An experimental study of a fixed bed chlor-alkali reactor

A 100 A continuous ‘flow-by’ chlor-alkali membrane reactor was constructed with both anode and cathode consisting of fixed beds of 0.6 to 1 mm diameter graphite particles. The reactor was operated over a range of conditions with and without co-current flow of air or oxygen to the cathode. With an anolyte of 5 M NaCl and catholyte 1.4–3 M NaOH the reactor produced sodium hydroxide and chlorine with ≥80% efficiency at temperatures 28–100°C, absolute pressure 270–970 kPa and superficial current density up to 3.3 kA m^?2. For operation at 100°C and an average pressure of 870 kPa with no gas delivered to the cathode, the cell voltage increased linearly from 2.5V at 0.3 kA m^?2 (10 A) to 4.0 V at 3.3 kA m^?2 (100 A). When oxygen was delivered to the cathode at 1 litre min^?1 under 870 kPa average pressure, the corresonding cell voltages were 1.6 V at 0.3 kA m^?2 to 3.4 V at 3.3 kA m^?2. In operation with air under the same conditions the cell voltage rose from 1.6 V at 0.3 kA m^?2 to 3.1 V at 1.6 kA m^?2. The performance of the oxygen cathode deteriorated with lower pressure and temperature due to mass transfer constraints on the oxygen reaction in the fixed bed electrode.     

PAIRED ELECTRO-GENERATION OF GLYOXYLIC ACID USING BIPOLAR MEMBRANE MADE FROM SODIUM ALGINATE AND CHITOSAN

Sodium alginate (SA) and chitosan (CS) were modified with Ca²⁺ and glutaraldehyde linking reagents to prepare the mSA/mCS bipolar membrane (BPM). The morphology of the membrane was characterized by SEM. The membrane was used as a separator in an electrolysis cell for the production of glyoxylic acid simultaneously at both the cathode and the anode. The catholyte consisted of a mixture of saturated oxalic acid and 0.1 mol/L HCl, and the anolyte was a mixture of glyoxal (10 wt.%) and KBr (10 wt.%). A nickel mesh was placed on the surface of the mSA cation exchange layer to act as the cathode, and the anode was PbO₂. The electrolysis voltage was as low as 2.7 V during operation at room temperature with a current density of 20 mA · cm^?2. Current efficiencies reached 82.9% in the cathode chamber and 75.7% in the anode chamber.     

Durability of ABPBI‐based MEAs for High Temperature PEMFCs at Different Operating Conditions

High temperature PEMFCs based on phosphoric acid‐doped ABPBI membranes have been prepared and characterised. At 160 °C and ambient pressure fuel cell power densities of 300 mW cm^–2 (with hydrogen and air as reactants) and 180 mW cm^–2 (with simulated diesel reformate/air) have been achieved. The durability of these membrane electrode assemblies (MEAs) in the hydrogen/air mode of operation at different working conditions has been measured electrochemically and has been correlated to the cell resistivity, the phosphoric acid loss rate and the catalyst particle size. Under stationary conditions, a voltage loss of only –25 μV h^–1 at a current density of 200 mA cm^–2 has been deduced from a 1,000 h test. Under dynamic load changes or during start–stop cycling the degradation rate was significantly higher. Leaching of phosphoric acid from the cell was found to be very small and is not the main reason for the performance loss. Instead an important increase in the catalyst particle size was observed to occur during two long‐term experiments. At high gas flows of hydrogen and air ABPBI‐based MEAs can be operated at temperatures below 100 °C for several hours without a significant irreversible loss of cell performance and with only very little acid leaching.     

