NITRIC ACID AND CAPROLACTAM PRODUCTION IN AMMONIA OXIDATION PROCESSES
HEAT EXCHANGER SYSTEMS FOR AMMONIA OXIDATION PLANT FOR NITRIC ACID AND CAPROLACTAM PRODUCTION
SCHMIDTSCHE SCHACK | ARVOS GmbH supplies highly sophisticated heat exchange systems, globally unrivaled in size, for mono and dual pressure catalytic ammonia oxidation processes in nitric acid and caprolactam production plants. The world’s largest heat exchanger ever built, with a diameter of 7 meters, was designed and fabricated by SCHMIDTSCHE SCHACK.
For the ground-breaking tertiary catalytic cracking process for adipic acid production SCHMIDTSCHE SCHACK provides heat exchanger systems downstream of the decomposition stage of nitrous oxide (“laughing gas”) laden exhaust gases.
Nitric acid production by Ammonia Oxidation takes place in two steps. An ammonia/air mixture is oxidized to nitrogen oxides in the presence of platinum/rhodium catalyst gauze. Then nitrogen oxide is oxidized and absorbed by water to produce nitric acid. The oxidation is a strongly exothermic process which follows the equation: 4 NH3 + 5 O2 + 4 NO + 6 H2O + 905 kJ 4 NH3 + 2 O2 + 2 N2 + 6 H2O + 1260 kJ The result is a process gas with typically 68 vol.% N2, 10 vol.% NO, 6 vol.% O2, 16 vol.% H2O. The hot process gas of 4 to 13 bar pressure and up to 920 °C is cooled by generating superheated steam and by preheating the tail gas.
Depending on the specific process requirements typical steam parameters are 18 bara to 55 bara at temperatures between 210 °C and 500 °C.
SCHMIDTSCHE SCHACK has developed and put into service reliable, maintainable heat exchanger solutions at reasonable cost. Depending on the specific plant parameters forced circulation or natural circulation systems are provided.
The heating surface bundles are fabricated from flat coil elements to allow a compact single vessel design. If natural circulation is selected vertically arranged shell & tube evaporator heating surfaces are integrated in the heat exchanger unit.
The start-up of a heat exchanger as process gas cooler is a critical phase from the perspective of corrosion. Nitric acid (HNO3) will be formed on the “cold” heating surfaces at the moment the process gas reaches the relevant dew point: 3 NO2 + H2O + 2 HNO3 + NO - 72 kJ If NH3 gas slips through the catalyst gauze during plant start-up, ammonium nitrate is formed in the presence of HNO3 immediately at temperatures below 170 °C. The build-up of ammonium nitrate, a corrosive and strongly adhesive mass, is represented in the following formula: HNO3 + NH3 = NH4NO3
To avoid this corrosion mechanism it is of great importance to warm up the “cold” heating surfaces to a safe temperature level during start-up. With optimized heat exchanger design and ideal material selection SCHMIDTSCHE SCHACK has developed outstanding methods to minimize corrosion. At the same time, our customers benefit from significantly reduced plant operation costs, e.g. due to lower start-up steam requirements. Furthermore optimal material selection helps to avoid corrosion mechanisms.
SCHMIDTSCHE SCHACK´S HEAT EXCHANGER SYSTEMS AND SERVICES FOR THE AMMONIA OXIDATION PROCESS:
- Nitric Acid Waste Heat Boilers
- Catalyst Baskets
- Burner Hoods
- Steam Drums