HIx concentration by electro-electrodialysis using stacked cells for thermochemical water-splitting IS process

In the thermochemical water-splitting iodine–sulfur process for hydrogen production, efficient concentration/separation of HI from HIx solution, a mixture of HI–H₂O–I₂, is very important. In this paper, an experimental study on concentrating HI in HIx using stacked electro-electrodialysis (EED) cells was carried out under the conditions of 1atm, 80 °C and the current density of 0.10 A/cm². The performance of EED stacks including 1, 2 and 4 EED units was evaluated. The results showed that multi-unit EED cells could concentrate HI in catholyte much faster than single-unit cells. The apparent transport number (t₊) of all the experiments were very close to 1, while the ratio of permeated quantities of water to H⁺ (β) changed in a relatively larger range of 1.98–2.89. Although the current efficiency will degrade faster when using a multi-unit stack than a single-unit cell at the late stage of EED process, at high iodine content multi-unit stack could maintain quite high current efficiency.     

Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production

As a critical subsection in the sulfur-iodine (SI) thermochemical cycle, HI concentration and separation must cope with the pseudo-azeotropy of HIx (HI-I₂-H₂O) and excess iodine in HIx solution. Although electro-electrodialysis (EED) coupled with conventional distillation is a validated method of HI separation from HIx solution in the SI process, the iodine deposition, resulting from changes in temperature and HI molality in HIx solution, can lead clogged flow channels of the EED anode and other tubes. A precipitator can address this problem by recovering excess iodine from HIx solution after the HIx purification column. The energy duty and required input flow rate per mole HI was investigated in this study using a process flowsheet simulation. A decrease in iodine concentration in the streams to EED could reduce cell duty effectively. An increase in HI molality in the EED cathode outlet resulted in an increase in EED duty; however, the amplitude was slight. The iodine molar concentration in the feed of the distillation column exhibited an appreciable influence on the distillation duty. However, with an increase in distillation column pressure, the effect of diminished iodine in feed on the HI distillate duty continued to decline. To assess the utilization of an iodine precipitator in the HI separation subsection, the energy demands and required input flow rates of three different flowsheets were calculated using Aspen Plus and Microsoft Office Excel. Results indicated that the flowsheet that only recovered iodine in the stream to the EED anode chamber exhibited the least HI separation duty and the lowest required input flow rate.     

Evaluation on the electro-electrodialysis stacks for hydrogen iodide concentrating in iodine–sulphur cycle

In thermochemical water-splitting iodine–sulfur cycle, concentrating hydrogen iodine in the HI–H₂O–I₂ solution is crucial for the efficient hydrogen production. Electro-electrodialysis (EED) is a very promising HI concentrating method. In this paper, EED experiments were carried out using stacked cells, aiming at the scale up of EED equipment. Compared with the single-unit EED cell, the multi-cell EED stacks could concentrate HI in catholyte much more rapidly. During the EED process, the cell voltages increased gradually with the expansion of the concentration difference between catholyte and anolyte. For the stacks with more EED cells, the voltage increased much more steeply. High operating temperature ensured EED process carried out under low cell voltages and avoided voltage swelling. The apparent transport number (t₊) of all the experiments were very close to 1, while the ratio of permeated quantities of water to H⁺ (β) changed in a range of 1.79–3.05, influenced by temperature, I₂ content and current density.     

Energy requirement of HI separation from HI–I2–H2O mixture using electro-electrodialysis and distillation

