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.