Aluminium chloride, storage

In the first alternative, trichloroethane is prepared by the liquid phase chlorination of vinyl chloride at 30—SO C under pressure. In the second alternative, trichloroethane is obtained by liquid phase chlorination of ethylene dichloride at about 60°C in the presence of aluminium chloride as catalyst. The trichloroethane is then dehydrochlorinated by agitating with an aqueous suspension of calcium hydroxide at about 50 C, Crude vinylidene chloride distills off as it is formed and is then purified by distillation under pressure. The dehydrochlorination of trichloroethane may also be accomplished by heating at 400°C. Vinylidene chloride is a colourless liquid (b.p. 32" C). It is rather difficult material to handle since it readily polymerizes on standing. Polymerization occurs rapidly on exposure to air, water or light but even storage under an inert atmosphere does not completely prevent polymer formation. The monomer is therefore commonly inhibited with a phenol, such as p-methoxyphenol, which is removed by distillation or alkali-washing before polymerization. 

Dangerous materials may require special equipment. Chlorination with gaseous chlorine requires quite expensive storage facilities. Chlorination with chlorine, thionyl chloride, sulphuryl chloride, phosphorus oxychloride, phosphorus trichloride, or phosphorus pentachloride, all of which are fairly hazardous, requires off-gas treatment. Some of these reactants can be recycled. Pyrophoric solids such as hydrogenation catalysts, anhydrous aluminium trichloride for Friedel-Crafts reactions, or hydrides used as reducing agents should usually be handled using special facilities. Therefore, all of the above proce.sses are usually carried out in dedicated plants. 

The chloride is usually (but not always) stabilised in storage by addition of aqueous alkali or anhydrous amines as acid acceptors. A 270 kg batch which was not stabilised polymerised violently when charged into a reactor. Contact of the chloride (slightly hydrolysed and acidic) with rust led to formation of ferric chloride which catalysed an intermolecular Friedel-Craft reaction to form polybenzyls with evolution of further hydrogen chloride. Contact of unstabilised benzyl chloride with aluminium, iron or rust should be avoided to obviate the risk of polycondensation. See Benzyl bromide Molecular sieve... 

Sludge removal Sludge should not be allowed to accumulate in the fuel storage basin but should be removed periodically by vacuuming or other methods. This material can contain chlorides, heavy metals, etc., and deposit on fuel assemblies, initiating pitting of the aluminium cladding. 

Crevices. Crevices between the aluminium clad assemblies and fuel storage racks or hangers should be avoided. Reduced pH, concentration of chloride ions and oxygen concentration cells in these crevices can lead to accelerated corrosion of the cladding. 

The corrosion processes associated with aluminium clad fuels in storage basins are electrochemical. Hence basin water plays a key role in the flow of electric current and ions in the process. The amount of metal removed by corrosion is directly related to the current flow. By increasing the resistance of the water, the corrosion of aluminium can be reduced. Fontana [10.4] pointed out that the low corrosion rates in high purity water are primarily due to the low conductivity of the water. Aluminium is passive and protected by its oxide film in the pH range 4-8.5. It has been observed that the pitting potential of aluminium in chloride solutions is independent of pH in the range 4-9 [10.7]. 

Aluminium of 99.5% minimum purity is the preferred material for storage vessels and pipelines. Hydrogen peroxide, as delivered, will not significantly corrode aluminium over long periods, and the corrosion products do not seriously affect the stability of the chemical. However, in the presence of chloride ions, serious pitting can occur, so dilution water must be of acceptable purity and contamination with chlorides must be avoided. 

Aluminium is very widely used in equipment for the transportation and the storage of ammonia water, and in production plants for nitric acid. As in gaseous ammonia, the presence of carbon dioxide CO2, sulphur dioxide SO2, hydrogen sulphide H2S, ammonium carbonate NH4CO3 and ammonium sulphide S(NH4)2 in ammonia does not alter the corrosion resistance of aluminium. On the other hand, the presence of chlorides (and of salts of certain heavy metals such as copper) has a noxious effect. 

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