The oxygen production step of a copper-chlorine thermochemical water decomposition cycle for hydrogen production: Energy and exergy analyses

In the copper-chlorine (Cu-Cl) thermochemical cycle water is decomposed into its constituents (oxygen and hydrogen) by a series of chemical reactions. The cycle involves five steps in which three thermally driven chemical reactions and one electrochemical reaction take place. Oxygen is produced during one of the main chemical reactions. In the present study, the O₂ production step is described with its operational and environmental conditions, and energy and exergy analyses are performed. The cycle is assumed driven using nuclear energy. Various parametric studies are carried out on energetic and exergetic aspects of the step, considering variable reaction and reference-environment temperatures. At a constant reference-environment temperature of 25 °C, the exergy destruction of the O₂ production step varies between 4500 and 23,000 kJ/kmol H₂ when the reaction temperature increases from 450 to 1000 °C. At a 500 °C reaction temperature and a 25 °C reference-environment temperature, the exergy destruction for this step is found to be 5300 kJ/kmol H₂. At a reaction temperature of 500 °C and a reference-environment temperature of 25 °C, the exergy efficiency of the step is determined to be 96% and to decrease with increasing reaction temperature and/or reference-environment temperature.     

Thermodynamic modeling of a nuclear energy based integrated system for hydrogen production and liquefaction

A nuclear based integrated system for hydrogen production and liquefaction with a newly developed four-step magnesium–chlorine cycle is proposed. The system uses nuclear energy to supply heat for the Rankine cycle and Mg–Cl cycle, where the power produced by the Rankine cycle is used to run the electrolysis steps of the Mg–Cl cycle and liquefaction cycle compressors. The four-step Mg–Cl cycle is specifically designed to decrease the electrical work consumption of the cycle by capturing HCl in dry form with an additional step to conventional three-step cycle. A performance assessment study is undertaken based on energy and exergy analysis of the subsystems, and total energy and exergy efficiencies of the plant are found to be 18.6%, and 31.35%. The comparisons of the subsystem efficiencies and total exergy destructions show that highest irreversibility ratio belongs to the Mg–Cl cycle by 41%, respectively.     

Factorial design of experiment (DOE) for parametric exergetic investigation of a steam methane reforming process for hydrogen production

Hydrogen is expected to play a significant role in future energy systems. The efficient production of hydrogen at a minimum cost and in an environmentally acceptable manner is crucial for the development of a hydrogen-including economy. The exergy analysis is a powerful tool to quantify sustainable development potential. An important aspect of sustainable development is minimizing irreversibility. The purpose of this study is to perform the exergy analysis of a steam methane reforming (SMR) process for hydrogen production. As a first step, an exergy analysis of an existing process is shown to be an efficient tool to critically examine the process energy use and to test for possible savings in primary energy consumption. The results of this investigation prove that the exergetic efficiency of the SMR process is 65.47%, and the majority of destroyed exergy is localized in the reformer with a 65.81% contribution to the whole process destroyed exergy. Next, an exergetic parametric study of the SMR has been carried out with a factorial design of experiment (DOE) method. The influence of the reformer operating temperature and pressure and of the steam to carbon ratio (S/C) on the process exergetic efficiency has been studied. A second-order polynomial mathematical model has been obtained through correlating the exergetic efficiencies with the reformer operating parameters. The results of this study show that the rational choice of these parameters can improve the process exergetic performance.     

Application of oxygen ion-conductive membranes for simultaneous electricity and hydrogen generation

