Current efficiency mercury cell

The conventional electrochemical reduction of carbon dioxide tends to give formic acid as the major product, which can be obtained with a 90% current efficiency using, for example, indium, tin, or mercury cathodes. Being able to convert CO2 initially to formates or formaldehyde is in itself significant. In our direct oxidation liquid feed fuel cell, varied oxygenates such as formaldehyde, formic acid and methyl formate, dimethoxymethane, trimethoxymethane, trioxane, and dimethyl carbonate are all useful fuels. At the same time, they can also be readily reduced further to methyl alcohol by varied chemical or enzymatic processes. 

Udapa et al.16 showed that C02 was reduced to formic acid at a mercury electrode in a 0.05 M phosphate buffer (pH 6.8) solution. A current efficiency of 81.5% was obtained at a current density of 20 mA/cm2 and a cell voltage of 3.5 V. On the other hand, Bewick and Greener17 reported that malate and glycolate were produced at Hg and Pb electrodes, respectively, using aqueous quartenary... 

Other remaining technical concerns with membrane cells relate to somewhat lower current efficiencies and to relatively short membrane lifetimes. At present, this is limited to 2-3 year of operation when coupled to much more careful brine pretreatment than is required for conventional asbestos diaphragm cells. A combination of mercury cell and membrane cell technologies has been recently tested for commercial feasibility [19]. The economics of the three primary chloralkali technologies have also been reviewed [20]. 

Nitrobenzene, C6H5NO2, can be reduced to anfline, C6H5NH2, at a mercury cathode. If the current efficiency is 80 %, how long must a current of 3.0 A flow in the cell to produce 1.0 kg of aniline ... 

H. Current Efficiency Equations for Mercury Cells. A schematic of the mercury cell is depicted in Fig. 4.4.4. As stated earlier, the cathodic reaction in the mercury cell is the discharge of the Na" " ions to form sodium amalgam as described by reaction (9). The material balance for the CI2 and the Cl species for the mercury cells are the same as in the membrane cells and hence, the chlorine current efficiency of the mercury cells,, is given by Eq. (49), which can be simplified by using Eq. (97) obtained from Eq. (25) to result in Eq. (98). (See ref. [15] for details.)... 

Equation (106) assumes that aU the sodium amalgam is decomposed to form caustic, and that there are no caustic losses during caustic collection. Rearrangement of Eq. (106) with p = q leads to the caustic current efficiency expression (107) for the mercury cells. 

The anolyte leaves the cell as depleted brine. This must be recycled to the cells to prevent a massive loss of salt, and it must be resaturated with salt to keep the current efficiency of the cells high. Technically, it is possible to operate with once-through brine. While losing salt, this option eliminates handling and treatment of the recycle stream. This practice was fairly common in the past but is almost unknown in today s mercurycell plants. The overriding issue is not the loss of salt but rather the escape of mercury from the system. 

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... 

Increases operating voltage reduces diaphragm-cell current efficiency promotes hydrogen formation on mercury cathode Reduces current efficiency and increases operating voltage disrupts membrane Same as calcium (see above)... 

Mercury Cell Process. The mereury cell process is not energy-efficient and is falling out of favor as a result of government regulations restricting the use of mercury. Less than 16% of Canadian chloralkali production currently uses mereury cells, and the process is gradually being phased out. 

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