The regeneration of CG-4 hydrogen sulfide adsorbent by ammonia leaching

Abstract

CG-4 H2S adsorbent, an iron oxide based solid, has high sulfur capacity (20-25% w/w) and thus finds favor in users’ eyes in North America. This product, imported from CLEAN Catalysis and Purification Technologies Development Company in Shanxi Province, China, is now being used in gas processing companies in Alberta, Canada and Texas, USA. However, due to the elemental sulfur deposition on the adsorbent, the recovered sulfur capacity by regeneration is only about 1/3 as that of fresh adsorbent. This limits the adsorbent use to be once, which results in higher operating cost due to the frequent changeover and cost for landfills. The problem of sulfur deposition is also the limitation to the utilization and regeneration of other desulfurization adsorbent or catalyst. This study developed a process to recover the sulfur capacity of CG-4 adsorbent by ammonia leaching to remove elemental sulfur. The leaching was conducted in a stainless steel cylindrical reactor at room temperature and a pressure higher than the vapor pressure of liquid ammonia. The leaching process does not deleteriously change the physical strength, but improve the properties of surface area, pore volume and pore size distribution. The new regeneration process is able to recover over 90% sulfur capacity in the first adsorption-regeneration cycle. The sulfur capacity recovery declined when CG-4 had been leached for more than one time. Nonetheless, even after the third time leaching, the sulfur capacity was recovered by 60%. The conditions of leaching process were optimized in a laboratory-scale experiment. Additionally, the elemental sulfur collected from leaching process has 91.5% w/w purity and can likely be used as an additive to asphalt or used as a soil amendment for agricultural applications. The separation of solid wash-offs and liquid ammonia was simply fulfilled by depressurizing the leaching vessel and vaporizing the ammonia. CG-4 adsorbent is verified capable of at least three times reuses, which results in 60% reduction in disposal amount per unit H2S being treated. This not only reduces the cost in disposal to landfills but also the cost in CG-4 adsorbent and brings the revenue from the recovered elemental sulfur. The vapor ammonia is recommended to be recycled and reused by compressing it back to liquid.