Hydrogen Removal Rate Research Articles

Optimization of impurity removal and purification flux components and their application in the production of A356 aluminum alloy

Abstract Taking into account the thermodynamic and kinetic conditions of impurity removal and purification, three impurity removal and purification fluxes were designed for comparative experiments. It was found that the impurity and hydrogen removal rates of experimental flux II were the best, with a decrease of 28.1% in micro grain size 28.1 μ m. The spacing between secondary dendrites is significantly reduced. The eutectic silicon phase of the alloy treated with experimental flux II is granular, with small and dispersed inclusions. The fracture structure of the alloy exhibits ductile fracture characteristics, mainly in the form of transgranular microporous aggregation, which helps to improve the plasticity and fatigue resistance of the alloy. The flux purification method, which focuses on impurity removal, can provide a reference for the actual impurity removal in aluminum alloy smelting.

Experimental investigation on reactivation characteristics of liquid–metal heat pipes after hydrogen inactivation

The use of a hydrogen window provides an effective way for liquid–metal heat pipes (LMHPs) to prevent hydrogen inactivation when operated in a hydrogen-containing atmosphere. This work experimentally investigates the effects of various operational conditions on the reactivation characteristics of LMHPs after hydrogen inactivation. Theoretical analysis and sensitivity analysis are conducted to enhance our understanding and optimize reactivation operations. The results indicate that a slope-shaped distribution of hydrogen buffer enable faster reactivation compared to a near-column shape. A sweep-gas flow rate in 200 mL/min is preferred under test conditions, when both the reactivation time and the pure N2 consumption rate are considered. The working temperature significantly affects the reactivation time, as higher temperatures accelerate the NaH decomposition and increase the hydrogen permeability of hydrogen window. The varying of pressure and preheating temperature of sweep gas under test conditions just have little influence on the reactivation time. In practical conditions, vacuum operations are not recommended due to its extremely low hydrogen-removal rate.

Experimental assessment of hydrogen removal rate for passive auto catalytic recombiners

Generation and accumulation of hydrogen in containment atmosphere could pose a potential threat to the integrity of containment as hydrogen can form flammable mixture with air in the containment during postulated severe accident scenario involving multiple failures. Studies on hydrogen generation and distribution for various accident scenarios in reactor containment of Indian pressurized heavy water reactors (PHWRs) indicate that during severe accident with core damage, the possibility of global deflagration in absence of mitigating features is high and may challenge the integrity of containment. Therefore, it is essential to incorporate mitigating provisions inside the containment to prevent the hydrogen reaching deflagration/ detonation limits during such a scenario.Out of several options available for hydrogen mitigation, Passive Auto-catalytic Recombiner (PAR) is one of the most effective and recommended (IAEA -TECDOC-1661(2011)) technology for the removal of hydrogen inside the containment. The testing and evaluation of the performance of PAR for different conditions envisaged in the containment (such as pressure, temperature, steam concentration and poison) is a vital requirement for the deployment of these devices in the nuclear power plants (NPPs). The behavior of catalytic recombiners in terms of hydrogen removal rate with varying hydrogen concentrations is required to be evaluated essentially to work out the numbers and location of recombiners inside the containment. Hence, for testing of catalytic recombiners, an instrumented test facility simulating different reactor containment environment under controlled conditions, has been commissioned. Recombiner Test Facility (RTF) is a test rig for testing of full scale prototypes of catalytic recombiners.Results of Hydrogen Removal Rates (HRR) for varying hydrogen concentration, method for the calculation of HRR and its validation, model equation developed for Corrected Hydrogen Removal Rates (CHRR) usable in numerical simulations are presented in this article. With the proposed method for CHRR, it was established that the Recombiner performance can be evaluated.

Degassing of Aluminum Alloy Melts by High Shear Melt Conditioning Technology: An Overview

The search for more efficient methods for degassing aluminum alloy melts has always been of great interest for the metal industry because the presence of hydrogen and oxides in the melts’ prior casting was detrimental to the integrity and properties of the final products. In this work, we present an overview of the progress and key findings from the research and development of an innovative High Shear Melt Conditioning (HSMC) degassing technology during the Liquid Metal Engineering (LiME) Research Hub project. Compared to conventional rotary degassing, this novel technique was capable of working at higher rotor speeds to efficiently break and disperse the naturally occurring oxide bifilms in the melt and to capture and disperse each supplied inert gas bubble into many tiny bubbles throughout the whole melt. This resulted in the elimination of the need to degas fluxes to remove the oxides in the melt, the reduction in the gas flow required to reach the same level of hydrogen removal rate, and the minimization of the regassing effect after processing. The increased process efficiency allowed for reduced melt processing costs and, at the same time, improved the melt quality, which resulted in fewer defects and improved mechanical properties.

