Purification of Caustic

The extract passes to an ammonia stripper operating at an intermediate pressure (about 1,450 kPa). Much of the ammonia flashes, and the injection of steam at the bottom of the column drives off the rest. Most of the ammonia is recovered for recycle by way of the anhydrous ammonia storage tank. At different levels in the stripper column, other ammonia-containing streams are recycled. These are identified below. 

The stripper bottoms, at nearly 200°C, can be a valuable source of heat. First, it can be interchanged with a dilute ammonia stream (carrying the anunonia fix m the scrubber bottoms) that is being recycled to the stripper column. Then, it is flashed at a positive pressure of about 1 atm to generate some of the steam used in the feed ammonia and caustic preheaters. The resulting solution will contain 2.3-2.5% NaOH at a temperature of about 125°C. It is a waste stream from the process. In some cases, it has served as a source of OH for the chemical treatment of brine. 

The caustic from the bottom of the extractor is flashed to remove most of the dissolved ammonia. Like the ammonia stripper, the primary flasher operates at the intermediate pressure of about 1,450 kPa. The flashed vapor abstracts its latent heat from the remaining liquid. A trim heater on the caustic fed to the flasher supplies the necessary heat to offset the drop in temperature that would otherwise occur. The flashed ammonia, along with some water vapor, then can go to the stripper for separation into ammonia vapor and water. 

the flashed caustic, which still contains about 2% ammonia, goes to an evaporator. This operates under vacuum (about 40 kPa absolute). The remaining ammonia is driven off as an approximately 15% w/w vapor, the balance being water. The condensate from the overhead exchanger is collected. 

The bottoms from the evaporator is the product of the purification plant. It will contain less than 0.05% NaCl and will be at about 125°C. It can be used in other processes or cooled for storage and shipping. 

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