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     

THERMODYNAMIC CONSIDERATIONS ON THE PURIFICATION OF H2SO4 AND HIX PHASES IN THE IODINE-SULFUR HYDROGEN PRODUCTION PROCESS

The reaction equilibrium and phase equilibrium in H₂SO₄ and HIx phases produced by the Bunsen reaction of the iodine-sulfur thermochemical hydrogen production process were examined using a chemical process simulator, ESP, with a thermodynamic database based on the mixed solvent electrolyte model. At temperatures lower than ca. 110°C, the reaction of HI and H₂SO₄ produced elemental sulfur in both phases. At higher temperatures, the reverse Bunsen reaction occurred, and SO₂ was produced in the H₂SO₄ phase. In the HIx phase, conversely, SO₂ formation predominated in a narrow temperature range and H₂S was produced with the increase in temperature. The presence of N₂ gas lowered the temperature of the predominant reaction change. A feed of O₂ for purification was proposed to suppress the consumption of objective components in the H₂SO₄ phase purification, and an O₂ feed to the HIx phase for the suppression of H₂S and S impurities was proposed by the simulation.     

Dehydration through pervaporation from HIx solution (HI-H2O-I2 mixture) using a cation exchange membrane for thermochemical water-splitting iodine-sulfur process

Pervaporation (PV) of water from HIx solution (HI-H₂O-I₂ mixture) using Nafion-117 was evaluated aiming at the application to dehydrate the azeotropic composition in HI decomposition reaction of thermochemical IS process. PV experiment was carried out by using HI solutions of 40–65 wt% and an I₂/HI molar ratio of 0–3 in the feed at the room temperature. The permeation flux decreased with increasing HI weight fraction in the feed. The permeation flux is dependent on the I₂ concentration in the feed having an I₂/HI molar ratio. A long time PV experiment was carried out using I₂/HI molar ratio of 1 (in HI solution of 55.9%) in the feed at room temperature. It is expected that the permeation component in the permeate zone using the PV process was mainly H₂O, and H₂O permeation was constant with increasing operation time.     

Application of Combinatorial Methodologies to the Synthesis and Characterization of Electrolytic Manganese Dioxide

A combinatorial method has been used to investigate the effects of anodic current density, and Mn(II) and H₂SO₄ concentrations on the electrochemical synthesis and characterization of electrolytic manganese dioxide (EMD). The combinatorial method involved rapid parallel and series electrochemical deposition of EMD from electrolytes with various Mn(II)(0.15–1.82 M) and H₂SO₄(0.05–0.51 M) concentrations, at various anodic current densities (25–100 A m^–2), onto individual 1 mm² titanium electrodes, in an overall array consisting of 64 electrodes. Electrode characterization was then by average plating voltage (recorded during deposition), and open circuit voltage and chronoamperometric discharge in 9 M KOH. The applicability and benefit of the method was demonstrated by identifying the conditions of 0.59 M Mn(II), 0.17 M H₂SO₄ and 62.5 A m^–2 anodic current density as leading to the best performing EMD. These are comparable with existing knowledge regarding the synthesis and electrochemical performance of EMD, demonstrating clearly the capabilities of the combinatorial method, and providing a starting point for future experimentation. An added benefit of the method in this work was the considerable time saved during experimentation.     

Studies on promising cell performance with H2SO4 as the catholyte for electrogeneration of Ag2+ from Ag+ in HNO3 anolyte in mediated electrochemical oxidation process

Electrochemical performance of a divided cell with electrogeneration of Ag²⁺ from Ag⁺ in 6 M HNO₃ anolyte has been studied with 6 M HNO₃ or 3 M H₂SO₄ as the catholyte. This work arose because in mediated electrochemical oxidation (MEO) processes with Ag(II)/Ag(I) redox mediator, HNO₃ is generally used as catholyte, which, however, produces NO _x gases in the cathode compartment. The performance of the cell with 6 M HNO₃ or 3 M H₂SO₄ as the catholyte has been compared in terms of (i) the acid concentration in the cathode compartment, (ii) the Ag⁺ to Ag²⁺ conversion efficiency in the anolyte, (iii) the migration of Ag⁺ from anolyte to catholyte across the membrane separator, and (iv) the cell voltage. Studies with various concentrations of H₂SO₄ catholyte have been carried-out, and the cathode surfaces have been analyzed by SEM and EDXA; similarly, the precipitated material collected in the cathode compartment at higher H₂SO₄ concentrations has been analyzed by XRD to understand the underlying processes. The various beneficial effects in using H₂SO₄ as catholyte have been presented. A simple cathode surface renewal method relatively free from Ag deposit has been suggested.     

