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Posted On: 09/25/2006

Bayer charges ahead with novel cathode

ChemicalProcessing.com

Bayer MaterialScience, Leverkusen, Germany, will build a plant for electrolysis of hydrochloric acid in Shanghai, China that will mark the first large-scale use of oxygen-depleting cathode (ODC) technology. That unit, with an annual capacity of 215,000 metric tons of chlorine, should be commissioned in 2008.

The technology has been developed by Bayer in cooperation with UhdeNora, Milan, Italy, a joint venture of Uhde, Dortmund, Germany, and De Nora, Milan and De Nora North America, Somerset, N.J. Bayer already operates a 20,000-mt/yr plant at Brunsbüttel, Germany, having put the first of two units there online in late 2003. The Chinese plant will employ cells similar in size to those at Brunsbüttel.

The Shanghai unit will use hydrochloric acid made as a byproduct in the manufacture of polyurethane raw materials toluene di-isocyanate (TDI) and methylene diphenyl di-isocyanate (MDI) at the complex. The chlorine generated will be recycled back for isocyanate production.

“The ODC process uses around 30% less electrical energy than the diaphragm process…,” notes Christian Ohm, head of Bayer’s Inorganic Basic Chemicals Business Unit, where work on the ODC first started. “The technical principle is the same as a fuel cell process. By feeding in oxygen, we can perform electrolysis at a much lower voltage,” he adds. This lower voltage stems from the production of water instead of hydrogen at the cathode. Oxygen reacts with protons formed during electrolysis, preventing formation of hydrogen.

Novel cell design

“ODC represents a departure from traditional diaphragm technology,” says Emory De Castro, executive vice president of the E-Tek division of Pemeas, Somerset, N.J., which made the gas-diffusion electrodes being used at Brunsbüttel, “and offers advantages in energy efficiency, safety and ease of operation.” (E-Tek was owned by De Nora North America at that time.)

Each cell contains a nanoporous cathode that is permeated with a rhodium-sulfide-based catalyst and bonded to an ion-exchange membrane, and a conventional anode (Figure 1).

Figure 1. Gas-diffusion cathode bonded to membrane produces water instead of hydrogen. (Click to enlarge.)

The cells also feature all-metal construction. A proprietary coating developed by De Nora resists corrosion of the metal. Conventional units use graphite, notes De Castro, which, while impervious, has relatively poor conductivity, is hard to machine, and can be difficult to assemble.

The cells consume about 1,000 kWh per ton of chlorine, which is produced at about 99.8% purity.

Avoiding the production of hydrogen not only cuts energy requirements but also obviates the risk of hydrogen explosively reacting with chlorine, De Castro explains. Having only chlorine gas in the cell eases shut downs and start ups, and enhances flexibility to respond the changes in demand.

Investment is comparable to that of conventional cells, says De Castro, adding that cuts in capital cost are possible. “Potential savings under study would come from alternative catalysts, either with a lower precious metal content or a different metal,” he notes.

Overall operating costs, which he reckons are at least one-third less than conventional diaphragm cells, could also go lower. “Improvements in performance could come from optimization of catalyst and electrode structure.”

The technology may also suit the electrolysis of sodium chloride. However, notes De Castro, work there is still only at the research stage.