دانلود رایگان مقاله اصلاح بخار هیدروکربن مدل تجزیه در اثر حرارت پلاستیک و غیر فعال کردن کاتالیزور

ABSTRACT

Catalytic steam reforming of n-hexane, 1-hexene, tetradecane and toluene over a Ni commercial catalyst has been carried out in a fluidized bed reactor at 700 ◦C. These compounds have been selected as model compounds of the volatiles formed in the pyrolysis of waste plastics in order to study in detail the performance of the catalyst in the pyrolysis-reforming of different plastic wastes. High carbon conversions and hydrogen yields are obtained at zero time on stream, with peak values being 96.5% and 82.8%, respectively, when n-hexane is used as model compound. Similar reactivity has been observed for tetradecane and 1-hexene, whereas lower carbon conversion (82%) and hydrogen yields (65%) are obtained for toluene. Concerning catalyst stability, olefinic compounds (1-hexene) and aromatic compounds (toluene) cause faster catalyst deactivation than paraffinic compounds (tetradecane and n-hexane). These disparities are explained by the different nature of the coke deposited and the different potential of the compounds to block Ni active sites, with olefins and aromatics being encapsulating coke precursors (amorphous and structured, respectively) and paraffins being filamentous and inert coke precursors.

4. Conclusions

Steam reforming of different model compounds (n-hexane, 1-hexene,tetradecane and toluene), representative of plastic pyrolysis volatiles, has shown that the composition in the reaction medium has great influence not only on the conversion and H2 yield obtained at zero time on stream but also on the amount and nature of the coke formed, and therefore on catalyst deactivation. Although slight differences are observed, high carbon conversion and H2 yields are obtained at zero time on stream (>92% and >76%, respectively) for linear hydrocarbons. As the paraffin molecule is longer the reforming rate is lower, but a comparison of the paraffin (n-hexane) with the olefin (1-hexene) shows that olefin susceptibility to thermal cracking leads to a decrease in the reforming reaction rate. Besides, lower conversion and H2 yield is observed for toluene, 82% and 65%, respectively, which evidences a lower reactivity of aromatic hydrocarbons compared to linear hydrocarbons. Catalyst deactivation is highly dependent on the composition in the reaction medium due to its effect on the nature of the coke. The presence of light olefins leads to the formation of amorphous carbon, which is deposited on Ni active sites and causes fast catalyst deactivation. Thus,the high amount of light olefins in 1-hexene reforming due to its thermal cracking causes faster catalyst deactivation than in paraffin reforming. Furthermore, as the paraffin molecule is longer, the reforming rate of the compound is lower and the thermal cracking rate is higher, with the latter leading to products causing catalyst deactivation. These products form preferably filamentous carbon, which is not deposited on Ni active sites, and therefore cause slow catalyst deactivation. Besides, aromatic compounds have high capability for condensing and forming a structured coke deposited on the catalyst surface, which is responsible for fast catalyst deactivation.