(357b) Sensitivity of Desulfurization Cost for Small-Scale Natural Gas Sweetening Units

Natural gas is primarily made up of methane but also contains impurities such as sulfur.^[1] The concentration of sulfur in natural gas can range from several hundred parts per million to several percent, and should be reduced below the sales gas specification limits (typically 4ppm).^[1] Therefore, removing hydrogen sulfide from the natural gas, which is also called gas sweetening, is common practice. There are a variety of gas sweetening processes that are currently used by the industry such as triazine-based adsorption, Fe-Chelate Redox Systems (LO-CAT is investigated in this research), amine-based adsorption, and the Claus process. ^[3]

Triazine-based adsorption is generally used for removing low levels of hydrogen sulfide (<100 ppm) at relatively high gas flow rates (>20 MMSCFD) or high levels of hydrogen sulfide (>100 ppm) at relatively low gas flow rates (<20 MMSCFD).^[3] The use of triazine scavengers is not recommended due to its cost when the hydrogen sulfide levels of sour gas is above 600 ppm.^[3] For LO-CAT, the gas processing capacity varies from 100 to 5000 MMSCFD.^[4] Amine-based adsorption is used for gas streams containing 600 ppm to 700 ppm hydrogen sulfide and limited carbon dioxide amounts.^[5] Claus sulfur recovery plants can decrease the hydrogen sulfide levels to 250 ppm with gas processing capacities greater than 4500 MMSCFD.^[6] Additional sulfur removal processes may be required after amine-based adsorption and Claus sulfur recovery plants because hydrogen sulfide levels of the treated gas may be higher than tail gas specifications at large gas flow rates.^[6] A new small-scale desulfurization process, SourCat,^[7] is under-development by Sour Gas LLC and IntraMicron, Inc., an Auburn University spin-off. The feasibility of natural gas sweetening from distributed small-scale production wells, i.e., from stranded resources, depends on the desulfurization costs of these technologies. Among the desulfurization processes discussed, triazine-based adsorption, LO-CAT and SourCat can be used for small-scale gas sweetening operations.

In this work, a systematic sensitivity analysis is performed to investigate how desulfurization costs of the triazine-based adsorption process, LO-CAT, and SourCat change with changes in inlet sour gas specifications, utility prices and raw material costs at small scales relevant for stranded gas sweeting operations. Sour gas specifications considered are gas flow rate (100 MSCFD – 100 MMSCFD) and hydrogen sulfide concentration (500 ppm – 2000 ppm). Process simulations of these three gas sweetening processes are built using Aspen Plus. Capital and operating costs are calculated using an economic analysis tool called ECON.^[8] Desulfurization costs ($/MSCF natural gas) are then calculated before and after the payback period (which is assumed to be three years). This presentation will discuss in detail the robustness of the desulfurization costs for these three processes with respect to the changes in the inlet conditions, raw material costs and utility prices.

References

1. Madsen, H. T. "Investigation of fouling formation during H2S scavenging with 1, 3, 5-tri-(2-hydroxyetthyl)-hexahydro-s-triazine." Esbjerg Institute of Technology, Aalborg University (2011).
2. Wanek, Rick. "Monitoring H2S to meet new exposure standards." Occupational health & safety (Waco, Tex.) 80.9 (2011): 20-22.
3. Fundamentals of Low-Tonnage Sulfur Removal and Recovery. Echt, B., D. Leppin, D. Mamrosh, D. Miradian, D. Seeger, and B. Warren.February 26, 2017. “Fundamentals of Low-Tonnage Sulfur Removal and Recovery.” Presented at the Laurance Reid Gas Conditioning Conference, Norman, OK.
4. Rouleau, W., – Small Capacity Sulfur Recovery Units for Coal Gasification, Merichem Company, 2011
5. Ankur J., Cameron. Wanted: Efficient Gas Treatment System To Fit Tight FPSO Footprint, HARTENERGY Company, 2015
6. Cheremisinoff, Nicholas P., and Paul E. Rosenfeld. Handbook of Pollution Prevention and Cleaner Production Vol. 1: Best Practices in the Petroleum Industry
7. Catalyzing Commercialization: Sour Gas Has a Sweeter Future. (2017, January). Retrieved from https://www.aiche.org/resources/publications/cep/2017/january/catalyzing-commercialization-sour-gas-has-sweeter-future
8. Kalakul, S., Malakul, P., Siemanond, K. and Gani, R., 2014. Integration of life cycle assessment software with tools for economic and sustainability analyses and process simulation for sustainable process design. Journal of cleaner production, 71, pp.98-109.