Electrochemical systems suffer from poor management of evolving gas bubbles. Improved understanding of bubbles behavior helps to reduce overpotential, save energy and enhance the mass transfer during chemical reactions. This work investigates and reviews the gas bubbles hydrodynamics, behavior, and management in electrochemical cells. Although the rate of bubble growth over the electrode surface is well understood, there is no reliable prediction of bubbles break-off diameter from the electrode surface because of the complexity of bubbles motion near the electrode surface. Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA) are the most common experimental techniques to measure bubble dynamics. Although the PIV is faster than LDA, both techniques are considered expensive and time-consuming. This encourages adapting Computational Fluid Dynamics (CFD) methods as an alternative to study bubbles behavior. However, further development of CFD methods is required to include coalescence and break-up of bubbles for better understanding and accuracy. The disadvantages of CFD methods can be overcome by using hybrid methods. The behavior of bubbles in electrochemical systems is still a complex challenging topic which requires a better understanding of the gas bubbles hydrodynamics and their interactions with the electrode surface and bulk liquid, as well as between the bubbles itself.

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Review-Physicochemical hydrodynamics of gas bubbles in two phase electrochemical systems.

Electrogenerated bubbles induced convection in narrow vertical cells: PIV measurements and Euler–Lagrange CFD simulation

The present paper focuses on Vertical Plane Electrode Reactors with Gas Electrogeneration, VPERGEs, which are involved in various industrial applications in large scale productions. It is well known that the efficiency of such electrochemical reactors is closely linked to the hydrodynamics of the electrogenerated bubbles which act as moving electrical insulators. Therefore, an accurate understanding of the two-phase flow hydrodynamics in these reactors is of prime importance to optimise their geometry and operation. However, the small inter-electrode space in VPERGEs is problematic for both experimental and numerical (CFD) investigations. In fact, because of the narrowness of the gap separating the electrodes, many key aspects of the multiphase flow field are inaccessible to experimental measurements, while on the other hand, CFD models typically used nowadays are at the limit of validity. This paper presents a review of articles dealing with experimental and numerical investigations of the flow in VPERGE reactors that were classified into four reactors type depending on their geometry and on the operating conditions. Such classification is necessary to enable extracting general information on the flow features. Focus is put in particularly on the advantages and drawbacks of the experimental techniques and numerical approaches encountered in the literature.

Electrogenerated bubbles induced convection in narrow vertical cells: A review

With the increasing use of Computational Fluid Dynamics to investigate multiphase flow scenarios, modelling surface tension effects has been a topic of active research. A well known associated problem is the generation of spurious velocities (or currents), arising due to inaccuracies in calculations of the surface tension force. These spurious currents cause nonphysical flows which can adversely affect the predictive capability of these simulations. In this paper, we implement the Continuum Surface Force (CSF), Smoothed CSF and Sharp Surface Force (SSF) models in OpenFOAM. The models were validated for various multiphase flow scenarios for Capillary numbers of 10 − 3 –10. All the surface tension models provide reasonable agreement with benchmarking data for rising bubble simulations. Both CSF and SSF models successfully predicted the capillary rise between two parallel plates, but Smoothed CSF could not provide reliable results. The evolution of spurious current were studied for millimetre-sized stationary bubbles. The results shows that SSF and CSF models generate the least and most spurious currents, respectively. We also show that maximum time step, mesh resolution and the under-relaxation factor used in the simulations affect the magnitude of spurious currents.

Comparison of Surface Tension Models for the Volume of Fluid Method

A Review of Inorganic Photoelectrode Developments and Reactor Scale‐Up Challenges for Solar Hydrogen Production

Liquid–gas flow patterns in a narrow electrochemical channel

On the electrode boundary conditions in the simulation of two phase flow in electrochemical cells

We show that the gas-liquid flow pattern in a single gas-evolving electrochemical channel can be remarkably different from the flow pattern in multiple submerged gas-evolving electrochemical channels. This is due to the fact that in a single channel there is a higher accumulation of small bubbles and these can considerably affect the liquid velocity pattern which in turn may affect the performance of a cell. Since experimental work is often carried out in single channels, while industrial applications almost always involve multiple channels, this study provides insight into the factors that affect the flow pattern in each situation and establishes the basis for relating the behavior of single-and multiple-channel devices.

