Coking and corrosion of the crude oil heat exchanger tube bundle in the atmospheric and vacuum units of petrochemical plants
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The image below shows the atmospheric and vacuum distillation unit in the petrochemical industry, specifically the tube bundle of the crude oil heat exchanger in the second atmospheric distillation column. Model: FB700－95－25－4, Material: 10# steel. This tube bundle has been in operation for nearly two years. Operating conditions: Shell side: crude oil, temperature 118°C, pressure 2.3 MPa. Gangcheng: Medium, usually medium pressure; temperature 246°C, pressure 0.818 MPa. During maintenance, when the heat exchanger was opened, it was found that the metal on the crude oil side was severely corroded.
The reasons for severe coking and metal corrosion in the tube bundle may include: 1. Temperature factors. Although the temperature on the crude oil side is not high (118°C), prolonged operation can lead to localized increases in temperature near the tube walls, causing the crude oil to coke. This reduces the heat transfer efficiency on the surface of the tube bundle and triggers localized corrosion. 2. Corrosion caused by impurities and sulfur-containing compounds in crude oil: Crude oil often contains corrosive substances such as chloride ions and sulfur, which accumulate on the surface of the pipe bundles over time, leading to pitting or crevice corrosion of the metal surface. In particular, 10# steel has poor corrosion resistance, making it prone to significant corrosion. 3. Low flow rate or abnormal flow conditions: When the flow rate of crude oil on the shell side is low, impurities tend to deposit and gradually form scale on the surface of the tube bundle, thereby creating localized concentration difference corrosion conditions that further exacerbate corrosion. 4. Selection of pipe materials: 10# steel has poor corrosion resistance; in acidic, sulfur-containing, or saline environments, it is prone to severe corrosion. It is advisable to consider using pipe materials with better corrosion resistance. 5. Long operating time and lack of cleaning and maintenance: Having been in operation for nearly two years, if regular and proper cleaning and maintenance are not carried out, impurities in the crude oil settle and form thick layers of scale, leading to localized overheating and corrosion. Suggested solutions: 1. Clean regularly to prevent the accumulation of scale over time. 2. Optimize operating parameters to increase fluid flow velocity and reduce impurity deposition. 3. Replace the heat exchange tube bundles with corrosion-resistant materials, such as stainless steel or other alloy steel tube bundles. 4. Strengthen the pretreatment of crude oil by removing impurities such as moisture, chloride ions, and sulfides, in order to effectively control the corrosive environment. .