CDU Overhead Dew Point Calculation

Inherently, crude contains some amount of salts which is present with water in a dissolved state. This salt is required to be removed from crude, other wise the salt will form acids on hydrolysis at high temperature inside crude heaters and will cause corrosion in the equipment. For removal of these salts, desalters are provided. However, the desalters are not 100% efficient in removing the salt. Hence some amount of salts is always present at the outlet of the desalters which gets hydrolysed inside crude heaters and form acids. Pure acids is present in vapour phase in the crude overhead circuit and is not corrosive. However when it comes in contact with water vapour and condenses along with the water droplet it is corrosive and detrimental to the equipment. Hence water dew point in the crude overhead circuit is an important parameter to control corrosion.

By definition, dew point is the maximum temperature at which 1st drop of liquid will form, in a mixture at a given pressure. Now what are the factor affecting the dew point? The major factor affecting dew point in the overhead circuit is Partial Pressure of water vapour which is in turn is governed by amount of stripping steam injected and amount of water being carried over along with the crude. The other factors are molecular weight of the water vapour and Operating pressure at the top of the column.


 
 
 
 

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