Development of Instant Fermented Cereal-Legume Mix Using Pulsed Microwave Vacuum Drying

Cereal-legume-based instant fermented food (Dhokla) is one of the most popular, indigenous fermented foods of India. Central composite design (CCD) was used to conduct fermentation experiments and optimization was carried out using response surface methodology (RSM). The effect of fermentation time (5.5–12.5 h), fermentation temperature (26.5–35.5°C), moisture content of batter (55–65% wb), and rice- to- bengal gram ratio (1.2–2.4) was evaluated with respect to total titratable acidity (TTA), total lactic count (TLC), firmness and overall acceptability scores (OAA) of steam-cooked Dhokla. The optimized condition for fermentation process was TTA 0.64 g mL^?1, total lactic count 221.62 cfu g^?1, firmness 146.35 g, and OAA score 6.82 at 12.5 h fermentation time, 26.5°C temperature, 65% moisture content of batter and 1.2 rice to bengal gram ratio. Further, the optimized fermented batter was dried by microwave vacuum using independent variables like thickness of batter (10–17 mm), microwave power density (3.5–10 W g^?1), and pulsating ratio (1.3–2). The responses studied were bulk density, rehydration ratio, color difference (ΔE), and OAA score. The best combination was found with bulk density 1014.22 kg m^?3, rehydration ratio 4.55, ΔE 9.57 and OAA score 6.88 at 17 mm thickness of batter, 10 W g^?1 microwave power density, and 1.3 pulsating ratio.     

Activity and active sites of nitrogen-doped carbon nanotubes for oxygen reduction reaction

Nitrogen-doped carbon (CNx) nanotubes were synthesized by thermal decomposition of ferrocene/ethylenediamine mixture at 600–900 °C. The effect of the temperature on the growth and structure of CNx nanotubes was studied by transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. With increasing growth temperature, the total nitrogen content of CNx nanotubes was decreased from 8.93 to 6.01 at.%. The N configurations were changed from pyrrolic-N to quaternary-N when increasing the temperature. Examination of the catalytic activities of the nanotubes for oxygen reduction reaction by rotating disk electrode measurements and single-cell tests shows that the onset potential for oxygen reduction in 0.5 M H₂SO₄ of the most effective catalyst (CNx nanotubes synthesized at 900 °C) was 0.83 V versus the normal hydrogen electrode. A current density of 0.07 A cm^?2 at 0.6 V was obtained in an H₂/O₂ proton-exchange membrane fuel cell at a cathode catalyst loading of 2 mg cm^?2.     

Porous PTFE separator for high temperature water electrolyzers

We have developed and tested a PTFE porous diaphragm for high temperature and high pressure alkaline water electrolyzers. The separator was prepared by a film-pressing technique. A mixture of PTFE and iron oxide is molded and heated above 370°C. The oxide is removed by washing with aqueous HCl. Open cell porous mats are thus obtained having a density of 0.6 g cm^?3. These diaphragms resist corrosion by 30% aqueous KOH at 160°C and they perform well in alkaline water electrolyzers regarding electrolyte permeability, gas-tightness and impedance. The voltage drop introduced by the membrane is 0.1–0.2 V at 500 mA cm^?2.     

Response of a proton exchange membrane fuel cell to a sinusoidal current load

The load-following capability of a proton exchange membrane fuel cell was studied by measuring the cell voltage response to a sinusoidal current load with large amplitude and varying frequency. A mathematical model was developed, incorporating mass transport and capacitive effects as well as the membrane resistance. The model was capable of separating the faradaic and capacitive currents and predicting the observed hysteresis. At frequencies of the sinusoidal current load below 1 Hz, no appreciable hysteresis in the polarisation curve was observed. When increasing the frequency above 1 Hz, a hysteresis appeared at current densities below 0.2 A cm^?2. The model related this hysteresis to capacitive effects. When using air as the cathode feed, hysteresis in the current density range 0.5 A cm^?2 and higher appeared above 1 Hz compared to 100 Hz for pure oxygen. The model revealed that hysteresis observed in this current density range was caused by oxygen transport limitations.     