The high temperature of the air in power generation gas-turbine cycles involving natural gas (mainly methane) oxidation accounts for the utilization of ion-conductive membranes within solid oxide fuel cells (SOFCs) and membrane reactors (MRs). In SOFCs, the electricity is directly derived from the chemical exergy of methane (SOFCs with internal methane reforming are considered here). Within a membrane reactor (MR), which is considered a substitute for combustion chambers in traditional gas-turbine units, the ion-conductive membranes separate oxygen from air and allow the flow of the hot combustion products (carbon dioxide and steam) to be separated from air. It permits the use of combustion products which are not diluted in nitrogen in the process of methane conversion into hydrogen. A modified gas-turbine cycle that includes a SOFC stack, an MR (instead of a traditional combustion chamber), and a catalytic reactor to convert methane to hydrogen is proposed. An exergy analysis of the proposed system is conducted to evaluate its exergy efficiency and the exergy losses for the processes occurring within the system. It is shown that, in comparison to the traditional gas-turbine cycle, there is a significant reduction (more than three times) in the exergy losses for the most irreversible process occurring in the system, natural gas combustion. It is also found that the proposed cogeneration scheme, including both power generation and the industrial catalytic conversion of methane to hydrogen, permits improved efficiencies for both technologies. The efficiency of this cogeneration, as well as the reduction in exergy losses, is demonstrated by the following observation: if the value of energy (exergy) efficiency of hydrogen production is considered equal to that for a traditional process, the corresponding thermal (energy) efficiency for electricity generation would reach values of 80–96% depending on the efficiency of a SOFC stack. The combined SOFC and MR application also eliminates the possibility of toxic nitrogen oxides formation and, at the same time, makes carbon dioxide removal from flue gases feasible (due to its high concentration). The development of the proposed technology is especially important, within the context of the hydrogen economy, if the produced hydrogen is used as a fuel for fuel cell vehicles.     

A hybrid adsorbent-membrane reactor (HAMR) system for hydrogen production

Design characteristics and performance of a novel reactor system, termed a hybrid adsorbent-membrane reactor (HAMR), have been investigated for hydrogen production. The recently proposed HAMR concept couples reactions and membrane separation steps with adsorption on the membrane feed-side or permeate-side. Performance of conventional reactors has been significantly improved by this integrated system. In this paper, an HAMR system has been studied involving a hybrid-type packed-bed catalytic membrane reactor undergoing methane steam reforming through a porous ceramic membrane with a CO₂ adsorption system. This HAMR system is of potential interest to pure hydrogen production for fuel cells for various mobile and stationary applications. Reactor behaviors have been investigated for a range of temperature and pressure conditions. The HAMR system shows enhanced methane conversion, hydrogen yield, and product purity, and provides good promise for reducing the hostile operating conditions of conventional reformers, and for meeting the product purity requirements.     

A zinc nanoparticles-dispersed multi-scale web of carbon micro-nanofibers for hydrogen production step of ZnO/Zn water splitting thermochemical cycle

Water splitting via the two-step ZnO/Zn thermochemical cycle is a promising and environmentally benign method for producing hydrogen from steam. In this study, we focus on the second step of the cycle, which is the exothermic hydrolysis reaction of Zn nanoparticles with steam. The unique Zn nanoparticle-dispersed carbon micro-nanofibers (Zn-CNFs) were prepared by impregnating carbon microfibers (ACFs) with a sodium dodecyl sulfate (SDS)-mediated mono-dispersed aqueous solution of Zn(II), followed by calcinations and reduction. The surfactant increased the metal (average crystal size ∼ 25 nm) loading by approximately two-fold on the ACFs. The CNFs were grown on Zn-ACF at 700 °C by catalytic chemical vapor deposition (CCVD) using acetylene as the carbon source. Zn has dual roles in this system, one as a catalyst for CNF growth and the other as a reactant in the hydrolysis reaction. The water-splitting reaction was performed at different steam and N₂ flow rates and reaction temperatures. The production rate and yield of H₂ at a reaction temperature of 600 °C were calculated as 1.66 × 10⁻⁶ mol/g s and 80%, respectively, which are comparable to or higher than results reported in the literature. The Zn-CNFs prepared in this study are a potential candidate for the H₂ production step of the ZnO/Zn thermochemical cycle.     

隔膜法和离子膜法烧碱生产中氯氢处理扩改及并轨工艺

介绍了湖北沙隆达股份有限公司13万t/a氯气处理、氢气处理工序的扩改工艺。给出9万t／a隔膜法与4万t／a离子膜法烧碱装置的氯氢处理工序的并轨方案及实施办法。扩改过程中,在氯气系统配置了4个安全水封,在氢气系统配置了2个安全水封,起到了保证安全生产、保护电解槽的作用。     

Performance Assessment of a Combined PEM Fuel Cell and Triple‐Effect Absorption Cooling System for Cogeneration Applications

In this paper, a parametric study of a combined proton exchange membrane (PEM) fuel cell and triple‐effect absorption cooling system (TEACS) is undertaken to investigate the effect of different operating conditions and system parameters on the COPs, efficiency of the fuel cell and the integrated system's overall utilisation factor. It is found that the fuel cell efficiency increases from 40% to 44.5% as the operating temperature of the fuel cell increases. However, as the fuel cell's temperature and current density increase, the COPs decrease from 2.4 to 0.9 as a result of the increase in the energy output of the fuel cell ranging from 7.4 to 10.7 kW. The efficiency of the fuel cell decreases from 41% to 32% with an increase in both fuel cell's current density and membrane thickness. The overall utilisation factor of the integrated system decreases from 84% to 35% with an increase in the current density and molar flow rate. Finally, this study reveals that the present integrated PEM fuel cell unit with a TEACS can be considered as an attractive and environmentally benign option for cogeneration purposes in sustainable buildings.     

