Thread Content
When producing syngas, the feed gas obtained from any type of raw material contains a certain amount of sulfides. Petroleum fractions contain thiols (RSH), thioethers (RSR), sulfur dioxide (CS2), thiophene (C4H4S), etc., which are mostly concentrated in heavy oil fractions, especially residue. Coal often contains organic sulfur (COS) and pyrite. The syngas produced from these raw materials has a low sulfur compound content, but its conversion into hydrogen sulfide and organic sulfur gases can poison catalysts and corrode metal pipes and equipment, posing a serious threat to ammonia synthesis; therefore, these sulfur compounds must be removed and these sulfur resources recovered. There are several methods for desulfurizing feed gas: (1) Dry desulfurization. The method that uses solid absorbents or adsorbents to remove hydrogen sulfide or organic sulfur is known as dry desulfurization. Such desulfurization methods are further divided into adsorption methods and catalytic conversion methods. The adsorption method uses solids with strong adsorption capacity for sulfides to remove sulfur. The adsorbents mainly include zinc oxide, activated carbon, iron oxide, molecular sieves, etc. Dry desulfurization is generally used in situations with low sulfur content; if the sulfur content is high, it leads to frequent regeneration and significantly increased costs. (2) Wet desulfurization: Dry desulfurization achieves a high degree of purification and can remove various organic sulfides, but its operation is intermittent, the equipment required is large, and the regeneration costs are high; it is only suitable for situations where the sulfur content in the gas is low. For gases containing large amounts of hydrogen sulfide, the solution absorption method is commonly used to remove hydrogen sulfide, namely wet flue gas desulfurization. Wet desulfurization agents are liquids, generally used for the desulfurization of gases with high sulfur content and large flow rates. Based on their different desulfurization mechanisms, they are further divided into chemical absorption methods, physical absorption methods, physico-chemical absorption methods, and wet oxidation methods. I. Chemical absorption method The chemical absorption method is a commonly used wet desulfurization process. There is the ethanolamine method (MEA), diethanolamine method (DEA), diglycolamine method (DGA), diisopropylamine method (DIPA), as well as the modified methyldiethanolamine method (MDEA) which has seen rapid development in recent years; MDEA contains accelerants and offers a very high level of purity. II. Physical absorption method The physical absorption method involves using an organic solvent to physically absorb sulfur under certain pressure; subsequently, the pressure is reduced to release the sulfide gases, thereby regenerating the solvent. The main method is the cold methanol process (Rectisol); in addition, there are also the propylene carbonate process (Fluar) and the N-methylpyridinium process (Purisol), etc. III. Physico-chemical absorption method: This involves combining solutions with both physico-absorption and chemico-absorption properties, resulting in a high desulfurization efficiency. The commonly used absorbents are mixtures of sulfolane and alkylol amines (such as methyl diethanolamine, MDEA); the former carries out physical absorption of sulfides, while the latter performs chemical absorption. IV. Wet oxidation method: Its basic principle involves using an alkaline solution containing a catalyst to absorb H2S; the catalyst acts as an oxygen carrier, enabling the oxidation of H2S to elemental sulfur, with the catalyst itself being reduced in the process. Air is passed during regeneration to oxidize and restore the catalyst to its reduced state. Source: Chemical Engineering Technology