Revisiting Gas to Solids Ratio for Activated Sludge Clarification by Electrolytic Hydrogen Bubbles: Theoretical and Experimental Evaluations

Molecular hydrogen (H2), as the green energy carrier from water electrolysis, can be utilized for separation of suspended micro-particles as electroflotation (EF). This study provides practical guidelines for the gas to solids (G/S) ratio as the governing parameter in EF, based on theoretical estimations and experiments for clarification of activated sludge. The G/S ratio in EF was controlled linearly by current density (j), under quasi-consistent current efficiency (at j > 8 mA/cm2) for H2 (~1) and O2 (~0.4) bubble generations on Ti cathode and IrTaOx anode, respectively. Based on the measured sizes of bubbles (approximated to 35 µm) and biological flocs (discretized to mean sizes of 22.5, 40, 60, 135, and 150 µm), batch flotation experiments estimated the maximum collision-attachment efficiency of 0.057. The rise velocities of floc-bubble aggregate were computed to derive the limiting G/S ratio to overcome the given influent hydraulic loading. Consequently, the estimates (5.23 × 10−4 and 5.92 × 10−4 at hydraulic loading of 0.87 and 1.73 cm/min, respectively) were compatible with the continuous EF experiments.

Stress-driven surface swell and exfoliation of copper as the plasma-facing materials in NBI ICP source

Neutral beam injection (NBI) heating is a significant auxiliary heating method used in Tokamak fusion devices. The material of faraday shield (FS) and accelerator grids in the NBI inductively coupled plasma (ICP) source can be damaged during operation by the high-density hydrogen plasma irradiation, and thus affecting the stability of the NBI system. In this paper, a series of hydrogen plasma exposure experiments are performed on oxygen-free copper (OFC) specimens at 400-850 K with ion energy of 20-200 eV and irradiation fluence up to 1.0×1025 /m2. Meanwhile, the rate equation model is adopted for numerical simulation of the bubble growth and hydrogen retention. The influence of OFC surface temperature, hydrogen ion energy and fluence on OFC damage are experimentally and numerically investigated. Surface observations show that swell and exfoliation are formed on the OFC samples at 400 K and 600 K by scanning electron microscopy (SEM). The hydrogen ion energy varying from 20 to 200 eV at 400 K is observed to have little effect on OFC surface microstructure. The simulative results show that there exist different critical temperatures when the initial bubble radius changes. The bubble surface density rises and the bubble size decreases with increasing temperature (below the critical temperature). In addition, adjacent bubbles get closer to each other with the growth of hydrogen bubbles, and the strong tensile stress is produced inside the surrounding material of hydrogen bubbles. Some cracks caused by hydrogen bubbles appear on the surface of the OFC to relax the pressure-induced stress, ultimately leading to OFC FS/grids material damage. This investigation helps to understand hydrogen retention and failure mechanisms of OFC materials under extreme operation conditions in the NBI devices.

A model of the growth of hydrogen bubbles in the electrolysis of water

The growth of attached bubbles during the electrochemical evolution of hydrogen at a horizontal cathode at the base of a quiescent, dilute aqueous solution is analysed using a simple model of the process that includes the Butler–Volmer reaction model, the diffusion and migration of electroactive species and a symmetry condition that approximately accounts for the presence of periodically spaced bubbles on the electrode surface. The diffusion controlled growth of a bubble approximately follows a $t^{1/2}$ law when the spacing of the bubbles on the electrode is large, departing slightly from it due to the non-uniformity of the concentration of dissolved hydrogen in the supersaturated solution into which the bubble grows, and approaches a $t^{1/3}$ law when the spacing decreases. The space- and time-averaged current density increases exponentially with the applied voltage for an alkaline solution when the consumption of water in the reaction is not taken into account. For an acidic solution, the average current density saturates to a transport limited value that depends on bubble spacing. For a given voltage, the presence of attached bubbles increases the average current density due to the decrease of the concentration overpotential caused by the bubbles. The spacing of the bubbles on the electrode surface decreases when the voltage increases if the maximum supersaturation at the electrode is imposed to be constant. The result suggests that coalescence of attached bubbles will occur above a certain voltage.

