Caustic evaporation boiling point rise

A single-effect evaporator is used to concentrate 0.075 kg/s of a 10 per cent caustic soda liquor to 30 per cent. The unit employs forced circulation in which the liquor is pumped through the vertical tubes of the calandria which are 32 mmo.d. by 28 mmi.d. and 1.2 m long. Steam is supplied at 394 K, dry and saturated, and the boiling-point rise of the 30 per cent solution is 15 degK. If the overall heat transfer coefficient is 1.75 kW/m2 K, how many tubes should be used, and what material of construction would be specified for the evaporator The latent heat of vaporisation under these conditions is 2270 kJ/kg. 

Multiple-Effect Evaporation. The discussion of brine evaporation in Section 7.1.5.2A showed how installing a number of effects and using process vapor as a source of heat reduces the amount of energy that must be added to the process. The boiling point rise (BPR) of brine solutions makes some of the available temperature potential useless as a driving force for heat transfer. This limits the number of effects and the achievable steam economy. Similar limitations exist in caustic evaporation but are much more severe. 

Volhard s Method.—39 gms. potassium cyanide and 10 gms. caustic potash are dissolved in 100 c.cs. of water in a large flask. 63 gms. potassium permanganate dissolved in 1 litre of water are then added, drop by drop, from a funnel, the flask being placed in a freezing mixture. The temperature should not rise above 8°. This is filtered, and a solution of 80 gms. ammonium sulphate is added, and the whole evaporated to dryness, The residue is powdered and extracted with 80 c.cs. absolute alcohol under a reflux at boiling point for hour. It is then filtered and the residue washed with boiling alcohol. The alcohol is removed by distillation until the volume is about 50 c.cs. It is then placed in a glass dish and allowed to stand. The crystals which separate are filtered, washed with alcohol, and dried. A second crop of crystals can be obtained from the mother liquor. 

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