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What is the role of filmers in reducing overhead corrosion?

Jan-2022

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Simon Calverley, KBC (A Yokogawa Company), simon.calverley@kbc.global

Filmers are often used in crude overheads corrosion control. The oil soluble filmer causes a thin layer of process hydrocarbon (in the case of the crude unit overheads, this is naphtha) to adhere to the metal surfaces that it contacts. For hydrocarbon soluble filmers, it is the layer of process hydrocarbon that is actually the protective film, not the filmer itself; the filmer is there to ‘stick’ the hydrocarbon to the metal. By preventing any corrosive species contacting the metal surfaces, the corrosion cell is disrupted and the corrosion rate will decrease. Actually, the filmer adsorbs and desorbs in equilibrium, maintaining a constant film on the metal surfaces. Filmers provide protection from aqueous corrosion due to weak acids such as hydrogen sulphide, carbon dioxide, and small organic acids. Filmers also provide more limited corrosion protection from stronger acids.

Filmers can enter the jet product through their use in the crude unit overheads. After the filmer is injected into the overheads, it dissolves in the naphtha, a portion of which is usually refluxed back to the tower. As filmers are heavy molecules, they will fall down the tower and leave in the higher boiling fractions, one being jet. If the jet is hydrotreated, the process destroys the filming amine contamination, producing harmless by-products. If it is not hydrotreated then clay filters are used to remove the filmer from the jet product.

Jan-2022
Berthold Otzisk, Kurita Europe, berthold.otzisk@kurita-water.com

Filmers are substances which reduce or eliminate corrosion potential by forming a protective layer on the metal surface. Galvanic corrosion is an electrochemical process of two half cell reactions with movement of electrons between areas with different electrochemical potentials. The metal surface acts as the anode. The basic corrosion cell requires an anode, cathode, metallic conductor, and electrolytes. If one of these is missing, aqueous corrosion will not occur. The formation of a thin protective film interrupts the flow of electrons from the anode to the cathode through the electrolyte and prevents or at least significantly reduces corrosion attack.

Absorption type oil-soluble filming amines are the most common corrosion inhibitors. The organic compounds are absorbed, where the polar head creates a stable bond with the metallic surface. Hydrocarbons, salts, and water can only contact the unpolar tail of the organic compounds, but cannot contact the metallic surface. At many overhead systems the oil soluble filmers are applied with a hydrocarbon slipstream for quick and good distribution into the product flow. Suitable filmers can help to significantly extend the run time of process units.

Jan-2022