Nitric acid, dimerization reactions with nitrogen dioxide

The nitric oxide which is formed reacts with oxygen to form nitrogen dioxide. Nitrogen dioxide exists in equilibrium with its dimer, dinitrogen tetroxide. The nitrogen dioxide/dimer mixture is sent to a column, sometimes called an absorption tower. Water is added at the top of the column. The nitrogen dioxide is converted to nitric acid. Byproduct nitric oxide is oxidized to nitrogen dioxide by means of a stream of air passed into the absorption column. The aqueous nitric acid is removed continuously from the base of the column. Overall, the reaction can be written as ... 

An exception is the case that a critical reaction step takes place in the gas phase only. An interesting example is the nitric acid process, where evolved nitrous oxide is oxidized to nitrogen dioxide in the gas phase. The dioxide (or its dimer) dissolves again and reacts with water. In this process both the gas volume and the gas/liquid interfacial area determine the overall reaction rate. Of course, gas/liquid mass transfer has then to be taken into account (section 4.6). 

---