HI Molality Research Articles

Cation exchange membrane based on side chain grafted sulfonic acid with poly(vinylidene fluoride-co-hexafluoropropylene) for electro-electrodialysis of HIx mixture

Acid and oxidative resistant sulphonated copolymer cation exchange membrane (CEM) was prepared and electro-electrodialysis experiments were performed to improve HI molality in the HIx (HI + I2) mixture. Reported membrane was prepared by chemical grafting of 2-methyl-2-(N-(3-sulfopropyl)acrylamido)propane-1-sulfonic acid with dehydofluorinated poly(vinylidene fluoride-cohexafluoropropylene) in presence of radical initiator. Different CEMs were assessed by their morphology, physicochemical properties, and stabilities in compare to Nafion117 membrane. Most suitable PVSU-1.72 membrane showed superior proton conductivity (5.15 × 10−2 S/cm) and ion exchange capacity (1.72 m eq./g) values in comparison to other acid resistant membranes reported in the literature. Further, PVSU-1.72 membrane consumes low electrical power (65.5 kJ/mol) in comparison to Nafion 117 membrane (156.0 kJ/mol) during electro-electrodialysis. Also in previous case, close to 100% current efficiency (ηc and ηa) demonstrates membrane suitability. Further, PVSU-1.72 consumes about 290 kJ mol−1-H2 energy to produce 1 mol H2, while Nafion117 membrane consumes 280.80 kJ/mol-H2 energy. These observations is helpful for devising high performance electro-electrodialysis process to develop the HI concentration as H2 source.

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.

Modeling and validation of the iodine‐sulfur hydrogen production process

The iodine‐sulfur thermochemical water‐splitting cycle (I‐S process) is one of the highly efficient, CO2‐free, massive hydrogen production methods. We simulated the I‐S process through commercial software programs Aspen Plus and OLI database with the aid of self‐developed models to analyze the overall running status of the process and to decrease the investment and time consumption of experiments. A two‐phase separator model operating at 353 K and an electro‐electrodialysis (EED) cell model working at 338 K were built on the basis of experimental data. The entire flow sheet of the I‐S process was modeled based on the two self‐developed models. The simulation models were validated through the experimental results obtained from the closed cycle I‐S facility (IS‐10) in our laboratory. By employing the simulation program, sensitivity analyses of the important parameters in the process were carried out, including the ratio of the distillate to the feed rate of the H2SO4 distillation column, reflux ratio of the H2SO4 column, H2SO4 conversion ratio, HI molality in the EED cathode outlet stream, and HI mole fraction in the liquid and vapor distillates of the HI distillation column. The key parameters significantly affecting the input duty were determined; that is, the ratio of the distillate to the feed rate of the H2SO4 distillation column and the HI molality in the EED cathode outlet stream. The optimal values of the analyzed parameters were also discussed. The simulation program we developed is a useful tool that can evaluate and optimize the I‐S process. © 2013 American Institute of Chemical Engineers AIChE J 60: 546–558, 2014

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 temperature on electro-electrodialysis of HI–I2–H2O mixture using ion exchange membranes

Abstract In the thermochemical water-splitting iodine–sulfur process for hydrogen production, an electromembrane process (electro-electrodialysis) was applied to increase the HI molality of a HI–I 2 –H 2 O mixture. In order to investigate the temperature dependence of the membrane performance for increasing the HI concentration, a theoretical formula for the performance indexes (proton transport number, initial cell voltage, and ratio of water permeation to proton permeation in the membrane) was derived on the basis of the Nernst–Planck equation and electrophoresis theory. The obtained formula could reproduce the experimental values of the grafted membranes and Nafion 212 in the temperature range 313–373 K and was validated. The temperature dependence of the performance indexes was then clarified as follows: (1) The initial cell voltage decreased with temperature upon increasing the diffusion coefficient of H + and (2) the proton transport number decreased with temperature because the diffusion coefficient of I − increased more markedly than that of H + . For the ratio of water permeation to proton permeation in the membrane, we find the optimum regression parameters for achieving low water permeation, and controlling the ion exchange capacity might help in optimizing the membrane structure so that water permeation can be minimized.

