Ostwald process

The Ostwald process is used for industrial production of nitric acid by oxidation of ammonia, which is preferably obtained by the Haber -Bosch process. It goes back to the Baltic German chemist Wilhelm Ostwald, who received the patent on the process in 1902. The basic chemical reaction of ammonia with air on a platinum catalyst was, however, already been patented in 1838 by Frédéric Kuhlmann. The obtained with the Ostwald process about 60 percent nitric acid can be concentrated only to 68.5 percent, because this composition, then an azeotrope with a boiling point maximum of 122 ° C. A higher concentration can be achieved in a Gegenstromdehydratisierung with drying agents such as phosphorus pentoxide or sulfuric acid.

Description

The method proceeds in three steps. A starting material is produced by the Haber-Bosch process, ammonia gas. Other starting materials are air and water.

Step 1: Heterogeneous catalyzed combustion of ammonia gas

In the first step, ammonia ( NH3), oxygen (O2) is mixed and in the presence of a platinum -rhodium catalyst at 800 ° C to 900 ° C to water ( H2O) and nitrogen monoxide (NO ) is reacted ( On newer systems, a special recovery network used to recover the expensive platinum). The gas mixture the catalyst may only be very short - only about a thousandth of a second - touch, otherwise the nitric oxide decomposes again into the elements:

An undesirable side reaction which occurs even without a catalyst, the oxidation of the ammonia to elementary nitrogen with the formation of water:

Another undesired side- reaction is the formation of nitrous oxide:

The side reactions are suppressed according to the principle of Le Chatelier by as high temperature and low pressure mains.

Step 2: Generation of nitrogen dioxide

In step 2, lowering the temperature of the nitrogen monoxide (NO) to below 50 ° C and mixing it with air. It runs from a further oxidation with oxygen (O2) to nitrogen dioxide (NO2), which is then dimerized to dinitrogen tetraoxide:

Step 3: reaction in the oxidation and absorption column

The nitrogen oxides are converted in the third step in scrubbers with water to form nitric acid ( HNO3):

As intermediates in this case arise nitric oxide ( NO) and nitrous acid ( HNO 2 ), because the reaction proceeds via the following intermediate steps: