Naphthenic acid is an acidic substance existing in oil, so it is likewise called oil acid. It is a carboxyl by-product of cycloalkane, as well as its main element is a five-carbon ring. The general formula is CnH2n-1COOH. Some are oily liquids, some are solid, as well as commercial items are dark oily mixes. The molecular weight arrays from 180 to 350, has an unique odor, is virtually insoluble in water, and also soluble in hydrocarbons. Harsh to steels such as lead and also zinc. Unrefined naphthenic acid has low acid worth and high neutral oil material, which impedes its popularization and application. The content of naphthenic acid differs from place to area, and also the general material is 0.3% to 7%. It is a blend of the complying with 3 carboxylic acids: ① saturated monocyclic carboxylic acid (CnH2n-1COOH); ② saturated polycyclic carboxylic acid (CnH2n-3COOH); ③ fatty carboxylic acid (CnH2n +1 COOH). Amongst them, ③ is considered as a contamination as well as does not belong to this post. In ① as well as ②, the cycloalkyl team is mostly pentyl, and a small amount is hexyl. The variety of alkyl groups is generally 1-5, that is, m=1-5. A substituent R might or may not be present on the cycloalkyl group. The carboxyl group is normally not directly attached to the cycloalkyl team, yet is attached to the cycloalkyl team through one or several (mainly n=6 ~ 18) methylene groups. There is generally one carboxyl team, that is, most naphthenic acids are monobasic acids. Naphthenic acid has the generality of carboxyl team, as well as it can react with steel to develop salt, which is called “naphthenic acid soap”.
Naphthenic acid is primarily an acid divided from petroleum items, which can be used to make steel naphthenates, as paint drier and also wood chemical. It is also used to make synthetic detergents, pesticides, special lubricating oils as well as dry cleaning soaps, and can be made use of as a solvent. Given that naphthenic acid is an organic acid including five-membered or six-membered rings, it can react with alcohols to create esters. Utilizing this particular, naphthenic acid as well as n-butanol are made use of as resources to prepare butyl naphthenate plasticizer via esterification. The ring of butyl naphthenate gotten in this way, like the benzene ring of phthalate, can be inserted between polyvinyl chloride particles, and also its longer alkyl side chain can wind around polyvinyl chloride molecules to develop a precise The framework has a better plasticizing impact as well as can be utilized as a supporting plasticizer for polyvinyl chloride.
Naphthenic Acid Property
160-198 °C (6 mmHg)
0.92 g/mL at 20 °C (lit.)
31.4Pa at 25℃
Practically insoluble in water
5[at 20 ℃]
Clear colorless to yellow-brown
88.1mg/L at 20℃
Naphthenic Acid Uses
Naphthenic acid is used in the manufacturing of numerous naphthenates.
Naphthenic acid is primarily made use of to prepare naphthenic acid salts, as well as its sodium salt is a low-cost emulsifier, farming growth agent, and also detergent in the fabric market; lead, manganese, cobalt, iron, calcium and other salts are utilized in printing inks as well as layers Desiccants; copper salts, mercury salts are made use of as wood preservatives and chemicals, fungicides; aluminum salts are used for grease, napalm and flares. Its nickel, cobalt, and also molybdenum salts can be made use of as organic synthesis stimulants and also drier, as well as some salts can additionally be used as ingredients for unique oils.
Naphthenic acid is utilized as a resources for the manufacturing of naphthenic acid salts as well as a drying out agent for finishings, and also is additionally used for the conservation of timber as well as cables.
Naphthenic acid is used as drier, bio-auxiliary as well as chemical of paint.
Corrosion Mechanism of Naphthenic Acid
The corrosion reaction between naphthenic acid and metal is:
2RCOOH + Fe→ Fe(RCOO)2 + H2
Corrosion products are soluble in oil, and it is easy to dissociate from the metal surface, making corrosion develop in depth. When the metal is in contact with naphthenic acid for a long time and the flow rate of crude oil is large, a characteristic groove-like corrosion can be observed on the metal surface. This is the hallmark that distinguishes naphthenic acid corrosion from other corrosions. If the crude oil contains active sulfide, at high temperature, the active sulfide begins to decompose to produce hydrogen sulfide. Hydrogen sulfide reacts with metals:
H2S + Fe→ FeS + H2
The resulting corrosion product FeS film has a certain protective effect under certain conditions. While the FeS film is dissolved in naphthenic acid, the reaction is:
FeS + 2RCOOH→ Fe(RCOO)2+ H2S
The generated hydrogen sulfide causes corrosion of downstream equipment, and the corrosion cycle formed in this way intensifies the corrosion of metals. When RCOOH-H2S-Fe coexists, the relationship of the corrosion system is shown in Figure 2.
