Operation of diaphragm cells

Normal operation of diaphragm cells does not permit independent control of anolyte and catholyte concentrations or temperatures. The catholyte composition is the more important, since it has a greater effect on cell operating efficiency and also determines the load on the caustic evaporators. The anolyte concentration must remain above some minimum, and this can become a constraint on the catholyte concentration. 

For safe operation of diaphragm cells, the header pressures must be maintained at the... 

The components of the diaphragm, membrane, and mercury cell voltages presented ia Table 8 show that, although the major component of the cell voltage is the term, ohmic drops also contribute to the irreversible energy losses duting the operation of the cells. 

Operation of membrane cells The same processes take place on the anodes and cathodes as in diaphragm cells. Activated titanium is used for the anodes and stainle.ss steel or nickel is preferred for the cathodes. No water transport takes place in the absence of current, but upon application of current solvation-water is transported by the current-carrying Na" ions as they travel from the anode chamber to the cathode chamber. 

Another emission control aspect of diaphragm cell operation concerns the use of the crude cell product, still containing sodium chloride, to carry out base-catalyzed reactions such as ring closure of propylene chlorohydrin (Eq. 8.27) or hydrolysis of chlorobenzene (Eq. 8.45). 

Recently a number of companies have offered synthetic fibre diaphragms, made primarily from PTFE, which mimic the performance of asbestos and are applied in a similar way. If these are shown to be successful commercially, the operational life of diaphragm cell units will be significantly extended. 

Comparison of Eq. (94) with Eq. (91) clearly shows that Eq. (92) is a crude approximation for estimating the chlorine current efficiency of diaphragm cells operating with acidic or basic feed brine. Calculations show that the six equation (Eq. 93) overestimates the efficiency by 1-2% compared with the rigorous Eq. (91). 

In hybrid plants that operate diaphragm cells in combination with other types, there are other options for use of the evaporator salt. One of the advantages of diaphragm cells is their ability to operate without penalty on salt supplied in the form of brine. With the other types, a brine supply presents a problem with the water balance. With mercury cells, for example, solid salt is needed to resaturate the depleted brine for recycle. Evaporator salt can fill this need, and, with the right division of production between the two types of cell, it is possible to run both types without a supply of solid salt. Several plants operate this way. With membrane cells, there are other ways to integrate the two types of cell (Section 9.4.1), but the basic idea is that the availability of salt from the evaporators again allows the combination to operate from an all-brine supply. 

And what of the original LeSueur cells The Brown Company, successor to Burgess Fibre, operated them, and others like them, for more than fify years — in competition with later designs of diaphragm cells, such as, the Townsend-Hooker (first used in 1906), the Billiter (1907) and the Gibbs. Of the last named more anon. 

There are basically three types of construction of molten carbonate cells. In the most developed type, the electrolyte is contained in a porous diaphragm of magnesia. This type of construction reduces corrosion, but increases electrolyte resistance. The second type, that of free flowing electrolyte, has not been developed because of the serious corrosion problem at the temperatures of operation of these cells. The third type uses electrolyte which is mixed with magnesia powder to a stiff paste. This structure seems to have the merits of both the other structures in that it reduces corrosion without much affecting the resistance of the cell. 

Table 10. Operating Capacities and Characteristics of OxyTech Diaphragm Cells...

Table 11. Design and Operating Characteristics of HU Series Diaphragm Cells...

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