Analysis of coking causes
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Analysis of Coking Causes: Coking has a great relationship with the properties of raw materials, the degree of crackability, the atomization of raw materials, the ease of vaporization, the ratio of agent to oil, the type of reactor and the operating conditions of the nozzle. Coking is also directly related to the final distillation point of the catalytic cracking feedstock. The final boiling point of our petrochemical heavy catalysis unit is generally above 560°C. The higher the final boiling point, the harder it is to vaporize the raw material, and the easier it is to form unvaporized oil. The catalyst absorbs the droplets of unvaporized oil to form a "wet" catalyst, causing the catalyst surface temperature to drop sharply to close to or lower than the riser outlet temperature. In a plug flow environment, relying on other catalysts The heating degree of the chemical agent is very limited. This oil-containing "wet" catalyst may adhere to the upper part of the riser nozzle or deposit on the outer wall of the settler and cyclone separator riser and in the "dead zone" space of the settler to form "liquid coke" (liquid coke). Under the action of the catalyst, following a series of complex chemical reactions, solid coke is finally formed. The atomizability of raw materials is also related to coking. In general, the oil droplets of easily atomized raw materials are smaller, and the number of oil droplets contacted by each catalyst particle will be greatly increased, improving the mass and heat transfer efficiency between the catalyst and the oil droplets.
There are many factors leading to coking between the bottom of the fractionating tower and the oil slurry system, including the chemical composition of the oil slurry, the operating conditions of the bottom of the fractionating tower and the oil slurry system (such as: The liquid level at the bottom of the fractionation tower, temperature, solid content of the catalyst, flow rate of process pipelines and heat exchanger tube bundles, etc.) are the main reasons. In addition, they are also related to the emergency treatment method in the accident state, the structural form of the bottom of the fractionation tower, etc. In order to increase the unit load, a large amount of pipeline transportation and northwest bureau residual oil with poor properties was supplied to heavy catalysts. In order to increase the light oil yield of the unit, partial back-refining of the oil slurry was adopted, which directly led to the deterioration of the oil slurry properties. The specific gravity of the oil slurry was operated at 1.05~1.1g/cm3 for a long time. ； In terms of operation, the temperature at the bottom of the fractionation tower is controlled too high, the oil slurry pump operates alone, the circulation volume is only 350t/h, and the flow rate of the oil slurry circulation system is less than 1.0m/s. ； In order to reduce energy consumption, a large amount of steam is restricted in each part of the reaction system. During low-load operation, the settler cyclone deviates from the allowed operating conditions, resulting in the oil slurry solid content exceeding the standard for a long time. These all aggravate the speed of coking between the bottom of the fractionation tower and the oil slurry system.
Study outside* When I was a kid, a teacher Fu told me this: The straight line direction of the raw oil nozzle corresponds to the inner wall, and the angle is the coking area. The top of the reactor is prone to coking. If the fluidization is not good and there is a certain temperature difference, the coke blocks will fall, which will cause difficulty in fluidization and cause danger. Now, of these two situations, the first has never happened, and the second has often happened.
Reply 3# shihua469469 What is the coking temperature?