4.1 CONTINUOUS PRODUCTION OF FUELS
First, by decomposing waste plastics in a steam stream, it was shown that waste plastics could be degraded without producing a carbonaceous residue. Furthermore, it was found that the decomposition of oxygen-containing plastics can proceed at a uniform reaction rate. Second, a new reactor was proposed to achieve high hold-up, high heat transfer, and good contact between melted plastics with steam in order to accelerate hydrolysis. Using this reactor, a mixture of waste plastics can be degraded, and further decomposition over an FeOOH catalyst can also be achieved. Finally, the oil obtained was upgraded to a variety of fuels, indicating gasoline and kerosene, over a Ni-REY catalyst in a steam atmosphere. On the basis of these laboratory-scale experimental results, a pilot-scale plant was built and the validity of the proposed chemical recycling process was examined.
Figure 6.25(a) shows a novel process for the continuous production of fuels from waste plastics. The proposed process consists primarily of three reactors. A mixture of waste plastics is fed into a pyrolytic reactor with heat-medium-particles stirred by a helical impeller (Figure 6.25(b)), where melted plastics are hydrothermally decomposed with steam and the random scission of C-C bonds. The produced mixture of a heavy oil containing wax and sublimate material is carried by steam stream to the next reactor, which is filled with an FeOOH catalyst (i.e. a catalytic hydrolysis reactor). The gaseous
Figure 6.24 Carbon number distribution of products obtained by the catalytic cracking of oil derived from a mixture of PE and PET (reaction conditions: T = 400°C, W/F = 1 h). (Reproduced with permission from Elsevier)
compounds, including the vapors of the wax and sublimate materials, are passed through an FeOOH catalyst bed, and the oxidative decomposition over the FeOOH catalyst with steam proceeds. The quality of the oil thus obtained is further upgraded over Ni-REY zeolite catalysts in the catalytic cracking reactor, and fuels, such as gasoline, kerosene, and gas oil are produced. The weights of the FeOOH catalyst and the Ni-REY zeolite catalyst were 45 and 20 kg, respectively. The feed rate of the waste plastics, F, was 45 kg/h (the time factor in the pyrolytic reactor, W/F = 1.0 h).
Table 6.9 Product yield at the outlet of the catalytic hydrolysis reactor and the catalytic cracking reactor in the pilot-scale plant
Gas (C1-C4) Fuel (C5-C19) Heavy oil (C20+) Others (wt%) (wt%) (wt%) (wt%)
Outlet of reactor with FeOOH 2.1 46.7 50.3 0.9
The product yields at the outlet of the catalytic hydrolysis reactor and at the catalytic cracking reactor are listed in Table 6.9. The product yields of liquid fuel (gasoline-kerosene) and heavy oil reached to 46.7 and 50.3 wt%, respectively, even at the outlet of the catalytic hydrolysis reactor. The catalytic cracking reactor yielded 13.5 wt% gaseous fuel, 52.4 wt% liquid fuels, and 33.9 wt% heavy oil, indicating that the heavy oil obtained at the outlet of the catalytic hydrolysis reactor efficiently upgraded the quality of the fuel oil. Using this method, the storage of carbonaceous residue and sublimate materials in reactors, valves, and pipelines can be avoided. Moreover, recycling heavy oil at the outlet of the catalytic cracking reactor into the pyrolytic reactor can enable an improvement in the yield of liquid fuel to 72.4 wt%. Accordingly, the validity of the proposed process for the continuous conversion of waste plastic mixtures to various fuels was demonstrated on the basis of this pilot-scale plant.
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