The separation of HI from HI–I₂–H₂O mixture is an essential subsection of the Iodine–Sulfur (IS) process for thermochemical hydrogen production. The energy requirement of the separation determines, to a large extent, the hydrogen production efficiency of the IS process. In order to examine duty of the separation using electro-electrodialysis (EED) and distillation, a process simulation study was carried out using an analytical model of EED based on ideal membrane properties and properties of the reported EED experiments using a Nafion^® membrane and graphite electrodes. For both of the ideal-membrane case and Nafion-membrane case, effects of the operating parameters on heat duty were estimated, which comprised column pressure, HI molality in the column feed, and the flow rate ratio of the input from Bunsen section to distillate rate. Low column pressure, and high HI molality in the column feed were preferable for the ideal-membrane case; column pressure of 1.0 MPa and optimized HI molality in the column feed were desired for the Nafion-membrane case. The flow rate ratio had little effect on the minimum heat duty in the ideal-membrane case; a value in the vicinity of the lower limit of the flow rate ratio was optimal for the Nafion-membrane case. The difference of the inclination of parameters resulted from the fractional vaporization of the column feed in the ideal-membrane case and weight of the EED cell duty on the total duty due to the membrane voltage drop. The optimization of these parameters was also carried out. The minimum total heat duty of the Nafion-membrane case was 3.07 × 10⁵ J/mol-HI, and that of the ideal-membrane case was 12.5% of this value.     

Effect of sulfuric acid on electro-electrodialysis of HIx solution

The effect of sulfuric acid on the concentration of HIx solution by electro-electrodialysis (EED) was examined for the thermochemical water-splitting iodine–sulfur process. Presence of sulfuric acid in the anolyte HIx solution did not affect the concentration behavior. However, sulfuric acid in the catholyte solution caused side reaction(s) producing whitish precipitates, which indicates that the sulfur compound should be removed prior to the EED operation.     

Energy consumption in iodine section of I–S: Effect of operating parameters of electro-electrodialysis on overall efficiency of cycle

Reducing heat demand for increasing concentration of HI in the HI_x solution of the iodine circuit of the Iodine–Sulphur cycle is considered the most effective way of increasing efficiency of the cycle. Electro-electrodialysis has emerged as an energy efficient way of increasing the HI_x concentration above azeotropic value. Simulation of the iodine circuit consisting of an EED, a flash and a decomposer was carried out in Aspen Plus™ simulation platform to study the effect of EED current density and outlet HI concentration on the efficiency of the cycle. Efficiency reduced strongly with increase in current density. For EED current density of 5 A/dm², maximum efficiency was ∼35.9% and the optimal range of EED catholyte's exit HI concentration, iodine-free, mole fraction was 0.19–0.21. Simulation results showed that reducing EED resistance was most effective, among all EED parameters, in increasing the cycle's thermal efficiency and if the EED resistance is completely eliminated the thermal efficiency value would increase to 39.4%.     

Electrochemical investigation of the Bunsen reaction in the sulfur–iodine cycle

The Bunsen reaction, as a part of the sulfur–iodine thermochemical cycle, was studied using an electrochemical cell. The effects of current density, operating temperature, H₂SO₄ concentration in the anolyte, HI concentration and I₂/HI molar ratio in the catholyte were explored. Both the H₂SO₄ in anolyte and the HI in catholyte were concentrated during electrolysis. Increasing current density amplified this H₂SO₄ and HI concentration, while the other operating parameters also varied the anolyte and catholyte concentration. The transport properties of the cation exchange membrane were examined. The electrode current efficiency remained close to 100% for most runs except those at high current density. Both the average cell voltage and the heat equivalent of electric energy were determined at different conditions.     

Effect of operating variables on performance of membrane electrolysis cell for carrying out Bunsen reaction of I–S cycle

Membrane electrolysis is coming up as one of the alternatives to direct contact mode of carrying out Bunsen reaction of I–S cycle. It has potential to reduce the use of excess iodine and water. A two-compartment membrane electrolysis cell with graphite electrodes and Nafion 117 membrane was used for Bunsen reaction. Effect of six independent variables on cell voltage was determined for current density values of up to 80 A/dm². The variables were anolyte pressure, catholyte pressure, temperature, sulphuric acid concentration, HI concentration, and I₂/HI molar ratio in catholyte. Flow rate of anolyte and catholyte were identified where mass transfer resistance was not significant before performing experiments with different independent variables. Cell voltage was analysed by identifying three different regimes based on its variation with current density and current density ranges where electrode resistance or ohmic resistance dominated are identified. Current efficiency was measured for 1 A/dm² and was found to be close to 100% irrespective of values of the independent variable. Minimum amount of heat equivalent of electric energy required for membrane electrolysis was calculated and increase in its value with increase in sulphuric acid concentration was compared with estimate of reduction in heat required for concentration of sulphuric acid.     

