Mass transport processes convective

There are three types of mass transport processes within a microfluidic system convection, diffusion, and immigration. Much more common are mixtures of three types of mass transport. It is essential to design a well-controlled transport scheme for the microsystem. Convection can be generated by different forces, such as capillary effect, thermal difference, gravity, a pressurized air bladder, the centripetal forces in a spinning disk, mechanical and electroosmotic pumps, in the microsystem. The mechanical and electroosmotic pumps are often used for transport in a microfluidic system due to their convenience, and will be further discussed in section 11.5.2. The migration is a direct transport of molecules in response to an electric field. In most cases, the moving... 

Mass transfer (continued) in monolith, 27 89 in porous catalyst, 27 60-63, 68 in tubular reactor, 27 79, 82, 87 Mass transport processes, 30 312-318 convective, 30 312-313 diffusive, 30 313-315 selectivity, 30 316... 

The two major causes of uneven current distribution are diffusion and ohmic resistance. Nonuniformity due to diffusion originates from variations in the effective thickness of the diffusion layer 8 over the electrode surface as shown in Figure 10.13. It is seen that 8 is larger at recesses than at peaks. Thus, if the mass-transport process controls the rate of deposition, the current density at peaks ip is larger than that at recesses since the rate of mass transport by convective diffusion is given by... 

There are three kinds of mass transport process relevant to electrode reactions migration, convection and diffusion. The Nernst—Planck equation... 

Mass transport processes - diffusion, migration, and - convection are the three possible mass transport processes accompanying an - electrode reaction. Diffusion should always be considered because, as the reagent is consumed or the product is formed at the electrode, concentration gradients between the vicinity of the electrode and the bulk solution arise, which will induce diffusion processes. Reactant species move in the direction of the electrode surface and product molecules leave the interfacial region (- interface, -> interphase) [i-v]. The - Nernst-Planck equation provides a general description of the mass transport processes. Mass transport is frequently called mass transfer however, it is better to reserve that term for the case that mass is transferred from one phase to another phase. 

Tubular electrode — Working electrode design employing a tube of the electrode material to be studied with (i.e. secondary battery) the electrolyte solution flowing through the orifice of the tube. This way well-defined forced convection of solution can be established. -> Mass transport processes can be treated mathematically. 

The experimental technique and electrode geometry should be selected to match the kinetic time-scale (the time domain over which a chemical process occurs, e.g. 1/k, where k is a first-order rate constant) of the reaction being studied. This is achieved by varying the rate of mass transport via convection, electrode size/shape or potential scan rate. 

In general, electrochemical systems are heterogeneous and involve at least one (or both) of the fundamental processes - mass transport and an electron-transfer reaction. Moreover, electrochemical reactions involve charged species, so the rate of the electron-transfer reaction depends on the electric potential difference between the phases (e.g. between the electrode surface and the solution). The mass transport processes mainly include diffusion, conduction, and convection, and should be taken into account if the electron-transfer reaction properties are to be extracted from the experimental measurements. The proper control of the mass transport processes seems to be one of the main problems of high-temperature electrochemical studies. 

Phenomena that arise in these materials include conduction processes, mass transport by convection, potential field effects, electron or ion disorder, ion exchange, adsorption, interfacial and colloidal activity, sintering, dendrite growth, wetting, membrane transport, passivity, electrocatalysis, electrokinetic forces, bubble evolution, gaseous discharge (plasma) effects, and many others. 

When the mass transport process is rate determining, and in the presence of a supporting electrolyte, a convective-diffusion equation for this tertiary current distribution must be solved ... 

The semiconductor model is analogous to processes created in a system of redox reactants namely, after a redox reaction occurs the products must be moved apart as quickly as possible. The processes of diffusional mass transport or convection are the only means of separating the products in solution, and these processes are slow compared to the mechanism involved in semiconductors. To enhance mass transport, it is possible to introduce intermediates so that the oxidant and reductant are separated by fast electron-transfer reactions. In this case, recombination is prevented (to some extent) by the physical separation of oxidant and reductant using intermediary donors and acceptors (63). 

Electrochemical timescales are of importance when one desires to measure the kinetics of an electrochemical process. Changing the rate of mass transport by convection, the shape or size of the electrode, and scan rate can vary the timescale. There is a quadratic dependence of the timescale upon the radius of the electrode, favoring microelectrodes, therefore, for rapid kinetic measurements. A timescale as short as 10 ps can be accessed using a microjet electrode. The reader is referred to an excellent analysis by Bond and co-workers. ... 

Diffusion is a process by which mass is transported down a gradient in chemical potential (often simplified to a gradient in concentration) by random thermal motion. It is distinguished from alternative mass transport processes such as convection in that it does not require bulk motion or mechanical action to move particles from one place to another. 

Mass transport in amperometric systems in which the reagent stream is forced to flow along the surface of the electrode may be described in terms of convective diffusion. Effectively this means that at sufficiently high values of Pg, the Peclet number, the liquid above an electrode may be divided into two distinct zones. In one zone, far away from the electrode surface, convection is important, and the concentration profile is substantially flat. In the other zone, adjacent to the electroactive surface, there is a sharp concentration gradient here diffusion is the predominant mass transport process. The Peclet number is given by v l/D, where is the main stream fluid velocity, and / is the length of the electrode (measured in the direction of fluid flow). Under these conditions, the mass transport limited current z l for a reversible electrode couple (i.e. the concentration of the electroactive form is zero at the electrode surface) is given by... 

It has already been noted that the flux of material to the rotating disc electrode is uniform over the whole surface, and it is therefore possible to discuss the mass transport processes in a single direction, that perpendicular to the surface (i.e. the z direction). Furthermore, it has been noted that the velocity of movement of the solution towards the surface, is zero at the surface and, close to the surface, proportional to Hence, even in the real situation it is apparent that the importance of convection drops rapidly as the surface is approached. In the Nernst diffusion layer model this trend is exaggerated, and one assumes a boundary layer, thickness 6, wherein the solution is totally stagnant and transport is only by diffusion. On the other hand, outside this layer convection is strong enough for the concentration of all species to be held at their bulk value. This effective concentration profile must, however, lead to the same diffusional flux to the surface (and hence current density) as it found in the real system. 

The transport of reactants to or from an electrode usually proceeds through diffusion and convection (charged species can also transport through migration under an electrical field). Diffusion is the dominant process for the transport of the species around the surface of an electrode immersed in a stagnant electrolyte solution. There may be some natural convection going on due to the impact of environmental vibration and the uneven temperature distribution in the electrolyte, but its effect is minimal. In such cases, diffusion is exclusively used to account for the mass transport process. 

Diffusion, migration, and convection are the three possible mass transport processes accompanying an electrode reaction. In the first case, the particles move due to the formed concentration gradient. The flux in the case of planar (onedimensional or linear) diffusion can be described by Pick s first law ... 

Forced convection is often applied to enhance the rate of the mass transport process, ft can be achieved by stirring the solution with the help of a separate stirrer, or the electrode itself can rotate, vibrate, or even simply expand its volume (which is a movement of its surface against the solution) as the dropping mercury electrode does. In the case of forced convection, the fluid flow can be laminar flow or turbulent flow. The flux of species i driven by one-dimensional motion along the a -axis can be given as follows ... 

Diffusion, migration and convection are the three possible mass transport processes. Diffusion should always be considered because, as the reagent is con-... 

Of equal importance in the mass transport process is the convection of the solvent into the gas phase. The boundary condition for diffusion in this convective case is given by... 

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