11 - Ammonia, Nitric Acid and Their Derivatives

Abstract

Today, most ammonia is produced by the direct combination of nitrogen and hydrogen, and other synthetic processes are found to be impractical for large-scale commercial exploitation. Successful ammonia conversion requires a catalyst that promotes a sufficiently rapid reaction at 100–300 atm and 400–500˚C to utilize the moderately favorable equilibrium obtained under these conditions. A measure of the efficiency of ammonia conversion is the “space velocity,” which is the volume of reactants fed to a reactor per hour divided by the volume of the reactor. Nitrogen for ammonia synthesis may be obtained either by separation from air by liquid air distillation or by consumption of the oxygen of air by the burning of a fuel in an air-restricted oxidative process, which leaves a nitrogen residue. The separation of nitrogen from liquid air is done by cryogenic or low-temperature technology. Purification is an important preliminary step in operating areas near a refinery or petrochemical complex because accumulation of combustible dusts or condensed vapors, such as methane or acetylene in cryogenic lines containing liquid oxygen, introduces serious operating risks. Even with the large recent price increase for petroleum-based feedstocks, natural gas is still likely to remain a significant, if not the dominant, source of the hydrogen required for ammonia synthesis for the foreseeable future. Modern nitric acid production uses catalytic oxidation of ammonia in air followed by absorption of the oxidation products in water to yield nitric acid. The production of super phosphate and triple super phosphate involves procedures with potential emission problems that differ from those of phosphoric acid production.