Zeolite Proton Conducting Membrane for Micro Fuel Cell Applications

Supported and self-supporting ZSM-5 membranes (Si/Al = 25) were prepared, assembled and tested for hydrogen fuel cell performance. A Grotthus-like mechanism is proposed for the proton conduction in hydrated zeolites, where the protons hop between neighbouring aluminium sites along a water bridge in the zeolite pores. The supported ZSM-5 membrane was prepared by seeding and secondary regrowth method on cellulose paper, whereas the self-supporting zeolite membrane consists of an array of micromembranes on silicon substrate. The self-supporting HZSM-5 membrane had an open-circuit voltage (OCV) of 0.77 V compared to 0.90 V for supported HZSM-5 and 0.98 V for Nafion 117. The differences are attributed to the contact between the electrode, catalyst and membrane layers in the assembly. The self-supporting HZSM-5 yielded a maximum power density (MPD) of 19.4 mW cm^?2 compared to 14.4 mW cm^?2 for supported HZSM-5 and 35 mW cm^?2 for Nafion 117.     

Fuel cell performance of polymer electrolyte membrane based on hexafluorinated sulfonated poly(ether sulfone)

The potential-current fuel cell characteristics of membrane electrode assemblies (MEAs) using hexafluorinated sulfonated poly(ether sulfone) copolymer are compared to those of Nafion^® based MEAs in the case of proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC). The hexafluorinated copolymer with 60 mol% of monosulfonated comonomer based acid form membrane is chosen for this study due to its high proton conductivity, high thermal stability, low methanol permeability, and its insolubility in boiling water. The catalyst powder is directly coated on the membrane and the catalyst coated membrane is used to fabricate MEAs for both fuel cells. A current density of 530 mA cm^?2 at 0.6 V is obtained at 70 °C with H₂/air as the fuel and oxidant. The peak power density of 110 mW cm^?2 is obtained at 80 °C under specific DMFC operating conditions. Other electrochemical characteristics such as electrochemical impedance spectroscopy, cyclic voltammetry, and linear sweep voltammetry are also studied.     

High Flux Layer by Layer Polyelectrolyte FO Membrane: Toward Enhanced Performance for Osmotic Microbial Fuel Cell

Combination of microbial fuel cell (MFC) and forward osmosis (FO) is called an osmotic microbial fuel cell (OMFC). Because of the high cost of FO membranes, for the first time laboratory made FO membrane has been used in OMFC. This study investigates the performance of FO membrane in OMFC treating glucose as substrate and 2M NaCl as draw solution. The FO membrane was able to achieve 18.43 lm^?2 h^?1 (LMH) and for fouled FO membrane it was 15.26 lm^?2 h^?1. The OMFC constantly produced bioelectricity and achieved maximum current density 139.52 A/m³ and power density 27.38 W/m³. The energy production of OMFC was 0.438 kWh/m³.     

Synthesis and Physicochemical Study of High-Performance Ether-Sulfonate Copolymer

Ether-sulfonate copolymer of 1,1′-bis(4-hydroxyphenyl) cyclohexane, bisphenol-A and 4,4′-disulfonyl chloride diphenyl ether [PESAC] has been synthesized and characterized by IR, ¹H NMR, viscosity ([η] = 0.23–0.25 gdl^?1), density (1.3172 g/cm³), tensile strength (11.8 MPa), electric strength (64 kV/mm), volume resistivity (4.24 × 10¹⁴ Ω cm) and dielectric constant (1.0). PESAC possesses excellent solubility, good hydrolytic stability against water, acids, alkalis and salt, high T_g (170°C) and high thermal stability (290°C). The associated kinetic parameters namely energy of activation E (386.6 kJ mol^?1), order of the reaction n (3.1), frequency factor A (1.53 × 10³¹ s^?1) and entropy change ΔS* (346.1 kJ mol^?1) have been determined and discussed.     