Experimental studies of a hybrid adsorbent-membrane reactor (HAMR) system for hydrogen production

An experimental study of the performance of a novel reactor system—termed the hybrid adsorbent-membrane reactor (HAMR)—is described. In the HAMR the reaction and membrane separation steps are coupled with adsorption. It was shown previously by our group for esterification reactions that this system results in significantly improved performance. The focus in this paper is on the use of the HAMR for hydrogen production. We present experimental investigations of the HAMR for the water-gas-shift (WGS) reaction using layered double hydroxides as adsorbents for CO₂ and nanoporous H₂-selective carbon molecular sieve membranes. The reactor characteristics are investigated for a range of temperatures and pressures relevant to the WGS application, and are compared with the predictions of a mathematical model previously developed by our group.     

Studies on the activation of hydrogen peroxide for color removal in the presence of a new Cu(II)-polyampholyte heterogeneous catalyst

In this work we describe the application of a new non-soluble and non-porous complex with copper ion based on ethylene glycol diglycidyl ether (EGDE), methacrylic acid (MAA) and 2-methylimidazole (2MI) in the decolorization of an azo dye Methyl Orange (MO) as a model pollutant at room temperature.The complex with copper ion was studied by ESR and SEM and was tested as a heterogeneous catalyst for H₂O₂ activation. A possible mechanism of interaction involves the production of hydroxyl radicals (confirmed by ESR), dioxygen and water.The Cu(II)-polyampholyte/H₂O₂ system acted efficiently in the color removal of MO. The adsorption and oxidative degradation of the azo-based dye followed pseudo-first-order kinetic profiles, and the rate constant for degradation had a second-order dependence on copper ion content in the mixture.A removal of MO higher than 90% was achieved in 20 min at pH 7.0, combining 0.8 mM of complexed copper ions in the mixture with 24 mM hydrogen peroxide.The dye adsorbed on the polyampholyte following a L4-type isotherm with 4.9 μmol g⁻¹ maximum loading capacity and 3.1 μM dissociation constant for the first monolayer.     

Preparation of mesoporous In–Nb mixed oxides and its application in photocatalytic water splitting for hydrogen production

A series of mesoporous In–Nb mixed oxides was synthesized using NbCl₅ and In₂O₃ as the starting material and triblock copolymer P123 as template. We investigated the influence of indium content on the synthesis and characteristics of the mesoporous In–Nb mixed oxides, and their photocatalytic activities for water splitting. The materials were characterized by small angle X-ray scattering, powder X-ray diffraction, extended X-ray absorption fine structure, transmission electron microscopy, scanning electron microscopy, energy dispersive spectrometer, N₂ sorption and UV–vis spectroscopy. The surface area of mesoporous In–Nb mixed oxides was greater than 90 m²/g with a wormhole framework. The optimization of synthesis condition of the mesoporus In–Nb oxides catalyst contained a small fraction of highly dispersed indium (In/Nb = 0.13) species intercalated into the framework of mesoporous niobium oxides and exhibited a high photocatalytic activity for water splitting reaction which was about 2.7 times as compared to mesoporous Nb₂O₅ and was about 19 times higher than commercial bulk Nb₂O₅.     

Thermodynamic analysis of a cyclic water gas-shift reactor (CWGSR) for hydrogen production

The cyclic water gas-shift reactor (CWGSR) is a cyclically operated fixed bed reactor for the removal of carbon monoxide from reformate gases. It is based on the repeated reduction of iron oxide by reformate gases and its subsequent oxidation by steam. To evaluate the thermodynamic limits of this reactor, we develop a model under the assumption of chemical equilibrium. For this purpose, we conduct a wave analysis which shows that the reactor behaviour is dominated by the movement of sharp reaction fronts. Depending on the positions of these fronts at cyclic steady state, five different operating regimes of the CWGSR can be identified. Besides the qualitative analysis of the regimes, the equilibrium model also offers a first quantitative analysis regarding the two performance parameters, i.e. fuel utilisation and product concentration. At 750 °C, a fuel utilisation of 55% can be achieved, and the molar hydrogen fraction in the product stream is up to 70%. The equilibrium model can be used for a first estimate of favourable design and operating parameters of the CWGSR.     

