At what level of H2S is cyclic hydrogen desulfurization generally carried out?
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When the H₂S content in the recycled hydrogen exceeds 2%, considering the installation of a desulfurization unit for the recycled hydrogen is necessary. This indicator is a common standard in the industry; it is designed to prevent a decrease in hydrogen partial pressure caused by high hydrogen sulfide concentrations, to inhibit desulfurization and aromatic saturation reactions, and to reduce the risk of corrosion to equipment. Furthermore, to protect catalyst activity, the H₂S concentration in the recycled hydrogen should generally not be lower than 1000 ppm (i.e., 0.1%), in order to prevent the catalyst from becoming reduced and deactivated. Therefore, in practical operations, the H₂S concentration must be kept within a reasonable range; the target is generally 500–1000 ppm, but the specific value may vary depending on the plant design, the sulfur content of the feedstock, and the type of catalyst.
The original poster explained quite clearly the range of H₂S concentration control for circulating hydrogen desulfurization; here are two points from practical experience for reference: 1) In our plant, fluctuations in the sulfur content of the feedstock caused the H₂S level to exceed 3%. When we temporarily activated the desulfurization tower, we observed a significant increase in pressure drop, so it is advisable to assess the processing capacity of the equipment in advance； 2) Different catalyst suppliers may have varying requirements regarding the minimum H₂S concentration; for example, brand A that we use requires a level of over 800 ppm, while brand B only requires 500 ppm. The specific requirement should be determined in accordance with the manufacturer’s technical specifications. It is better to have the process engineer conduct a sensitivity analysis for these parameters.