| In order to expand the scope of the ethylene pyrolysis material and recycle the waste polyolefin, this study has employed the directly heated technology, rather than steam cracking (indirectly heated pyrolysis), to perform the high pyrolysis of the non-traditional feed. This study has combined the two-step method of waste polyolefin monomer recovery with the directly heated pyrolysis of petroleum hydrocarbon for the first time. Meanwhile, two sets of polyolefin cracking apparatus and four sets of petroleum hydrocarbon pyrolysis apparatus indirectly heated by superheated steam have been designed and built. As a result, the thermal cracking of polyolefin, co-cracking of polyolefin, co-cracking of polyolefin/hydrocarbon, high temperature pyrolysis of petroleum hydrocarbon/polyolefin cracked products have been studied. And especially, the influence of experimental conditions on the cracked products, the interactions between the different components, the kinetics of materials and the influence of high temperature pyrolysis reactor on the pyrolysis results were focused on. This new technology will be energy saving, low carbon emissions, reclamation of wastes. The main conclusions are as follows.Firstly, it is stated that the thermal cracking of polyolefins is efficient for producing liquid hydrocarbons as feedstock of subsequent olefin recovery by high temperature pyrolysis. The liquid product yield will decrease with the increasing of temperature/ residence time, at the temperature of 643-693 K. The advantages of this work can be summarized as high yield of light fraction, low aromatics content and mild cracking conditions, compared with other publications. That is to say, more viscosity-reducing and less volatile loss can be simultaneously achieved in thermal cracking of polyolefins by selecting the reasonable residence time-temperature combination.Secondly, the co-cracking of PE/PP has been studied. The experimental results indicate that a significant interaction can be observed when the mass fraction of PP is more than 30 wt%. And 30/70 LDPE/PP gives more volatile products due to the synergetic effect of LDPE and PP. The synergetic effect of LDPE and PP is not only related to the mass fraction of PP, but also related to the reaction time. The interactions mechanism are related to intermolecular hydrogen transfer between LDPE/its cracked products and PP/its cracking products, and such intermolecular hydrogen transfer occurs not only in liquid phase but also in gaseous phase due to the long cracking time in closed batch reactor. PP/its cracked products provide more radicals, while PE/its cracked products provide more secondary H for intermolecular hydrogen transfer, and the combination of these two factors enhances the chain breakage. It is also shown that presence of synergistic effect not only exists in thermal cracking using closed batch reactor/tube reactor, but also exists in the thermal degradation process of thermogravimetric analysis. And The interactions between PE and PP are mainly related to experimental conditions such as temperature, residence time and the degree of mixing for PE/PP. So closed batch reactor with agitation equipment is useful for thermal cracking of polyolefin to produce more light oils.Thirdly, co-cracking of polyolefin/hydrocarbon has obtained better results. The i-alkane can enhance the cracking of HDPE and HDPE/i-alkane gives more volatile products. This enhancement is related to the intermolecular hydrogen transfer. The volatile product is further divided into wax/oil and gaseous product. And the the co-cracking of HDPE/(HDPE/PP cracked product), HDPE/HGO, HDPE/C10H12 not only influences the wax/oil product, but also affects the gas product. For the co-cracking HDPE/(HDPE/PP cracked product) and HDPE/HGO, the ratio of HDPE and hydrocarbon will influence the interaction results.2/1 HDPE/hydrocarbon cracking not only enhances the wax/oil yield, but also improves the gaseous product yield.Fourthly, this study shows that petroleum hydrocarbon/polyolefin pyrolysis directly heated by superheated steam is feasible. The superheated steam, not only provides the energy for pyrolysis, but also performs a diluent. Three series experiments including indirectly heated pyrolysis without superheated steam, indirectly heated pyrolysis with superheated steam and directly heated pyrolysis by superheated steam have been carried on using GW-1 and the results show that directly heated pyrolysis by superheated steam has obtained the higher olefins yield, higher ratio of ethylene to methane, and better operational continuity. In addition, the results also indicate that the carbon distribution will influence the olefin yield.Fifthly, it is shown that the experimental conditions is closely related to the pyrolysis reactor. So enhancing the mixing effect, increasing the reaction temperature and shortening the residence time by changing reactor structure, can obviously improve the pyrolysis results. When LGO is used as raw material, on one hand, both the mole content of ethylene and the ratio of ethylene to methane has been increased by increasing the temperature and shortening the residence time, on the other hand, both the mole content of ethylene and the ratio of ethylene to methane has also been increased by diminishing feed inlet area. These results indicate that improving the reactor structure can increase the ethylene yield, promote the ethylene selectivity and inhabit excessive methane and hydrogen production. In addition, the mixing effect between feed and superheated steam can be strengthened by diminishing feed inlet area. Thus, the liquid product has changed from brown liquid with black sediment to light yellow liquid without sediment. This result indicates that the pyrolysis are mainly chain scission reactions and the secondary reactions, such as cyclization reaction and dehydrogenation reaction, are also reduced significantly due to the mixing effect improvement. In the end the pyrolysis mechanism scheme has been built. |
PDF Full Text Request