases of different compositions by selecting different process technologies to meet different environmental protection requirements will be discussed in detail. 2. Acid-rich gas treatment process In production practice, a variety of improved Claus processes can be used to treat acid-rich gas. For example, two-stage or three-stage Claus process, direct oxidation process, cold bed adsorption process, etc. For the convenience of explanation, a set of two-stage Claus equipment used to process acid gas with a H2S content of 93% is used as the basis for discussion, and the effective processing of the acid gas is used as the standard to analyze and compare the above-mentioned processes. The total sulfur conversion rate of this secondary Claus unit is 91.8%, which is lower than the current industry standard. Inspection revealed that sulfur recovery could be improved through several small operational or design improvements. The acid gas composition and basic secondary Claus process operating data of this unit are shown in Table 1. Table 1 Composition of acid-rich inlet gas and basic secondary Claus process operating data http://www.zshg.com/wenzhang/jishujiaoliu/klsl-1.gif 2.1 The improved two-stage Claus process is based on the basic two-stage Claus process. The outlet temperature of the waste heat boiler is reduced from 449°C to 371°C, and a sulfur condenser is added to recover all the elemental sulfur generated in the combustion furnace. The outlet temperature of the first reactor bed dropped from 386°C to 343°C. This lower temperature promotes an increase in Claus conversion and ensures complete hydrolysis of COS and CS2 generated in the furnace. Similarly, by controlling the temperature of the second preheater, the outlet temperature of the second reactor is maintained at 30°C above the dew point temperature of sulfur. The closer the bed operating temperature is to the sulfur dew point, the higher the equilibrium conversion. By controlling the flow rate of the air fan at the inlet of the combustion furnace, the ratio of H2S:SO2 can be changed from 1.2:1 to 2:1 to optimize the performance of the device and increase the sulfur recovery rate from 91.8% to 96.1%, an increase of 4.3 percentage points compared with the design data. 2.2 The standard three-stage Claus process adds a third reactor after the second Claus reactor to form a three-stage Claus device. The sulfur recovery rate of a typical three-stage Claus unit using acid-rich gas as raw material is 96% to 97.5%. The first reactor was operated at 343°C to ensure complete decomposition of COS and CS2. Subsequent beds are operated at 30°C above the sulfur dew point to obtain the highest sulfur recovery. By adding a third reactor, the total sulfur conversion rate of the secondary Claus unit is increased by 1.9%. A comparison of the acid gas-rich feedstock process is shown in Table 2. Table 2 Comparison of sour gas raw material processes http://www.zshg.com/wenzhang/jishujiaoliu/klsl-2.gif 2.3 Two-stage Claus + direct oxidation process The original third reactor of three-stage Claus is replaced by the oxidation reactor of direct oxidation process (such as super Claus process). The direct oxidation process uses a special catalyst to directly convert H2S into elemental sulfur. Typical processes include Super Claus super-99 and super-99.5. The numbers "99" and "99.5" represent the total sulfur recovery that can be achieved when the last stage reactor is filled with Super Claus catalyst. To achieve a total sulfur recovery rate of 99%, the exhaust gas from the second Claus reactor directly enters the reactor equipped with a direct oxidation catalyst. ； To achieve a total sulfur recovery rate of 99.5%, a hydrogenation reactor must be added upstream of the direct oxidation reactor to convert all residual sulfides into H2S before entering the direct oxidation reactor, thus achieving a higher total sulfur recovery rate. The process operation of the direct oxidation reaction is different from the traditional Claus process. Unlike maintaining the H2S:SO2 ratio of 2:1, this process requires operating under excess H2S conditions. Sulfur species other than H2S do not react and pass directly through the catalyst bed. Therefore, the prereduction of non-HS species can lead to higher conversion rates in the direct oxidation reactor, allowing the device to achieve higher total sulfur recovery rates. By using a direct oxidation reactor instead of the third reactor, the total sulfur recovery rate can be increased to 98.8%. It can be seen from Table 2 that compared with the three-stage Claus process, the total sulfur recovery rate is increased by 0.8%. 2.4 Two-stage Claus + cooling bed adsorption process (sub-dew point process) The cooling bed adsorption process uses cold bed adsorption instead of the third-stage Claus reactor. Since the Claus reaction is an exothermic reaction, the lower the reaction temperature, the closer the reaction is to the equilibrium conversion rate. The cold bed adsorption process is different from the traditional Claus process in that the reactor operates below the dew point of sulfur, and all elemental sulfur generated in the catalyst bed is adsorbed on the catalyst. The process requires multiple reactors because the catalyst needs to be periodically regenerated to remove adsorbed elemental sulfur. When one of the reactors is in the reaction phase, the remaining reactors are in the regeneration phase. The gas coming out of the third condenser does not need to be preheated and enters the cooling bed directly. Therefore, the elemental sulfur produced condenses directly on the catalyst. Similar to the operating mode of the traditional Claus device, the ratio of H2S:SO2 must be controlled at 2:1. As can be seen from Table 2, the total sulfur recovery rate achieved by this process configuration is 99.0%. At the same time, Table 2 also lists the total sulfur recovery rate that can be achieved by the three-stage Claus + direct oxidation reactor and the three-stage Claus + cold bed adsorption process. 