Modeling and experimental assessment of the novel HI-I2-H2O electrolysis for hydrogen generation in the sulfur-iodine cycle

To improve the sulfur-iodine (SI or IS) cycle for renewable hydrogen production, direct electrolysis of HIx solution (HI-I₂-H₂O) from Bunsen reaction has been recently proposed. This work concerns the detailed microscopic physical performance and electrolytic processes of HIx electrolysis through theoretical simulation and experimental exploration. A two-dimensional mathematical model of the electrolytic cell for HIx electrolysis was developed, and was verified by the relevance between the simulated and experimental hydrogen production. The concentration, electric and flow field distributions were characterized. The decrease of anodic HI and increase of cathodic H₂ were found along the flow direction. The local potential distribution declined from anode to cathode region. Higher pressure drop from the inlet to outlet of channel as well as the pumping power were required for anolyte than catholyte. More detailed electrolytic processes along the flow channel at different operating conditions were analyzed. Increasing temperature from 303 K to 343 K improved the H₂ production rate. A low anode flow rate of 0.2 m/s was favorable for achieving high conversion rate of reactants and low consumed pumping power. The developed models and the clarified characteristics of HIx electrolysis will be further applied to the improved SI cycle.     

Membrane electrolysis of Bunsen reaction in the iodine–sulphur process for hydrogen production

In traditional IS process for production of hydrogen by water decomposition, the Bunsen reaction (SO₂ + I₂ + 2H₂O → H₂SO₄ + 2HI) was carried out by direct contact of SO₂ with aqueous solution of I₂ where a large excess of I₂ (8 mol) and H₂O (16 mol) were required. Excess amounts of these chemicals severely affected the overall thermal efficiency of the process and new ways including membrane electrolysis was reported in literature for carrying out Bunsen reaction where the amount of excess chemicals can be greatly reduced. We have carried out Bunsen reaction in a two-compartment membrane electrolysis cell containing graphite electrodes and Nafion 117 membrane as a separator between the two-compartments. Electrolysis was carried out at room temperature with continuous recirculation of anolyte and catholyte. Electrolysis was done in constant-current mode with current density in the range of 1.6 A/dm² to 4.8 A/dm². Initial concentrations of H₂SO₄ and HI were about 10 and 5 N, respectively and I₂/HI molar ratio in the catholyte was varied in the range of 0.25–1.5. Current efficiency was found to be close to 100% indicating absence of any side reaction at the electrodes. Cell voltage was found to vary linearly with current densities up to 80 A/dm² and for I₂/HI molar ratio in the range of 0.25–1.5 the cell voltage was found to be lowest for the value of 0.5.     

Effect of asymmetric variation of operating parameters on EED cell for HI concentration in I–S cycle for hydrogen production

EED process for HI concentration was studied for the effect of individual operating parameters such as I₂/HI ratio, concentration of HI(xHI_/H₂O)$\text{HI}\left(x_{\text{HI}/{\text{H}}_2\text{O}}\right)$, temperature and pressure. Studies were conducted in an asymmetric system where the effects of operating parameters were varied for anolyte and the catholyte separately. Open circuit voltage (OCV) was found to be a contributor toward the net potential drop across the EED cell. Ohmic resistance was found to decrease with increase in I₂/HI ratio in catholyte and was found to increase with increase in I₂/HI ratio in anolyte. Increase in xHI_/H₂O$x_{\text{HI}/{\text{H}}_2\text{O}}$ decreased the resistance for anolyte section whereas caused an increase in resistance for catholyte section. Increase in temperature reduced the voltage drop and the resistance across the EED cell. A non-zero differential pressure between the two compartments of the cell increased the resistance across the cell without affecting the OCV value. Electrode potential studies at the graphite electrodes showed an increase in the electro potential with increase in the iodine concentration and decrease with the increase in the HI concentration. Energy required for concentrating acid increased linearly with current density favoring operation at low current densities. Energy consumed in overcoming OCV contributed substantial fraction of the total energy consumed in EED process at lower current densities.     