Nine years of research and development on advanced water electrolysis. A review of the research programme of the Commission of the European Communities

The development of advanced water electrolysis was one of the main tasks of the R & D programme on hydrogen funded, within its main R & D programme on Energy, by the Commission of the European Communities. Most of the work has been concentrated on the development of alkaline water electrolysis, as this process appears particularly promising. (Water electrolysis based on ‘acidic’ solid polymer electrolytes, developed during the last 10 years, seems to be a potentially attractive alternative technology, at least for electrolysers of smaller scale (up to 100kW). Even at this size, however, there is not yet evidence of any overall economic advantage over advanced alkaline cells.) The results of 9 years of R & D in this field are critically examined, by reviewing the improvements achieved on the components of the electrolytic cell as well as the overall modification of the cell design. The anode, cathode and diaphragm have been the components investigated, but also the constituent materials, the nature of the electrolyte and its operating conditions have been dealt with. Three main lines of advanced electrolyser development were identified in the course of these investigations. The corresponding charcteristics are:

1. low temperature (70°C to 90°C), low current density (i=0.1–0.3 A cm^?2);
2. moderate temperature (<120°C), high current density (i up to 1 A cm^?2), medium pressure (5–10 bars);
3. medium temperature (120–160°C), high current density (i=1–2 A cm^?2), moderately high pressure (30 bars).

In cell design, very compact cell units have been devised, in which a ‘zero gap’ configuration (anode and cathode are placed directly on the diaphragm) is generally adopted, resulting in very low internal cell resistance (about 0.2 Ω cm²). Potential energy savings of 20 to 30% can be anticipated for the advanced electrolysis. In addition to this work on advanced alkaline water electrolysis, some limited research efforts on high temperature (>1100 K) water vapour electrolysis have been made and are reported. The latter work has been concentrated on the production of thin-layer doped zirconia solid electrolytes (d=50μm), potentially leading to high performance cells. The economic implications of high-temperature vapour electrolysis, however, cannot be judged at the present status of development.     

MODELING PROCESS DYNAMICS USING A NOVEL NEURAL NETWORK ARCHITECTURE: APPLICATION TO STIRRED CELL MICROFILTRATION

A novel neural network architecture is presented for dynamic process modeling, using stirred cell microfiltration of bentonite suspensions as a model system. Unlike previous studies that include time explicitly as a network input and have a single output at that time, the network architecture presented contains the process variables as inputs and many outputs representing the output (filtrate flux in this case) at different selected times. The network is shown to represent the stirred cell microfiltration of bentonite suspensions over a range of pressures (0.2–1.5 bar), initial concentrations (0.5–2.0 g/L), stirrer tip speeds (0.04–0.17 m/s), membrane resistances (3.09 × 10¹⁰–6.85 × 10¹⁰ m^?1), pH values (2.5–10.4), and temperatures (20°–24°C) with good accuracy (R² = 0.91 on network test data). With this network architecture, it becomes easy to track the time dependence of the relative effect of the various process parameters on the system output. Thus, for example, the network weights show that the effect of stirring rate on flux increases as time progresses, while the opposite effect is seen for membrane resistance, as expected.     

Corrosion of the zinc negative electrode of zinc–cerium hybrid redox flow batteries in methanesulfonic acid

Corrosion of zinc in aqueous methanesulfonic acid has been evaluated over a wide range of concentrations of acid (0.5–5 mol dm^?3), dissolved zinc (0.5–2 mol dm^?3), and electrolyte temperature (22–50 °C). The corrosion rate of zinc, in terms of weight loss and the volume of hydrogen evolved, varied with time and it was found to be highly dependent on the surface state and electrolyte conditions. With an initial active layer of zinc present, the corrosion rate rapidly increased following a decline when the proton concentration in the solution decreased to ca. 0.56 mol dm^?3. Organic and inorganic inhibitors were added to the electrolyte to suppress the zinc corrosion in 1 mol dm^?3 methanesulfonic acid. The strong adsorption and blocking effects of cationic organic adsorption inhibitors, such as cetyltrimethyl ammonium bromide and butyltriphenyl phosphonium chloride, led to a significant decrease in zinc corrosion over a 10 h immersion period. With the addition of indium and lead ions inhibitors, the zinc surface showed less activity. Zinc corrosion continued to a smaller extent in the presence of these metallic inhibitors during the first few hours, but the metallic layer of the inhibitors did not cover the surface completely resulting in continued hydrogen evolution and making the inhibitors less effective at longer times.     

