Diaphragm cells power consumption

Another aspect that may be taken into account is that of membrane electrolysers having a lower power consumption (Fig. 15.4). Not only does the new technology save power but it also requires less steam to evaporate the cell caustic product to 50%. Additionally, salt removal equipment required in diaphragm plants uses power. This benefit can also be turned around so that for the same power consumed by a diaphragm cell room extra volumes of rayon-grade caustic soda can be produced from the membrane electrolysers. 

The use of polyperfluorosulfonic acid membranes as the cell separator was first demonstrated about three decades ago. Yet it was not until the mid-1980s when the economic advantages of membrane cells over the traditional mercury- and diaphragm-cell technology were fully demonstrated—consequent to better membrane performance, higher caustic product concentrations, and lower power consumption. Retrofitting chlor-alkali facilities with membrane cells accounted for much of the growth and sustenance of this industry over the past two decades. 

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. 

The operating voltage and current efficiency of a cell fix its unit DC power requirement. This varies with time, usually in the direction of increasing power consumption, and so cells and their auxiliaries must be designed with this fact in mind. In a diaphragm or membrane cell, performance declines over the lifetime of the anolyte/catholyte separator, usually as a result of the accumulation of impurities. The performance of a mercury cell... 

Current density in diaphragm cells is relative low, usually only up to 2-3 kA m . It is difficult to adapt it to changes in the required chlorine production. Special low-cost cell constructions allow an economic operation at decreased current density and correspondingly low electrical power consumption, in spite of the needed larger electrode area. 

The principles needed to understand the efficient operation of the diaphragm process involve the current efficiency, cell voltage, power consumption, and optimization of the operating conditions [113], [114]. 

Table 11. HU series diaphragm cells specific load, cell voltage, and power consumption... 

The combination of the Modified Diaphragm and expandable DSA anodes reduces power consumption by 10-15% from that of regular asbestos diaphragms and standard, fixed DSA anodes [127]. Table 12 presents performance data for the two most common MDC cell sizes [125]. The OxyTech MDC-29 is shown in Fig. 41. The licensed chlorine capacity of OxyTech cells now exceeds 20 000 t/d. 

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