Chemistry Corner (Nitric Acid):
8.8.1 General1-2
In 1991, there were approximately 65 nitric acid (HNO3) manufacturing plants in the U. S. with a total capacity of 11 million tons of HNO3 per year. The plants range in size from 6,000 to 700,000 tons per year. About 70 percent of the nitric acid produced is consumed as an intermediate in the manufacture of ammonium nitrate (NH4NO3), which in turn is used in fertilizers. The majority of the nitric acid plants are located in agricultural regions such as the Midwest, South Central, and Gulf States because of the high demand for fertilizer in these areas. Another 5 to 10 percent of the nitric acid produced is used for organic oxidation in adipic acid manufacturing. Nitric acid is also used in organic oxidation to manufacture
terephthalic acid and other organic compounds. Explosive manufacturing utilizes nitric acid for organic nitrations. Nitric acid nitrations are used in producing nitrobenzene, dinitrotoluenes, and other chemical intermediates.1 Other end uses of nitric acid are gold and silver separation, military munitions, steel and brass pickling, photoengraving, and acidulation of phosphate rock.
8.8.2 Process Description 1,3-4
Nitric acid is produced by 2 methods. The first method utilizes oxidation, condensation, and
absorption to produce a weak nitric acid. Weak nitric acid can have concentrations ranging from 30 to 70 percent nitric acid. The second method combines dehydrating, bleaching, condensing, and absorption to produce a high-strength nitric acid from a weak nitric acid. High-strength nitric acid generally contains more than 90 percent nitric acid. The following text provides more specific details for each of these processes.
8.8.2.1 Weak Nitric Acid Production 1,3-4 -
Nearly all the nitric acid produced in the U. S. is manufactured by the high-temperature catalytic oxidation of ammonia as shown schematically in Figure 8.8-1. This process typically consists of 3 steps: (1)ammonia oxidation, (2) nitric oxide oxidation, and (3) absorption. Each step corresponds to a distinct chemical reaction.
Ammonia Oxidation -
First, a 1:9 ammonia/air mixture is oxidized at a temperature of 1380 to 1470EF as it passes through a catalytic convertor, according to the following reaction:
(1) 4NH3 + 5O2 <=> 4NO + 6H2O
The most commonly used catalyst is made of 90 percent platinum and 10 percent rhodium gauze constructed from squares of fine wire. Under these conditions the oxidation of ammonia to nitric oxide (NO) proceeds in an exothermic reaction with a range of 93 to 98 percent yield. Oxidation temperatures can vary from 1380 to 1650EF. Higher catalyst temperatures increase reaction selectivity toward NO production. Lower catalyst
temperatures tend to be more selective toward less useful products: nitrogen (N2) and nitrous oxide (N2O). Nitric oxide is considered to be a criteria pollutant and nitrous oxide is known to be a global warming gas. The nitrogen dioxide/dimer mixture then passes through a waste heat boiler and a platinum filter.
Nitric Oxide Oxidation -
The nitric oxide formed during the ammonia oxidation must be oxidized. The process stream is passed through a cooler/condenser and cooled to 100EF or less at pressures up to 116 pounds per square inch absolute (psia). The nitric oxide reacts noncatalytically with residual oxygen to form nitrogen dioxide (NO2) and its liquid dimer, nitrogen tetroxide:
(2) 2NO + O2 --> 2NO2 <=> N2O4
This slow, homogeneous reaction is highly temperature- and pressure-dependent. Operating at low temperatures and high pressures promotes maximum production of NO2 within a minimum reaction time.
Absorption -
The final step introduces the nitrogen dioxide/dimer mixture into an absorption process after being cooled. The mixture is pumped into the bottom of the absorption tower, while liquid dinitrogen tetroxide is added at a higher point. Deionized process water enters the top of the column. Both liquids flow countercurrent to the nitrogen dioxide/dimer gas mixture. Oxidation takes place in the free space between the trays, while absorption occurs on the trays. The absorption trays are usually sieve or bubble cap trays. The exothermic reaction occurs as follows:
(3) 3NO2 + H2O --> 2HNO3 + NO
A secondary air stream is introduced into the column to re-oxidize the NO that is formed in Reaction 3. This secondary air also removes NO2 from the product acid. An aqueous solution of 55 to 65 percent (typically) nitric acid is withdrawn from the bottom of the tower. The acid concentration can vary from 30 to 70 percent nitric acid. The acid concentration depends upon the temperature, pressure, number of absorption stages, and concentration of nitrogen oxides entering the absorber.
There are 2 basic types of systems used to produce weak nitric acid: (1) single-stage pressure
process, and (2) dual-stage pressure process. In the past, nitric acid plants have been operated at a single pressure, ranging from atmospheric pressure to 14.7 to 203 psia. However, since Reaction 1 is favored by low pressures and Reactions 2 and 3 are favored by higher pressures, newer plants tend to operate a dual-stage pressure system, incorporating a compressor between the ammonia oxidizer and the condenser. The oxidation reaction is carried out at pressures from slightly negative to about 58 psia, and the absorption reactions are carried out at 116 to 203 psia. In the dual-stage pressure system, the nitric acid formed in the absorber (bottoms) is usually sent to an external bleacher where air is used to remove (bleach) any dissolved oxides of nitrogen. The bleacher gases are then compressed and passed through the absorber. The absorber tail gas (distillate) is sent to an entrainment separator for acid mist removal. Next, the tail gas is reheated in the ammonia oxidation heat exchanger to approximately 392EF. The final step expands the gas in the power-recovery turbine. The thermal energy produced in this turbine can be used to drive the compressor.
8.8.2.2 High-Strength Nitric Acid Production1,3 -
A high-strength nitric acid (98 to 99 percent concentration) can be obtained by concentrating the weak nitric acid (30 to 70 percent concentration) using extractive distillation. The weak nitric acid cannot be concentrated by simple fractional distillation. The distillation must be carried out in the presence of a dehydrating agent. Concentrated sulfuric acid (typically 60 percent sulfuric acid) is most commonly used for this purpose. The nitric acid concentration process consists of feeding strong sulfuric acid and 55 to 65 percent nitric acid to the top of a packed dehydrating column at approximately atmospheric pressure. The acid mixture flows downward, countercurrent to ascending vapors. Concentrated nitric acid leaves the top of
the column as 99 percent vapor, containing a small amount of NO2 and oxygen (O2) resulting from dissociation of nitric acid. The concentrated acid vapor leaves the column and goes to a bleacher and a countercurrent condenser system to effect the condensation of strong nitric acid and the separation of oxygen