Diaphragm process cell cost

The optimum current density of the membrane process (including the SPE cell process) is higher than that of the diaphragm process, because of the relatively high cost of the perfluoro ionomer membrane and its greater sensitivity to impurities, which requires the use of more expensive material for equipment. 

The choice of technology, the associated capital, and operating costs for a chlor—alkaU plant are strongly dependent on local factors. Especially important are local energy and transportation costs, as are environmental constraints. The primary difference ia operating costs between diaphragm, mercury, and membrane cell plants results from variations ia electricity requirements for the three processes (Table 25) so that local energy and steam costs are most important. 

This yields a solution of highly pure alkali (free of chloride ions), which can be used in the manufacture of synthetic fibers. The mercury, which has been stripped of sodium, is returned to the electrolyzer. The cost of chlorine is higher in the mercurycell than in the diaphragm-cell process. In addition, the mercury-cell process is ecologically dangerous, owing to the possible escape of mercury into the environment hence, it has increasingly been discontinued in all countries. 

In this chapter brief information on the origin and cost of titanium will be discussed. A definition of the term unique properties will be given and how these properties are exploited in the membrane, diaphragm and in the mercury cell processes will be considered and miscellaneous applications touched upon. The chapter will conclude with a summary of the financial benefits which, after all, propel the use of this challenging material. 

One of the most common industrial methods for the production of sodium hydroxide depends on the electrolysis of brine in a diaphragm cell. The products of the electrolysis are chlorine, hydrogen, and cell liquor, which is a solution of sodium hydroxide and sodium chloride. A large fraction of the cost of commercial sodium hydroxide results from the concentration, separation, and purification of the alkali. The sodium hydroxide required in the sea water descaling process need... 

A comparison of the relative merits of the three electrolytic routes for the production of chlorine and caustic soda must be based on economics, although there are social factors which must also be taken into account. It is the intention here to discuss the factors determining the economics of the processes only in qualitative terms because the uncertainties in a more exact calculation are too great. It might be noted, however, that recent published calculations would indicate that the costs of chlorine and caustic soda produced by the processes are very similar and this must be the case since mercury, diaphragm and membrane cells are all currently operated and all are still considered when new plant is to be purchased. 

These include the cost of labour, the cost of replacing components and the rent and rates on the area of land. A comparison of the latter is very unfavourable to diaphragm cells where almost twice as much land is necessary because of the low current density and the need for evaporation plant. Membrane and mercury cells have similar land requirements because the former are packed together closely in a filter press and to some extent the electrodes scale vertically. All the processes are largely automated and hence the difference in labour costs is likely to arise because of a variable incidence of component and pipework failure. Certainly the need to replace diaphragms on a routine cycle increases the labour costs for the diaphragm cell process. 

The energy consumed in the evaporation of cell liquor to 50% NaOH is a major disadvantage of the diaphragm-ceU process. The evaporation process also brings some offsetting advantages. First, the removal of water from the process makes it possible for a plant to operate with a brine feed. Most mercury- and membrane-cell plants use the more expensive solid salt or return depleted brine, at a cost, to a brine well for reconcentration. Second, the evaporation yields refined salt as a by-product. 

In a balanced plant, all the chemically treated brine flows through ion exchange, not just that equivalent to the membrane-cell production. This adds a cost to the process, because in a segregated operation only that brine corresponding to the membrane-cell production would be so treated. At the same time, it improves the quality of the brine fed to the diaphragm cells. This in turn improves the current efficiency of those cells, but little information is available in the literature, and it is not possible to quantify the phenomenon here. 

As mentioned in Section 17.1, the anodic and cathodic compartments of an electrochemical cell can be separated by an ion-exchange membrane or a porous diaphragm. The division of a cell is often practiced in industrial processes, despite the additional costs, the need for additional seals and possible maintenance problems. A separator may indeed allow a more independent choice of anode/anolyte or cathode/catholyte, enable current eftkiency to be maintained due to the exclusion of redox shuttles and help to isolate electrode products or prevent the formation of explosive or toxic mixtures, for example H2-O2. However, if possible, undivided cells are preferred, as they lead to lower ohmic drops and to much simpler technologies. 

A detailed discussion of the capital investment and operating costs for the three processes for a 200 000 t/a-plant in 1991 is given in [186]. A comparison of the investment costs does not make sense today for the mercury process, because no mercury cell plant and only a few diaphragm cell plants were built since then. All new plants are using the membrane process. 

Caustic Concentration. The elaborate multistage evaporators required for the concentration of the diaphragm-cell caustic and the separation of NaCl and Na2S04 must be nickel plated because of the corrosiveness of the cell liquor containing NaCl and NaC10 j. These evaporators cost 20 - 35 % of the total. The evaporators for the membrane process may be constructed of stainless steel and are much smaller because the essentially salt-free cell liquor is more concentrated, costing 3 - 4 % of the total. The mercury process produces 50 % caustic directly, evaporation is not required. 

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