Effect of sodium and potassium sulphates on zinc electrowinning

In order to evaluate the intrinsic effect of high concentrations of sodium and potassium sulphates in zinc electrowinning solutions, measurements of coulombic efficiency were carried out under mass transfer-controlled conditions in synthetic solutions of very high purity. A solution composition of 1 mol dm^–3 ZnSO₄+1.5 mol dm^–3 H₂SO₄ was employed with and without additions of 0.5 mol dm^–3 Na₂SO₄ and/or 0.25 mol dm^–3 K₂SO₄. With temperature and current density similar to plant practice (37° C, 650 A m^–2) and electrode rotation rates of 10 and 45 s^–1, the coulombic efficiency for three successive batch tests (200 mg zinc) increased by an average of 1.2% (from an average of 96.0%) for additions of 0.5 mol dm^–3 Na₂SO₄+0.25 mol dm^–3 K₂SO₄. The results were evaluated in terms of available theories, solution purity and predicted changes in solution composition (zinc and hydrogen ion activities) and physical properties following additions of Na₂SO₄/K₂SO₄. It was concluded that in the plant situation the increase in coulombic efficiency would probably be offset by an increase in cell voltage of about 2%, the net effect on power efficiency being a decrease of about 1%. The zinc deposit morphology and preferred orientation were also studied. The addition of sodium and/or potassium sulphate to the solution resulted in rougher, darker zinc deposits, a slight grain refining effect, and a change from random to predominantly basal (002), (004) crystal orientation (at 45 s^–1).     

Potentiometric measurements of ionic transport parameters in poly(ethylene oxide)-LiX electrolytes

An electrochemical technique based on concentration cell e.m.f. measurements is used to determine the lithium transference number and diffusion coefficient in poly(ethylene oxide)-lithium salt complexes. Measurements were carried out at 90°C on PEO–LiI, PEO–LiClO₄ and PEO–LiCF₃SO₃ electrolytes. According to the phase diagram of the PEO-lithium salt system these complexes are fully amorphous at 90°C. Accurate determination oft _Li ⁺ by the e.m.f. concentration cell method generally requires knowledge of the mean salt activity coefficients. However, this becomes unnecessary when the two electrolyte concentrations differ only slightly. As a first step the mean salt activity coefficient was estimated using a galvanic cell of the lithium/PEO-LiX/MX _n /M type withM ⁿ⁺=Ag⁺ or Pb²⁺, and X^–=I^– or CF₃SO₃ ^–. The resulting lithium transference numbers are 0.34 for the PEO–LiI complex and 0.7 for PEO–LiCF₃SO₃. Discrepancies between thet _Li ⁺ values can be explained by the formation of triplets in the PEO–LiCF₃SO₃ electrolyte. By recording concentration cell potential versus time and comparing with theoretical curves, the salt lithium diffusion coefficient was obtained.D _LiI was found to be around 4×10^–8 cm² s^–1 in PEO–LiI and 8×10^–8 cm² s^–1 in PEO–LiCF₃SO₃ at 90°C. These results suggest a liquid-like behaviour for the microscopic transport mechanism.     

