Why is sodium hydroxide used for the alkaline washing in ethylene plants?
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During the alkaline washing process in ethylene plants, why is sodium hydroxide solution used to absorb carbon dioxide, hydrogen sulfide, COS, and RSH, rather than methyl-diethanolamine? In hydrocracking units, methyl-diethanolamine is used to absorb hydrogen sulfide from the dry gas; after the amine solution is regenerated, the hydrogen sulfide is sent to the Claus sulfur recovery unit. In ethylene plants, the sulfur-containing waste alkali solution obtained after absorption with sodium hydroxide has a high sulfide content; it is toxic and malodorous, as well as difficult to treat. Only the patented technologies of a few companies such as Siemens and Fushun Petrochemical Research Institute can meet the requirements for treating sulfur-containing waste alkali solutions.
The original poster has confused the requirements for desulfurizing dry gas or light hydrocarbons with those for desulfurization in ethylene cracking. Alcohol amine desulfurization and decarburization is generally used in LPG purification and syngas purification processes, with the amide being reused through continuous regeneration. The sulfide content in this process is generally high. As for the ethylene cracking gas (after rapid cooling at the outlet of the cracking furnace), sodium hydroxide is used as a solvent for desulfurization and deacidification, taking advantage of its strong alkalinity and the ability to absorb sulfides and acidic substances rapidly and thoroughly. During this process, the sulfuride content remains essentially constant, and the amount of material to be processed is large; therefore, a rapid desulfurization process is required, which necessitates the use of sodium hydroxide with high alkalinity. The above are my personal opinions, and not necessarily all of them are correct.
If the hydrogen sulfide content is high, ethanolamine can be used in combination with an alkaline solution, as ethanolamine can be regenerated and reused, making it cost-effective. Since the raw material for ethylene plants has a low sulfur content, alkali washing is generally used, as it is a relatively simple method. Additionally, ethanolamine can absorb butadiene, resulting in a loss of butadiene.
There’s also ethanolamine; desulfurization using it is not particularly effective. Sodium hydroxide achieves cleaner desulfurization. In ethylene plants, even though ethanolamine is used to remove the main amount of sulfur, sodium hydroxide is still necessary for further desulfurization.