Experimental study of the vapour–liquid equilibria of HI–I2–H2O ternary mixtures,Part 1: Experimental results around the atmospheric pressure

In the framework of the massive production of hydrogen using the sulfur–iodine thermochemical cycle, the design of the reactive distillation column, chosen by CEA for the HIx section, requires the knowledge of the partial pressures of the gaseous species (HI, I₂, H₂O) in thermodynamic equilibrium with the liquid phase of the HI–I₂–H₂O ternary mixture in a wide range of concentrations up to 270 °C and 50 bar. In the first of these two companion papers, we describe the experimental device which enables the measurement of the total pressure and concentrations of the vapour phase (and thus the knowledge of the partial pressures of the different gaseous species) for the HI–I₂–H₂O mixture in the 20–140 °C range and up to 2 bar. This device is used to carry out a large set of experiments investigating various mixtures with optical on-line diagnostics (FTIR for HI and H₂O, UV–visible for I₂). This leads to the determination of the concentrations in the vapour phase for many experimental conditions, results of which are given in this paper. The companion paper (part 2) describes the experimental device which enables measurements of the total pressure and species concentrations in the vapour phase in the process domain.     

Influence of the initial HI on the multiphase Bunsen reaction in the sulfur–iodine thermochemical cycle

In the sulfur–iodine cycle flowsheet, HI may exist in the feeds of Bunsen reaction. The effects of the initial HI and the operating temperature on the kinetic process and thermodynamic equilibrium of the multiphase Bunsen reaction were investigated. Increasing initial HI concentration (HI/H₂O = 0–1/18) or temperature (303 K–358 K) amplified the reaction kinetic rate, and led to the earlier appearance of liquid–liquid separation and less time to reach the thermodynamic equilibrium. But the separation became difficult for further increase of the initial HI content. The liquid–liquid equilibrium (LLE) phase separation was enhanced with rising temperature. An increase in the initial HI content slightly weakened the LLE phase separation at a lower temperature, while at 345 K and 358 K, the LLE phase separation characteristics showed little variation in the HI/H₂O molar ratio range of 0–1/18. A hyper-azeotropic HI concentration in the HIx phase was obtained with feeding HI. The conversion of SO₂ lowered as the initial HI content and the temperature increased.     

Demonstration of the I–S thermochemical cycle feasibility by experimentally validating the over-azeotropic condition in the hydroiodic acid phase of the Bunsen process

Conventional I–S cycle flowsheets suffer from low thermal efficiency and highly corrosive streams. To alleviate these problems, KAIST has proposed the optimal operating condition for the Bunsen reaction and devised a new flowsheet that produces highly enriched HI through spontaneous L–L phase separation and simple flash processes under low pressure. A series of experiments were performed at KAIST to validate the new flowsheet and extend its feasibility. The experimental procedure, measurement method with a rich iodine condition, and results of experiments are discussed in this paper. When the molar ratio of I₂/H₂SO₄ in the feed increased from 2 to 4, the molar ratio of HI/(HI + H₂O) in the HIx phase improved from 0.157 to 0.22, which is high enough to generate highly enriched HI gas through flashing. An inverse Bunsen reaction and a sulfur formation were observed when the temperature was increased from 313 K to 343 K and the molar ratio of I₂/H₂SO₄ was decreased from 4 to 1. 10–50 wt% of HI in the feed turned into I₂ when an inverse Bunsen reaction and a sulfur formation occurred. The experimental data utilized in the previous parametric study of KAIST has been validated.     

