Heat released when nitrogen and hydrogen are reacted in the catalytic reactor. However, the ammonia formed in the reactor must be liquefied to better storage. Ammonia stays in liquid form below -33oC at atmospheric pressure. Better liquefaction system avoids the loss and decrease the ammonia concentration in the recycle. Conversion effects directly with the increase in ammonia content in the recycle.
Ammonia synthesis process flowsheet with liquefaction system description:
Process flowsheet of ammonia synthesis contains the reaction loop, the liquefaction (by refrigeration) system and recovery (by absorption) system.
Most ammonia reactors are designed for the catalysts bed operation between 360oC to 520oC. In case the ammonia reactor operates at temperature 250oC with the pressure of 140 kg/cm2 and due to exothermic reaction, the outlet temperature increases up to 440 oC. At the time of inlet just before every catalyst bed, a definite least temperature of 360 to 380oC is necessary to make sure to obtain the required reaction rate. If the temperature at the catalyst inlet is less than 360oC, the reaction rate may perhaps slow down and the heat liberated by the reaction becomes too small to carry on the temperature in the converter, and the reaction quickly turn off itself if appropriate process adjustments are not made instantaneously. In contrast, it is desirable to maintain the catalyst temperatures as low as possible to extend the catalyst life.
Ammonia Synthesis reaction affected by the following parameters:
- Ammonia content percent in the feed gas
- Inert gas content percent in the feed gas
- H2 to N2 ratio in the feed gas
- Reaction temperature
- Circulation Rate
- Operating pressure
- Catalyst bed activity
For start-up of the reactor
Natural gas fired start-up heater is installed with bypass loop connection of synthesis gas. Catalyst heat up during start up, done by continuous circulation of the synthesis gas between reactor and fired heater to obtain the reaction temperature.
The turbine compressor maintains pressure of the reactor. To its suction side, makeup synthesis gas introduced continuously along with the circulating synthesis loop.
Reactor effluent heat recovery and cooling
The effluent from the reactor passes through the waste heat boiler to recover the heat generated by the reaction. Further cooling of the reactor effluent takes place in the boiler feed water preheater and further cooled in counter current with the inlet gas to the reactor, finally cooled to 40oC in the water cooler