The impact of corrosion plays a major impact on the refinery economics, causing heavy losses to the crude distillation units and downstream process units, such as the FCC or thermal crackers. Corroding of the walls of the processing unit is not the sole problem. A lot of damage is caused by reaction products of H2S and HCl with iron surfaces, such as iron chlorides and iron sulfides, which plug and foul pipelines. This reduces the efficiency of heat transfer in heat exchangers, affects the flow between different distillation plates in the CDU, and increases corrosion in the overhead system.
Replacement and repairing of corroded and damaged parts, including related human resources, are heavy, but not major maintenance expenses. It should be well understood, that most of the financial losses are assigned to the shutdown time for repairing when the refinery unit does not produce.
H2S, Mercaptanes, and Sulphur are important key components in the corrosion of the crude distillation units, but their impact can be controlled by blending sulfur-rich crude oils with sweet crude oil, and by diluting the sulfur concentration, or by chemical treatments.
Major corrosion influencers of the CDU, are chloride salts, which are dissolved in the water-oil emulsion of crude oils.
Mainly three different chloride salts are available in crude oils, Sodium Chloride (NaCl), Calcium chloride (CaCl2), and Magnesium Chloride (MgCl2). Whereas Sodium Chloride is quite stable under the refining temperatures, magnesium chloride and calcium chloride react easily with water (humidity) when heated, while liberating hydrochloric acid (gas) and precipitating alkali hydroxides.
Hydrogen chloride that enters the CDU, is absorbed by condensed water that is formed when the temperature decreases below the Dew Point. Especially the acidic condensate will corrode the walls of the CDU, which will further result in the blocking and plugging of the CDU, plates, and pipelines. Salt that enters the vacuum tower also decomposes and contaminates the HVGO which serves as feedstock for the FCC, which aside from corrosion also shortens the lifetime of the catalyst.
In case salt remains in bottom residues, it will partially find its way to the FCC, but also to other cracking processes such as the visbreakers. The presence of sodium chloride, but merely the presence of unreacted calcium 
and magnesium chlorides will attack not only the processing unit itself but also pipelines and heat exchangers. As a result of corrosion and resulting fouling and plugging of pipelines and heat exchangers, more energy will be required to pump the petroleum products through the pipelines, and due to a reduced heat transfer capacity of the heat exchangers, more heating will be required to keep these thermal processes running.
Amines are injected in the overhead system of the CDU to neutralize HCl and form amine chlorides. Partially, these amine chlorides return to the top of the CDU through the reflux system. When the temperature drops below the salt point, which is about 8 °C below the dew point, and about 24°C below the top temperatures, these highly corrosive salts start to precipitate and enhance corrosion and fouling. By lowering the HCl concentrations, less amounts of amines chlorides will be formed and influence the salt point. Less salt will precipitate, which makes it possible to lower the distillation temperatures in the head of the CDU, and by that to increase the naphtha production.
It is clear, that desalting forms the platform for corrosion reduction. The desalting process takes place in the desalter and includes several processes, such as de-emulsification, electric and chemical, of the water-crude oil emulsion, water washing of the crude oil, and phase separation of water and crude oil.
It is of the highest importance to ensure that the effectiveness of the desalting process reduces the salt content to its absolute reachable minimum, preferably below values of 5-10 PTB of salt.
Online and real-time determination of the salt content of the crude oil leaving the desalter is also essential to fine-tune the process parameters of the desalter.
Continuous monitoring of the salt content by the MOD 4100 S Salt in crude oil analyzers, is highly beneficial to guarantee the highest degree of desalting at the lowest cost of operation, also including reducing the amount of wash water to its minimum.
Reducing the salt content also influences the salt point. Less amine chlorides precipitate in the head of the atmospheric tower. This allows the distillation temperatures in the heads of the column to be reduced, and by that to increase the production capacity of naphtha, and reduces the production of low-valued gas, which increases directly the refinery the refining margin.
Furthermore, effective desalting prevents superfluous corrosion and minimizes the cost to repair and clean corroded, fouled and plugged parts, and that reduces the shutdown time for maintenance. This saves annually dozens of millions of dollars for the refineries, which makes the ROI for a salt-in crude analyzer symbolic only.