3D simulation of hydrogen distributions affected by a passive auto-catalytic recombiner in an oxygen-starved condition

In this paper, we simulated the change in hydrogen behavior according to a Passive Autocatalytic Recombiner (PAR) under oxygen depletion conditions and compared the results with the THAI project experimental results. In order to reduce calculation cost, we calculated the hydrogen-oxygen bonding reaction in the catalyst region through correlation equations of the hydrogen removal rate according to the PAR type, and we did not consider the shape of the catalyst with a relatively small size and the detailed hydrogen-oxygen bonding reaction. The hydrogen concentration at the PAR inlet and outlet, mixed gas temperature, flow rate at PAR inlet, pressure and total hydrogen removal rate were similar to the experimental results and analysis values in all cases and the reduction in the hydrogen removal rate agreed well under oxygen depletion conditions. However, after the hydrogen injection was stopped, errors of the hydrogen concentration and flow rate at the PAR inlet increased as the buoyancy decreased because the high temperature of the catalyst was not taken into account.

Enhancing removal of hydrogen from granular polysilicon by innovating vacuum separation model and method for SoG-Si

Silane process of granular polysilicon has become a promising method for the preparation of polysilicon due to its continuous low temperature, simple process and low energy consumption. However, granular silicon contains more hydrogen than conventional columnar silicon, which can result in the deterioration of single crystal furnace thermal field life and rod stability by the “hydrogen jump” in the process of Czochralski method. Hydrogen removal has become an important problem to be solved in the industry development. Thus, we propose a hydrogen separation model suitable for silicon system based on vacuum experiment and thermodynamic calculation, which can provide a theoretical basis for the research and development of silicon dehydrogenation method. The predicted removal rate of hydrogen at different temperatures and vacuum pressures is in good agreement with the experimental results, reflecting the reasonability of the model. The results show that the hydrogen removal rate increases with the increasing of temperature and the decreasing of pressure, where temperature plays a leading role in the removal of hydrogen in silicon. At less than one atmosphere, the increase in dehydrogenation rate by 1 °C ranges from 0.01% to 0.25% in the temperature range from 1450 to 1800°CAt temperatures below 1800°Cthe maximum dehydrogenation rate is less than 0.001% for each 1pa reduction in pressure from one atmospheric pressure to 1000pa. According to the model calculation results, a hydrogen removal method is designed by using the vacuum electromagnetic induction. The deep removal of trace hydrogen in silicon has been realized, hydrogen in silicon drops rapidly from around 20 ppm to less than 5 ppm.

Long-Term High-Temperature Longevity Testing of Thermosyphons with Actual Dimensions

The results of a 20-year-long longevity bench testing of nineteen full-size thermosyphons (TSs) made of steel 20 at a steam-water medium temperature of 240–265°C are presented. For the manufacture of TSs, various methods for processing the inner surface and different compositions of filling aqueous solutions have been used. The thermosyphons were periodically removed from the tests and cooled for monitoring the conditional relative vacuum $${{p}_{{{\text{vac}}}}} = 1 - {{{{p}_{0}}} \mathord{\left/ {\vphantom {{{{p}_{0}}} {{{p}_{{{\text{atm}}}}}}}} \right. \kern-0em} {{{p}_{{{\text{atm}}}}}}},$$ where $${{p}_{0}}$$ and $${{p}_{{{\text{atm}}}}}$$ are the absolute pressure in the steam-gas volume of the thermosyphon and the atmospheric pressure, respectively. The value of $${{p}_{0}}$$ was determined using a thermal method described in this paper, which does not require thermosyphon depressurization. After the last inspection of pvac, four TSs were removed from the tests. The solution contained therein was taken for the chemical analysis of its composition. For the inspection of the inner surface, samples were cut out of the pipes. For the samples of one thermosyphon, metallographic studies were performed to assess changes in the structure and mechanical properties of the pipe metal in the course of hydrogen diffusion through the pipe wall. It was revealed that 16 of 19 TSs exhibit a decrease in pvac less than 6% during the test period. A high performance was also obtained for TSs manufactured according to the economical technology reported in this paper. The oxygen-free environment inherent in the thermosyphons provide self-passivation of the thermosyphon internal surface with the formation of a protective layer of magnetite. This layer reduces the corrosion rate with releasing hydrogen up to the rate of hydrogen removal via diffusion through the wall of a carbon steel pipe. Retaining the initial vacuum, reducing the wall thickness of the thermosyphon less than by 0.1 mm, and the positive results of metallographic studies confirmed the potentialities of long-term thermosyphon operation with retaining high heat-transfer characteristics and with the absence of hydrogen influence on the structure and mechanical properties of the thermosyphon metal.

