Simulation of a triple effect evaporator of a solution of caustic soda, sodium chloride, and sodium sulfate using Aspen Plus

Worldwide, the Chlor-Alkali process is the most well-known method for the production of chlorine (Cl2) and sodium hydroxide (NaOH). NaOH, also known as caustic soda, is a very important alkali with many applications in the processing and production of paper, detergents, aluminum, petrochemicals, inorganics, and in the food industry. The aqueous solution of caustic soda, known as “cell liquor," produced in this process must be concentrated from 11 to 50 percent weight, which is achieved through a multiple effect evaporator system. In some cases, the brine used as a raw material carries a few other components that cannot be separated before the feeding of the brine into the process. The presence of sulfates and chloride ions in addition to high NaOH concentrations (0.44 - 0.5 mass fraction) and high temperatures (above 60 °C, at 86 mmHg) causes the precipitation of a triple salt (Na2SO4 · NaCl · NaOH). This work focuses on using and validating the model of a triple effect evaporator in Aspen Plus using plant data. According to our results, lower temperatures and the extraction of sulfates could reduce the proportion of triple salt that precipitates in the last stages of the evaporator.