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

Abstract

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–H2–N2 system. The coating’s 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 °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 °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.

Comparison of the anti-coking performance of CVD TiN, TiO 2 and TiC coatings for hydrocarbon fuel pyrolysis

“…Because the characterization of TiN and TiO 2 coatings has been described in detail in our previous work [19][20][21], only the characterization of TiC coating is shown here. Fig.…”

“…The thermal cracking of supercritical RP-3 Chinese jet fuel was employed to compare the anti-coking performance of TiN, TiO 2 and TiC coatings with intuitive amount of carbon deposits and running time at high temperatures as the index. [19]. Furthermore, more details about the preparation of TiN and TiO 2 coatings were also described in previous work [19,21].…”

“…[19]. Furthermore, more details about the preparation of TiN and TiO 2 coatings were also described in previous work [19,21].…”

“…In previous work [19][20][21], the anti-coking performances of TiN and TiO 2 coatings were investigated. The results show that TiN coating has an excellent anti-coking performance but poor oxidation resistance [22].…”

Oxidation behavior of CVD star-shaped TiN coating in ambient air

“…Owing to these properties, TiN is used in a wide range of applications, such as protective materials for machine parts and cutting tools, wear-resistant coatings for orthopedic implants and diffusion barriers in semiconductor technology [4][5][6]. Recently, TiN coatings have been designed as anti-coking coatings for hydrocarbon steam cracking [7] and liquid hydrocarbon fuels pyrolysis [8], and exhibit a very good protective layer to prevent the substrate from metal dusting and severe coking. In our recent work [7], star-shaped TiN coatings were prepared by chemical vapor deposition (CVD) and their anti-coking ratios above 90% was obtained during cyclohexane pyrolysis at 770 1C for 1.5 h. For anti-coking applications, CVD TiN coating is an ideal material due to the following reasons: (i) CVD TiN coating has a good adhesive strength due to higher linear expansion coefficient.…”