Della Zassa, Micol (2008) Feedstock recycling di plastiche: recupero di monomero da polietilene e polistirene mediante trattamenti termici. [Tesi di dottorato]
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By this work, we want to emphasize the importance of the chemical waste recycling, since it allows the reprocessing of the polymer waste into raw materials, which can be utilized again in the refineries or in the petrochemical industry: turning a polymer back into monomers, amenable to further polymerization, closes the life-cycle of the material.
Unfortunately, selective depolymerization is not easy to achieve for the largest class of polymers, i.e. those obtained by free-radical polymerization. Polyolefins belong to this category and xxPE and PP together are the largest production of synthetic polymer. Decomposition of polyolefins into olefins can be thermally achieved indeed, but the process is expected to operate through a random, rather uncontrollable, free-radical reaction mechanism.
Products distribution is dramatically affected by temperature and residence time.
A serious analysis of the state of the art in the waste recycling field and the reutilization of plastics in Europe, and particularly in Italy, shows that feedstock recycling is still little developed. Consequently recycling could be an interesting support in the traditional methodology of mechanical and energetic recycling.
Nowadays, the integrated waste management of plastics seems the best solution to safeguard the word environment.
With the target of demonstrating the chemical recycling effectiveness, the tests in laboratory have taken place starting from the depolymerization of plastic waste by heating in absence of oxygen: we gave great attention especially to the non-catalyzed pyrolysis of polyethylene, considering the great presence of this polymer in solid urban waste, due to the versatility and to the many uses allowed by this plastic.
Pyrolysis is a process of thermal decomposition in inert atmosphere (nitrogen, helium, argon) of a substrate, usually solid, and generally characterized by a fairly elevated molecular complexity.
This substrate, thanks to the heating action, produces a solid fraction (called char), a liquid fraction (formed by products called tars, which condense at room temperature) and gases.
During the experiment we tested different plant configurations in order to final out the best operating conditions to maximize the yield in the base monomer which constitutes the fed polymer.
Since we worked with polyethylene, it was obvious for us to try to maximize the productivity in ethylene. In this line of action, we also tried to look for optimal contact-time of the polymer in the reactor and optimal operative temperatures.
In fact, operating on some parameters, it is possible to improve the selectivity of some compound instead of others: this is the reason why we tried to identify the right level of heating, the suitable temperature of cracking and the appropriate contact-time in order to find out, more specifically, the thermal decomposition reaction towards the desired olefins .
Beside the study of the best plant processing, we also optimized the gas-chromatographic sampling technique, with the purpose to obtain a quality and quantity analysis of the gas mixture, produced by the pyrolysis reaction.
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