| The aromatization of butenes is an important route to the value-added use of C4 LPG by-product sources. However, the fast coking deactivation of conventional zeolite aromatization catalyst has been a major obstacle to be overcomed. Therefore, this dissertation is focused on the anti-coking deactivation of aromatization catalyst during the low-temperature aromatization of C4 LPG. Emphasis was given to the effects of crystal size, acidity, and micropore diffusivity of ZSM-5 zeolite catalyst on its coking-deactivation rate. Following main results are obtained:(1) The aromatization of C4 LPG over different crystal size ZSM-5 zeolite catalysts with similar acidity was studied. Results show that these catalysts exhibit similar initial activity but different stability. The order of reaction stability of different zeolites is:micro-sized ZSM-5< small crystal sized ZSM-5< nano-sized ZSM-5 zeolite catalyst. The nano-sized ZSM-5 zeolite catalyst possesses unique coke deactivation resisting ability, which is attributed to its short channels, large external surface area, abundant intercrystal pores, and much more pore mouths which decrease the chance of coke blockage.(2) Four series of zeolite catalysts are prepared by high-temperature steaming and acid leaching combination. XRD, NH3-TPD, pyridine-FT-IR, adsorptions of n-hexane and c-hexane are used to study the relationships between the zeolitic acidity and micropore diffusivity with the coking deactivation of nano-sized ZSM-5 zeolite catalyst in the low-temperature aromatization of C4 LPG. It is found that, nano-sized ZSM-5 zeolite catalyst with stronger acidity generally shows faster coke deactivation speed. Meanwhile, the diffusivity of the zeolitic micropores also has remarkable influence on the coking deactivation of the catalyst. The stronger the micropore diffusion constraint (the adsorption capacity ratio of n-hexane to c-hexane, denoted by n-h/c-h), the more serious coke deactivation is. By subjecting the nano-sized HZSM-5 zeolite catalyst to steaming treatment under 500-550℃, and leaching the catalyst with dilute HNO3 before and after steaming, the modified catalyst shows proper acidity and micropore diffusibility, and thus possesses satisfactory anti-coking deactivation ability.(3) By optimizing zeolitic acidity and micropore diffusivity with modification combination, DLG-1 catalyst was successfully developed by the nano-sized HZSM-5 zeolite, which exhibits satisfactory performances in both anti-coking deactivation and low-temperature aromatization of butene when fed with C4 LPG. Effects of reaction conditions, C4 LPG feedstock composition and purity, and with or without carrier gas on the low-temperature aromatization of C4 LPG was investigated over this catalyst. It is found that, beyond catalyst, the suitable reaction conditions, high purity feedstock (i.e., low diene and S-compounds content and alkaline N-compounds free) and the use of hydrogen as carrier gas are all very important to further enhance the catalyst’s anti-coking deactivation ability. Under the optimal range of reaction conditions (T=420-450℃, P=2.0-3.0 MPa, WHSVLPG=0.8-0.9 h-1 and VH2/VLPG=260), the conversion of butene and aromatization activity over DLG-1 catalyst, in the low-temperature aromatization, can keep above 99% and 60% after 1104 h time-on-stream, and the coke amount and C/H mole ratio of coke are 20.8% and 1.32 separately.(4) DLG-1 catalyst exhibits excellent regeneration performance. It can keep aromatization activity and butene conversion 60% and 99%, respectively, during more than 1000 hours time on stream after ten times repeated in-situ coke-burning regeneration. The catalyst was also subjected to a scale up reactor (200 ml catalyst loading) evaluation, which was carried out at the conditions of T=410℃, P=2.0 Mpa, WHSVLPG=1.2 h-1, and VH2/VLPG=260 for 600 h (operation was limited by C4 LPG supply), results obtained including butene conversion above 99%, aromatization activity around 48%, the percentage of dry gas and coke are below 2, hydrogen loss no more than 0.6 (wt%). Products study indicated that this scaled up test gave 76% gasoline fraction yield (calculated on butene), said gasoline product is most desired gasoline blending cut for low contents of olefin (1.6%), benzene (1.3%) and sulfur (1 ppm), and high octane number (RON> 97). The low-temperature aromatization also produced large amount of high quality Propane-butane LPG in which olefins content is less than 1%. Such LPG can be used as ethylene cracking feedstock. It is believed that the low-temperature aromatization of C4 LPG over modified nano-sized HZSM-5 zeolite catalyst, DLG-1, has very bright commercialization prospect. |
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