Membrane operation, cell

Catalytic cathodes in membrane cell operations exhibit a voltage savings of 100—200 mV and a life of about 2 + yr using ultrapure brine. However, trace impurities such as iron from the caustic recirculation loop can deposit on the cathode and poison the coating, thereby reducing its economic life. 

A majority of membrane cells operate with the membrane resting on the anode. It is conceivable that the structure of the anode is an important design parameter. Asahi Chemical Industry claims to have optimized the structure of the anode by the use of a perforated plate. However, it is theoretically possible to design... 

Diaphragm cell operating at 2.32 kA Membrane cell operating at 3.5 kA Mercury cell operating at lOkA m ... 

Anion Transport. The anions of concern for caustic purity that are transported from the anolyte to the catholyte during electrolysis are Cl and CIOJ. These species cannot be easily removed by simple chemical treatment of the caustic from a membrane cell. However, one can optimize the membrane cell operations to minimize the ingress of these ions into the caustic. 

Early membrane cells operated at relatively low current efficiencies and were able to tolerate correspondingly higher concentrations of impurities. As membranes were improved and better results became possible, the requirements for brine purity became stricter. For a time, the addition of phosphate to the brine to sequester the hardness ions and prevent them from entering the membranes mitigated some of the effects of hardness. Finally, it became necessary to devise a process to increase the purity of the brine well beyond that obtained by chemical treatment, and ion exchange is now the standard technique. Several general reviews of the brine ion-exchange process itself are available [119-121]. 

Next, we consider the possibility of purging a major part of the brine to another salt user. The outlets most likely to be available to a membrane-cell operator are other electrochemical processes. 

When membrane cells operate in tandem with mercury or diaphragni cells, there is an opportunity to purge sulfate by sending at least part of the depleted brine that otherwise would be recycled to the other cells. This approach is less effective with mercury cells. The water balance is extremely tight in a mercury-cell plant, and any import of water must be balanced by an export. Since export would spread the potential for mercury pollution, we reject it as an operable solution. Only merciuy-cell plants with some capacity for addition of water to the brine loop, as for example a plant with a brine evaporator, could be integrated in this way. 

T.F. O Brien, Dechlorination of Brines for Membrane Cell Operation. In N.M. Prout and J.S. Momhouse (eds). Modem Chlor-Alkali Technology, vol. 4, Elsevier Applied Science, London (1990), p. 251. 

Developments in cell and control system technology since Weedon s paper suggest possible variations. If membrane cells operate at a significant positive pressure, the cell header pressure control and compressor suction pressure control can be made less interdependent. Some variation in suction pressure can be accepted in the interest of overall process efficiency. On the control system side, computing power has advanced enormously in the past decade. Some logic can be built into the control system without compromising process dynamics. 

T. F. O Brien, Considerations in the Conversion of Existing Chlor-alkali Plants to Membrane-cell Oper-.ation. In C. Jackson (ed.). Modem Chlor-Alkali Technology, vol. 2, Ellis Horwood, Chichester (1983),... 

Recirculated Caustic Feed to Electrolyzers. Most membrane cells operate at 30-35% caustic with a slightly lower feed caustic concentration. Thus, most of the caustic collected from the cells is diluted, cooled, and recycled to the cells. ITie circulating pumps must continue to run in case of a power failure and so should connect to a backup power supply. 

---