Characterization of polypyrrole films incorporating various anions

The conductivity of polypyrrole films has been enhanced by electrochemical post-deposition doping with various anions. The change of conductivity was found to depend on the type and concentration of the anion. Results for the polypyrrole films doped with anions of H₂SO₄, (C₂H₅)₄N(O₃SC₆H₄CH₃), KI, CH₃C₆H₄SO₃H · H₂O (p-toluene sulphonic acid monohydrate), AlCl₃, KBrO₃ and HNO₃ showed that in the case of H₂SO₄, (C₂H₅)₄ N(O₃SC₆H₄CH₃) and CH₃C₆ H₄SO₃ H · H₂O the conductivity can be enhanced by up to a factor of two, from a value of 67 S cm^–1 up to 165, 102 and 95 S cm^–1, respectively. Doping with I^– had a negligible effect on the conductivity which was about 71 S cm^–1, while in the case of AlCl₃, KBrO₃ and HNO₃ the conductivity of the polypyrrole decreased significantly for certain anion concentrations.     

Characterization and use of aqueous caesium chloride as an ultra-concentrated salt bridge

Five strong aqueous binary electrolytes — one symmetrical (CsCl) and four unsymmetrical (Li₂SO₄, K₂SO₄, Rb₂SO₄, Cs₂SO₄) — have been examined, for possible use as salt bridges for the minimization of liquid junction potentials (E _L), up to the highest concentrations practicable, by the method of homoionic transference cells: Pt–Ir | Cl₂ | CsCl (m ₂) CsCl (m ₁) | Cl₂ | Pt–Ir and Hg | Hg₂Cl₂ | CsCl (m ₂) CsCl (m ₁) | Hg₂Cl₂ | Hg for CsCl, and Hg | Hg₂SO₄ | Me₂SO₄ (m ₂) Me₂SO₄ (m ₁) | Hg₂SO₄ | Hg for the Me₂SO₄ sulphates where Me=Li, K, Rb and Cs. CsCl, K₂SO₄, Rb₂SO₄, and Cs₂SO₄, prove to belong to the class obeying close equality of transference numbers for their ions, that is,t ₊= |t _–|=0.5, over the whole concentration range (namely, from infinite dilution up to saturation). This result qualifies aqueous CsCl as an unrivalled salt bridge, whose equitransference is obeyed more stringently than any other salt. This is now demonstrated experimentally over the whole molality range, the saturation molality being as high as 11.30 mol kg^–1 at 25°C. The observed propertyt ₊=|t _–|=0.5 excludes K₂SO₄, Rb₂SO₄, and Cs₂SO₄, as possible salt bridges because the equitransference conditions for minimization ofE _L's are ₊ = |_–| = l/(z ₊ + |z _–|) = 0.333, i.e.,t ₊=0.333 andt _–=2t ₊=0.667. Finally, Li₂SO₄, though behaving quite differently from the other three sulphates studied, does not sufficiently approach the required conditions, contrary to what one might have hoped from its known infinite-dilution transference numbers.     

Electrotransformations at some lead-antimony alloy/sulphuric acid interfaces

The electrochemical behaviour of Pb-(0–8%) Sb alloys in H₂SO₄ solutions was studied using impedance and cyclic galvanostatic measurements. The experimental results indicated that the alloy containing 2.5% Sb is critical in many respects. The potential arrest corresponding to the formation of the sulphate layer was absent for the alloy containing 2.5% Sb while it was present for the other alloys. The efficiency of the charging was found to increase with decreasing anodizing current. The electrochemical transformations during cyclic galvanostatic polarization and self-discharge are discussed.     

Development of a novel redox flow battery for electricity storage system

A novel cylindrical battery which uses carbon fibres with high specific surface area as electrodes and a porous silica glass with high chemical stability as membrane has been fabricated. The results obtained from electrolysis of 0.5 M K₃Fe(CN)₆–0.5 M KCl and of 85 mM V(IV)–1 M H₂SO₄ indicate that the cell possesses excellent electrolytic efficiency. As a redox flow battery (RFB) its performance was investigated by employing all-vanadium sulfate electrolytes. The results of the cyclic voltammetry measurements indicate that at a glassy carbon electrode the electrochemical window for 2 M H₂SO₄ solution could reach 2.0 2.4 V. Constant current charging–discharging tests indicate that the batteries could deliver a specific energy of 24 Wh L^–1 at a current density of 55 mA cm^–2. The open-circuit cell voltage, after full charging, remained constant at about 1.51 V for over 72 h, while the coulombic efficiency was over 91%, showing that there was negligible self-discharge due to active ions diffusion through the membrane during this period.     

