Neutralization, overhead systems

Figure 1.3 shows a single drum overhead system. Double drum systems are also used. The difference between the two systems is the reflux temperature at the top of the tower. In the single drum system, total liquid condensation occurs in the overhead condensers. The reflux will be cool and will keep the tower top cool. It is advisable to check the hydrochloric acid dew point vs partial pressure to determine the anticipated location of corrosion. For example, tower top temperatures above 250° F (120°C) can transfer corrosion to the cold reflux. Where dew point conditions exist in the tower, It may be desirable to add ammonia to the reflux to neutralize the acid. 

Check the pH of the water withdrawn from the reflux drum. If it is low (5-6), the operators may not be adding sufficient NHj or neutralizing chemicals to the overhead system. 

Cracking of admiralty metal (C 44300) heat-exchanger tubes has been a recurring problem in a number of refining units and petrochemical process units. For example, ammonia is often used to neutralize acidic constituents, such as hydrogen chloride or sulfur dioxide, in overhead systems of crude distillation or alkylation units, respectively. Stripped sour water containing residual ammonia is used as desalter water at some crude distillation units. This practice causes ammonia contamination of the overhead system even if no ammonia is added intentionally. 

The sulfate ester hydrolysate is stripped to give a mixture of isopropyl alcohol, isopropyl ether, and water overhead, and dilute sulfuric acid bottoms. The overhead is neutralized using sodium hydroxide and refined in a two-column distillation system. Diisopropyl ether is taken overhead in the first, ie, ether, column. This stream is generally recycled to the reactors to produce additional isopropyl alcohol by the following equiUbrium reaction ... 

Electricity is taken from the National Grid by appropriately located substations which eventually transform the voltage down to 11 kV at a local substation. At the local substation the neutral conductor is formed for single-phase domestic supplies and three-phase supplies to shops, offices and garages. These supplies are usually underground radial supplies from the local substation, but in rural areas we still see transformers and overhead lines suspended on wooden poles. Figures 3.11 and 3.12 give an overview of the system from power-station to consumer. 

As an example, in TN-C-S supplies to domestic premises a break in the combined neutral/earth (CNE) conductor of an overhead service line would cause the metalwork in the premises to become live at or about the supply voltage if any apparatus, such as a dishwasher, were to be connected and switched on. To prevent this type of hazard, the integrity of the CNE conductor, throughout the system, has to be maintained, so all joints have to be properly made and reliable, and fuses and solid state devices are prohibited in these conductors. 

The foregoing description has purposefully been kept as general as possible in order to illustrate the basic criteria for wet refinery corrosion and its solution by chemical treatment with neutralizers and film-forming inhibitors. Use of neutralizers and inhibitors has been described in the crude still and overheads. The same concepts can be applied in other systems where there is a hydrocarbon product in contact with liquid water containing corrosive constituents, usually hydrochloric acid and hydrogen sulfide. 

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