Principle of the Claus process for sulfur production

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A large amount of H2S gas is generated during the processing of oil and gas. To protect the environment and recover elemental sulfur, the Claus process is widely used in industry to treat acidic gases containing H2S. The reaction equations are as follows: H2S + 3/2 O2 = SO2 + H2O (1) 2H2S + SO2 = 3Sx + 2H2O (2). Reactions (1) and (2) take place in high-temperature reactors; in the catalytic reaction zone (below 538°C), in addition to reaction (2), the following hydrolysis reactions of organic sulfides also occur: CS2 + H2O = COS + H2S (3) COS + H2O = H2S + CO2 (4). This article reviews the development history of improved Claus sulfur recovery processes, explains the basic principles, influencing factors, and operating conditions of these processes, and provides a brief evaluation. 1. Development History of the Process 1.1 The Original Claus Process In 1883, British chemist C.F. Claus first proposed a patented technology for recovering elemental sulfur, and this technology has been in use for over 100 years now. The original Claus process is a two-step procedure, whose flow diagram is shown in Figure 1; it is specifically designed to recover the sulfur consumed in the production of sodium carbonate via the Leblanc process. The reaction process for the latter is described by the following equations: 2NaCl + H2SO4 = Na2SO4 + 2HCl (5); Na2SO4 + 2C = Na2S + 2CO2 (6); Na2S + CaCO3 = Na2CO3 + CaS (7). To recover elemental sulfur, in the first step, CO2 is introduced into a slurry composed of H2O and CaS (an alkaline waste product), thereby producing H2S according to the aforementioned reactions. In the second step, H2S and O2 are mixed and then introduced into a container containing a catalyst; the catalyst bed has been preheated to the required temperature by some means. The reaction then proceeds according to the equation: CaS(s) + H2O(l) + CO2(g) = CaCO3(s) + H2S(g) (8), which corresponds to reaction (9). After the reaction begins, the bed temperature is maintained at a constant level by controlling the flow rate of the reactants. Obviously, this process can only proceed at very low space velocities on the catalyst. It is reported that H2S + 1/2 O2 = 1/X Sx + H2O (9). If oxides of iron or manganese in the form of hydrates are used, it is not necessary to preheat the catalyst bed before starting the reaction. However, since the reaction between H2S and O2 is a highly exothermic reaction, and the heat generated can only be dissipated through radiation, this limits the Claus furnace to handling only small amounts of H2S gas. To achieve a yield of 80–90%, only 2–3 volumes of H2S gas can be processed per hour per volume of catalyst ； To accelerate heat dissipation and thereby increase processing capacity, attempts were made to install cooling coils in the kiln to allow cold exhaust gas to circulate through it; however, these attempts were unsuccessful. Despite further efforts, this method was not adopted on an industrial scale before the 1930s. The reason for this, as mentioned above, is that when H2S and O2 undergo oxidation reactions directly in the reactor, the large amount of heat generated cannot be removed, resulting in a sharp rise in temperature inside the reactor. To control the reaction temperature, it was necessary to use a relatively low space velocity, which in turn limited the production capacity and hindered the industrial application of this method. 1.2 Improved Claus process: An early modification of the Claus process was the I·G·Claus process (1932). In this process, part of the H2S was burned in a boiler, and the resulting SO2 was then combined with unreacted H2S. Since a large amount of heat was already released during the combustion to produce SO2, less heat was generated during the subsequent catalytic reactions, preventing catalyst damage due to overheating. This is what is known as the “split-flow method.” In 1938, the German company I·G·Farbenindustrie AG made significant improvements to the Claus process, not only increasing the processing capacity substantially but also finding a way to recover the energy that had previously been wasted. The key point is to carry out the oxidation of H2S in two stages. The first stage is called the thermal reaction stage, during which 1/3 of the volume of H2S is oxidized to SO2 in the steam boiler, as shown in equation (1); simultaneously, a large amount of reaction heat is released and recovered in the form of water vapor ； The second stage is called the catalytic reaction stage, during which the remaining 2/3 volume of H2S reacts further with the generated SO2 on the catalyst to form elemental sulfur, as shown in equation (2). The process flow of the modified Claus process is shown in Figure 2. By comparing Figures 1 and 2, it can be seen that thanks to the installation of waste heat recovery equipment, approximately 80% of the heat released during the reactions in the furnace can be recovered. Moreover, the temperature of the catalytic conversion reactor