TECHNOLOGY measure about 0.8 Χ 2 mm. The cell reactions are
Trickle-Bed Electrolytic Cell For Peroxide Developed Dow Chemical has developed a new type of cell for the electrolytic reac tion of a gas and a liquid in which one of the electrodes is porous and self-draining, being composed of a bed of graphite chips coated with Du Pont's Teflon fluorocarbon and carbon black. A number of potential applica tions are envisioned for the cell, but its first use probably will be in production of hydrogen peroxide. At present Dow has entered into a joint development project with Hu ron Chemicals, a Kingston, Ont., producer of specialty chemicals for the pulp and paper industry. One of those chemicals is hydrogen per oxide, which is experiencing consid erable growth for use in the me chanical and chemical bleaching of
New cell permits gas/liquid reaction Gas out
Oxygen in Anolyte compartment
Trickle-bed cathode
Porous screei support for separator
Anode..
Electrolyte in Liquid permeable separator
16
March 12, 1984 C&EN
Product out
wood pulp. In practice, alkaline per oxide usually is added directly to the pulp slurry. The peroxide conventionally is made by mixing dilute hydrogen peroxide with caustic soda, sodium silicate, and other stabilizers. Most pulp mills purchase and transport the peroxide to the consumption site. This is expensive and has spurred interest in producing the peroxide on site via local electro chemical reduction of oxygen. Past attempts to do this, Dow engineers say, have not succeeded because of lack of fundamental knowledge of the electrochemistry involved a n d because of i n a d e q u a t e cell design. Dow's answer to these problems is the flow-control tricklebed cell. In Dow's trickle-bed cell, the liq uid electrolyte enters the side of the bed. The cell separator was cho sen to give flow control of the entering liquid. The flow rate is con trolled to give desired product com position in a single pass through the cell. This simplifies the cell de sign and eliminates the need for electrolyte recycle. By keeping the trickle-bed highly porous, the drain age rate through the bed is high enough to ensure that the bed is essentially dry except for that por tion adjacent to the separator. The oxygen enters the bed at the top and flows downward through the porous cathode. The essentially dry bed means pressure drops of usual ly less than 1 psig per foot of bed height. One of the modifications of cell construction is the inclusion of an ion exchange membrane that acts as a control barrier to ion migration into and out of the cathode cham ber. The key development in the cell design is the new electrode material. The bed is filled with graphite chips coated with a Teflon/carbon black matrix. Chips are typically flat and
2 0 H - — 1/2 0 2 + H 2 0 + 2e 0 2 + H 2 0 + 2e — H 0 2 - + OH~ The net reaction is OH-+
1
/202^H02-
Sodium hydroxide is fed to the an ode compartment and an alkaline peroxide solution is obtained as the product effluent. Probably the most important op erating variable is the liquid flow rate. Temperature has a minor ef fect on operation, but very high tem peratures must be avoided; there is a provision for cooling the cell. There are two major undesirable side effects. One is the electrochemical consumption of H 0 2 ~ ion by re verse reactions of the peroxide gen eration scheme H 0 2 - + H 2 0 + 2e — 3 0 H O H - + H 0 2 - — 0 2 + H 2 0 + 2e The other is catalytic decomposi tion of the peroxide by the reaction 2H202 — 2H20 + 0 2 due to the presence of metal ions in the electrolyte. Gas electrodes require three-phase contact to operate, and the usual arrangement is a porous electrode filled with electrolyte. Mass trans fer in such a system is often severe ly limiting. The Dow trickle-cell re moves this limitation. The Dow-Huron cooperative proj ect has shown that it is possible to produce a solution of 2% hydrogen peroxide in 1M sodium hydroxide in a cell operating at 2 volts and about 67% efficiency. Continued op eration of a pilot plant by the part ners will provide the necessary de sign data for a commercial plant. Preferred size of a commercial plant for installation in paper and pulp mills is about 10 tons per day of peroxide in caustic. Data gathered so far suggest that the capital cost for such a plant would be about $3.25 million and that the product cost would be about 42 cents per lb. If the data continue to be encour aging, the partners believe that an on-site pilot unit could be operating at a pulp mill within two years. Joseph Haggin, Chicago
