(514x) Hydrothermal Depolymerization of Polyolefin Using Supercritical Water Powered By Renewable Solar Thermal Energy

Plastic pollution has become one of the most pressing environmental issues, as the world’s ability to deal with plastic wastes is challenged by the rapid accumulation of plastic wastes caused by slow degradation rate, increasing sorting fee due to the complexity of the plastic products, and most important, little driving force from the society. Chinese import ban on global plastic wastes forces us to think about where to find a home for them [1]. Production of plastic wastes can also be a financial burden for related businesses due to enormous number of social costs added to industrial operations based on emissions [2]. Conventional thermal depolymerization process can convert the polymers into shorter hydrocarbons, but it still depends on the non-renewable fossil fuels. Therefore, a depolymerization process driven by a renewable energy source (such as solar thermal energy) will become the best solution as the fossil fuels combustion limitation era takes hold.

The proposed hydrothermal depolymerization process involving supercritical water is driven by renewable solar thermal energy instead of non-renewable fossil fuels used in the conventional process. The use of supercritical water enables fast, selective and efficient reactions to convert organic wastes to crude oil equivalent, in a result comparable to conventional thermochemical conversion methods like pyrolysis and gasification.

The proposed process starts by grinding the feedstock polymer in a granulator and mixed with water, and this mixture is then fed into the depolymerization reactor. The temperature and pressure are regulated in the reactor and the supercritical water depolymerization reactions are carried out in the supercritical water conditions (temperature: 380~500ºC, pressure: 7~30MPa), breaking down the long hydrocarbon chains of the polymer. The product stream then flows through a heat recuperator where the heat can be recycled. The separation of products is conducted in the distillation column, generating oil and gas products. The heat recycled from the heat recuperator is firstly used in the reboiler of the distillation system, boiling up the base flows. The “cool-down” fluid is then fed into the condenser of the distillation system, and eventually back to heat recuperator for the next cycle of the heat recycling. Clearly, the core of this process is the depolymerization reactor, which needs to be suitably designed using adequate knowledge of the process kinetics, with the specific processing conditions set to optimize the distribution of products desired. Likewise, an important aspect of the project is the integration of renewable Solar Thermal energy into the process. The solar energy is concentrated through Cassegrainian optics using inexpensive blow molded/vacuum formed mirrors, with the output of several optics combined and transmitted to the reactor using rectilinear sections of pipe. Finally, the radiant energy needs to be efficiently converted to heat with minimal radiative losses, and the heat likewise efficiently injected into the process. A heat pipe has been developed, characterized, and modeled to convert the concentrated solar thermal beam(s) to heat and injects it to the process. Because heat pipes can be designed to optimize operation at specific temperatures, this approach offers the ability to precisely control the temperature of the thermal transmission, enabling better process control.

Reference:

[1] A. L. Brooks, S. Wang and J. R. Jambeck, "The Chinese import ban and its impact on global," Science Advances, vol. 4, no. 6, p. eaat0131, 2018.

[2] B. C. Howard, S. Gibbens, E. Zachos and L. Parker, "A running list of action on plastic pollution," National Geographic, 10 Jun 2019.