(443d) Thermodynamic Modeling of the Hybrid Sulfur (HyS) Cycle for Hydrogen Production

The hybrid sulfur (HyS) cycle has received much attention over the past few decades as a possible means of efficient large-scale production of clean hydrogen from high-temperature nuclear or solar heat sources [1]. A typical HyS cycle consists of a thermochemical step and an electrochemical step, the net result of which is the splitting of water into hydrogen and oxygen. In order to fully understand the limiting HyS heat and electric power requirements and to optimize the operating and design parameters, it is important to perform a thermodynamic analysis of the highly nonideal aqueous SO₂-H₂SO₄ system. Therefore, we develop a comprehensive thermodynamic model for the sulfuric acid-sulfur dioxide-water ternary system based on the symmetric electrolyte NRTL (eNRTL) [2] activity coefficient model framework in conjunction with the PC-SAFT [3] equation-of-state. This model is an extension of the previously published thermodynamic model of the sulfuric acid-water [4] binary system and additionally considers one step hydration of sulfur dioxide molecules. The eNRTL binary interaction parameters and the chemical equilibrium constants are regressed from the experimental SO₂solubility data in aqueous sulfuric acid solutions and the ternary model is validated over a wide temperature range from 283.15 to 383.15 K, and acid concentrations from pure water to pure acid. The model should be very useful in supporting process research, development, and design of the advanced direct solar water splitting process based on the HyS cycle.

Keywords:

Hybrid-Sulfur (HyS) cycle, Electrolyte NRTL, Hydrogen, SO₂ solubility, Sulfuric Acid

References:

[1] Gorensek MB, Summers WA, â??Hybrid sulfur flowsheets using PEM electrolysis and a bayonet decomposition reactor,â? Int. J. Hydrogen Energy(2009) 34, 4097-4114.

[2] Song Y, Chen C-C, â??Symmetric electrolyte nonrandom two-liquid activity coefficient model,â? Industrial & Engineering Chemistry Research(2009) 48, 7788â??7797.

[3] Gross J, Sadowski, G, â??Application of the Perturbed-Chain SAFT Equation of State to Associating Systems,â? Ind. Eng. Chem. Res. (2002) 41, 5510-5515.

[4] Que H, Song Y, Chen C-C, â??Thermodynamic Modeling of the Sulfuric Acid-Water-Sulfur Trioxide System with the Symmetric Electrolyte NRTL Model,â? J Chem Eng Data (2011) 56, 963-977.