Electrolysis separation factor

Electrolysis. For reasons not fiiUy understood (76), the isotope separation factor commonly observed in the electrolysis of water is between 7 and 8. Because of the high separation factor and the ease with which it can be operated on the small scale, electrolysis has been the method of choice for the further enrichment of moderately enriched H2O—D2O mixtures. Its usefiilness for the production of heavy water from natural water is limited by the large amounts of water that must be handled, the relatively high unit costs of electrolysis, and the low recovery. 

Isotopic Concentration. A number of techniques have been reported for concentrating tritium from naturally occurring sources. For example, separation factors (H/T) of 6.6 to 29 were observed (49) for the concentration of tritium by electrolysis of tritiated water. Tritium is concentrated in the undecomposed water. 

In aqueous solutions, approximately one atom of deuterium, D, is present for every 7000 atoms of the ordinary hydrogen isotope (protium, H). In the evolntion of heavy hydrogen, HD, the polarization is approximately 0.1 V higher than in the evolution of ordinary hydrogen, H2. Hence during electrolysis the gas will be richer in protium, and the residual solution will be richer in deuterium. The relative degree of enrichment is called the separation factor (S) of the hydrogen isotopes,... 

The isotope effect (i.e. the difference in the rates of evolution of hydrogen from H20 and D20) on hydrogen evolution is very important for theoretical and practical reasons. The electrolysis of a mixture of H20 and D20 is characterized, like in other separation methods, by a separation factor... 

Figure 2.64 Effect of permeation temperature on permeance and separation factor for a membrane fabricated at 40 mA cm-2 current density and 45 °C electrolysis temperature [94] (by courtesy of Springer-Verlag).

The emichment of tritium is usuaUy determined by the tritium separation (fractionation) factor during electrolysis, and by measuring the initial and final amounts of water. However, many workers have reported that the value of the separation factor of tritium depends on the electrode material, the type of electrolytic emiclunent ceU, the current density, the mode by which water is fed into the electrolytic cell, and the temperature of the electrolytic cell. In 1991, a rehable method was proposed for estimating tritium concentrations in water, based on a rehable correlation between the water electrolytic emiclunent of deuterium and tritium. The constancy of the ratio, k, during the electrolysis, k = a(fi — — 1), was... 

One could use the method of electrolysis to separate other iso-topes, for instance, to produce O, but the separation factors are low, and the method is not expedient, unless the product has a very high value and other competing methods of separation (e.g., gas diffusion) cannot be used for one reason or another. 

In a detailed laboratory investigation of the effect of cell variables on the deuterium separation factor in electrolysis of water, Brun and co-workers [B13] have found that a depends on the cathode material, electrolyte composition, and cell temperature, generally as follows. The separation factor is higher for an alkaline electrolyte than for an add. With KOH, at 15°C, a pure iron cathode gave the highest value reported, 13.2. The separation factor for mild steel, the material used in most commercial electrolyzers, was 12.2. Values as low as S were reported for tin, zinc, and platinized steel. At 2S°C the separation factor with a steel cathode was 10.6, and at 75°C it had dropped to 7.1. 

In this analysis of electrolysis, the somewhat optimistic assumption will be made that a separation factor of 7 can be obtained at a cell voltage of 2.1. At 95 percent current efficiency, the power consumption per gram-mole of water electrolyzed is then... 

Separation factors in electrolysis for other elements are much lower than for hydrogen. A few values that have been reported are listed in Table 13.15. These values are so low, and the cost of electric energy per unit electrolyzed is so high, that electrolysis is uneconomical for separating isotopes of any element other than hydrogen. Some concentration of takes place in an electrolytic deuterium plant. 

It was mentioned above that there is no isotope effect in a cathodic reaction involving a water molecule[434]. If this reaction corresponds to electrochemical desorption of hydrogen, the isotope separation factor S should be < 2 for electrolysis in alkaline solutions, in accordance with formula (4.12) (see section 4.3). Experimental data given in sections 4.5 and 6.4 show that = 4.2... 

Design possibilities for electrolytic cells are numerous, and the design chosen for a particular electrochemical process depends on factors such as the need to separate anode and cathode reactants or products, the concentrations of feedstocks, desired subsequent chemical reactions of electrolysis products, transport of electroactive species to electrode surfaces, and electrode materials and shapes. Cells may be arranged in series and/or parallel circuits. Some cell design possibiUties for electrolytic cells are... 

Another type of electrolyser uses polymer membranes to both support the electrolysis reaction and to separate the gases. Efficiency factors for PEM electrolysers are predicted to reach 94%, but this is only theoretical in 2002. These electrolysers are best suited for small plants that have a variable output of hydrogen173. 

Electrolysis offers an alternative route for organic synthesis via the formation of anion and cation radical intermediates. However, traditional electrolytic methods suffer from a number of limitations such as heterogeneity of the electric field, thermal loss due to heating and obligatory use of supporting electrolytes. These factors either hamper electrosynthetic efficiency or make the separation process cumbersome. The combination of electrosynthesis and microreaction technology effectively overcomes these difficulties. 

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