Parallel plate cell, mass transport

Mass Transport. Probably the most iavestigated physical phenomenon ia an electrode process is mass transfer ia the form of a limiting current. A limiting current density is that which is controlled by reactant supply to the electrode surface and not the appHed electrode potential (42). For a simple analysis usiag the limiting current characteristics of various correlations for flow conditions ia a parallel plate cell, see Reference 43. 

Improving the mass transport regime In parallel plate cells this is achieved either by increasing the electrolyte flow rate or introducing turbulence promoters into the interelectrode gap. 

THE PROBLEM An electrolytic reaction takes place in a parallel plate cell of length L, width w, and interelectrode gap d, with w d. The reaction takes place under mass transport control with a mass transfer coefficient kj. The flow regime is turbulent and it can be assumed that the electrolyte volume V= w x d x L. Use dimensional analysis to obtain a scale-up criterion that will keep conversion constant. 

With such a flat plate, the boundary layer will increase in thickness indefinitely, if slowly (Fig. 1.10(c)). On the other hand, if the flow is in a restricted channel (e.g. a circular-tube or a parallel-plate cell) the boundary layers at the two walls must merge at some point and beyond, a steady-state situation or fully developed laminar flow will result (Fig. 1.11). Fundamental mass transport studies in electrolytic cells are usually carried out in cells with an entry length without electrodes so that the boundary-layer thickness is uniform over the current-carrying surface. 

Figure 22.9 shows a very simple beaker-type glass cell with parallel plate electrodes and a cooling mantle. Mass transport is performed by a magnetic stirrer. A Teflon stopper is used to hold the connections for the electrodes. 

The importance of high rates of mass transport for a clean and efficient electrosynthesis using a filter press reactor has been stressed, and the effect of inclusion of a platic mesh turbulence promoter considered [56]. A multipurpose filterpress cell for continuous electrolysis of organic compounds has been described [57], and a mathematical model of the startup of a continuous parallel-plate reactor has been published [58]. 

In industrial practice, it is possible to find examples of cells which use unstirred solutions (e.g. electrorefining, batteries), stirred or agitated solutions (e.g. electroplating) and flowing electrolytes (e.g. synthesis, water treatment). Moreover, it is unusual for the flow of solution and the current path, which depends on cell geometry, to be parallel and the electrode may not be equivalent to a flat plate (e.g. bed electrodes, cathodes for plating) as a result, we must write our mass transport expressions in three dimensions. Nor is it always possible to assume that migration of the electroactive species is unimportant. 

It will be seen in the next and subsequent chapters that a wide variety of cell geometries (e.g. parallel plates, concentric cylinders, Swiss roll), types of electrode (e.g. plates, beds, porous, expanded metals and gauzes) and flow patterns are used in industrial electrochemistry. In most the flow is too complex to warrant a detailed fluid-mechanical calculation. Rather the normal approach to mass transport in electrolytic cells is to treat the cell as a unified whole and to seek expressions in terms of space-averaged quantities which permit some insight into the mass transport conditions within the cell. 

The electrolyte inlets and outlets must be designed to give a low pressure drop over the reactor and the required flow characteristics (for mixing or mass transport reasons) with due regard for the nature of the reactants and products. In the case of a filterpress, parallel-plate reactor, internal manifolding provides a neat, compact method of distributing the catholyte and anolyte flows but it requires precise sealing and offers little control over bypass currents in bipolar cells. 

By decreasing the channel size from 2 mm to 1 mm, the number of dual channels and the ratio of gas contact area to land area can be progressively increased considerably as shown in Figure 10.12d through 10.12f. A high-performance bipolar plate with four 0.5 mm serpentine parallel channels is also considered (Boddu et al., 2009) as shown in Figure 10.12g. With a combination of an increase in number parallel channels and a decrease in channel size, the effective gas contact area can be optimized in terms of heat and mass transport and pressure drop in the gas flow channels and performance of the fuel cell. 

Stack Configuration The individual cells in bipolar plate stacks such as the PEFC are typically connected in series, with current collection across the entire electrode surface along the interface between the bipolar plate landings and the DM. The flow fields in AFCs are similar to those used in other fuel cells, and various parallel and serpentine configurations are used to optimize mass, heat, and reactant/product transport. 

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