can also be adjusted by controlling the temperature of the incoming process gas. This essentially eliminates the problem of difficult reactor temperature control, while simultaneously increasing the processing capacity of the plant significantly, thus laying the foundation for modern sulfur recovery processes. “The advent of the \"direct current method\" or \"partial combustion method\" represented a groundbreaking advancement for the Claus process, and it was only after that that the Claus process came to be widely used in industry. To meet the demands of production development, the Claus process itself was also subjected to numerous improvements. The main advancements after 1938 involved the addition of more catalytic reactors, as well as the removal of sulfur and heat between these reactors, thereby shifting the reaction equilibrium in favor of a higher sulfur yield. 1.3 Modern improved Claus process: Over the course of more than half a century, through continuous research and improvements in catalysts, equipment, materials, processes, and control methods, the improved Claus process has evolved into the simple, reliable, cost-effective sulfur recovery method that is widely used today. Depending on the H2S content in the feed gas, the modern modified Claus process can be roughly divided into three basic types: partial combustion method, split-flow method, and direct oxidation method. All of these types consist of a series of vessels such as high-temperature reaction furnaces, condensers, reheating furnaces, and catalytic conversion reactors. The difference between these types lies in the different methods of generating SO2 before the primary catalytic conversion reactor. Based on these three methods, various technical measures such as preheating and supplementary fuel gas supply are added to each of them, resulting in different variants, the general details of which are shown in Table 1. It should be noted that the classification ranges shown in the table are not entirely strict. Table 1: Various process methods and their applicable ranges. H2S content in the feed gas, % (V): Process method – 50–100, 40–50, 25–40, 15–25, <15. Partial combustion method; Partial combustion method with preheating of the feed gas and/or air; Split-flow method; Partial split-flow method with preheating of the feed gas and/or air; Direct oxidation method and other special methods for treating lean acidic gases. The key point is that the heat released from the combustion of H2S in the reaction furnace must be sufficient to maintain a stable flame; otherwise, the equipment will not be able to operate properly. 1.3.1 Partial combustion method: All of the feed gas enters the reaction furnace, while the amount of air supplied is sufficient only to burn 1/3 of the H2S in the feed gas to produce SO2, thereby ensuring the required stoichiometric molecular ratio of H2S:SO2 of 2:1 in the process gas. Although there is no catalyst in the reaction reactor, H2S can still be effectively converted into sulfur vapor. The conversion rate varies depending on the temperature and pressure in the reactor; generally, the conversion rate of H2S within the reactor can reach 60% to 75%. The remaining sulfur compounds will continue to undergo catalytic reactions as shown in reactions (2), (3), and (4) in the catalytic conversion reactor. The operating temperature of the catalytic conversion reactor is generally maintained at 20–30°C above the dew point temperature of the gaseous sulfur in the process gas. In catalytic conversion reactors beyond the second stage, the H2S conversion rate is around 20–30%; therefore, in systems that use partial combustion, the H2S conversion rate can reach 90–98%. Currently, there are several commonly used partial combustion process flows in industry. Figure 3 shows the principle process flow diagram of the externally mixed partial combustion method. The main feature of this process is to draw a stream of high-temperature process gas from the outlet of the waste heat boiler and mix it into the inlet streams of the primary and secondary conversion reactors, in order to reheat the process gas. The advantages of the external mixing process are simple equipment, a compact layout, and flexible temperature control ； The disadvantage is that it is difficult to control the blending at high temperatures, the blending valves suffer severe corrosion, which affects the overall sulfur conversion rate. Figure 4 shows the principle process flow diagram of the internal mixing-heat exchange partial combustion method. The main feature of this process is the combination of the mixing pipe (also known as the internal bypass pipe) and the furnace tubes of the waste heat boiler; the mixing process takes place at the rear of the waste gas boiler. The temperature of the process gas in the primary catalytic conversion reactor is adjusted by varying the opening degree of the outlet valve of the mixing pipe, while the inlet temperature of the secondary catalytic conversion reactor is regulated using a self-heating heat exchanger. The principle of internal