The formation of another reaction phase

It is possible that reactants evaporate from a liquid reaction mixture, and that the evolved gas phase becomes a second reaction phase. In general, the effect of gas phase reactions as compared to liquid phase reactions will be negligible (as long as there is no mist formtion, see next section). 

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). 

Chemical reaction in a mist that forms in a gas phase 

When in a gas phase reaction a liquid condenses, the liquid phase may absorb reactants, and reactions may proceed in the liquid phase with much higher rates than in the gas phase. In many cases the liquid phase will form a mist that is difficult to separate. The temperature in the mist particles may rise, promoting certain reactions within the particles. An example is the oxidation of nitrous oxide in moist air, where eventually nitric acid mist is formed. The contribution of the mist phase to the total conversion may be considerable. However, the estimation of reaction rates in this situation is usually very difficult, since often the relative volume of the mist and the size of the droplets cannot be predicted. This makes this type of process difficult to control. It may be better to avoid this situation, either by raising the temperature so that no mist will form, or by lowering the temperature so much that a well controlled amount of liquid will condense and scrub the mist. 

Mist formation may also occur in gas/liquid reactions reactants may evaporate, react in the gas phase and form a liquid mist that will not coalesce with the other liquid phase. The temperature of the mist may rise, and uncontrolled reactions may take place in the mist particles. Examples of these are found in the processes for the manufacture of nitric and sulfuric acids, and in the preparation of ammonium nitrite from ammonium carbonate solution and nitrous oxides. In the nitric acid production, the product in the mist may be recovered by an effective separation of the mist particles (with demisters, wet scrubbers or electrostatic filters). But in the ammonium nitrite process, the product formed in the mist phase may subsequently decompose into nitrogen and water, thus reducing the process yield. 

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