Committee Members Information

• Ph.D. Advisor: Professor Mani Sarathy (KAUST Chemical Engineering)
• External Examiner: Professor Meihong Wang (University of Sheffield, Process and Energy System)
• Committee Chair: Professor Luigi Cavallo (KAUST Chemistry)
• 4th Committee Member: Professor Pedro Castano (KAUST Chemical Engineering)

Abstract

Hydrogen Production from Hydrogen Sulfide with Deep Hydrogen sulfide (H2S) is a hazardous gas primarily produced in the oil and gas industry. Traditionally, it is thermally transformed via the Claus process into solid sulfur. However, the drawbacks of Claus and the need for low-carbon methods of hydrogen (H2) production have led to the recognition of H2S as a hydrogen carrier. In this thesis, the H2S-Iodine (H2S-I2) thermochemical cycle is studied as a novel method for H2S transformation and hydrogen production, based on the reaction of H2S with I2 in aqueous media, forming hydrogen iodide, an intermediate in H2 production. The cycle is characterized by two stepwise mechanisms that depend on the concentration regime: at high H2S concentrations (>1000 ppm), sulfur is a by-product; at low H2S concentrations (<30 ppm), sulfur dioxide is a by-product. Using quantum chemistry calculations, both mechanisms were elucidated in the gas and liquid phases, revealing highly thermodynamically and kinetically hindered rate-limiting steps. On this basis, three deep eutectic solvents (DESs) were selected and evaluated as potential catalysts for the H2S-I2 cycle through gas-phase conformational exploration, including the effect of aqueous media as single-point energy calculations. The results showed that the presence of hydroxyl functional groups and the flexibility of ethylene glycol (EG) fragments in the ChCl:2EG DES enabled the formation of unique covalent interactions, thereby substantially reducing the energy requirements of the rate-limiting step. Later, liquid-phase optimizations, including the ChCl:2EG DES, revealed multiple pathways by which the mechanisms can develop. Finally, the H2S-I2 cycle was simulated, with and without the ChCl:2EG solvent, at an industrial scale for both concentration regimes. In general, the DES reduced water recirculation within the system, thus lowering heating and cooling requirements and improving HI decomposition. The simulations were used in a techno-economic assessment to estimate capital and operational costs, as well as the levelized cost of hydrogen (LCOH) and gas treatment (LCOH2S). As a result, the cost of treating sour gas (60%mol) is 0.09 USD/kgH2S, and the LCOH is 3.02 USD/kgH2, indicating that the process is competitive to green hydrogen production methods. In contrast, treating sour natural gas (10 ppm H2S) via the ChCl:2EG-mediated cycle demonstrates a suitable technology for low-temperature gas sweetening.