Hydrogen Degassing in a Vacuum Arc Degasser Using a Three‐Phase Eulerian Method and Discrete Population Balance Model

A three‐phase Eulerian model incorporating slag‐steel interactions to predict the rate of hydrogen removal from molten steel in a full‐scale industrial vacuum arc degasser (VAD) has been developed. The interfacial area for hydrogen transfer is calculated using a bubble population balance model. This accounts for bubble growth due to changes in hydrostatic pressure as well as coalescence and breakup. The predicted velocity field and bubble distribution are compared with experimental data in the literature. The bubble size predictions under atmospheric pressure conditions are sensitive to the initial bubble size, while under vacuum conditions they are relatively independent of the initial size. The omission of the slag layer from the model results in a 12% increase in the hydrogen removal rate. Variation in the slag eye diameter as a function of argon flowrate is simulated and compared with empirical correlations in the literature. Hydrogen measurements from a full‐scale VAD unit at Sheffield Forgemasters International Ltd. steelworks are compared to the model predictions for a series of melts of varying initial hydrogen content. Based on the initial hydrogen content of the liquid steel, the model predicts the amount of hydrogen removed to within ±20% of final experimental measurements obtained from the melt.

Modeling the Effect of Plug Positions and Ladle Aspect Ratio on Hydrogen Removal in the Vacuum Arc Degasser

The rate of hydrogen removal from molten steel in a vacuum arc degasser (VAD) is simulated using a three‐phase (slag‐argon‐steel) Eulerian model. The time required to degas a 100 tonne melt from 5 to 1.5 ppm is predicted for a series of ladle aspect ratios and plug layouts. Compared to an axisymmetric single plug system, the degassing time can be reduced by 36% with the use of three equiangular plugs. Increasing the aspect ratio (AR) of the melt from 0.8–1.2 leads to an improvement in degassing performance, followed by a reduction in performance between AR = 1.2–1.6. A radial plug position of 0.5R is optimal for achieving low hydrogen levels in the melt. Reducing the inter‐plug angle from θ = 180° (for double plug) and θ = 120° (for triple plug) to θ = 45° further reduces this time by 18 and 3.8%, respectively. The fastest rate of hydrogen removal is obtained through the use of three plugs at positioned at an angle of θ = 45° and plug radial position of 0.5R.

Experimental study of effect of ambient flow condition on the performance of a passive autocatalytic recombiner

This paper examines the effect of ambient flow conditions on the function of a passive autocatalytic recombiner (PAR), including external flows blowing horizontally at the PAR inlet or outlet. A full size PAR unit was tested at a large-scale test facility. The PAR self-start hydrogen concentration threshold, capacity and ignition limit were determined based on global hydrogen concentration, and the ignition events were monitored using an infrared camera. The test results show that the PAR self-started at 1–2.1vol.% global H2 for all the test conditions and it tended to self-start at slightly higher global H2 concentration under higher ambient flow speed. The overall hydrogen removal rate (apparent capacity) by a given PAR calculated on the basis of global hydrogen concentration could be significantly affected by the local flow pattern. The ignition could occur at higher global H2 concentrations if there is a flow, comparable in size to the PAR self-sustained buoyancy flow, blowing at the PAR outlet. These experiments provide a more detailed understanding of the effect of local flow conditions on PAR effectiveness and indicate the importance of 3-D modeling to capture the local effects.

