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In the sulfur recovery units of the two-stage Claus process and SCOT flue gas treatment methods used in most petroleum refining and chemical plants, special sulfur recovery catalysts are filled in both the primary and secondary converters as well as the flue gas hydrogenation reactor; these catalysts facilitate further reactions to increase the sulfur recovery rate. The active components of these catalysts are primarily active alumina with a large surface area and numerous active sites. In the operation of sulfur recovery units, the level of catalyst activity is of critical importance; once its activity declines or it becomes inactive, it will inevitably lead to a reduction in sulfur recovery rates, and may even cause the entire unit to stop operating. This article mainly discusses the main reasons for the decline in catalyst activity and the measures that should be taken to maintain high catalyst activity and extend its service life, thereby ensuring the long-term, safe, stable, and efficient operation of sulfur recovery units. 1 Analysis of the reasons for activity decline: On the surface of sulfur recovery catalysts, especially on the inner surfaces of the pores, there are numerous active centers. Reactants (H2S and SO2) are adsorbed on the surfaces of these active centers where reactions take place. The elemental sulfur produced is then desorbed from the surface and diffuses into the process gas. Therefore, the greater the surface area of the catalyst and the more active centers it has, the higher its catalytic activity. However, the activity of the catalyst gradually decreases over time during use. The main reasons for this are the blockage of pores or the loss of active centers, which leads to a reduction in sulfur recovery rate and a decrease in the temperature rise of the bed layer; this process results in a decline in the catalyst’s activity. There are two types of factors that cause a decline in catalyst activity: the first is changes in the catalyst’s internal structure, which lead to a gradual reduction in its activity and prevent it from being regenerated; the second type consists of external factors, whose effects are rapid, but which can sometimes be prevented, and by taking certain measures, the catalyst’s activity can be restored partially or entirely. 1.1 Changes in the internal structure of the catalyst: The change in its internal structure during use, which leads to a reduction in its surface area, is the process of catalyst aging. The aging process can be divided into thermal aging and hydrothermal aging; high temperatures and the presence of liquid water cause thermal aging and hydrothermal aging. Below 500 ℃, this process proceeds slowly; if the temperature of the catalyst bed exceeds 550 ℃, phase changes occur in the catalyst components, high-temperature alumina is gradually formed, the surface area drops sharply, and the pores in the porous alumina begin to collapse, causing the catalyst to lose its activity permanently. Special attention should be paid to changes in the catalyst bed temperature during operation, and overheating is strictly prohibited. 1.2 Activity degradation caused by external factors There are mainly 3 external factors that affect the degradation of catalyst activity: sulfur deposition, carbonaceous substance deposition, and sulfation. The decrease in catalyst activity caused by these factors is temporary and can be reversed. 1.2.1 Sulfur deposition During use, the active sites on the surface of the catalyst are covered by elemental sulfur; the deposition of elemental sulfur may occur as a result of both condensation and adsorption. Condensation: When the operating temperature of the reactor is below the sulfur dew point of the process gas, sulfur vapor condenses and deposits on the catalyst, blocking the micropores within the catalyst particles as well as the pores between them. This not only affects the catalyst’s activity but also leads to an increase in the pressure drop across the catalyst bed. Adsorption: Even when the reactor operating temperature is above the sulfur dew point of the process gas, due to its large surface area and microporous structure, sulfur vapor is adsorbed onto the surface active sites through adsorption and capillary condensation, thereby reducing the catalyst’s activity. This process is reversible. To restore catalyst activity, the bed temperature is generally increased to vaporize the condensed sulfur and desorb the adsorbed sulfur, allowing it to volatilize into the process gas. 1.2.2 Deposition of carbonaceous substances During the combustion of acidic gases, the hydrocarbons present may not burn completely, resulting in the formation of coke and tar-like carbonaceous substances. They are easily adsorbed by the catalyst and deposited at the top of the primary reactor. If the amount of deposited coke is too large and spreads throughout the entire bed, it will increase the pressure drop in the bed, affecting the quality of the sulfur product. Tar-like hydrocarbon-sulfur polymers are formed when heavy hydrocarbons or organic solvents present in the process gas react with elemental sulfur at high temperatures. These tars deposit on the surface of the catalyst, blocking the micropores on its surface and reducing its activity. When the mass fraction of tar deposited on the catalyst surface reaches 1%–2%, the catalyst loses all of its activity. 1.2.3 Formation of sulfates: Due to the action of SO2, SO3, and O2 present in the catalyst and process gas, alumina and iron oxide in the catalyst are converted into sulfates, which occupy the active surface sites of the catalyst and reduce its activity. Sulfation on the catalyst surface can occur in several ways. During normal operation of the device, SO2 reacts with oxygen or hydroxyl groups chemically adsorbed on the Al2O3 surface to form sulfates, but in small quantities. If SO3 or O2 is present in the process gas, even at just a few mg/L, it will accelerate the sulfation of the catalyst. The amount of sulfate formed on the catalyst surface does not increase indefinitely, as the sulfate produced can react with H2S in the process gas to be reduced back into Al2O3. When the rate of formation equals the rate of reduction, the amount of sulfate stops increasing, and an equilibrium state is reached. 