Caustic evaporation

Fig. 30. Flow diagram of a triple-effect caustic evaporator.

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. 

Fig. 21 12 Steel ring-shaped cathode in the heati ng chamber of a caustic evaporator.

Use of a novel Alfa-Laval plate heat exchanger system for 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. 

Typically, a single-effect membrane caustic evaporator takes around 1.2 tonnes of steam per tonne of caustic soda while a double-effect uses around 0.7 tonne of steam per tonne of caustic soda. An equivalent single-effect diaphragm evaporator uses 4.2 tonnes per tonne. However, a well-run multiple four-effect diaphragm evaporator consumes about 2.1 tonnes of steam to produce one tonne of caustic soda. At a price of US 20 per tonne of steam, a saving of US 4 million per year for a 200 000 tonnes per year plant can be achieved (Fig. 15.6). So what extra equipment is required besides 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. 

Nickel s immunity to alkalies has been successfully used in caustic evaporator tubes and the corrosion rate is <0.005 mm/yr in 50% NaOH at 100 C. It is recommended that Alloy 201 be used at 316°C and above in order to avoid graphitization at the grain boundaries. 

The aqueous caustic recovered from the first coim-tercurrent washing circuit is limed to remove the coal-derived mineral matter, sulfur compounds, and carbonates. The mixture is provided with sufficient residence time to permit precipitation of the impurities before being centrifuged. The purified hquid is preheated and sent to a caustic evaporator where the water is recovered for recycle to the first wash train, while producing anhydrous caustic (as either a molten liquid or as flakes) for reuse in the initial leaching of the coal. 

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). 

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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