Convective heat transfer and solid conversion of reacting particles in a copper(II) chloride fluidized bed

This paper develops a predictive model of convective heat transfer and conversion of cupric chloride particles in a fluidized bed reactor of a copper–chlorine (Cu–Cl) cycle of thermochemical hydrogen production. The hydrolysis reaction of particles in the fluidized bed is endothermic and it requires excess steam for complete conversion of cupric chloride solid. The excess steam supply may be used for partial heat supply to the endothermic reaction, and also to avoid defluidization in the bed. To avoid defluidization, the change of gas flow in the bed due to the reaction should be minimized at a given operating condition. The model predicts the maximum possible steam inlet temperature, steam conversion, amount of partial heat supply, and also gas flow rates through the bed to avoid defluidization. The new model presents significant new insight by analyzing the hydrodynamic and mass transport processes, considering the equilibrium limitation on the conversion of cupric chloride solid. The model results indicate that the chemical reaction requires a high mole ratio of steam for complete conversion of cupric chloride particles. The maximum steam conversion is limited by temperature, pressure, and the presence of hydrogen chloride gas. The maximum conversion of steam at 400 °C is 3.75% and it requires excess steam of 12.8 moles per unit mole of cupric chloride solid for complete conversion of solid. The heat supply by steam for the reaction, as well as raising the solid feed to the reaction temperature, varies with reaction temperature. The paper also adds significant new insight by analyzing the steam flow requirement in terms of temperature, conversion rate, and quality of fluidization. Additional new results are presented and applications discussed for the Cu–Cl cycle of nuclear hydrogen production.     

Modelling and simulation of a hybrid process (pervaporation–distillation) for the separation of azeotropic mixtures of alcohol–ether

This work reports the modelling and simulation of a hybrid process, based on the combination of distillation and pervaporation, for the separation of azeotropic mixtures of alcohol–ether. After having selected the separation of methanol‐2‐metoxi‐2,2‐dimethyl ethane (ETHER) as a motivating example the mathematical modelling of the distillation column was achieved and used together with a mass transfer model previously reported for the pervaporation operation in order to simulate the behaviour of the hybrid process for different compositions of the feed stream (case 1: 3.2 wt% methanol, 55.4 wt% C₄, 41.4 wt% ETHER, and case 2: 5.2 wt% methanol, 42 wt% C₄, 52.8 wt% ETHER). Simulation tasks were carried out with the process modelling system gPROMS and the results of alternative process configurations that result from the relative location of the separation technologies have been compared on the basis of the required membrane area. Finally, the design of the pervaporation unit including the overall processing costs is reported. Copyright © 2001 Society of Chemical Industry     

Synthesis of a highly active carbon-supported Ir–V/C catalyst for the hydrogen oxidation reaction in PEMFC

The active, carbon-supported Ir and Ir–V nanoclusters with well-controlled particle size, dispersity, and composition uniformity, have been synthesized via an ethylene glycol method using IrCl₃ and NH₄VO₃ as the Ir and V precursors. The nanostructured catalysts were characterized by X-ray diffraction and high-resolution transmission electron microscopy. The catalytic activities of these carbon-supported nanoclusters were screened by applying on-line cyclic voltammetry and electrochemical impedance spectroscopy techniques, which were used to characterize the electrochemical properties of fuel cells using several anode Ir/C and Ir–V/C catalysts. It was found that Ir/C and Ir–V/C catalysts affect the performance of electrocatalysts significantly based on the discharge characteristics of the fuel cell. The catalyst Ir–V/C at 40 wt.% displayed the highest catalytic activity to hydrogen oxidation reaction and, therefore, high cell performance is achieved which results in a maximum power density of 563 mW cm⁻² at 0.512 V and 70 °C in a real H₂/air fuel cell. This performance is 20% higher as compared to the commercial available Pt/C catalyst. Fuel cell life test at a constant current density of 1000 mA cm⁻² in a H₂/O₂ condition shows good stability of anode Ir–V/C after 100 h of continuous operation.     

