Cell Caustic Evaporation

Yet more research and development effort concentrates on the diaphragm cell caustic evaporator, finding ways to aid the evaporation of the 10-12% caustic soda in brine to make it into a saleable product. Work is directed into methods of removing the salt products and impurities and preventing corrosion of the equipment. Recovery of the salt from the evaporated caustic soda is an important part of a diaphragm cell plant as the recovered salt is used in the strengthening of the feed brine. 

FIGURE 9.66. Diaphragm-cell caustic evaporator—vapor and condensate flows. 

FIGURE 9.67. Diaphragm-cell caustic evaporator—solution and sluny flows. 

Many centrifuges are effective filters in which the separating force is intensified by the action of the machine. The major application in the chlor-alkali industry is in diaphragm-cell caustic evaporation. The salt precipitated in that operation comprises about 3% of the evaporator product. Most of this is removed by centrifuges. Section 9.3.2.5 describes the process and the use of two different types of centrifuge. 

This section describes the controls for a triple-effect membrane-cell caustic evaporator. Section 9.3.3.1 explains some of the reasoning behind the selection of the number of effects and the progression of flow of caustic through the evaporators. To illustrate the control systems here, we assume backward feed of the caustic. 

There are two major differences between diaphragm- and membrane-cell caustic evaporation. EM hragm-cell liquor is much lower in caustic content (and therefore consumes more steam) but contains large amounts of dissolved salt, which precipitates during the evaporation process and must be handled in slurry form. Section 9.3.3.3 covers this subject in detail. 

E. H. Cook and M. P. Grotheer, Energy S avingDevelopments for Diaphragm Cells and Caustic Evaporators, 23rd Chlorine Plant Manager s Seminar, New Orleans, The Chlorine Institute, Inc., Feb. 6, 1980. 

Another aspect that may be taken into account is that of membrane electrolysers having a lower power consumption (Fig. 15.4). Not only does the new technology save power but it also requires less steam to evaporate the cell caustic product to 50%. Additionally, salt removal equipment required in diaphragm plants uses power. This benefit can also be turned around so that for the same power consumed by a diaphragm cell room extra volumes of rayon-grade caustic soda can be produced from the membrane electrolysers. 

Many caustic evaporator suppliers have schemes to separate the two types of caustic whilst making 50% solutions of both. A particularly good option is to install an extra first-effect evaporator for the membrane caustic soda and split the steam usage between the diaphragm first-effect and the new membrane first-effect, then recombine the steam again to feed to the rest of the diaphragm cell evaporator system. 

Not all diaphragm plants would consider conversion. In the case of plants where the diaphragm caustic soda from the cells is fed to a downstream plant there is no economic driver to convert. Indeed, ICI Chlor-Chemicals has a diaphragm plant where the sole requirement is the use of the diaphragm cell caustic as a reagent for treating raw brine for multiple chlor-alkali plants, ammonia soda plants and evaporated salt plants. 

The membrane cell produces about 35% caustic soda, which is concentrated by evaporation to 50%. Membrane cell caustic soda is the preferred product compared to the diaphragm and mercury-cell caustic soda. 

The membrane-cell caustic is concentrated in an evaporator, following the scheme in Fig. 3.4. The multiple-effect evaporator yields a high steam economy. The 50% caustic from the membrane cell generally has 30 ppm salt and 5-10 ppm sodium chlorate. 

The chlor-alkali industry s development of ion-exchange membrane-based electrolytic cells in the early 1970s was driven by environmental concerns associated with mercury and asbestos and the desire to reduce energy costs associated with electrolysis and caustic evaporation [1] (Ihble 4.8.1). 

It is usually necessary to concentrate diaphragm- and membrane-cell caustic by evaporation. Simple concentrating evaporators suffice in membrane-cell service, but diaphragm-cell evaporators reject most of the dissolved salt in the process of concentration. This salt must be removed from the resultant slurry, and it is used to prepare... 

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. 

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