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The Flow Pattern in Single and Multiple Submerged Channels with Gas Evolution at the Electrodes

Different flow regimes have been observed, both experimentally and in CFD simulations, in narrow channels with gas evolution. In this manuscript, we examine, using the Euler–Euler model, the flow in a narrow channel, where gas is evolved from a vertical wall. We find some pseudo-turbulent features at conditions described in this manuscript. The transition to this pseudo-turbulent regime is associated with the value of a specific dimensionless group.

Transition to pseudo-turbulence in a narrow gas-evolving channel


 Acidizing of high-temperature carbonate reservoirs faces many challenges and requires a superior retarded acid system with high thermal stability, controlled reaction rate, and acceptable corrosion profile as compared to lower-temperature formations. In this study, a novel retarded acid system is introduced to address the shortcomings of the available retarded acid systems in the market. The proposed retarded acid system is based on a unique formulation of HCl and the sodium salt of monochloroacetic acid and does not require gelation by a polymer or surfactant or emulsification in diesel.
 The proposed acid system combines the use of a strong mineral acid (i.e., hydrochloric acid) with sodium monochloroacetate (HCl/SMCA). The acid system benefits from two mechanisms: 1) hindering the fast reaction of HCl and 2) in-situ acid generation by hydrolysis of SMCA towards glycolic acid which provides dissolution capacity for deeper penetration. The hydrolysis of SMCA occurs over time as acid penetrates through the formation. The HCl/SMCA system has an initial pH of 2-3, which significantly reduces corrosion rates at high temperatures. In this study, the dissolution capacity of the acid system was first measured. Then the potential risk of unwanted precipitation of the reaction products was investigated. Finally, the performances of the SMCA system at various formulations were investigated by performing coreflood experiments at high temperatures. The coreflood experiments were conducted at different injection rates to obtain the acid efficiency curve or pore-volume-to-breakthrough (PVbt) curve. Finally, corrosion experiments were conducted at high temperatures using three SMCA formulations.
 From the dissolution experiments, it was found that the dissolution capacity of the HCl/SMCA acid system, containing only 6 wt% HCl, can be as high as 1 lb CaCO3 scale/gal. It was shown that the reaction products from the calcite dissolution are fully soluble and the chelation by sodium gluconate is the main responsible mechanism. From the coreflood results, it was found that the new HCl/SMCA system can efficiently stimulate limestone formations with no face dissolution. It improves the wormholing performance significantly over HCl acid only and the PVbt decreases from 2.6 to 1 at 130°C. Benefiting from the gentle nature of the acid/SMCA system, tighter formations can be treated at much lower injection rates. CT scan images confirm the favorable wormhole propagation characteristics of the SMCA formulations. It was shown that 60% of the acid capacity remained unused even at very low injection rate, showing the retardation properties of the proposed system. According to the corrosion data, when SMCA used as retarding agent the corrosivity of HCl is decreased and much lower inhibitor concentrations are needed.
 The new HCl/SMCA system effectively retards initial non-uniform HCl acidizing and adds in-situ acid generation, thereby improving overall the uniformity of the formation acidizing process. This slow-release HCl/SMCA acid system has a low viscosity and less aggressive initial pH, making its use attractive in a broad range of stimulation applications and offering the oilfield industry a high performing and a cost-effective alternative to acid retardation via polymers/surfactants or emulsification in diesel.

A. Bokkers

Papers

Liquid–gas flow patterns in a narrow electrochemical channel

On the electrode boundary conditions in the simulation of two phase flow in electrochemical cells

The Flow Pattern in Single and Multiple Submerged Channels with Gas Evolution at the Electrodes

Transition to pseudo-turbulence in a narrow gas-evolving channel

New Retarded Acid System for High Temperature Applications: An Efficient Alternative to Emulsified and Viscosified Acid Systems