Comparative study of propane steam reforming in vanadium based catalytic membrane reactor with nickel-based catalysts

The performance of catalytic membrane reactor with Pd-coated V membrane was examined for steam reforming of propane. The long term reforming experiment confirmed the stability of the V membrane with high hydrogen selectivity and permeability. The effect of types of hydrogen permeable membranes on the performance of the catalytic membrane reactor was studied by comparing Pd-coated V, Pd–23Ag, and Pd–10Ag membranes. The types of hydrogen separation membranes (i.e. hydrogen removal rates) did not have a marked effect on the propane conversion rates, while the product compositions were largely influenced by the hydrogen removal rate. Varying metal oxide supports of Ni-catalysts resulted in significant differences in the product compositions. Further, the evaluation of various catalyst-support systems (9wt%Ni–1wt%M/CeO2, M = Co, Pt, Ag, Ru) revealed that hydrogen yield was the highest when 1wt%Ag was added to Ni/CeO2. However, it was also found that excessive secondary metal additions can have negative impact on the catalytic behaviour of parent catalysts.

Contributions to steel semi-finished parts quality improvements

The quality of finite steel products depends on steel content of hydrogen. On steel elaboration, in order to remove the dissolved hydrogen, at least one ladle secondary treatment is needed. The paper introduces the results obtained in the increase of hydrogen removal output, during steel secondary treatment inside the LF installation. Correlations were established between the hydrogen removal rate and the parameters of the secondary treatment (bubbling flow, bubbling time and bubbling pressures).

Removal of Impurities in Aluminum by Uses of Fluxes

A new method for removal of impurities in pure molten aluminum has been presented in this paper. The basic principle of this method is that the molten fluxes are forced to inject into the molten aluminum under counter-gravity, creating strong vortex to mix fully with the molten aluminum. In this way, the impurities in the aluminum will transfer into the flux because of the absorption of the flux to the inclusions. As the result, the molten aluminum is purified. The experiments were carried out for pure aluminum combined with the flux (40wt.% NaCl，30wt.% KCl，10wt.% NaF and 20wt.%Na3AlF6). The results show that after 3 purifying cycles (6 minutes), the inclusion contents decreased from 2. 1% to 0.35%, a removal rate of 83.3%; and hydrogen concentration decreased from 0.37ml/100gAl to 0.12ml/100gA, with hydrogen removal rate being 68%.

Flux Jet Technique for Purification of Molten Aluminum

In this paper a new technology for purification of molten aluminum, namely flux jet technology has been presented. The basic principle of this method is that the molten flux is forced to inject into the molten aluminum and create strong vortex and is uniformly mixed with the molten aluminum under the pressure differential. The flux and the molten aluminum will mix fully to contact each other. In this way, the inclusions in the aluminum will transfer into the flux because of the absorption of the flux to the inclusions. As the result, the molten aluminum will be purified. The experiments were carried out for A357 aluminum foundry alloy with the flux (40% NaCl, 30% KCl, 10% NaF and 20% Na3AlF6). The results show that after 3 cycles, the inclusion contents dramatically decreases from 1.61% to 0.333%, a decline rate of 79.4%; and hydrogen concentration decreased from 0.32ml/100gAl to 0.09ml/100gAl, with hydrogen removal rate being 72%.

The preparation of a metal foam support of Pt/Al 2O 3 for combustion of hydrogen

A catalyst bed is an essential component of a passive autocatalytic recombiner (PAR) used to remove hydrogen that is accidentally released during a meltdown of a nuclear reactor power plant. Metal foam was selected as a catalytic support in this study, due to its excellent thermal conductivity and its high surface-to-volume ratio. The support surface area was additionally increased by the application of an Al 2O 3 washcoat. An investigation of optimum coating conditions of the Pt/Al 2O 3-coated metal foam was performed with respect to the hydrogen conversion rate and the thermal behavior, with the goal of achieving high hydrogen removal rates. The experimental results showed that the hydrogen conversion rates were affected by washcoat weights and platinum weights, and the majority of the experimental conditions of the Pt/Al 2O 3-coated metal foam showed hydrogen conversion rates over 95%. For cost-effective fabrication, the optimum conditions are an Al 2O 3 washcoat weight of 45 wt.%, and a platinum weight of 3 wt.%. These will lead to a complete removal of the hydrogen at a hydrogen concentration of 4 vol.%. Furthermore, since a region of high hydrogen concentrations has a temperature lower than about 560 °C, which is the ignition limit at a hydrogen concentration of 4 vol.% and ambient pressure, the hydrogen risk can be avoided using the metal foam support.

