Diaphragms voltage drop

FIGURE 1.7. Voltage distribution in an electrochemical reactor = cell voltage E = anodic electrode potential 1 = voltage drop in anolyte = voltage drop in diaphragm = voltage drop in catholyte = cathodic overpotential. 

Point (a) only concerns simple metal electrodes and needs to be tested for each case. Point (b) is important for the measuring instrument being used. In this respect, polarization of the reference electrode leads to less error than an ohmic voltage drop at the diaphragm. Point (c) has to be tested for every system and can result in the exclusion of certain electrode systems for certain media and require special measures to be taken. 

Novel, thin diaphragms, which, in conjxmction with changes in the cell configuration, were to minimize the ohmic voltage drops (iR in Eq. 11.7). 

C. Voltage Drop Across the Separator. The voltage drop across the diaphragm, AV is ... 

The voltage drop in each monopolar electrolyzer is equal to the voltage drop of a single cell. This depends primarily on the type of cell, the current density, and the choice of membrane or type of diaphragm. The number of electrolyzers contained in a cell line then fixes its total voltage. The design voltage must be sufficient to allow for losses in buswork and interceU connectors, the maximum current density to be used, and the deterioration with time of the components of the circuit. 

Vertical Electrolyzers. Diaphragm and membrane cells are genoally vertical. In the case of a bipolar-type electrolyzer, electricity is supplied ftom both ends of the cell stack, and the current is perpendicular to the electrode surface. On the other hand, electricity is supplied to a tank-type vertical electrolyzer from the top or bottom of the electrode, and this results in ohmic voltage drops within the electrode [1,13,14]. 

A diaphragm is selected that produces the minimum voltage drop. If possible, the diaphragm is dispensed with. 

The components of the diaphragm, membrane, and mercury cell voltages presented ia Table 8 show that, although the major component of the cell voltage is the term, ohmic drops also contribute to the irreversible energy losses duting the operation of the cells. 

The transverse voltage is the drop of potential across the double layer and is of the order of 0-01 to 0 05 volt. It will be noted that the amount of liquid transported is dependent on the nature of the liquid and on the current and is independent of the diameters or lengths of the tubes of the diaphragm. Somewhat divergent views are held as to the actual thickness of the double layer (Lamb, Phil. Mag. xxv. 52, 1888) a point which we have 7 referred to. 

While the combination of the apphed current and current efficiency in an electrochemical reactor is a measure of the overall rate of product output, it is the product of the current and cell voltage that will determine the reactor s electrical power consumption, as indicated by Equation (26.103). The overall voltage in an electrochemical reactor is composed of the following components (1) thermodynamic cell potential, (2) anode kinetic and mass transfer overpotentials, (3) anolyte IR drop, (4) diaphragm/membrane IR drop, (5) catholyte IR drop, and (6) cathode kinetic and mass transfer overpotentials. For more information on each of these terms, the reader should refer to Section 26.1. 

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