The effect of “heavy metal content” in naphtha on coking in the reactors used for ethylene production via steam cracking

This post was last edited by qqgd on 2024-7-31 09:47. It’s not clear what background information this statement is based on. Naphtha, which is used as a feedstock for the pyrolysis of ethylene, is generally a straight-chain hydrocarbon that has been hydrogenated and refined, with impurities such as heavy metals and silicon arsenic present at extremely low concentrations. It is indeed difficult to find direct evidence regarding whether these impurities can affect coking during the pyrolysis process. Since the ethylene cracking process is a non-catalytic thermal cracking process, the impact of trace heavy metal elements on the cracking and coking processes may require reference to some mechanistic research literature. From my understanding, coking in ethylene cracking occurs mainly because, during the cracking process, some hydrocarbons undergo excessive cracking to form conjugated dienes as well as certain aromatic or aromatic-like compounds, which then polymerize to form tar. It is recommended that the original poster take a look at a book titled \"Ethylene Engineering\".

In the process of producing ethylene via steam cracking, the quality of the raw materials has a significant impact on the efficiency of the entire reaction as well as the operational stability of the equipment. Naphtha, being a commonly used raw material in this process, indeed has a heavy metal content that can affect coking behavior. Coking refers to a phenomenon in which, due to prolonged high-temperature pyrolysis reactions inside the pyrolysis furnace tubes, carbon molecules deposit on the tube walls. Such deposition reduces thermal efficiency, causes corrosion to the equipment, lowers production efficiency, and ultimately requires shutdowns to remove the coked substances. The main mechanisms by which heavy metals affect naphtha are as follows: 1. Catalytic decomposition: Heavy metals such as nickel (Ni), vanadium (V), and iron (Fe) can act as catalysts, promoting the decomposition of hydrocarbon molecules and accelerating carbon deposition, thereby speeding up the coking process. 2. Catalyzed recombination: Heavy metals can promote the recombination of free radicals, resulting in the formation of polycyclic aromatic hydrocarbons and other high-molecular-weight compounds; these substances are more prone to decomposition at high temperatures, forming solid carbon. 3. Reduced reaction selectivity: Catalysts generally increase the rate and selectivity of specific chemical reactions, but side reactions that are not desired during cracking processes (such as carbon deposition) are also accelerated by the catalyst, thereby reducing the yield of ethylene and increasing the likelihood of coking. Therefore, in order to mitigate the negative effects of coking, it is usually necessary to carry out demetallization treatment on the naphtha feedstock, such as using sodium dihydrogen phosphate (NaH2PO4) to treat the crude oil and reduce the content of heavy metals. At the same time, process optimization, control of reaction conditions, the use of anti-coking coatings, and periodic cleaning of the furnace tubes are also important measures for controlling coking. Overall, although the olefin content in naphtha is the main cause of coking, the level of heavy metals also affects coking. Controlling the quality of raw materials and properly designing process conditions are key to reducing coking in pyrolysis furnaces. .

Ethylene cracking is a thermal cracking process that is independent of catalysis; furthermore, during ethylene cracking the residence time of the material in the cracking furnace tubes is very short, so the conditions for catalytic cracking are not met. Regarding what you mentioned, could you provide some reference materials so I can study it?