The influence of processing parameters on the ultrasonic degassing of molten AlSi9Cu3 aluminium alloy

The effect of high intensity ultrasound on the degassing of AlSi9Cu3 alloy using the novel MMM (Multi-frequency Multimode Modulated) technology was studied. Different ultrasonic parameters (power and frequency), melt temperature and processing times were tested and their influence on the degassing efficiency evaluated. RPT (Reduced Pressure Test) was used to evaluate the hydrogen content in the alloys and the samples porosity level and density. For the experimental conditions used in this research, it was found that ultrasonic frequency has no influence on the hydrogen removal rate that, in turn, strongly depends on the ultrasonic power, the processing time and the melt temperature. The experimental results suggest that the MMM ultrasonic technology is an important improvement to the fixed-frequency ultrasonic systems by significantly decreasing the processing time to achieve a quasi-equilibrium hydrogen concentration in aluminium melts.

Coupling of ethylbenzene dehydrogenation and benzene hydrogenation reactions in fixed bed catalytic reactors

The application of dual-functionality via a well-mixed catalyst pattern in an integrated fixed bed catalytic reactor for simultaneous production of styrene and cyclohexane has been investigated. A rigorous mathematical model based on the dusty gas model is used for the simulation. Equilibrium shifting is achieved by the drain-off mechanism of hydrogen in dehydrogenation–hydrogenation reaction media. The simulation results show that the fixed bed reactor (FBR) with auxiliary reactions achieves substantial increase in ethylbenzene conversion (≈100%) compared to the fixed bed reactor. Optimal conditions are observed and an effective length criterion for the optimal conditions is presented. The effects of the operating conditions on the reactor performance are examined. It seems that the overall performance of the reactor is a balance between the operating conditions and the rate of hydrogen removal. The results suggest that appreciable energy saving can be achieved by maintaining the reactor beds at different temperatures.

CH4 Decomposition with a Pd−Ag Hydrogen-Permeating Membrane Reactor for Hydrogen Production at Decreased Temperature

The decomposition of CH4 into C and H2 over Ni/SiO2 was investigated with a Pd−Ag hydrogen-permeating membrane reactor. The CH4 decomposition reaction hardly proceeded at temperatures lower than 773 K in a conventional fixed-bed reactor because of a chemical equilibrium limit. However, the conversion of CH4 was dramatically increased by removing the formed hydrogen by the Pd−Ag membrane. Because higher CH4 decomposition conversions were achieved at higher hydrogen removal rates in the Pd−Ag membrane, the removal of the formed hydrogen seems to be the key step for CH4 decomposition reaction at decreased temperature. Because a higher hydrogen permeation rate was achieved on the Pd(77)−Ag(23) membrane than on the Pd(90)−Ag(10) membrane, the CH4 conversion was always higher in membrane reactors using Pd(77)−Ag(23) than in reactors using Pd(90)−Ag(10) for the hydrogen separation membrane. The CH4 conversion increased with increasing contact time of the reactant and/or with increasing sweep Ar flow rate, becaus...

Removal of Particles by ICRF Cleaning in HT-7 Superconducting Tokamak

The ICRF (Ion Cyclotron Range Frequency) cleaning technique has been used as a routine wall cleaning method in the HT-7 superconducting tokamak. In a wide range of toroidal field, the removal rate of residual gas by ICRF cleaning was about twenty times higher than that of glow discharge cleaning (GDC). At different gas pressure and RF power levels, the ICRF cleaning is studied carefully. A good impurity cleaning effect and a very high hydrogen removal rate were obtained. The removal rate of hydrogen by 5 kW ICRF cleaning achieved was 1.6 × 10-5 Torr.l/s. And the relationships among pressure P, outgassing rate Q, atomic layers L absorbed on surface and the cleaning mode were discussed briefly.

Hydrogen removal by annealing from C-doped InGaAs grown on InP by metalorganic chemical vapor deposition

Hydrogen removal from C-doped InGaAs grown on InP substrate by metalorganic chemical vapor deposition is discussed based on the dependence of hole concentration on various annealing conditions. It is revealed that the hydrogen removal rate becomes higher as the annealing temperature is higher and the activation energy of hydrogen removal is about 1.9 eV regardless of C-doped layer thickness. The hydrogen removal rate is also found to be inversely proportional to C-doped layer thickness, suggesting that the hydrogen removal reaction is mainly governed by hydrogen diffusion.