In the figure, n is the number of naphthenic acid methylene groups. The phase diagram is divided into corrosion-free zone (Fe), passivation zone (FeS) and corrosion zone [Fe(RCOO)2]. It can be seen from the phase diagram:
(1) There is a critical pressure in the reaction between naphthenic acid and iron. When the naphthenic acid is lower than the critical pressure, naphthenic acid will not corrode iron. Corrosion occurs when the critical pressure is exceeded.
(2) The partial pressure of hydrogen sulfide will affect the composition of the sulfide film formed on the iron surface, thereby affecting the ability of the film to resist corrosion by naphthenic acid.
(3) As the value of n increases, the molecular weight increases, and the critical pressure for the reaction between naphthenic acid and iron increases. It shows that the corrosiveness of naphthenic acid decreases with the increase of molecular weight.
Factors Affecting Naphthenic Acid Corrosion
01. Effect of acid value
At a certain temperature, the corrosion rate of metal is related to the content of naphthenic acid in crude oil (see Figure 4). The research results show that: at a certain temperature, the corrosion rate of A3 steel in naphthenic acid is: dc/dt=kc3/2. Experience shows that within a certain temperature range, there seems to be a certain critical value between the corrosion rate and the acid value, above which the corrosion rate will be significantly accelerated. For carbon steel, the acid value of crude oil is above 0.5mg/g, and the corrosion rate of A3 steel increases obviously
Figure 4 Effect of acidity and temperature on corrosion
02. Effect of temperature on corrosion rate of naphthenic acid
At low temperatures, naphthenic acid hardly causes corrosion to equipment, but above 200 °C, as the temperature rises, the degree of corrosion to equipment gradually intensifies, which is related to the high activation energy of naphthenic acid and the reaction. The research shows that the activation energy of the reaction between naphthenic acid and A3 steel is: Ea=346kJ/mol, and the relationship between the corrosion rate and temperature of A3 steel is: Ink =a-b/T. Gutzeide’s experiment shows that the corrosion rate of carbon steel and low alloy steel increases twice for every 55°C increase in temperature. The relationship with temperature (see Figure 5). It can be seen from the figure that the corrosion rate of 316 stainless steel is the lowest, and that of carbon steel is the highest. The Deruygs report pointed out that the corrosion of carbon steel starts at 149 °C, and the corrosion rate rises linearly with the increase of temperature. At about 204 °C, the corrosion caused by naphthenic acid is already quite serious, which should be paid enough attention to by refineries. Figure 6 shows the relationship between metal corrosion rate and acid value.
Fig.5 Corrosion rate of various materials in naphthenic acid
Fig.6 Relationship between metal corrosion rate and acid value
03. Effect of flow rate on corrosion rate of naphthenic acid
Flow rate and flow regime are very important factors affecting naphthenic acid corrosion. The turbulence created in refinery equipment’s elbows, tees and pumps accelerates corrosion of the equipment. When the gas volume is greater than 60% and the steam velocity is greater than 60m/s, the corrosion is the most serious. Under such conditions, the corrosion rate of certain equipment, such as furnace tubes, elbows, and pipelines, can increase by two orders of magnitude. At a certain temperature, the relationship between the corrosion rate and the flow rate of a certain material in crude oil seems to have a critical speed, below which the corrosion rate of naphthenic acid is very low. Experience shows that: in the absence of turbulent flow, when the flow velocity is less than 25-30m/s, carbon steel is resistant to naphthenic acid corrosion. The flow rate parameters of crude oil in domestic refinery equipment are controlled: the flow rate of the low flow rate to oil line shall not exceed 62m/s; the flow rate of the high flow rate to oil line shall not exceed 94m/s.
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