Severe coking of the tube bundle in the crude oil and residue oil heat exchangers of petroleum refining units

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The image below shows the heat exchanger tube banks of the atmospheric and vacuum distillation units in the petrochemical industry, specifically those in Line 1 and Line 3. Model: FLB500-55-25-2, Material: 10#, Operating conditions: Shell side: Normal pressure line, temperature 180°C, pressure 0.80 MPa. Tube bank: medium, three-wire length, temperature 330°C, pressure 1.1 MPa. As can be seen from the graph, the constant-pressure pipeline oil tends to coking under the influence of high temperatures inside the pipe, along with significant corrosion. The normal cut is located at a specific fractionation point in atmospheric distillation, with a boiling point that lies between that of gasoline and other fractions such as diesel. Generally speaking, its boiling point is relatively moderate – higher than that of light fractions such as the gasoline components at the top of the tower, and lower than that of heavy fractions such as those in the normal second fraction and below. Sulfur content in the normal first fraction is one of the key factors that cause corrosion. Sulfur can exist in various forms, such as elemental sulfur, hydrogen sulfide, thiols, etc. Under certain conditions, these sulfur-containing compounds react chemically with metals, causing corrosion of the metals. For example, hydrogen sulfide reacts with iron to form ferrous sulfide; if this layer of ferrous sulfide is unstable, it is continuously destroyed and reformed, thereby causing ongoing corrosion of the equipment.