CONTEMPORARY ADVANCES ON CONVERTING MARINE PLASTIC WASTE INTO READY TO USE FUELS


Introduction Since 1990, the global demand for plastic has increased by 5% every year. In 2019, the global plastic market size was valued at 568.9 billion USD, with an expected compounded annual growth rate of 3.2% over the next decade [1]. As the use of plastic in both industrial and municipal settings increases, plastic waste pollution becomes an increasingly major threat to both ecological and human health.


Marine ecosystems in particular have been negatively impacted by global reliance on plastics. The ecological consequences of marine plastic waste cannot be understated. According to Barnes et al., up to 80 percent of waste that accumulates on shorelines, the seabed, and the ocean surface is plastic. As plastics do not biodegrade, the waste eventually deteriorates into microplastics, which are then consumed by marine life [3]. According to Wabnitz et al., the bodies of most marine species, ranging from plankton to whales, now contain plastic. Plastic waste has become dangerously intertwined with the ecology of marine life, with plastic being seen in bird nests, worn by hermit crabs, and appearing in the intestines of turtles and albatross [5]. Nearly 300 marine species have been shown to ingest or become entangled in plastic waste, resulting in biological harm and death [6]. Because of its light weight, durability, and buoyancy, plastic waste is a serious biological hazard once seaborne. The inherent buoyancy of most plastic waste means the waste is easily swept by ocean currents, where it contaminates even remote regions of the oceans.


Roughly 50% of waste plastic is deemed “unrecoverable” and subsequently disposed of [7]. Among conventional techniques, landfi lls and incineration are the most common methods of disposal [8]. Incineration as a means of energy recovery is directly linked to the release of harmful compounds, including dioxins and furans, into the environment, which negatively impacts human and ecological health [9]. Recycling is regarded as the best possible solution for the plastic waste crisis. While there is unmistakable value in primary and secondary recycling of plastic solid waste (PSW), or the repurposing of plastics into new plastic products, through some blending of virgin materials, tertiary recycling is the ideal approach for converting PSW into high- value chemicals: namely, hydrocarbon fuels. Tertiary recycling, or cracking, is a process of degrading plastics, typically through pyrolysis or catalytic cracking. Pyrolysis and catalytic thermal cracking are the most promising and effective methods of depolymerizing post-consumer plastics.


Methods of Plastic to Fuel Conversion


Pyrolysis is a method of thermal depolymerization of plastics in the absence of oxygen. Typically, plastic is fed into a chamber, where it is heated to a narrow temperature range, without excessive variations. Oxygen is then purged from the pyrolysis chamber. The formation of carbonaceous char by-products is


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managed before it lowers the heat transfer to the plastic. Finally, the pyrolysis vapors are condensed to produce consistent, high- quality distillate [10].


The pyrolysis process can be further broken down into process subsets. Slow pyrolysis involves the use of a slow heating rate, while fast and fl ash pyrolysis requires rapidly heating the plastic to temperatures as high as 1300K [11]. Catalytic thermal degradation is a subset of traditional pyrolysis, in which catalysts are added to pyrolysis reactions. The addition of catalysts has been shown to offer numerous advantages over purely thermal pyrolysis. In polyolefi n pyrolysis, the addition of a catalyst lowered pyrolysis temperatures and residence time. The reduction of the degradation temperature and reaction time results in an increase of conversion rates from many polymers compared to thermal pyrolysis alone [12-15]. Specifi cally, catalytic pyrolysis has been shown to provide better control over the hydrocarbon product distribution in the pyrolysis of low and high-density polyethylene, as well as with polypropylene and polystyrene [16-20]. Several catalysts, such as zeolite [21], silica-alumina, and clay [22-23], have been shown to improve fuel quality, increase selectivity, and lower both the pyrolysis temperature and residence time [22].


Also promising is the method of hydrocracking. Hydrocracking, the cracking of larger hydrocarbons into fuel-range hydrocarbons. Hydrocracking is simply pyrolysis performed in the presence of hydrogen at elevated temperatures [24].


By far, the most discarded plastic types are polyolefi ns, such as polyethylene, polypropylene, and polystyrene. Overall, polyolefi ns account for more than half of all plastic waste [25-26]. As a result, a notable amount of recent research has been focused on methods of converting this waste into useful materials. Polyethylene (PE) and polypropylene (PP) are polymers containing only carbon and hydrogen [27]. As a result, fuels derived from this plastic waste are comparable to fossil fuels, and typically don’t require further upgrading.


Process Optimization


While effective, thermal pyrolysis is often economically infeasible, as the reaction requires signifi cant energy. As catalysts lower the activation energy required for pyrolysis, their use is essential for widespread adoption. Reducing energy input through catalysts reduces the reaction temperature and time, narrowing the product distribution.


Optimization of the plastic to fuel conversion process has been the subject of several studies. Several factors, including catalyst type, feed composition, particle size, and catalyst-to-polymer ratio have been shown to impact both fuel and conversion quality. Notably, zeolite (aluminosilicate mineral) based catalysts are also more effective, and substantially reduce the temperature of cracking. Reforming catalysts, including Pt/SiO2Al2O3 are shown to increase the octane number (i.e., the measure of a fuel’s ability to withstand compression in an internal combustion engine without detonating) while maintaining the carbon number. Feed composition can affect the speed of plastic decomposition. The decomposition of polyethylene and polypropylene is faster when mixed with polystyrene because it catalyzes the radical formation reaction. Acidic sites also play a role in cracking. With an increase in pore size, hydrocarbons degrade into smaller hydrocarbons, such as gas. Particle size should be considered. Catalysts with smaller particle sizes have larger surface areas for catalytic activities. However, this can translate to a smaller pore size for cracking. Catalyst loading methods are also of interest. Two methods of catalyst loading can be employed. Liquid phase contact is an effective method in which the catalyst is mixed with plastic. While conversions using this method are effective, catalyst recovery is extremely poor [22]. The alternative method utilizes vapor phase contact and requires the loading of the catalyst into a basket [23, 28].


In plastic pyrolysis, mass and energy transfer, due to the high viscosity and low thermal conductivity of plastics, are notable constraints. The reactor type, operating conditions, and product distributions all impact mass and energy transfer [29]. Uniform heat distribution is needed for effective mass and energy transfer. The use of fl uidized bed reactors (Figure 1) can facilitate a uniform distribution of energy, which is a crucial component in obtaining desirable hydrocarbons. Because of the rapid circulation and the turbulent gas fl ow in fl uidized bed reactors, a much higher effi ciency in heat exchange is attained, as well as better temperature control [30]. Sharratt et al. tested the catalytic cracking of HDPE (high-density polyethylene) with ZSM-5 (Zeolite Socony Mobil–5) in a fl uidized bed reactor, obtaining more than 65% of hydrocarbons, with a range of C3-C5. Screw kiln reactors have been shown to be more effective than batch reactors, producing lower percentages of heavy products. Another area of interest in optimizing heat transfer is the viscosity reduction of molten plastic. This can be achieved through the incorporation of solvents or oils, such as lubricating oil or vacuum gas oil. The


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