PTQ Q4 2023 Issue

converted into the corresponding water-soluble imidazo - line salt by reaction with an acid. However, weaker acids are preferred to be used and not hydrochloric acids to form the corresponding chloride salts. At higher temperatures, chlorides may be knocked out of the molecule and thus become very corrosive again. Therefore, it is better to use corrosion inhibitors with alkylammonium chlorides only at lower temperatures as active ingredients. At higher dosages, water-soluble filmers tend to form stable emulsions or foam. For this reason, special care must be taken to avoid overdosing when using this type of fil - mers. It is of great advantage that water-soluble filmers can be combined well with Kurita’s ACF Technology. ACF Technology uses solid organic bases that react immedi - ately with corrosive acids to form an ACF salt with very low corrosion potential and a neutral pH of 7. Already formed ammonium chloride or ammonium sulphide deposits, which have a very high corrosion potential, are dissolved in the running process and removed as water-soluble ACF salts with the sour water. With a combined application of a filmer and ACF technology, the metal surface can be pro - tected against corrosion, and the deposition of ammonium salts can be prevented simultaneously. Corrosion monitoring equipment It does not matter whether the filmer is oil-soluble or water-soluble. However, it is essential to know if the mol - ecule has a structure that allows it to enter the corrosion product lattice (see Figure 1 ). The selection of the coating agent (filmer) is determined by the ratio of oil and water in the product stream to protect against corrosion. The effectiveness of a film-forming additive and its behaviour in a liquid medium must be controlled under standardised laboratory conditions. This also helps to improve the formulation of a corrosion inhibitor and to better compare corrosion inhibitors with each other. Some well-known laboratory methods are used for comparative tests. One example is ASTM G31- 72 Standard Practice for Laboratory Immersion Corrosion Testing of Metals. C1018 steel coupons are dipped in the corrosion inhibitor formulation in such laboratory tests and then submerged in a standard brine at 60°C (140°F) under ambient pressure for some hours. The corrosion attack on the metal surface of the coupons is then evaluated. Kettle test method Kurita uses the modified Kettle test method based on the NACE (The National Association of Corrosion Engineers) test standard 1D1966. Please refer to Figure 2 , which describes the schematic structure of the Kettle test method. A pH adjustment function and hydrogen sulphide injection (H2S/Ar gas) are added to simulate the conditions of the actual distillation equipment as much as possible. Under standard test conditions (chloride concentration of 100 mg/L), the relationship between the amount of filming agent and corrosion rate can be determined when the pH is adjusted to 4.0, 5.0, 6.0, or other pH levels. The temperature is maintained at 65°C (149°F) as the adsorptive power of adsorption-type filmers decreases as

N H

Naphthenic acid

Imidazoline

Sarcosine

3 - n

Alkoxylated phosphate ester

Figure 1 Filming corrosion inhibitor molecule structures

Water-soluble filming corrosion inhibitors For several years, the need for water-soluble filmers has become increasingly important. The average volume of water recovered from crude oil rises to provide superior protection in systems with high water cuts. High system pressures increase the concentration of dissolved gases. Carbon dioxide forms carbonic acid, which reacts with the metal to form iron carbonate. This leads to pitting corrosion and weakens the equipment. Sulphate-reducing bacteria in some crude oils can lead to the formation of hydrogen sul - phide gas. It is a corrosive gas and reacts with steel to form iron sulphide with pitting corrosion. Like oil-soluble filmers, the water-soluble additives build a protective barrier to keep corrosive components away from the metal surface. In this case, however, no hydrocar - bon-containing media is needed, as the non-polar compo - nents of the filmer combine and thus build up the barrier. This effect can then be used as a protective function in sys - tems with high water cuts. Imidazoline salts, naphthenic acid salts, alkoxylated phos - phate esters or sarcosines are often used as corrosion inhib - itors for aqueous systems. Chemically, imidazolines belong to heterocyclic compounds. They have a five-membered ring structure with two nitrogen atoms in the ring. Many well-established imidazolines are available on the market, and the imidazoline molecules have different structures and hydrocarbon chain lengths. Oil-soluble imidazolines can be

pH controller

Micro pump

HCl

Flow meter

Gas

Oil

Test coupons

Temperature controller

Magnetic stirrer

Water

Oil bath

Figure 2 Kettle test method

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PTQ Q4 2023