Design process in Chemical Engineering

Design process in Chemical Engineering Example: Synthesis of ammonia In 1995, the US produced 35 billion pounds of ammonia. The basic reaction is: N2 + 3H2 2NH3 Prior to the development of a catalyst to promote this reaction, the large-scale production of nitrogenous fertilizer was not feasible.

In practice, some amount of the reactants will appear in the product stream. These can be separated using a condenser.

Component NH3 H2 N2

Boiling point at 1 atm (°C) -33 -253 -196

Note that the N2 and H2 from the condenser are recycled back to the reactor.

Sources of N2 and H2 are required. N2 can be extracted from the air. The boiling point of O2 is -183°C. How can H2 be produced?

One method is to crack water H2O + 242 kJ  H2 + ½ O2 Note that this reaction requires 242 kJ of energy per mole of water which is a great deal of energy.

How can this reaction be accomplished?

Electrolysis is too expensive since electricity is an expensive source of energy. There is a need to look for cheaper sources of energy. Consider using “Chemical Energy”.

Methane is a relatively inexpensive fuel. Burning methane leads to the following: CH4 + 2O2  CO2 + 2H2O + 803 kJ Evidently: CH4+ 2O2  CO2 + 2H2O + 803 kJ 3.3 (H2O + 242 kJ  H2 + ½ O2) -----------------------------------------------------------CH4 + 0.35O2 + 1.3H2  CO2 + 3.3H2 + 0 kJ

Again the reactor output must be purified to extract unreacted components. It is critical to remove O2 since it will react with the H2 in the ammonia reactor. Instead of using a condenser it is cheaper to remove O2 by burning methane in excess.

The boiling point of methane is -164 °C, so this will also be expensive to separate using a condenser.

However, methane will not interfere with the ammonia reactor. For the main reaction its presence is inert. Instead of removing it, it will be allowed to enter the ammonia reactor but will be minimised by adjusting the input so it is only present in trace concentrations.

The CO2 has a boiling point of -79°C.

So the final process is:

Note there are two purge streams to balance the process. There will be a build-up of unburned methane and unused nitrogen in the process which need to be balanced by bleeding off (purging) the streams to run the process at steady state.