Inhibition Effect of APCVD Titanium Nitride Coating on Coke Growth during n-Hexane Thermal Cracking under Supercritical Conditions

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• 作者：Shiyun Tang ; Shuang Gao ; Shengwang Hu ; Jianli Wang ; Quan Zhu ; Yaoqiang Chen ; Xiangyuan Li
• 刊名：Industrial & Engineering Chemistry Research
• 出版年：2014
• 出版时间：April 2, 2014
• 年：2014
• 卷：53
• 期：13
• 页码：5432-5442
• 全文大小：593K
• 年卷期：v.53,no.13(April 2, 2014)
• ISSN：1520-5045

文摘

In this work, titanium nitride (TiN) coating was used as a passive layer to inhibit metal catalytic coking during hydrocarbon fuel cracking on the microchannel inner surface of stainless steel 304 (SS304) tubes. In order to obtain an inert and effective passive coating, TiN coating was prepared in SS304 tubes with 2 mm inside diameter and 700 mm length by atmospheric pressure chemical vapor deposition (APCVD) using a TiCl₄鈥揌₂鈥揘₂ system. The coating鈥檚 thickness, phase composition, morphology, and chemical composition were investigated by metalloscopy, X-ray diffraction, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX), respectively. Characterization results indicated that TiN coating had a relatively complete cubic-phase crystal form with a N/Ti ratio of 1:1, presenting small star-shaped crystals on the whole. The inhibition effects of TiN coating on the morphologies and amounts of coke were studied by SEM and EDX after n-hexane thermal cracking at 600 掳C and 3.3 MPa for 20 min. Under these conditions, different contributions to carbon deposition were discussed including oxidative reactions and pyrolysis of n-hexane. Along the axial length of the bare tube, stunted and clubbed cokes formed by autoxidation near the distance of 100 mm; granular metal carbides and filamentous cokes formed by metal catalysis near the distances of 350 and 600 mm, respectively. However, no morphologies of carbon deposits on a TiN-coated tube surface were observed after thermal cracking of n-hexane at 600 掳C and 3.3 MPa for 20 min. At distances of 100, 350, and 600 mm away from the tube inlet, the carbon atomic percentages of coke in these three areas were 27.28%, 58.04%, and 99.69% for the bare tube, larger than those of 5.76%, 15.73%, and 30.66% for the TiN-coated tube, respectively. The results showed that the inhibition effect of APCVD TiN coating on coke growth is superior to that of other coatings (e.g., alumina coating). The reason is that TiN coating not only creates a barrier between the hydrocarbon fuels and metal surface to inhibit related catalytic coke formation but also minimizes carbon deposits by absorbing C atoms.