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 walls of the process 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 relating human resources, are heavy, but not the major maintenance expenses. It should be well understood, that most of the financial losses are assigned to the shut down 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 its impact can be controlled by blending sulfur rich crude oils with sweet crude oil, and by that 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 blocking and plugging of the CDU, plates and pipelines. Salt that enters the vacuum tower also decomposes, and contaminates the HVGO which served 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 the visbreakers. The presence of sodium chloride, but merely the presence of unreacted calcium and magnesium chlorides will attack not only the process 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 corrosives salts start to precipitate and enhance corrosion and fouling. By lowering the HCl concentrations, less amounts of amines chlorides will be formed and influences 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 in 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, phase separation of water and crude oil.
It is of highest importance to ensure that the effectiveness of desalting process reduces the salt content to its absolute reachable minimum, preferably below with values of below 5-10 PTB of salt.
On-line and real time determination of the salt content of the crude oil leaving the desalter 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 analysers, is highly beneficial to guarantee the highest degree of desalting at 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 reducing 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 by that reduces the shut down time for maintenance. This saves annually dozens of millions of dollar to the refineries, which makes the ROI for a salt in crude analyser being symbolic only.