Nitric acid

Uses of Nitric Acid• starting material in the manufacture of

nitrogen fertilizers e.g; ammonium nitrate, nitrophosphates and potassium nitrates• the use of ammonium nitrate as a

fertiliser is declining because of concerns about nitrate groundwater contamination

• nitrating agent in the preparation of explosives and organic intermediates such as nitroalkanes and nitroaromatics

• in the production of adipic acid• used as an etchant and pickling agent

for stainless steels• in rocket fuel production

Nitric Acid Production

1. Chilean saltpeter or nitrate

2. Arc process or Birkeland and Eyde process

3. Ostwald's process or Ammonia oxidation process

Nitric Acid Production1. Chilean saltpeter or nitrate

Nitric acid occurs in nature in the form of nitrate salts. Initially, large scale production of nitric acid began with sodium nitrate as a feed stock

NaNO3 (s)+ H2SO4 (l)NaHSO4 (s)+ HNO3 (l)

Nitric Acid Production

N2 + O2 2NO - heat2NO + O2 2NO2 + heat 4NO2 + 2H2O + O2 4HNO3

2. Arc process or Birkeland and Eyde process

Near the beginning of the 20th century, world reserves of sodium nitrate were low and processes were being developed. Arc process was abandoned because of poor energy efficiency.

Production of Nitric Acid

The critical step in nitric acid production, the catalytic combustion of ammonia, was developed by Ostwald around the turn of the century and was in production by 1906.

3. The Ostwald Process

1) Catalytic Combustion

4NH3 + 5O2 4NO + 6H2O

2) Oxidation of NO

2NO + O2 2NO2 N2O4

3) Absorption

3NO2 + H2O 2HNO3 + NO

Raw materials

Three raw materials are needed for the nitric acid process:

• Ammonia

• Air

• Water

The process

Ostwald Process

Production of Nitric OxideCatalytic Combustion

4NH3 + 5O2 4NO + 6H2O

According to Le Chatelier’s principle, the increase in volume in Equation implies that conversion declines as pressure rises

Combustion of ammonia is one of the most efficient catalytic reactions with possible conversions up to 98 percent.

Oxidation of ammonia

Ammonia is oxidised by oxygen in air to make nitrogen monoxide (common name: nitric oxide),4NH3 + 5O2 4NO + 6H2O ∆Ho

298 = −940 kJ mol−1

The reaction is catalysed by an alloy of platinum and rhodium.

Other reactions also occur such as4NH3 + 3O2 2N2 + 6H2O ∆Ho

298 = −11268 kJ mol−1

4NH3 + 4O2 2N2O + 6H2O ∆Ho298 = −1140 kJ mol−1

The yield of nitric oxide depends on pressure and temperature as indicated in the table.

These exothermic reactions release energy to produce steam and/or to preheat the waste gas. After this energy transfer, the temperature of the gas is 100 to 200 oC, depending on the process. It is cooled further with water.

Pressure (kPa) Temperature (oC) NO yield (%)

<170 810 - 850 97

170 to 650 850 - 900 96

>650 900 – 940 95

Burner Head

Platinum Gauze

Super Heater

FeedwaterPreheaterNitrous Gas

Outlet

Burner Design

The catalyst is platinum and the reaction occurs at 900 oC. The catalyst is a Pt-Rh fine-mesh gauze, where the Rh provides strength.

The biggest issue with this method is the loss of precious metal at the reactor temperatures.

Recovery gauze is typically used to absorb platinum oxide vapor and form an alloy. This gauze can periodically be removed and platinum recovered.

Production of Nitric Oxide

Catalytic Combustion

Production of Nitric Oxide

Catalytic Combustion

Burner Efficiency

Effect of Rh on the Platinum Gauze

Oxidation of Nitric Oxide

Further air is mixed with the nitrogen monoxide to oxidise it to nitrogen dioxide2NO + O2 ⇌ 2NO2 ∆Ho

298 = −112 kJ mol−1

Nitrogen dioxide dimerises to dinitrogen tetraoxide2NO2 ⇌ N2O4 ∆Ho

298 = −57.2 kJ mol−1

The NO is cooled en route to the absorption tower and, if necessary, compressed.

As a result of this process, part of the NO is oxidized to nitrogen dioxide and dinitrogen tetroxide.

To describe the kinetics of NO oxidation, a third-order rate equation is used.

This reaction is unusual because the reaction is quicker at lower temperatures. The reaction rate has a negative temperature coefficient.

Absorption

Dissolving oxides of nitrogen in water

The gas entering the absorption tower is a mixture of nitrogen dioxide and dinitrogen tetraoxide.

A counter-current flow of water is used to absorb the nitrogen dioxide. A number of reactions happen, but the overall reaction may be summarised by the equations:

4NO2 + 2H2O + O2 4HNO3

2N2O4 + 2H2O + O2 4HNO3

The contributing reactions include:

3NO2 + H2O ⇌ 2HNO3 + NO

N2O4 + H2O HNO2 + HNO3

3HNO2 HNO3 + H2O + 2NO

A secondary air stream is introduced to re-oxidise NO and to remove move NO2 from the product acid.

Environmental improvements

Special reactors are included in most modern plant and refitted into older plant to reduce emissions of oxides of nitrogen. The reduction takes place over a catalyst, and the reaction may be summarised as:

6NO2 + 8NH3 7N2 + 12H2O

Higher Concentrations of Nitric Acid

Nitric acid can be concentrated to about 68% in water with the above described process.

This concentration is adequate for most fertilizer applications but not for chemical nitration reactions.

However, there are ways to overcome the azeotrope (68% in water) and go to higher acid concentrations.

Concentrated Acid Processes:1) Direct - injecting or absorbing NOx into weak nitric acid under pressure.2) Indirect - extractive distillation and rectification with sulfuric acid or

magnesium

Monopressure process

BFW = boiler feed water, CH = high level compression, CM = medium level compression, CW = cooling water, and D = makeup driver, EX = expander, and F = filter

Monopressure (single-pressure) process

• Ammonia combustion and NOx absorption take place at the same working pressure.

• These include medium-pressure (230 – 600 kPa) and high-pressure (700 – 1100 kPa) processes.

• Very few plants currently employ low pressure (100 – 220 kPa) for both combustion and absorption.

• Higher operating pressures reduce equipment size and capital cost.

Monopressure process• The capital cost of the high monopressure process is

about 10–14% lower than that of the dual-pressure process.

• Higher gauze temperatures and operating pressures accommodate a more efficient recovery of process energy, either as steam or as reheated tail gas, providing power for air compression.

• Additional cost advantages by going to a vertical equipment layout, resulting in a smaller process footprint, less piping, and the elimination of weak acid pumps.

Dual pressure Process

RC = refrigerated cooling

Dual-pressure (split-pressure) processes

• The absorption pressure is higher than the combustion pressure.

• Modern dual-pressure plants feature combustion at 400 – 600 kPa and absorption at 900 – 1400 kPa.

• Some older plants still employ atmospheric combustion and medium pressure absorption.

Dual pressure process

• Some older plants still use atmospheric pressure for ammonia conversion.

• Compared to high monopressure plants, the lower oxidation pressure improves ammonia yield and catalyst performance.

• Platinum losses are significantly lower and production runs are extended by a longer catalyst life.

• Reduced pressure also results in weaker nitric acid condensate from the cooler condenser, which helps to improve absorber performance.

• Due to the split in operating conditions, the dual-pressure process requires a specialized stainless steel NOx compressor.