blending is the same as that of external blending, so their advantages and disadvantages are similar as well; the only difference is that internal blending requires less space. However, since the mixing pipe is located inside the waste heat boiler, maintenance becomes difficult in the event of a malfunction. Figure 5 shows the partial combustion method using an acidic gas reheater. This process is characterized by the use of a series of reheat furnaces as a means to regulate the temperature of the process gas. The reheat furnace uses acidic gas as fuel; the required amount of air is still determined based on the calculation that 1/3 of the volume of H2S in the incoming acidic gas is converted to SO2. The temperature inside the furnace is controlled by the amount of acidic gas fed in. As for reheating furnaces, there are also various types of them. In addition to acid gas reheating furnaces, common ones include fuel gas reheating furnaces and tubular reheating furnaces. The former uses natural gas or fuel gas as fuel for the reheating furnace, and mixes the burned flue gas with the process gas to regulate its temperature, while the latter regulates the temperature of the process gas through indirect heating using a tubular furnace. 1.3.2 Split-stream method: The split-stream method is recommended when the H2S content in the feed gas is in the range of 25–40% (V). In this method, 1/3 of the volume of H2S in the feed gas is first sent to a high-temperature reactor, where it burns in combination with an appropriate amount of air to produce SO2; this process is shown in reaction (1). The resulting SO2 gas then mixes with the remaining 2/3 of the H2S, and they undergo a low-temperature catalytic reaction in a catalytic conversion reactor. The split-flow process generally employs two-stage catalytic conversion, with a total sulfur conversion rate of approximately 88–92%, making it suitable for smaller sulfur recovery units. Figure 6 shows the principle process flow diagram of the blending-heat exchange type shunt method. A feature of this process is that the two reheating methods mentioned above, namely blending and heat exchange, are applied separately to the split-flow method. 1.3.3 Direct oxidation method: Essentially, the direct oxidation method is a form of the original Claus process. This method is recommended for raw gas with an H2S content in the range of 2–15% (V). The characteristic of the direct oxidation method is that it does not require a high-temperature reaction furnace; instead, the feed gas is preheated to an appropriate temperature, mixed with air, and then directly fed into the catalytic conversion reactor where low-temperature catalytic reactions take place according to equations (1) and (2). The amount of air required is still 1/3 of the volume of H2S needed to complete combustion and produce SO2. Figure 7 shows the principle process flow diagram of the direct oxidation method. Furthermore, depending on whether the acidic gas contains NH3, it can also be divided into an NH3-free combustion process and an NH3-combustion process. If the ratio of the volume of macropores with an NH3 volume concentration of 1 μm to the volume of pores with a diameter > 0.1 μm is as high as 0.7 or more, it reduces the diffusion limitations within the particles and increases the sulfur adsorption capacity. Therefore, compared to conventional Al2O3 catalysts, it allows operation at lower temperatures, enabling the system to achieve higher conversion rates. 5.8 Selection and Use of Catalysts The selection and use of catalysts are directly related to the overall sulfur conversion rate and sulfur recovery rate of the plant. On the condition that both mechanical strength and wear rate meet the requirements for use, catalysts with a large specific surface area and pore volume should also be chosen, in order to maximize the area of active centers and minimize the impact of diffusion resistance on reactant and product molecules. To achieve optimized production of recovered sulfur, from a technical and economic perspective, the most effective strategy is to develop a range of functional catalysts; by utilizing catalytic technology, it is possible to improve the efficiency of the plants while keeping the on-site production conditions essentially unchanged or with only minor variations. This has been confirmed by industrial production practices both domestically and internationally. According to available data, when the H2S content in the acidic gas is also 60% (V), using different catalyst loading schemes can yield unexpected results under the same equipment and process conditions; the details are listed in Table 8. Table 8 Efficiency of Seriated Sulfur Recovery Catalysts* Catalyst Loading Total Sulfur Conversion % Sulfide Content in Exhaust Gas % (V) CR Second Reactor 93.8 1.178 AM·CR Second Reactor 95.5 0.979 CRS-31 Second Reactor 96.7 0.854 CRS-31 Second Reactor with Additional Third Reactor 98 0.468 *Note: The first reactor uses CR Al2O3 catalyst; AM is a Claus process “O2 leakage” protection catalyst, and CRS-31 is a TiO2 catalyst.