Electrolyzers

In order to lower the capital cost of the electrolyzer plant, it is necessary to operate at as high a current density as possible, i.e., often more than 500 mA cm Some commercial electrolyzers run at a current density as high as 3000 mA cm The drawbacks are a greater cell resistance, the need for a 

In order to minimize electricity consumption, it is important to choose an electrolyte of maximum conductivity. Usually, a concentrated aqueous solution of potassium hydroxide (30-40 wt.%) is employed. This must be prepared from very pure water, otherwise impurities will accumulate during electrolysis the chloride ion, which is usually present in water, is particularly harmful in that it causes pitting of the protective films formed on metal surfaces in alkaline solutions. 

Alkaline water electrolyzers that operate at pressures up to 5 MPa are under development. At present, they are commercially available with a capacity range of 1-120 N-m h h The goals are to optimize the units for energy efficiency and to make them responsive to fluctuating current supply that would occur with 

Compact design No busbars required Low-cost electrical equipment Higher operating voltage Lower operating current Parasitic electrical bypass currents Consequent reduced electrical efficiency Inhomogeneous current distribution Increased corrosion rate of electrodes 

hydrated protons are eondueted through the polymer membrane elee-trolyte from the positive to the negative eleetrode. 

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It is now obtained commercially by the electrolysis of absolutely dry fused sodium chloride. This method is much cheaper than that of electrolyzing sodium hydroxide, as was used several years ago. 

L. calx, lime) Though lime was prepared by the Romans in the first century under the name calx, the metal was not discovered until 1808. After learning that Berzelius and Pontin prepared calcium amalgam by electrolyzing lime in mercury, Davy was able to isolate the impure metal. 

Current is fed into the electrolyzer by means of anodic and cathodic end elements. The anodic compartment of each cell is joined to an independent brine feed tank by means of flanged connections. Chlorine gas leaves each cell from the top, passing through the brine feed tank and then to the cell room collection system. Hydrogen leaves from the top of the cathodic compartment of each cell the cell Hquor leaves the cathodic compartment from the bottom through an adjustable level connection. 

Table 9. Design and Operating Characteristics of Glanor Bipolar Diaphragm Electrolyzers...

Oy Tech MonopolarE,lectroly rs. OxyTech Systems (a joint venture company of Occidental Chemical and Eltech Systems) suppHes monopolar diaphragm electrolyzers of two designs the OxyTech "Hooker" H-Type (27,28) shown in Figure 11 and the "Diamond" MDC-Type (28,29) in Figure 12. 

Eig. 19. CME monopolar electrolyzer a, membrane b, cathode element c, half-cathode element d, current distributor e. Teflon tube f, CI2 + depleted brine manifold g, conductor rod h, CI2 + depleted brine outlet nozzle i, base frame j, recycled NaOH manifold k, recycled NaOH inlet nozzle 1, gasket (the gasket-to-element ratio is quite small) m, tie rod n, anode element o, H2 + NaOH manifold p, end plate, q, under cell bus bar (simplifies piping... 

ICIFM-21SP Monopolar Electrolyzers. Id s EM-21 SP monopolar electrolyzer incorporates stamped electrodes that are 2 mm thick and of a relatively small (0.2 m ) size (50). The electrolyte compartments are created by molded gaskets between two of the electrode plates the electrode spacing is finite and is estabHshed by gasket thickness. The electrode frames are supported from rails and are compressed between one fixed and one floating end plate by tie rods. Inlet and outlet streams are handled by internal manifolds. A crosscut view of the electrolyzer is shown in Eigure 21. As of 1989, ICI had Hcensed 20 plants having an annual capacity of 468,250 t of NaOH. 

The plant incorporating the air cathode electrolyzer must include a high performance air scmbbing system to eliminate carbon dioxide from the air. Failure to remove CO2 adequately results in the precipitation of sodium carbonate in the pores of the cathode this, in turn, affects the transport of oxygen and hydroxide within the electrode. Left unchecked, the accumulation of sodium carbonate will cause premature failure of the cathodes. 

Most of the voltage savings in the air cathode electrolyzer results from the change in the cathode reaction and a reduction in the solution ohmic drop as a result of the absence of the hydrogen bubble gas void fraction in the catholyte. The air cathode electrolyzer operates at 2.1 V at 3 kA/m or approximately 1450 d-c kW-h per ton of NaOH. The air cathode technology has been demonstrated in commercial sized equipment at Occidental Chemical s Muscle Shoals, Alabama plant. However, it is not presentiy being practiced because the technology is too expensive to commercialize at power costs of 20 to 30 mils (1 mil = 0.1 /kW). 

Table 14. Typical Specifications for Feed Brine to Electrolyzers...

Table 9. Operating Conditions for Hydrogen Production Electrolyzers ...

Electrolyzer Corp. Ltd. BBC Norsk Hydro de Nora Lurgi ... 

In another development (32), the sodium sulfate solution produced during the desulfurization of paste with caustic soda is electrolyzed in a membrane ceU to produce caustic soda and high purity sulfuric acid. The caustic soda is recycled to the desulfurization stage the sulfuric acid, after concentration, can be reused in battery production. 

The zinc electrolyte contains ca 60 kg/m zinc as sulfate and ca 100 kg/m free sulfuric acid. It is electrolyzed between electrodes suspended vertically in lead or plastic-lined, eg, poly(vinyl chloride), concrete tanks. The insoluble anodes are made of lead with small amounts of silver. The anodic... 

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