The phenomenon, causes, and preventive measures for coking in the heating furnace tubes?

It is caused by uneven heating of the furnace tubes, local overheating, and the heavy and dirty nature of the raw materials.

Why is your question so much like a short-answer question? Answer: Reasons: 1. Uneven heating of the furnace tubes, with the flame hitting the tubes and causing localized overheating. 2. Fluctuations in the feed rate, deviation in flow direction, and prolonged residence time of the oil, leading to cracking. 3. Polymerization of heavy ring compounds in the raw materials, as well as the presence of impurities. 4. Incomplete removal of coke during maintenance; the existing coke in the furnace tubes after restart acts as a catalyst, promoting the formation of new coke. Phenomena: 1. In a bright furnace chamber, dark spots on the furnace tubes indicate that coking has occurred in those areas. 2. The processing capacity remains unchanged, yet the furnace chamber temperature and the pressure entering the furnace increase. 3. A slow response of the furnace outlet temperature suggests that coking has formed around the thermocouple sleeves. Measures: 1. Maintain uniform furnace chamber temperatures to prevent localized overheating of the furnace tubes. 2. During operation, closely monitor and adjust parameters such as the feed rate, pressure, and furnace chamber temperature. 3. Carry out thorough cleaning tasks when the furnace is shut down. 4. Prevent any uneven distribution of materials

Coking is a phenomenon in which, when the temperature of the oil inside the furnace tubes exceeds a certain limit, thermal cracking occurs, resulting in the formation of free carbon that accumulates on the inner walls of the tubes. It mainly occurs in vacuum distillation furnaces, coking furnaces, visbreaking furnaces, lubricating oil heating furnaces, and ethylene cracking furnaces. Coking causes a sharp rise in the tube wall temperature, exacerbating corrosion and high-temperature oxidation of the furnace tubes, leading to bulging and cracking of these tubes. It also increases the pressure drop inside the tubes, deteriorating the operation of the furnace; in some cases, this even forces the facility to shut down ahead of schedule. Therefore, how to prevent or reduce tube coking has become one of the most prominent issues in the design and operation of various high-temperature heating furnaces. Coking process: Coking initially involves thermal pyrolysis, condensation, and the separation of polymer substances, primarily due to reactions induced by heating ； Due to the low flow velocity, a turbulent state is not achieved; as a result, the aforementioned reactants remain on the tube walls and gradually accumulate, forming a fouling layer. To prevent coking, in coking furnaces or lubricating oil heating furnaces, it is not advisable to overly pursue advanced heat transfer parameters and thus set the radiation heat intensity too high. For vertical coking furnaces with horizontal tubes and bottom heating, Kellogg recommends an average radiant heat intensity of only 31,600–34,700 W/m2 (when the inlet cold oil flow rate is 2.14–3.04 m/s), while also emphasizing the need for uniformity in the heat intensity distribution.

An analysis of the factors affecting coking and salt formation in the tubes of coking furnaces: Delayed coking involves heating heavy oils in tubular heaters, using high flow rates and intense heat to raise the temperature of the oils to that required for coking reactions in a short time. After that, the oils leave the heater and enter the coke tower, so that the coking reaction takes place mainly in the coke tower rather than in the heater. This is a thermal processing method that transforms heavy oils into light oils, intermediate distillates, and coke. Residue is a complex mixture containing aromatic compounds; it has a high boiling point and a large average molecular weight. Under high temperatures, it decomposes into smaller molecular gases and light oils on the one hand, and condenses to form coke on the other hand. The entire delayed coking process can be considered to take place in three steps: first, as the feed oil passes through the heating furnace, it partially vaporizes and undergoes mild cracking; second, cracking occurs as it passes through the coke tower; third, the heavy oil separated out in the coke tower continues to undergo pyrolysis and condensation until it is converted into coke along with gas. We begin our analysis by examining the factors that affect coking. First, let’s identify the main factors that influence coking: 1. Properties of the feed oil: Factors such as the residue content, high sulfur levels, and levels of metals and their salts in the feed oil have a significant impact on delayed coking units. These are the primary reasons for coking and salt deposition in the furnace tubes. 2. Outlet temperature of the heating furnace: The outlet temperature of the furnace has a direct impact on the degree of reaction, and it is influenced by coking in the furnace tubes as well as the conditions under which coke is formed. Since delayed coking is an endothermic reaction, the heating furnace can provide the heat required for the coking reaction. If the temperature is too low, the coking reaction cannot proceed adequately, and soft coke will be formed, reducing the yield of liquid products ； Furthermore, at low temperatures, the foam layer inside the coke tower increases, as foam is essentially a product of incomplete reaction—resinous gums and asphaltenes. Therefore, increasing the temperature at the furnace outlet allows the products resulting from incomplete reactions to undergo further reactions at higher temperatures to form coke, thereby reducing the foam layer. The height of this foam layer is related not only to the foaming properties of the raw materials but also closely to the temperature at the furnace outlet. When the temperature is too high, the hardness of the formed coke increases, making it difficult to remove the coke and leading to coking in furnace tubes and other components. The heating furnace uses dual-feed routes, so the temperature deviation between these routes (i.e., uneven heating within the furnace) is also one of the main factors contributing to tube coking. Regarding uneven combustion in the furnace, since our heating furnace uses a single control valve to regulate the gas flow, it is difficult to control, which makes uneven combustion in the furnace more likely to occur. 3. Others: The amount of water injected into the coking furnace has a direct impact on coking in the furnace tubes; less water injection will accelerate coking in these tubes, so it is essential to maintain an appropriate level of water injection. In short, all process facilities and auxiliary equipment should be designed with the goal of minimizing coking in the furnace tubes, ensuring that the carbon residue level of the raw materials does not exceed specified limits, maintaining the temperature at the furnace outlet, and controlling the amount of water injected. Whether under normal conditions or abnormal conditions (power outages, water shortages, lack of air or steam supply, equipment failures, etc.), the key points for operation are to ensure as much as possible a proper material flow through the heating furnace tubes, and to keep the temperatures inside the furnace chamber and at the furnace outlet within acceptable limits.