Continuous treatment of filament with a dosed amount of reagent

The characteristics of treatment of filaments with a dosed amount of reagent were examined on the example of elimination of sodium ions from PAN twist. It was shown that a 2 g/liter concentration of H₂SO₄, required for elimination of sodium ions from 0.026–0.35 to 0.01–0.03%, is attained over the entire section of the twist on contact of PAN twist with a solution of H₂SO₄ of high concentration, 20–30 g/liter, for 0.2–0.8 sec. The diffusion coefficients of H₂SO₄ and the thermal conductivity of the twist were determined. It was shown that heating of the twist takes place two orders of magnitude more rapidly than diffusion of sulfuric acid.All-Russian Scientific — Research Institute of Polymer Fibres, Mytishchi. Translated from Khimicheskie Volokna, No. 3, pp. 20–23, May–June, 1994.     

Influence of ionic equilibrium in the CuSO4-H2SO4-H2O system on the formation of irregular electrodeposits of copper

The relative concentration of hydrogen ion (H⁺) as a function of sulfuric acid (H₂SO₄) concentration (calculated using Pitzer's model) and the electrochemical processes by which irregular copper deposits are formed were correlated. Irregular deposits are formed by potentiostatic electrodeposition at a high overpotential where the hydrogen evolution reaction occurs parallel to copper electrodeposition. Two sets of acid sulfate solutions were analyzed. In one set of experiments, the concentration of CuSO₄ was constant while the concentration of H₂SO₄ was varied. The other set of experiments was performed with a constant concentration of H₂SO₄ and different concentrations of CuSO₄. Then, the volumes of the evolved hydrogen (calculated as the average current efficiencies of hydrogen evolution) and the morphologies of copper deposits, characterized by the SEM technique, obtained for the same ratio of CuSO₄/H₂SO₄ were mutually compared and discussed in terms of the relative concentrations of hydrogen ions (H⁺) as a function of the H₂SO₄ concentration. Good agreement between the ionic equilibrium in the CuSO₄-H₂SO₄-H₂O system and the results of the electrochemical processes was obtained. In this way, it was shown how this ionic equilibrium can be applied to predict and analyze the solution composition in electrolytic copper deposition processes.     

H2SO4 as a passivating medium on the localised corrosion resistance of surgical 316L SS metallic implant and its effect on hydroxyapatite coatings

The localised corrosion resistance of 316L SS metallic implant due to H₂SO₄ treatment is being studied through electrochemical studies involving cyclic polarisation experiments and impedance studies. The efficiency of hydroxyapatite (HAP) coatings on H₂SO₄ treated 316L SS is also been investigated through electrochemical studies and the dissolution characteristics of the coatings. The study reveal that 15% H₂SO₄ treatment was found to be efficient in the corrosion resistance of 316L SS and dissolution of alloy is considerably reduced in the hydroxyapatite coatings on 15% H₂SO₄ treated 316L SS.     

A new approach to electrochemical production of benzaldehyde from toluene in an undivided cell in the presence of the couple V5+/V4+

Oxidation of toluene to benzaldehyde in the presence of the redox couple V⁵⁺/V⁴⁺ was carried out in an undivided cell where oxygen gas was continuously bubbled over the cathodic surface and the electrolyte was a mixture of aqueous H₂SO₄ solution containing V⁵⁺ and toluene. Some experimental conditions affecting the current efficiency for benzaldehyde production, such as H₂SO₄ concentration, current density, V⁵⁺ concentration and surfactants, were determined. The maximum current efficiency for benzaldehyde production at ambient temperature was 156.3% under the conditions of 11 M H₂SO₄, 2.7 × 10^–4 M CTAB, current density 1.25 mA cm^–2 and 0.0128 M V⁵⁺.     