A development of direct hydrazine/hydrogen peroxide fuel cell

A direct hydrazine fuel cell using H₂O₂ as the oxidizer has been developed. The N₂H₄/H₂O₂ fuel cell is assembled by using Ni-Pt/C composite catalyst as the anode catalyst, Au/C as the cathode catalyst, and Nafion membrane as the electrolyte. Both anolyte and catholyte show significant influences on cell voltage and cell performance. The open-circuit voltage of the N₂H₄/H₂O₂ fuel cell reaches up to 1.75 V when using alkaline N₂H₄ solution as the anolyte and acidic H₂O₂ solution as the catholyte. A maximum power density of 1.02 W cm⁻² has been achieved at operation temperature of 80 °C. The number of electrons exchanged in the H₂O₂ reduction reaction on Au/C catalyst is 2.     

Thermal efficiency evaluation of HI synthesis/concentration procedures in the thermochemical water splitting IS process

Effects of operation parameters of HI synthesis and concentration procedures on thermal efficiency in the thermochemical hydrogen production IS process were investigated based on heat/mass balance. Concentration of HI was carried out by an electro-electrodialysis (EED) cell. Transport number of proton, electroosmosis coefficient and ratio of the flow rates at the inlets of the EED cell were considered as parameters of the EED cell in different over potential and temperature difference at the heat exchangers. The parameters of the EED cell had little effect on the thermal efficiency at optimized over potential and temperature difference. The relation between these parameters and thermal efficiency was dependent on over potential or temperature difference. These parameters should be optimized by a heat/mass evaluation. Composition of the solution produced at the HI synthesis procedure influenced thermal efficiency significantly. High thermal efficiency was expected at low I₂ concentration and high HI concentration.     

A thermo-physical model for hydrogen–iodide vapor–liquid equilibrium and decomposition behavior in the iodine–sulfur thermo-chemical water splitting cycle

We developed an improved thermo-physical model for the hydrogen–iodide (HI) VLE and decomposition behavior in the iodine–sulfur (IS) cycle. We reproduced the Neumann's modified NRTL model by an optimization scheme from the experimental data that he used. Then, we improved the model by correcting his unphysical assumption for the non-randomness parameter, and used the two-step equilibrium approach for the HI decomposition modeling. The KAIST model (improved Neumann's model) with a new set of binary parameters predicts the total pressure of HI solution within 2.98% of RMS error for the HI–H₂O binary mixture, and within 7% of RMS error for the HI–H₂O–I₂ ternary mixture. We also proposed a simple logic to predict the liquid–liquid phase separation condition within 2.07% of relative deviation in the temperature range of 70–149 °C.     

硫碘热化学循环水分解制氢流程优化与模拟

选择大型化工流程模拟软件Aspen Plus对硫碘循环制氢系统进行流程优化设计与模拟,计算系统的质量平衡、能量平衡,并对系统热效率进行评估.碘化氢相中HI浓度采用本生反应实验中的过恒沸浓度,避免高能耗电渗析的使用,从而大大提高系统效率.在不考虑废热发电情况下,与文献值56.8％相比,系统产氢热效率高达68.46％.     

Experimental density-composition data and thermal expansion coefficient of HI-I2-H2O solution

In thermochemical water-splitting iodine-sulfur cycle for hydrogen production, basic physico-chemical data of HI-H₂O-I₂ (HIx) solution are very important. Detailed and systematic studies on density/coefficient of thermal expansion (CTE) are in great need. In this work, the density values of 53 HIx samples with different compositions are measured at atmospheric pressure and temperatures ranging from 20 to 90 °C. HIx solution's density varies dramatically when changing HI or I₂ contents. Increasing either HI or I₂ concentration will cause increase of density. When heated, HIx's density decreases because of thermal expansion. With the help of density-temperature curves, the CTE values are calculated for HIx solutions of different compositions. It is found that increase of either I₂/HI or H₂O/HI will bring rise of HIx's CTE. Although the CTE value is relatively small, it is very sensitive to the change of composition. In the range of this work, the HIx's CTE value changes within 5.45E-4/ºC to 9.17E-4/ºC. Polynomial regression is conducted to model the relationship between CTE and the composition. The obtained approximate quadratic polynomial model has good accuracy to reproduce most of the experimental CTE values within a deviation of ±5%.