Application of Hydrophobic Membranes for Hydrogen Sulfide Removal from Sour Water

Background Hydrogen sulfide (H2S) is a highly toxic and corrosive gas generated as a byproduct in both upstream and downstream operations in the oil & gas industry. If H2S is present in water streams, its removal is generally required before water disposal or reuse. At gas processing facilities in Qatar's North Field, wastewater containing H2S, referred to as sour water, is generated during natural gas conditioning and processing. These sour water streams need to be treated before disposal or reuse and that is currently achieved using conventional sour water strippers. With recent advances in membrane materials and fabrication techniques, there is increasing interest from both academia and industry to adopt membrane-based separation processes. For H2S removal from sour water, hydrophobic membranes contactors are believed to be a cost-effective alternative to conventional sour water strippers. In a membrane contactor, the feed stream containing H2S flows on one side of the hydrophobic membrane while a receiving stream flows on the other side. As opposed to other membrane-based processes (e.g. reverse osmosis, microfiltration), the hydrophobic membrane in this application does not perform any selective separation but instead facilitates mass transfer of H2S gas by providing a large surface area to pass through. The receiving solution acts as a trap which also enhance the driving force required to maximize mass transfer. Membrane contactors have been commercially used for the treatment of natural gas but not for sour water. In the past decade, most published literature focused on removing carbon dioxide (CO2) and H2S from natural or flue gas streams. The removal of dissolved gasses such as oxygen and ammonia from aqueous streams has also been studied with successful commercial products and applications. To the author's knowledge, the application of membrane contactors for H2S removal from sour water has not been investigated, which presents the basis for this innovative research. Objectives The overall objective of this project was to evaluate a novel adaptation of membrane contactors for H2S removal from process water generated by onshore gas processing operations located in Ras Laffan Industrial City. A hollow fiber membrane contactor was used to remove H2S from sour water using sodium hydroxide (NaOH) as a receiving solution. The caustic solution immediately reacted with H2S gas converting it into sodium sulfide according to equation 1: H2S+2NaOH –> Na2S+2H2O (1) The rapid conversion of H2S to sodium sulfide in the receiving solution maximized the H2S concentration gradient and hence gas mass transfer through the membrane. Once the sulfide was trapped, ultraviolet (UV) light was applied to oxidize the sulfide to other sulfur species (thiosulfate and sulfate). This oxidation step enabled the receiving solution to be safely disposed, at the appropriate pH, without the risk of H2S release. Methodology A custom-built bench scale unit was used to evaluate H2S removal rates as a function of the feed sulfide concentration, pH and temperature, as well as the sulfide oxidation rate by UV light. For the experiments, a hollow fiber membrane contactor (Minimodule G543, Liqui-Cel, Membrana, 3M Corp., USA) was selected as the separation module and process water collected Qatari gas processing facilities was used as feed solution. The feed water, pretreated to remove suspended solids, had a pH of 8, H2S concentration of 100 mg/L and a total organic carbon content of 60 mg/L. Experimental data were compared with mass transfer equations developed for gas transfer through microporous membrane. Results Results showed that hydrophobic membrane contactors are effective in removing H2S from sour water. The removal rate is directly proportional to the mass transfer coefficient and sulfide concentration in the feed solution. Feedwater pH had an impact on the mass transfer coefficient as well as the removal rate since the H2S speciation in water is pH dependent. At low pH ( < 4.5) the majority of the sulfide was present as volatile H2S gas, enhancing the mass transfer across the membrane. At pH above 8, the mass transfer rate decreased significantly due to the small amount of volatile H2S in solution. Test results indicated that the mass transfer coefficient was proportional to the sulfide speciation in solution. At pH 4, the measured mass transfer coefficient was 0.243 cm/min while at pH 7 it was 0.090 cm/min for experiments performed with actual process water. Feedwater temperature also impacted the mass transfer rate, with higher rates achieved at higher temperatures since the H2S diffusivity increases with temperature. At 45 oC the mass transfer rate was 0.336 cm/min, while at 25 oC was 0.243 cm/min. UV light oxidation experiments showed that sulfide can be converted first to disulfide and then to thiosulfate and sulfate. The conversion from disulfide to thiosulfate is slow when only UV light is used; however, aerating the receiving solution enhances the conversion rate. When UV light alone is used, the oxidation of sulfide to disulfide follows a first order reaction with a rate of 0.015 min-1. When UV was combined with aeration, the oxidation also followed a first order reaction with a rate of 0.035 min-1 (more than 2X higher compared to UV alone), converting the sulfide to thiosulfate and sulfate. Outcome This is the first evaluation of membrane contactors for the removal of H2S from sour water from oil and gas operations. Qatar can be one of the primary beneficiary of this technology since large volumes of sour water are generated during sour gas conditioning and processing in the North Field. This process can also be applied in other operations worldwide and could have a major impact on how sour water is treated in the petroleum industry. The authors had already filled a patent application on this technology (publication # US2016/0355414) and plan to work with the Qatar Science and Technology Park (QSTP) to identify a possible startup company that may be interested in further developing the technology.