1.2.4 Solid mechanical impurities: Solid mechanical impurities resulting from catalyst wear and those carried in from upstream sources can also cause a decline in catalyst activity. 2 Preventive Measures 2.1 Strict Control of Operating Parameters From a process perspective, in addition to selecting catalysts with good stability under active conditions, resistance to sulfidation, thermal stability, and resistance to sulfation, it is also necessary to strictly control the operating parameters at various stages within the specified range. This is to prevent the reactor bed temperature from rising too high, which could cause changes in the catalyst’s internal structure and result in its permanent loss of activity. On the other hand, the temperature should not be too low either, to avoid reduced catalyst activity due to excessive sulfur deposition. Further improve the operational skills of the operators in order to minimize catalyst deactivation caused by improper operation. 2.2 Desulfurization of the catalyst 2.2.1 Desulfurization measures during plant operation Under normal operating conditions of the plant, it is possible to periodically increase the operating temperature of the converters; starting from the first-stage converter, the inlet temperature of each bed layer can be raised by 15–30 °C and maintained for 36–48 hours. This approach allows some of the deposited sulfur to be removed, thereby restoring the catalyst to a high level of activity. 2.2.2 Desulfurization measures during shutdown Before shutting down the unit, it is necessary to remove the sulfur deposited inside the unit and within the catalysts. The first step is to increase the inlet temperature of the converter by 15–30 °C and maintain it for 36–48 hours; the second step is to switch the acidic gas to fuel gas, burn it on a chemical equivalent basis, and inject water vapor into the combustion furnace to prevent damage to the refractory materials in the furnace as well as the formation of carbon. The combustion exhaust gases are then passed through the catalyst beds in each stage of the converter at a temperature of 315–370 °C. Once no liquid sulfur more flows out of the condenser, continue the purging for another 12–24 hours. Then reduce the purging temperature; once it falls below the sulfur combustion temperature (180–208 °C), start introducing a small amount of air. Control the air flow rate so that the oxygen content in the gas entering the converter is below 1%–2%. Closely monitor the bed temperature as well as the temperature difference between the inlet and outlet gases – an increase in this temperature difference indicates sulfur combustion, and the oxygen content should be reduced immediately. After that, gradually increase the amount of air while reducing the amount of fuel gas, until the equipment reaches ambient temperature. Cooling can also be carried out under an inert gas atmosphere. If there is superheated steam at temperatures above 350 °C, steam purging can also be used for desulfurization; however, after desulfurization, the steam must still be displaced with an inert gas to prevent water from condensing in the system and causing equipment corrosion. In the case of a short-term shutdown, it is sufficient to increase the inlet temperature of the bed layer by 15–30 °C and maintain this level for 36–48 hours before resuming operation. 2.3 Carbonization measures for carbon-containing substances: When the amount of tar deposited on the catalyst is small, it is generally possible to restore the catalyst’s activity by replacing the catalyst at the top of the bed; if the tar deposition extends throughout the entire bed, then it is necessary to replace all of the catalyst or to employ carbonization measures. Before carbonization, desulfurization must be carried out according to the desulfurization procedure before it can proceed. The steps for carbon burning are as follows: after sulfur removal is complete, gradually replace the acid gas with fuel gas, and burn it in accordance with chemical equivalence. Raise the temperature gradually to 450–500 °C, then adjust the amount of air supplied to the combustion furnace so that the oxygen content in the gas flowing through the converter does not exceed 1%. Pay close attention to the temperature changes in various parts of the system, ensuring that it does not exceed 550 °C. To protect the refractory materials in the combustion furnace and control the formation of carbon, an appropriate amount of steam should be injected into the furnace. The effectiveness of carbon burning can be determined by checking the levels of O2 and CO2 in the gas entering and leaving the converter. When the levels of O2 and CO2 remain constant, stop the carbon burning process, lower the temperature, switch back to using acid gas, or shut down the operation. Burning carbon can easily cause localized overheating of the catalyst, accelerating its aging. Although carbon burning increases the catalyst’s activity, its surface area decreases significantly. Under normal circumstances, the hydrocarbon content in acidic gases should be strictly controlled, and the combustion conditions should be improved to prevent carbon deposition. Charcoal burning is not recommended unless necessary. 2.4 Reduction of sulfates By changing the conditions, namely by increasing the H2S concentration and the bed temperature, aluminum sulfate can be converted back into alumina. The procedure is as follows: Increase the inlet temperature of the primary converter to 340–370 °C, and raise the inlet temperature of the secondary converter by 30 °C above the normal operating temperature; maintain these conditions for 12–24 hours in order to remove elemental sulfur adsorbed on the catalyst surface. Then, adjust the molecular ratio of H2S to SO2 in the process gas to 2.5 or higher, and keep this ratio for 24–36 hours. Finally, restore the temperature and the H2S/SO2 ratio to the normal operating conditions, after which a recovery in catalyst activity can be observed. This process will lead to a decrease in the H2S conversion rate, and an increase in the levels of H2S and SO2 in the exhaust gases, thereby causing pollution. 3 Conclusion (1) In sulfur recovery production, it is necessary to strictly control the operational parameters and improve the level of operation, in order to minimize any reduction in catalyst activity caused by improper operations. (2) Different preventive measures can be taken to restore the activity of catalysts that has decreased due to external factors.