Zinc deposition and dissolution in sulfuric acid onto a graphite–resin composite electrode as the negative electrode reactions in acidic zinc-based redox flow batteries

Electrodeposition and dissolution of zinc in sulfuric acid were studied as the negative electrode reactions in acidic zinc-based redox flow batteries. The zinc deposition and dissolution is a quasi-reversible reaction with a zinc ion diffusion coefficient of 4.6 × 10^?6 cm² s^?1 obtained. The increase of acid concentration facilitates an improvement in the kinetics of zinc electrodeposition–dissolution process. But too high acid concentration would result in a significant decrease in charge efficiency. The performance of the zinc electrode in a three-electrode system with magnetic stirring was also studied as a function of Zn(II) ion concentration, sulfuric acid concentration, current density, and the addition of additives in 1 M H₂SO₄ medium. The optimum electrolyte composition is suggested at high zinc(II) concentration (1.25 M) and moderate sulfuric acid concentration (1.0–1.5 M) at a current density range of 20–30 mA cm^?2. Whether in acid-free solution or in sulfuric acid solution with or without additives, no dendrite formation is observed after zinc electrodeposition for 1 h at 20 mA cm^?2. The energy efficiency is improved from 77 % in the absence of additives in 1 M H₂SO₄ medium to over 80 % upon the addition of indium oxide or SLS–Sb(III) combined additive as hydrogen suppressants.     

Advanced visible-light photocatalytic property of biologically structured carbon/ceria hybrid multilayer membranes prepared by bamboo leaves

Biologically structured carbon/cerium dioxide materials are synthesized by biological templates. The microscopic morphology, structure and the effects of different oxidation temperatures on materials are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) ultraviolet-visible light spectrum (UV–Vis) and X-ray Photoelectron Spectroscopy (XPS). Moreover, by splitting water under visible light irradiation, the hydrogen production is measured to test the photocatalytic property of these materials. The results show that materials made with bamboo biological templates which are immersed in 0.1 mol L^?1 of cerium nitrate solution, then carbonizated in nitrogen (700 °C) and oxidized in air (500–600 °C), can obtain the biological structure of bamboo leaves. The product is in the composition of hybrid multilayer membrane, which one is carbon membrane form plant cell carbonation and another is ceria membrane by nanoparticle self assembly. The best oxidation temperature is 550 °C and the band gap of carbon/cerium dioxide materials synthesized at this optimum oxidation temperature could be reduced to 2.75 eV. After exposure to visible light for 6 h, the optimal hydrogen production is about 302 μmol g^?1, which is much higher than that of pure CeO₂.     

Recovery of chlorine from dilute hydrochloric acid by electrolysis using a chlorine resistant anion exchange membrane

A new chlorine resistant anion exchange membrane enables innovative possibilities for hydrochloric acid electrolysis for recovery of chlorine. This is of interest for hydrochloric acid that is neutralized in the chemical industry because purity and concentration are not sufficiently high for recycling. In the common electrolysis process hydrochloric acid is fed into the anode compartment and needs a satisfactory HCl concentration for supplying the anode with chloride ions. Using an anion exchange membrane as a cell separator the feed flows into the cathode chamber where a low HCl concentration is acceptable because Cl⁻ ions at the anode can be supplied by addition of a salt which is not consumed. Experimental data of the membrane and the process are presented: membrane permselectivity improved up to above 97% using CaCl₂ as added salt, chlorine current efficiency up to 98% and oxygen content as low as 0.5 vol%, cell voltage at 4 kA m⁻² 2.3 V, equivalent to 1740 kWh per t produced chlorine, even at low HCl concentrations. Thus, the power consumption is comparable with the common process. A problem of the new process is the high water transport through the membrane. Therefore, experiments for two process alternatives were carried out. Disadvantages of water transport can be avoided by using a high concentrated CaCl₂ solution as anolyte and catholyte and as absorption medium for diluted HCl gas streams. Additionally, a cell design was investigated where the anode is directly connected to the membrane in an empty (gas filled) anode compartment.     

