Introduction to the Claus process for sulfur removal—suitable for beginners

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For sulfur recovery, the method currently used to extract sulfur from refinery acidic gases is primarily the Claus process. The main reactions involved are: 2H2S + O2 → 2H2O + 2/xSX (1), 2H2S + 3O2 → 2SO2 + 2H2O (2), and SO2 + 2H2S → 3/xSX + 2H2O (3). Depending on the level of H2S content in the acidic gases, there are three variants of the Claus process available: partial combustion, split-flow, and direct oxidation; among these, the partial oxidation method is the most commonly used at present. The principle and process flow of the Claus process for sulfur recovery are shown in the figure below. Principle and process flow of the Claus process for sulfur recovery 1. Combustion furnace ； 2. Waste heat boiler ； 3. Primary converter ； 4. Secondary converter ； 5. Separator ； 6. In the alkali scrubber tower, the acidic gas coming from the desulfurization unit undergoes partial combustion with an appropriate amount of air inside the furnace, where reactions (1) and (2) take place. The amount of air used is sufficient to oxidize 1/3 of the H2S in the acidic gas to SO2; this SO2 then enters the converter along with the unoxidized H2S, where catalytic conversion (3) occurs. For the partial combustion of H2S, the amount of air fed into the combustion furnace must be strictly controlled; this is a key aspect of the Claus process. The temperature in the combustion furnace is around 1200°C, and in addition to SO2, H2O, and N2, the combustion products also contain a small amount of elemental sulfur generated directly from the decomposition of H2S. To recover heat, the combustion products are first converted into steam in a waste heat boiler before entering the converter. The converter is equipped with a natural bauxite or synthetic alumina catalyst. Reaction (3) is a reversible exothermic reaction; therefore, reducing the reaction temperature is beneficial for increasing the equilibrium conversion rate. However, its temperature must be at least 30°C higher than the dew point of sulfur vapor to prevent sulfur from depositing on the catalyst surface. The inlet temperature of the converter is generally around 230–280°C; due to the heat release during the process, the outlet temperature rises to about 270–300°C. The reactant exiting the converter is cooled by condensation to yield sulfur. To achieve a higher sulfur recovery rate, industrial plants typically also have secondary, tertiary, or even quaternary converters. With two-stage conversion, the sulfur recovery rate is 93%–95%, 94%–96% with three-stage conversion, and can reach 95%–97% with four-stage conversion. It can be seen that the more catalytic conversion stages there are, the higher the overall conversion rate, but the equipment investment also increases accordingly; a comprehensive consideration is necessary. The purity of sulfur recovered by this method is approximately 99.8%. When the H2S content in the feed gas is between 15% and 50%, if the partial combustion method is also used, the reaction heat is insufficient to maintain the high operating temperature required for conversion in the combustion furnace; therefore, the split-flow method is preferable. One-third of the acidic gases are introduced into the combustion furnace, and air is supplied (with a H2S/O2 ratio of 1) to convert all the H2S there into SO2; then the remaining two-thirds of the acidic gases are fed at the furnace outlet for catalytic conversion. When the H2S content in acidic gases is below 15%, the feed gas cannot burn properly; it must therefore be preheated to a certain temperature in a heating furnace along with an adequate amount of air, and then fed directly into the reactor where H2S and O2 are catalytically converted into sulfur.