Catalytic pyrolysis of high-density polyethylene on HZSM-5 and HY zeolite catalysts in a conical spouted bed reactor
Special Symposium - Environmental Protection & Sustainability
Environmental Protection & Sustainability (EPS - Poster)
Keywords: plastic pyrolysis, catalytic pyrolysis, conical spouted bed, HDPE, zeolites
There is a sharp increase in the the production and use of plastics, of around 4% per year, due to their fundamental contribution to all daily activities. At present, only a small percentage, less than 15%, of plastic waste is recovered or recycled. Pyrolysis is an alternative process for converting and upgrading plastic wastes.
In the pyrolysis of polyolefins, the use of catalysts in the reactor decreases the temperature required for pyrolysis and modifies product distribution. Moreover, it improves the yield of volatile products and provides selectivity in product distribution, especially for obtaining products of high commercial interest (gasoline and monomers) [1, 2].
In this study, one of the most commonly used plastics, high-density polyethylene (HDPE), has been pyrolysed at 500 oC over HZSM-5 and HY zeolite catalysts. A conical spouted bed reactor has been used in order to perform flash pyrolysis of polyolefins under conditions similar to industrial units [3].
Batch runs have been carried out to monitor the evolution of the different lumps. The catalyst bed weighs 100 g, and batches of 3 g of plastic are fed into the reactor for product monitoring. The nitrogen flowrate is 20% in excess of that required for minimum spouting velocity. Product analysis has been carried out by means of a gas chromatograph, being connected on-line to the reactor by means of a thermostated line. Furthermore, product identification has been carried out by mass spectrometry.
Products have been grouped into 6 lumps: light olefins, light alkanes, C5-C10 non-aromatic compounds, single-ring aromatic compounds, heavy fraction and coke.
The degradation rate of the plastic is very similar in both cases. Thus, degradation does not start for a few seconds (3 or 4), due to the low thermal conductivity of plastics. The maximum product formation rate takes place at approximately 50 s in both cases and the polymer is totally degraded at 150 s.
Product distributions obtained with the two catalysts are very different. Thus, using HZSM-5, the main fraction consists of light olefins, around 55%, whereas with HY C5-C10 non-aromatic fraction prevails, around 45%. The yield of light alkanes obtained using HZSM-5, 14%, is slightly higher than that with HY, 6%. Furthermore, the yields of aromatics and heavy fraction are higher in pyrolysis with HY catalyst.
In conclusion, HZSM-5 catalyst is appropriate for obtaining light olefins that can be used as monomers, whereas HY catalyst is more suitable for obtaining gasoline fraction products.
References
1. R. Bagri and P.T. Williams, J. Anal. Appl. Pyrol., 63 (2002) 29.
2. S. Ali, A.A. Garforth, D.H. Harris, D.J. Rawlence and Y. Uemichi, Catal. Today, 75 (2002) 247.
3. R. Aguado, M. Olazar, B. Gaisán, R. Prieto, J. Bilbao, Ind. Eng. Chem. Res., 41 (2002) 4559.
Presented Monday 17, 13:30 to 15:00, in session Environmental Protection & Sustainability (EPS - Poster) S-7P.