Digestion of pre‐treated food waste in a hybrid anaerobic solid–liquid (HASL) system

The hybrid anaerobic solid–liquid (HASL) system was developed to be used in industrial‐scale operations to minimize the amount of food waste for disposal in Singapore. Thermal pre‐treatment of food waste at 70 °C for 2 h (experiment E1) or at 150 °C for 1 h (experiment E2) facilitated the hydrolytic and acidogenic processes in the acidogenic reactor and methanogenesis in the methanogenic reactor in the HASL system. The highest dissolved chemical oxygen demands in the effluents from the acidogenic reactors were 17 575, 19 980 and 24 235 mg dm^?3 in the control with food waste without thermal pre‐treatment and experiments E1 and E2, respectively. The maximum concentrations of methanogens in the methanogenic reactor were 2.3 × 10⁷, 3.8 × 10⁷, 4.3 × 10⁷ cells cm^?3 for the control and experiments E1 and E2, respectively. However, the performances of the methanogenic phase in terms of specific activity of methanogens did not differ significantly for the control and experiments E1 and E2. Use of thermally pre‐treated food waste halved the time to produce the same quantity of methane in comparison with anaerobic digestion of fresh food waste. The fluorescent measurements of co‐enzyme F₄₂₀ and oligonucleotide probe Arc915 specifically bound (hybridized) with 16S rRNA were used for monitoring of methanogens during anaerobic digestion of food waste. There was a linear correlation between these parameters and the concentration of methanogens in the effluent from the methanogenic reactor. Copyright © 2005 Society of Chemical Industry     

Scanning electrochemical microscopy characterization of bimetallic Pt–M (M = Pd, Ru, Ir) catalysts for hydrogen oxidation

A combinatorial screening method, combined with scanning electrochemical microscopy (SECM) in a tip-generation–substrate-collection (TG–SC) mode, was applied to systematically and rapidly identify potential bimetallic catalysts (Pt–M, M = Pd, Ru, Ir) for the hydrogen oxidation reaction (HOR). The catalytic oxidation of hydrogen on the candidate catalysts was further examined during cyclic voltammetric scans of the substrate with a tip close to the substrate. The quantitative rate of hydrogen oxidation on the candidate substrates was determined for different substrate potentials from SECM approach curves by fitting to a theoretical model. SECM screening results revealed that Pt₄Pd₆, Pt₉Ru₁ and Pt₃Ir₇ were the optimum composition of the catalysts from the Pt–Pd, Pt–Ru and Pt–Ir bimetallic systems for hydrogen sensors. The catalytic activity of the candidate catalysts in HOR was highly dependent on the substrate potential. The kinetic parameters for HOR were obtained on Pt₄Pd₆ (Tafel slope = 124 mV, k° = 0.19 cm/s, α = 0.52), Pt₉Ru₁ (Tafel slope = 140 mV, k° = 0.08 cm/s, α = 0.58) and Pt₃Ir₇ (Tafel slope = 114 mV, k° = 0.11 cm/s, α = 0.48) and compared with Pt (Tafel slope = 118 mV, k° = 0.17 cm/s, α = 0.5). Among the bimetallic catalysts studied, Pt₄Pd₆ exhibited the highest activity toward HOR with a high standard rate constant value in a wide range of applied potentials.     

Thermoeconomic design of a multi-effect evaporation mechanical vapor compression (MEE–MVC) desalination process

Desalination of seawater accounts for a worldwide water production of 24.5 million m³/day. A “hot spot” of intense desalination activity has always been the Arabian Gulf, but other regional centers of activity emerge and become more prominent, such as the Mediterranean Sea and the Red Sea, or the coastal waters of California, China and Australia. Despite the many benefits the technology has to offer, concerns rise over potential negative impacts on the environment. Key issues are the concentrate and chemical discharges to the marine environment, the emissions of air pollutants and the energy demand of the processes. To safeguard a sustainable use of desalination technology, the impacts of each major desalination project should be investigated and mitigated by means of a project- and location-specific environmental impact assessment (EIA) study, while the benefits and impacts of different water supply options should be balanced on the scale of regional management plans. In this context, our paper intends to present an overview on present seawater desalination capacities by region, a synopsis of the key environmental concerns of desalination, including ways of mitigating the impacts of desalination on the environment, and of avoiding some of the dangers of the environment to desalination.