High temperature naphthenic acid corrosion

1 High temperature corrosion caused by naphthenic acid 

Naphthenic acid is a kind of organic acid in crude oils and has a saturated ring structure. Its content is typically measured and indirectly expressed by non-aqueous titration with potassium hydroxide (mg KOH per 100 ml for clean oils) or total acid number (TAN, as milligrams of KOH required to neutralize the acid in one gram of oil). Naphthenic acid in crude oils is a rather complex mixture. Its molecular weight ranges from 180 to 700, mainly from 300 to 400 [4]. 

2 Characteristics of high temperature naphthenic acid corrosion 

During the refining processes, naphthenic acid will be heated, distilled, vaporized and condensed. It then can dissolve into the distillate oil which has a similar boiling point and result in corrosion problems. Naphthenic acid has the typical characteristics of monobasic aliphatic acid and the corrosion processes will mainly take place in liquid phases. In general, the naphthenic acid corrosion rate is relatively low when the temperature is lower than 240°C. At temperatures higher than 240°C, especially in water-free enviromnents, corrosion becomes very serious. The reactions between naphthenic acid and metal can be described by the following equations  

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The application of the FSM-IT system has since been developed by iicorr Ltd (formerly CorrOcean Ltd) within over 20 refineries worldwide, for monitoring the effects of high temperature Naphthenic acid corrosion, which is an increasing concern with the processing of high acid - opportunity crudes. 

Many factors can affect high temperature corrosion processes in the petrochemical industry. Current investigations therefore aim to find the potential influences of some major factors on the corrosion processes and rates, such as temperature, velocity of flow and vacuum on high temperature naphthenic acid corrosion, and temperature and active sulfur content on high temperature sulfidation. 

Because the data collected in laboratories are significantly different from those collected directly from the field corrosion inspection of the processing units, the pilot simulation apparatuses will bridge the gap between laboratory tests and field practices. For example, the dynamic simulation apparatuses for high temperature naphthenic acid corrosion can simulate the temperature, velocity of flow and acid number of the working media in industrial enviromnents and can run continuously. 

High temperature naphthenic acid corrosion can normally be observed at temperatures ranging from 230°C to 400°C. Some of the literature has reported two peaks on the corrosion rate-temperature curves, the first peak at 270-280°C and the second at 350-400°C. 

The total acid number (TAN) of crude oils can be decreased through suitable mixing processes, i.e addition of crude oils with a low TAN into crude oils with a high TAN may obtain a mixed crude oil with a low TAN. If the TAN of this mixed crude oil is lower than the critical value for the initialization of high temperature naphthenic acid corrosion, the problems associated with this type of corrosion could be partially solved. 

Addition of a small amount of dilute caustic solution into the oils can neutralize naphthenic acid and then is the most economical way to control high temperature naphthenic acid corrosion. However, a disadvantage of this method is that salt corrosion of furnace tubes may become serious. Furthermore,... 

The addition of oleo-soluble inhibitors can decrease the high temperature naphthenic acid corrosion rate at 316-400°C. These inhibitors can form adherent and protective thin films on the inner surfaces of the refinery process units to prevent the exposure of metallic surfaces to the corrosive media. 

High temperature naphthenic acid corrosion can only happen at temperatures below 500°C since naphthenic acid will completely decompose when the... 

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