Alternative catalytic processes for the valorization of plastic wastes to fuels

Resumen

The first chapter analyses the state of the art on plastic production and main sectors of application, and on generation of plastic wastes, focused on the environmental problem generated by uncontrolled dumping and the different alternatives and processes, at the research or industrial stage, proposed for the management and valorization of plastic wastes. Pyrolysis has been found to be one of the most common strategies to produce liquid hydrocarbons from plastic wastes. However, the oil product obtained requires upgrading through hydrotreatment to be used as a fuel. Catalytic hydrocracking is proposed as an alternative treatment to pyrolysis and hydrotreatment, in a single stage. In a previous work, the conditions of catalytic hydrocracking of PS for working under kinetic control were established, and the kinetic model was proposed. Hydrocracking of plastics has been carried out in a three-phase stirred reactor under kinetic control with bifunctional catalysts in slurry and hydrogen pressure. Bifunctional Pt/zeolite catalysts have been prepared and characterized, in order to establish a relationship between properties and performance. Several virgin plastics and plastic wastes have been characterized and tested, so that their behavior in reaction can be related to their composition. The required experimental equipments and procedures used all over the work for reaction, preparation and characterization are described in detail in Chapter 2. Chapters 3 and 4 have been devoted to optimization of the bifunctional catalyst for hydrocracking of plastics, using virgin PS as the base polymer for comparison. In Chapter 3, the effect of the acid properties of the zeolitic supports on activity and selectivity has been studied; the acid properties of the supports have been modified by either dealumination or desilication. On the other hand, optimization of the metallic function has been studied in Chapter 4, where the effect of platinum content and preparation procedure on activity and selectivity has been analyzed. Platinum contents in the range 0.1¿ 1.0 wt.% and prepared by either ionic exchange or impregnation on HBeta zeolite have been studied. Plastic wastes are composed by a mixture of different polymeric materials. Hence, once catalyst was optimized for PS, its performance in hydrocracking with other polymeric materials was studied. In Chapter 5, common polymers with only carbon and hydrogen atoms in their composition, such as PB and HIPS, were chosen. HIPS, besides, is composed by a mixture of PS and PB. Thus, the effect of mixture has been also analyzed. Thermal hydrocracking experiments were also carried out for comparison. Hydrocracking of more complex polymers has been studied in Chapter 6. Polymers in plastic wastes are not only composed of carbon and hydrogen. Thus, ABS, with also nitrogen, has been first studied. ABS, besides, is composed of PS, PB and PAN, and the effect of mixture can be also analyzed. Additionally, halogens are commonly found in plastic wastes, either in the polymer molecules or in additives such as flame retardants. Thus, a residual ABS, with additives and fillers, has been also studied in Chapter 6. The effect of pressure, temperature and concentration of polymer has been analyzed, mainly focused on the yield to gasoline and its composition. Actual, more complex, plastic wastes have been also studied in Chapter 6. Plastic wastes from cellular phones (CP) have been chosen because of their content of ABS and complexity, focused also in the yield to gasoline and its composition. Plastic wastes with a high content of inorganic components, such as many of the wastes from electrical and electronic equipment, are not suitable for catalytic hydrocracking. Thus, steam gasification to produce hydrogen has been proposed and studied as an alternative valorization process in Chapter 7, applied to phenolic boards, the main plastic component in printed circuits. Electronic wastes usually include a significant amount of metals in their composition. These metals could act as catalysts in steam reforming of the plastics. This catalytic effect has been also studied, with nickel or tantalum capacitors (TC). The catalytic effect of the presence of molten carbonates in a vessel together with the wastes was the object of a previous study. In this work, the combined effect of nickel and molten carbonates has been also analyzed. Finally, Chapters 9 and 10 summarize the nomenclature and the literature used all over the work, respectively.