3. Lean Acid Gas Treatment Process The processing of lean acid gas requires special considerations for the operation of the burner. Since the concentration of H2S in the feed to the Claus burner is relatively low, it is impossible to generate a stable flame, and the incomplete combustion of hydrocarbons in the sour gas can lead to poisoning of the catalyst in the reactor due to coke deposition. Therefore, processing lean acid gas is mainly about how to make up for the lower acid gas concentration through technology, rather than obtaining a higher sulfur recovery rate. These processes mainly include: a. Four-stage Claus process with sour gas preheating and combustion gas burner ； b. Fully catalytic Selectox process ； c. Acid gas bypass combustion furnace technology ； d. Adopt oxygen-rich air into the combustion furnace process. A comparison of several processing techniques for lean acid gas is shown in Table 3. Table 3 Comparison of lean acid gas processing technology http://www.zshg.com/wenzhang/jishujiaoliu/klsl-3.gif 3.1 Acid gas preheating process A four-stage Claus device used to treat H2S content of 21% is used as the basis for discussion on lean acid gas treatment. In order to obtain a relatively stable flame at the burner, the acid gas can be preheated to 260°C, and a special burner needs to be used to burn the fuel gas alone. The burner temperature of the four-stage Claus device is 927°C, and the sulfur recovery rate is 96.3%. The acid gas composition and basic working condition results of this device are shown in Table 4. Table 4 Lean acid gas composition and basic operating condition results http://www.zshg.com/wenzhang/jishujiaoliu/klsl-4.gif 3.2 Selectox process Selectox process refers to the oxidation of H2S into SO2 through a catalytic burner at approximately 370°C. Selectox process includes two types: through Selectox process and circulating Selectox process. The straight-through Selectox process is used to treat acid gas with a H2S content of 5%. ； The circulating Selectox process is used to treat acidic gas with an H2S concentration of 5% to 40% (mol), and can also effectively treat acidic gas with a H2S content of 65% to 70% (mol). The Cyclic Selectox process is a fully catalytic process, which means there is no flame anywhere in the process. A special catalyst bed replaces the sour gas burner of the traditional Claus unit. Selectox catalyst only occupies a small part of the upper part of the first reactor bed and is used to oxidize H2S into SO2. The remainder of the bed is filled with a conventional Claus catalyst, and 80% of the reaction is completed here. Since the reaction is exothermic, it is required to control the H2S concentration in the raw gas to avoid over-temperature. The circulating Selectox process uses a circulating fan to circulate a part of the Selectox condenser outlet tail gas to dilute the inlet gas so that the H2S concentration is less than 5%. This mainly inert gas circulation limits the reactor inlet temperature to approximately 371°C. After the Selectox reactor, two traditional Claus reactors are connected, and the sulfur recovery rate is 96.3%. 3.3 The acid gas bypass process allows part of the acid gas to bypass the combustion furnace, which can solve the problem of insufficient combustion of lean acid gas. The bypass gas is mixed with the burner outlet gas in front of the waste heat boiler. The amount of oxygen entering the burner is the same as the amount of oxygen required for complete combustion of all gases. The flame temperature can be maintained at 1010~1204℃. The consequence of bypassing a part of the acidic gas to the combustion furnace is that the hydrocarbons in the bypass gas are not burned, which may cause coking of the downstream catalyst bed. Moreover, due to the shortened residence time in the reactor and the reduced inlet temperature of the waste heat boiler, the Claus conversion rate of the thermal reaction section is reduced. The system has a sulfur recovery rate of 96.2%. 3.4 The oxygen-enrichment process increases the oxygen content in the combustion air entering the burner, which can increase the flame temperature and flame stability, reduce the volume of bypass air, and improve the processing capacity and sulfur recovery rate of the device. Increasing the burner temperature can prevent catalyst poisoning caused by coking. 4. Treatment of Acid Gas The ammonia in the acid gas of the Claus unit comes from the upstream acid water stripper. Ammonia must be completely decomposed in the Claus burner to avoid the deposition of ammonium salts in subsequent catalyst beds. The decomposition of ammonia can be promoted by using an oxygen-enriched process and/or using special burners. There are three main types of ammonia burning burners that can be used: Cope, Oxyclaus and Comprimo. The Cope process increases the burner temperature by using oxygen enrichment. The burner temperature is 1482°C. The burner temperature is adjusted by a circulating air flow from the first sulfur condenser. To use 100% oxygen enrichment, special burners must be used, and acid gas, circulating gas, air and oxygen must be controlled. ； The Oxyclaus process and Comprimo process do not require circulating gas to adjust the burner temperature. All gases enter the special burner to mix and burn acidic gas, air, and oxygen. Typical burner temperature is 1204~1288℃, which is lower than the temperature of Cope burner. 5 Conclusion a. Direct oxidation or cold bed adsorption can be used as the last stage reactor to treat acid-rich gas. ； b. The operation of the burner can be improved or the traditional burner can be replaced by a catalytic burner to produce a stable flame and a higher flame temperature to handle lean acid gas. ； c. Oxygen-enriched and/or special burners can be used to treat ammonia-containing gas to produce a higher flame temperature, promote the decomposition of ammonia, and prevent the deposition of ammonia salts on the subsequent catalyst bed.