Interfacial Reactions between AlSi10 Foam Core and AISI 316L Steel Sheets Manufactured by In-Situ Bonding Process

Aluminum foam sandwiches (AFS) with AlSi10 foam cores and AISI 316L steel skins are manufactured by an in-situ bonding process. The precursor of the core foam was made with the powder compacted method. The precursor and skins, coupled together, were then heated up to the melting point of the Al alloy. The gas released by the blowing agent formed hydrogen bubbles in the melt. producing the foam. Such a porous structure was kept frozen at room temperature via cooling in cold water. To optimize the process conditions, some foaming experiments have been conducted with different holding times and temperatures. Such manufactured AFS were cut, chemically etched and studied with an optical microscope associated with image analysis software to get information about pores morphology in terms of circularity and equivalent diameter. The interface AlSi10-AISI316L has been characterized by SEM and EDX to investigate the bonding conditions between cores and skins. Finally, the AFS have been polished and etched to analyze the microstructure. Quasi-static compressive tests have been performed on the AFS. Obtained results showed that the interface formed during the foaming can be characterized by the inter-diffusion of alloying elements, as confirmed by the good quality of metallurgical joints.

The effect of magnetic field on the dynamics of gas bubbles in water electrolysis

This study determines the effect of the configuration of the magnetic field on the movement of gas bubbles that evolve from platinum electrodes. Oxygen and hydrogen bubbles respectively evolve from the surface of the anode and cathode and behave differently in the presence of a magnetic field due to their paramagnetic and diamagnetic characteristics. A magnetic field perpendicular to the surface of the horizontal electrode causes the bubbles to revolve. Oxygen and hydrogen bubbles revolve in opposite directions to create a swirling flow and spread the bubbles between the electrodes, which increases conductivity and the effectiveness of electrolysis. For vertical electrodes under the influence of a parallel magnetic field, a horizontal Lorentz force effectively detaches the bubbles and increases the conductivity and the effectiveness of electrolysis. However, if the layout of the electrodes and magnetic field results in upward or downward Lorentz forces that counter the buoyancy force, a sluggish flow in the duct inhibits the movement of the bubbles and decreases the conductivity and the charging performance. The results in this study determine the optimal layout for an electrode and a magnetic field to increase the conductivity and the effectiveness of water electrolysis, which is applicable to various fields including energy conversion, biotechnology, and magnetohydrodynamic thruster used in seawater.

In situ investigation on the role of hydrogen evolution on the estimated metastable pit sizes in an Al-Mg alloy

The fraction of pit dissolution charge that contributes to hydrogen evolution (HE) occurring within metastable pits (analyzed in real-time) was determined to evaluate the error it introduces in the pit size estimations derived merely from the analysis of dissolution current. With in situ optical time-lapse imaging, the charge consumed by HE (C<sub>HE</sub>) during the current transients was determined from the size of the hydrogen bubbles evolving at real-time metastable pit locations in an aged Al-Mg alloy. The ratio C<sub>HE</sub>/C<sub>pit</sub> was observed to range between 0.019 and 0.052 depending on the charge of the transient. This difference was attributed to the catalytic nature of the pit surface that develops as a consequence of the pertinent pitting mechanism. Therefore, it was concluded that the HE within the metastable pits might not play a significant role in the underestimation of metastable pit sizes determined from the current transients recorded at anodic potentials below E<sub>pit</sub>.

The Effect of Magnetic Field on the Dynamics of Gas Bubbles in Water Electrolysis

In this work, the movement of the gas bubbles evolved from the platinum electrodes in the influence of various magnetic field configurations are experimentally investigated. The oxygen and hydrogen bubbles respectively evolve from the surface of anode and cathode have distinctive behaviors in the presence of magnetic fields due to their paramagnetic and diamagnetic characteristics. The magnetic field perpendicular to the surface of the horizontal electrode induces the revolution of the bubbles. The opposite revolution direction between the oxygen and hydrogen bubbles cause the swirling of the flow and spread out the bubbles between the electrode which enhances the conductivity and electrolysis effectiveness. On the other hand, the vertical electrodes in the influence of a parallel magnetic field induce horizontal Lorentz force which effectively spells out the bubbles and increases the conductivity and electrolysis effectiveness as well. However, when the layouts of the electrode and magnetic field result in upward or downward Lorentz forces which competes with the buoyancy force, the sluggish flow in the duct would hinder the movement of the bubbles and decrease the conductivity and charging performance. This phenomenon affects the corresponding natural convection and mass transport as well. These results propose the optimal layout of the electrode and magnetic field which is useful to enhance the conductivity or the effectiveness in water electrolysis.