Corrosion: protection, 26 overhead

Both neutralizers are injected in the fractionator overhead line in order to be present when the dew point of hydrochloric acid in solution is reached. It is important to use a quill to inject neutralizers or inhibitors because drip injection can cause dissolution of the protective scale on the inside of the pipe, which can result in corrosion and erosion in that area. Often, however, neutralization is not accomplished, and severe corrosion from hydrochloric acid still occurs at the dew point. The pH is controlled at the overhead receiver water draw because dew point pH measurement is not feasible. One method of controlling the dew point pH is to recycle water from the drum to the overhead line. This water buffers the condensate at the hydrochloric acid dew point and also provides water in which the ammonia can dissolve. 

Aluminium is the most abundant metallic element in the Earth s crust. It is extracted by purifying bauxite, an impure hydrated oxide, which may be represented as AI2O3.3H2O, and the subsequent electrolytic extraction of the metal. Many of the uses of aluminium — in canning, aircraft construction, and overhead transmission lines — depend on its low density and resistance to corrosion in air and water. This resistance is a kinetic effect, caused by a coherent oxide film, which protects the underlying metal. Some of the important aluminium chemistry that we discuss is summarized in Figure 9.16. 

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