The effects of SO2 flow rate, SO2 mole fraction, iodine content and water content on the Bunsen reaction were experimentally studied for the thermochemical iodine–sulfur cycle for hydrogen production. One multistage reaction mechanism on the Bunsen reaction has been hypothesized, considering the kinetic characteristics. The appearance of a liquid–liquid equilibrium (LLE) phase separation was in the range of 20–40 min in the apparatus we chose. An increase in iodine content and a decrease in water content improved LLE separation characteristics to the H2SO4 phase and HIx phase. The H2S formation side reaction easily occurred with the increase in water content and decrease in iodine content. The optimal operating condition that the inlet SO2 mole fraction exceeds 0.12 coupled with initial I2/H2O molar ratio exceeding 0.284 has been proposed based on the experimental results, for the criteria of improving the separation characteristics and preventing side reactions.
Ke Zhang,Xiaoling Li,Liping Chang,Weiren Bao,Hui Wang
Hydrogen sulfide (H 2 S) emitted from oil industry's hydrotreating processes can be converted into hydrogen and used back to the same processes through a H 2 S splitting cycle, where the Bunsen reaction and HI decomposition are two participating reactions. To overcome the difficulties and complications posted in the scaling up of the cycle, direct electrolysis of the Bunsen reaction product solution was proposed and has been studied in a batch electrolysis cell in our earlier work. This paper studies the direct electrolysis using a customer-made, continuous-flow electrolysis cell. The effects of the operating parameters including the current density , the entering HI concentration and flow rate of the anolyte, the toluene to aqueous phase ratio and stirring speed in anolyte cell, the H 2 SO 4 concentration and circulation rate of the catholyte on the performing parameters such as the conversion of iodide ions, the yield of iodine transferred to toluene, and the anodic and cathodic current efficiencies for iodide conversion and hydrogen production were carefully investigated. The results show that the cathodic current efficiency for hydrogen production is nearly 100% for all the runs and that the anodic current efficiency for iodide ion conversion to iodine is relatively low (20%–70%) and varies with the changes in operating parameters. Running at high levels of the current density, the volumetric ratio of toluene to aqueous phase in anolyte, or the stirring speed in anolyte, and low levels of the entering concentration of I − in anolyte or the flow rate of anolyte in electrolysis operation are in favor of having a high iodide conversion and high I 2 -toluene yield. Iodide anions at a few mmol L −1 level (a few thousandths of the entering concentration) are found in the cathodic chamber caused by its diffuse against the electric field and the proton exchange membrane . The continuous, direct electrolysis of the Bunsen product solution can be considered being adapted in the sulfur-iodine (S–I) water splitting cycle for hydrogen production.
Biyi Huang,Yanqun Zhu,Yong He,Shaojie Xu,Yanwei Zhang,Zhihua Wang
Bunsen reaction is the initial step of the sulfur-iodine (SI) cycle for hydrogen production , and it has a significant effect on the subsequent reactions. Compared with traditional methods, electrochemical Bunsen reaction is more advantageous for SI cycle as it requires less amounts of iodine for the reaction. In this method, the two acid phases are generated separately in two chambers of the electrolytic cell . In a typical electrochemical Bunsen method, sulfur dioxide is oxidized at the anode to form sulfuric acid while iodine is reduced at the cathode to generate hydrogen iodide . However, the high cell voltage can lead to high energy consumption. In this work, two Catalyst Coated Membranes (CCMs) were synthesized to study the effects of Pt/C catalysts on the electrolytic reaction with a two-chamber membrane electrolyzer . The properties of the membranes were characterized by constant current electrolysis curves, polarization curves , cyclic voltammetry method and electrochemical impedance spectroscopy (EIS). All the electrochemical measurements were carried out at 298 K. At the current density of 50 mA/cm 2 , the average cell voltage with commercial Nafion 115 membrane was 1.37 V, while the cell voltage with the home-made CCMs was as low as 0.5 V. With the help of Pt/C catalyst coated on the membrane, a significant reduction in charge transfer resistance ( R ct ) was observed according to the Nyquist plots . The titration results indicated that the permeation decreased with the increase in catalyst loadings. Furthermore, the current efficiency increased by 54.23% (from 63.16% to 97.41%) and the energy required to produce 1 mol hydrogen reduced by 75.68% (from 1084.546 kJ to 263.785 kJ).
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