In situ controlled electrochemical promotion of catalyst surfaces: Pd-catalysed ethylene oxidation

The catalytic activity of polycrystalline Pd films deposited on 8 mol% Y₂O₃-stabilized–ZrO₂ (YSZ), an O^2–-conductor, can be altered reversibly by varying the potential of the Pd catalyst film via the effect of nonfaradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion. The complete oxidation of ethylene was investigated as a model reaction in the temperature range 290–360 °C and atmospheric total pressure. The rate of C₂H₄ oxidation can be reversibly enhanced by up to 45% by supplying O^2– to the catalyst via positive current application. The steady-state rate change is typically 10³–10⁴ times larger than the steady-state rate I/2F of electrochemical supply or removal of promoting oxide ions. The observed behaviour is discussed on the basis of previous NEMCA studies and the mechanism of the reaction.     

Performance evaluation of poly (methacrylic acid) as corrosion inhibitor in the presence of iodide ions for mild steel in H2SO4 solution

The inhibition performance of poly (methacrylic acid) (PMAA) and the effect of addition of iodide ions on the inhibition efficiency for mild steel corrosion in 0.5 M H₂SO₄ solution were investigated in the temperature range of 303–333 K using electrochemical, weight loss, scanning electron microscopy (SEM), and water contact angles measurements. The results show that PMAA is a moderate inhibitor for mild steel in 0.5 M H₂SO₄ solution. Addition of small amount of KI to PMAA significantly upgraded the inhibition efficiency up to 96.7%. The adsorption properties of PMAA and PMAA + KI are estimated by considering thermodynamic and kinetic parameters. The results reveal that PMAA alone was physically adsorbed onto the mild steel surface, while comprehensive adsorption mode characterized the adsorption of PMAA + KI. Adsorption of PMAA and PMAA + KI followed Temkin adsorption isotherm. The SEM and water contact angle images confirmed the enhanced PMAA film formation on mild steel surface by iodide ions.     

Synergistic effect of 2-mercapto benzimidazole and KI on copper corrosion inhibition in aerated sulfuric acid solution

2-Mercapto benzimidazole (MBI) was used as a copper corrosion inhibitor in aerated 0.5 mol L^–1 H₂SO₄ solutions. The inhibition efficiency (IE) increased with increasing MBI concentration to 74.2% at the 1 mM level. A synergistic effect existed when MBI and iodide ions were used together to prevent copper corrosion in sulfuric acid. It was found that IE reached 95.3% in 0.5 mol L^–1 H₂SO₄ solutions containing 0.75 mmol L^–1 MBI and 0.25 mmol L^–1 KI. X-ray photoelectron spectroscopy (XPS) analysis of the copper samples showed that a (Cu⁺MBI) complex film formed on the surface to inhibit the copper corrosion and the iodide ions did not participate in the formation of the inhibitor film. The synergistic effect was attributed to the adsorption of iodide anions on the copper surface, which then facilitated the adsorption of protonated MBI and the formation of an inhibitive film.     

An electrochemical technique for the reduction of aromatic nitro compounds

A preparative technique for the electroreduction of aromatic nitro compounds using a copper cathode, a lead anode and a porous pot diaphragm with 30–40% H₂SO₄ as the anolyte and 30–40% H₂SO₄ containing 2–3% Ti(SO₄)₂ as the catholyte is reported. Nitrobenzene, dinitrobenzenes, nitrotoluenes, phenols, phenetoles and nitrochlorobenzenes have been reduced by this method. Slightly modified techniques were employed for some compounds such as 2-nitro-m-xylene. The results of some pilot plant studies are also reported. The studies indicate that on smaller scales this technique may be competitive with metal/acid systems with the additional advantage of less pollution problems.     

Effect of precipitation bath composition on the process of spinning viscose yarn

Conclusions A connection between the change in characteristics of the viscose yarn spinning process and the structure of the precipitation bath solutions has been shown.To create optimum conditions for spinning viscose yarns, a bath is necessary in whose solutions the structure of water is broken up.Possibilities for improving conditions for spinning yarns into precipitation baths containing the H₂SO₄-Na₂SO₄-H₂O system are limited by the solubility of sodium sulfate.Translated from Khimicheskie Volokna, No. 1, pp. 40–42, January–February, 1987.