The Other Strong Acid: Nitric

     In the U.S. alone, there are over 65 nitric acid manufacturing facilities with a total capacity of over 11 million tons per year.  Plants can range in size from 6,000 to 700,000 tons per year.   Approximately 70% of all nitric acid produced is used in the manufacturing of ammonium nitrate which is then used in fertizilers.  Nitric acid is also a key component in the manufacturing of adipic acid and terephthalic acid.   Other uses include the making of explosives, mine leaching, and stainless steel pickling.

     Nitric acid production can be composed of one or two processes depending on the required concentration of the end product.   We'll explore both processes and outline the main features of each.

Weak Nitric Acid Process

     Much of the nitric acid produced in the world is manufactured via a high-temperature catalytic oxidation of ammonia.  This process consists of three main steps:  ammonia oxidation, nitric oxide oxidation, and absorption.  This process can be performed at one or multiple pressures.  We'll focus on the single pressure process here to cover the basics.  Newer processes typically operate at a low and a high pressure to favor the reactions.  You can learn more about the other processes here.

Figure 1: Single Pressure Nitric Acid Process

     A mixture composed of a 1:9 ratio of ammonia and air is oxidized at a temperature near 1400 °F (760 °C) in a catalytic converter according to the reaction:

4 NH₃  +  5O₂ -->  4 NO  +  6H₂O

The most common catalyst is composed of about 90% platinum and 10% rhodium (by weight).  The catalyst is formed into wire gauze and inserted into the converter.  The exothermic reaction proceeds to a nitric oxide yield of about 93-98%. 

     The nitric oxide is cooled (and water condensed) to a temperature of 100 °F (37.8 °C) or less at a pressure up to 115 psia (7.8 bara).  The nitric oxide reacts (noncatalytically) with oxygen to form nitrogen dioxide and nitrogen tetroxide via the reaction:

2 NO  +  O₂ --> 2 NO₂  +  N₂O₄

This reaction is very dependent on both temperature and pressure.  Low temperatures and high pressures favor the production of nitrogen dioxide (preferred) over nitrogen tetroxide. 

     After being cooled, the nitrogen dioxide/nitrogen tetroxide mixture enters an absorption column.  The gaseous mixture is introduced at the bottom of the column while liquid dinitrogen tetroxide and deionized water enter at the top.  Liquids flow countercurrent to the gases in the system while the oxidation takes place between the trays and absorption takes place on the trays (usually bubble cap trays).  The reaction in the absorption column proceeds by:

3 NO₂   +  H₂O --> 2 HNO₃  +  NO

A second air stream entering the column further oxidizes the NO and removes the NO₂ from the product acid.  Acid concentrations leaving the absorption tower are typically between 55-65% by weight.

Concentrating Weak Nitric Acid

Figure 2: Concentration Unit for Production of Strong Nitric Acid

      Dilute nitric acid from the process in Figure 1 is fed to the concentration unit as shown in Figure 2 along with concentrated sulfuric acid (usually 60-67% by weight).  The addition of sulfuric acid is necessary due to the azeotrope formed between nitric acid and water.

Figure 3:  Vapor-Liquid Equilibrium Date for Nitric Acid/Water System

Figure 4: Shifting of Equilibrium Data with the Addition of Sulfuric Acid

     The use of extractive distillation allow the concentration of nitric acid past the azeotropic point at 70% shown in Figure 3.   Packed columns are typically used and are operated nearly atomospheric pressure.   The nitric acid vapor exiting the top of the colum labeled T-103 is near 99% concentrated (also containing NO₂ and O₂ from nitric acid dissociation).  For more details on the separation process, click here.

By: Christopher Haslego, Owner and Chief Webmaster (read the author's Profile)
chris.haslego@cheresources.com