A numerical assessment of bubble-induced electric resistance in aluminium electrolytic cells

This paper reports on an assessment of the bubble-induced electrical resistance in the Hall-Héroult process for primary aluminium production through a combined use of physical and numerical modelling. Using a physical air–water model, the transient bubble dynamics beneath the bottom surface of an anode was captured using a digital camera. Bubble morphology information obtained from the experiment was used to set up a numerical model. Computational fluid dynamics (CFD) modelling was applied to predict the current flow and the corresponding voltage drop across the electrolytic cell with and without the presence of bubbles. The predicted bubble-induced voltage drop for a current density of 0.7 A cm^?2 is about 0.11 V for a bubble coverage of 37 % and 0.29 V for a bubble coverage of 50 %. These values are within the range of experimental measurements reported for commercial cells. The predictions show that the presence of bubbles does not greatly affect global current distribution within the whole cell, but it does significantly affect the local current flow at the anode-bath interface. Locally high current flow occurs at the contact point of the anode bottom surface, bubble and liquid. In addition to the effect of bubble coverage, the bubble size and bubble thickness affect the voltage drop significantly.     

Performance and cycling of the iron-ion/hydrogen redox flow cell with various catholyte salts

A redox flow cell utilizing the Fe²⁺/Fe³⁺ and H₂/H⁺ couples is investigated as an energy storage device. A conventional polymer electrolyte fuel cell anode and membrane design is employed, with a cathode chamber containing a carbon felt flooded with aqueous acidic solution of iron salt. The maximum power densities achieved for iron sulfate, iron chloride, and iron nitrate are 148, 207, and 234 mW cm^?2, respectively. It is found that the capacity of the iron nitrate solution decreases rapidly during cycling. Stable cycling is observed for more than 100 h with iron chloride and iron sulfate solutions. Both iron sulfate and iron chloride solutions display moderate discharge polarization and poor charge polarization; therefore, voltage efficiency decreases dramatically with increasing current density. A small self-discharge current occurs when catholyte is circulating through the cathode chamber. As a result, a current density above 100 mA cm^?2 is required to achieve high Coulombic efficiency (>0.9).     

Electrochemical removal of the insecticide imidacloprid from water on a boron-doped diamond and Ta/PbO<Subscript>2</Subscript> anodes using anodic oxidation process

The removal of pesticides from water is a major environmental concern. This study investigates the electrochemical removal of the insecticide imidacloprid (IMD) from aqueous solutions on a boron-doped diamond (BDD) and Ta/PbO₂ anodes under galvanostatic electrolysis. The influence of operating parameters, such as applied current density (50–100 mA cm^?2), initial chemical oxygen demand COD (0) (281–953 mg L^?1), temperature (25–65 °C) and pH (3.0–10.0) on COD and instantaneous current efficiency (ICE), was studied using the BDD electrode. The degradation efficiency of IMD increased by increasing current density and temperature, but noticeably decreased by the increase of initial pH value and initial concentration of IMD. The COD decay follows a pseudo-first-order kinetic, and the process was under mass transport control. COD removal reaches 90% when using an apparent current density of 100 mA cm^?2, initial COD of 953 mg L^?1, pH of 3.0 and at 25 °C after 4.5 h electrolysis time. Compared with Ta/PbO₂, BDD anode has shown better performance and rapidity in the COD removal using the same electrolysis device.