Electrolytes flow patterns

Changes in the bnlk electrolyte velocity far from the electrode and/or variations in the electrolyte flow pattern (as might occnr if the cell geometry were altered) can be accounted for in Equation (26.73) by a change in the valne of 5, as is well known from bonndary-layer theory in nonelec-trochemical systems. For given valnes of and C , an increase in bulk electrolyte velocity will decrease 5, resulting in an increase in the current density. Frequently, 5 is related to the mass transfer coefficient (k, a common mass transfer parameter,... 

Electrolytic gas evolution can be discussed on two scales of length. The macroscopic or process scale is important to the overall design of equipment and includes modeling the overall distribution of gas in the reactor and the effects of gas bubbles on the gross electrolyte flow pattern. The microscopic scale is where the details of bubble events and their consequences are found. In this review, I concentrate on the latter, microscopic scale. 

The design and operation of electrolytic processes is the province of the electrochemical engineer. It is convenient for him to concentrate on the electrolysis cell and, indeed, its design is a complex business requiring an understanding of potential and current distribution, electrolyte flow patterns, electrode kinetics, etc., and the consideration of the cost and performance of cell components (e.g. electrodes, separators, rectifiers). The cell, however, must fit into the overall process and hence it is normal to develop figures of merit which indicate cell performance and permit a discussion of its interaction with other parts of the process. 

Figure 5.8 Electrolysis cell used by the Cams Chemical Co. for the production of potassium permanganate, (a) Electrolyte flow pattern, (b) Design of bipolar electrodes.

In the electrochemical cell the workpiece is the anode and the tool is the cathode. The electrolyte is fed through the tool at a rapid flow rate, 9—60 m s in such a way that the supply of electrolyte is uniform over the surface being machined. In practice the electrolyte flow pattern is as important as the arrangement of conducting surfaces on the tool in determining the current density distribution and both factors must be considered in the design of the tool. 

The tool will be constructed of both conducting (copper, copper/tungsten alloy or steel) and non-conducting (epoxy resins and rubber materials) surfaces and the positioning of the holes for electrolyte entry will determine the electrolyte flow pattern between the workpiece and the tool. In order further to ensure even flow of electrolyte it is also common to use restrictors particularly when forming a feature in a flat surface. 

Real-time tracking of zinc-side electrode activity, linked to electrolyte flow patterns, and zinc deposition and de-plating behavior... 

Consider coupling with fluid flow modelling studies to optimize electrolyte flow patterns... 

Whilst acknowledging the need to consider any chemical process as an integrated package, it is convenient in this chapter to focus attention on the electrochemical stage and particularly upon the key component, the electrochemical reactor The detailed design and characterization of such reactors may involve complex, interactive procedures. An understanding of factors such as potential and current distribution, electrolyte flow patterns, electrode ther-... 

Gas and liquid flow up along the membrane, then turn to the inlet of a narrow channel at the top of the chamber. This flow pattern enhances continuous replacement of electrolyte over the whole membrane surface. It is especially effective in eliminating gas stagnation at the top zone of the electrolysis area. The DAM-type system ensures that the fine-bubble flow is constant through its narrow channel and that smooth gas separation occurs at the outlet of the channel. Gas and liquid flow separately through an upper duct, an outlet nozzle and an outlet hose, then to a... 

The interaction of forced and natural convective flow between cathodes and anodes may produce unusual circulation patterns whose description via deterministic flow equations may prove to be rather unwieldy, if possible at all. The Markovian approach would approximate the true flow pattern by subdividing the flow volume into several zones, and characterize flow in terms of transition probabilities from one zone to others. Under steady operating conditions, they are independent of stage n, and the evolution pattern is determined by the initial probability distribution. In a similar fashion, the travel of solid pieces of impurity in the cell can be monitored, provided that the size, shape and density of the solids allow the pieces to be swept freely by electrolyte flow. 

None of the set-ups discussed so far provides stirring of the electrolyte for bubble removal or for enhancement of the reaction rates. A standard set-up developed to study kinetic electrode processes is the rotating disc electrode [11]. The electrode is a small flat disc set in a vertical axle. The hydrodynamic flow pattern at the disc depends on rotation speed and can be calculated. An additional ring electrode set at a different potential provides information about reaction products such as, for example, hydrogen. However, because this set-up is designed to study kinetic processes and is usually equipped with a platinum disc, it becomes inconvenient if silicon samples of different geometries have to be mounted. 

Certain criteria have to be met in the construction of hydrodynamic electrodes, such that the laminar flow pattern, which is used in the derivation of the theoretical equations, is conformed to. Thus edge effects, which are due to the fact that electrode and surrounding mantle are not of infinite size and which are also dependent on cell dimensions, must be minimised. The shape of the electrode and mantle is important the surfaces must be smooth and there must be no discontinuities or electrolyte penetration at the electrode/mantle junction. 

Voltammetry experiments are occasionally undertaken in the form of a tubular or rectangular channel through which the electrolyte solution is pumped at a more or less constant velocity. The electrode may form the channel itself or be embedded in the wall of an inert material, which defines the flow pattern. Sometimes the channel is packed with small particles of electrode material in contact with each other. The latter situation is designed to improve the conversion efficiency of the cell. When all the electroactive molecules are converted during passage through such a porous bed, the efficiency is 100% and the cell is said to be operating coulometrically (see Sec. IV.F). 

Rote et al. (1993, 1994) used a carotid thrombosis model in dogs. A calibrated electromagnetic flow meter was placed on each common carotid artery proximal to both the point of insertion of an intravascular electrode and a mechanical constrictor. The external constrictor was adjusted with a screw until the pulsatile flow pattern decreased by 25 % without altering the mean blood flow. Electrolytic injury to the intimal surface was accomplished with the use of an intravascular electrode composed of a Teflon-insulated silver-coated copper wire connected to the positive pole of a 9-V nickel-cadmium battery in series with a 250000 ohm variable resistor. The cathode was connected to a subcutaneous site. Injury was initiated in the right carotid artery by application of a 150 xA continuous pulse anodal direct current to the intimal surface of the vessel for a maximum duration of 3 h or for 30 min beyond the time of complete vessel occlusion as determined by the blood flow recording. Upon completion of the study on the right carotid, the procedure for induction of vessel wall injury was repeated on the left carotid artery after administration of the test drug. 

Tin, in contrast to the other metals, deposited spontaneously onto Pt (that is, at open circuit, without the need for current flow in an external circuit) [51]. Auger spectra following spontaneous deposition showed a strong oxygen signal. Anodic electrolysis (oxidation) increased the oxidation state of the surface layer somewhat and rendered the surface passive except for evolution of H2 at very negative potentials and 02 at very positive potentials. Once immersion of Pt(lll) in Sn2+ (C1 or Br ) solution had taken place, the Sn deposit could not be removed from the surface by electrolysis in the same electrolyte. LEED patterns of the Sn layer were diffuse, indicating that the tin oxide layer was disordered. The pathway of spontaneous Sn deposition probably involves disproportionation, followed by oxidation of the metallic tin. 

The driving force for the diffusion of across the solid electrolyte membrane is the difference in O2 concentration between the anode and cathode side. This difference is a result of depletion of O2 by partial oxidation of CzHe on the anode side. Since the concentration of oxygen in the anode has a profound effect on the concentration gradient of 0 across the electrolyte membrane, the flow pattern of fuel/air mixture around the fuel cell has to be carefully controlled through the flow geometry. 

---