Pyrolysis has received much attention as a technology capable of breaking down waste plastics in recycling processes, but existing pyrolysis approaches often give rise to products with broad molar mass distributions and poor yields. Catalysts can improve yield and selectivity, but they are expensive and have limited lifetimes. Researchers at Yale University (New Haven, Conn.; www.yale.edu) have developed a method that could improve both of those parameters. The team, led by Yale engineering professors Liangbing Hu and Shu Hu, developed a highly selective, energy-efficient and catalyst-free pyrolysis method that can convert plastic into valuable chemicals.
A critical component of their work is a 3D-printed, electrically heated carbon column reactor consisting of three sections with decreasing pore sizes (see diagram). Using 3D printing to build the structure allowed the researchers to precisely control the dimensions of the reactor pores and investigate the effects of pyrolysis.
The first section of the reactor contains 1-mm pores, while the next section contains 500-µm pores, and the third section is made of 200-nm pores. As the chemicals pass through the reactor, the hierarchical porous structure plays a pivotal role in controlling the reaction progress of the chemicals, the researchers say.
The reactor design prevents larger molecules from advancing through the reactor before they’ve been adequately broken down, while also providing a way to control the temperature in the reactor, which prevents coking and other effects that can inhibit the process.
The researchers tested the reactor with polyethylene, and reported a record-high yield of nearly 66% of the plastic waste converted into hydrocarbons with sizes relevant to vehicle fuels (C8 –C18).
To demonstrate a more scalable design, the researchers also used a device made up of commercially available carbon felt. They found that this design — even without the optimization that a 3D-printed structure provided — still improved the selectivity of the pyrolysis products and achieved a satisfactory yield, converting more than 56% of the plastic into useful chemicals.
“These results are very promising and show a great potential for putting this system into real-world application and offering a practical strategy for converting plastic waste into valuable materials,” says Shu Hu, Yale University assistant professor of chemical and environmental engineering. The study is published in the journal Nature Chemical Engineering.