Determination and Correlation of Vapor–Liquid Equilibria for the Nitric Acid + Water, Nitric Acid + Water + Phosphoric Acid, and Nitric Acid + Water + Phosphoric Acid + Sulfuric Acid System

The wet-process production of phosphoric acid by the nitric acid method is considered as a promising technology with great development prospects due to no production of the byproduct phosphogypsum. However, the traditional nitro-phosphate fertilizer producing process cannot separate nitrogen and phosphorus thoroughly, which limits its applications. The key to solving this problem is to remove nitric acid from the crude phosphoric acid product obtained in the nitric acid wet process by evaporation. Therefore, it is important to measure the vapor–liquid equilibrium (VLE) data of the nitric acid + water + phosphoric acid + sulfuric acid quaternary system under different pressures and concentrations. In this study, the VLE data of the following systems were measured at 10, 20, 40, 60, 80, and 97.4 kPa by the quasi-static ebulliometric method: (1) nitric acid + water; (2) nitric acid + water + phosphoric acid; and (3) nitric acid + water + phosphoric acid + sulfuric acid. The electrolyte non-random two-liquid parameters for the following pairs were correlated with the VLE data by using Aspen Plus as a tool: HNO3-H2O, H3PO4-H2O, HNO3-H3PO4, HNO3-H2SO4, H3PO4-H2SO4, H+[H2PO4]−-H+[NO3]−, H+[NO3]−-H+[HSO4]−, and H+[H2PO4]−-H+[HSO4]−. The obtained VLE data and interaction parameters can provide the basis for the nitric acid method of mother liquor denitrification in the subsequent industry.