ICONE19-43742 FLOWSHEET STUDY OF HI SEPARATION PROCESS FROM HI-H_2O-I_2 SOLUTION IN THE THERMOCHEMICAL HYDROGEN PRODUCTION IODINE-SULFUR (IS) PROCESS

Heat demand was investigated by flowsheet calculation of the subsection of HI separation from HI-H_2O-I_2 solution in the thermochemical hydrogen production iodine-sulfur (IS) process. Concentration of HI by electro-electrodialysis (EED) and conventional distillation of HI were applied to the subsection. Heat/mass balance of HI distillation column was calculated using the Environmental Simulation Program (ESP), a process simulation software. Experimental data of Nafion^[○!R] 117 cation exchange membrane, including upper limit of HI molality difference between outlet streams, was applied for the EED cell modeling. HI molality at the cathode outlet of the cell, pressure in the HI distillation column, and flow rate ratio of the feed to the subsection to distillate of the column were focused on as variable parameters. Optimum EED HI molality to minimize the heat demand of the subsection appeared because the tendency of the advantageous HI molality was different for the cell and the column. Though the minimum heat demand decreased as the increase of the column pressure, the demands at 1.0 MPa and at 2.0 MPa were similar. Though total heat demand was small in smaller flow rate ratio, the ratio had an attainable lower limit. The heat/mass balance of the subsection with the optimized parameters, which was more realistic than our previous analysis, was shown.

Electro-electrodialysis of HI–I 2–H 2O mixture using radiation-grafted polymer electrolyte membranes

In the thermochemical water-splitting iodine–sulfur process for hydrogen production, new polymer electrolyte membranes were applied in an electro-membrane process (electro-electrodialysis, EED) to increase the HI molality of HIx solution (HI + I 2 + H 2O mixture) to be over quasi-azeotropic. Radiation grafting of a styrene monomer into a poly(ethylene- co-tetrafluoroethylene) base film and subsequent sulfonation provided electrolyte membranes that had ion exchange capacities (IECs) of 1.1–1.6 mmol/g. With the EED of the HIx solutions using [HI] = [I 2] = 10 mol/kg at 40 °C the transport number of protons, ratio of permeated quantities of water to the protons, and current efficiency all appeared to depend on the IEC of the resulting membranes. When compared to Nafion, the self-made membranes exhibited lower electric cell resistance, and thereby decreasing up to 32% of the overall energy required in the concentration operation.

Concentration of HIx solution by electro-electrodialysis using Nafion 117 for thermochemical water-splitting IS process

An experimental study of applying electro-electrodialysis (EED) for improved HI concentration in the HIx solution, a mixture of HI–I 2–H 2O of approximately quasi-azeotropic compositions has been carried out in the conditions of around 90 °C and using Nafion 117 and graphite electrodes. A range of 25–80% increase in initial current efficiency of HI molality in catholyte is measured with the use of EED. In general, the efficiency increases with increasing iodine molality and weight ratio of anolyte solution to catholyte solution. The EED performance degrades in time. In some cases, the HI concentration limits are observed. Electric conductivity of the HIx solution, overvoltage of electrode reaction, and the membrane voltage drop is measured in a temperature range of 20–120 °C. It is found that the EED cell voltage, which is an important cell performance parameter, is governed by the membrane voltage drop.

05/02479 Process analysis for hydrogen production by reaction integrated novel gasification (HyPr-RING)

The IS process to produce hydrogen from water requires efficient separation procedures. Effects of three typical membrane techniques (an electro-electrodialysis (EED), an electrochemical cell (EC), a hydrogen permselective membrane reactor (HPMR)) on total thermal efficiency were evaluated by heat/mass balance calculations based on the experimental data. The EED to concentrate HI solution is the most important membrane technique to obtain high thermal efficiency among the three techniques. The maximum thermal efficiency was 40.8% at 12.5molkg-H2O-1 of HI molality after the EED. The second important technique is the EC at the reaction of H2O, SO2 and I2. The maximum thermal efficiency was 38.9% at 15.3molkg-H2O-1 of H2SO4 molality after the EC. The HPMR at the decomposition reaction of HI was effective to improve one pass conversion of HI to 76.4%, and the amounts of recycled HI was reduced by 91.5% using this membrane technique. The required heat at the reactor was small compared with that at the EED or at the EC. Total thermal efficiency was improved only 0.7% by the application of the HPMR.

