Preparation Of Anti-Coking Coating In The Thermal Cracking Of Hydrocarbon And The Evaluation Of Coking Inhibition Performance

Coking is inevitable in the ethylene production by thermal cracking and other chemical processes. The feasible technology of inhibiting coking is an important way to reduce the coking rate and prolong the operation period of cracking furnace, which has a remarkable effect on energy saving and reduction of consumption. It has become a technical difficulty needed to make big efforts to solve in the world academic and engineering fields.In order to implement the industrial application of the anti-coking coating technology in thermal cracking from laboratory scale, the experimental study of pilot-scale preparation of the SiO2/S coating was conducted in this paper. The anti-coking property of the coating was also evaluated in the pilot plant unit. And the effects of processing parameters on the behaviors of coating deposition were studied by numerical modeling. The difference of structures, morphologies and mass of cokes formed on the HP40alloy with different surface conditions were studied. Meanwhile, the coking inhibition properties of SiO2/S coating, sulfur/phosphorus-containing compound and potassium acetate were investigated experimentally. The main research contents and conclusions are as follows:(1) The coking behaviors on the HP40alloy with different surface conditions were studied, and the structures of cokes on the alloy surface were analyzed by characterization methods used in catalyst research. The results showed that the content of H in cokes deposited on the alloy during thermal cracking was only0.7%, which had a highly condensed level with the amorphous carbon structure composed of sp2and sp3and a low graphitization degree. The coking rate decreased with the cracking time. The morphologies of cokes formed on the oxidized HP40alloy surface changed from filamentous coke to granular coke, indicating that the coking mechanism in coking process turned from catalytic coking to pyrolytic coking. Cokes formed on the inner surface of after-service HP40cracking tube in industry with the long time erosion by oxidation/reduction atmosphere repeatedly were mainly filamentous cokes. Under the same experimental condition, there were more cokes deposited on the inner surface of industrial HP40cracking tube than those on the oxidized HP40alloy surface.(2) The effects of SiO2/S coating, sulfur/phosphorus-containing compound and potassium acetate on coking during light naphtha thermal cracking were investigated. When the coking time was3h, the coking inhibition rates of SiO2/S coating, sulfur/phosphorus-containing compound and potassium acetate were61%,55%and48%respectively. The SiO2/S coating showed the best anti-coking performance because it hampered the contact of hydrocarbon gas with catalytic metal and inhibited the nucleation and growth of catalytic coke.(3) Based on the similarity theory, the pilot plant unit for the thermal cracking of hydrocarbons was developed with the industrial GK-VI cracking furnace as a reference. The pilot tube was a coiled pipe reactor with the diameter and length of10mm and14.4m, which had10passes in the cracking section. The experimental study of pilot-scale preparation of the SiO2/S coating was conducted. And the even coating was obtained with smooth surface, good adhesion with the substrate and the ability of bearing20thermal shock cycles in water quenching at900℃. When the outlet velocity were lOm/s,20m/s,30m/s and40m/s, the coating thickness of the tube cross section at the10th pass (the highest temperature pass) were5μm,10μm,15μm and18μm.(4) The comparative coking experiments were conducted between the SiO2/S coating and coking inhibitor in the pilot plant unit with naphtha. The results of24h coking tests showed that the coking inhibition rates of the SiO2/S coating and coking inhibitor were39%and40%respectively. Their anti-coking properties were almost the same. Cokes deposited at high temperature sections under the condition of high velocity (with outlet velocity of about30m/s) with smooth surfaces were mainly formed by the solidification of tar droplets, which still had a highly condensed level with the amorphous carbon structure composed of sp2and sp3.(5) The effects of processing parameters of coating preparation on the CVD process were studied by numerical modeling. The results showed that the intermediates transportation was enhanced with the inlet carrier gas flow rate increasing, which led to a reduction of fluctuations of deposition rates along the reactor tube. Meanwhile, the mass conversion rates of intermediates reduced. The increase of mass concentration of source materials made the fluctuations of deposition rates along the reactor tube aggravated. The increase of outlet gas temperature resulted in the increase of mass conversion rates of intermediates. The calculation results showed that Damkohler numbers on the deposition surface were the magnitude of10"2-10"’, indicating that the control step for the SiO2/S coating deposition was surface reaction (Da<<1).