Optimization of the process parameters of an electrochemical cell in the IS process

One of the key reactions for efficient hydrogen production using the water-splitting iodine–sulfur (IS or S–I) process is the Bunsen reaction ( SO 2 + I 2 + 2 H 2 O = H 2 SO 4 + 2 HI ) . The Bunsen reaction was examined using an electrochemical cell with a cation exchange membrane as the separator. The optimal molalities of anolyte and catholyte were evaluated by total thermal efficiency using the heat/mass balance of the IS process. The I 2 / HI ratio had little effect on the required total voltage; the I 2 / HI ratio can be reduced to 0.5 without decreasing the total thermal efficiency. On the other hand, HI and H 2 SO 4 molality greatly affected the total thermal efficiency. The total thermal efficiency increased with increasing HI molality up to 16.7 mol kg - H 2 O - 1 and the maximum thermal efficiency was found at 15.3 mol kg - H 2 O - 1 of H 2 SO 4 . Membrane resistances are very important parameters affecting the efficiency. The total thermal efficiency increased by 3.0% at a current density of the electrochemical cell of 10.0 A dm - 2 by increasing the operating temperature from 313 to 363 K.

Evaluation of the IS process featuring membrane techniques by total thermal efficiency

The IS process to produce hydrogen from water requires efficient separation procedures. Effects of three typical membrane techniques (an electro-electrodialysis (EED), an electrochemical cell (EC), a hydrogen permselective membrane reactor (HPMR)) on total thermal efficiency were evaluated by heat/mass balance calculations based on the experimental data. The EED to concentrate HI solution is the most important membrane technique to obtain high thermal efficiency among the three techniques. The maximum thermal efficiency was 40.8% at 12.5 mol kg - H 2 O - 1 of HI molality after the EED. The second important technique is the EC at the reaction of H 2O, SO 2 and I 2. The maximum thermal efficiency was 38.9% at 15.3 mol kg - H 2 O - 1 of H 2SO 4 molality after the EC. The HPMR at the decomposition reaction of HI was effective to improve one pass conversion of HI to 76.4%, and the amounts of recycled HI was reduced by 91.5% using this membrane technique. The required heat at the reactor was small compared with that at the EED or at the EC. Total thermal efficiency was improved only 0.7% by the application of the HPMR.

Electro-electrodialysis of hydriodic acid using the cation exchange membrane cross-linked by accelerated electron radiation

Electro-electrodialysis (EED) of hydriodic acid with HI molality of ca. 9.5 mol/kg was examined in the presence of iodine using a commercial cation exchange membrane (CMB) as a separator. For the increase of the selectivity of proton permeation, the membrane was cross-linked by accelerated electron radiation. The membrane properties (area resistance, ion exchange capacity (IEC), water content) of the cross-linked membranes were measured. The area resistance in 2 mol/dm 3 KCl solution of the cross-linked membranes decreased as compared with that of the non-cross-linked membrane (original of CMB membrane). The IEC and water content of cross-linked membranes at each dose rate had almost the same value as that of non-cross-linked membrane. Electro-electrodialysis of hydriodic acid with HI molality of ca. 9.5 mol/kg was examined at 75 °C with 9.6 A/dm 2. The cross-linked cation exchange membrane by accelerated electron radiation had higher selectivity of the proton permeation by cross-linking structure of polymer than that of the non-cross-linked membrane.

Electro-electrodialysis of hydriodic acid in the presence of iodine at elevated temperature

Electrolysis of hydriodic acid with HI molality of ca. 10 mol/kg was examined in the presence of iodine using a commercial cation-exchange membrane, CMH, as the separator. Redox reaction of iodine–iodide ions that occurred at the glassy carbon electrodes, the selective proton permeation and the relatively low water permeation through the membrane enabled the increase of HI molality in the catholyte by sacrifice of HI in the anolyte even at the temperature of 80°C. Effects of temperature and iodine molality on the membrane properties such as proton transport number and electroosmosis coefficient were clarified in the ranges of 15–80°C and 4–15 mol/kg, respectively. The effects on the electric resistance of test cell were also determined. Carbon cloth was effective in reducing the cell resistance.

Electrodialysis of hydriodic acid in the presence of iodine

Electrodialysis of hydriodic acid was studied in the presence of iodine and in the HI molality range of 8–12 mol/kg using commercial ion exchange membranes, CMH and APS. Owing to the high transport numbers of counter ions and low water permeation of the membranes, the increase of HI molality was possible by electrodialysis in spite of the high HI molality of the test solution. Interesting phenomena with respect to the anion exchange membrane, APS, were observed in the electrodialysis. Firstly, the membrane strongly suppressed the H 2O permeation. Secondly, apparently, neutral iodine molecules were transported through the membrane even against the concentration gradient. The transportation occurred toward the reverse direction of the anion permeation. These phenomena were discussed in terms of an iodine adsorption and a charge transfer interaction between iodine and the polymer matrix.